Evolution and Latest Trends in Cooling and Lubrication Techniques for Sustainable Machining: A Systematic Review

Abstract

This document presents a review on cooling and lubrication methods in machining. A systematic search of information related to these methods was carried out based on the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) methodology. The importance of the sustainability of machining processes is highlighted, as they represent between 10 and 17% of the total manufacturing cost of the final part and have negative environmental and health impacts. Although dry machining completely eliminates the use of cutting fluids, in many cases it produces unsatisfactory results due to the increase in temperature inside the tool, which requires prior analysis to ensure its viability compared to conventional techniques. On the other hand, semi-dry machining, which significantly reduces the volume of cutting fluids, is a more competitive alternative, with results similar to those of conventional machining. Other sustainable cooling and lubrication methods are also being investigated, such as cryogenic and high-pressure cooling, which offer better machining results than conventional processes. However, they have a high initial cost and further research is needed to integrate them into industry. While the combination of these cooling and lubrication methods could lead to improved results, there is a notable lack of comprehensive studies on the subject.

1. Introduction

Throughout history, machining processes have required solutions to minimize the heat and wear generated during the contact between the tool and the piece material. In their early stages, cutting fluids consisted of simple natural oils designed to enhance lubrication and reduce heat, thereby improving precision and extending tool life. However, as industrialization progressed and processes became more complex, it became necessary to innovate in the development of more effective and specific fluids for different applications [1,2,3].

During the 20th century, there was a major evolution of cutting fluids, with advances such as the introduction of emulsions and synthetic oils, designed to provide better cooling and lubrication (C/L) during the process. The wider use of these fluids led to remarkable progress, such as increased cutting speeds and improved accuracy of the machined parts. However, in the last decades of the century, the growing interest in environmental sustainability raised concerns about the negative impacts associated with conventional cutting fluids [4]. The generation of toxic waste and water pollution became critical issues. It was in this context that the ideas of dry and semi-dry machining emerged as more sustainable alternatives. Dry machining completely eliminates the use of fluids while semi-dry machining significantly reduces their volume [5,6].

As the viability and effectiveness of these methods were demonstrated, the industry gradually adopted them, focusing not only on environmental sustainability but also waste reduction and their potential to reduce tool wear [7]. Despite their benefits, dry and semi-dry machining present technical challenges in terms of temperature control and its application to certain materials. The goal remains to overcome these obstacles in order to establish these practices as the standard for machining processes [6].

These advancements are closely aligned with global sustainability goals and the transition to Industry 4.0, where efficient and environmentally friendly manufacturing is a priority. Leading industries such as aerospace and automotive manufacturing are adopting sustainable machining solutions to increase productivity and ensure compliance with environmental regulations [8]. The aerospace sector, for example, is increasingly adopting semi-dry machining techniques for processing difficult-to-cut materials like titanium and nickel alloys, which are crucial for manufacturing critical components such as turbine blades and structural parts [6].

This document presents a review that focuses on dry and semi-dry machining, but also includes other sustainable techniques such as cryogenic machining or a combination of different techniques. In order to contextualize the research, a systematic search of the literature on the subject was carried out using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses), from its origins to its current state, identifying possible future lines of development. Additionally, considering the current context, the review was conducted with a sustainability approach, looking at other processes aimed at more environmentally friendly machining practices.

2. Materials and Methods

As mentioned above, this document conducts a systematic review within the conceptual framework presented in the introduction, using a PRISMA-based literature search methodology adapted to the field of engineering. This research uses the guidelines defined by the PRISMA methodology in its latest 2020 revision. Furthermore, the document published by Blanco et al. in 2021 [9] is used as a reference, describing in detail the methodology to be used to carry out systematic reviews with PRISMA, including a case study focused on the most recent trends in the use of magnesium, aluminium and titanium in sectors such as automotive and aeronautics industries.

While PRISMA provides a robust basis for conducting systematic reviews, it is not without its inherent limitations. One of these constraints is its reliance on the availability and quality of the included studies, which can vary considerably depending on factors such as the scope of the database or the quality of the search terms used. Additionally, PRISMA does not evaluate the methodological adequacy or relevance of the selected studies, delegating this critical responsibility to researchers, which can introduce potential bias if not carefully managed. Lastly, another important limitation arises from the variability among the studies identified. These differences between the articles may manifest in aspects such as the methodologies employed, the approaches used for data collection, or the types of studies themselves, among others. This lack of uniformity can significantly complicate the synthesis, analysis, and discussion of the results, which, once again, remains the responsibility of the researchers.

As a consequence, the methodology used in this study is based on a systematic approach structured in several key steps. First, the research question was defined, with a focus on investigating the latest trends in lubrication and cooling systems, with an emphasis on sustainability. Subsequently, inclusion and exclusion criteria were defined in advance to filter the articles of the search. These criteria focused on aspects such as publication mode and quality standards. Concurrently, search criteria were established, comprising carefully selected keywords and their synonyms to ensure comprehensive coverage of the topic and to minimize bias during the search process. Once the initial set of articles was retrieved, a preliminary selection process was conducted. This involved using database tools to apply filters based on the predefined criteria. The texts were then reviewed, and articles that did not align with the research scope or failed to meet the established quality standards were excluded. The final selection of the literature was subjected to detailed reading and statistical analysis, supported by database tools, to categorize and organize them according to factors such as type, topics addressed, and relevance. This process facilitated a thorough analysis and ensured that the selected articles provided valuable insights into sustainable machining practices. Finally, the conclusions drawn from the analysis were synthesized to develop the results and discussion sections of this article. The search and analysis strategies are shown as outlines in Figure 1 and Figure 2.

2.1. Inclusion and Exclusion Criteria Used in the Search

The establishment of inclusion and exclusion criteria in advance provides a solid foundation for the research and ensures that the results obtained are consistent with the stated objectives. These criteria not only define the scope of the study but also minimize the risk of bias and ensure that the selection of the literature is relevant and representative for subsequent analysis [9].

In order to achieve a comprehensive understanding of the topic from its origins, no publication date restrictions were applied. This approach allowed for the inclusion of both historical developments and recent trends in sustainable machining techniques.

The primary focus was on journal articles, which are recognized for their comprehensive methodological procedures and reliable results. In addition, review articles were incorporated into the selection, as they provide a comprehensive overview of the current state of the field, including significant advancements. Conference proceedings were also included, acknowledging their role in presenting recent advancements, innovations, and emerging trends. These contributions are particularly valuable for identifying novel approaches to sustainable cooling and lubrication techniques.

The refinement of search queries within the specified topic area was achieved through the formulation of a series of keywords and synonyms that were meticulously arranged into Boolean equations (see Figure 3). This methodology ensured comprehensive coverage, thereby reducing the risk of omitting any potentially relevant studies.

The bibliographic resource selected for this review was Web of Science (WoS), chosen for its extensive repository of high-quality, peer-reviewed publications and its advanced tools for precise searches and detailed analysis. WoS ensures access to credible studies from high-ranking journals, enhancing the review’s reliability. Its advanced filtering options and Boolean search capabilities enable comprehensive and targeted literature searches, while its bibliometric tools support trend analysis and citation tracking. The selection of WoS as a bibliographic resource is consistent with the principles of PRISMA, namely those of rigor and transparency, thereby ensuring the validity of the review and the inclusion of impactful studies on sustainable machining.

The selection of publications was constrained to those of an Open Access nature, with the objective of ensuring global accessibility and thereby fostering collaboration in research. The application of strict quality criteria was a further essential element of the research process. The inclusion of articles was contingent upon their having undergone the process of peer review and having been published in journals that were ranked in the top quartiles (Q1 and Q2) of the Journal Impact Factor and Journal Citation Reports. This ensured a high standard of reliability and rigor across the analyzed texts. These criteria are summarized in

Table 1. Inclusion and exclusion criteria used in the search.

Inclusion/Exclusion Criteria	Feature
Date of search	28 November 2024
Type of study	Articles: journals, reviews and conference proceedings
Keywords and synonyms	See Figure 3
Information sources	Web of Science (WoS)
Databases in WoS	Option ‘All databases’
Language of publication	English
Publication mode	Exclusively Open Access
Quality criteria required for each publication	Peer-reviewed articles included in WoS. Journal articles limited to Q1–Q2
Quality criteria reviewed for each publication	Reviewed by Journal Impact Factor and Journal Citations Reports

.

2.2. Definition of Search Criteria

The objective of the present literature review is to address the issues of dry and semi-dry machining, as well as other sustainable processes, from their origins to the present day and possible future directions. In accordance with the search methodology described above, the utilization of keywords and their synonyms in the form of Boolean equations is used as the search method.

These terms are shown in Figure 3. The initial column defines the cooling/lubrication (C/L) methods that are being sought. The second column specifies the process type, in this case machining, along with its predominant variations, which are commonly used in dry and semi-dry machining. The third column is used for the correct selection of articles, narrowing down the field of machining. The subsequent column, containing the word ‘review’, is used to refine the search to texts containing state-of-the-art reviews on the subject, as these articles are particularly useful for developing monographs or similar researches. The last column contains words that focus on sustainability and energy efficiency. As this is a very broad topic with a large number of unrelated articles expected, the advanced ‘exact search’ option of WoS is used to ensure that the resulting texts from the search contain at least one term from each column.

The Boolean equations employed in the search process are as follows: TS = (dry OR semi-dry OR near-dry OR minimal quantity cooling OR minimal quantity lubrication OR minimal volume cooling OR minimal volume lubrication) AND TS = (milling OR machining OR turning OR drilling OR boring OR reaming OR cutting OR tapping) AND TS = (machin* OR CNC OR manufactur* OR fabrication OR substractive OR precision) AND TS = (review) AND TS = (sustainabili* OR carbon dioxide OR carbon emission* OR energy efficiency OR fossil fuel OR fuel consumption OR fuel saving OR global warming OR sustainable OR green OR energy-efficiency OR environment*).

With ‘exact search’ enabled in more options, the results of WoS search, when entered with these Boolean equations, yielded a total of 166 documents.

2.3. Literature Selection

This section describes the final selection process of the documents obtained in the previous search. Initially, the search was filtered according to the inclusion and exclusion criteria listed in Table 1. This filter was applied using the WoS search tools, including only ‘Open Access’ and English language documents, which resulted in the exclusion of 115 (69.3%) documents from the search. The literature was then subjected to a first reading to apply the remaining inclusion and exclusion criteria. Consequently, 5 (2.4%) were excluded on the basis that they did not meet the defined quality criteria. Finally, the remaining 14 (9%) were discarded because they did not relate to dry and semi-dry machining or other sustainable C/L processes.

The final selection of literature amounted to 32 documents, which constitutes 19.3% of the total. The selection process of the literature found, as previously explained, is summarized in the following flowchart shown in Figure 4.

2.4. Synthesis and Analysis of the Documents Included in the Literature Review

The 32 selected texts were subjected to a detailed analysis, encompassing content analysis, with the aim of highlighting methods, approaches and results. This process identifies common patterns, key contributions to the topic in question and possible gaps in existing knowledge. For the purpose of this study, the documents found were subjected to a more thorough reading, resulting in the collection of the data presented in

Table 2. Primary analysis of data from selected search documents. The symbol ‘X’ denotes that the documents under consideration pertain to or incorporate content relevant to the specified category.

Documents	Number of References	Publication Type	Publication Year	State of the Art	Process Optimization	Experiments Design	Conventional C/L Machining	Dry Machining	Semi-Dry Machining	Other Sustainable Processes	Country
[1]	138	Q1	2017	X			X	X	X	X	Spain
[2]	6	Q2	2022	X			X	X	X	X	Saudi Arabia
[3]	38	Q2	2012		X	X	X			X	Germany
[4]	2	Q2	2024	X	X		X	X			India
[5]	110	Q2	2021	X						X	India
[6]	7	Q1	2024	X			X	X	X	X	Russia
[7]	0	Q2	2019	X			X			X	Romania
[8]	21	Q2	2020	X				X	X	X	South Africa
[10]	7	Q2	2020	X	X			X	X	X	China
[11]	19	Q2	2020	X			X	X	X	X	Pakistan
[12]	48	Q2	2019	X			X	X	X	X	Spain
[13]	1	Q2	2020		X	X	X		X		India
[14]	2	Q1	2023	X			X	X	X	X	Egypt
[15]	53	Q2	2012		X		X	X	X	X	Alemania
[16]	19	Q2	2022	X	X		X	X	X	X	United Kingdom
[17]	24	Q2	2019	X			X	X		X	Italy
[18]	15	Q2	2019	X			X	X	X	X	Nigeria
[19]	23	Q2	2020	X			X			X	India
[20]	0	Q2	2024	X			X			X	Romania
[21]	3	Q2	2020	X			X	X	X	X	India
[22]	12	Q2	2022	X				X	X	X	Argentina
[23]	26	Q2	2012		X	X		X			Germany
[24]	7	Q2	2016	X				X			Malaysia
[25]	1	Q1	2023	X	X				X	X	China
[26]	1	Q1	2023	X	X			X	X	X	China
[27]	1	Q1	2024	X	X		X		X	X	United Arab Emirates
[28]	0	Q2	2024	X	X		X		X		Malaysia
[29]	0	Q1	2023		X	X		X			India
[30]	0	Q2	2020	X			X			X	Indonesia
[31]	11	Q2	2022	X			X			X	Inglaterra
[32]	55	Q2	2022	X				X	X	X	India
[33]	45	Q2	2012	X						X	Japan

.

As shown in Figure 5, a total of 31 texts (96.9%) were identified in the engineering field, which significantly exceeds those found in the other areas. It is also noteworthy that 16 (50%) documents were found to be on materials science and 10 (31.3%) on economics, environmental sciences, ecology and physics. These results serve to confirm the selection of keywords used in the search, since the main objective was to identify engineering texts with a focus on environmental, social and economic sustainability.

The majority of the analyzed texts correspond to recent publications, with 24 (75%) published since 2019. As illustrated in Figure 6, there is clear evidence of a growing concern for sustainability in recent years, with all the papers emphasizing this concept by highlighting dry and semi-dry machining as viable alternatives to consider in terms of economic improvement and reducing the environmental impact of machining processes. This suggests an enhancement in social awareness and the development of technologies in recent years that focus on economic, social and environmental responsibility.

As previously outlined, the word ‘review’ was incorporated into the search criteria in order to identify articles that contained the state of the art on the searched topic. Of the total texts analyzed, 27 (84.4%) carried out a state-of-the-art evaluation, which could also include an experiment to improve certain parameters of machining processes. A total of 12 (37.5%) documents concentrated on process optimization, of which 4 (12.5%) involved experiments with the aim of enhancing or offering a different perspective on machining processes.

The concept of sustainability was also a major focus of the research, with all processes geared towards this goal. This involved improving health and safety conditions for the operator and reducing the cost of machining processes. It is essential to improve dry and semi-dry C/L techniques, or other sustainable techniques such as cryogenic or high-pressure cooling machining, to reinforce the previous aspects. Of the documents examined, 20 (62.5%) texts used conventional C/L machining as the basis for the comparisons made in their documents, which serve to test sustainable C/L techniques. Of these, 21 (65.6%) texts were related to dry machining, 19 (59.4%) to semi-dry machining and 26 (81.3%) to other sustainable processes. The potential for integrating these processes, such as cryogenic cooling with semi-dry machining, is also noteworthy. Furthermore, dry machining was a recurrent theme across all articles, even if only in combination or in comparison with the main process of the article.

In relation to the provenance of the studies, with the initial author designated as the main author for the present analysis, India is the country with the highest number of published studies, with a total of 7 documents (22%), followed by Germany and China with 3 (9.4%) texts each. Finally, countries such as Spain and England also feature, though their contributions are comparatively less substantial.

To provide a more detailed vision of the state of the art of C/L techniques, the papers that performed experimental studies were classified according to the machining process as shown in

Table 3. Analysis of the literature reviewed according to the type of machining process. The symbol ‘X’ denotes that the documents under consideration incorporate content relevant to the specified category.

Documents	Machining Processes	Surface Integrity	Tool Wear	Tool Life	Chip Morphology	Tool Material	Workpiece Material
[3]	Drilling	X				Not specified	34CrNiMo6, AlMgSi1
[13]	Turning	X	X	X	X	CNMg 120408 coated with TiAlN	AISI 52100
[23]	Milling			X	X	PM-HSS + AlCrN	20MnCr5
[29]	Milling	X	X		X	Tungsten carbide coated with AlTiN	AZ91

. This table highlights the main variables studied in the experiments carried out in the papers, as well as the materials used in the workpiece and tools. In addition to the experimental designs developed in the 4 papers listed in Table 3, a large number of real cases were found in the other papers where the variables listed were studied and compared for different C/L techniques. These are not included in this table to avoid making the list excessively large.

3. Results

3.1. Sustainability in Machining

3.1.1. The Need for Sustainability in Machining

Sustainable manufacturing is widely recognized as the major industrial development trend of recent decades, driven by the need to balance economic growth with environmental and social responsibility. Sustainability can be defined as the ability to produce using strategies and techniques designed to minimize environmental and economic impacts reducing resource and production costs while supporting employment and improving the safety of machinery and equipment used in manufacturing processes. This approach intends to ensure that manufacturing processes are viable in the long term, in response to social and environmental demands, while maintaining competitiveness in the global marketplace [8,10].

It is an irrefutable fact that every manufacturing process is associated with costs, environmental impacts and risks to operator safety. Machining in particular is one of the most energy- and materials-intensive processes in this field. At each stage there are key considerations such as the use of machinery, the handling of raw materials, C/L fluids, energy optimization, tool life and the recycling of the chips produced. In the quest for sustainability in machining, the following objectives are being pursued [10]:

• An increase in energy efficiency and optimization of resource use;
• A reduction in process waste;
• An improvement in health and safety conditions for people involved in the process;
• A minimization of emissions of harmful substances.

3.1.2. Cooling and Lubrication in Machining

The literature reviewed emphasizes the critical role of C/L in ensuring the sustainability of machining processes. This is explained by the fact that the traditional cutting fluids utilized in industrial contexts constitute between 10% and 17% of the total manufacturing cost of the final part (see Figure 7) [11]. This variability arises from multiple technical and operational factors. First, direct costs associated with the procurement of cutting fluids vary considerably depending on the type of fluid and their specific properties, such as cooling capacity, viscosity, and oxidation resistance [12]. Second, indirect costs, such as the maintenance of application systems and pre-disposal treatment, can significantly increase the initial cost of the cutting fluid [13]. Furthermore, operational conditions and equipment configurations also play a critical role in influencing these costs. For instance, machining operations involving high cutting speeds or difficult-to-machine materials may necessitate the use of more sophisticated equipment and advanced cooling and lubrication technologies compared to conventional setups, thereby increasing both operational expenses and initial costs [1,6].

In addition to their economic impact, these fluids raise significant environmental and health concerns. Chemical compounds present in cutting fluids can generate hazardous waste, contaminate the environment and affect the health of exposed operators, reinforcing the need for strategies to minimize their use [12].

