**6. Critical Analysis and Discussion**

This manuscript brings an accessible joining of most relevant improvements made in the last two decades regarding the multi-material machining operations. Through several tables containing the state-of-the-art of different themes and respective SWOT analyses comparing the different research performed around the globe, this article serves as an aggregated encyclopedia, so that less time is needed to consult the necessary information on this theme. In order to show the opportunities for each of the main areas related to the machining process on FML and high-hardness materials, SWOT analyses have been developed. The first important issue explored in this paper relates to the FML manufacturing process. To obtain an FML with excellent mechanical properties, it is important to choose the right surface treatment in order to improve the adhesion between layers of dissimilar materials. To present the SWOT analysis, four papers were compared: Roth et al. [8], Drozdziel-Jurkiewicz and Bienia [22], Dieckhoff et al. [50] and Park et al. [51]. These authors were chosen due to the new approach of surface treatments on metal sheets to produce FMLs. The SWOT analysis of the surface treatment applied to FML stacks production is presented in Figure 16.


**Figure 16.** SWOT analysis showing the impact of surface treatment on the adhesion of dissimilar materials during the production of FML stacks.

The most common surface treatments are mechanical abrasion and chemical treatment or a combination of them. Generally, these treatments create a rougher surface promoting mechanical interlock, chemical interlock, or both. Thus, it is important to have a deep analysis about what parameters can influence the interfacial adhesion. Combination of surface treatments improves the bonding energy between the dissimilar materials. A mechanical surface treatment, followed by a chemical one, creates deeper valleys that improve mechanical interlock between the polymeric matrix and metal. Some chemical binders are also used to improve the chemical bonding between metal and composite. New surface treatment techniques, such as laser and plasma techniques with a sustainable bias, should be tested to replace conventional, less sustainable techniques, in order to protect the environment and reduce costs.

In a conventional manufacturing process using an autoclave, the void content can be reduced, and the mechanical properties can be improved, but the production costs are high using this manufacturing process. The thermoforming process can produce a high-quality workpiece with lower cost when compared with the autoclave process.

The second SWOT analysis is related to drilling and milling processes in high hardness materials (Figure 17). The results of two papers found in the literature, Barman et al. [63] and Bolar et al. [65], were discussed to elaborate the SWOT analysis of drilling and milling processes. The authors chose to compare the efficiency of these machining processes in the titanium alloy Ti6Al4V and in CARALL. These papers were chosen because they address and compare the drilling and milling techniques of these materials.

**Figure 17.** SWOT analysis obtained by comparing between drilling and milling processes in FML.

The helical milling process has superior response to high-hardness materials when compared to the drilling process in all parameters studied by the authors, such as machining forces, chip morphology, machining temperature, surface roughness, hole size and burr size. The drilling process is in danger of becoming obsolete compared to milling, so it is necessary to renew the drilling process, such as the development of new tools, new coatings, lubricants and coolants, as well as a more detailed study of the drilling process parameters.

The last one is a SWOT analysis on the use of lubricants/coolants in FML machining. The results of three papers found in the literature, Bertolini et al. [26], Kumar et al. [76] and Giasin et al. [78], were discussed. The authors chose to compare the efficiency of dry and cryogenic conditions during the FML drilling process of three different FML materials: GLARE, Ti/CFRP/Ti and Mg-based FML, respectively. All three papers indicated that cryogenic cooling is the best compared to MQL or dry cooling (Figure 18). Cryogenic cooling is also eco-friendly, as MQL and dry cooling are, but its advantages are better than the others. It promotes a hole quality in terms of lower average hole size, circularity error, perpendicularity error, and burr height due to a significant reduction in machining temperature and a substantial reduction in surface roughness. Although the use of cryogenic drills has shown satisfactory results, it is important to consider the type of drill selected in terms of geometry and coating, as well as the process parameters. In cryogenic machining, cutting forces and torque were more pronounced compared to dry and MQL environments. Poor choice of process parameters can lead to material failure during machining, such as delamination at the metal/composite interface, resulting in material waste and increased process costs.

