1. Introduction
Transport is responsible for more than 30% of CO
2 emissions in the European Union (EU); 72% comes from road transport, 13.6% from maritime industry, 13.4% from civil aviation, and 1% from rail transport and others. The EU has committed to reducing emissions from transport by 60% by 2050, compared to 1990 levels [
1].
Regarding road transport, new cars sold in 2017 emitted on average 118.5 g CO
2/km, i.e., an increase compared to 2016 of 0.4 g CO
2/km. On the other hand, the mass of vehicles strongly influences their consumption and pollutant emissions. The average mass of new cars sold in 2017 in the EU was 1390 kg. The mass of an average diesel car sold in 2017 was 20 kg lower than in 2016 [
2]. On the other hand, CO
2 emissions from heavy-duty vehicles, such as trucks, buses and coaches, represent about 6% of total EU emissions and 25% of road transport emissions, and are expected to increase by 9% between 2010 and 2030 [
3]. Concerning legislation, the average CO
2 emissions of new passenger cars registered in the EU must decrease by 15% by 2025 and by 30% by 2030, compared to 2021 data. These requirements imply an increase in the manufacturing cost of a new car of about EUR 1000 per year by 2030. However, this cost is much lower than the fuel savings over the lifetime of the vehicle. Most cars today are powered by combustion engines, and it is expected that at least 80% of the new car fleet in 2030 will still use a combustion engine [
4].
As for the aviation sector, the worldwide aviation industry produces about 2% of all human-generated carbon dioxide emissions [
4,
5]. On the other hand, the EU air transport sector directly employs between 1.41 and 2.02 million people and supports between 4.83 and 5.54 million jobs globally, contributing directly to the gross domestic product (GDP) with EUR 110,000 million. Its global impact, tourism included, reaches EUR 510,000 million [
6]. In 2001, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) was created, and among its most important objectives are the reduction of CO
2 emissions by 75% compared to 2000 data, and the design of aeroplanes to optimise their recycling [
7]. In the design of fuselages and cabin interiors, it is essential to promote lightweight structural materials. Their use will require new design and manufacturing methods, with multifunctional materials and structures to save weight and reduce manufacturing costs, seeking high recyclability and reusability [
8]. With these requirements, the aircraft industry is making significant efforts towards sustainability. For example, each new generation of Boeing aircraft is between 15% to 25% more efficient than the previous one [
9]. Regarding Airbus, the A350 model is made of 53% lightweight and composite materials, and saves 25% of fuel compared to the previous model, and the A220 family of aircraft is the most efficient in its class [
5]. New production methods, such as 3D printing, help to produce lighter parts with significantly less waste, and help to reduce consumption. Together with Tarmac Aerosave, Airbus reuses or recycles up to 92% of its aircraft. Emissions of volatile organic compound (VOC) were reduced by 60% from 2006 to 2017. These innovative solutions aided Airbus in reducing greenhouse gas emissions up to 14% between 2006 and 2017 [
5,
10]. On the other hand, the Intergovernmental Panel on Climate Change (IPCC) is the United Nations body charged with assessing the science related to climate change. In October 2018, the IPCC published a special report on the impacts of global warming of 1.5 °C above pre-industrial levels to support the Paris Agreement process. This report estimated that climate warming due to human activities is increasing by 0.2 °C per decade. To stabilise, global net CO
2 emissions from human activities would need to reach net zero by 2050 [
10,
11]. For this reason, in 2019, all airlines worldwide with international routes started to officially track and communicate their emissions, following the guidelines defined by the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), approved by the International Civil Aviation Organization (ICAO) of the United Nations in 2016 [
5,
9].
Maritime transport is responsible for approximately 940 million tons of CO
2 emissions annually, accounting for around 2.5% of worldwide greenhouse gas emissions (GHG). If no new measures are implemented, it is estimated to increase from 50% to 250% by 2050. The International Maritime Organization (IMO) has set targets to reduce annual GHG emissions from shipping by at least 50% by 2050, compared to 2008 levels [
12]. The IMO has established guidelines for the consumption of each type of ship according to its cargo capacity, and newly built ships will have to adapt to these regulations, which will become progressively more stringent over time. For example, by 2025, all new ships must be 30% more efficient than those manufactured in 2014 [
13]. By transferring Airbus’ expertise in aeronautics to shipping, AirSeas is expected to enable ship owners to reduce fuel consumption and CO
2 emissions by 20% [
5].
