*3.3. Multi-Material Machining Processes*

Conventional machining processes such as drilling, milling, turning, and water jet cutting can be applied to composites, as long as the correct tool design and operating conditions are used. An FML is difficult to machine due to its anisotropic properties, inhomogeneous structure, and high abrasiveness of its constituents. These operations typically result in damage to the laminate such as fiber debonding, spalling, matrix cracking, fiber failure or pullout, and very fast wear of the cutting tool [55,56].

In the aerospace industry, the FML is used because of its light weight and stability at elevated temperatures. In these composite structures, large numbers of cut-outs and holes need to be produced. As mentioned above, FMLs are basically composed of a sheet of metal, aluminum, or titanium, and composite, carbon or glass fiber reinforcement, and a thermoset or thermoplastic matrix. Composites require high speed and low feed, drilling titanium requires low speed and high feed, while drilling aluminum requires a balance between speed and feed. Accordingly, the great challenge to drilling the FML is the right choice of parameters processes because of the materials' constituent properties that also vary with the environmental conditions [57].

Kumar et al. [58] compared the machinability of conventional drilling of hybrid titanium/carbon-fiber-reinforced polymer/titanium (Ti/CFRP/Ti) stack laminates in a single shot under dry and cryogenic conditions. The results indicate that the low temperature affects the hole quality but increases the thrust force due to the increase in the hardness of the Ti sheet at low temperatures. Azwan et al. [59] investigated the effect of different drilling parameters on FMLs, such as the drilling speed, feed rate and thickness of the FMLs on the strength of composite materials. They concluded that drilling at a lower spindle speed and a lower feed rate generates higher workload than at a higher speed, for the same feed rate. The thicker FML induces an higher workload compared to the one with less thickness. Zitoune et al. [60] studied the influence of cutting variables on thrust, torque, hole quality and chip during the drilling of a CFRP/Al stack. They observed that the magnitude of thrust force and torque during the drilling of Al compared to CFRP is doubled at a low feed rate. Hole circularity and surface roughness increase with increasing feed rate. The aluminum layer also has a better finish compared to the CFRP one.

Another machining process to produce holes commonly used in aircraft part finishing is milling. The milling process uses rotary cutters to remove material by advancing a cutter into a part. This method is used as an alternative to drilling these joints with conventional twist drills, named after helical milling, where a milling tool rotates in a helical path and creates the hole [61]. Helical milling has also been investigated for making holes in FMLs, with several advantages such as lower cutting forces, lower heat generation and easy chip evacuation [62]. The main difference between drilling and helical milling is that in the former, the hole diameter is determined by the tool diameter, whereas in the latter, the hole is defined by a combination of the tool diameter and the helical path, resulting in greater flexibility in the hole diameter [63].

One of the compounds used in the aerospace industry that is difficult to machine is a unidirectional CFRP and Ti6Al4V, due to the hardness characteristics of titanium sheets and the abrasive characteristics of CFRP. They observed diameter variations, which may be due to the different Young's moduli of titanium and CFRP, as well as variations in surface roughness caused by material-specific chip formation mechanisms [61]. Hemant et al. [64] evaluated the helical hole milling process in GLARE laminates. Although the material and parameters are different from those used by [61], similar process dissimilarities were observed, with variations in hole diameter depending on the material layer, production of discontinuous powdery chips and surface roughness. Therefore, to obtain an excellent quality finished product, the milling process parameters need to be adjusted to produce holes of uniform diameter throughout their depth, continuous chips in the metal zone, low surface roughness and a composite layer without delamination.

#### 3.3.1. Comparison of Drilling and Milling Processes Parameters

The different properties of the constituent materials in FMLs are a challenging task when hole-making is needed. Improper selection of the process and process parameters can result in poor surface quality, inadequate dimensional quality, dimensional inaccuracy, or even component failure [65].