Cutting fluids are defined as mixtures formulated for the purpose of enhancing the efficiency and quality of machining processes. These mixtures are constituted by oil as the primary base to which various additives are added to optimize specific properties such as lubrication, cooling, corrosion protection and reduction in tool wear. Based on their chemical composition, cutting fluids can be categorized into two distinct groups [1]:

• Straight oils: These types of oil are water-immiscible cutting fluids composed mainly of mineral, vegetable or animal oils, which provide excellent lubrication and corrosion protection, making them ideal for low-speed or high-friction machining operations such as threading and broaching. However, their cooling capacity is limited due to their low thermal conductivity, making them less suitable for high-speed machining. In contrast, these oils offer advantages such as long life and high lubricating efficiency, but they generate mist during use, which poses a health hazard, and they have a significant environmental impact, especially if they are mineral-based. They are mainly used in specific applications where lubrication is a priority, although their limitations have led to the development of more sustainable alternatives [1].
• Water-soluble oils: These types of oil are water-diluted cutting fluids that combine the lubrication of oil with the high cooling capacity of water. They are primarily used in high-speed machining processes where thermal control is a priority. Depending on their oil content, they are classified as emulsions (high lubrication), semi-synthetic fluids (balance between lubrication and cooling) and synthetic fluids (excellent cooling, less lubrication). The advantages offered by these fluids include effective cooling, reduced flammability and enhanced visibility, but they have disadvantages such as limited life, susceptibility to corrosion, bacterial growth and foaming. These fluids are particularly well suited for machining materials that generate elevated levels of heat but require careful maintenance to avoid performance problems [1].

While these cutting fluids provide optimum levels of cooling, lubrication and cleanliness, they contain chemical elements that have the potential to be hazardous to the environment and human health. Their composition may include substances that can cause respiratory problems, allergies, skin problems and even more serious diseases such as cancer. These risks serve to reinforce the importance of their proper handling, the use of personal protective equipment and the development of less toxic or biodegradable alternatives that reduce their negative impact on health and the environment [1].

The attempt to minimize or eliminate the utilization of cutting fluids constitutes a significant major challenge due to their indispensable role in machining processes and their correlation with process parameters such as cutting speed, depth of cut and feed rate. The main functions of these cutting fluids are to provide the following benefits [14]:

• The reduction in frictional heat generated at the tool–workpiece interface, which allows for higher cutting speeds;
• The improvement of surface roughness and surface morphology of the machined part. They also protect the machined surface as well as the tools and machine parts against corrosion, thus extending their service life;
• The delivery the cutting fluid at high pressure to help break up the chips and remove them from the cutting zone. As a result, the lubricating effect of the cutting fluid on the cutting edges leads to improved cutting performance.

Cooling plays a fundamental role in machining processes as it directly influences the surface quality of the machined parts as well as the tool life. During machining, the contact between the tool and the workpiece generates high temperatures in the cutting zone due to friction and the plastic deformation of the material [7]. If these temperatures are not controlled, defects such as residual stresses and thermal deformation can occur on the workpiece surface and on the tool, affecting not only the structural integrity but also the dimensional accuracy and surface roughness of the machined part [1,2]. Residual stresses can cause microcracking, localized deformations and alteration of the material’s microstructure, which decreases the wear and fatigue resistance of the part. Thermal deformations can alter the appearance and mechanical properties of the workpiece, often manifesting as surface burns [12]. Additionally, thermal control prevents the part from undergoing uneven expansion or contraction, which improves dimensional stability and ensures specified tolerances are precisely maintained. Another important aspect is the roughness achieved on the machined part, which is also directly related to cooling, in addition to other factors such as cutting forces and vibrations generated during the process. Without proper cooling, excessive heat can locally soften the surface of the material, compromising machining accuracy and generating visible marks on the surface. On the other hand, the generation of excessive heat also accelerates the wear of the cutting tool, especially at the cutting edges, causing defects such as burr formation or material sticking, which affect the stability of the process [1]. This increased wear significantly reduces tool life and cutting quality, forcing frequent tool changes. This can mainly lead to accelerated edge wear, resulting in cracking, deformation and loss of hardness in the tool material. This is particularly relevant in difficult-to-machine materials where tools are often exposed to high pressures and temperatures for long periods of time [1,15].

Lubrication also plays a crucial role in machining processes as it has a direct impact on the quality of the machined parts, tool life and cutting efficiency [1]. Its main function is to reduce friction between the cutting tool and the workpiece, which, importantly, reduces the forces required in the process. This is essential to maintain process stability and avoid defects in the machined parts. Proper lubrication helps to extend tool life by significantly reducing tool wear modes [1]. The main mode of cutting tool wear that is dependent on the lubrication used in the process is the adhesion of material to the tool, which affects process stability. Correct lubrication reduces the build-up of material on the cutting edge of the tool, known as the build-up edge, and ensures a cleaner and more uniform cutting surface. This results in parts with superior surface quality, characterized by reduced roughness and increased dimensional accuracy [3]. Furthermore, lubrication facilitates the evacuation of chips generated during cutting, preventing them from interfering with the tool or damaging the surface of the part, a critical aspect in high-speed processes. In parallel with these two aspects, many lubricants are designed with additives that protect tools and parts against corrosion, which is particularly valuable for sensitive materials or in adverse environmental conditions [2]. Finally, the impact of lubrication also has an indirect effect on productivity and process sustainability. By reducing cutting forces and tool wear, it enables higher speeds and feeds to be achieved without compromising quality, optimizing production time and reducing resource and energy consumption [16].

The lubricating effect is more significant than the coolant when machining at low speeds and vice versa when machining at high speeds. This is due to the fact that at low speeds, the heat generated by friction at the tool–workpiece interface is moderate and the contact time between the two is longer. Under these conditions, lubrication significantly reduces tool wear and improves the quality of the machined surface by promoting smooth sliding between the contacting surfaces. In contrast, as the cutting speed increases, the heat generation during the process rises due to the elevated friction and plastic deformation of the material.

In summary, cooling and lubrication are essential elements in machining processes as their effectiveness has a significant impact on tool life, surface quality and overall process efficiency. Their performance is closely related to the application conditions and the properties of the fluid, such as density, viscosity and specific heat. But it is also dependent on a wide range of factors, including the type of process, the parameters associated with it, such as cutting speed or shear rate, the type of tool used and the environmental conditions surrounding the process [12].

3.1.3. Conventional C/L in Machining

The conventional lubrication and cooling strategy, also known as flood C/L, consists in applying large volumes of cutting fluids directly to the machining zone. As previously discussed, these fluids fulfill critical functions, including cooling, lubrication of the machined component and tool, and chip removal. This strategy has been extensively adopted on a global scale, attributable to its numerous benefits in terms of enhancing productivity across a wide range of machining processes [1,12].

The application of flood C/L has been demonstrated to offer numerous advantages in machining operations. A primary benefit is its ability to ensure a uniform and precise surface finish on the machined parts, thereby producing superior quality outcomes across a diverse spectrum of environments and parameters. This approach effectively mitigates thermal defects such as deformations and residual stresses, leading to enhanced dimensional stability and part tolerances [7]. Furthermore, flood cooling enhances operational efficiency by enabling the machine to operate at elevated cutting speeds and feeds, thus optimizing process parameters. This is achieved by applying a significant amount of cutting fluid, which maintains low temperatures in the cutting zone while providing adequate lubrication. It also acts as a means of efficiently removing chips, preventing them from accumulating and potentially damaging the tool or work surface, which is particularly important in high-speed or continuous machining operations. The aforementioned properties of the cutting fluid also offer notable benefits in the context of cutting tools, as discussed in the previous section. These benefits include improved tool life, which in turn reduces the need for frequent tool changes and the associated costs [7,12].

However, this strategy faces significant limitations that challenge its sustainability and efficiency in machining operations. One of the main concerns is its environmental impact. The cutting fluids used in this technique contain chemical components that can generate hazardous waste, contaminate soil and water, and adversely affect operator health. The environmental impact is also reflected in energy consumption, as the continuous flow of fluids requires energy for both pumping and recirculation, potentially accounting for up to 50% of the system’s total energy demand [2]. Additionally, these fluids are prone to microbial growth, which increases toxicity risks and necessitates frequent maintenance [1]. From an operational perspective, although flood C/L achieves remarkable results across various environments and processes, its effectiveness diminishes in high-speed operations or when machining difficult-to-machine materials. Under these conditions, insufficient fluid transfer to the cutting zone can reduce its C/L capacity, leading to accelerated tool wear and deteriorated surface quality [12]. Another significant challenge is the associated cost. Implementing flood systems requires considerable investment in specialized equipment, such as recirculation and filtration systems, as well as the ongoing maintenance of the cutting fluids. As noted earlier (see Figure 7), these costs can represent between 10% and 17% of the total machining process cost, limiting its economic feasibility for small operations or limited budget industries [2]. Furthermore, in unfavorable scenarios, such as high-speed processes, the total cost of cutting fluids can exceed 17% and even reach up to 30% of the total cost. Such extreme cases highlight the inappropriateness of this strategy, which is therefore not included in the standard cost representation of cutting fluids shown in Figure 7 [17].

The implementation of flood cooling and lubrication strategies in industrial applications is initiated by with the design and installation of advanced systems that ensure uniform fluid distribution within the machining zone. These systems generally comprise high-pressure pumps, recirculation filters, and specialized nozzles that are engineered to direct the flow with precision toward the tool–chip interface [1,3]. Furthermore, it is imperative to provide comprehensive training to machine operators, equipping them with a comprehensive understanding of the system’s technical intricacies and ensuring their proficiency in fluid handling protocols to ensure safety. Regular maintenance of the systems and efficient fluid management are essential to sustain their effectiveness. This involves monitoring parameters such as pH levels and additive concentrations, as well as performing regular filter cleaning to prevent blockages. A lack of maintenance can lead to reduced fluid lifespan and negatively impact the machining process quality [13,14]. From an economic perspective, the initial implementation of these systems can be costly due to the required infrastructure. However, the advantages in terms of enhanced productivity, superior machining quality, and extended tool life justify this investment, particularly in applications demanding high precision and durability [12].

The ensuing sections provide detailed analyses of this method, intended to serve as a basis for comparison with the sustainable strategies discussed in the subsequent sections.

Surface quality is a critical indicator of the integrity of machined components, and it is primarily measured by the average surface roughness (Ra), which is calculated as the arithmetic mean of the absolute deviations of the surface profile from the median line within the evaluation length. Another common parameter is the average height of the five highest peaks and the five deepest valleys (Rz). These variables are typically correlated with cutting speed and feed rate, as these factors directly influence surface quality by affecting temperature, tool wear and chip formation [12]. At lower cutting speeds, the tool–material interaction may result in higher surface roughness due to increased shear stress and burr formation. As the cutting speed increases, surface roughness generally improves, reaching an optimal point. However, it is important to note that excessively high cutting speeds can lead to degradation in surface quality due to accelerated tool wear and excessive heat accumulation [18]. As previously discussed, flood C/L strategy has been shown to achieve notable outcomes across a wide range of scenarios [1].

In the turning process conducted by Makhesana et al. [13], a range of Ra between 2.2 and 2.8 µm was achieved for a cutting speed of 130 m/min, a feed rate of 0.2 mm/rev, a depth of cut of 1.2 mm, and a lubricant flow rate of 5 L/h. The selected cutting speed falls within an ideal intermediate range to balance machining efficiency and surface quality. The material utilized in this study was AISI 52100 steel, which is known for its moderate machinability. Consequently, it can be deduced that the obtained Ra values are consistent with the prevailing standards for turning processes. In a separate study, Khan et al. [2], a turning process was reviewed using various types of lubricants applied via flooding. For a feed rate of 0.2 mm/rev, Ra values ranging from 3.1 to 4.3 µm were reported for cutting speeds between 55 and 100 m/min, with the best Ra values achieved at a cutting speed of 80 m/min. This represents the optimal point between cutting speed and surface quality, as discussed in the previous section. In the work by Biermann et al. [3], a drilling process was performed using flood C/L, with two different materials utilized for the workpieces. For 34CrNiMo6 steel, a cutting speed of 200 m/min yielded Rz values ranging from 1.7 to 6 µm for feed rates of 0.1–0.15 mm/rev. Conversely, for the AlSiMg1 aluminum alloy, an elevated cutting speed of 300 m/min yielded Rz values ranging from 6.5 to 8.5 µm for feed rates ranging from 0.15 to 0.25 mm/rev. These outcomes underscore the impact of material properties under analogous machining conditions and underscore the pivotal role of feed rate, wherein an escalation in feed rate results in an augmentation in surface roughness. The results presented herein demonstrate the significant influence of cutting speed and feed rate on surface quality. However, it should be noted that surface quality of machined components is highly dependent on process conditions, including the machine tool, cutting tool, materials used and the operational environment. Consequently, considerable variability in results can occur even for similar processes.

Another variable directly associated with surface quality is the machining-affected layer, which refers to the microstructural and mechanical changes induced in the surface and subsurface of the material as a direct consequence of the thermal, mechanical and chemical forces present during the cutting process. Typically, this variable is assessed through the hardness of the affected layer, which is influenced by various factors [7,12]. The generation of heat during machining can result in recrystallization, phase transformations, and grain growth, leading to either surface hardening or softening. Plastic deformation at the cutting interface may also contribute to surface hardening. In addition to the hardness of the affected layer, the depth of the layer, the distribution of residual stresses within the workpiece, and the formation of burrs on its surface are commonly measured to evaluate the impact of machining processes [1].

The hardness of the machining-affected layer is a critical parameter for evaluating the surface quality of machined components, as it reflects alterations in the material’s resistance to permanent deformation. These changes can considerably influence the performance of the component, particularly in terms of wear, fatigue and dimensional stability [19]. The assessment of the hardness of the affected layer is typically conducted through the implementation of techniques such as Vickers micro-hardness testing, a method that facilitates the generation of a hardness profile extending from the surface to the material’s interior [12]. Alternatively, Rockwell hardness testing is frequently utilized for layers situated deeper within the material [2]. The present study undertakes an analysis of the hardness of the machined layer under the flood C/L method, a technique that has been demonstrated to effectively preserve the stability of the mechanical properties in close proximity to the machined surface. In the research conducted by Makhesana et al. [13], a drilling process was carried out under the previously specified conditions. The microhardness of the surface layer was measured at depths ranging from 100 µm to 1600 µm using a Vickers microhardness tester with a 200 g load applied for 15 s. The resulting measurements yielded values ranging from 725 to 775 HV, indicating minimal changes in the hardness of the machined surface. These values were close to the base material’s hardness of 55 ± 5 HRC, suggesting that the consistent cooling capability of the method minimized adverse thermal and mechanical effects on the machined surface. This approach has been shown to reduce the tendency for work hardening or potential alterations in the material’s microstructure, which are often observed under less efficient machining conditions. The findings of this study underscore the importance of effective cooling strategies in preserving the integrity and performance of machined components.

The correlation between burr height and the machining-affected layer is well documented, with burr formation having a substantial impact on the mechanical and thermal properties of the material’s surface. The formation of burrs is typically observed along the edges of the workpiece, resulting from the ‘pushing’ of material rather than a clean cut, a phenomenon that is particularly pronounced in ductile materials. It is noteworthy that material ductility increases with temperature, thereby amplifying deformation and consequently increasing burr height. Furthermore, residual stresses generated within the affected layer have the capacity to influence the geometry and size of these burrs [1,3]. Burr height is typically measured using white-light optical microscopy, a technique that provides the precision required to quantify the vertical dimension of the burr from the material’s base. Burr height has direct implications for product quality, as taller burrs can compromise surface quality and necessitate additional deburring processes [15]. In the study conducted by Biermann et al. [3], a drilling process was performed under previously described cutting conditions, and burr height was examined in a conventional C/L environment, correlating it with feed rate. Burr height values for 34CrNiMo6 steel ranged between 350 and 500 µm, while those for the AlSiMg1 aluminum alloy ranged between 625 and 725 µm, with higher values observed at greater feed rates. The aluminum alloy produced taller burrs due to its higher ductility. Furthermore, it was observed that an increased feed rate correlates with higher thrust forces, which can lead to greater deformation of the remaining material and consequently larger burr formation. Therefore, it is crucial to appropriately control cutting parameters to avoid compromising the characteristics of the machined component.

Cutting forces are defined as the mechanical interactions that occur between the tool and the workpiece during machining processes. These forces are a critical factor in evaluating process performance and machining conditions. These forces can be classified into three primary components: the main cutting force (Fc), which acts in the cutting direction and is responsible for material removal; the feed force (Ff), associated with the tool’s movement in the feed direction; and the radial force (Fr), which acts perpendicular to the feed plane and is generally smaller in magnitude but significantly impacts system stability [1,18]. The influence of these cutting forces on machining efficiency and stability is determined by a variety of operational and material parameters. A comprehensive analysis of these forces is typically conducted by examining several key variables, including cutting time, feed rate, cutting speed, and the number of machining steps. Increases in cutting time have been shown to result in heightened tool wear, leading to elevated forces due to diminished cutting efficiency [13]. The feed rate is another crucial parameter; as it increases, the main cutting force also rises because more material volume is removed per unit of time, especially in the case of hard-to-machine materials. Cutting speed also plays a significant role; at very high speeds, excessive heat generation can lead to accelerated tool wear, thereby increasing cutting forces. Conversely, at elevated cutting speeds, the tool–material interface experiences a reduction in contact time, thereby diminishing cutting resistance [2]. Furthermore, elevated temperatures within the cutting zone can transform the material’s physical state, enhancing its malleability and facilitating its machining. Finally, machining steps are directly related to machining time. As the number of steps increases, cumulative heat generation and progressive tool wear contribute to higher cutting forces. This underscores the critical need to optimize machining parameters to ensure performance and stability [2,12].

A review of the documents reveals several significant trends in the analyses of cutting forces. In the study by Nita et al. [20], the authors examined a turning process involving an aluminum alloy 7075 workpiece. The experiment was conducted using an uncoated tungsten carbide insert under flood C/L. Cutting forces were measured using a Kistler 9257B piezoelectric dynamometer, and different cutting speeds and feed rates were evaluated. The study found that the magnitude of the main cutting force increased with an increase in feed rate from 0.079 to 0.205 mm/rev, which can be attributed to greater material removal rate (MRR) per unit of time. The force values ranged from 65 to 105 N at a constant cutting speed of 51 m/min. Conversely, the cutting forces decreased with increasing cutting speed, primarily due to the reduced contact time between the tool and the material. For a constant feed rate of 0.079 mm/rev, the main cutting forces exhibited a range between 65 and 55 N as cutting speeds increased from 51 to 181 m/min. Additionally, under conventional wet conditions, the formation of built-up edges was observed. These edges consist of machined material accumulating and adhering to the tool’s cutting edge during machining, leading to increased cutting resistance. However, this phenomenon diminished at higher cutting speeds due to reduced material adhesion to the tool. Regarding machining steps, the work by García-Martínez et al. [12] examined a milling process under conventional C/L conditions, with a cutting speed of 72 m/min and a feed rate of 0.1 mm/tooth. A cutting length of 1200 mm was performed, corresponding to a total of 12 steps. It was observed that the magnitude of the main cutting force exhibited a progressive increase from 1201 N to 1435 N over the course of these steps, a phenomenon primarily attributed to tool wear and the accumulation of heat. During the initial phases of machining, conventional cooling methods effectively maintained cutting forces at moderate levels by dissipating the generated heat. However, as the steps progressed, the effectiveness of conventional cooling decreased due to the accumulation of residue and a reduction in temperature.

