New Advances and Future Possibilities in Forming Technology of Hybrid Metal–Polymer Composites Used in Aerospace Applications

Abstract

Fibre metal laminates, hybrid composite materials built up from interlaced layers of thin metals and fibre reinforced adhesives, are future-proof materials used in the production of passenger aircraft, yachts, sailplanes, racing cars, and sports equipment. The most commercially available fibre–metal laminates are carbon reinforced aluminium laminates, aramid reinforced aluminium laminates, and glass reinforced aluminium laminates. This review emphasises the developing technologies for forming hybrid metal–polymer composites (HMPC). New advances and future possibilities in the forming technology for this group of materials is discussed. A brief classification of the currently available types of FMLs and details of their methods of fabrication are also presented. Particular emphasis was placed on the methods of shaping FMLs using plastic working techniques, i.e., incremental sheet forming, shot peening forming, press brake bending, electro-magnetic forming, hydroforming, and stamping. Current progress and the future directions of research on HMPCs are summarised and presented.

1. Introduction

In recent years, polymer matrix composites reinforced with continuous fibres have become one of the basic and most promising groups of materials in many engineering sectors, especially in the automotive and aerospace industries. Fibre metal laminates (FMLs) were developed due to the limitations of metals, which relate primarily to their large weight, and the limited use of classic composites. These consist of alternating layers of metal and a polymer composite reinforced with fibres [1,2,3]. The first FMLs were developed in the laboratory in the mid-20th century. Among these, we distinguish two groups. The first group consists of composites based on aluminium, while the second group consists of composites in which other metals are used, such as magnesium [4,5,6], titanium [7,8], or steel [9,10,11]. The most commercially available FMLs are glass reinforced aluminium laminates (GLARE), aramid reinforced aluminium laminate (ARALL), and carbon reinforced aluminium laminate (CARALL). The first generation FML, ARALL, was developed in 1978 at Delft University of Technology [12]. There are four grades of ARALL composites, differing mainly in the grade of the base material (ARALL: 1-7075-T6; 2-2024-T3; 3-7075-T76; 4-2024-T8). ARALLs have not been widely used due to their low strength and durability on the metal–composite interface, low strength of the fibre–matrix connection, and high production costs [13,14,15]. CARALL is a laminate made of layers of a polymer composite reinforced with carbon fibres preimpregnated with epoxy resin and layers of aluminium [16,17,18]. CARALL exhibits good fatigue properties under high cycle loading [19]. This property of FMLs is one of the main concerns for aerospace applications, especially during the service life of aircraft structures [20]. GLARE laminates consist of composite layers reinforced with high strength glass fibres (GFs) in an epoxy resin matrix and metal layers made of 2024-T3 aluminium alloy (GLARE 2 through GLARE 6) and 7475-T761 (Glare 1) [21,22]. The industrially produced varieties of GLARE differ in the number of composite layers and the angular orientation of the reinforcing fibres [23]. FMLs are widely used in aviation. The Boeing 787 was the first commercial aircraft to be 80% constructed of composites by volume [24,25]. Each Boeing 787 aircraft contains approximately 32,000 kg of carbon fibre reinforced polymer (CFRP) composites. Now, each A380 contains 440 m² of FMLs in vertical stabiliser leading edges and fuselage panels [26].

The Boeing 787, Bombardier C-Series, and Airbus A350 aircraft are fabricated with FRP composites with 50%, 47%, and 53% by weight of composites, respectively [24,27,28]. In the aircraft industry, FMLs are often used in upper fuselage skin panel structures, and CFRPs are used in the aircraft wing boxes, wing panels, and stabilisers [29]. Drones and unmanned aerial vehicles (UAVs) are currently being fabricated using fibre reinforced polymer (FRP) composites [30,31]. It is clear from Figure 1 that the importance of composite materials in passenger aviation is increasing exponentially.

The properties of FMLs are clearly more desirable than those of the constituent elements. By using various combinations and numbers of metal layers as well as fibre reinforced polymer composites, the properties of FMLs can be controlled to give a practically unlimited range. FMLs exhibit very high strength in relation to their weight [33,34,35]. These properties derive from the properties of the individual components and depend on the properties and orientation of the reinforcing fibres, the kind of metal layer and its thickness [36,37,38]. The orientation of the fibres in relation to the successive layers of the FML determines the anisotropy or isotropy of the resultant composite [39,40]. Fibres used for their production can be continuous or discontinuous (staple fibres, whiskers) [41,42]. Numerous products made of single fibres, such as fabrics, mats, rovings, or preimpregnates [43,44], can also be used as reinforcement.

The FML production process uses various production methods, which depend mainly on the type of matrix (thermosetting or thermoplastic) due to significant differences in the properties of the polymers used [45,46]. The most common methods of manufacturing composites using thermosetting polymers include hand lay-up, forming with the use of a flexible vacuum bag or autoclave, infusion methods, low and high pressure pressing, and automated fibre placement (AFP). Composite materials consisting of thermoplastics are mainly produced by the method of “hot” pressuring in presses using chilled moulds, AFP, and methods of continuous pultrusion [47]. In terms of the mechanical strength of FMLs, the most advantageous method of producing FMLs is the autoclave method, which uses the simultaneous action of pressure, vacuum, and temperature [15,48].

The main advantage of autoclave technology is the obtaining of a laminate of very high quality in terms of the degree of porosity and metal–composite adhesion. The adhesive bonding of metal and composite layers and hardening of the preimpregnate take place in the autoclave process; however, this method requires a long cycle time and complex tool set. Therefore, directly formed FMLs are gaining more attention. The most common FML forming methods include press brake bending, lay-up technique, shot peening forming, incremental sheet forming, and die forming (stamping, hydroforming, and electromagnetic forming).

The research work currently being conducted is focused on new types of metal–fibre laminates for specific applications, such as use underwater [49,50], and techniques for their production [51,52]. The development priorities of FMLs in industrial use are titanium/CFRP hybrid laminates [53] and composites with a thermoplastic polyether ether ketone (PEEK) matrix reinforced with carbon fibres [54,55]. Currently, there is an intensification of research on the use of thermoplastic preimpregnates in the production of FMLs [56,57] due to their high resistance to dynamic loads [58,59].

In order to reduce the environmental impact without compromising the strength requirements, there is a growing interest in the production of FMLs reinforced with carbon, sugar palm, and flax fibres [60]. The environmental concerns lead to the development of FMLs containing natural fibre composites [61,62]. Hussain et al. [63] developed novel 3D jute reinforced natural fibre aluminium laminates (JuRALs). Mohammed et al. [64] tested compression, tensile, and flexural strengths of FMLs made from natural/synthetic fibre composites. The compressive and tensile strengths obtained showed that the carbon and flax fibre reinforced aluminium alloy (CAFRALL) composite had superior properties over the carbon and kenaf fibre reinforced aluminium alloy (CAKRALL) composite. It was also found that both composites can be used as materials in the fire-designated zone. At the same time, investigations have been conducted on various nanofillers, including clay nanoparticles, graphene nanoplatelets, oxide nanoparticles, and carbon nanotubes, to improve the properties of FMLs [65,66,67].

Due to the growing interest in hybrid metal–polymer composites (HMPC) and the emergence of a rapidly growing number of publications in this field, this paper presents a systematic review according to the PRISMA guidelines [68] on the new advances in and future potential of forming technology for HMPCs. A brief classification of the currently available types of FMLs and details of their methods of fabrication are also presented, with particular emphasis on research carried out in recent years. Review articles found in the literature are mainly devoted to production methods and methods of surface preparation of materials intended for the production of FMLs. Meanwhile, this article places particular emphasis on the methods of forming FMLs using plastic working, incremental sheet forming, shot peening forming, press brake bending, electromagnetic forming, hydroforming, and stamping. The article ends with a summary and conclusions.

