**3. Multi-Material Machining Processes**

#### *3.1. Multi-Material Process Preparation*

The manufacturing process for FMLs is similar to that for polymer composites, using an autoclave process. To produce an FML, six steps must be followed: (i) sheet metal surface preparation, (ii) material deposition: prepreg and metal layers, (iii) mold cleaning and vacuum bag preparation, (iv) curing process, (v) stretching process, and (vi) inspection, usually imaging techniques, and mechanical testing [1].

All these steps must be carefully followed to obtain an FML with excellent mechanical quality, which depends on the correct choice and execution of the material preparation methods. A weak interfacial interaction between the prepreg and the metal can lead to a delamination process in composite structures, which is a major defect. The correct choice of surface treatment is then required to improve the bonding energy between the materials [47].

Adhesive bonding processes such as chemical, coupling agent, dry, electrochemical and mechanical surface treatments can be used as single or combined methods to produce an FML. All these surface treatments change the surface topography and increase the surface roughness, as well as the interaction between the metal and the bonding site.

Some surface treatments were used as a single method. Kwon et al. [21] used a single method, sanding, a mechanical surface treatment, using three types of sandpaper and different sanding times. They concluded that longer sanding times increased surface energy and improved mechanical adhesion between metal and composite. Drozdziel-Jurkiewicz et al. [22] investigated some aluminum and titanium surface treatments using mechanical, chemical and electrochemical methods to achieve high adhesion at the metalcomposite interface. These surface treatments improve the interfacial fracture toughness of the FML, increase the bond strength and provide high adhesion at the FML interface. The study showed that chromic acid anodization is still the most effective in improving the expected bond strength. In their studies, Thirukumaran et al. [20] used the first step, mechanical abrasion, to create a roughened surface at the macro level and remove an unwanted oxide layer. They then tested the chemical process, also known as acid etching, using two chemical solutions: potassium dichromate and ferric sulfate. The treatment with ferrous sulfate, which contains SO4 2e ions, had a better performance, promoting pitting, which increased the roughness and favored good adhesion between the substrate surfaces, as indicated by the higher tensile strength values.

Laser and plasma techniques are also used for the surface treatment of aluminum sheets to produce FML. Dieckhof et al. [50] treated AA2024 and AA5028 sheets and found that both techniques allowed the formation of a structured anodic oxide layer, which improved corrosion protection and interfacial adhesion. Park et al. [51] prepared the metal surface by mechanical abrasion, followed by phosphoric acid anodization and alkaline acid etching techniques. They also observed that the rough substrates were essential to improve the bond strength of the metal sheet-prepreg interface. In addition, they understood that selecting different autoclave pressures can influence the quality of the FML, reducing the void content and avoiding premature failure of the FML. Furthermore, Cheng et al. [52] optimized the production of pores on an aluminum substrate. They chemically treated the metal surfaces with three different electrolytes, all containing SO4 2e ions combined with oxalic acid and/or ferrous sulfate. To improve the interfacial adhesion between the composite and the metal sheet, they also used CNTs for interfacial bridging and final bond strength. They concluded that a rougher substrate surface potentially helps to improve wetting, contact area, and mechanical occlusion of the bonded joint.

Analyzing the surface treatment process in aluminum alloy found in the literature, the combination of mechanical and electrochemical methods is the best used to reach excellent surface preparation. All the papers analyzed showed that a good surface preparation favors an adequate roughness of the sheet metal surface, contributing to an optimal mechanical interlock and excellent interfacial adhesion, thus guaranteeing an FML of sufficient quality so that it can be machined without suffering delamination processes.

#### *3.2. Numerical Simulation Applied to FML Machining*

The numerical simulation technique is an economical option used during the development of a product or process. In the numerical simulation, the cost of equipment and man-hours is lower compared to the equipment used in lubrication processes, finishing, and the qualification of the finished product, as it includes the cost of acquisition, maintenance, and man-hours per machine. In this way, numerical simulation helps to reduce costs if it is carried out before experimental tests, reducing the number of tests, and making the choice of parameters more reliable. In addition, numerical simulation can be used to help understand the mechanism of the machining process by predicting cutting forces and temperature effects.

In the aeronautical field, the FML machining process requires a proper design of the processing parameters to avoid the occurrence of various problems and to minimize defects dependent on the process temperature variation, which can be detrimental to the integrity of the parts. These aspects become more critical in dry machining, such as matrix burning, fiber extraction and delamination.

In their work, Parodo et al. [53] monitor the temperature measured on the tool and the workpiece during dry drilling of Al/GFRP (GLARE) and Al/CFRP (CARALL). The influence of cutting speed on the temperature trends was analyzed. In addition, a numerical model was developed to analyze the process of temperature evolution during drilling. The numerical simulation also indicated that the temperature fields are dictated by the thermal properties of the carbon and glass fibers: temperature profiles within the CARALL were found to be smoother than those observed in GLARE. Giasin et al. [40] studied the machinability of GLARE laminate by experimental techniques and analytical simulation. The chosen parameters were the effect of feed rate and spindle speed on the cutting forces and hole quality. A 3D FE model was also developed to help understand the mechanism of drilling GLARE. To evaluate the numerical model's efficiency, the collected thrust force and torque data were compared, and it was shown that the FE drilling model can predict the cutting forces within acceptable levels. It was the first contribution to the simulation of the drilling process of FMLs. Zitoune et al. [54] investigated the effect of machining parameters and tool geometry on cutting forces, hole quality, and CFRP/Al interface using experimental and numerical simulation. From the experimental study, it was found that the tool-enhanced geometry induced less thrust force compared to the standard one. The

numerical analysis was based on the linear fracture mechanics of the CFRP and the plastic behavior of the aluminum with isotropic hardening. The results showed the critical thrust force responsible for the delamination of the last layer as a function of the aluminum thickness and, on the other hand, the maximum shear force responsible for the separation of the CFRP/Al interface was predicted as a function of the aluminum thickness.

Kim et al. [41] presented a method for predicting a cutting force model to optimize the feed direction. In this study, they used a CFRP with six different absolute fiber orientation angles. The fiber cutting angle significantly affects the cutting characteristics of the material and can be easily changed by varying the feed direction. By changing the feed direction angle, which is a simple adjustment in the milling process, this method effectively reduces the cutting force in material milling. In addition, since a predictive cutting force model is used, it is possible to derive the optimum feed direction under different cutting conditions with minimal experimentation.

Numerical simulation of the drilling process is relatively new; therefore, it is an area in the machining process that can be explored. It was observed that many of the authors investigate the influence of tool geometry on cutting forces and the delamination process of the material. However, a smaller number of authors study the temperature change in the profiles and how this can affect the properties of the material, the tool or can even be useful to study new types of lubricants that are more sustainable.