One of the primary challenges in machining processes is the generation of heat at the tool–workpiece interface. The calculation of cutting temperature is based on the heat generated in the cutting zone. This heat is primarily generated by the plastic deformation of the material and is subsequently transferred to the chips, the tool, and the workpiece, exerting a detrimental effect on the tool’s lifespan, dimensional accuracy, and the surface quality of the machined components [12]. Conventionally, cutting temperature measurement is facilitated by thermocouples and infrared radiation sensors. Each method has its own set of advantages and limitations. Thermocouples are cost-effective and accurate but are less effective in capturing rapid temperature changes. Infrared sensors, although more expensive, are ideal for high-speed processes [16]. The correlation between this parameter and machining time and cutting speed is well documented. At low cutting speeds, the generated temperatures are moderate due to the lower energy converted into heat. However, this scenario results in prolonged contact time between the tool and the workpiece, increasing heat accumulation. As cutting speed increases, the initial temperature rises significantly due to greater mechanical stress. It has been demonstrated that at higher levels of cutting speed, the temperature of the workpiece decreases as a result of enhanced thermal transfer to the chips, which function as heat conductors [7,11]. Machining time exerts a direct influence on thermal accumulation. Prolonged machining times leads to elevated temperatures in both the tool and the workpiece, potentially resulting in thermal deformations and surface defects in the machined component [3].

In the study conducted by Ishfaq et al. [11], the turning process of AISI 4037 steel was analyzed using a Sandvik carbide insert tool coated with TiCN + Al₂O₃. The study revealed a direct correlation between cutting speed and temperature, with an increase in temperature from 660 °C to 750 °C as cutting speed increased within a range of 160–260 m/min for a feed rate of 0.10 mm/rev. When the feed rate was increased to 0.20 mm/rev, the cutting temperature rose to a range of 750–840 °C within the same cutting speed interval. This phenomenon is attributed to the augmented MRR, which engenders elevated cutting forces and friction at the interfaces, thereby diminishing the time available for heat dissipation. In this process, the preponderance of heat generation is attributable to primary deformation, sliding at the tool–chip interface, and contact between the tool and workpiece. In a study by Nita et al. [20], the milling process of a Ti6Al4V titanium alloy was examined using an APMT tool coated with TiN. The study focused on the tool–material interface temperature as a function of cutting speed and for three different feed rate values. The experiment considered a range of cutting speeds from 60 to 180 m/min. For a feed rate of 0.04 mm/rev, the temperature ranged from 110 to 160 °C. For a feed rate of 0.06 mm/rev, the temperature ranged from 115 to 170 °C. Finally, for a feed rate of 0.08 mm/rev, the temperature ranged from 125 to 190 °C. As was the case in the preceding scenario, the temperature exhibited an upward trend in alignment with the cutting speed and feed rate. As with the preceding case, the temperature exhibited a parallel increase with both cutting speed and feed rate. Despite the observed paralleling trends, a significant discrepancy exists in the reported temperature values. This discrepancy is primarily attributed to variations in parameters, materials, and machining processes, as well as the distinct environments and machines utilized in the respective experiments.

Tool wear is defined as the progressive deterioration of the cutting tool surface due to the interaction conditions between the workpiece material and the tool during the machining process. Tool wear has been shown to have a direct impact on the geometry and functional properties of the tool itself. Additionally, it can have a significant effect on the quality of the machined component, particularly with regard to dimensional accuracy, surface finish, and the presence of thermal defects [1,12]. The measurement of tool wear is typically conducted using optical or tactile probes, which evaluate the extent of wear along the tool flank. The quantification of tool wear is typically accomplished by measuring the width of the wear band (VB) on the flank surface. This parameter is influenced by several factors, including cutting speed, the materials used in both the tool and the workpiece, cutting length, and the temperature generated during the process. A notable correlation exists between tool wear and machining time, with extended machining periods often resulting in increased wear, as evidenced by numerous studies [2].

Tool life is a critical parameter that is closely linked to the previously discussed phenomenon of tool wear. It directly affects operation time and production costs. A longer tool life reduces the frequency of tool changes during machining, thereby minimizing downtime and enabling a more continuous workflow. This is particularly advantageous in mass production environments, where each interruption significantly impacts operational efficiency. By extending tool life, process costs are reduced, thereby improving profitability and overall process efficiency [1]. Tool life is typically quantified in terms of either the elapsed time or the number of components produced until the acceptable wear limit is attained. Generally, this value is considered acceptable until the VB falls within the range of 0.3–0.6 mm, depending on the type of material being machined, the process conditions, and the established quality standards [3,12].

As machining time increases, tool wear accumulates progressively due to prolonged exposure to the interaction conditions between the workpiece material and the cutting tool. In the study conducted by Makhesana et al. [13], a drilling process was performed using a CNM 120408 tool coated with TiAlN under the previously described process conditions. Tool wear analysis revealed that after 10 min, the accumulated wear reached 0.2 mm, indicating the initial effectiveness of flood C/L in controlling the heat generated in the cutting zone. After 20 min, the wear increased moderately to 0.3 mm. By 35 min, the accumulated wear reached 0.36 mm, reflecting a progressive yet controlled increase. These findings suggest that flooding is an effective method of minimizing tool wear by providing continuous C/L. In a related study, Nita et al. [20] reviewed a practical case of a milling process that involved tools coated with AlCrN for machining the titanium alloy Ti6Al4V. Utilizing a cutting speed of 86 m/min and a feed rate of 1.026 mm/rev, the average tool wear was measured at 82.2 mm. This outcome is deemed suboptimal, given the material’s challenging machinability and the inability of conventional C/L methods to adequately dissipate the generated heat. A significant proportion of the heat, approximately 80%, remained within the tool, thereby expediting its deterioration. These findings underscore the limitations of the flood C/L method in effectively managing thermal challenges during the machining of difficult-to-machine materials.

The conventional C/L method in machining continues to serve as a foundational technique within the industry, ensuring consistent outcomes in terms of surface quality, tool life, and overall process efficiency. Nevertheless, its reliance on cutting fluids based on mineral oils and water poses significant challenges, including high environmental impact, health risks to operators, and substantial operational costs, particularly those associated with waste management and disposal [12]. While its effectiveness in certain contexts remains unquestionable, current demands for more sustainable and efficient processes highlight its limitations. In the context of an industrial landscape that is increasingly focused on sustainability, the flood C/L must either evolve or be superseded by more sustainable alternatives that not only minimize environmental impact and operational costs but also enhance the performance standards traditionally imposed [14,15].

3.1.4. Sustainable Machining

As previously stated, it is imperative to promote sustainable machining that optimizes C/L, with the objective of minimizing the use of conventional cutting fluids applied by flooding. In addition to the deleterious environmental impact, these fluids present a range of health risks to operators, including respiratory and skin problems. The reduction in these fluids’ use contributes to enhanced sustainability of the process and a notable reduction in associated operating costs, benefiting both industry and the environment [21].

In addressing this issue, alternative non-traditional C/L techniques have been developed, including dry machining, semi-dry machining, cryogenic cooling or high-pressure cooling (HPC) or even the combined use of these methods [22]. Furthermore, there has been a focus on the investigation of alternative cutting fluids, with a particular emphasis on biodegradable fluids that exhibit reduced aggression towards the environment and human health. These fluids are formulated to meet sustainability requirements while maintaining the requisite performance for machining operations [22].

The implementation of these alternatives, whether employed individually or in combination, provides customized solutions to a wide range of conditions and applications (see Figure 8). Their implementation has been demonstrated to markedly reduce the issues associated with conventional cutting fluids, consequently facilitating cleaner, safer and more cost-effective processes within the industry [22].

3.2. Dry Machining

Dry machining is a process that does not utilize conventional cutting fluids. The primary benefit of dry machining is the prevention of contamination from atmospheric or aqueous sources. Moreover, the absence of fluid residues on the workpiece or chips reduces the necessity for subsequent cleaning and residue treatment, thereby lowering costs and energy consumption. This approach also eliminates the potential for fluid residue contamination of the workpiece and chips, facilitating their handling and subsequent processing [18].

Nevertheless, the elimination of cutting fluids considerably reduces the capacity to dissipate the heat generated during the machining process. Consequently, there is an increase in the temperature of both the cutting tool and the workpiece, which gives rise to the following effects [16]:

• Accelerated tool wear and reduced tool life, resulting in increased operating costs;
• Thermal deformation of the workpiece, cutting tool and machine, causes a reduction in machining accuracy. These issues are primarily manifested as dimensional inaccuracies;
• The surface layers of the workpiece destabilized by phase transformations, residual stresses and other thermally induced defects that affect the mechanical properties of the machined component. This results in micro-cracks in the surface as well as damage from oxidation and corrosion.

The absence of cutting fluids can result in inadequate chip evacuation, particularly in processes where chips are produced in large quantities or are challenging to manage. This phenomenon can lead to clogging of the cutting zone, resulting in reduced efficiency and the emergence of surface defects. Furthermore, there is an augmented probability of material adhesion to the cutting edge of the tool, which can compromise both cut quality and dimensional accuracy [23]. In the document published by García-Martínez et al. [12], a turning process was reviewed under flooding C/L and dry machining for the Ti6Al4V material, utilizing uncoated carbide round insert tools. The experiment was conducted at cutting speeds ranging from 90 to 150 m/min and a feed rate of 0.15 mm/rev. The study identified significant chip adhesion to the cutting tool due to the low thermal conductivity of the titanium alloy in the workpiece, which caused the heat generated during machining to concentrate at the tool–chip interface. This factor, in conjunction with the absence of C/L at the tool–material interface, was responsible for premature tool breakage and higher main cutting forces when the cutting speed exceeded 100 m/min during dry machining. The tool life values in the dry machining process were approximately 50–60% lower compared to the same process carried out under flood C/L conditions at the studied speeds. Conversely, the main cutting force values obtained under dry conditions were 0–10% lower than those recorded under flood C/L across the same speed range. This reduction was attributed to the material softening caused by the high cutting temperatures generated during dry machining, making the material more ductile [12].

The employment of dry machining is reserved exclusively for scenarios in which the quality of the product is guaranteed to be, at a minimum, equivalent to that obtained with conventional cutting fluids [2]. Consequently, the implementation of dry machining necessitates a comprehensive examination of the conditioning factors of the process. These encompass the type of process, the type of tool and workpiece, the machine to be utilized, the process parameters, and the appropriate management of the chips produced [1]. The viability of dry machining is frequently demonstrated at reduced cutting speeds and in instances where the workpiece does not demand high levels of dimensional or shape precision.

An example of a successful industrial implementation is described in the article by Bernhard Karpuschewski et al. [23], wherein gear milling was executed employing powder metallurgy high-speed steel (PM-HSS) under dry cutting conditions. The primary objective of this study was to evaluate the technical and economic feasibility of this method. The study utilized PM-HSS tools with advanced AlCrN coatings, which were specifically designed to withstand high temperatures and reduce wear. The workpieces, case-hardened steel gears manufactured from 20MnCr5, exhibited modules ranging from 1.5 to 4 mm and gear VB between 29 and 87 mm. Cutting speeds ranging from 200 to 300 m/min were employed in this study. The primary focus of the study was on the service life of the tools, with the objective of achieving acceptable tool life values without compromising gear quality at cutting speeds of 200 m/min. The investigation revealed that the primary factors influencing tool life were the average chip thickness, the average cutting length, and the number of impacts. The average chip thickness is indicative of the cross-sectional area of the generated chip. An increase in chip thickness leads to an increase in the mechanical load on the cutting edge, thereby accelerating tool wear. The average cutting length is defined as the distance the tool travels in contact with the material. Increased heat generation due to friction is a consequence of longer cutting lengths, leading to escalation in thermal wear of the tool. Finally, the number of impacts indicates the frequency with which a tool tooth contacts the material per meter of cutting length. A higher number of impacts distributes thermal and mechanical loads more evenly, thereby extending tool life. In conclusion, the service life of tools is contingent on a multifaceted interaction between these parameters, necessitating meticulous optimization to ensure tool durability and process efficiency [23].

In the study conducted by Ishfaq et al. [11], the turning process of 17-4 PH stainless steel was reviewed under various C/L environments, including dry and flood machining. The experiment was conducted with a cutting speed of 78.5 m/min and a feed rate of 0.143 mm/rev. The variables of interest were analyzed as a function of the depth of cut, which ranged from 0.2 to 1 mm in increments of 0.2 mm. The experiment revealed that the highest recorded temperature during dry machining was 151 °C at a cutting depth of 1 mm. This temperature was the highest compared to other C/L strategies, being 84% higher than that obtained with flood C/L. This disparity was found to amplify with increasing depths of cut, with the cutting temperature during dry machining at a cutting depth of 0.2 mm being 30% higher than that under flood C/L. The primary factors contributing to this increase in temperature were identified as augmented tool wear and elevated cutting forces, which resulted in enhanced frictional heat generation at the tool–workpiece interface. In regard to tool wear, the values recorded were also higher compared to other C/L strategies, with a maximum VB of 184 μm observed at a depth of cut of 1 mm, representing a 24% increase over flood C/L. Once more, the difference in VB between flood C/L and dry machining became more pronounced with increasing depths of cut. The absence of lubrication resulted in elevated friction at the tool–workpiece interface, thereby contributing to accelerated tool wear. Finally, a minimum Ra of 2.37 μm was obtained at a depth of cut of 1 mm. In this case, the results obtained during dry machining were also the least favorable, with Ra values 18% higher than those obtained with flood C/L at its maximum value. Furthermore, more pronounced defects, such as thermal and mechanical marks caused by accelerated tool wear and the lack of temperature control, were observed in dry machining. Consequently, while dry machining presents itself as a viable application, it is essential to recognize its significant limitations and the specific environments in which it can be implemented to maximize efficiency [11].

An alternative method for enhancing the efficacy of dry machining is to apply a coating to the tool comprising a material with superior properties, such as diamond. This approach is also a widely employed in conventional C/L processes, where it has been demonstrated to yield substantial improvements in process performance [2]. The extant literature indicates that tool coating results in an improvement in tool life compared to uncoated tools during the dry machining process. In contrast, the use of an uncoated tool has been shown to result in higher surface roughness and the potential formation of undesirable edges may form on the machined part. Studies of carbide-coated tools have also demonstrated comparable values to those obtained by flooding. However, it is important to note that tool coating is subject to inherent limitations in demanding processes where high rates of tool wear can occur [2]. This claim is further substantiated by the findings of the study conducted by Pawanr et al. [4], which examined a practical case analyzing the performance of cutting tools coated with different materials. Three coatings were applied to cemented carbide tools as the base material: a thick TiN/TiAlN coating, a thin TiN/TiAlN coating, and a TiAlSiN nanocomposite coating. These coatings were applied using physical vapor deposition (PVD). A turning process was performed on malleable cast iron KTZ 700-02, and different variables were analyzed. The Fc was measured as a function of cutting speed. For a feed rate of 0.2 mm/rev, the values presented in Figure 9 were obtained. These results show that for all tools, the forces remained nearly constant or even decreased as the cutting speed increased. This behavior is primarily attributed to the balance between thermal softening and dynamic hardening. Among the tools examined, the nanocomposite-coated tool and the thin-coated tool exhibited the lowest forces across all speed ranges due to their lower coefficient of friction, higher hardness, and better wear resistance. In contrast, the thick-coated tool generated the highest forces due to its greater surface roughness and relatively lower hardness, resulting in a 50 N difference compared to the other coatings at the highest cutting speed. The surface roughness behavior was also studied under different conditions, and improved surface quality was observed as the cutting speed increased, with a maximum value of 1.2 µm at the highest cutting speed of 250 m/min. The values obtained at elevated speeds were analogous to those attained under C/L conditions, thereby substantiating the efficacy of tool coatings and the optimization of process parameters. This outcome is concomitant with the satisfactory machinability of the cast iron utilized as the workpiece material in the process. The study culminates with the recommendation to operate at elevated cutting speeds, minimal depths of cut, and reduced feed rates to mitigate cutting forces and specific energy consumption while attaining optimal surface quality [4].

Another instance of the utilization of coated tools can be observed in the turning process conducted by Makhesana et al. [13], as previously delineated. This process was performed under flood C/L, as well as under conditions of dry machining. The experiment was conducted using a CNM 120408 tool, coated with TiAlN, on AISI 52100 steel. The cutting speed was set at 130 m/min, the feed rate at 0.2 mm/rev, and the depth of cut at 1.2 mm. The outcomes of the experiment under dry machining conditions were the least optimal in comparison to other C/L methodologies, with Ra values ranging between 3.5 and 3.8 µm, which is approximately 46% higher than that obtained under the flood C/L. Furthermore, the microhardness measurements taken from a depth of 100 µm to 1600 µm revealed a range of 725 to 760 HV. These values were analogous to those obtained under flood C/L; however, the variability of the values at each machined surface depth was pronounced. This variability was particularly evident at a depth of 800 µm, where the maximum value under flood C/L was 775 HV, while the minimum value under dry machining was 725 HV. The influence of various parameters, including the heat generated during the process, was considered. Despite the differences in values at each point, no significant deviation from the base material hardness was observed, and the results obtained from both methods were deemed acceptable. In the analysis of tool wear, it was observed that over a period of 35 min, the tool wear increased progressively from 0.12 mm to 0.45 mm, representing a 25% increase compared to the maximum values recorded under the conventional C/L strategy. Consequently, while the microhardness outcomes under dry machining conditions were deemed acceptable, the elevated surface roughness, accelerated tool wear, and the absence of uniformity in mechanical properties underscore the inefficiency of dry machining relative to flood C/L. This underscores the imperative for employing conventional C/L strategies in demanding applications [13].

In order to enhance the accessibility of dry machining in industry, a number of experimental papers have been identified in the extant literature. Conventional machining frequently utilizes the technique of internal tool cooling [24]. This technique enables the maintenance of the key benefits of dry machining in terms of the environmental impact and operator safety while simultaneously reducing process costs in comparison to conventional methods. The implementation of this technique may entail the creation of a chamber within the toolholder, through which the cooling fluid is conveyed, or the incorporation of a copper plate to facilitate heat transfer, thereby markedly reducing the temperature generated within the tool. Furthermore, the heated fluid is returned to a condenser to complete the cooling circuit. The utilization of a solid lubricant is also proposed as a solution to the issue of friction between the tool and the workpiece. This is achieved by drilling a micro-hole in the cutting tool to hold the solid lubricant. This method has been shown to reduce wear and extend tool life in comparison to conventional dry machining [24].