On the other hand, it is inevitable to mention the effect that the COVID-19 pandemic has caused on transportation-related sectors. In the aviation sector, compared to 2019, the year 2020 experienced a reduction of 2.7 billion passengers and losses of USD 371 billion, and forecasts indicate that in 2021 there will be a reduction of between 1.9 and 2.4 billion passengers, and losses between USD 282 and 343 billion. [
14]. In the automotive sector, production losses in the EU due to COVID-19 amounted to 4,024,036 motor vehicles from January 1 to September 30, 2020. This quantity represented 22.3% of total EU production in 2019. The jobs of at least 1,138,536 Europeans working directly in automotive manufacturing were affected by factory closures as a result of the pandemic, and the impact on the automotive supply chain, in general, has been even more critical [
15].
As previously stated, the need to reduce energy consumption, and therefore pollutant emissions, is one of the critical targets for transport-related sectors, such as the aeronautical, automotive, or maritime industries. An important research line is focused on reducing the mass of vehicles and ships through components made of light metal alloys. Within this range of alloys, aluminium, titanium, and magnesium alloys stand out for their excellent weight/strength ratio. Moreover, these alloys can be combined with one other or with other light and resistant materials to create hybrid structures, offering the combined advantage of low weight and high strength while maintaining good wear and fatigue characteristics [
16,
17,
18], although it is estimated that the maximum reduction in environmental impact by reducing vehicle mass is limited to 7% [
19].
Aluminium alloys are characterised by their light weight, corrosion resistance, and good thermal and electrical conductivity. Their mechanical properties at room temperature, such as tensile strength, yield strength, and modulus of elasticity, are moderate, and their fatigue strength is acceptable. They are materials that are widely used in lightweight structural parts for the automotive and aerospace industries [
16,
20], and the transportation industry is responsible for 35–40% of their total consumption [
21]. They are frequently applied through aluminium matrix composites (AMMs); these multi-materials have superior corrosion, wear, electrical, temperature, and elastic modulus properties compared to conventional alloys, and good cost/strength ratios, which make them good candidates for aerospace, automotive, chemical, and transportation applications, where, for example, in the case of the transportation industry, their use is forecast to double by the year 2025 [
22,
23]. In addition, they are also frequently used associated in multi-material composites with steels [
24], FRPs [
25], magnesium [
17], and titanium [
16].
Concerning magnesium alloys, magnesium is a metal with many advantages of use. It is very abundant, representing 2.7% of the earth’s crust, and it can be produced from seawater with a purity of 98.8%. Its density is 66% that of aluminium and 25% that of steel, making it an ideal candidate to replace them. Magnesium is the lightest structural alloy on the planet, but its use is limited by its low formability. In machining processes, the forces and temperatures in the cutting zone are low. In addition, its chips are short, and tool life is long, so it is considered an easy material to machine and a good candidate for machining through sustainable cooling/lubrication. However, it is a material that presents some drawbacks, such as the risk of ignition from 450 °C and the risk of explosion when it is in powder form. It generates flammable and potentially explosive hydrogen atmospheres when reacting with water, so it is of interest to reduce the use of water-based coolants [
18]. There is a lot of literature associated with its use in companies in the automotive sector, and it is currently used by automotive companies such as Volkswagen, General Motors, Ford, Toyota, and BMW to manufacture some of their components [
26].
Regarding titanium alloys, they are characterised by light weight, high wear and corrosion resistance, and a great ability to maintain high strength at high temperatures; however, their low thermal conductivity and high reactivity make them tend to produce premature tool wear, so they are considered a difficult material to machine [
27,
28]. It is estimated that Ti6Al4V alloy represents 50% of the global production of titanium metal, and 80% of this corresponds to the aerospace and medical industries [
28,
29]. Titanium is a material with a clear interest in being recycled, reused, and manufactured in near-final forms because of its environmental impact when using primary material [
30,
31].
On the other hand, in studies performed on existing studies, not all publications, papers, or research articles provided similar reliability, methodological quality and interest [
32]. For this reason, a methodology adapted from the PRISMA statement to the field of engineering was established. The objective is to limit possible biases in selecting and analysing scientific literature to provide the most relevant and representative articles, with a contrasted quality, and transparent and uniform inclusion criteria. The resulting selection comprises the 40 most cited research papers over the period 2015–2020, with an average of 20.6 citations per article, according to information from the WoS search engine on 7 December 2021.
Among the selection criteria adopted for the search were: Open Access literature, because it is a trend in academic evaluation [
33,
34] that seeks to democratise access to knowledge and educational materials [
35,
36,
37,
38]; English-language publications, because it is the language employed in more than 90% of articles in science [
39] and has the highest number of citations [
40]; use of the Web of Science (WoS) repository, as it is the world’s leading search platform for scientific citations and analytical information [
41] and overlaps 94% of the citations included in Scopus in the engineering domain [
42], and the average number of citations per year has been included as it is widely used and accepted as a bibliometric indicator [
42,
43,
44].