However, there are several interrelated factors. The most crucial factors which affect the machinability of the material are cutting forces, tool geometry, materials, coatings, chip formation, analysis of tool wear, hole metrics such as the hole size and circularity error, surface roughness and burrs formation [66]. Bolar et al. [65] compared two hole-making techniques used in CARALL: drilling and milling. In their study, the two hole-making techniques were evaluated using various performance measures including thrust, radial force, chip morphology, surface roughness, machining temperature, hole diameter accuracy and burr size. Comparing the results, the authors found some advantages of the helical hole milling process in terms of lower thrust and radial force. The intermittent cutting and convenient chip evacuation and heat dissipation helped to lower the temperature and prevent some material damage. The discontinuous aluminum chips produced by the helical milling process proved to be beneficial, with the holes showing a superior surface finish. However, excessive axial feed in helical milling resulted in tool deformation and chatter, leading to surface quality degradation. Further analysis of the machined surface using microscopic inspection showed that the delamination process occurred when conventional drilling was used. On the other hand, the helically milled hole was free of such defects and showed no signs of delamination. They also found that the formation of oversized holes after the drilling process was incredibly significant. Finally, the exit burr's height was much lower with helical milling due to the lower thermal impact and lower thrust. Considering all the aspects presented here, it is safe to say that helical milling is a suitable alternative for machining holes in FMLs.

Another study comparing drilling and milling was carried out by Barman et al. [63]. In their work, they evaluated the two-hole machining process in titanium alloy Ti6Al4V material. They carried out the machining tests considering the thrust force, surface roughness, hole diameter and machining temperature. The morphology of the chips produced and burr formation were also investigated. The magnitude of the force components (thrust and radial force) and the cutting temperature was lower in the milling process, which produced discontinuous powdery chips that evacuated easily without damaging the machined hole surface. The final quality of the holes and the diameter are better in the helical milling process, which produced burr-free holes, contrary to drilling. A similar study using helical

milling and drilling techniques to machine AISI D2 tool steel was previously carried out by Iyer et al. [67]. They found that helical milling produced H7-quality holes with good surface roughness compared to drilling. Another comparative study on helical milling of a larger thickness CFRP/Ti stack and its individual layers was conducted by Wang et al. [5]. Their experimental results showed that as the number of holes increased, the cutting forces increased due to tool wear and its dependence on the material type. Indeed, the abrasive nature of the CFRP resulted in an increase in cutting force. The hole edge quality is good while machining the titanium alloy, and low delamination is registered at the tool entry and exit points in the material, indicating that if quality problems of the holes appear, it refers to the milling of CFRP. Hole size was inversely proportional when comparing a single layer to a stack. In the drilling process, with CFRP, smaller hole sizes can be achieved compared to the titanium plate; however, oversized CFRP holes and undersized titanium alloy holes are observed when helically milling stacks.

Therefore, there is the agreement in the literature when comparing the drilling and milling processes. All results showed similarity regardless of the material used, Ti/CRFP stacks, Al/CRFP stacks, Ti metal alloy, CRFP or steel, indicating that the best process and the one that promotes less material damage is the helical milling process.

#### 3.3.2. Lubrication Processes during Machining

During the machining process, a large amount of cutting heat and friction heat can be generated due to the abrasive nature of the material and tool wear, which usually makes the temperature elevate rapidly. Temperature is one of the most important factors affecting the machined hole surface quality, especially for the CFRPs. When drilling CFRPs, the temperature can reach about 150~250 ◦C, leading to a risk of thermal degradation of matrix resin, carbonization of thermosetting matrices, the fusion of thermoplastic matrices, and sometimes, burning of the carbon fibers [68]. In addition, the thermal effects can affect the quality of machined holes. These, however, can be minimized using coolants supplied either directly or indirectly to the cutting tool-workpiece interaction zone, to remove part of the generated heat. Nevertheless, the use of coolants adds extra costs for handling, disposal, and environmental impact [69].

The coolants generally used in machining processes are water-oil emulsion, a mixture of a soluble oil cutting fluid and mineral oil lubricant [70], micro-lubrication, also known as minimum quantity lubrication (MQL) [71], cryogenic coolants CO2 [72] and liquid nitrogen (LN2) [73,74], air cooling [62] and vegetable oil [75], among others.