It is also noteworthy to mention the utilization of high-speed dry machining on very demanding materials. The work by Jin Zhang et al., published in 2023 [25], provides an in-depth analysis of the characteristics and challenges associated with high-speed dry milling techniques when applied to difficult-to-machine materials, including titanium alloys, nickel and high-strength steels. A study was conducted to investigate the milling of these materials by varying the process parameters, such as speed and depth of cut. The present study investigates the implementation of the method in industrial processes, emphasizing the optimization of parameters to achieve optimal results, particularly in comparison to other C/L methods. To enhance the capabilities of the method, complementary methods and technologies are also being investigated. The employment of high-performance tools, such as SiAlON ceramic tools and composite-coated tools (TiN/TiCN/TiAlN, TiCN-NbC), has demonstrated potential in reducing cutting force, cutting temperature, and surface roughness while concurrently minimizing tool wear during the machining process. Energy-assisted machining constitutes another promising alternative, involving the use of various forms of external energy to optimize cutting processes. These techniques are particularly advantageous for materials that pose significant challenges in machining operations due to their complexity and resistance to conventional machining processes. The employment of energy-assisted machining techniques has been shown to enhance efficiency, reduce tool wear, and improve surface quality. These methods typically involve the use of technologies such as laser applications or ultrasonic vibrations, which is explored in greater detail in forthcoming sections [25].

Despite its many limitations, dry machining offers a significant advantage in that it eliminates all types of cutting fluids. From an environmental and economic standpoint, it remains a viable option, provided the process is carried out effectively to avoid material losses, as it is a method with reduced capabilities [25]. Cutting fluids contribute significantly to environmental pollution and incur high costs in terms of acquisition, maintenance, and disposal, which can sometimes amount to twice their initial cost [26]. From an ecological standpoint, the elimination of cutting fluids prevents the release of toxic and carcinogenic compounds, thereby protecting both the environment and operators’ health [28]. Furthermore, the absence of chemical treatments and liquid waste management simplifies operations and reduces the overall environmental impact. Economically, this technique eliminates costs associated with cutting fluids, which typically account for 10–17% of total machining expenses [2]. The elimination of auxiliary equipment, such as pumps and filtration systems, reduces the initial infrastructure investment and subsequent maintenance costs [11]. While the initial investment in advanced coated tools may be necessary to withstand the operational conditions of dry machining, these expenses are rapidly offset by the cumulative savings derived from the aforementioned areas. However, as previously highlighted, tool wear is significantly higher in dry machining compared to other strategies, which increases tool-related costs [6,11].

The implementation of dry machining in industry is hindered by significant challenges, primarily due to inherent limitations associated with the generation of high temperatures during the process, which can have detrimental effects on the surface integrity of machined parts and the tool wear and life, particularly when machining materials classified as difficult to machine, such as titanium alloys and heat-resistant steels [5,23]. In order to ensure the continued viability of dry machining, it is imperative to prioritize the development of technologies that address its current limitations and expand its application. Advancements in materials science, such as the development of advanced materials and self-lubricating coatings for tools, are poised to play a pivotal role in mitigating heat and reducing friction during operation. Innovative C/L technologies, including internal cooling systems and solid lubricants, are also deemed essential to enhancing dry machining performance [14]. The extant literature in this field underscores the importance of designing machines that are optimized to handle heat and chips, incorporating elements like internal channels for focused heat dissipation and chip evacuation. Integration with Industry 4.0 is a crucial step in enhancing the accessibility of dry machining to industry, including complete digitization with sensors and artificial intelligence for real-time monitoring and process optimization [11,27]. These methodologies contribute significantly to the optimization of cutting parameters, ensuring precision and efficiency during machining operations. Despite recent advances, challenges persist in adapting existing machinery to the elevated thermal and mechanical loads associated with dry machining. Primary challenges include the retrofitting of conventional machines with advanced heat-resistant components and the conception of effective chip evacuation mechanisms to prevent chips from adhering to the tools. This adherence can lead to tool wear and increased heat generation during the process [14,23]. Furthermore, the variability of materials and machining conditions calls for the integration of adaptive systems that can dynamically adjust to ensure optimal performance, emphasizing the necessity of incorporating real-time analysis and adaptive controls into machining setups. By addressing these challenges and emphasizing continuous innovation, dry machining has the potential to transition from a specialized technique to a mainstream solution, thereby combining sustainability, efficiency and precision in industrial applications [11].

3.3. High-Performance Cooling and Lubrication Techniques

3.3.1. Semi-Dry Machining

Semi-dry machining can be classified into three main categories: minimum quantity lubrication (MQL), minimum quantity cooling (MQR) and minimum quantity cooling lubrication (MQCL). In the extant literature, this technique is commonly referred to as MQL, and the terminology is used throughout this document. MQL involves the use of a combination of compressed air and a minute quantity of oil in droplet form to generate a spray that is applied directly to the cutting zone. This approach utilizes a markedly reduced oil flow, ranging from 0.01 to 2 L/h, in contrast to the 50–1000 L/h typically employed in conventional C/L systems [1].

This method primarily utilizes fatty alcohols and synthetic esters, which possess the inherent characteristic of biodegradability. The method primarily utilizes vegetable oils, including those derived from soybean, rapeseed, palm and sunflower due to their biodegradability and good lubricating properties. However, these oils exhibit low thermal stability, which is a limitation of this method [14]. The utilization of mineral and synthetic oils is also prevalent, with synthetic alternatives demonstrating enhanced stability but exhibiting reduced sustainability. The atomization of the lubricant is achieved by means of compressed air, which facilitates direct delivery to the cutting zone, ensuring uniform distribution and optimizing effectiveness with minimal fluid usage. Additionally, it provides supplementary cooling to assist in temperature control and facilitates the removal of chips and debris from the work area to prevent the formation of damaging build-up [3]. The primary benefits of this method over C/L systems are as follows [14]:

• Reduced cutting fluid consumption and costs. The amount of cutting fluid used is so small that it is virtually consumed in the process, eliminating fluid disposal problems;
• Reduced environmental and worker health risks;
• It produces almost dry chips, making them easy to recycle;
• In favorable cases, it can help improve the surface quality of the machined part by minimizing chip formation and optimizing thermal control during the process.

In order to successfully integrate MQL technology, it is imperative to address its applicability issues in scenarios where its use is restricted [26]. In optimal conditions, the MQL technique can prove to be highly advantageous. For instance, in the study published by Wang et al. [26], a system was utilized comprising equipment engineered to atomize minute quantities of lubricant in a compressed air stream, with a flow rate of 60 mL/h and air pressures adjustable between 3 and 6 bar. The MQL system was then compared to flood C/L, which employed a flow rate of 8.4 L/min. The MQL method utilized the 0.01% of the fluid volume required by flood C/L. The study examined variables such as surface roughness, the hardness of the machined-altered layer, and the forces generated during the process, based on key parameters such as cutting speed, feed rate and depth of cut. For a cutting speed of 40 m/min and a variable depth of cut ranging from 2 to 7 µm, Ra values were observed to range from 0.21 to 0.25 µm. These values were found to be slightly higher (7%) in comparison to those obtained with flood C/L. These values are notable when considering the grinding process that was utilized. Under identical cutting conditions, Ffs were observed to range from 11 to 42 N, which were 40% lower than those achieved with conventional methods. This reduction can be attributed to enhanced lubricant penetration in the contact zone. Furthermore, for a constant cutting speed and a variable depth of cut ranging from 2 to 150 µm, hardness values between 195 and 290 HV were obtained, exhibiting slightly higher values at intermediate depths in comparison to the flood C/L. This enhancement is ascribed to a reduction in heat generation and more localized cooling during the grinding process. The outcomes demonstrate that the MQL technique is effective in reducing grinding forces and minimizing thermal damage to the workpiece surface, thereby promoting better surface integrity compared to the conventional C/L method. However, surface roughness under MQL was slightly higher, attributed to the preservation of the sharpness of abrasive grains, which resulted in more defined marks [26].

Despite the limited cooling capacity, the findings emphasize that implementing sustainable strategies can enhance the efficiency of machining processes while concurrently reducing their environmental impact, thereby propelling progress toward a more environmentally sustainable industrial sector. However, it is imperative to undertake a comprehensive examination of the process to ensure the optimal benefits of the MQL method are realized.

In the study by García-Martínez et al. [12], an end milling process was reviewed for a Ti6Al4V titanium alloy workpiece using an uncoated tungsten carbide insert end mill. A MQL system was utilized, employing refined palm oil at a flow rate of 350 mL/h and a pressure of 2 bar. The variables of interest encompassed temperature, surface roughness, and flank wear of the tool, which were evaluated as a function of cutting speed and feed rate. The experiments were executed at a cutting speed of 90 m/min and a feed rate of 0.025 mm/rev. To monitor the temperature generated during the process, a FLIR T460 thermal camera with a resolution of 320 × 240 pixels and the capability to capture 76,800 temperature points per frame at a rate of 30 frames per second was employed. The maximum recorded temperature was 321 °C under the MQL system at the highest cutting speed and feed rate. While these values are notable, they can be improved, as they are attributed to the limited capacity of the MQL system to provide consistent and effective lubrication at the tool–workpiece interface due to the rapid evaporation of refined palm oil at elevated temperatures generated during machining. The range of Ra values was from 0.24 to 0.65 µm, with optimal results observed at a cutting speed of 120 m/min and a feed rate of 0.025 mm/rev. At these speeds and feed rates, the combination of relative velocity and moderate friction facilitated the formation of a more stable lubricating film at the tool–chip interface, thereby reducing friction and surface damage. The tool’s lifespan was measured at 11.1 min in the conducted tests, with VB of 502 µm observed at the end of its life. This behavior was attributed to the high temperatures generated at the tool–chip interface, which increased friction and promoted adhesion of the machined material to the tool. This is especially true due to the chemical affinity of titanium with carbon in tungsten carbide. The study concluded that the results obtained with MQL are notable, including a reduction in cutting temperatures, lower cutting forces, decreased tool wear and improved surface quality of the machined part. However, these results can be further enhanced, and the use of sub-zero air is proposed to cool the tool–material interface and mitigate the effects of these limitations [12].

In a manner analogous to dry machining, semi-dry machining can also employ techniques to enhance its capabilities. One such technique involves the use of minimal solid lubrication, wherein a small amount of solid lubricant is utilized in conjunction with a base oil to optimize the machining process.

In the paper published by Makhesana et al. [13], the turning process of AISI 52100 steel using CNM 120408 carbide inserts coated with TiAlN was examined. The investigation explored the potential of MQL and Minimum Quantity Solid Lubrication (MQSL) methods as sustainable alternatives to the flood C/L. The MQSL method is a lubrication technique that utilizes a reduced amount of lubricant mixed with a solid lubricant, which is directed directly into the cutting zone. This method creates a thin lubricating film that reduces friction and wear at the contact interfaces. Both the MQL and MQSL methods employed a flow rate of 300 mL/h of SAE 40 base oil. However, in the MQSL method, 20% by weight of calcium fluoride (CaF₂) particles, with a size of 10 µm, was mixed with the base oil. For an intermediate cutting speed of 130 m/min, a feed rate of 0.2 mm/rev, and a depth of cut of 1.2 mm, Ra values of 2.8 µm and 1.9 µm were obtained for the MQL and MQSL methods, respectively. The results for MQL were comparable to the conventional C/L method, showing a 12% increase in the Ra value, while the results obtained with the MQSL method were 24% better compared to the conventional average value. In terms of hardness in the machined layer, for depths ranging between 100 and 1600 µm, hardness values between 735–800 HV and 750–780 HV were recorded for the MQL and MQSL methods, respectively. While the MQL method exhibited reasonably uniform distribution, it was less consistent than the MQSL method, which achieved the lowest variation across depths. This outcome underscores the efficacy of the solid lubricant in forming a protective film at the tool–material interface, thereby mitigating thermal and mechanical stresses. In the context of tool wear, the study observed a tool life enhancement with MQSL, as evidenced by a reduction in VB from 0.36 mm to 0.30 mm. Notably, the MQL method yielded equivalent results to the conventional method after 35 min, although higher tool wear was observed when conventional methods were employed over this period. The findings obtained with MQL demonstrated a 17% decrease in wear when compared to the conventional approach. In summary, the MQL technique exhibited outcomes that were analogous to those of the conventional C/L method, thereby substantiating its equal efficacy while markedly diminishing the consumption of cutting fluids. Nevertheless, the MQSL method was identified as the most efficacious strategy for minimizing tool wear, consequently prolonging tool life in comparison to conventional methods [13].

It is also noteworthy to mention the application of the MQL method in high-speed machining. A study by Rahman et al. [28] examined various instances of high-speed machining of AISI 4340 steel. In this study, a turning process was subjected to analysis under both flood C/L and the MQL method. The study examined various variables across three conditions, each with distinct process parameters. Condition 1 corresponds to the finishing regime, where the priority is achieving a high-quality surface finish. The experimental setup involved a spindle speed of 1000 rpm, a feed rate of 0.2 mm/rev, and a minimal depth of cut of 0.2 mm. Condition 2 represents the semi-finishing regime, aiming to strike a balance between MRR and surface quality. In this condition, a moderate spindle speed of 500 rpm, a feed rate of 0.25 mm/rev, and a depth of cut of 1 mm were employed. Finally, Condition 3 represents the roughing regime, which is designed to maximize productivity through rapid MRR. This condition utilized a low spindle speed of 250 rpm, a high feed rate of 0.28 mm/rev, and a maximum depth of cut of 2 mm. For the MQL method, Ra values of 1.5 µm, 2.0 µm, and 6.5 µm were obtained for Conditions 1, 2, and 3, respectively. These values demonstrated improvements of 66%, 60%, and 56% over conventional C/L methods for the respective conditions. In terms of tool wear, the VB was measured as 0.17 mm, 0.22 mm, and 0.26 mm for Conditions 1, 2, and 3, respectively. These measurements also exhibited improvements of approximately 10% over conventional results across all conditions. Consequently, the MQL method demonstrated a substantial superiority over flood C/L in the studied variables for the turning process of AISI 4340 steel. Statistical and optimization analyses indicated that the optimal parameters to minimize these effects were a feed rate of 0.2 mm/rev, a spindle speed of 250 rpm, and a depth of cut of 0.5 mm [28].

The incorporation of nanoparticles into cutting fluids has emerged as a particularly salient solution to the limitations of MQL. The combined utilization of these nanoparticles significantly enhances the properties of the process, offering notable benefits such as reduced friction, shear forces, and cutting temperatures [29]. A comprehensive review of the use of nanoparticles in cutting fluids as a means to improve machining performance in sustainable MQL systems is provided in the study published by Panjaya et al. [30]. The low fluid viscosity facilitates precise distribution in the form of microdroplets that efficiently reach the cutting zone, contributing to reduced friction, effective heat dissipation, and improved overall machining performance. The addition of nanoparticles such as MoS₂, TiO₂, and SiO₂ adjusts the fluid viscosity and enhances its thermal and lubricating properties. For instance, in studies involving AISI 1045 steel, a standard MQL fluid reduced cutting temperatures by 21% compared to dry machining. By incorporating 0.5% MoS2 nanoparticles, Fc was reduced by 37%, cutting temperature decreased by an additional 24%, tool life increased by 44%, and Ra improved by 39%. In tests with hardened stainless steel, a standard MQL system increased tool life by 30% and reduced cutting temperatures by 36% compared to dry machining. With the incorporation of nanoparticles, these improvements reached up to a 40% increase in tool life and a 50% reduction in cutting temperatures. Nanoparticle-infused fluids also demonstrated a significant reduction in cutting forces. In cutting experiments, the cutting force values in standard MQL systems ranged from 100 to 650 N, while incorporating nanoparticles reduced this range to 30–300 N [30].

As demonstrated by numerous real-world case studies and experimental investigations, the MQL method is distinguished by its sustainability, as it utilizes significantly smaller amounts of lubricant compared to conventional C/L systems. This reduction in lubricant usage leads to a decrease in environmental impact and disposal costs. Typically, MQL methods use fluid volumes of up to 350 mL/h, rarely reaching the extreme of 2 L/h, whereas flood lubrication systems can consume up to 100 L per hour (L/h), except in highly demanding cases [30]. The employment of biodegradable oils, such as vegetable-based oils, contributes to enhanced sustainability by minimizing toxicity and facilitating the natural decomposition of the lubricant [12]. Additionally, it reduces occupational health risks by preventing the formation of vapors and mists that could cause respiratory and skin issues for operators [11]. In terms of energy consumption, MQL systems exhibit reduced energy demands related to fluid pumping and circulation, attributable to their diminished fluid volume. This, in conjunction with enhanced cutting efficiency, contributes to a reduced overall energy expenditure for the process [28]. From an economic perspective, MQL also reduces the total costs associated with cutting fluids when compared to conventional systems, constituting approximately 2–4% of the total process costs [30]. The extant literature encompasses studies that underscore the beneficial impact of integrating single and hybrid nanoparticles into cutting fluids. A multitude of experiments have investigated the utilization of nanoparticles, such as MoS₂, in conjunction with diverse lubricants and base fluids. The findings indicate substantial enhancements in properties such as thermal conductivity, friction coefficient, tool life, and overall machining efficiency when compared to conventional methods [29].

Despite its occasional superiority over conventional C/L methods, the MQL technique has certain limitations and challenges that affect its effectiveness in certain machining processes. Its cooling capacity is limited, which can be problematic for difficult-to-machine materials where cutting temperatures reach extremely high levels, leading to the defects discussed previously [31]. In certain operations, such as deep drilling or machining extremely hard materials, the fluid does not always penetrate effectively into the cutting zone, reducing its effectiveness. In addition, the dependence on parameters such as droplet size and air pressure make it difficult to implement, while excessively small particles can pose health risks [20]. Although MQL reduces frictional forces, it does not always adequately cool the tools, especially in processes with high thermal demands, which can shorten tool life. While MQL is a more sustainable option, the use of non-biodegradable fluids or fluids with toxic additives could undermine its environmental benefits. Economically, despite the long-term cost benefits, the initial investment in specialized equipment can be challenging, especially for small businesses. In addition, integrating MQL into existing machinery requires the installation of an MQL fluid delivery system, which typically consists of an atomizer or injector that mixes compressed air with small amounts of lubricant [10]. This system must include special nozzles to direct the flow to critical areas to ensure proper fluid penetration at the tool–material interface. Machine software configuration may also be required to program-specific MQL parameters, such as synchronizing fluid delivery with the machining cycle [10]. These challenges highlight the need to optimize MQL through innovation, such as the incorporation of nanoparticles into the MQL technique. Consequently, research in this area focuses on the development of advanced, sustainable and efficient cutting fluids, as well as innovations in application technologies, such as more precise nozzles and improved atomization systems. Similar to dry machining, the integration of Industry 4.0 technologies is being promoted, where automation through sensors and real-time monitoring is essential to optimize critical parameters [12]. In addition, adaptation to demanding processes, such as high-speed machining and difficult-to-machine materials, is being studied by optimizing process parameters. To advance in this direction, further research is needed to provide more data on different scenarios and to facilitate the industrial integration of MQL [25].