The following conclusions can be drawn from the analysis: machining is included in 40% of the studies, and turning is the most trending topic found in 38% of the machining studies, followed by drilling in 31%. In addition, 75% of the selected machining studies include sustainable cooling, 63% dry machining, and 19% cryogenic cooling and MQL. Moreover, studies on the replacement of conventional fluids by biodegradable oils are emerging. On the other hand, studies that include multi-material components are also growing in number. Within this group, the joining of dissimilar materials with 46%, and the machining of multi-material composites with 38% of the multi-material articles, are identified as topics of special interest. Multi-material combination of metal–metal, with 62%, appears in the most extensive number of studies on multi-materials, followed by AMMs, found in 31%. Finally, additive manufacturing is another topic identified as a subject of current interest, appearing in 18% of the selected studies. The work is complemented by tables summarizing the main topics covered and the most cited articles on sustainable machining and re-cooling, multi-materials and additive manufacturing.
2. Methodology for Article Selection and Analysis
The methodology followed for the review and analysis of the literature published during 2015–2020 on lightweight aluminium, titanium, and magnesium alloys with potential application to the aeronautical and automotive fields, involving machining processes and/or innovative manufacturing processes, is described below. This methodology was established beforehand and applies to the selection and information analysis stages. The main objective is to minimise the probability of bias in the study. First, quality, inclusion and exclusion criteria are predefined. Then, keywords and resulting Boolean equations are defined. A search engine is predefined, and a search is performed by applying the Boolean equations and the predefined inclusion criteria. Afterwards, the application of the methodology carries out an initial preselection and an individual review of each preselected study, to ensure compliance with the inclusion criteria that make it possible to arrive at a final selection. In the case that a larger number of articles than necessary was obtained for the study, the papers with the highest average number of citations per published year are selected. Finally, the desired information is extracted through closed questions, and the information contained in the studies is analysed to answer the questions that gave rise to the research. The flow chart in
Figure 1 shows the applied methodology graphically.
The purpose is to obtain a selection of relevant, comparable articles chosen based on pre-established criteria, and thus minimise the risk of bias in their choice or analysis by establishing protocols for all the decisions to make over the process. The Web of Science (WoS) database was defined as the search engine. This database is provided by Clarivate Analytics and allows for tracking more than 1700 million references cited in more than 159 million records [
45].
WoS is the world’s leading search platform for scientific citations and analytical information. It is used as a research tool, supports a wide range of scientific papers from different disciplines, and allows the analysis of large-scale datasets. Therefore, it has been extensively employed in academic studies published over the last 20 years [
41,
42,
43]. The choice of English as the language of publication is based on the fact that in international journals more than 90% of the articles in the natural sciences are written in English, so many authors believe that it is necessary to publish in English, or even only in English [
39]. Furthermore, articles published in non-English languages have fewer citations [
40,
42], so including articles in non-English languages may lead to heterogeneity in the selection, and the aim is to obtain a representative selection based on homogeneous selection criteria, so that we can analyse current trends. As a publication format, only Open Access publications are chosen. According to the declaration from the Budapest Open Access initiative in 2002, “Removing access barriers to this literature will accelerate research, enrich education, share the learning of the rich with the poor and the poor with the rich, make this literature as useful as it can be, and lay the foundation for uniting humanity in a common intellectual conversation and quest for knowledge.” [
37,
38]. According to the Director of the Harvard Open Access Project, “open access is the convergence of tradition and a new technology, which makes a public good possible” [
36]. The open access movement seeks to democratise access to knowledge and educational materials, and is integrated in important institutions such as Harvard or MIT, where the OpenCourseWare project allows access to a major part of MIT’s course materials in open access [
35]. In addition, universities such as the Dutch University of Utrecht are beginning to stop using the impact factor as a parameter for recruitment and promotion decisions, and their professors will be evaluated for their commitment to open science [
33]. Furthermore, a study on the impact of open access on the number of citations in academic publications in civil engineering shows that papers from top-ranked journals (first quartile and second quartile) obtained more citations than non-OA articles, according to data from the WOS and Scopus databases [
34]. Although it is interesting to include in the literature selection a weighting of the importance of the knowledge transmitted by the articles, no reproducible method has been found so far. Therefore, the number of citations, a widely accepted criterion, was used as a bibliometric indicator [
42,
43,
44]. At this point, it is important to emphasise that this is an open access literature review that seeks to identify the most recent trends through the systematic analysis of representative publications, with proven acceptance by the scientific community, peer-reviewed, and published in recognised journals and conferences, and it does not intend in any case to be a ranking of the best publications. Finally, although there is great interest in nanomaterials based on titanium [
46], aluminium [
47], or magnesium [
48], they were excluded from the search to avoid noise in the results, because the authors’ study focuses on the macroscopic level.
