**1. Introduction**

Sustainable machining is being adopted all over the world in manufacturing units as a common practice, as all economic and business activities demand sustainability. It would not be wrong for sustainable manufacturing to be characterized as a branch or extension of the sustainable development philosophy [1]. The sustainable manufacturing philosophy adds value to the final product while keeping the quality environment for the upcoming generations [2]. A wide range of parameters are included in sustainable manufacturing, such as the personal health of the workers, environmental issues, and the safety related to machining operation. As all the basic ingredients of sustainability are an integral part of sustainable manufacturing processes, which include the cost associated with machining operation, safety of the environment, and society, it therefore has a broader perspective than just green and eco-friendly machining operation [3]. The beginning of sustainable manufacturing processes start from the selection of the raw materials, into the early process of manufacturing, and until the finishing of the final product, keeping in view the integrity

**Citation:** Khan, M.A.A.; Hussain, M.; Lodhi, S.K.; Zazoum, B.; Asad, M.; Afzal, A. Green Metalworking Fluids for Sustainable Machining Operations and Other Sustainable Systems: A Review. *Metals* **2022**, *12*, 1466. https://doi.org/10.3390/ met12091466

Academic Editors: Francisco J. G. Silva and Jorge Salguero

Received: 30 May 2022 Accepted: 17 August 2022 Published: 31 August 2022

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and objectives of the organization and its performance. The major manufacturing activity is machining, which encompasses a wide range of operational variables that have the room or potential for transformation towards sustainable development. These operational variables include but are not limited to the cooling and lubricating fluids used in machining operation, disposal of water or other working fluids, energy conservation, life of the tool, and recycling of the chips [4]. MWFs are generally used to cool the workpiece during the machining process and serve to lubricate the workpiece from the beginning, and it is well-known that these fluids are generally required to achieve a high quality output as well as a smoother and higher efficiency in the machining process. Additionally, MWFs are used to decrease the friction between the tool and the workpiece during machining operation, thereby reducing the potential for detrimental effects such as adhesion, galling, and welding; they remove the heat generated at the interface and carry away the chips and other debris that are generated during the machining operation [5,6].

The widely used mineral-based MWFs are the primary cause of many diseases in the machine operators such as skin infections, lung problems, and may also lead to the development of cancer. In addition, studies have found that they are not biodegradable, therefore it is required to treat them before disposing them off into the environment. Otherwise, they may cause serious issues to the environment [7]. In order to achieve sustainability in machining operation, several improvements are needed in this regard, such as developing new materials and applications methods; newer technologies are also needed to dispose-off MWFs [8]. Furthermore, green MWF development will also allow for cutting-edge technology to make processes more sustainable and ensure the safety of the workers and environment. The opportunities for performing sustainable machining are illustrated in Figure 1, and these opportunities can be used in order to address the issues pertaining to MWFs that are based on mineral oil. The most important aspect, in terms of the quality and economical perspectives, is the dimensional exactness of the workpiece [9,10]. Therefore, the machining operators should be able to identify the conditions which result in the precise dimensions for most of the used working materials [11–13].

**Figure 1.** The opportunities for performing sustainable machining.

MWFs hold a major percentage of the effluents that are disposed into the environment [14], and in a study by Cheng et al. [15], it was quoted that the volume of MWF waste had been estimated to be more than 20 billion liters. To curb this issue, environmental regulation authorities have been urging companies to adopt or develop new ways of controlling and discharging the industrial MWFs to mitigate their detrimental effects to the environment and natural habitats. Consequently, there is a need for environmentally friendly MWFs to achieve sustainability in machining operations [16]. New MWFs such

as the ones based on vegetable oil provide better results than mineral oil-based MWFs. This is because of the fact that a far more effective layer of lubricant is formed between the tool and workpiece, developed by the saturated fatty acids present in vegetable oil [17–19]. The vegetable oil-based MWFs have shown an enhanced performance compared to the mineral oil-based MWF for the drilling operation performed on AISI 316L steel, increasing tool life up to 177% and reducing the thrust force up to 7%. It was also demonstrated by Lawal et al. [12] that the presence of triglycerides in the vegetable oil gives better properties that are needed in the lubricants.

Several studies have been conducted to assess the economic impact of MWFs. Adler et al. [20] provided figures that over two billion gallons of machining fluids were consumed by manufacturers in North America in the year 2002. Similarly, another research by Marksberry and Jawahir [6] showed that the total annual consumption of MWFs was 640 million gallons globally in 2007, whereas around 100 million gallons were utilized in US manufacturing sectors; the actual consumption was far larger than this figure, according to other sources. Lawal et al. [21] revealed that in 2005, the global consumption of MWFs was quite high, i.e., more than 1200 million gallons, and the projected increase over the decade was 1.2 percent. The actual estimate was not possible, due to the pervasive nature of filed processes. Pusavec et al. [22] revealed that 15% to 20% of the overall cost of machining processes is due to the MWFs utilized for cooling and lubrication purposes. Replacing the cutting fluids with sustainable machining processes so that it can save up to 20% of overall machining costs would be a huge achievement for manufacturers. King et al. [23] also discussed that about 7% to 17% of the total manufacturing costs is related to the cutting fluids, and 4% is related to tooling expenses. Fluid expenses in industries include the purchase of fluids, setup of a fluid dispensing system, maintenance, waste treatment, and fluid disposal [10]. Brinksmeier et al. [24] showed that MWFs have expenditures of around 16.9% of the overall manufacturing sectors in European automotive industries. Hence, it is obvious from all of these studies that the cost for the handling of MWFs is almost 17–20% of the total manufacturing cost.

### **2. Scientometric Analysis**

Scientometric analysis [25–28] is usually carried out after importing the databases from authentic libraries. Usually, the Scopus and Web of Science databases are selected for the analysis, but it has been reported and observed that Scopus provides a wider and more inclusive coverage of content. The access to profiles of all authors, institutions, serial sources, and the availability of the interrelated databases interface makes the use of Scopus more convenient and comfortable for practical use [29]. Therefore, the Scopus database has been selected for analysis.

Scientometric analysis usually starts by selecting some of the most frequent or widely used keywords on the topic. Therefore, some relevant keywords were used to start the analysis after a preliminary literature review. A total of 1834 documents were filtered out and only the published articles were selected. Articles that were in press were omitted from the analysis. After the search was complete, the database was exported to the commercially available integrated development environment (IDE) R Studio [30], which was used to analyze the database.

#### *2.1. Annual Scientific Publication*

Figure 2 shows the annual scientific publications, which range from 1975 to 2021. It can be seen from the figure that research on metalworking fluids and sustainable machining started from 1975 and only had a few articles published until the early 2000s. However, a spike was observed in the research from 2003 onwards, where the number of annually published papers increased and in the past seven years, a substantial advancement has been made in the research area of sustainable MWFs and sustainable machining operations. Therefore, the prime focus of this article was to review the papers published in the past

10 years, but for the sake of establishing some basic concepts and forming the bases, some earlier literature has also been cited.

**Figure 2.** Annual scientific publication on metalworking fluids and sustainable machining operations.