3.3.2. Cryogenic Cooling Machining

Cryogenic cooling has been proposed as a sustainable alternative to conventional cooling in metal machining, offering an environmentally friendly solution. In this method, a cryogenic fluid, such as liquid nitrogen (LN₂) or liquid carbon dioxide (CO₂), is applied directly to the cutting zone of the tool or workpiece instead of traditional oil-based cutting fluids to improve material properties and optimize process performance. This analysis primarily focuses on liquid nitrogen, a commonly used fluid due to its efficacy, but findings from several studies identify carbon dioxide as a potential alternative. The subsequent discussion enumerate some of the advantages of this method [7]:

• Greater cleanliness and sustainability in machining operations;
• Increased tool life and MRR, leading to reduce energy consumption;
• Improved dimensional accuracy and enhanced coefficient of friction between the tool and the workpiece;
• Enhanced chip breaking, which facilitates chip evacuation;
• Cryogenically machined components have improved properties, such as longer fatigue life, wear resistance and corrosion resistance;
• Significantly reduced equipment footprint compared to conventional C/L strategies.

Cryogenic cooling offers substantial advantages by harnessing low temperatures to optimize various processes. These temperatures stabilize molecular and cellular structures, thereby contributing to the preservation of the integrity of organic substances and biomaterials. Additionally, cryogenic applications enable the precise and controlled manipulation of materials, thereby minimizing deformations and defects in processes where accuracy is paramount [12]. A comprehensive review of the available literature reveals the remarkable applications of this process. In the work developed by Khanna et al. [5], a comprehensive review is conducted on the design and development of cryogenic machining setups for difficult-to-machine materials. In one of the cases reviewed, a turning process was performed using cryogenic cooling machining and flood C/L. The material machined was a Ti6Al4V titanium alloy, and the cutting tools used were CNMG 120408 inserts of grade PR 1535. Cryogenic cooling was employed using LN₂ at a pressure of 0.6 MPa and a flow rate of 2 L/min. The study investigated the impact of critical variables, including the cutting speed, ranging from 100 mm/min, the feed rate of 0.4 mm/rev, and a constant depth of cut of 0.5 mm, on MRR ranging from 67 to 333 mm³/s. The analysis encompassed the assessment of Ra, tool life, and energy consumption as a function of these parameters. The range of Ra values obtained was from 0.3 to 0.6 µm, representing an improvement of 64% compared to the wet machining process. This enhancement is attributed primarily to cryogenic cooling, which minimizes the formation of built-up edges and irregular tool wear, thereby facilitating cleaner cuts. Additionally, tool life increased by 80% compared to wet machining due to the effective heat dissipation in the cutting zone, which reduced adhesive and diffusion wear. In terms of energy consumption, for a constant MRR, energy consumption ranged from 14 to 45 kJ, decreasing with an increase in the MRR. This is due to the fact that at lower MRR, the process takes longer as material is removed at a slower pace, resulting in higher energy consumption over a longer period. In comparison to the flood C/L, the energy consumption was reduced by 61%, primarily due to the elimination of the need for pumping and filtration systems for the coolant. In conclusion, cryogenic cooling machining offers significant improvements over conventional methods by efficiently controlling temperature and stresses in the cutting zone, making it a more efficient, sustainable, and cost-effective alternative for machining materials with low machinability [5].

Proud et al. [31] published a comprehensive review of the use of the CO₂ coolant, exploring its advantages, limitations and applications. One of the case studies reviewed in the document involved a turning process using cryogenic cooling, which was compared to flood C/L on a Ti10V2Fe3Al titanium alloy, utilizing cemented carbide cutting tools. For the purpose of cryogenic cooling, CO₂ snow was utilized at a flow rate of 2.72 kg/min. The CO₂ was stored in a high-pressure cylinder and delivered to the cutting edge through the tool holder. The phase change of the CO₂ resulted in a mixture of approximately 40% snow and 60% cold gas, reaching a temperature of −79 °C. The study systematically analyzed the impact of cutting speed and cutting length on critical parameters such as cutting forces, burr formation on the machined surface, tool wear and tool life. The constant feed rate and depth of cut were maintained at 0.1 mm/rev and 0.3 mm, respectively. The study’s findings revealed that the primary cutting force, feed force and radial force were 60–100 N, 40–75 N and 70–100 N, respectively, at a cutting speed of 100 m/min. The findings indicated that the Fc remained comparable for both methodologies. However, the use of CO₂ resulted in a diminished Ff and a modest augmentation in Fr. This phenomenon can be ascribed to the diminished lubrication properties of CO₂ in comparison to flood C/L. The results suggest that while the cryogenic method does reduce thermal wear, it does not lubricate as effectively as flooding. In regard to burr formation at the built-up edges, higher levels of burr formation were observed with flood C/L, particularly during extended cuts, due to critical wear along the depth-of-cut line. This accelerated wear led to the weakening of the primary cutting edge, thereby enabling the accumulation of uncut material along the workpiece edge. The burrs that resulted from this process exhibited a jagged or serrated morphology, a consequence of the interaction between the worn edge and the plasticized material. Furthermore, an increase in cutting speed led to an escalation in burr formation, resulting in a deterioration of surface finish quality and an acceleration in fatigue wear on the tool. Conversely, the implementation of CO₂ snow exhibited a substantial reduction in burr formation. This cryogenic method maintained lower temperatures in the cutting zone, suppressing wear along the depth-of-cut line and reducing plastic deformation of the material. Consequently, the presence of burrs was found to be virtually non-existent, even at elevated cutting speeds. This enhancement in surface quality, coupled with a reduction in mechanical stress on the tool, signifies a notable advancement in the machining process. In terms of tool wear, the average and maximum flank wear values observed with CO₂ were between 60 and 130 µm, 60 and 230 µm, respectively, for cutting lengths of up to 2500 mm. In comparison to the use of emulsion, these wear values were reduced and exhibited greater uniformity. The use of CO₂ led to a substantial enhancement in tool life, particularly at cutting speeds ranging from 50 to 100 m/min. For instance, at a cutting speed of 100 m/min, the tool life with CO₂ reached 23 min, while with emulsion it was only 13 min. However, at higher cutting speeds (150 m/min), there was a substantial increase in thermal and mechanical wear, leading to a significant reduction in tool life. The tool exhibited greater wear when cooled with flooding, attributable to heat accumulation in the cutting zone, build-up edge formation, and flank oxidation. Conversely, the use of CO₂ contributed to the maintenance of reduced temperatures, thereby mitigating wear and enhancing the overall efficiency of the process [31].

In the document published by García-Martínez [12], sustainable C/L methods for machining titanium alloys were analyzed, including cryogenic cooling machining. In one of the cases examined in the study, a milling process was performed on the titanium alloy Ti6Al4V using tungsten–cobalt carbide tools coated with TiAlN. The process was studied under flood C/L conditions and also under cryogenic conditions, utilizing an internal LN₂ injection system operating at a pressure of 0.4 MPa and a nozzle outlet temperature of −190 °C. The cutting parameters encompassed cutting speeds ranging from 50 to 300 m/min, a constant depth of cut of 0.3 mm, and a feed rate ranging from 0.01 to 0.07 mm/rev. The cutting temperature was examined, and at a cutting speed of 300 m/min, it reached approximately 497 °C (see Figure 10). This method demonstrated 20–25% reduction in the average cutting zone temperature compared to conventional methods, thereby hindering the diffusion of elements such as Ti, W and Co, reducing friction and extending the tool life. The cutting forces generated during the process were also examined, with Fc and Ff ranging between 140 and 195 N, 42 and 68 N, respectively. These values decreased as cutting speed increased due to elevated temperatures at higher speeds. However, a slight increase was observed in comparison to conventional C/L conditions, attributable to the enhanced stiffness and reduced plastic deformation of the material resulting from the low temperatures. Of particular note was the significant reduction in Fr, a critical factor in friction and resistance at the tool–workpiece contact surface, which exhibited values ranging from 16 to 22.5 N under cryogenic cooling conditions. This reduction in Fr was also observed to increase with increasing cutting speed. This study demonstrated that LN₂ cryogenic cooling is highly effective in improving the machining of difficult-to-machine materials, such as the Ti6Al4V alloy. The benefits achieved in terms of cutting temperature reduction, suppression of element diffusion between the tool and the workpiece, and process stability resulted in prolonged tool life and improved machining efficiency [12].

Cryogenic cooling finds application not only as a coolant during machining but also as a complementary cryogenic treatment, serving as an alternative to conventional heat treatments such as quenching and tempering [6]. By cooling materials to extremely low temperatures, such as −190 °C with LN₂ or −79 °C with CO₂, permanent and irreversible transformations are induced in the microstructure [12,31]. This phenomenon is particularly evident in steels, where the transformation of retained austenite into martensite increases hardness by approximately 20–30% compared to conventional methods. Furthermore, carbide refinement has been shown to reduce their size by up to 50% and improve their distribution, thereby enhancing wear resistance by 15–25%. Furthermore, residual stresses are reduced by 40–60%, thereby enhancing dimensional stability and reducing the probability of service failures. These modifications have been shown to result in a 150% increase in tool life and a 35–50% reduction in cutting edge wear [6,11]. The cryogenic treatment has also been observed to yield beneficial effects on other materials, such as titanium alloys. In the case of titanium alloys, for instance, grain size refinement has been shown to enhance hardness by up to 20% while concurrently reducing adhesive wear by 30–40%. These phenomena are of particular relevance in the context of high-precision machining operations, where these critical factors must be taken into account. Additionally, improvements in fatigue resistance can reach up to 25%, a critical factor for components subjected to repetitive load cycles, such as medical implants or aircraft engine parts. Additionally, cryogenic cooling has been shown to enhance the surface integrity of machined components, reducing residual stresses by 50–70% and producing more stable surfaces for high-reliability applications [6,12]. However, it should be emphasized that these microstructural alterations are not uniform and may on occasion yield unfavorable variations in the quality of the final product [6].

Cryogenic cooling machining is a technique that has been identified as a solution to the environmental challenges associated with traditional C/L methods in manufacturing processes. This technique has been shown to reduce energy consumption and address multiple environmental issues. In comparison to oil-based cutting fluids, which generate hazardous waste that is difficult to manage and recycle, cryogenic fluids have a significantly lower environmental impact [12]. The evaporative nature of these fluids eliminates the need for liquid waste disposal and its subsequent treatment, thereby reducing water and soil contamination. In addition, cryogenic fluids do not emit toxic gases, thereby enhancing safety and health conditions for operators [18]. In terms of energy consumption, cryogenic cooling has been shown to reduce friction and temperature at the tool–material interface, as evidenced by the research conducted by Khanna et al. [5]. This reduction in friction leads to decreased tool wear, higher cutting speeds, and fewer process interruptions, resulting in optimized energy use. In comparison to conventional flood C/L methods, which necessitate the use of substantial volumes of fluid due to their lower thermal efficiency, cryogenics demand significantly less fluid [5]. Moreover, as evidenced by the cases examined in this section, cryogenic cooling has been observed to enhance productivity when machining challenging materials, such as titanium and nickel alloys, by reducing the formation of built-up edges and achieving enhanced surface finish [5,12,31].

Despite these benefits, the implementation of cryogenic cooling in industrial settings remains challenging due to its nascent stage of development. While cryogenic coolants are considered environmentally friendly in terms of direct emissions, concerns regarding their sustainability arise when considering their production and transportation, which generate a substantial carbon footprint when compared to alternative methods. Additionally, the handling of these coolants during the process necessitates rigorous safety measures due to associated risks such as frostbite or reduced visibility caused by condensation. Economically, the initial and operational costs are high, as the implementation of specialized systems and the continuous supply of coolants like LN₂ or CO₂ require significant investment, exceeding the costs of traditional methods [5,31]. Moreover, the design of cryogenic supply systems is complex and requires high precision to ensure uniform distribution of the coolant, complicating their standardization and optimization [25]. The integration of these systems into existing machinery also necessitates a detailed analysis of all components, as specialized devices such as nozzles, high-pressure pipelines and flow controls must be incorporated to apply cryogenic coolants precisely and uniformly. This process also requires updates to the monitoring and control systems of machines to manage specific variables, ensuring thermal stability and synchronization with machining processes without compromising precision or productivity. While the initial investment in specialized equipment and machinery modifications may be substantial, the accumulated benefits can justify this expense [17]. Cryogenic cooling offers a compelling long-term viability due to its capacity to enhance process efficiency and reduce associated operational costs, such as extending tool life and achieving energy savings the processes [31].

In summary, the utilization of cryogenic cooling in machining has emerged as a technologically advanced approach, offering notable advantages such as diminished tool wear, enhanced surface quality, and environmental sustainability through the minimization of conventional cutting fluids. Notwithstanding the initial financial investment and the intricacy involved in its integration into existing machinery, the long-term economic benefits substantiate its implementation in industrial domains characterized by stringent precision and efficiency requirements. The extant literature underscores the necessity for further research, namely the optimization of cryogenic coolant supply systems, the development of standards for their implementation, and the exploration of their economic feasibility across a range of industrial applications [5,6,17]. The combination of cryogenic cooling with the MQL method is a pivotal development area with considerable potential. This subject is examined in the subsequent section.

3.3.3. Cryogenic Cooling-MQL Machining

The cryogenic cooling-MQL (CryoMQL) C/L system represents a sophisticated solution that has emerged as an evolution of the MQL and cryogenic cooling machining methods. This system was developed to overcome the inherent limitations of these techniques when applied individually, as previously discussed. Cryogenic cooling, as outlined earlier, relies on the use of low-temperature liquids, such as LN₂ or CO₂. This technique has been shown to lead to substantial improvements in thermal dissipation, which in turn reduces thermal stress generation and extends tool life. In addition to these benefits, the use of cryogenic coolants eliminates the toxic residues associated with conventional fluids, thereby fostering safer and more environmentally responsible working conditions [19,25]. In contrast, the MQL technique involves the direct application of minimal quantities of lubricants to the cutting zone, resulting in a significant reduction in fluid consumption and the costs associated with waste treatment and disposal [27]. The hybrid Cryo-MQL system integrates the distinct advantages of each technique, optimizing both C/L to enhance surface finish, reduce cutting forces, and extend tool life. This integrated approach signifies a substantial advancement in sustainable and efficient machining practices [6].

The system’s implementation can be categorized into three distinct modes: external, internal and hybrid. In the external mode, the delivery of lubricant and cryogenic coolant occurs through dedicated or hybrid nozzles. Dedicated nozzles supply the mediums independently, while hybrid nozzles either pre-mix the mediums or eject them simultaneously. A Cryo-MQL generator can also be utilized to mix both mediums in a chamber before delivery [27]. In the internal mode, the lubricant and the cryogenic coolant are transported through internal channels in the tool or spindle. These systems can feature single-channel configurations, where both mediums are mixed prior to delivery, or dual-channel setups, where the mediums are transported separately. This method provides high delivery precision, achieving more effective C/L. However, it requires significant modifications to the equipment, resulting in higher initial costs [14]. The hybrid mode combines internal and external delivery; one medium, either the lubricant or the cryogenic coolant, is delivered internally, while the other is supplied externally through nozzles. This approach balances precision and flexibility, offering optimized performance in terms of C/L. Nevertheless, its configuration is more complex compared to the other two methods [19].

This synergy has proven to be particularly effective in the manufacturing of advanced alloys and critical components for industries such as aerospace, where high standards of precision and surface quality are required. In the document published by Iruj et al. [27], an exhaustive review of hybrid C/L systems is presented. One case reviewed in this document involves a turning process of the titanium alloy Ti3Al2.5V using uncoated carbide insert tools. The CryoMQL method was implemented through two distinct nozzles, one for the lubricant and the other for the cryogenic coolant, LN₂. The present application was executed in a manner that entailed the use of a biodegradable oil as the lubricant, with a flow rate of 50 mL/h, and LN₂ supplied at a flow rate of 0.35 L/min under a pressure of 4 bar. In this instance, surface roughness, cutting temperature, and tool wear were analyzed as functions of cutting length, with values up to 1200 mm. The constant parameters included a cutting speed of 150 mm/min, a feed rate of 0.20 mm/rev, and a cutting depth of 0.5 mm. The results indicated that hybrid machining produced the lowest Ra, with initial values of approximately 0.5 µm, which remained consistently low throughout the process. Under independent MQL and LN₂ conditions, Ra was moderate, with values of approximately 0.8 µm and 0.7 µm, respectively. In terms of cutting temperature, the hybrid combination achieved the lowest temperatures, ranging between 100 and 200 °C along the cutting length. The MQL method resulted in a slight reduction in temperature, remaining within the range of 350–400 °C, while cryogenic cooling produced a range of 250–300 °C. This phenomenon was attributed to the synergistic interaction of LN₂, which rapidly cooled the cutting zone, and the MQL oil, which reduced friction and prevented lubricant evaporation. In terms of tool wear, MQL exhibited a maximum VB of 0.2 mm, attributable to the lubricating effect of the biodegradable oil. However, its effectiveness was constrained by moderate temperatures. Conversely, cryogenic cooling with LN₂ demonstrated superior wear control, attaining maximum VB values of approximately 0.15 mm by maintaining low temperatures and averting severe thermal damage. However, the most optimal outcomes were attained through a hybrid approach, which entailed a combination of liquid nitrogen’s effective cooling properties and the localized lubrication of MQL. This strategy effectively minimized friction and mechanical damage, resulting in a maximum wear value of only 0.1 mm. The findings of this study demonstrate that the hybrid strategy is the most effective approach for extending tool life and enhancing overall process performance [27].

In the document published by Khanna et al. [5], an additional turning process was conducted using the CryoMQL method and dry machining for titanium alloy Ti6Al4V with cutting tools. The CryoMQL method employed water-miscible oil with specialized additives at a flow rate of 35 mL/h as a lubricant, and LN₂ as a cryogenic coolant. The experimental process was executed with cutting speeds of 80 and 120 m/min, a feed rate of 0.2 mm/rev, and a depth of cut of 1 mm. The study’s objective was to analyze the surface roughness, micro-hardness of the machined layer, and the cutting forces generated during the process. The analysis revealed that at a cutting speed of 80 m/min, Ra was 0.65 µm in a dry environment and 0.35 µm under CryoMQL, indicating a 46% reduction in CryoMQL compared to dry machining. At an increased cutting speed of 120 m/min, Ra was recorded as 0.72 µm in dry machining and 0.49 µm with CryoMQL, indicating a 32% reduction. These enhancements can be attributed to the reduced friction and lower temperatures associated with CryoMQL, which lead to minimized tool wear and enhanced surface quality. The microhardness measurements of the altered surface layer during dry machining exhibited a range from 405 HV at the surface to 370 HV with increasing depth. In contrast, the hybrid method resulted in a range of 400 HV at the surface to 345 HV at greater depths, also decreasing with increased distance from the surface. With regard to cutting forces, at a cutting speed of 80 m/min in a dry environment, Ff was 205 N and the Fc was 330 N. In the hybrid method, these forces were significantly lower, at 141 N and 240 N, representing reductions of 31% and 27%, respectively. At a cutting speed of 120 m/min, the Ff and Fc in the dry environment were 215 N and 345 N, respectively. The CryoMQL method produced values of 144 N and 270 N, corresponding to reductions of 33% and 22%, respectively. This reduction in forces was attributed to decreased friction during machining. The CryoMQL method exhibited superior performance in all aspects when compared to dry machining. In summary, it significantly reduced cutting forces by up to 33%, improved Ra by up to 46%, and reduced material adhesion to the tool, with 31% of material adhesion in CryoMQL compared to 87% in dry machining [5].