The Boolean Equation (1) indicates the search strategy. The definition of the search aims to select representative and quality literature published on magnesium, aluminium, and/or titanium light alloys, with studies of individual materials and those of hybrid or multi-material materials being of interest, whenever the materials included are lightweight structural materials. In addition, the latest trends in machining processes are explored, particularly those related to the aeronautical or automotive industries, and all this is defined under a pronounced sustainability-oriented perspective. Other inclusion criteria required for the final selection are publication in Open Access, in journals with an impact factor of Q1, Q2, or proceedings of renowned conferences, and the use of the English language.
By applying the methodology, the final selection of the 40 articles ranking the highest average number of citations per year of publication during the period 2015–2020 was reached. Finally, a literature selection with an average of 20.6 citations/article and a total of 825 citations was obtained. For the number of total citations and for the calculation of the averages per year of publication for each article, the data provided by WoS on the date of the last search on 7 December 2021 were used. An initial pre-selection and the final selection of articles were made based on the data provided by WoS at the last search. The calculation of average citations by year of publication was made, considering for each article the difference in years between the year of publication and the year 2020, both included.
Once the pre-selected articles had been reviewed and the final selection was obtained, a simple database was prepared in Excel in which the relevant information extracted from each article was stored—mainly the title, authors, scientific journal with its classification by percentile or congress, year of publication, Digital Object Identifier (DOI), and total and average number of citations per year of publication. In addition, univocal questions were defined to be asked to each article to obtain a complete picture of the interests and trends of the topics and materials investigated. Subsequently, during the review of the studies, the information extracted through closed questions was stored in the file.
Table 1 shows the main characteristics and the inclusion criteria required for the selected bibliography, and
Table 2 summarises the metrics of the publications where the selected studies were published.
4. Conclusions
Transport-related industries, such as aeronautics and the automotive industry, have established the reduction of consumption and pollutant emissions as a top priority, pushed by increasingly restrictive environmental legislation and a society that is more demanding and aware of the need for sustainability. In this context, one line of work is the replacement of conventional structural materials with lightweight materials that meet the stringent requirements demanded and, at the same time, contribute to reducing the consumption of vehicles and aircraft. Among these materials are metallic alloys of aluminium, titanium, and magnesium, which have excellent weight-to-strength ratios and can be used individually, in combination with one another or with other lightweight structural elements. For this reason, the authors considered it interesting to carry out a state-of-the-art review on these materials to know the trend on materials studied, combinations used, current sustainable processes applied, and machining processes and sustainable cooling/lubrication involved.
In addition, a methodology adapted from the PRISMA statement was established. The aim is to minimise potential biases in the selection and analysis of the literature. This methodology ensures the availability of relevant and representative articles, with contrasted quality, and clear and homogeneous inclusion criteria.
The main criteria chosen were publications in Open Access, in English, using Web of Science as the search engine. As a bibliometric indicator for the selection, the average number of citations per year of publication was adopted. In addition, the search was carried out using predefined Boolean equations based on keywords. The criteria aim to review the open-access literature to identify the most recent trends by using representative publications, with proven acceptance by the scientific community, that are peer-reviewed, and are published in recognised journals and conferences. It is important to emphasise that the purpose is not to make a ranking of the best publications but to study the trend. As a next step, a future review employing similar criteria, but including both Open Access and non-Open Access, is considered to be of interest, in order to analyse differences and deepen the study. The final selection consists of the 40 most cited related research papers during the period 2015–2020, with an average of 20.6 citations per article, according to information from the WoS search engine on 7 December 2021.
Machining appears in 40% of the studies, including turning with 38% and drilling with 31% of the machining studies, the most trending topics present. On the other hand, 75% of the selected machining studies include sustainable cooling, 63% for dry machining, and 19% for cryogenic cooling and MQL. In addition, studies on the replacement of conventional fluids with biodegradable oils are emerging. Studies involving multi-material components are also abundant. Within this group, the joining of dissimilar materials and the machining of multi-material parts are identified as topics of interest, including 46% of the studies addressing multi-materials. The multi-material combination involved in most studies is the metal–metal combination, included in 62% of the studies involving multi-materials, followed by AMMs, which are present in 31%.
Finally, additive manufacturing is another subject identified as increasing in interest and appears in 18% of the selected studies. The work is supported by tables summarising the main topics covered, and summarising the most cited articles on sustainable machining and cooling, multi-materials or hybrid components and additive manufacturing.