**1. Introduction**

Composite materials have been gaining visibility for the last few decades due to their advanced properties in comparison to the other material families, specifically in the aeronautic industry, but also with a growing interest in the automobile industry. This tendency began after the Second World War with innovations for military applications, where these materials brought a significant weight reduction in structural design and presented excellent fatigue properties and corrosion resistance as well [1,2].

Fiber-reinforced polymers (FRPs) are heterogeneous composite materials which combine lightweight, stiff and brittle reinforcing fibers, such as aramid, glass and carbon (known, respectively, as AFRPs, GFRPs and CFRPs) bound together by a polymeric matrix (thermoplastic or thermoset) [2]. The fibers, the reinforcing phase, contribute to the improvement of the mechanical properties of the laminated composite, whereas the matrix transfers the load to the inner fibers and at the same time it protects them from external damage and provides the composite material with its high fracture toughness [3].

In spite of this, there was a need to improve even further the properties of these materials so they could sustain the harsher conditions existent in aircraft, a requisite satisfied with

**Citation:** Costa, R.D.F.S.; Sales-Contini, R.C.M.; Silva, F.J.G.; Sebbe, N.; Jesus, A.M.P. A Critical Review on Fiber Metal Laminates (FML): From Manufacturing to Sustainable Processing. *Metals* **2023**, *13*, 638. https://doi.org/10.3390/ met13040638

Academic Editor: Francesco Colangelo

Received: 16 February 2023 Revised: 12 March 2023 Accepted: 17 March 2023 Published: 23 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the appearance of the fiber-metal laminates (FMLs) [4]. These are, by definition, hybrid structures which combine alternate phases of FRPs in the form of plies with thin sheets of a metal alloy, usually aluminum (FRP/Al) or titanium (FRP/Ti) [5,6]. An example of a FML with the fibers incorporated in epoxy resin is observable in Figure 1, which presents a three/two lay-up, since three metal layers alternate with two reinforcement ones.

**Figure 1.** Fiber metal laminate configuration (adapted from [7]).

Nowadays, several types of FMLs exist, depending on the chosen metal to use or even the polymer, based on the intended application for the new material. For example, if these are composed of elastomer interlayers, their designation becomes FMEL, for fiber-metalelastomer laminates [8]. On the other hand, considering just the metal counterpart, the most common FMLs are the CARALL (Carbon Reinforced Aluminum Laminate), GLARE (Glass Reinforced Aluminum Laminate) and ARALL (Aramid Reinforced Aluminum Laminate) [9–12]. Apart from the metal and reinforcement constituents, as well as the lay-up configuration in which the metal layers can be outside or inside the multi-material, the direction of the laminate must also be considered, namely, if it is a unidirectional hybrid laminate or a cross ply (woven), as shown in Figure 2.

**Figure 2.** Classification of FMLs [13].

Through this union, FMLs combine the durability and easiness of fabrication associated with metal alloys with the outstanding specific properties and excellent fracture and fatigue resistance of high-performance composite materials [14,15]. Furthermore, metals lack fatigue strength and corrosion resistance, whereas composites have a low bearing

strength and impact strength, in addition to their problem of reparability [16]. Hence, the combination of both materials overcomes the existing negative issues individually. The result is a high-strength, lightweight material with improved thermal, mechanical and tribological properties [17].

Regarding the manufacturing of multi-materials, this is a difficult process, due to the different properties of the materials to be joined and relatively weak adhesion between them [18–21]. Surface treatments can also be executed to enhance the adhesion at the metal-composite interface [22–25]. An adequate surface treatment of the metallic layer is indispensable to guarantee a good mechanical and adhesive bond between the composite laminates and metal surfaces. This treatment can be mechanical, through an abrasion to produce a macro-roughened surface and to remove undesirable oxide layers; chemical, with the immersion of the substrate in an acid solution; or electrochemical, with a coupling agent or dry surface, such as plasma-sprayed coating or ion-beam-enhanced deposition [1]. After the treatment, metal sheets may also be annealed, in order to relieve mechanical and thermal stresses, another step to facilitate the adhesion [26].