2. Classification of FMLs

Over the past decade, most industrial demands, especially in the aircraft and automotive industries, have emphasised the use of high-performance, high strength, and lightweight structures. The hybrid material concept has been suggested to meet these kinds of demands and to overcome the disadvantages of lightweight materials. This has spurred the development of different FMLs. Fibre–metal laminates can be considered a family of hybrid materials [69] that can exhibit the advantages of both metals and composites. Ding et al. [52] presented the development of FMLs chronologically, starting with the metal bonding technology in the 1940s and ending with a new approach utilised by the LEIKA project in 2010. ALCOA and AKZO have produced the first generation of FMLs [70]. In 1971 and 1972, fibre–metal laminates were presented by embedding laminates of titanium and layers of fibres as part of a performance study in the space department of Fokker, a contractor to NASA, together with a department in Schliekelmann and in the Philips laboratory [71]. In 1978, the first tests were carried out on carbon and aramid fibre reinforced laminates with flight simulation [72]. Later, the demand was for FMLs with high strength characteristics together with much stiffer laminates, and CARALL was investigated and developed from ARALL [73]. In the 1980s, ARALL was developed by Delft University and used as a panel in the lower skin of the Fokker F-27 wing structure. ARALL is made from layers of aluminium and aramid fibre reinforced epoxy [74,75]. GLARE is a unique material for aircraft applications [76] and, nowadays, is mostly used for the fuselage structure of modern aircraft. GLARE was also developed at Delft in 1987, and the first tests were on the Airbus A380 super-jumbo on 16th May 2001 [71]. To find promising properties and applications, different FMLs have been investigated and developed since the 1990s using different materials and fibres. There are two main classical ways of bonding the layers of metals with fibre reinforced laminate, which are mechanical and adhesive, and the commercially available types of FML that have been most extensively used are ARALL, GLARE, and CARALL based on aramid fibres, high strength glass fibres, and carbon fibres, respectively [17,77]. A review paper has been presented by Sinke [78], which describes the manufacturing principles of FMLs. Overall, in view of all that has been mentioned in the brief literature survey presented here, this explains that FMLs are made of different metals as base materials and reinforced by different fibres. Moreover, FMLs can be fabricated using an alternating laminate of thin metal sheets and thin composite layers [79]. Therefore, the classification of FMLs is important and is needed for the performance requirements and manufacturing considerations of different applications. FMLs may be classified based on the structural arrangements of the different layers or by the materials of which it is composed. In terms of structural arrangements, FMLs may be divided into asymmetric, sandwich, and multi-stack [52]. As an alternative to the classification based on the structural arrangement, the constituent materials can be used as a more accepted classification of FMLs. Constituent materials can, in theory, be used and mixed freely to fabricate FMLs [80]. To this end, Table 1 lists the FMLs based on their constituents, and the reader should bear in mind that the classification does not consider the different treatment of the materials.

Figure 2 and Figure 3 can be derived from Table 1, showing that aluminium alloy is the most frequently used base material in the fabrication of FMLs, with a percentage use of 62%, as indicated in Figure 2. Figure 3 shows that glass is the reinforcement most commonly used in FMLs, with a percentage use of 36.89%.

3. Fabrication of FMLs

The procedure for preparing the surface of the layers of metal composites is similar to the procedure used for the adhesive bonding of materials. The surface preparation should help to obtain the strongest possible adhesive bonds. To meet this condition, it is necessary, [166,167,168], to remove impurities on the surface of the joined surfaces to obtain the appropriate surface roughness of the joined materials and to obtain the proper surface energy of the treated surface. There are three basic methods of surface treatment [17,169,170,171]:

• applying an adhesion promoter;

• mechanical treatment (e.g., grinding and sandblasting);

• the use of chemical or electrochemical treatment (e.g., etching with hydrofluoric acid and dry surface treatments).

Among the basic treatment methods, in addition to the methods mentioned above, there are special methods such as corona discharge, plasma treatment, laser treatment, flame treatment, electromagnetic radiation, and ozone oxidation. Examples of surface treatment methods used during the production of selected FML are listed in Table 2. All aluminium alloy sheets were initially degreased prior to further steps in the surface pretreatment.

Mechanical treatment involves abrasive scrubbing of the substrate surface with sandpaper to introduce physico-chemical changes that modify the surface topography and yield a wettable surface [183]. The most commonly used chemical treatments are based on etching with a solution of potassium dichromate and sulphurous acid [184,185]. Acid etching is typically an intermediate step in production between degreasing, alkaline cleaning, and electrochemical treatment [186]. Three chemical etching methods have become the most popular: those using sulpho-ferric acid [17], Forest Product Laboratory [187], and chromic–sulphuric acid [17,188].

Electrochemical degreasing is one of the most effective methods of cleaning metal surfaces, but it requires strict adherence to the composition of the solutions used and strict operating parameters. The degreasing process is intensified by the emission of hydrogen (cathode cycle) or oxygen (anode cycle). It consists of emulsifying fats and oils with the help of gas bubbles emitted in the electrolysis process: hydrogen at the cathode and oxygen at the anode. Cathodic degreasing is more effective than anodic degreasing due to the evolution of twice as much gas. Too low a current density does not ensure complete degreasing, and too high a current density has a negative effect on the quality of the substrate. Anodic oxidation in chromic acid anodising (CAA) or phosphoric acid anodising (PAA) solutions is the preferred stabilising treatment for the structural adhesive bonding of high strength aluminium alloys in critical applications such as aircraft components [17]. Although the European aviation industry uses chromic acid anodising as its preferred stabilisation treatment, Boeing invented phosphoric acid anodising, which led to better joint durability [17,184,189].

Silane-based coatings are environmentally friendly multi-metal precoats that can be used on various metals, e.g., aluminium and its alloys [190]. Due to their environmental compatibility, these products can be used as substitutes for traditional pretreatment methods. Coupling agent treatment creates a permanent covalent bond between silane and the metal (oxide), which creates a layer with intermediate properties between the metal and the polymer [191]

To eliminate the use of non-ecological chemical treatment for the surface preparation of aluminium alloy sheets, dry surface treatments were developed: plasma-sprayed coating [192,193], excimer laser texturing [194,195], and ion beam enhanced deposition [188,196,197]. The plasma method consists of subjecting the outer layer of the modified material to the action of low temperature plasma, which is created because of high-frequency partial discharges taking place in a vacuum chamber. A review of the methods for treating metallic surfaces for the production of FMLs can be found in the papers by Sinmazçelik et al. [17] and Molitor et al. [198].

After the surface of the sheet and the fibre for the reinforced plastics have been prepared, they are stacked alternately, with the metal layers constituting the outer elements of the composite. Instead of producing FML from the metallic sheets and prepregs, metallic sheets can be combined with alternate layers of polymer films and fibres [199]. The suitably stacked materials are then bonded under pressure and temperature. In addition to conventional autoclave fabrication and hot press methods, out-of-autoclave methods are being developed [60,200,201]. Thermal treatment during the production of composites introduces residual stresses in the structure of the composite, which reduce the fatigue strength of FML components.

FMLs have a low density and a low rate of crack propagation in the layers of such hybrid materials. Additionally, FMLs, when compared with metallic materials, have increased:

• fatigue strength [99,176,202,203],

• fire resistance [204,205,206],

• resistance to electric (atmospheric) discharge [207],

• impact strength [114,208,209], and

• corrosion resistance [204,210,211].

Compared to polymer composites, FMLs show better strength, impact strength, and fracture toughness [46,64,212]. One of the major disadvantageous properties of FMLs is their susceptibility to delamination under unfavourable loads.

4. Press-Brake Bending

Single curvature FML components are formed by press-brake bending (PBB) in which a plastic deformation is created by clamping the material between the punch and the die (Figure 4a). This process takes place on a classic press brake and the degree of deformation of the material is limited by the properties of the composite, primarily by the low failure strain of the fibres of the matrix layer. Basic elements formed using PBB include single curvature sections (Figure 4b). Thin FMLs, where the distance of the deformed fibres from the neutral axis is small, do not cause problems during bending [213]. The most favourable bending conditions are for composites with 2/1 lay-up, in which the fibres are located near to the neutral axis. The basic parameter determining the possible degree of deformation of the composite sheet is the minimum bending radius, which is a function of the failure strain of the fibres [213]:

(1)$r_{min}\cong \frac{t}{2ϵ}$

where $ϵ$ is the failure strain of the fibres and t is the distance of the outermost fibre layers to the neutral line.

In a similar manner to the bending of metallic sheets, the minimum bending radius increases exponentially as the thickness of the composite increases. Apart from the failure strain of the fibres, the delamination of boundary layers outside the bending zone may occur when bending laminates with a thickness greater than approximately 1.5 mm. The greater the thickness, the more dominant is delamination as the destruction mechanism of FMLs. In general, the greater the thickness of the composite, the smaller the minimum bending radius [52,214]. Moreover, when the bending direction is parallel to the fibre direction, it enables smaller radius bending for GLARE [215].

The bending behaviour of CFRP and GFRP panels fabricated using the lay-up technique has been studied by Rajkumar et al. [202]. It was found that the interlaminar shear strength (ILSS), computed using Equation (2), decreased as strain rate increased.

(2)$ILSS\cong \frac{3F_{max}}{4wt}$

where F_max is the maximum load, t is the FML thickness, and w is the specimen width.

Placing the carbon fibre layer exterior to the glass fibres will enhance the mechanical strength of FMLs. The good bonding strength between aluminium and CFRP leads to crack arresting in the flexural strength of 3/2 lay-up with stacking configuration Al/C/C/C/Al/C/C/C/Al (C—one layer of carbon fibre, Al—6061 aluminium alloy) [202].

The mechanical properties of composite materials depend on the adhesion between the fibre and the matrix and on the strength of the fibres. The basic strength tests for determining the adhesion of composite layers include inter alia and bending tests. The damage tolerance of the 2024-T3-based GLAREs and an aluminium roll-bonded laminate in the three-point bend test has been investigated by Cepeda-Jimiénez [103]. During the bend tests (Figure 5a), different fracture mechanisms of laminates were activated that depend on their interfaces and the constituent materials. There was clear debonding between fibres and the resin matrix in GLARE after bend testing (Figure 5b). Optical macrograph analysis of GLARE laminate has shown several delaminations and extensive plastic deformation of aluminium alloy layers.