As with the MQL method, it is possible to incorporate nanoparticles into the hybrid CryoMQL system, representing a significant innovation for improving machining efficiency, particularly for difficult-to-cut materials. These nanoparticles, typically composed of materials such as aluminum oxide (Al₂O₃), copper oxide (CuO) or boron nitride (BN), are dispersed in small quantities within the lubricant applied via the MQL method. Their nanoscale size allows them to effectively penetrate the tool–workpiece interface, acting as friction-reducing agents and facilitating heat transfer. The combination of these effects with those of cryogenic coolant offers unique advantages. The literature reviewed indicates that their use can reduce friction by up to 30% compared to conventional MQL and significantly decrease tool wear, extending tool life by approximately 25% to 50% compared to conventional MQL [5,27]. To achieve these benefits, precise regulation of the nanoparticle concentration in the lubricant is paramount. The optimal range for this concentration is typically between 0.1% and 0.5% by volume, as elevated concentrations may lead to an increase in viscosity, potentially impeding their efficacy. It is noteworthy that extant literature underscores the necessity for further development to fully harness the potential of this method. Moreover, there is a compelling need for additional research to broaden its implementation in industrial settings [14].

With regard to economic and environmental sustainability, CryoMQL demonstrates inferior performance in comparison to the standalone methods, as it integrates the adverse effects of both approaches. Nevertheless, when juxtaposed with conventional methods, it offers numerous advantages. Notably, it has been shown to reduce cutting fluid consumption by up to 90%, a feat accomplished through the precise application of minute quantities of lubricant and the utilization of cryogenic coolants such as liquid nitrogen or carbon dioxide [31]. This approach not only reduces costs associated with fluid management but also, as previously discussed, ensures that cryogenic coolants evaporate without generating contaminating residues. This contributes to a substantial reduction in the disposal of cutting fluids, thereby mitigating health risks for operators by decreasing their exposure to hazardous chemicals [20]. From an environmental perspective, the CryoMQL method has been shown to reduce the generation of toxic waste and contribute to addressing issues such as soil and water contamination, which are commonly associated with the extensive use of conventional cutting fluids. In terms of energy consumption, this hybrid method enhances the thermal efficiency of the machining process by more effectively reducing temperatures in the cutting zone than traditional methods. This reduction in the necessity for additional energy to counteract the heat generated during machining leads to a diminished overall energy demand [14].

The CryoMQL method, despite its promise, faces several limitations and challenges in its implementation within machining processes. One of the primary difficulties lies in the efficient integration of supply systems for cryogenic coolant and MQL, as precise synchronization is required to ensure that both components are adequately delivered to the cutting zone. Ensuring the chemical and thermal compatibility between the lubricant and the cryogenic coolant remains a technical challenge that warrants further investigation [6]. The economic implementation of the CryoMQL method entails a substantial initial cost, primarily due to the necessity of specialized equipment and cryogenic gas supply. However, this approach generates significant long-term savings by reducing lubricant consumption, minimizing waste treatment costs, and substantially extending tool life. Its additional energy efficiency also contributes to lower operational costs. While this method offers substantial advantages in large industries, its adoption may be constrained in small enterprises due to the substantial initial investment required [14,31]. The retrofitting of existing machinery with this system, which necessitates individual MQL and cryogenic cooling equipment, as well as specialized systems for synchronized application, is a potential avenue for expansion. Examples include the installation of dual nozzles and advanced control systems to synchronize the application of coolant and lubricant. Finally, sustainability-related challenges, such as the proper handling of cryogenic gases, which can pose safety risks without adequate ventilation and containment systems, warrant consideration. The development of more environmentally friendly strategies for the production and supply of these gases is critical to maximizing the environmental benefits of the method. The literature also highlights the need for research into the design of tools specifically optimized for working under cryogenic and minimal lubrication conditions to improve their performance and longevity [19,31]. Moreover, the development of advanced predictive models is expected to optimize parameters such as cutting speed, coolant flow rate, and lubricant concentration. These models can leverage tools like artificial intelligence and computational simulation to enhance process efficiency and reliability [6].

3.3.4. High-Pressure Cooling Machining

High-Pressure Cooling (HPC) machining has been identified as a pioneering technique in the realm of sustainable machining, owing to its emphasis on optimizing the efficiency of the cutting fluid. This method utilizes high pressures to inject the fluid directly into the cutting zone, thereby enhancing penetration in the tool–chip contact zone and on the side face of the tool. This enhancement in cutting conditions is accompanied by a reduction in overall fluid consumption, thereby contributing to the sustainability of the process [4]. A comprehensive review of the extant literature reveals that HPC not only significantly extends tool life but also provides enhanced cooling. This effect can be attributed to the capacity of the high-pressure cutting fluid to decrease the contact distance between the tool and the chip. The force of the coolant displaces the chip from the tool faces. This reduction in chip thickness, as compared to conventional C/L methods, can be attributed to the fragmentation of the chip into a greater number of parts [31]. Consequently, HPC is proving to be a highly efficient method for optimizing machining performance [8].

HPC has been demonstrated to be advantageous in scenarios involving the machining of difficult-to-machine materials, high cutting speeds, high feed rates, deep-hole drilling processes, and continuous chip production [11]. The document published by Khan et al. [2] presents a review of various C/L methods, including HPC. One of the cases reviewed in this study involved the turning process of the nickel alloy Inconel 718 using uncoated tungsten carbide insert tools. The process was executed using both flood C/L and HPC under cutting speeds of 30, 60, 90, and 120 m/min, a constant feed rate of 0.25 mm/rev, and a constant depth of cut of 1.2 mm. The conventional method employed a vegetable-based emulsion at a flow rate of 6 L/min, whereas the HPC method used the same emulsion at a flow rate of 1.5 L/min under 500 bar. The tool life was evaluated under these machining conditions, where at lower cutting speeds the tool life increased by 95% with HPC compared to conventional machining. These enhancements were ascribed to the advanced C/L capabilities of the alternative methods within the cutting zones. At elevated cutting speeds, the discrepancy in tool life became more pronounced, underscoring the inadequacy of conventional machining due to its propensity for accelerated tool wear. In contrast, the HPC method exhibited a substantially superior performance. Furthermore, an economic analysis was conducted, encompassing factors such as machine usage, material costs, energy consumption, cutting fluids, and tool life. The operating costs were calculated on an hourly basis, resulting in EUR 31.13/h for conventional machining and EUR 37.31/h for HPC. The use of oil-based fluids incurred costs of EUR 0.20/h and EUR 0.17/h, respectively, for conventional C/L and HPC. The conventional method exhibited slightly lower energy consumption, with consumption of 5 kW, in comparison to 7 kW for HPC, attributable to the utilization of a high-pressure pump. Tool life, measured by edge wear, was found to be significantly shorter in conventional machining, resulting in increased replacement costs. Conversely, the HPC method mitigated this impact. The total cost per component was determined by aggregating various cost components, including operating expenses, tool expenditures, fluid costs and energy costs. The analysis revealed that the HPC method led to a cost reduction of up to 71% at high cutting speeds, attributable to enhanced efficiency and sustainability in the process [2].

While not all extant literature classifies this as a sustainable technique, García-Martínez’s document [12] highlights several cases in which significantly higher cutting fluid flow rates were used compared to conventional C/L methods. This suggests that HPC may not be considered a green, environmentally friendly technique [12]. For instance, one case examined in this document involved a turning process for titanium alloy Ti6Al4V, utilizing TiCN/Al₂O₃/TiN-coated cemented carbide tools under dry and HPC conditions. The HPC system utilized a flow rate of 12 L/min, distributed through two nozzles at a pressure of 80 bar. The jets were directed at the tool–workpiece and tool–chip interfaces through nozzles with an internal diameter of 0.5 mm. This flow rate is notably higher than those employed in conventional C/L methods previously reviewed, which typically measures up to 100 L/h, except in cases of high demand. In this study, a comprehensive analysis was conducted on the impact of various parameters on the performance of the HPC system. The analysis encompassed the surface roughness, cutting forces, interface temperature and tool wear, with the study focusing on cutting speeds of 78, 112, and 156 m/min, feed rates of 0.12, 0.14 and 0.16 mm/rev and a constant depth of cut of 1 mm. The analysis of Ra revealed that the employment of HPC enhanced the quality of the machined surface in comparison to dry cutting. For instance, at a cutting speed of 156 m/min and a feed rate of 0.14 mm/rev, the Ra value under HPC conditions was approximately 1.2 µm, while under dry cutting, it was around 1.5 µm. The HPC method was found to enhance outcomes by 15–20% in comparison to dry cutting. The enhancement in surface roughness with HPC over dry cutting was more evident at elevated cutting speeds and moderate feed rates. An analysis of cutting forces revealed that HPC led to a substantial reduction in the forces necessary during machining when compared to dry cutting. For instance, at a cutting speed of 78 m/min and a Ff of 0.16 mm/rev, the cutting force under dry cutting was 804 N, whereas it was 734 N under HPC, representing an 8.6% reduction. At elevated cutting speeds, such as 156 m/min, in conjunction with a feed rate of 0.12 mm/rev, Fc exerted by HPC was recorded at 585 N units in comparison to 640 N units under dry cutting conditions. This observation signifies an 8.6% reduction in Fc. The enhancement of cutting speed tends to diminish cutting forces due to the thermal softening of the machined material. However, this effect is more pronounced with HPC, as coolant jets not only efficiently cool the tool and the workpiece but also enhance lubrication, thereby reducing friction at the cutting interfaces. On average, the utilization of HPC led to a reduction in Fc by 8–9% in comparison to dry machining. The analysis of the temperature at the tool–chip interface revealed a substantial decrease when employing HPC in contrast to dry cutting. Specifically, at a cutting speed of 78 m/min and a feed rate of 0.12 mm/rev, the interface temperature decreased from 870 °C under dry conditions to 685 °C with HPC, representing a 21.3% reduction. At higher speeds such as 156 m/min and a feed rate of 0.14 mm/rev, the interface temperature decreased from 1220 °C under dry cutting conditions to 1030 °C with HPC, equating to a 15.6% reduction. On average, HPC reduced the tool–chip interface temperature by 15–22%. This reduction, in conjunction with the decrease in cutting forces relative to dry machining, contributed to reduced thermal tool wear, thereby extending tool life and enhancing machining quality. In the analysis of tool wear, under dry conditions, the increase in interface temperature gave rise to wear mechanisms such as adhesion, excessive rubbing, and the formation of built-up edges, which were especially pronounced at higher cutting speeds. For instance, at a cutting speed of 112 m/min and a feed rate of 0.12 mm/rev, VB reached 300 µm within a mere five minutes of dry cutting. Under identical cutting conditions, VB using HPC was 220 µm after six minutes of machining, thereby extending tool life by 55–60%. HPC minimized adhesive wear and the formation of built-up edges, with crater wear on the tool edge predominating due to the impact of high-speed jets [12].

The extant literature also mentions the potential combined use of HPC with other C/L techniques. The integration of HPC with MQL offers numerous advantages. Specifically, HPC rapidly cools the cutting zone, thereby extending the tool’s lifespan and preventing heat-related issues such as accelerated wear and thermal deformation [18]. Conversely, MQL serves to complement the cooling by reducing friction at the tool–chip interface, enhancing surface quality and further minimizing tool wear. This integration of HPC with MQL enhances the sustainability of the process by minimizing fluid consumption, thereby reducing operational costs and liquid waste. However, implementing this technique presents certain challenges. The implementation of hybrid systems necessitates specialized equipment, such as nozzles that effectively combine minimal lubrication with high-pressure application. This results in increased initial costs, which are counterbalanced by long-term savings [18]. In addition to MQL, the integration of HPC with cryogenic cooling is also a viable option. This approach capitalizes on the benefits of high-pressure cooling, such as efficient chip removal and active lubrication, while leveraging the extreme thermal control and waste reduction capabilities of cryogenic cooling. In a hybrid configuration, high-pressure systems are employed to apply cutting fluids to the machining zone, while cryogenic cooling is selectively directed to critical areas to reduce temperatures [15,20]. This technique also contributes to improved sustainability by requiring fewer cutting fluids and utilizing non-toxic, residue-free cryogenic liquids. Despite these advantages, the implementation of this technique is hindered by several challenges. These challenges include high initial costs, operational complexity and energy consumption. The generation of cryogenic fluids and the synchronized operation of high-pressure systems are two examples of processes that consume energy and pose additional challenges [20].

HPC signifies a substantial technical innovation in the domain of machining, attaining exceptional outcomes in highly demanding applications. However, it should be noted that this approach may not always prioritize process sustainability. In some instances, it has been observed to consume more oil-based cutting fluids compared to conventional methods [12]. Conversely, in processes that are less demanding, HPC can lead to a substantial reduction in the use of cutting fluids by enhancing the efficiency of coolant application, thus decreasing the total flow volume required [2]. The financial implications of the process can vary depending on its specific requirements. As evidenced by Khan et al. [2], the adoption of HPC led to a reduction in the cost per component by up to 71% in comparison to conventional methodologies. In regard to energy consumption, the HPC method employs a high-pressure pump, which increases the total energy consumption of the process compared to conventional machining, as well as the associated costs. Despite this increase, the HPC method has been shown to enhance tool life, thereby offsetting the additional energy consumption by reducing downtime and the frequency of tool changes [2].

Consequently, the HPC method confronts substantial challenges concerning its environmental impact and energy consumption. It also entails considerable initial costs associated with the acquisition and configuration of the specialized equipment necessary to operate under high pressure. These systems necessitate robust components capable of withstanding extreme pressures, further increasing the initial investment, particularly for small and medium-sized enterprises [6,20]. Conversely, there is potential for integrating these systems into existing machinery, facilitating their implementation in smaller companies. This integration necessitates specific modifications to the equipment, such as adapting coolant supply systems. This necessity arises because conventional machines are typically designed to operate with standard cooling methods and are generally not equipped to handle the high pressures required by HPC. The integration of HPC necessitates the implementation of specialized equipment such as high-pressure pumps, reinforced piping, and precision nozzles capable of directing coolant flow to the cutting zone with sub-millimeter accuracy. The HPC method is confronted with challenges related to environmental impact, energy consumption, and initial costs. However, these challenges can be mitigated through technical improvements, leading to long-term savings and a favorable return on investment. Moreover, HPC can be integrated with eco-friendly methodologies, such as MQL and cryogenic cooling, to address these issues. This approach is proposed to minimize cutting fluid usage. However, further research is necessary to develop these hybrid systems to achieve a balance between sustainability, performance and cost [20].

3.4. Hybrid Cutting Processes

3.4.1. Vibration Assisted Machining

Vibration-assisted machining (VAM) is a machining technique that utilizes controlled vibrations to enhance efficiency and quality by complementing other processes. The principle of VAM lies in momentarily interrupting the contact between the tool and the material during the cutting process, thereby generating a pulsed effect that reduces friction and cutting forces. Vibrations are generated using piezoelectric, magnetic or mechanical devices integrated into the tool or tool holder. In certain systems, these vibrations are synchronized with the tool’s feed motion, achieving specific effects such as more uniform cuts [32].

VAM offers numerous advantages, positioning it as an innovative solution for machining difficult and high-precision materials. One of its main strengths is the significant reduction in cutting forces, which decreases tool wear and extends tool life. This translates into enhanced economic and operational efficiency, especially in applications where tool-related costs are high [12]. Additionally, the induced vibrations facilitate more efficient chip evacuation, preventing undesirable accumulations and improving surface quality by reducing defects such as scratching or burr formation [5]. Another notable advantage is the improvement in dimensional precision and surface finish, as VAM minimizes heat generation in the cutting zone, thereby reducing thermal deformation and damage to the edges of machined parts. This makes it particularly useful in the manufacturing of high-precision components. Furthermore, VAM enables the machining of advanced and difficult materials, such as titanium alloys, ceramics and superalloys, which are typically challenging in conventional processes due to their high hardness and thermal resistance. The possibilities of VAM extend beyond conventional machining. This technique can be integrated with non-traditional processes, such as MQL, cryogenic cooling or HPC, to further optimize its sustainability and efficiency [1].

A review of the extant literature reveals numerous cases in which VAM has been utilized as a complementary process to enhance machining outcomes. In the study published by García-Martínez et al. [12], various machining processes under different C/L conditions are examined. One case in the study investigates the milling of titanium alloy TC4 using uncoated cemented carbide tools. In this case, conventional C/L is assisted by ultrasonic vibration, utilizing an ultrasonic generator and a transducer to convert electrical signals into mechanical vibrations. The ultrasonic currents ranged from 50 to 150 mA, in conjunction with spindle speeds varying from 1200 to 6000 rpm, a constant feed rate of 40 mm/min and cutting depths ranging from 0.2 to 1 mm. A comprehensive analysis of the machined surface was conducted, with a particular focus on Ra and residual stresses. For surfaces machined with conventional C/L-VAM, the arithmetic average deviation and root mean square deviation were slightly worse compared to those obtained with conventional C/L alone, with average values ranging from 0.25 µm to 0.32 µm for the former and 0.22 µm to 0.28 µm for the latter. This was attributed to the formation of stepped structures induced by ultrasonic vibration. However, this defect led to the generation of isotropic textures with higher texture aspect ratio values, indicating greater surface uniformity and improved lubricant retention, thereby enhancing efficiency during turning. Regarding residual stresses, conventional milling assisted by ultrasonic vibration generated 22% higher compressive stresses than conventional methods. These stresses increased proportionally with higher ultrasonic currents, improving fatigue and corrosion resistance. The study’s conclusion asserts that the implementation of conventional C/L-VAM facilitates effective lubricant retention during the process, enhances the wear resistance of the final component and produces a machined surface with isotropic and compressive properties [12].

