A review on conventional and nonconventional machining of SiC particle-reinforced aluminium matrix composites

Among the various types of metal matrix composites, SiC particle-reinforced aluminum matrix composites (SiCp/Al) are finding increasing applications in many industrial fields such as aerospace, automotive, and electronics. However, SiCp/Al composites are considered as difficult-to-cut materials due to the hard ceramic reinforcement, which causes severe machinability degradation by increasing cutting tool wear, cutting force, etc. To improve the machinability of SiCp/Al composites, many techniques including conventional and nonconventional machining processes have been employed. The purpose of this study is to evaluate the machining performance of SiCp/Al composites using conventional machining, i.e., turning, milling, drilling, and grinding, and using nonconventional machining, namely electrical discharge machining (EDM), powder mixed EDM, wire EDM, electrochemical machining, and newly developed high-efficiency machining technologies, e.g., blasting erosion arc machining. This research not only presents an overview of the machining aspects of SiCp/Al composites using various processing technologies but also establishes optimization parameters as reference of industry applications.


Introduction
Metal matrix composites (MMCs) are prepared by combining a metallic matrix with hard ceramic reinforcements. Usually, metals including aluminum, magnesium, cobalt, titanium, copper, and various alloys of these materials can be adopted as a matrix. Meanwhile, the reinforcement material is generally a hard ceramic material, such as SiC, TiC, B 4 C [1], Si 3 N 4 , AlN, Al 2 O 3 , TiB 2 , ZrO 2 , and Y 2 O 3 [2]. The most widely used metal matrix materials for producing MMCs are aluminum and its alloys, because their ductility, formability, and low density can be combined with the stiffness and load-bearing capacity of the reinforcement [3]. Among numerous reinforcement materials, SiC is usually employed because it has some unique advantages, e.g., low cost, good hardness, and high corrosion resistance, compared to other reinforcements [4]. With the combined advantages of aluminum matrix materials and SiC reinforcement, SiC p /Al MMCs have been certified and are steadily advancing owing to their excellent properties such as high strength, low density, and high wear resistance. They are widely used in the automobile and aircraft industries, structural applications, and many other systems [5]. Since SiC p /Al composites consist of a metal matrix and a SiC reinforcement, different volume or weight percentage SiC in the matrix materials forms different SiC p /Al composites, e.g., 10% (mass fraction), 20% (volume fraction), 45% (mass fraction) and 65% (volume fraction) SiC p /Al matrix composites. A typical micrograph of a SiC p /Al MMC with 65% (volume fraction) SiC particle reinforcement is shown in Fig. 1 [6].
The specific properties of SiC make it very suitable for the production of Al MMCs [7]. However, on the processing aspect, the hard reinforcement causes an inevitable and severe problem of limiting the machining performance and rapid tool wear [8], which results in poor machin ability and cost increase [9]. Consequently, it is not surprising that SiC p /Al composites are considered difficultto-machine [10]. To date, many attempts have been made to improve the machinability of this hard material.  Figure 2b depicts the distribution of SiC p /Al machinability studies conducted in industrial countries. In Fig. 3a, the statistics of the studied SiC (volume or weight fraction) according to appearance frequency in the literature are presented, and the SiC fractions are classified into 10 divisions. It is indicated that most studies are focused on SiC p /Al composites with low SiC fractions, e.g., 5%-20% (volume fraction). Nevertheless, in recent years, increasing attention has been paid to the machining investigation of SiC p /Al with high-SiC fractions, such as 50%, 56% and 65% (volume fraction).