Isiktas and Taskin [216] investigated the springback properties of 5754-H24/CFRP laminates prepared using adhesive. The effect of core thickness and bending angle on the springback behaviour of FMLs is studied using bending dies with different bending angles (Figure 6). They observed that (i) the amount of springback angle decreased as core thickness increased, and (ii) the amount of springback was higher at all bending angles when the thickness of the face sheet increased.

Impact and fatigue resistance of CARALLs was studied by Bellini et al. [217] in the three-point bending test. Two different interfaces were considered: one obtained with prepreg resin and the other with a structural adhesive. It was found that the laminate with only one metal sheet bonded with structural adhesive presented a higher ILSS than the laminate with the sheet metal bonded only with prepreg resin. In another paper, Bellini et al. [218] analysed the influence of both the adhesion interface between the CFRP layer and the aluminium sheets as well as the layer thickness. Four different laminates, bonded with prepreg resin or with structural adhesive alone and with one or two metal sheets (Figure 7), were investigated in the three-point bending test. It was found that the laminate with two metal sheets bonded with the adhesive had the lowest flexural strength, whilst the laminate with a single aluminium sheet bonded with just the prepreg resin resulted in a better solution. The effect of the stacking characteristics on the mechanical behaviour of the FML was the subject of many studies that have considered the thickness of the composite laminate [99,219,220] or worked with one standard thickness of the metal sheet [5,221].

Kim et al. [173] investigated the springback characteristics of GLARE 3 2/1-0.4 in the brake forming process. When the load is removed, most metallic materials exhibit springback as a result of the relaxation of the elastic part of the internal stress. After unloading, the material undergoes elastic deformation in the direction opposite to the direction of the applied load (Figure 8a). The outer layers of the composite are subjected to tensile stress, and the inner layers are subjected to compressive stresses (Figure 8b). When the specimen was heated up to a temperature of 100 °C (limited by the degradation of the composite matrix), the springback was decreased by 19%, compared to that of the process at room temperature [173]. This is a result of the lower yield stress and elastic modulus of the aluminium alloy and the composite at elevated temperatures [222,223]. Springback characteristics are mainly influenced by the forming load [173].

Li et al. [224] analysed springback and failure modes of GLARE 2A-3/2-0.3, GLARE 3-3/2-0.3, and GLARE 2B-3/2-0.3 in the roll bending process, which is considered a cost-effective and efficient sheet forming process. Experimental and numerical results illustrated that springback depended strongly on the lay-up configurations of the laminates tested. GLARE laminates with different fibre orientations signified different resistance abilities to springback; the springback of GLARE 2B was at a minimum. Failures, including delamination, matrix cracking, and fibre fracture, occurred with increasing of the curvature of laminates after bending.

5. Shot Peening Forming

Shot peening forming (SPF) consists of the production of local deformation of the sheet as a result of interaction with hard balls. These balls impact with the surface of the FML at high speed, and the kinetic energy is transferred into a localised plastic strain in the material, which changes the curvature of the entire panel. The characteristics of the influence of the balls on the workpiece surface is elastic–plastic. In aviation, the SPF technique was originally used to form metallic sheets. Kulkarni et al. [225] used SPF to generate curvatures in thin 7075-T6 aluminium alloy sheets for the forming of airplane wing skins to controlled contours. Later, Friese et al. [226] re-shaped laser-welded fuselage panels for the Airbus A380 using SPF. These had varying thicknesses and geometries of stringers and skin. The method that was developed provided design flexibility combined with a high level of automation. After much positive research, the use of SPF to manufacture metal components has focused on forming FMLs [227].

Another effective non-destructive method to detect material defects and to monitor the complete manufacturing process “in-situ” is X-ray computed tomography (XCT) [228]. XCT, due to its high spatial resolution and very short acquisition time, is successfully applied to determine the process parameters in aerospace composite manufacturing techniques (Figure 9). Naresh et al. [229] reviewed the application of XCT to assess the autoclave and out-of-autoclave manufacturing processes. He concluded that current aerospace manufacturing processes, combined with advanced modelling approaches using XCT, show great potential as tools for the design of better manufacturing processes for the next generation of composite aerospace parts.

The results of the application of SPF to form an aluminium–lithium alloy indicated that shot peening caused non-negligible work hardening in the external metal layers, which increased the tensile strength of laminates consisting of 2060 aluminium alloy sheets and S4-glass/epoxy prepregs used as a fibre layer [140]. The compressive stresses introduced by SPF (Figure 10) effectively improved the fatigue crack growth properties of FMLs. Li et al. [140] also found that the failure was mainly dominated by the limitation of fibre failure strain. In another paper, Li et al. [138] studied the deformation behaviour of novel fibre metal laminates (NFMLs) (Figure 11) based on metal sheets from the Al–Cu–Li family, with a Cu/Li ratio of 5.29 after SPF.

The curvature radius of NFML strips decreases when the upper metal layers are etched, indicating that the processed upper layer experiences a compressive state of stress [45]. It was found that the decreased curvature radius is only achieved for NFMLs after etching the lower metal layer (Figure 12). This proves that a state of tensile stress exists in the lower layer. The residual stresses vary at the fibre–metal and 0°–fibre–90° interface due to the different material modulus of each component (Figure 13). The change in stresses at the interphase boundaries results from a rapid change in the deformation behaviour of the layers. Undoubtedly, due to the layered structure of FMLs, the residual stresses are distributed in a more complex manner than in homogeneous metals.

The previous paper by Li et al. [136] on the failure behaviour of shot peened NFMLs shows that the failure mode of NFMLs after SPF mainly included fibre breakage around the shot peened surface and metal–fibre interface delamination. C-scan ultrasonic testing results indicated that ball size significantly affected the curvature of formed NFMLs due to the complex effects of fibre layers on the forming behaviour.

Effects of SPF parameters on the forming characteristics of Ti–CF–Polyimide and Ti–CF–PEEK laminates were investigated by Hu et al. [230]. They observed that the interfaces between the metal layers and the prepreg layers after SPF are continuous and well preserved. SPF did not generate visible micro-cracks and delaminations. Moreover, interfacial bonding within the laminate was not damaged after the forming process. The powerful compressive stresses in the shot peened surface make FML cracking, caused by tensile stresses, difficult. In this way, the mechanical properties of the outer metal layers of FMLs [231] are improved.

Russig et al. [232] developed the Rotary Peening Forming (RPF) process as a new shot peening forming process in which the shot is moved on a circular trajectory held by a flexible connection. GLARE-3 5/4, GLARE-2 3/2, and GLARE-1 3/2 were used as the test materials. It was found that the curvature obtained by RPF is less than that obtained using conventional SPF.

RPF is an evolution of the flap peening process [233], which uses elastic flaps with embedded shot to peen part surfaces. Compared to conventional SPF, the RPF does not require particle recirculation, and, consequently, the machine can be made more compact. RPH induces a similar stress state as SPF, but also involves tangential components that can create shear deformation in the deformed layer.

Wang et al. [234] improved the fatigue strength of CFRP laminate by application of shot peening onto the surfaces of welded flange plates (Figure 14) of high strength low alloy steel. They found that shot peening significantly increases roughness and hardness, in contrast to those samples without treatment. Moreover, an improvement in fatigue life of 20% to 30% and an enhancement in fatigue strength were observed in shot peened specimens.

6. Incremental Sheet Forming

6.1. Background

Single-point incremental forming (SPIF) is known as the simplest process variant within the incremental sheet forming technologies. It has been studied intensively, which has led to its progressive development since the 2000s [235]. In SPIF, there is no need for dedicated dies; a simple clamping rig alone is used to support the blank edge. The sheet is incrementally formed into the desired shape using a hemispherical end tool following a predefined toolpath [236,237]. The process works as illustrated in Figure 15.

Due to its high degree of flexibility, low cost, ability to process complex shapes, high formability, as well as low-forming forces, compared to conventional sheet forming processes, SPIF has mainly proven itself for small batch and rapid prototype production [238,239,240].

Some examples of SPIF applications are given in [241,242]. The potential markets for this technology are the medical sector [243,244,245,246], architecture [247,248], aircraft [249,250,251], and the automotive industry in particular [252,253].

Furthermore, the suitability of SPIF and the accuracy of the products thus formed are enormously influenced by numerous factors: in particular, the sheet material, tool path strategy, technological parameters (punch diameter, punch rotational speed, toolpath strategy, step size, friction), and design parameters (initial sheet thickness, target geometry) [254,255,256].

On the other hand, single-point incremental forming originated as a technique for forming metallic materials [257,258,259]. Some research has, however, been carried out in order to produce components made of non-metallic material, such as:

• bimetal composite [260,261],

• polymers [262,263],

• bilayer polymeric sheets [264],

• polymer-based composite materials [265,266,267,268,269,270,271].