Pimenov et al. [6] published a study that examined the use of cemented carbide tools coated with TiAlN to process the titanium alloy Ti6Al4V under both conventional conditions and a process combining HPC with VAM. The utilized fluid was a 6% soluble oil emulsion in water, delivered at a flow rate of 21 L/min and variable pressures ranging from 50 to 200 bar. The amplitude of the employed waves was a peak-valley amplitude of 16 µm, with a vibration frequency of 22,338 Hz. This method has been shown to meet stringent requirements, operating at cutting speeds ranging from 200 to 500 m/min, a feed rate of 0.005 to 0.015 mm/rev and a constant depth of cut of 0.05 mm. In this study, various key variables were analyzed. Initial surface quality analysis revealed similar results for both processes, with Ra values of approximately 0.2 µm, as geometric interaction dominated the process. However, as machining progressed, tool wear significantly influenced Ra increases, revealing clear differences depending on the conditions applied. Under HPC conditions at 200 bar, Ra remained below 0.4 µm for a cutting length of 225 km, while under conventional conditions, Ra exceeded the average value of 0.4 µm after just 17 km. This finding demonstrates the improvement in surface quality achieved with HPC-VAM. A subsequent analysis of cutting forces revealed that both the Fc and the Ff exhibited an increase with increasing cutting speed and feed rate. The HPC-VAM method resulted in a reduction of forces by 15% to 45% compared to the conventional method, particularly at cutting speeds between 200 and 400 m/min. However, at a cutting speed of 500 m/min, the advantage of the HPC-VAM in reducing forces diminished significantly, with an only 5% improvement over the conventional process observed. In terms of cutting temperature, the HPC-VAM method at 200 bar pressure resulted in a substantial reduction in cutting temperature compared to the conventional method, with a maximum difference of up to 55%. At a cutting speed of 300 m/min, the HPC-VAM method yielded a temperature of 189 °C, compared to 420 °C for the conventional method. This finding demonstrates that the assisted method maximizes heat dissipation, thereby enabling more efficient machining with reduced thermal accumulation in the tool and the workpiece. Finally, tool wear analysis revealed that the HPC-VAM method significantly prolonged tool life. At 200 bar pressure, tool life improvement reached up to 6.3 times compared to the conventional method, reflecting a substantial reduction in adhesion and oxidation wear. This prolongation of tool life underscores the merits of the HPC-aVAM method under demanding cutting conditions [6].

VAM has a considerable impact on the sustainability of machining processes due to its capacity to optimize energy consumption and minimize waste generated during production. In terms of energy efficiency, VAM can be more effective than conventional methods in specific applications. The incorporation of controlled vibrations facilitates material removal and reduces the mechanical effort required by the tool. This reduction in mechanical effort by the tool can lead to a decrease in energy demand during the cutting process [12,32]. Nevertheless, it is imperative to acknowledge that generating vibrations, particularly in ultrasonic systems, necessitates additional energy supply [6]. Another critical aspect of VAM is its ability to enhance chip evacuation efficiency and reduce the occurrence of surface defects. Furthermore, the compatibility of VAM with sustainable technologies, such as MQL and cryogenic cooling, strengthens its potential as a tool for transitioning toward greener manufacturing processes. By integrating the benefits of sustainable techniques, VAM contributes to further improving process efficiency, although it does so at the cost of supplying energy for vibration generation [4].

Despite its advantages, VAM faces various limitations and challenges that must be addressed to maximize its applicability and effectiveness in industrial contexts. One of the primary challenges is the need for complex vibration generation systems, which often rely on piezoelectric or magnetic devices. These systems incur additional costs to machinery and require specialized maintenance, increasing both initial and operational costs. The integration of these devices into standard tools can also pose challenges, thereby limiting their widespread industrial application. A comprehensive study of the process and the operating environment is therefore essential to ensure the vibration system’s compatibility with the existing machine’s specifications, such as load capacity, structural rigidity, and operational precision [8]. Another significant challenge pertains to energy consumption, as previously mentioned. Additionally, VAM’s applicability to specific materials and processes is limited. While it has proven effective for difficult-to-machine materials, such as titanium alloys, it is not always suitable for softer materials or high-speed processes where vibrations may not yield significant benefits or could even negatively impact the results [4,6]. From an industrial standpoint, the adoption of VAM is constrained by a pervasive lack of awareness concerning its functionality and advantages. Many companies may be hesitant to allocate resources towards relatively novel technology, particularly if the immediate benefits are not readily discernible [6].

3.4.2. Laser-Assisted Machining

Laser-assisted machining (LAM) is a sophisticated technique that has been developed to enhance the efficiency and quality of machining processes. In LAM, a laser is utilized to apply heat to the material’s surface prior to or during the cutting process. This reduction in mechanical resistance facilitates the operation of the tool. LAM is particularly relevant for materials that are otherwise challenging to machine due to their brittleness, high hardness or low thermal conductivity, such as advanced ceramics and composite materials [25,26].

This method offers several significant advantages, rendering it an efficient solution for machining difficult-to-machine materials. A primary benefit of this method is the reduction in cutting forces required, as the localized heating of the material lowers its resistance, facilitating machining and reducing tool wear. Consequently, the lifespan of cutting tools is increased, particularly when dealing with high-hardness materials such as technical ceramics and advanced alloys [12]. LAM has been shown to enhance surface quality by reducing the formation of microcracks and other defects that are frequently observed in conventional machining processes [15]. Furthermore, LAM facilitates enhanced precision and efficiency in the machining of complex components, rendering it particularly advantageous in applications where tight tolerances and high-quality finishes are imperative, such as in the aerospace and medical industries. While the primary focus of LAM is thermal assistance via laser in conventional machining processes, this technology can also be complemented by other techniques, such as cryogenic machining or ultrasonic-assisted machining, forming hybrid processes that maximize both efficiency and quality [4].

The study by García-Martínez et al. [12] reviews several processes under different C/L conditions. One such process involves laser-assisted milling of the titanium alloy Ti6Al4V using carbide tools coated with TiAlN. Conventional C/L methods employ a 10% oil-water mixture as a cutting fluid, while the LAM process uses only argon as an auxiliary gas. This substitution has been shown to simplify the process, reduce costs and minimize the environmental impact associated with cutting fluids. The argon gas was supplied at a rate of 0.47 L/min with a pressure of 3.5 bar. The laser utilized in the process had a wavelength of 1.071 µm, a maximum power of 1 kW and a laser advance distance of 3.5 mm. Cutting speeds of 50 m/min and 75 m/min were employed, with a feed rate of 0.1 mm/tooth and a constant depth of cut of 1 mm. Various variables were analyzed in the study. In conventional conditions, maximum cutting forces of 679 N in the Ff and 361 N in the Fr were recorded. In contrast, the application of LAM led to a significant reduction of these forces, depending on the laser power. At a laser power of 170 W, the cutting forces were reduced to 500 N in Ff and 180 N in Fr, representing 30% and 50% decreases, respectively. This reduction was attributed to localized laser preheating, which lowers the material’s mechanical resistance during machining and is one of the key advantages of LAM. Temperature measurements revealed an average cutting zone temperature of approximately 23 °C for conventional C/L compared to 500 °C with the application of a 170 W laser, marking an increase of over 2000%. This substantial increase in temperature led to enhanced machinability of the titanium alloy without causing significant microstructural alterations. Hardness measurements of the machined surface revealed an average of approximately 350 HV under conventional conditions, while LAM reduced this slightly to 315 HV, meaning a 10% decrease. This reduction was attributed to the lower residual compressive stresses resulting from the reduced cutting forces in LAM. Microhardness analyses at depths of 2 mm confirmed that hardness variations were confined to a superficial layer of less than 200 μm, with no significant changes in the material properties of the unaffected zone. A cost analysis highlighted the economic benefits of LAM compared to conventional milling. Specifically, the cost analysis revealed that under optimal conditions, the cost of traditional milling at a cutting speed of 50 m/min was approximately USD 52 per part. In contrast, LAM, operating at a cutting speed of 85 m/min exhibited a cost reduction of 26%, with a cost of USD 38 per part. While LAM entails supplementary laser operation costs, estimated at USD 27.6 per hour, it facilitates a 70% augmentation in cutting speed, thereby markedly enhancing productivity. Moreover, the elimination of liquid coolants engenders further cost savings and environmental benefits. The optimization of cutting speed and the extension of tool life contribute to the overall cost-effectiveness of LAM, underscoring its economic viability in comparison to conventional methods [12].

As previously stated, LAM is a technology that contributes to sustainability in manufacturing by addressing key aspects related to resource efficiency, environmental impact reduction, and process performance improvement. From an energy perspective, LAM uses a laser to locally heat the areas to be machined, reducing the material’s resistance and consequently lowering the cutting forces required. This results in a reduction in energy consumption during the process when compared to conventional methods, which typically necessitate higher mechanical efforts [12]. Moreover, LAM eliminates or reduces the need for cutting fluids, which are a significant source of contamination and cost in traditional machining. By circumventing the use of such fluids, the process mitigates environmental risks associated with their handling and disposal while also enhancing health and safety conditions for operators by reducing exposure to potentially harmful chemicals [8,12]. Another salient benefit is the enhanced longevity of cutting tools, attributable to the localized heating that mitigates both mechanical and thermal wear, thereby reducing the frequency of tool replacement and consequently the generation of industrial waste. This in turn results in a reduction in the demand for resources necessary to manufacture new components. The capacity of LAM to machine advanced and difficult-to-machine materials, such as titanium alloys, ceramics, and superalloys, further bolsters its sustainability credentials. This capability enables the efficient production of lightweight and durable components that are essential in industries like aerospace and automotive, where weight reduction and energy savings are key priorities [15,25].

Notwithstanding its myriad advantages, LAM faces challenges regarding its initial sustainability due to the high costs and energy consumption associated with laser equipment. The initial financial investment in laser systems and their incorporation into machining setups entails not only the cost of the laser itself but also expenditures associated with maintenance, operation and the training required for personnel to utilize this sophisticated technology. Moreover, the energy consumption of laser systems can be substantial, particularly in configurations necessitating high power levels to process materials with high thermal resistance, thereby potentially counteracting some of the energy benefits of the process [12]. The integration of laser systems into existing machinery frequently necessitates retrofitting traditional equipment with laser modules and upgrading control systems to synchronize the laser with machine movements. Advanced sensors are also essential for process monitoring and thermal damage prevention. Another critical limitation is the sensitivity of LAM to process parameters. Ensuring proper configuration of variables such as laser power, feed rate and focal position is paramount to circumvent undesirable effects, including material overheating, the formation of heat-affected zones and potential deformation or changes in the mechanical properties of the material [25]. Furthermore, LAM exhibits compatibility challenges with certain materials. While its effectiveness with high-strength alloys and advanced ceramics is well documented, its application with heat-sensitive materials or those with low thermal conductivity is less clear, as the localized heating could potentially lead to undesirable outcomes. Furthermore, the integration of lasers into hybrid machining systems, such as those combining cryogenic or ultrasonic-assisted machining, introduces additional technical complexity and increases implementation costs. While these hybrid approaches can enhance the benefits of LAM, they require a greater degree of operational expertise and investment. In conclusion, while LAM offers significant advantages, its technical, economic and operational challenges necessitate innovative solutions and a balanced approach to maximize its potential in modern manufacturing [12,15].

3.4.3. Electric Discharge Machining

Electric Discharge Machining (EDM) is a process that utilizes controlled electrical discharges between an electrode and the workpiece, with a dielectric fluid that is ionized to facilitate the current to flow. The generation of sparks during this process leads to the erosion of the workpiece material, enabling the machining of high hardness conductive materials to be machined with high precision and without the application of mechanical forces. The process is particularly well suited for creating complex shapes and deep cavities in materials such as hardened steels and advanced alloys [1]. The elimination of problems induced by mechanical stress and vibration during machining is due to the absence of physical contact between the tool electrode and the workpiece. Notwithstanding these advantages, EDM is associated with environmental and health disadvantages. These disadvantages stem from the use of kerosene as the dielectric fluid, which functions as a cooling medium, waste disposal and electrical insulation. The use of kerosene results in the generation of hazardous waste, toxic gases and occupational health hazards. In light of these concerns, there has been a surge in interest in proposing more sustainable alternatives, such as the use of deionized water, which is less polluting, and dry EDM (DEDM), which uses gas instead of liquid, thus eliminating waste and reducing environmental impact. However, its low machining speed and high energy consumption limit its use to specific applications [33].

The aforementioned factors have led to the conclusion that EDM is not considered an inherently sustainable process. Nevertheless, its incorporation into production systems has generated interest within the field of sustainability, particularly due to the potential of combining it with more sustainable processes, such as dry machining or MQL. This concept is examined in the work of Singh et al. [32], which reviewed initiatives aimed at advancing green manufacturing through the EDM process. The study examined the performance of three EDM process variants—wet, dry, and near-dry—during drilling operations on SPK steel X210Cr12, a material widely used in the mold and die industry. In the DEDM process, a compressor supplied air at 4 bar through the tool. In the near-DEDM process, a mixture of deionized water (flow rate of 40 mL/min) and air (1.5 bar pressure) was delivered through a biaxial hose. In the wet EDM process, deionized water was used as the sole dielectric fluid at a flow rate of 80 mL/min. The tool employed in all cases was a copper electrode with 99.9% purity. The experiments were conducted under three levels of discharge energy: low (1.5 A, 4 ms pulse duration, 80 V), medium (6 A, 35 ms, 80 V), and high (21 A, 100 ms, 80 V). Each experiment was conducted for a duration of 35 min, with rotational speeds of 400 rpm for the wet and DEDM processes and 190 rpm for the near-DEDM process. The study analyzed surface roughness and tool wear as performance variables. The surface roughness analysis revealed that the wet EDM process resulted in higher roughness, attributable to the formation of larger and deeper craters, particularly at high discharge energy levels, where surface roughness values of 4 µm were recorded—1.4 times greater than those observed in DEDM and near-DEDM processes. This phenomenon was attributed to the enhanced debris evacuation facilitated by the water–air mixture in the dielectric, which reduced the adhesion of molten material to the machined surface and enhanced final surface quality. The evaluation of tool wear involved measuring the mass loss of the tool before and after the machining process. The values obtained were expressed in mg/min and calculated by dividing the mass loss by the total machining time of 35 min for each test. The wet EDM process demonstrated the highest tool wear, approximately eight times greater than that recorded in the DEDM and near-DEDM processes. This was attributed to higher thermal energy transfer to the tool, caused by the high viscosity and dielectric capacity of deionized water. Conversely, at low discharge energy levels, the near-DEDM process exhibited 1.8 times greater tool wear than the wet EDM process, attributed to increased thermal activity due to the oxygen content in the mixed dielectric air, which promoted MRR from the tool. The DEDM process demonstrated the lowest tool wear overall but was also associated with insufficient MRR, particularly at low discharge energies, where virtually no material was machined. The findings of this study suggest that the near-DEDM process achieves a more favorable balance under moderate to low energy conditions, while the wet EDM process exhibits higher tool wear in high-energy applications [32].

The role of EDM in sustainability is evident in both its challenges and its advancements toward more eco-friendly practices. Conventionally, EDM relies on hydrocarbon-based dielectric fluids such as kerosene which generate toxic vapor emissions and hazardous waste. These byproducts pose significant risks to the environment and operator health [33]. In response to the growing demand for sustainable manufacturing processes, alternatives such as DEDM, near-DEDM, water-assisted EDM and EDM using cleaner dielectric fluids, including deionized water or compressed gases, have been developed. These alternatives have been shown to eliminate or significantly reduce the use of hydrocarbons, thereby minimizing waste generation and pollutant emissions and improving the environmental impact of the process [32]. From an energy efficiency standpoint, while EDM inherently consumes electricity to generate discharges, advancements in the design of modern machines have successfully reduced energy consumption per unit of machined material. This enhancement in energy efficiency contributes to sustainability by improving resource efficiency and reducing indirect carbon emissions associated with energy use. Notwithstanding these advancements, EDM’s sustainability still faces challenges. The adoption of cleaner technologies, such as dry EDM, often necessitates a substantial initial investment and technical adjustments to match the efficiency of traditional methods. Moreover, the proper disposal of electrodes and dielectric fluids remains a logistical and environmental challenge for many industries [15].

EDM is a process that presents various limitations and challenges that affect its implementation and efficiency in industrial environments. One of the primary challenges is its dependence on conductive materials, which restricts its applicability exclusively to this category and excludes non-conductive materials that may require alternative processes [32,33]. Moreover, EDM is a relatively slow process compared to other machining methods, particularly for removing large volumes of material, which limits its use in high-production applications [8]. The cost of the process is another significant limitation. EDM equipment, including high-precision generators and control systems, is expensive to acquire and maintain. The consumption of electrodes, which undergo deterioration during the process, results in additional expenses, particularly for high-precision materials such as copper or graphite. This is further compounded by the cost of dielectric fluids, whose handling and disposal present both economic and environmental challenges due to the waste generated and contamination risks. From a technical perspective, precise control of EDM parameters is imperative to ensure quality outcomes, such as surface finish and dimensional accuracy. Failure to maintain these parameters consistently can lead to defects, such as microcrack formation, residual stresses or excessive electrode wear [32]. Additionally, the EDM process generates heat and micro-ejections of material, which can alter the mechanical properties of the treated surface [22]. EDM is advancing toward automated and integrated processes, which introduces additional challenges. While automation can improve efficiency and reduce human errors, it requires significant investment in infrastructure and technical training. Furthermore, while the energy requirements of EDM are lower compared to some conventional methods, further improvements are necessary to enhance the process’s energy efficiency and sustainability [15,17].

4. Discussion

As discussed in the preceding sections, dry and semi-dry machining strategies have been examined, accompanied by case studies that underscore their potential and key advantages over conventional methods. The primary benefits of these techniques have been identified as improvements in environmental, economic and social sustainability. Additionally, other sustainable processes closely related to dry and semi-dry machining have been investigated, and the prospect of combining two or more of these methods to optimize process outcomes has been emphasized.

As previously mentioned, dry and semi-dry machining strategies have exhibited significant advantages in terms of environmental, economic and social sustainability. Nevertheless, these techniques are inherently limited in their applicability. For instance, dry machining, which eliminates cutting fluids, encounters substantial challenges when applied to difficult-to-machine materials, such as titanium alloys. In these cases, the generation of excessive heat leads to thermal deformation and premature tool wear. This underscores the necessity for additional technological advancements to enhance its competitiveness [23,25,29]. Conversely, MQL has demonstrated its superiority over dry machining and, under optimal conditions, conventional C/L techniques, yielding comparable or even superior outcomes in terms of sustainability and operational cost reduction. However, it is important to note that certain processes such as drilling encounter specific challenges with this technique, including chip evacuation and dependency on cutting conditions. Furthermore, MQL’s effectiveness is contingent on the specific cutting parameters, including but not limited to speed, feed rate and material-specific conditions. Improper configuration can lead to inadequate lubrication, which may result in accelerated tool wear and inconsistent surface quality [11,21,28]. This phenomenon is illustrated in Figure 11, which presents the results for surface roughness in a case examined in the work of Makhesana and Patel [13]. The findings indicate that flood cooling yields the most notable results, followed by those obtained with the MQL method, and finally by dry machining. These observations underscore the necessity of accurately adjusting cutting parameters for the MQL method to ensure optimal performance, contradicting the notion that it invariably surpasses conventional methods.

Cryogenic cooling has emerged as a pioneering and promising technique in machining due to its ability to effectively address the thermal challenges associated with tool–material interaction. By utilizing cryogenic coolants such as liquid nitrogen LN₂ or carbon dioxide CO₂ at extremely low temperatures, this technique minimizes heat accumulation in the cutting zone, resulting in a range of tangible benefits for both process quality and the final product. A notable advantage of cryogenic cooling is its impact on the surface quality of machined components. By maintaining low temperatures at the tool–material interface, this technique reduces the risk of thermal deformation, microcracks and residual stresses on the surface and subsurface of the material. In such cases, the use of cryogenic cooling not only enhances dimensional accuracy but also extends tool life by mitigating abrasive and thermal wear [12,31]. Additionally, cryogenic cooling facilitates the machining of materials with high resistance to heat and abrasion which, under conventional or dry machining conditions, generate critical temperatures that compromise both the process and the final product [5]. As illustrated in Figure 12, a case examined in the work published by Proud et al. [31], various strategies, including cryogenic techniques, MQL and dry machining, are evaluated. It is evident from the analysis that cryogenic cooling techniques yield superior outcomes in comparison to dry machining and MQL, further augmenting performance through the enhancement of pressure within the cryogenic coolant flow.