Both conventional and nonconventional machining methods have been adopted for the processing of SiC p /Al matrix composites. Figure 3b displays the approximate distribution of the machining methods utilized in the studies. It can be observed that turning, milling, and drilling are the most commonly used conventional machining technologies, whereas electrical discharge machining (EDM) is the most frequently used nonconventional machining technology. Besides EDM, wire EDM, and electrochemical machining (ECM), there are some other nonconventional machining technologies that have been adopted for improving the machining of SiC p /Al matrix composites, e.g., the newly developed arc discharge machining (ADM) [11]. This review considers both conventional and nonconventional machining studies conducted by numerous researchers to summarize the machinability performance of SiC p /Al matrix composites and to offer transferable knowledge for industry application.

Fabrication and properties of SiC p /Al matrix composites 2.1 Fabrication
Different fabrication techniques are used for the preparation of aluminum MMCs, e.g., stir casting, powder metallurgy, squeeze casting, in-situ process, deposition technique, and electroplating [12]. The widely used processes are stir casting and squeeze casting [13]. Stir casting (vortex technique) is generally considered as the most  economical one among all the available methods of Al MMC production, and it allows fabrication of very large components. Its advantages lie in simplicity, flexibility, and applicability to large volume production [14]. In this process, the matrix material is superheated above its melting temperature. The particles are also preheated at approximately 1 000-1 200°C to oxidize the surface. The melted matrix is then stirred at an average stirring speed of 300-400 r/min as the vortex is formed during stirring [2,15]. The major problem with stir casting is segregation or dusting of reinforced particles [13]. The squeeze casting process combines casting and forging to overcome casting defects such as pitting, porosity, and segregation of reinforcements [16]. Squeeze casting is a nonconventional process. Solidification of the molten slurry is carried out at high squeezing pressures, which enhance the microstructure and mechanical properties [17,18]. In the fabrication of Al MMCs, many types of aluminum alloys have been adopted, e.g., Al6061

Properties
The machinability of MMCs differs from conventional metal materials because of the abrasive reinforcement element. It is known that SiC particles have some specific properties, e.g., high melting point (2 730°C), high modulus (250 GPa), good thermal stability, good hardness, high wear and impact resistance, and high chemical resistance [7]. These excellent properties enhance the characteristics of Al-SiC composites. Consequently, SiC-related issues (e.g., fraction and size) are the key factors that are affecting the properties of SiC p /Al matrix composites. It is believed that the mechanical properties of Al  A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 281 tools, chemical vapor deposition (CVD) diamond tools, and multilayer-coated carbide insert tools [40]. PCD cutters are the most commonly used tools. They are generally preferentially considered when turning highvolume fraction SiC p /Al composites. This is because these diamond-based turning tools both increase tool life and produce acceptable machining surfaces [41]. Durante et al.
[42] insisted that it was possible to use only the PCD turning tools for improving the cutter service time and reducing the cutter changing frequency because HSS cutters could be destroyed in several seconds, whereas conventional and coated carbides could only work for a few minutes. Karabulut and Karako [43] also advised that PCD cutting tools should be used considering their excellent mechanical properties, although these tools were generally not cheap. On the aspect of tool cost, carbide and rhombic inserts have been regarded as an economical alternative turning solution compared to PCD or CBN tools. Sahin [44] reported that multicoated carbide tools with TiN as the top layer presented a better wear property than those of other cutting tools when machining SiC p /Al matrix composites. In addition, Errico and Calzavarini [45] found that the deposition of a thin-film CVD diamond increased the cutting performance of hard metal substrates by more than 100%. Meanwhile, Andrewes et al.
[46] observed a faster flank wear rate on a CVD diamond insert than on a PCD insert, but that faster wear rate could be reduced by securing stronger adhesion between the carbide substrate and diamond coating.

Tool wear mechanism
The machinability of MMCs differs from that of conventional materials due to the heavy cutting tool wear caused by abrasive elements [29]. Flank wear is the main type of wear observed on the tip tool [47]. In terms of tool wear mechanism, Manna and Bhattacharayya [48] explained the following: as the SiC particle contacted with the cutting tool, the atoms from the harder material were likely to diffuse into the softer matrix during the sliding process, which increased the hardness and abrasiveness of the workpiece. In the rapid wear phase and steady wear phase, diffusion and abrasion caused tool flank wear, respectively.