Recently, there has been increased attention on possible applications of SPIF for hybrid metal–polymer composites (sandwich panels, FMLs, GLAREs) (Figure 16), which is proving a promising research field [272].

6.2. Application of SPIF to Hybrid Metal–Polymer Composites

Up to now, research on the use of the SPIF process for hybrid metal–polymer composites is very limited. In this context, Jackson et al. [272] demonstrated the mechanical viability of SPIF for two panels with metal faceplates and polymer cores: mild steel–polypropylene–mild steel (MS/PP/MS, Sollight^®) and aluminium–polypropylene–aluminium (Al/PP/Al, Hylite^®). As illustrated in Figure 17, four failure modes have been observed in SPIF of composites: faceplate fracture, core shear, local indentation, and delamination. In order to avoid indentation, panels must be ductile and have an incompressible core and faceplates.

It was shown that similar results are noted concerning the effect of step size and tool radius, the accuracy of the sine law, and the trends of forces on both sandwich panels and monolithic sheet metal. However, a reduction of 50% in the magnitude of the vertical force was noted for the composite, which is advantageous in process design. Moreover, for a step size of 0.5 mm and a punch radius of 10 mm, the maximum formable wall angles (a widely used parameter for assessing formability) are about 50° and 40° for MS/PP/MS and Al/PP/Al, respectively. It is noted that the limiting failure mechanism in both cases is faceplate fracture (Figure 18). Accordingly, these results revealed that, with these materials, the range of geometric shapes that can be formed is slightly more limited than that of sheet metals, for which the maximum achievable wall angle can reach 78° (2.1 mm of AA 3003-O), as reported by Jeswiet et al. [235].

Nevertheless, it cannot be assumed that these are the forming limits of the sandwich panels with SPIF in general due to the various process conditions that can be considered later. As is well known, a different combination of process parameters [245,273,274,275] will certainly lead to different conclusions.

Fiorotto et al. [265] evaluated the feasibility of producing complex geometry panels from two aluminium plates and a composite core in a cost- and time-effective manner. Composite laminates consist of two woven kevlar fabrics. During FML forming, circumferential cracking and the generation of wrinkles along the wall of the part have been noted (Figure 19). By contrast, by excluding the composite core, two aluminium plates were simultaneously formed without defects.

The effect of initial sheet thickness on the formability of laminated materials, Al/PP/Al (Hylite^®), has been investigated by Girjob et al. [276]. This material has exceptional properties, such as flexural stiffness and very low density. It was found that a non-uniform thickness distribution occurred throughout the circumference of the formed workpiece. Obviously, it is known that a proper selection of SPIF process parameters has, in fact, a great influence on the production of components with satisfactory structural and dimensional characteristics. Thus, further experimental tests should be performed to reach more accurate conclusions.

In this framework, Davarpanah et al. [277] examined the effect of some of the process parameters, i.e., step down Δz (0.2 mm and 0.4 mm), metal thickness t_Al (0.8 mm and 1.0 mm), and polymer thickness t_PA (0.79, 1.19, 1.58, 1.98 and 2.38 mm), on the formability and failure modes during incremental forming of adhesively bonded AA5052 aluminium–polyamide laminate sheets. The observed failure modes for the laminates are delamination, metal tearing, and galling, as shown in Figure 20.

The series of experiments and conclusions are summarised in Table 3. It has been noted that, at low polymer thicknesses, the composite undergoes delamination between the layers. Increasing polymer thickness improves formability because the tendency to delamination decreases until the transition thickness is attained. Thus, the mode of failure shifts from delamination to metal tearing, resulting in maximum formability. In addition, this critical t_PA value increases with greater Δz when the metal thickness is constant. In laminates with a thinner metal sheet (0.8 mm), only two types of failure appear: delamination and metal tearing. However, a small increase in the AA5052 thickness (1 mm), at t_PA > t_PA-critical, results in galling of the polymer before fracture.

However, more research is needed to reveal the effects of other parameters, such as adhesive strength and forming speed (rotation and feed rate).

Harhash et al. [278] conducted several experimental tests to prove that, like metal sheets, steel–polymer–steel sandwich composites can be formed successfully by SPIF, subject to different forming limits. It was found that shaping the SPS sheets is possible and similar to steel sheets, but with an earlier fracture at angles of more than 30%. Thus, the formability of the SPS is much lower than the uniform sheet in terms of the limiting wall angle and cracking height.

Additionally, after cutting the shaped conical part, delamination was observed in the unsupported SPS laminate regions (Figure 21a). Owing to the tool step down (Δz = 0.5 mm) and the friction, the inner side of the formed part shows galling marks, whereas the outer skin sheet is subject to failure by cracking (Figure 21b). The high ductility of the core layer (>200% elongation at failure) prevents it from cracking.

As a means of extending these SPIF limits for the metal–polymer–metal sheets, multi-stage forming was examined in the same paper [278]. It was concluded that two-stage SPIF improves the part height at cracking, in comparison with direct SPIF (Figure 22). This is explained by the fact that the direct SPIF forming was under plane strain conditions until failure.

On the other hand, a comparison of two-stage SPIF with stretch forming proves that both could produce a dome of 25 mm depth in the first stage, but with entirely different thinning behaviour (Figure 22b,d,e). In stretch forming, the thinning rate is more significant in the centre (at the bottom of the sample). As a result, cracking occurs in this zone (Figure 22e), while it appears in the wall area with SPIF.

Although promising results are obtained with multi-stage SPIF, the depth achieved in stretch forming is greater. Therefore, the authors recommended increasing the number of the sequential stages with an angle interval of ~5°.

The results of the previously cited investigations confirm the applicability of SPIF; however, the presence of some different failure types and limits was observed when compared to metal sheets. These defects are generally due to the non-uniform thickness properties and relatively insufficient metal/polymer adhesion strength. Forming at elevated temperature becomes, accordingly, the most suitable and promising solution to overcome the current process limitations, especially for hard-to-work alloys, polymers, and composites. Consequently, different concepts have recently been developed for warm SPIF to enhance a material’s formability and to improve geometrical accuracy. A review and analysis of the state-of-the-art research on hot incremental sheet forming has been conducted by Liu [279] and by Zhu et al. [280]. This hybrid forming technology has first been applied to metal sheets [281,282,283,284,285,286,287] and later to polymer sheets [267,288,289,290,291,292,293,294].

Within that framework, Al-Obaidi et al. [271] devoted their research to investigating the effectiveness of such process variants for glass fibre reinforced polymer (PA6GF47) sheets and to shorten the manufacturing process for orthotics using SPIF assisted by hot air heating (Figure 23). The PA6GF47 sheets were sandwiched between two Teflon layers and two metal sheets. Conical parts were shaped with a variety of wall angles. It is found that samples formed with a wall angle greater than 50° were distinguished by wrinkling, folding, internal cracks, and voids.

In another study, Al-Obaidi et al. [295] continue with the same hot SPIF technology to form a basalt fibre reinforced thermoplastic polymer (BFRTP).

As shown in Figure 24, two laminate types were considered during the forming of conical shapes: the first consists of BFRTP incorporated between two aluminium sheets, and the second consisted of only one aluminium sheet inserted between two layers of BFRTP. The Teflon layers were utilised to enhance the sliding of the laminate during hot SPIF. In addition, the metal sheets were employed to protect the BFRTP from excessive heating effects and punch friction. Conical shapes were formed with several wall angles. It was noted that parts with a wall angle ≥50° were suffering from delamination and void initiation. Owing to the large difference between the plastic properties of the aluminium sheet and the behaviour of the thermoplastic laminate during hot SPIF, the second type of laminate has a much greater percentage delamination than the first one.

Based on existing knowledge of SPIF for sheet metals, the economic benefits were the same when addressing hybrid metal–polymer composites, but product formability must still be enhanced. Therefore, use of the process with its different variants and conditions needs an extensive investigation in relation to these materials.

Up to now, numerical simulations and optimisation techniques have greatly contributed to the development of SPIF [296,297,298,299,300], especially for metallic components with complex geometries. Therefore, the application of such techniques should be extended to FMLs.

7. Lay-Up Technique

The lay-up technique is the oldest and simplest manual method of producing open-mould laminates. During lamination, the reinforcing component is placed in the mould in the form of layers with a filling (resin). The reinforcement layers can be in the form of unidirectional continuous filament roving and in the form of staple fibre fabrics or mats. The strength and stiffness of the laminate is directly dependent on the production method and the direction of the fibres [301,302]. Components made of laminates have lower weight and greater strength than the majority of structures made of homogeneous materials. The selection of the lamination method depends on the type of materials used, the desired shape and dimensions of the product, as well as the specific utility requirements [303,304].

For this reason, the following methods of obtaining laminates can be distinguished:

• hand lay-up,

• spray lay-up,

• compression moulding,

• poltrusion,

• resin transfer moulding (RTM), and

• filament winding.