**Figure 18.** SWOT analysis of lubricants and coolants obtained by comparing between MQL, dry and cryogenic cooling.

Despite all the gathered information related to the subject in analysis, studied in detail and deepened as much as possible, this paper still possesses some limitations. Due to the considered restrictions imposed in its realization, such as the chosen databases, selected keywords, and delimited time period, there may have been eventual scientific articles that were not used for being outside these limits. Therefore, if a study was carried out before the 21st century, it was not included in this review. Nevertheless, the intention was to reassemble the data in the most up-to-date way possible, without making the mistake of aggregating information that is already outdated and surpassed by more recent studies, according to the established boundaries.

#### **7. Conclusions**

FMLs, or Metal Fiber Laminates, are the most sought-after materials in the aerospace and automotive industries, because they combine the mechanical properties of metallic materials, high mechanical resistance and composite materials, as well as presenting a high strength to weight ratio. Nevertheless, the continuous use of these materials in these industries requires mastery of production and finishing techniques.

To this end, it is essential to know how to select the most appropriate surface treatment process for the production of an FML. In the literature review, it was observed that there are conventional and non-conventional techniques, but the conventional techniques combined mechanical treatments by abrasion followed by electrochemical treatment of anodizing using potassium dichromate and ferric sulphate. These are still the ones that present the best results due to the production of pitting caused by the presence of sulphate ions.

By comparing the machining processes, drilling and milling, the quality of the finished part and the response of process parameters such as machining forces, chip morphology, machining temperature, surface roughness, hole size and burr size, it has been concluded that helical milling is by far the best process for machining FMLs or high-hardness materials, and may make drilling obsolete for application in these materials.

Lubrication and coolant applied during the machining process are critical factors in providing quality holes. Data from the literature have been compared and it has been found that the best lubricants/coolants are cryogenic, followed by MQL, and finally dry cooling. These three methods are designed to be sustainable, to produce no waste that pollutes the environment, to be non-toxic and to be affordable.

The tooling used also greatly influences the final quality of the hole. Literature data showed that the best drills to be used are double helix tools containing a CVD diamondbased coating in order to minimize axial forces.

Finally, the SWOT analysis discussed three main topics: surface treatment applied to the production of FML stacks, drilling and milling in FML and lubricants and coolants. The SWOT analysis confirmed what had already been analyzed and discussed in the literature.

Applications based on FMLs will grow substantially in the near future, due to the differentiating characteristics of these materials, both in terms of resistance and toughness as well as their light weight, which makes them particularly useful in mobility applications or where requirements of transport and assembly are demanding. In addition to all already referred, they are sustainable materials, as they present increased durability and due to their lower weight, they can contribute to a lower emission rate when used in mobility applications. The use of thermosetting matrices still represents a challenge in terms of recycling. Since a substantial increase in the consumption of these materials is foreseen, processing techniques need to evolve, hence, the pertinence of this work for researchers who are just starting out in this field of knowledge.

**Author Contributions:** Conceptualization, R.D.F.S.C., R.C.M.S.-C. and F.J.G.S.; methodology, R.C.M.S.- C. and F.J.G.S.; formal analysis, R.D.F.S.C., R.C.M.S.-C. and N.S.; investigation, R.C.M.S.-C.; resources, F.J.G.S.; data curation, R.D.F.S.C. and R.C.M.S.-C.; writing—original draft preparation, R.D.F.S.C. and R.C.M.S.-C.; writing—review and editing, R.C.M.S.-C., F.J.G.S. and N.S.; visualization, R.D.F.S.C., R.C.M.S.-C., F.J.G.S. and A.M.P.J., N.S.; supervision, F.J.G.S. and A.M.P.J.; project administration, F.J.G.S. and A.M.P.J.; funding acquisition, F.J.G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research work was developed under the "DRIVOLUTION—Transition to the factory of the future", with the reference DRIVOLUTION/BM/01/2023 research project, supported by European Structural and Investments Funds regarding the "Portugal 2020" program scope.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank the ISEP, UPorto/FEUP and Centro Paula Souza—FATEC-SJC for the infrastructure offered for the development of this project.

**Conflicts of Interest:** The authors declare no conflict of interest.