For the PCD tool, the tool wear that occurs on the cutter is similar to that observed when machining other materials and may be interpreted as surface fatigue and a microfracture process. The wear may be exacerbated by adhesion between the tool and the workpiece [49], and vertical grooves are visible on the flank face of the tool [50]. For the TiN-coated tool, abrasion is the main tool wear mechanism and there is almost no evidence of chemical wear; moreover, tool wear occurs on the flank face and the cutting speed is found to be the most influential parameter [51]. For the CVD diamond-coated carbide tool, the tool wear process includes melting of the workpiece material onto the tool surface as well as alterations of the rake face and cutting edge by the consequent pullout. Tool failure of smooth coatings occurs by a process including work material transfer and welding on the tool surface as well as regular removal of the built-up layer and built-up edge (BUE), inducing coating pullout, which exposes the relatively soft tool substrate to abrasive wear caused by the hard SiC particles [52]. For the uncoated WC tool, the flank wear is high due to the formation of BUE and generation of high cutting forces at low cutting speeds. In addition, the formation of BUE enlarges the actual rake angle; thus, it is found that the increment of cutting forces may increase the cutting tool wear in turn [53]. Manna and Bhattacharayya [54] proposed that the feed rate was less sensitive to tool wear compared with the cutting speed during turning SiC p /Al with an uncoated WC cutter. For the CBN and diamond-coated cemented carbide cutting tools, abrasion and adhesion were observed as the predominant wear mechanisms. Scanning electron microscopy (SEM) investigation revealed that tool flank wear was the dominant wear mode. In contrast, machining of an MMC containing relatively large SiC particles (110 lm) using CBN cutting tools resulted in fracture of both the cutting edge and nose [55]. For the SCD tool, microwear, chipping, cleavage, abrasive wear, and chemical wear were the dominant wear patterns. It was pointed out that the combined effects of abrasive wear of SiC particles and catalysis of copper in the aluminum matrix had caused severe graphitization. Figure 4 displays SEM images of a worn SCD tool used for turning of 15% (volume fraction) SiC p / 2009Al [39].
As can be observed from Fig. 5, the cutting speed, depth of cut, feed rate, and nose radius are the main factors that affect tool wear significantly in most of the turning cases [56]. For instance, the tool wear increases with increasing cutting speed, depth of cut, and feed rate when turning 5%-20% (mass fraction) SiC-reinforced MMCs using an HSS cutting tool [36]. When turning SiC p /Al7075 MMC with multilayer TiN-coated WC inserts in a dry machining environment, the most significant parameter affecting tool flank wear was cutting speed, followed by feed rate and depth of cut [57].
Based on experiments and modeling of tool deterioration, it is found that the volume fraction of SiC reinforcement strongly influences the tool wear [58]. Higher percentages of SiC particles lead to higher tool wear. A higher surface contact rate between the SiC particles and cutting edge occurs in higher-percentage SiC p /Al matrix composites [47]. During turning, when the SiC particles gain higher kinetic energy, they will strike the tool insert surface, which causes severe wear [56]. Improving cooling and lubrication has significant impacts on the flank wear, adhesive wear, and tool breakage. It was demonstrated that adequate flushing as well as excellent lubricating and cooling properties would help to reduce the three-body abrasion at the boundary zones of the minor and major flanks [59].
Since tool wear is an important factor that contributes to the variation in spindle motor current, speed, feed rate, and depth of cut, on line tool wear state detecting is available. By analyzing the effects of tool wear and cutting parameters on the current signal, models on the relationship between the current signals and the cutting parameters were established with a partial design taken from experimental data and regression analysis. The fuzzy classification method was used to categorize the tool wear states to facilitate defective tool replacement at the appropriate time [60]. Besides, artificial neural networks (ANNs) and the coactive neuro-fuzzy inference system are available for predicting the flank wear [61].