The types of laminating forms can be either internal or external. A geometrical outline of the product is formed on the required forming surface. The next stage of the lamination process is the curing of the individual layers of laminate, which can take place both at ambient and elevated temperatures with the use of infrared heaters. The conditions and time for curing of laminate largely depend on its type, shape and dimensions.

Spray lay-up technique involves applying both resin and fibre as a liquid suspension using a special spray nozzle (Figure 25). The fibres in the form of roving are continuously fed to the spray nozzle, and, in the nozzle, they are cut into fibres with a length between 0.3 mm and 13 mm. As a result, the air stream “imposes” the fibres and the resin on the mould surface [17,305,306].

RTM is a method of manufacturing laminates in closed forms, which consists of forcing catalysed resin into a tightly closed mould, previously filled with reinforcing fibres and/or sandwich material [307,308,309]. The counter-mould is made similarly to the main mould. It consists of steel stiffeners and reinforcements, which allows the use of high injection pressure [310,311]. Light resin transfer moulding (LRTM) is a variation of the RTM method that uses a light, semi-flexible counter-mould that is clamped by applying a vacuum between the seals placed around the perimeter of the mould (Figure 26) [312,313]. The resin injection pressure is relatively low, and the entire process is supported by negative pressure [314,315,316]. The RTM/LRTM method allows the share of reinforcement in the total mass of the composite (over 60%) to be increased compared to, for example, hand lay-up lamination [317]. RTM and LRTM technologies are mainly used in the construction of boats and water equipment [318,319,320] and in aircraft construction [321] as well as automotive and medical composite moulding applications [322,323,324,325,326]. After the impregnation, the laminate component can cure, usually at room temperature [56,57]. The most important parameters determining the effective preforming saturation in the RTM process are [327]:

• an appropriate injection pressure gradient,

• good wettability in the fibre–resin system, and

• good tightness of the impregnation system.

In the case of very high-quality requirements regarding the moulded parts and the use of preimpregnates made from composites reinforced with high-modulus fibres and resins hardened at an elevated temperature, it is necessary to cure the laminates in an autoclave (Figure 27). An autoclave is a furnace that allows both heating and pressure to be applied to a material [328]. Several single preimpregnate/prepreg layers stacked on top of each other are covered with a high temperature resistant foil, sealed, and connected to the vacuum lines. Negative pressure and multiplied overpressure consolidate the preimpregnate layers and squeeze excess resin from the layers of fibres; the increased temperature causes the laminate to be curried [213,329]. Only single or shallow double curvature panels with relatively large radii can be formed with this technique [78,213]. However, it is an expensive process, especially when it comes to large parts [200,330,331]. In order to reduce production costs, out-of-autoclave techniques can be used that allow localised curing and/or bonding of thermosets [200,332]. Induction heating [333,334,335] and microwave radiation [336,337,338] have been proposed to cure fibre reinforced thermoset composites.

Bars and profiles can be produced by the pultrusion method. Fibre is pulled through a bath of resin, and then the fibre passes through dies forming and squeezing the excess resin and the heating zones of the mould, where it is hardened [47,339,340]. The technology is intended for the production of profiles with a constant cross-section. Reinforcement, and, at a later stage, the finished elements, are pulled through successive devices shown schematically in Figure 28 [341,342,343,344]. Several works have reported using the pultrusion process for various types of fibre, e.g., kenaf fibre [345,346,347], kenaf–glass fibre [348,349,350], and jute–glass fibre [351,352,353].

Filament winding (Figure 29) involves the continuous winding of fibres on a rotating core with the shape of a rotational solid (cylinder, cone, etc.) [355,356,357]. Circumferential, helical and planetary winding [358,359,360,361] can be performed depending on the direction of rotation of the core and the way the carriage is moved. The design of the carriage with roving and the core turning device enables both the carriage speed and rotational speed of the core to be changed, so that the winding angle can be adjusted. The roving tapes used in the filament winding process may be preimpregnated with resin. Before winding on the core, they must be heated so that the resin becomes liquid [362].

From the literature review, the reader should note that there remain two different approaches to obtaining the complicated shape of the FML part (Figure 30). In the first approach, the component layers of the material are first deformed, and then a laminate is fabricated based on the deformed layers. Forming single thin layers is costly and extremely demanding. A less costly alternative is to form the FMLs to be fabricated directly. In a later part of the article, methods of plastic working and laser forming for FML panels will be presented. Table 4 summarises the main advantages and disadvantages of lay-up techniques of FML fabrication.

8. Die Forming

In the die forming technique, sheet blanks obtain the desired shape according to the die configuration [372,373]. The energy required to cause the deformation can be applied from different sources: mechanical, hydromechanical, electromagnetic (Figure 31), by laser beam, etc. Recently, these forming methods were utilised to form steel-based FMLs, but the latest research papers have been focused mainly on aluminium-based composites. Researchers consider the configuration of the material layers [374,375,376,377] or the process parameter setup [86,378,379,380,381,382,383,384,385,386] to reduce material defects and go beyond the current forming limits caused mainly by breakage and wrinkling. In order to improve the formability of the layers of FRP, many studies are focused on die forming at elevated temperatures [387,388,389]. The forming process can be isothermal or non-isothermal [390]. The comparison of the forming behaviour of FMLs with the conventional deep drawing process leads to the conclusion that early fracture of the laminate arises without achieving a significant depth [391,392,393,394]. FMLs must usually undergo preprocessing in terms of heating and stacking before being properly formed.

8.1. Stamping

Stamping is the most common method of die forming, in which the desired shape is achieved with tool sets (a blank holder and dies). To increase the forming ability of FMLs, elevated temperatures are required. Rahiminejad and Compston [9] determined the forming limit curve for FMLs in the stamping process for blanks in configuration 2:1 made of 0.45 mm thick steel and 1 mm 0/90° weaved GFPP and heated at 140 °C (above the polypropylene crystallisation temperature) and 170 °C (polypropylene melt temperature). At a temperature of 170 °C, composite flow out of the bond and in thickness reduction were observed. Better strain results were obtained for the temperature 140 °C where the middle layer softened, allowing for better slip. Nam et al. [162] investigated stamping behaviour of steel–SRPP laminate in open die configuration at room temperature, applying various values of blank holder force (2 kN, 7 kN, and 14 kN). It was found that better strain distribution was obtained for FML than for sheet steel. Furthermore, a significant effect of blank holder force was noted, in that the selection of the higher forces (7 kN and 14kN) affects wrinkling reduction, but a delamination effect was also observed. Additionally, Blala et al. [378,379] performed drawing tests on 2/1 GLARE (0.5/0.2/0.5 mm) composite. Different blank holder forces, gaps, and cure states were considered. Deep forming cured GLARE seems not to be possible because of composite low elongation rate (5% compared to 20% Al 2024-T3 sheet) and its high coefficient of friction between Al sheets. An optimal blank holder gap (1.1 mm), 0.1 mm lower than the blank thickness (1.2 mm), and a blank holder force in the range from 3 to 6 kN was selected, achieving 31mm forming depth of semi-cured GLARE. Liu et al. [374] focused on the effect of fibre orientation on the stamp forming process of GLARE composites. FML blank containing multi-directional fibre layers (0/45−45/90°) achieved better formability depth (42 mm) than unidirectional ones (35 mm). In 2020, Blala et al. [308] improved the drawability of semi-cured Glare sheets in the deep drawing of cylindrical cups using a variable blank holder force (VBHF). Variation in the friction force of the holder and the laminate on contact surfaces varied as a function of the position of the outer flange edge (Figure 32). When there is constant blank holder force (BHF), the FML fails at the beginning of the forming process due to the fibre’s limited strain at rupture. When the BHF is too low, wrinkles occur at the flange of the drawpiece. The results of experimental tests and numerical modelling revealed that wrinkles can be eliminated with a decreased BHF trajectory.

In 2021, Blala et al. [86] proposed a novel approach to stamping automated small FML parts by hot-pressing non-cured FMLs (Figure 33), which was compared with the conventional forming method (Figure 34). A significant increase in formability was observed by this process, from the 13 mm drawing depth conventionally obtained compared to the 38 mm with an improved wall thickness homogeneity produced by hot-pressing (Figure 35). The thickness reduction values achieved were 11.66% for the top and 14.30% for the bottom aluminium sheet and 6.6% for the fibre layers. It was also concluded that the lower aluminium sheet is subjected to more thinning than the upper sheet. To eliminate defects in the FML during stamp forming, Blala et al. [22] developed a new method consisting of forming non-cured laminate followed by hot-pressing. The conventional process of forming autoclave-cured 2.1 GLARE specimens leads to the formation of typical failure modes during stamping, i.e., failure by tearing, sidewall wrinkling and excessive thinning, delamination, and fracture (Figure 36a–f). The results of numerical computations and experimental tests show that the approach developed can improve the stamping process of 2/1 GLARE parts, which translates into obtaining products without the defects typical of FMLs (Figure 37g).