Shyha et al. [70] analyzed the hole quality/integrity after drilling of titanium/CFRP/ aluminum stacks under flood cutting fluid water/oil emulsion of 7–8% volume solution and spray mist, a mixture of soluble oil cutting fluid and mineral oil lubricant. Some significant improvements in the machining process were observed. Burr height was generally less than 500 μm, except when spray mist was used. Delamination of the CFRP laminates was significantly reduced due to the support provided by the Al and Ti layers. Surface roughness was significantly lower when using through-spindle cutting fluid compared to spray mist application, especially on the Al section. Spiral-shaped continuous aluminum chips were prevalent, while both short and long helical chips were found with the titanium material when cutting under wet regime. In contrast, the CFRP layer typically produced dusty black composite particles suspended in the soluble oil of the coolant emulsion.

Kumar and Gururaja [76] investigated the cryogenic cooling effects during the drilling of Ti/CFRP/Ti stacks. The parameters such as thrust, torque, delamination, burr height and surface roughness were considered to investigate the effects of liquid nitrogen (LN2) as a coolant. The results are compared with those obtained from dry drilling of Ti/CFRP/Ti stacks, indicating that torque, top surface of metal composite interface, exit burr height and surface roughness decreased when drilling was performed under a cryogenic environment. Nonetheless, thrust force and damage to the lower surface of the metal composite interface are increased under LN2 cooling conditions. Biermann and Hartmann [72] analyzed a cryogenic process cooling with CO2 and found that the chosen coolant improved burr

formation in drilling compared to dry machining and resulted in higher sustainability comparing to machining with cooling lubricant. Many authors have investigated the influence of the application of cryogenic liquid nitrogen cooling and minimum quantity lubrication (MQL) during GLARE machining. Giasin et al. [77] observed that the use of MQL and cryogenic liquid nitrogen cooling increased the cutting forces; however, both reduced the surface roughness of machined holes, adhesion and built-up edge formation on the cutting tool compared to dry drilling. Examination of the microhardness of the top and bottom aluminum sheets near the hole edges after machining showed that it increased when both coolants were used. Pereira et al. [62] reported some benefits of air-cooling application. It was used to cool the cutting point and to remove the chips, obtaining better results in terms of cutting forces and temperature. The cutting temperature reduction in helical milling was 30%. When drilling the CFRP, the cutting heat becomes lower by increasing the revolution speed. Chips pulverized into small sizes reduce cutting temperature by absorbing heat on the cutting of CFRP composite plate. However, the powder-like chip is bad for the machining center, even promoting corrosion and wear of part of the equipment.

Lubricants and coolants play an important role in the machining process to prevent delamination damage in the material during the drilling process, especially in FMLs, because the difference in modulus of elasticity between composite and metal causes different machining deformations. The main defects observed after machining and their discussion are presented below.

### 3.3.3. Machined FML Defects and Analysis Techniques

Drilling holes in both CRFP and FML can result in material damage as the drill passes through the material. Several types of defects are associated with drilling operations, both at the entry and exit of the hole: dimensional defects, surface roughness and surface integrity problems of the hole wall. Defects induced by machining process conditions include matrix cracking, fiber fracture, debonding, delamination and fiber pull-out. Poor hole quality is the cause of nearly 60% of all scrapped parts [78]. As drilling is often one of the last machining operations, the damage that occurs at this stage results in huge economic losses when near-finished parts have to be scrapped. Understanding and detecting the type, size and location of defects that can occur during drilling operations is important for economical and sustainable process improvement [79]. This damage always occurs combined in hole-surrounding areas.