Cutting force, chip formation, and simulation
The resultant cutting force consists of components due to chip formation, ploughing and particle fracture, and displacement. Merchant's shear plane analysis, slip-line field theory, and Griffith's theory can be adopted for calculating these force components, respectively [62]. Generally, as the cutting forces increase with the flank wear of the turning inserts, the feed and depth forces show a corresponding increase [63]. Manna and Bhattacharayya [54] conducted a series of experiments and found that the cutting force was smaller at lower cutting speeds, whereas the feed force was larger at lower cutting speeds than at higher cutting speeds. Besides, the properties of the SiC particle reinforcement, such as size and volume fraction, contributed to the change in the cutting forces [64]. Gaitonde et al. [65] illustrated that a combination of a high cutting speed with a high feed rate was advantageous for minimizing the specific cutting force. It was demonstrated that the reinforcement percentage had an increasing effect on the resultant force when turning SiC p /Al composites [66], and the cutting force magnitudes were also sensitive to the size of reinforcement particles [67].
The chips formed from the workpiece material will indicate the material deformation behavior during  A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 283 machining [68]. Figure 6 shows that chip voids initiate around the particles along the inner surface first, and then some SiC particles become fractured [69]. In the turning process, the tool rake angle has a great influence on the chip formation. Normally, the material of the workpiece is removed under the tensile stress supplied by the cutting tool with a positive rake angle. On the contrary, the material is removed under the compressive stress supplied by the cutting tool with a negative rake angle. Therefore, it can be deduced that the plastic deformation of chips occurs more easily when using a tool with a negative rake angle than a tool with a positive rake angle [70]. During turning of SiC p /Al matrix composites, the primary chip forming mechanism should be the initiation of cracks from the outer free surface of the chip due to the high shear stress [71]. The particles can interfere with matrix plastic deformation and retard the growth of cracks formed in the chip [72]; thus, the size and volume fraction of reinforcement significantly influence the chip formation mechanism. In the case of finer reinforcement composites, the chip segments are longer and gross fracture occurs at the outer surface of the chips only. By contrast, in coarser reinforcement composites, complete gross fracture causes the formation of smaller chip segments [73]. Because the volume fraction of SiC increases the chip disposability, the chip thickness ratio and shear angle increase [53], and the sizes of chips are decreased during dry machining operation [36]. Ge et al. [74] discovered that a saw-toothed chip was formed during ultraprecision turning of SiC p /Al composites and the chip-formation mechanisms were dynamic microcrack behavior and strain concentration.
Generally, cutting force and chip formation in the turning processes are complex. Simulation models have been developed for a better understanding of these processes. For example, Kishawy et al. [75] reported an energy-based analytical force model for orthogonal cutting of MMCs. Dandekar and Shin [76] proposed a multistep three-dimensional (3D) finite element model using commercial finite element packages to predict the subsurface damage after machining of particle-reinforced MMCs. The particles and matrix were modeled as continuum elements with isotropic properties separated by a layer of cohesive zone elements representing the interfacial layer to simulate the extent of particle-matrix debonding and subsequent subsurface damage. A random particle dispersion algorithm was applied for the random distribution of the particles in the composite. Duan et al. [77] also simulated the chip formation and cutting force in SiC p /Al composite machining by developing a three-phase friction model that considered the influence of matrix adhesion, two-body abrasion, and three-body rolling. The schematic of the toolchip interface in SiC p /Al composites machining is depicted in Fig. 7. It was found that the change in the tool-chip interface friction coefficient with the particle volume fraction and particle size was reasonable. The chip root micrographs obtained from the experiments showed that two-body sliding, three-body rolling, and matrix sticking were the main contact forms that determined the tool-chip interface friction in SiC p /Al composite machining. As exhibited in Fig. 8, Wu et al. [78] developed a microstructure-based model for investigating the mechanisms of chip formation in the machining of particulatereinforced MMCs. The morphology and distribution of the particles, debonding of the particle-matrix interface, and fracture of particles and the matrix were comprehensively integrated into the model. Because of the high strain and strain rate throughout the cutting process, the Johnson-Cook (J-C) constitutive model is generally utilized to describe the properties of matrix materials in simulations [79][80][81][82]. The J-C equation is based on experimentally determined flow stresses and is a function of strain, temperature, and strain rate in separate multiplicative terms [83]. It is given by where r is the flow stress, r the plastic strain, _ e the strain rate, e 0 the reference plastic rate, T and T m the current temperature and material melting temperature, respectively, T r the room temperature, A, B, C, n, and m the material constants that can be obtained using dynamic Hopkinson bar tensile tests. In some conditions, e.g., if the strain exceeds a certain value (0.3) or under a high strain rate condition (higher than 10 3 /s), a modified J-C constitutive model with a correction of strain and strain rate hardening is used for the simulation of turning of particlereinforced MMCs [78]. A detailed summary of machining models for composite materials can be found in Ref. [84].