8.2. Hydroforming

Hydroforming is a technology for shaping tube profiles, shells, and flat sheets using fluid pressure. Elements made using this method have superior surface quality and dimensional accuracy. The main obstacle preventing the popularisation of this method is the high cost of the machines and the process, compared with conventional stamping [375]. Fibre orientation has a significant impact on FML performance; Sumana et al. [375] produced hydroformed cylindrically shaped FMLs made of 1 mm thick 6061-T6 aluminium alloy and three alternative FRP thicknesses: 1 mm, 2 mm, and 3 mm. The lowest buckling deformation and highest buckling strength were observed during hydroforming of a cylinder shape consisting of 0/90° woven fibre, and 60/30°, ±45°, and ±55° fibre orientation samples. An increase in the FRP layer thickness increases circumferential pressure causing initial buckling. Zafar et al. [382] presented an alternative method of hydromechanical deep drawing to produce FML blanks. Stacks of three Al 2024-O discs, each 0.5 mm thick, were used in the experiment at room temperature. The influence of input parameters blank holder force (7.8–14.9 kN) and cavity pressure (25–55 MPa) on wrinkles and facture were examined. As a result, lower wrinkling was observed for a 11 kN blank holder force and 42 MPa cavity pressure. The hydromechanical deep drawing process seems to be more effective than the conventional stamping process in terms of cost, productivity, and complexity. Using this approach, any type of composite bonding layer can be applied after forming and reheating and solidification treatments can be eliminated. As an extension, in their work, Zafar et al. [383] called this new approach the “3A method”, the use of which permits multiple metal blanks to be formed at once. Figure 37 shows a comparison between a forming method that is well known and thoroughly studied in the literature [395,396,397,398,399] (Figure 37a) and the novel “3A method” (Figure 37b). Three types of thickness of Al2024-O blanks were tested using this method: 0.5 mm, 1.5 mm, and 3 × 0.5 mm. The respective cavity pressures of 15, 20, and 40 MPa were established as optimal for the blanks. An elevated punching force was observed for 3 mm × 0.5 mm during forming and explained by additional friction between the layers. The proposed forming method eliminates the need to reheat and re-solidify the thermoplastic-based composite resin system at controlled forming pressures.

Additionally, Zhang et al. [376] analysed the springback and thinning effects using the “3A method” for stacked triple aluminium sheets with a thickness of 3 mm × 0.5 mm to form a hemispherical radius. A maximum reduction in thickness of the lower layer to 0.43 mm and successful 36 mm forming depth were noted. Liu et al. [400] compared different FMLs, ARALL, CARALL, GLARE, and the effect of their fibre layer type on the forming limit when a standard blank holding force was applied. The GLARE formability limit seems to be higher than the others while CARALL’s was the lowest—lower than for the 3 × Al stack. It was noted that fibre strain damage and thinning rate of the aluminium sheet are important factors in manufacturing this type of FML.

Numerical and experimental investigations of the effect of the blank holder gap on the hydromechanical deep drawing of FMLs made with 2024-T3 aluminium alloy sheet and W-9011 prepreg woven glass fibre sheets were conducted by Blala et al. [384]. Due to applying the BHF and cavity pressure in the laminate system, the resin plays the role of rubber, which decreases the thickness of the FMLs (Figure 38). Although the value of this reduction is very small, it plays an important role in the available formability of material. Cavity pressure higher than optimal causes the formation of wrinkling starting from the 2024-T3 layer, which is directly in contact with fluid pressure. By contrast, a cavity pressure lower than optimal causes fracture of the FML. FML parts manufactured by considering the optimal value of the blank holding gap (BHG) lead to a reduction in time and effort spent in mass production [401]. An appropriate setting of the BHF, BHG, and cavity pressure ensures the achievement of higher degrees of deformation and the forming of drawpieces with a more complex shape.

8.3. Electromagnetic Forming

Electromagnetic forming (EMF) is a contactless and high-speed technology dedicated to form flat and hollow components. A pulsed magnetic field is used to apply Lorentz force to the workpiece, which causes deformation. The EMF process influences such properties as residual stresses, impact toughness, and durability, and its major advantage is that it increases the formability of materials [380,402]. Chernikov et al. [127] compared the electromagnetic forming of five-layer fibre reinforced metal polymer laminates with forming using a rubber pad. The formation of rifts (Figure 39) eliminates the high-velocity impact of the material with the die and the resulting delamination of the blank. It was found that the rift zone was dominated by compressive circumferential strains, rather than tensile radial strains as during conventional rubber pad forming.

Glushchenkov et al. [380] applied EMF to a five-layer metal–polymer composite fabricated with a 0.3 mm thickness commercial aluminium with carbon fibre interlayers bounded with two-way reinforced epoxy laminate. The strains have been analysed using the Vic-3D system, before and after deformation, and the results were compared to the rubber pad forming method. The specimens achieved a rift depth of 4.5, 5.15, and 5.35 mm, which equalled an energy charge of 2.49, 2.86, and 3.25 kJ, respectively, compared to the 4.06 mm obtained by rubber pad forming. Compressive circumferential, radial, and equivalent strains on the convex side and lower radial strains were observed and compared to rubber stamping in which there appear circumferential and tensile radial strains. On the concave side, strains were similar in both methods. The occurrence of compressive strains in EMF of the bottom side could increase formability but, on the other hand, could initiate wrinkling. Khardin et al. [381] investigated an Al–polymer–Al (0.3 mm + 0.6 mm + 0.3 mm) sandwich using the electromagnetic bending method. Blanks were made of two AW-Al Mn1Mg0.5 sheets and a thermoplastic polyolefin middle layer. The authors stated that higher strain values cause an increase in energy level of the electromagnetic field. Part melting of the aluminium sheet was found, which set one of the forming limits. By applying the EMF method, a higher profile height was achieved than with conventional stretching without cracking (>30%).

8.4. Laser Forming

Laser forming technology applies photon irradiation to heat the component surface, which leads to deformation by thermal stress. This forming method is used to produce rapid prototypes, aligning and adjusting the FMLs. The surface of the composite, heated locally along the assumed path, undergoes elastic–plastic deformation as a result of thermal stresses.

Hu et al. [377] investigated laser peen forming (Figure 40) in the manufacture of large size, complex shape FMLs. GLARE 2–0°, GLARE 2–90°, and GLARE 3 specimens were each prepared with a 0.3 mm layer thickness to analyse the effect of fibre orientation on the process. Fibre orientation was a significant factor in the anisotropic properties of FMLs, bending being more advantageous in a perpendicular direction to fibres oriented unidirectionally. It was confirmed that, by this method, a small forming radius could be obtained with an increase in laser scanning time; GLARE 2–0°, GLARE 2–90°, and GLARE 3 specimens gained a 221.8, 91, and 177.3 mm radius concave curvature, respectively. Using this method, the creation of convex bending for different GLARE stacks was obtained with better results of deformation in the perpendicular direction to that of the fibres oriented unidirectionally.

Gisario and Barletta [385] have studied the impact of laser beam power of the operating parameters (power, scanning speed, and number of passes) on final GLARE 1 and GLARE 2 shape. The range of applicable parameters allowing the achievement of high-quality results and a precise bending angle without thermal effects has been identified. Applying a set of scans at steadily increasing distances can decrease the delamination effect, allowing more deformed profiles to be obtained. For aluminium layers, the occurrence of the tempering phenomenon caused by repeated laser irradiation should be considered in process design. In work by Gisario et al. [386], GLARE 1 and GLARE 2 FMLs were used in research and a multi-layer perceptron artificial neural network (ANN) while a Levenberg–Marquardt algorithm was employed to analyse the data. The artificial neural networks that were developed predicted the temperature and bending angle by a power set and velocity laser beam. The coefficient of determination for temperature R² = 0.978 and bending angle R² = 0.963 indicates that ANNs can be an excellent alternative to numerical and analytical models. Gisario and Barletta [385] developed two-stage laser forming of FMLs (Figure 41) to obtain components with precisely controlled bending radii.

9. Summary and Future Trends

Plastic working technologies allow material to be given appropriate functional properties, which depend on the rheological conditions of the forming process. Metal processing and manufacturing is one of the most important segments of the industrial processing sector. In addition to the continuous improvement of the existing methods of plastic forming, new technologies are also being brought into use, the purpose of which is to reduce the energy consumption of processing, modernise technological machines, tools, and create environmentally friendly forming conditions in line with the climate policy of the European Union countries.

The review of the literature on the methods of forming of hybrid metal–polymer composites presented here triggered the following conclusions in the minds of the authors:

• Up to now, the target industry associated with FMLs has been the aviation industry focused on the use of lightweight components with relatively high strength. Progress in plastic forming technologies is also stimulated by developments in the automotive industry, which are full of exogenous innovations, primarily new production technologies and a new machine park. In the context of the Industry 4.0 concept, the development of the technology of plastic processing of aluminium alloy-based FMLs can be a stimulus for the introduction of innovative materials in the shipbuilding industry, in the production of yachts, autonomous vehicles, and light personal air land vehicles.