Several methods can be employed to achieve the intended configuration, with the most prominent being the fusion, chemical bonding (bond dual materials using structural adhesive) [27–29], physical and mechanical bonding, such as screwing or riveting (SPR) [21] and friction-based joining processes [30]. Figure 3 presents some of the processing methods used in the fabrication of Carbon Fiber Reinforced Metal Matrix Composites (CF/MMCs).

**Figure 3.** Processing methods for fabrication of CF/MMCs (adapted from [17]).

The shown techniques can be of diverse natures. In the solid-state processing, the composites are formed as a result of bonding between the metallic matrix and carbon fiber due to mutual diffusion occurring between the two in solid states at high temperature/pressure. The liquid-based metallurgy methods include processes such as casting and gravity or vacuum infiltration. This technique has as benefits short processing times, high fiber contents and low cost; however, the carbon fibers are likely to separate and float on the surface due to the significant density differences. Finally, the deposition processing consists of depositing the matrix on the fibers through various methods, followed by consolidation of the final product [17].

Alternatively, other fabrication methods also exist, such as forming techniques [31,32], where the cured fiber and resin matrix layers are deformed elastically, and the metal layers are deformed plastically by deep drawing, with a combination of optimized inner glass fiber patches and non-cured FMLs [33,34] or vacuum infusion, where there is no need of an autoclave or press [35,36].

In the case of CFRP/Al and CFRP/Ti stacks, firstly, the metal surfaces are treated for an optimal adhesion between the alloy and epoxy resin. After that, they are cured in a hot press, in order to achieve their final configuration, with the adhesive impregnated fibers prepregs successfully joined with the metallic layers [1].

The multi-material is of the utmost importance to the construction of aircraft due to its enhanced properties, surpassing the other families of materials in specific applications [37]. The CFRP composites are applied in the fabrication of major structural members of aircraft, such as floor beams, frame panels and a significant portion of the tail sections [38]. Moreover, 25% of the Airbus A380 airframe consists of composites, from which 22% are carbonor glass-fiber-reinforced plastics and 3% are GLARE [39]. The application of GLARE in the upper fuselage shell of the Airbus A380 resulted in 15–30% weight savings over aluminum panels alone with significant improvement in fatigue properties [40]. Considering the Boeing Commercial Airplanes, 30% of the Boeing 767's outer structure is made from composites, and the Boeing B-787 contains almost 57% of its primary structure built only from composite materials, which confers it its wide range of flexibility. In the same way, the blades used in the cooler section of compressor and fan cases, usually employed in aero engines, are manufactured from CFRP, which reduced the assembly total weight by 180 kg and operational costs by 20% [41,42]. Furthermore, CARALL is also used in helicopter struts to absorb shocks, as well as plane seats. This FML shows an extreme fatigue fracture development resistance and outstanding pressure intensity and energy absorption [43]. Its high stiffness and strength with good impact properties also give CARALL laminates a great advantage for space applications [1].

Not only does the aeronautic industry benefit from the use of these materials, but the automobile sector is also adopting the same practice [44,45]. The reasons behind this are mostly the reduction in fuel consumption and the CO2 emissions of future car generations. The combination of high-strength steel alloys and CFRP prepregs in an FML is a promising approach to producing lightweight car structures with a high stiffness-to-weight ratio [33]. In an automotive chassis, for example, it is beneficial to use CFRP where the chassis needs to be light and stiff aluminum where the deformation needs to be controlled [46]. Due to the high damping and good vibration resistance of FRP, metal-FRP hybrid components can also be used to improve the noise, vibration, and harshness (NVH) performance of automotive parts, which enhances the driving experience [47].

Based on a literature survey between 2000 and 2023, the main contribution of this manuscript is to provide information in a structured way about composites and manufacturing and machining operations of various materials, mainly in the field of drilling, and how the process can be performed in a sustainable way. The major novelty brought by this study is to group this information to perform a SWOT study, performing the critical analysis of the results already existing in the literature to prospect opportunities for improvements in the manufacturing of FML, in the machining process making it as sustainable as possible. Thus, in a structured and organized manner, it is intended to facilitate reader access and guide to what can still be studied in depth to contribute to creating a robust database on the topic in question.