The integration of the MQL method and cryogenic cooling has emerged as a hybrid solution with exceptional potential, representing a novel and advanced approach to machining. This approach integrates the deep cooling capacity of cryogenic fluids with the localized and efficient lubrication of MQL, thereby achieving a unique balance between thermal efficiency and friction reduction. Cryogenic cooling machining alone lacks lubricating properties, which can increase cutting forces in certain materials; however, when combined with MQL, this limitation is effectively mitigated [6,27]. Additionally, the integration of cryogenics with MQL reduces the necessary volume of cryogenic fluids, thereby optimizing operational costs and enhancing the economic viability of the system. Consequently, this hybrid method not only addresses the limitations of each individual technique but also significantly improves machining performance in terms of surface quality, tool wear and operational efficiency [20].

The HPC method entails the direct application of cutting fluids at elevated pressures within the machining zone. This approach has been shown to enhance machining properties by providing efficient thermal control, reducing cutting forces and optimizing chip evacuation. Consequently, it leads to enhanced surface quality, reduced residual stresses and prolonged tool life [18]. The efficacy of HPC is particularly pronounced in the machining of hard-to-machine materials and high-speed processes, where it facilitates the maintenance of high MRR without compromising process stability. Despite the high initial costs, the benefits in terms of quality, efficiency and sustainability strongly justify its implementation in demanding industrial applications [2].

VAM and LAM are two advanced machining techniques that offer significant improvements in critical machining variables, such as cutting forces, surface quality and tool wear. VAM utilizes controlled vibrations, often ultrasonic, applied to either the tool or the workpiece, resulting in intermittent contact between the two. This mechanism reduces cutting forces by minimizing continuous friction, optimizes chip evacuation and extends tool life [8,12]. Furthermore, it enhances surface quality by preventing material build-up on the tool, thereby reducing the formation of burrs and defects [4]. Conversely, LAM employs a laser beam to locally preheat the material prior to cutting, reducing its resistance and facilitating machining. This localized preheating has been shown to significantly reduce cutting forces and enable the machining of advanced materials such as superalloys and composites while avoiding defects such as microcracks and residual stresses [12,26].

EDM employs the use of controlled electrical discharges to induce thermal erosion, thereby facilitating the removal of material. This process renders EDM an optimal technique for machining difficult-to-machine conductive materials, such as superalloys and hardened steels. The primary benefit of EDM lies in its ability to operate without the application of cutting forces, a characteristic that enables the machining of complex geometries and delicate microstructures without inducing mechanical deformation [17]. The technique’s versatility allows for the creation of both deep cavities and intricate shapes; however, significant challenges remain, including electrode wear and energy consumption. When operated with optimal configuration, EDM achieves precise surface finishes and tight tolerances, particularly in components with complex geometries or delicate microstructures. EDM does not rely on conventional cutting fluids; rather, it requires liquid dielectrics, whose environmental management must be carefully addressed [32].

As detailed in

Table 4. Comparison of outcome variables between different C/L techniques (* Bad ** Regular *** Good **** Best). ⁽¹⁾ Best for nano-MQL. ⁽²⁾ Better for nano-MQL.

C/L Technique	Surface Finish	Surface Integrity	Cutting Forces	Cutting Temperature	Tool Wear	Tool Life
Conventional C/L machining	**	**	***	***	***	***
Dry machining	**	*	**	*	*	*
Semi-dry machining (MQL)	** (1)	** (2)	*** (1)	** (2)	*** (1)	***
Cryogenic cooling machining	****	***	**	****	***	***
Cryogenic MQL cooling machining	****	****	****	****	****	****
High-pressure cooling	****	***	***	**	****	****
Vibration assisted machining	*	***	***	***	***	***
Laser assisted machining	***	**	****	*	***	***
Electric discharge machining	***	**	***	**	**	**

, the document presents a comprehensive summary of all the variables that were analyzed for each lubrication and cooling system. This table offers a synopsis of the capabilities associated with each C/L method, thereby facilitating the clear identification of their respective strengths and limitations.

It has been determined that conventional machining with cutting fluids provides efficient lubrication and adequate heat dissipation, making it ideal for standard applications and materials that are not excessively complex. This approach has proven to be highly effective in high-speed turning and milling operations, where the generated heat could compromise the integrity of the workpiece and cutting edge. It achieves favorable results with materials such as nickel alloys and hardened stainless steels due to its ability to minimize thermal generation.

Dry machining faces challenges such as accelerated tool wear and elevated cutting temperatures, yet it is better suited for materials with high thermal conductivity such as aluminum alloys and carbon steels. In these materials, the heat generated during cutting can dissipate rapidly through the chips and the workpiece. Additionally, dry machining is ideal for rough milling operations, where precise surface finishes are not required, as well as for low-depth, high-speed turning operations, where tool coatings resistant to high temperatures are essential. However, significant difficulties are encountered when working with materials that are difficult to machine, such as titanium or nickel alloys. Furthermore, this method operates with the lowest dimensional tolerances among all the methods reviewed.

MQL has been shown to significantly reduce the volume of fluids used, providing a sustainable alternative with favorable outcomes in terms of surface quality and tool life. It is particularly effective in high-speed milling and turning operations, where reducing heat in the cutting zone and ensuring adequate lubrication are crucial to preventing tool wear and achieving high-quality surface finishes. The applicability of MQL is further enhanced by its versatility with regard to machinability levels, making it suitable for a wide range of materials, including hardened steels and lightweight alloys. Additionally, its efficacy in chip evacuation, particularly in the context of continuous turning operations, is noteworthy. Furthermore, MQL’s performance in low-depth drilling operations is noteworthy, as its localized lubrication contributes to extending the lifespan of drill bits and mitigating the risk of chip jamming. Under favorable conditions, the performance of MQL is comparable to that of conventional methods. The incorporation of nanoparticles into this method offers significant improvements by optimizing the fluid’s properties, enhancing its thermal conductivity and lubricating capacity, and reducing tool wear and improving surface quality of machined parts. The nano-MQL technique in particular proves highly effective for hard-to-machine materials and high-speed processes, making it an advanced and versatile approach for modern machining applications.

Cryogenic cooling has been shown to significantly enhance heat dissipation, thereby improving wear resistance and extending tool life. Consequently, machining of challenging materials, such as titanium and nickel alloys, is possible due to the reduction in tool wear, as these materials tend to generate high cutting temperatures due to their low thermal conductivity. Cryogenic cooling is particularly well-suited for machining large components where thermal control is critical to maintaining dimensional tolerances. Furthermore, the application of liquid nitrogen (LN₂) to the cutting zone facilitates chip removal and reduces friction, rendering it advantageous for deep or precision applications. In conclusion, cryogenic cooling is imperative for processes that demand high-quality surface finishes.

The integration of cryogenics with MQL represents a synthesis of the advantages inherent in both methodologies, thereby attaining a balance between efficient cooling and sustainable lubrication. This approach has proven to be remarkably effective in milling operations involving challenging materials, such as titanium and nickel alloys, which generate substantial amounts of heat. Additionally, it demonstrates notable efficacy in high-speed turning operations and with advanced materials. The cryogenic coolant effectively manages the heat generated during prolonged cutting, while the minimal amount of lubricant ensures high-quality surface finishes and extends tool life. Additionally, it is particularly well-suited for precision drilling operations, where it prevents extreme temperatures from accumulating at the cutting edge, while the lubricant reduces friction and facilitates chip evacuation. Consequently, CryoMQL emerges as a remarkably versatile method for applications necessitating tight tolerances and impeccable surface finishes, such as the manufacturing of medical implants or aerospace components.

HPC has been demonstrated to enhance fluid penetration into the cutting zone, thereby significantly reducing temperatures and tool wear. This technique has been shown to be highly effective when working with challenging materials such as titanium alloys, nickel alloys and stainless steels. Consequently, it is widely employed in deep and high-speed milling operations, as it facilitates chip evacuation and controls cutting temperatures, which are critical factors. Furthermore, HPC is well suited for continuous turning operations, particularly in the machining of low-machinability materials. The high pressure prevents the accumulation of heat within the tool, thereby enhancing its durability and mitigating the risk of thermal deformation in the workpiece. Additionally, it minimizes the formation of built-up edges in materials with high surface energy, such as stainless steel and aluminum. This property renders HPC particularly well suited for deep drilling operations, as it facilitates efficient chip removal throughout the process.

VAM has been demonstrated to reduce cutting forces and enhance chip control, particularly when working with materials that are difficult to machine, such as aluminum and titanium alloys. This approach has been shown to minimize the formation of built-up edges, thereby improving surface quality and dimensional accuracy. It is particularly well suited for deep or precision drilling operations in materials such as carbon fiber composites, lightweight alloys, and stainless steels, as it facilitates chip breakage and reduces thrust forces.

LAM has been demonstrated to enhance machinability by reducing the surface hardness of the material prior to cutting, making it particularly effective for heat-resistant alloys. LAM has been shown to excel in machining hardened materials, such as heat-treated steels and titanium alloys, by preheating the workpiece with the laser. This reduces the material’s resistance in the cutting zone, improves tool life and enables higher cutting speeds. Furthermore, LAM’s efficacy extends to drilling brittle or hard materials, such as glass or ceramic composites, where the laser softens the area of penetration, thereby facilitating the drilling process.

EDM is a process that diverges from conventional methodologies. It is utilized for the machining of materials with extreme hardness and electrical conductivity, including tungsten carbides, titanium alloys, nickel alloys, and conductive ceramics. Drilling EDM is a widely employed technique for the creation of deep, small-diameter holes in materials that are challenging to machine. This renders it particularly prevalent in aerospace and turbine applications, where the necessity of holes for ventilation or cooling purposes is paramount.

As elucidated in the document, the impact of each C/L method on sustainability has been thoroughly examined. Conventional C/L, a prevalent practice in machining processes, confronts substantial challenges with regard to environmental and economic sustainability. This method involves the use of large volumes of cutting fluids that contain hazardous chemicals, which pose risks of environmental contamination and health hazards for operators. Economically, conventional C/L contributes to 10–17% of total process costs due to fluid acquisition, handling and disposal, in addition to requiring additional equipment, which increases operational costs [1,18]. In absolute terms, dry machining represents the most sustainable option, as it eliminates the need for cutting fluids and their associated handling, disposal and maintenance costs. However, its feasibility is limited by the need for advanced tooling and accelerated wear, which potentially increases long-term costs. Despite its restricted applicability to specific processes, dry machining is considered a benchmark in sustainability, as it entirely avoids chemical waste [13]. The MQL technique is a balanced approach that considers sustainability and functionality. This technique utilizes minimal amounts of biodegradable fluids, which reduces toxicity and facilitates natural decomposition. Additionally, MQL’s lower energy demands and operational costs make it a compelling alternative to conventional methods, especially in high-precision applications [21]. From an environmental perspective, cryogenic cooling is a superior option because it does not generate liquid or solid waste during the evaporation of refrigerants. This eliminates the need for traditional cutting fluid management, thereby minimizing soil and water contamination and significantly reducing operator health risks by avoiding exposure to harmful chemicals. However, the sustainability of cryogenic cooling is offset by the high energy demands associated with refrigerant production and storage, as well as the elevated implementation and operational costs [15]. CryoMQL integrates the benefits of both cryogenic cooling and MQL, achieving a 90% reduction in cutting fluid usage and employing evaporative cryogenic refrigerants. While this hybrid approach mitigates soil and water contamination, its overall sustainability impact is mitigated by the combined costs and energy consumption of the two methods [31]. HPC has been shown to optimize the use of cutting fluids by applying them efficiently; however, it can still consume significant volumes in demanding applications. Although it improves tool life and reduces per-piece costs, its elevated energy consumption due to high-pressure pumps challenges its environmental sustainability [4,5]. VAM stands out for reducing mechanical efforts and optimizing energy consumption in certain applications, enhancing overall process efficiency. However, the additional energy required to generate controlled vibrations may offset these benefits. Its compatibility with sustainable methods, such as MQL or cryogenics, underscores its potential in advancing toward greener manufacturing [17]. LAM offers high energy efficiency and eliminates cutting fluids, significantly reducing environmental and operator health risks. Its capacity to machine advanced materials while minimizing tool wear positions it as a sustainable choice in sectors like aerospace and automotive, though high initial costs limit widespread adoption [11]. EDM, traditionally constrained by hydrocarbon-based dielectrics, has evolved towards cleaner alternatives like DEDM and water-assisted EDM. These approaches have been shown to reduce toxic emissions and enhance sustainability. However, it should be noted that energy-intensive processes and logistical challenges in waste disposal remain significant barriers [6,25].

In order to assess the financial implications associated with diverse C/L systems in machining processes, it is imperative to take into account all pertinent factors, ranging from raw materials to the final product and subsequent disposal costs. This encompasses the costs of raw materials, fluid consumption, equipment and tool expenses, as well as cleaning and waste disposal costs. Furthermore, the time and resources necessary for cleaning the final product and chips must be taken into account, as these activities can have a substantial impact on the overall process costs. The ensuing comparative analysis, presented in

Table 5. Qualitative estimation of C/L techniques costs (* Very low ** Low *** Medium **** High ***** Very high). ⁽¹⁾ High for nano-MQL and nano-CryoMQL. ⁽²⁾ Very high for nano-MQL and nano-CryoMQL.

Cooling and Lubrication Technique	Raw Material Cost	Fluid Consumption	Equipment Costs	Tool Cost	Cleaning Costs	Disposal Costs
Conventional C/L machining	**	*****	****	***	*****	*****
Dry machining	*	*	*	*****	*	*
Semi-dry machining (MQL)	** (1)	** (2)	***	**	**	**
Cryogenic cooling machining	***	***	*****	***	*	*
Cryogenic MQL cooling machining	**** (1)	*** (2)	*****	***	**	**
High-pressure cooling	****	*****	****	***	*****	*****
Vibration assisted machining	***	*	****	***	**	*
Laser assisted machining	****	*	****	**	*	*
Electric discharge machining	****	****	****	****	****	*****

, elucidates the financial implications of these systems, offering a comprehensive perspective on the cost-effectiveness of the available alternatives [1].

In order to assess the environmental impact of different C/L techniques in machining, it is imperative to examine all factors contributing to their ecological footprint. These factors include but are not limited to waste generation, fluid consumption, emission of hazardous substances, mists and emissions produced during the process and health risks posed to operators. Each of these aspects significantly contributes to the environmental profile of each technique, highlighting their strengths and limitations in terms of sustainability and safety. The ensuing discussion employs a systematic approach, with the aid of

Table 6. Qualitative estimation of C/L environmental impact (* Very low ** Low *** Medium **** High ***** Very high). ⁽¹⁾ High for nano-MQL and nano-CryoMQL. ⁽²⁾ Very high for nano-MQL and nano-CryoMQL. ⁽³⁾ Very low for liquid nitrogen.

Cooling and Lubrication Technique	Residue	Fluid Drag Out	Dangerous Substances	Mist and Emissions	Workers Health Hazards
Conventional C/L machining	*****	*****	****	*****	*****
Dry machining	*	*	*	*	*
Semi-dry machining (MQL)	** (1)	** (2)	**	*** (1)	***
Cryogenic cooling machining	*	*	*	** (3)	*
Cryogenic MQL cooling machining	** (1)	** (2)	**	*** (1)	***
High-pressure cooling	*****	*****	****	*****	*****
Vibration assisted machining	*	*	*	*	*
Laser assisted machining	*	*	*	*	*
Electric discharge machining	*****	****	*****	*****	*****

, to elucidate the environmental impact of the various alternatives and facilitate the selection of the most suitable technique based on the specific requirements of the process.

Several alternatives to conventional C/L have been proposed, offering significant advantages in terms of sustainability, efficiency and reduction in operational costs. Among these, dry machining stands out for completely eliminating cutting fluids, while MQL achieves a balance between functionality and sustainability by utilizing small amounts of biodegradable lubricants. Conversely, cryogenic cooling and CryoMQL present advanced solutions for machining challenging materials, achieving minimization of waste. However, these methods pose challenges related to energy consumption and high initial costs. Methods such as HPC, VAM and LAM demonstrate increased efficiency in specific applications but require specialized equipment and have higher energy impacts. Collectively, these alternatives signify a substantial stride toward sustainable machining, and their selection should be informed by a thorough analysis of process requirements, prioritizing a synthesis of environmental sustainability and economic feasibility.

5. Conclusions

A review of the extant literature reveals that the implementation of sustainable C/L processes in industry confers numerous benefits, including a reduction in environmental impact and costs and improvements in machining results. Based on the literature review, the following conclusions can be drawn:

• Dry machining is the most sustainable alternative as it completely eliminates the use of cutting fluids. However, this method faces significant limitations, such as increased tool wear and a limited capability in high-precision operations. To address these challenges, ongoing innovation focuses on advanced tool coating materials. In contrast, MQL, with fluid consumption rates of up to 350 mL/h compared to over 100 L/h in conventional methods, provides a balance between functionality and sustainability. Additionally, the integration of nano-particles in MQL fluids has emerged as a highly promising technique, further enhancing the properties of the lubricants.
• Cryogenic cooling has been shown to reduce the cutting zone temperature compared to conventional machining, which minimizes the formation of built-up edges and extend the tool life. However, its adoption is limited by high energy consumption and implementation costs. The primary areas of development focus on the design of systems that facilitate a more uniform and controlled distribution of refrigerants. When combined with MQL, the advantages of both methods can be leveraged, leading to a reduction in cutting fluid usage while achieving exceptional cooling performance.
• In the realm of advanced industrial sectors, methodologies such as HPC, VAM and LAM emerge as targeted solutions. HPC, though accompanied by high energy and fluid consumption, has been observed to significantly enhance fluid penetration into the cutting zone. VAM’s effectiveness stems from its ability to reduce cutting forces and enhance chip removal, contributing to extended tool life and optimized energy consumption. LAM exhibits remarkable efficiency in machining advanced materials, leading to decreased tool wear and mechanical stress. Future research endeavors are directed to the exploration of hybrid techniques, such as MQL-HPC or MQL-LAM.
• Despite the technical efficacy of EDM in processing materials with extreme hardness or complex geometries, the process generates substantial amounts of waste. The primary advancements in this technique are focused on its potential application in dry or semi-dry environments, which face significant technical challenges.

A review of the literature shows that machining studies mainly focus on surface roughness, integrity, cutting forces, temperature, tool wear and tool life, while other key factors such as MRR or chip morphology remain underexplored. Future research should prioritize these aspects and extend the analysis to sustainable cooling and lubrication methods, including solid lubricants, nanoparticles and hybrid processes.