• Research on forming aluminium alloy-based FMLs commonly used in aerospace applications has been extended to other materials such as titanium and steel. The latter material, when used as a component of composites, can lower manufacturing costs in the automotive industry. Pressure from governments to reduce CO2 emissions from transport will increase. In this context, weight reduction will continue to be the focus of vehicle manufacturers, and composites and other lightweight construction materials will play an increasingly important role.

• Press-brake bending of single curvature components is being replaced by alternative laser bending technologies and the formation of shot peening. Existing research confirms their high efficiency when bending plates with large curvatures and in many planes at the same time. However, due to the longer processing time, these methods are most productive in small lot production.

• The growing interest of industry in lightweight and high strength composite structures brings the need to solve the problem of ensuring subsequent ecological recycling. It is necessary to develop new technologies for the management of post-consumer waste or to modify the current technology of producing FMLs. Thermoplastics have lower strength and a lower modulus of elasticity than epoxy or polyester resins, but show better recyclability.

• The new opportunities created by material engineering allow the fabrication of new FMLs with previously unknown properties, as well as modifying those already known in order to adapt them to new applications and reduce production and material costs. Currently, investigations are conducted on the use of nanofillers, such as single-walled or multi-walled nanotubes, to strengthen and stiffen composites.

• One of the ways to increase the deformability of composites using plastic working methods is to perform the forming process at an elevated temperature. Although an increase in temperature improved the formability of composites, the forming temperature is limited by the melting temperature of the polymer matrix and the glass transition temperature of HMPC. The temperature windows for HMPC forming are not well understood because most of the industrially used composites are still being shaped in cold forming conditions.

• Increasing the productivity and lowering the cost of producing composites is possible through the use of optimal material combinations of layers of thin metal sheets and composite. The introduction of cheap, high strength steel sheets and new thermoplastic polymers may open new windows for the use of composites that are cheap to produce and easy to recycle.

• Reducing the prices of components for the production of composites, as well as the dissemination of automated manufacturing methods, such as filament winding and RTM, have led to the development of 3D reinforcement-based spatial laminates. Designers show greater confidence in such composites than in classic composites due to the elimination of the risk of delamination.

• A critical failure phenomenon in metal–fibre laminates, in addition to poor adhesion at the metal–composite interface, is poor adhesion at the reinforcing fibre–matrix interface, leading to delamination. In order to achieve the desired shape-dimensional quality of FML elements and optimal formability, an in-depth understanding of the mechanisms of their forming, including the use of non-destructive methods of defect detection, is required.

Author Contributions

Conceptualization, T.T.; methodology, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; validation, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; investigation, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; resources, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; data curation, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; writing—original draft preparation, T.T., S.M.N., M.S. (Manel Sbayti), H.B., M.S. (Marcin Szpunar) and H.G.L.; writing—review and editing, T.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Trends in the use of composite materials in Airbus aircraft (reprinted with permission from [32]; copyright© 2021, Springer International Publishing AG, part of Springer Nature).

Figure 2. FMLs according to base material.

Figure 3. Reinforced composite used in FMLs.

Figure 4. (a) schematic diagram of PBB and (b) examples of FML components produced by PBB (reproduced with permission from [213]; copyright© 2021 Woodhead Publishing Limited).

Figure 5. (a) Schematic view of test specimen and (b) fracture of GLARE specimen (reproduced with permission from [103]; copyright © 2021 Elsevier Ltd.).

Figure 6. The punches and dies with different bending angles (reprinted with permission from [216]; copyright© 2021, King Fahd University of Petroleum & Minerals).

Figure 6. The punches and dies with different bending angles (reprinted with permission from [216]; copyright© 2021, King Fahd University of Petroleum & Minerals).

Figure 7. Stacking sequence of FMLs analysed by Bellini et al. [218] (reproduced with permission from [218]; copyright© 2021 Elsevier Ltd.).

Figure 8. (a) The springback phenomenon and (b) the stress state in the bending of GLARE (reprinted with permission from [173]; copyright© 2021, Springer-Verlag London Limited).

Figure 9. Potential application of XCT in aerospace composite manufacturing techniques (reproduced with permission from [229]; copyright© 2021 The Authors. Published by Elsevier Ltd.).

Figure 10. SPF principle of metallic sheets (reprinted with permission from [136]; copyright© 2021, Springer-Verlag London Ltd., part of Springer Nature).

Figure 11. The laminating design of novel fibre–metal laminates (reprinted with permission from [136]; copyright© 2021, Springer-Verlag London Ltd., part of Springer Nature).

Figure 12. Deformation of shot peened NFMLs after etching: (a) schematic diagram of layer removal and (b) deformation after etching (reproduced with permission from [138]; copyright© 2021 Elsevier Ltd. All rights reserved).

Figure 13. Distribution of residual stresses in shot peened NFML (reproduced with permission from [138]; copyright© 2021 Elsevier Ltd. All rights reserved).

Figure 14. Geometry of a typical test specimen [234] (this is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited).

Figure 15. Schematic illustration of the SPIF principle.

Figure 16. Materials used in the SPIF process.

Figure 17. Failure modes of sandwich panels in SPIF (reproduced with permission from [272]; copyright© 2021 Elsevier B.V.).

Figure 18. Sandwich panels formed using SPIF: (a) MS/PP/MS (45° angle without failure) and (b) Al/PP/Al (40° angle showing faceplate fracture) (reproduced with permission from [272]; copyright© 2021 Elsevier B.V.).

Figure 19. Mechanical failure on formed FML (reprinted with permission from [265]; copyright© 2021, Springer-Verlag France).

Figure 20. Types of failure: (a) delamination, (b) metal tearing, and (c) galling (reproduced with permission from [277]; copyright© 2021 The Author(s). Published by Elsevier B.V.).

Figure 21. Defects during SPIF of metal–polymer–metal (MPM) sandwich sheets: (a) delamination and (b) cracking of the outer skin sheet and galling marks in the inner surface of the formed part, reproduced from [278] (this is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited).

Figure 22. Evaluation of semi-spherical parts formed by direct, multi-stage, as well as conventional stretch forming in terms of: (a) minor–major strain distribution and (d) thickness reduction distribution determined by Digital Image Correlation of (b) the multi-stage and (c) direct SPIF compared to (e) stretch forming of Ø75 mm semi-spherical parts. Red arrows indicate cracking of the outer skin sheet, reproduced from [278] (this is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited).

Figure 23. Principle of hot SPIF: (a) workpiece combination, (b) clamping fixture, and (c) setup assembly (reproduced with permission from [271]; copyright© 2021 The Society of Manufacturing Engineers. Published by Elsevier Ltd.).

Figure 24. The BFRTP laminate structures utilised in the experiments (reproduced with permission from [295]; copyright© 2021 The Author(s). Published by Elsevier B.V.).

Figure 25. Scheme of spray lay-up technique: 1—mould, 2—gel coat, 3—pot of resin catalyst pot, 4—chopper gun, 5—fibre, 6—air pressurised resin, 7—laminate.

Figure 26. Schematic diagram of light resin transfer moulding (reproduced with permission from [312]; copyright© 2021 Elsevier Ltd.).

Figure 27. Schematic diagram of the autoclave setup: 1—vacuum bag, 2—breather cloth, 3—release film, 4—prepreg plies, 5—valve, 6—edge bleeder, 7—sealant, 8—tool.

Figure 28. Schematic diagram of a pultrusion process line (reproduced with permission from [354]; copyright© 2021 Taylor & Francis).

Figure 29. Schematic diagram of the filament winding process (reproduced with permission from [363]; copyright© 2021 Elsevier Science Ltd.).

Figure 30. The routes to create a HMPC (reproduced with permission from [230]; copyright© 2021 Elsevier Ltd.).

Figure 31. Variants of die forming methods: (a) stamping, (b) hydroforming, and (c) electromagnetic forming.

Figure 32. (a) 2024-T3 blanks, (b) W-9011 glass fibre, (c) 2/1 GLARE assembly, (d) semi-cured GLARE blank, (e) combined forming, and (f) curing of formed components (reprinted with permission from [308]; copyright© 2021, German Academic Society for Production Engineering (WGP)).

Figure 33. Preparation and forming of GLARE 2/1 using the new optimised process: (a) surface treatment of 2024-T3 sheets, (b) optimised 2/1 laminate assembly, (c) combined forming, and (d) hot pressing (reproduced with permission from [86]; copyright© 2021 Elsevier Ltd.).

Figure 34. Preparation and forming of GLARE 2/1 using the conventional process: (a) surface treatment of 2024-T3 sheets, (b) 2/1 laminate assembly, (c) vacuum curing of blanks, and (d) forming (reproduced with permission from [86]; copyright© 2021 Elsevier Ltd.).

Figure 35. Comparison of deformations between (left) the conventional process and (right) the optimised process: (a) wrinkling, (b,c) the failure of the aluminium alloy, (d) delamination failure of the laminate, (e,f) drawpieces free of wrinkles and delamination, (g,h) cross section of the final drawpiece (reproduced with permission from [86]; copyright© 2021 Elsevier Ltd.).