The great issue in FML drilling operations is the quality of the hole: several defects occur related to the process, mainly on the entry and exit sides of the hole, as well as dimensional and surface roughness issues of the hole wall. The detection of these defects is not trivial, especially when non-destructive methods are used. Several methods have been applied to evaluate the hole quality, such as Computed Tomography (CT)/X-ray Tomography Analysis [79–81]; C-Scan [82], Scanning Electron Microscopy (SEM), Stereoscopic Optical Microscopy (SOM), and Energy Dispersive Spectroscopy (EDS) [83]. Nguyen-Dinh et al. [84] analyzed the surface integrity of composites using x-ray tomography after trimming process. The application of the X-ray technique has enabled the measurement of the craters' volume compared with surface techniques, such as surface roughness and 3D optical topography. They observed several damaged zones with formation of craters in the material surface after trimming process, indicating material pull-out (Figure 7).

**Figure 7.** X-ray tomography images showing machining damage for cutting speed of 150 m/min and feed speed of 500 mm/min at cutting distance of 1.68 m with various depth of scanning of (**a**) 12 μm, (**b**) 60 μm, (**c**) 140 μm and (**d**) 152 μm [84].

Pejryd et al. [79] used X-ray computed tomography to detect defects caused by drilling holes in a CFRP. Surface defects and surface properties such as fiber debonding and surface roughness could be easily investigated. Figure 8a shows a typical surface image of a drilled hole based on the reconstruction of the X-ray images. Figure 8b illustrates one way of highlighting the glass fiber material. This technique allows other components to be identified by color. In this case, a red color is used to clearly distinguish it from the surrounding material.

**Figure 8.** (**a**) A 3D model reconstructed from CT scan of the drilled hole, outer surface. The hole has a nominal diameter of 9.5 mm and (**b**) CT image of the inner wall of a 9.5 mm diameter hole, with glass fiber material highlighted in red [79].

Wang et al. [85] compared the holes' quality of CFRP/Al and CFRP/Ti-6Al-4V by Scanning Electron Microscopy (SEM). To reduce the damage, a two-step process of helical milling process was proposed and compared with that of conventional drilling. In the first step, milling was executed starting on the composite part, and then, in the second step, milling started on the metal part. In the conventional drilling process, they noticed that the damage is superior when the tool entry starts on the composite part, showing many uncut fibers (Figure 9a). The cutting action of the second step eliminates the damage caused by the first step, but the surface next to the hole showed fiber pull-out (Figure 9b). The images showed that the damage induced by the helical milling process in both steps is irrelevant.

**Figure 9.** Scanning Electron Microscope (SEM) images comparing hole quality of both conventional and helical milling methods (steps 1 (**a**) and 2 (**b**)) [85].

The minimization of delamination damage is of great importance, because it is a critical parameter that determines the acceptance or rejection of composite components. To achieve this proposal, Bertolini et al. [73] analyzed the hole quality of an Al/CFRP stack obtained by different drilling processes: ultrasonic (UD) and cryogenic (CD) and compared with dry drilling/regular drilling (RD) using the SEM technique. The feed rates used were 0.05 mm/rev, 0.1 mm/rev and 0.15 mm/rev. The FML was composed of two layers: one of 5 mm thick aluminum alloy considered the entry face, and the second one composed of a 4 mm thick CFRP sheet, considered the exit face. They were evaluated solely at the exit, since the entrance appears defects free (Figure 10).

**Figure 10.** Entry delamination on the aluminum sheet at varying feed and drilling strategies [73].

Figure 11 shows the exit face where the delamination process occurred to a greater or lesser extent depending on the variable feeding and drilling strategy. Severe exit delamination occurred solely when drilling tests were conducted under CD, regardless of the adopted feed. This phenomenon can be correlated to the thrust force increase thanks to the hardening of the material as a consequence of the liquid nitrogen application. It is acknowledged that the higher the thrust force, the greater the exit delamination, because the deflection of the FML's last ply concerns a larger zone.

**Figure 11.** Exit delamination on the CFRP sheet at varying feed and drilling strategies [73].

These techniques already guarantee by themselves the analysis of the damage caused by the machining process, but when combined with other image analysis techniques cited here, they improve the accuracy of the analysis, complementing the information and being a determining factor for adjusting process parameters.

To reduce damage during the machining process, it is important to choose the correct tool and tool wear. Therefore, Section 4 deals with this subject in order to provide as much information as possible on the possible tools and tool wear used for multi-material machining.