## **2. Methodology**

#### *2.1. Literature Review*

To develop this review, scientific databases such as Clivarate/Web of Science and Scopus were searched for selected papers related to FML and their production steps. In both scientific databases, the advanced search by title, abstract and keywords was used. The chosen search range was between 2000 and 2023. The selection of publications started with the first level of keywords used: Fiber Metal Laminates (FML), process, fabrication, and machining. The following keywords were associated with topics related to manufacturing steps and material characterization, such as CFRP/Al stacks, CFRP/Ti stacks, cooling, damage, defects, drilling, milling, surface treatment, tools, microscopy, tomography and their comparison.

Using the Web of Science search tools, the publication selection started with the combination of the keywords FML and process (6504 papers found), FML and manufacturing (3014 papers found), FML and fabrication (2439 papers found) and FML and machining (856 papers found). The number of papers decreases when a specific keyword is used. If

the keyword FML is changed to "CFRP/Al stacks and machining", the number of papers decreases drastically to 37. Using the Scopus search tools, the combination of the keywords CFRP/Al stacks and processing or CFRP/Al stacks and machining yielded 4445 papers and 3104 papers, respectively, indicating that this research platform has more specific papers related to the topic discussed here. As mentioned above, the number of papers decreases when a specific keyword is used. In this case, when the keyword "CFRP/Al stacks" is changed to "FML and machinability", the number of papers decreases drastically to 24. The diagram presents the route followed to reach the best literature information about the FML machining process (Figure 4).

**Figure 4.** Route to reach best literature information about FML machining process.

Using the keyword "FML and machining", the first three publishers with more publications on the subject are Elsevier, Springer Nature and Sage, and using the keyword "FML and process", the first three publishers with more publications on the subject are Elsevier, Springer Nature, and Wiley, as can be seen in Figure 5a,b.

Another relevant piece of information extracted from the database was related to the countries that most published on the subject of FML machining, and it can be seen in Figure 6 that China leads the publications, followed by the USA and India, with Brazil being in the 13th position.

The literature review is divided into three main sections: (i) multi-material, (ii) machining tools and (iii) process sustainability. The first section discusses the multi-material manufacturing process, focusing on CRFP/Al stacks and CRFP/Ti stacks, followed by the main machining processes: drilling and milling, addressing the most used coatings and substrates for use in multi-material drilling/milling processes. The same section will also cover process parameters and the main lubrication methods used in multi-material machining. Finally, the main process failures according to quality levels and acceptance criteria will be presented, as well as how to analyze these process failures. The second section examines the types of tools used in the machining processes in terms of geometry, the presence or absence of coatings, and the tool wear. Finally, in the third section, the importance of studying the sustainability of the process is presented regarding the recycling or reuse of the chips produced for the different thermoplastic and thermosetting composite

materials, and regarding the products used for cooling during the machining process, considering the gains in operational efficiency.

**Figure 5.** Number of papers produced per publisher using the keyword (**a**) publishers with more publications on the subject and (**b**) FML and process.

**Figure 6.** Number of papers by country produced on the FML and machining area.

### *2.2. SWOT Analysis*

The SWOT analysis reinforces the vision of the strategic context, helping in the knowledge of characteristics on the theme that may not be apparent, in addition to also helping to understand its importance in the market. It is one of the tools that allows a broad view of the theme under study, considering the internal analysis of the process (Strengths and Weaknesses), which can be controlled, and the external analysis (Opportunities and Threats) are elements that cannot be controlled, but their understanding is essential to create the strategic plan of the process [48,49]. The SWOT analysis is carried out to identify where the multi-material machining process turns the threats into opportunities.

The information to carry out the SWOT analysis was extracted from the bibliographic review of the multi-material machining process and corresponding critical analysis. From the information collected, a list of strengths and weaknesses, opportunities, and threats was created by comparing the different sources of information. After compiling the lists, the most relevant items were prioritized, for an effective analysis of about 3 to 5 items per category that were highlighted as relevant. The items were crossed to check the relationships between them and were rated from 0 to 3, where 0 is no relationship, 1 is a weak relationship, 2 is a moderate relationship and 3 is a strong relationship. Based on the SWOT matrix, improvement actions will be established for the topic, identifying opportunities, weaknesses, sustainability and threats to the multi-material machining process.