Figure 36. Failure types and locations: (a) punch corner fracture, (b) flange fracture, (c) fibre tearing and aluminium fracture, (d) sidewall and flange wrinkles, edge delamination, (e,f) tearing of glass fibre, (g) successfully formed cup using the new process proposed by Blala et al. [22] (reprinted with permission from [22]; copyright© 2021, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature).

Figure 37. (a) Existing and (b) novel 3A method (reprinted with permission from [383]; copyright© 2021, Springer-Verlag France).

Figure 38. Effect of clamping force on the reduction in FML thickness (reprinted with permission from [384]; copyright© 2021, Springer-Verlag London Ltd., part of Springer Nature).

Figure 39. Schematic diagram of free electromagnetic forming: 1—workpiece, 2—coil, 3—die (reprinted with permission from [127]; copyright© 2021, KSAE/112-11).

Figure 40. (a) Schematic diagram of laser peening and (b) bending out of plane with concave and convex curvatures (reproduced with permission from [377]; copyright© 2021 Elsevier B.V.).

Figure 41. Visual appearance and cross-section of (a) GLARE 1 and (b) GLARE 2, and the experimental set-up of the (c) first and (d) second investigation (reproduced with permission from [385]; copyright© 2021 Elsevier Ltd.).

Table 1

Classifications of fibre–metal laminates.

Base Material	Alloy	Fibre Reinforced	References
Aluminum	Al 2024	glass fibre reinforced/epoxy	[41,48,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100]
		carbon fibre reinforced/epoxy	[91,94,101,102,103]
		glass fibre reinforced polypropylene (GFRP)	[104,105,106,107]
		polypropylene fibre reinforced polypropylene (PP/PP)	[108]
		thermoplastic nylon LFT with glass fibre reinforced	[109]
		Nanoclay reinforced polypropylene and glass fibre	[110]
		self-reinforced polypropylene (SRPP)	[106,111,112]
	Al 8090	glass fibre reinforced/epoxy	[113]
	Al 7075		[114]
		aramid fibre reinforced/epoxy	[115]
	Al 1100	aramid fibre reinforced polypropylene	[116]
	Al 5052		[117,118]
		carbon fibre reinforced/epoxy	[119,120]
		self-reinforced polypropylene (SRPP)	[121]
	Al 5005		[122]
	Al 6061		[123]
		glass fibre reinforced/epoxy	[114,124]
		carbon fibre reinforced/epoxy	[125]
			[126]
	commercial aluminium		[127]
	Al 7475		[100]
	Al 5083		[128]
	Al 6082		[129]
	Al 1060		[97]
	Al 1050		[130,131]
		Nanoclay reinforced polypropylene and glass fibre	[132]
	LY12M	glass fibre reinforced/J272	[133]
		carbon fibre reinforced/J272
	Al 5182	glass fibre reinforced (PA6)	[134]
Aluminium–lithium	Aluminium–lithium	glass fibre reinforced/epoxy	[135,136,137,138]
	V-1469		[139]
	2060		[140]
Magnesium	AZ31 alloy	carbon fibre reinforced/epoxy	[5,141]
		glass fibre reinforced polypropylene (GFPP)	[5]
		glass fibre reinforced/epoxy	[6,100,142,143]
		self-reinforced polypropylene (SRPP)	[111]
	Magnesium	silicon carbide fibre reinforced	[144]
Titanium	Ti 15-3-3-3	carbon fibre reinforced polyetheretherketone (PEEK)	[145]
	β-titanium alloy	glass fibre reinforced PEKK (GF/PEEK)	[146]
	grade-2 alloy	glass fibre reinforced polymer (GFRP)	[147]
	TA2	carbon fibre reinforced/epoxy	[33,148]
	Ti-6A1-4V		[149]
		glass fibre reinforced/epoxy	[7,150]
		graphite fibre reinforced	[151]
		graphite-fibre/epoxy and boron fibre/aluminum	[152]
		thermoplastic fibre reinforced polymer	[153]
Steel	TS275	glass fibre reinforced (PA6)	[134]
	galvanized steel		[154]
	galvanized steel	glass fibre reinforced polypropylene (GFPP)	[9]
	AISI 304	glass fibre reinforced/epoxy	[155]
		carbon fibre reinforced/epoxy	[156]
	steel		[157,158,159]
	galvanized steel		[160]
	DC03		[161]
	steel	self-reinforced polypropylene (SRPP)	[162]
	HC220Y	carbon fibre reinforced (PA6)	[163,164]
	HC260LAD + Z100	glass or carbon fibre reinforced (PA6)	[165]

Table 2

Review of surface treatments in the manufacture of fibre metal laminates.

Metal	Matrix	Metal Surface Treatment	Reference
aluminium(grade not specified)	self-reinforcing polypropylene	5% solution of sodium hydroxide for 5 min.	[172]
2024-T3 aluminium alloy	epoxy	etching with chromic–sulphuric acid	[173]
2024-T3 aluminium alloy	epoxy	anodising with phosphoric acid	[174]
2024-T3 aluminium alloy	epoxy	etching with chromic acid followed by anodising with phosphoric acid	[175]
35K cold-drawn steel	epoxy	5% etching solution for 10 min.	[176]
DP800 hot-dip galvanised steel	PA6	galvannealing	[154]
Ti-6Al-4V titanium alloy	epoxy	grit blasting	[177]
Ti-6A1-4V titanium alloy	polyimide	surface treatment with Pasa-Jell 107	[151]
β-titanium alloy	woven S-glass fibre reinforced PEKK	laser treatment	[146]
Ti-15–3-3–3 alloy	polietheroimid	wiping with ethanol	[178]
Pure titanium TA2	polyimide	sand blasting	[179]
AZ31 magnesium alloy	glass fibre reinforced polypropylene	abrading with a 1200 sandpaper and cleaning with acetone	[5]
AZ31 magnesium alloy	epoxy	grit blasting	[180,181]
magnesium	epoxy	abrading and solvent wipe treatment	[182]

Table 3

Summary of the experimental tests performed by Davarpanah et al. [277].

tPA (mm)	tAl (mm)	Δz (mm)	Failure Mode	Formability
<1.58	0.8	0.2	delamination	increases
1.58			metal tearing	maximum
>1.58			metal tearing	slightly reduced
<1.98		0.4	delamination	increases
1.98			metal tearing	maximum
>1.98			metal tearing	slightly reduced
<1.58	1	0.2	delamination	increases
1.58			metal tearing	maximum
>1.58			galling	slightly reduced
<1.98		0.4	delamination	increases
1.98			metal tearing	maximum
2.38			galling	slightly reduced

Table 4

Main advantages and disadvantages of selected methods of fabricating HMPC components.

Method	Advantage	Disadvantage
Hand lay-up	Low cost of tooling [364,365]The process is simple [364]Large, complex parts can be formed [365]Flexible design [364]Production technology is easy to master [365]Any combination of fibres and matrix materials can be used [364]	The process is time-consuming [364]Powder and bad smells are involved in the production environment [365]Quality of the final product depends on highly skilled manpower [364]The process is labour intensive [366]
Spray lay-up	Suitable for small- to medium-volume parts [364]Low-cost tooling [364]	It is difficult to control the fibre volume fraction [364]There is difficulty in removing trapped air from the moulding [365]The process does not provide a good surface finish [364]
RTM	Complex parts can be fabricated [307,309,366]Higher volume outputs and much lower cycle times than other methods [308,366]Production of smooth-surfaced parts on both inner and outer surfaces of the mould [307,308,366].Components can be produced up to 5–20 times faster compared with open moulding techniques [367]	High production volumes require offset high tooling costs [307,308,367]Difficult to effectively improve the fibre volume content of the product [367]The size of the part is limited by the mould [308,367].
LRTM	Produces finished surfaces on both sides of the part [318,368]Improved process controlenhance of dimensional stability [319,368]Significant saving in tooling costs as it only requires a single-piece mould [318,368]Cost-effective to manufacture large panels such as ship’s hulls [368,369]Fabrication of large composite panels takes place at room temperature [319,370]Scalability for large structures [320,371]Toughened infusion resins can be used [371]	Compared with conventional RTM, LRTM is characterised by limited compaction of the reinforcement, a limited injection pressure, and lower dimensional accuracy [318,319,320,372]Trial and error development can prove costly and ineffective [313,373]
Autoclave process	Large components can be fabricated [364]Good surface finish [364]	High initial cost of equipment [364]High cost of maintenance [364]Not suitable for small products [364]
Pultrusion	Requires low labour [348,349,350,364]Good surface finish [348,364]Process is economical [364]	High cost of heated die [346,350,353,364]Limited to constant or near-constant cross-section components [346,349,364]
Filament winding	Complex fibre patterns can be used [355,356,364]Good resin distribution in products [364]A favourable fibre orientation can be obtained [358,359]Process can be highly automated [355,356,357,358]	Process is limited to convex shaped components [358,359]The external surface of the component is not smoothly finished [360,361,364]High cost of mandrell [355,356,364]