Surface integrity and machining efficiency
With turning, the machined surfaces contain many defects of pits, voids, microcracks, grooves, protuberances, matrix tearing, and so on [85]. In investigations on the machining surface roughness (SR), R a (the arithmetic mean roughness), R t (the maximum peak-to-valley height of roughness) [86], and R z (the maximum peak-to-valley height within sampling length) [87] are generally considered. Ding et al. [88] studied the machining performance of SiC p /Al composites with various types of polycrystalline CBN and PCD tools; they explained that the adhesion property of the tool and the work material had a major influence on the surface finish. Sharma [89] studied the interaction effects of various factors and reported that an increase in nose radius improved the SR, while the feed rate has a more severe effect on the SR. Davim [90] proposed that the cutting velocity, cutting time, and feed parameters had statistical and physical significance on the SR of the workpiece. Palanikumar and Karthikeyan [91] insisted that feed rate was the main factor that had the greatest influence on the SR, followed by the cutting speed and SiC volume fraction. Muthukrishnan and Davim [92] also supported that the feed rate has the highest statistical and physical influences on the SR, whereas Manna and Bhattacharayya [48] considered that the cutting speed, feed rate, and depth of cut had equal influences on the R a and R t values. Aurich et al. [93] suggested that high cutting speeds and feed rates and moderate depths of cut needed to be used to decrease the thermal load of the workpiece. Muthukrishnan et al. [35] found that the surface finish was superior at lower feed rates and higher cutting speeds for Fig. 7 Schematic of the tool-chip interface in SiC p /Al composite machining a the tool-chip contact, b an enlarged view of matrix sticking, twobody sliding, and three-body rolling, c an enlarged view of the tool-chip contact face [77] A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 285 PCD inserts. When the cutting speed was 400 m/min, a steady low R a value could be obtained over the entire tool life, which made high-speed finishing of MMCs possible [94]. Ge et al. [95] reported that R a of 20-30 nm could be attained by using single-point diamond tools (SPDT) or PCD tools; moreover, the surface obtained by SPDT was smoother and the number of crushed or pulled out SiC particles was fewer. Dabade et al. [96] observed the lowest SR (R a = 0.13 um) on the machined surfaces of higherfraction reinforced MMCs (Al/SiC/30p), and the maximum SR (R a = 2.47 lm) was found on the machined surfaces of Al/SiC/20p composites. It was reported that the SR of the cutting surface decreased as the volume fraction of SiC decreased [97], and the change in size was more influential than the volume fraction [96]. Wang et al. [98] conducted precision turning experiments to study the influence of particle size on the surface quality and proved that the SR (peak-valley value) was close to the particle radius. The performance of cutting tool materials has been evaluated in terms of surface quality from the best to the worst, which are PCD, CBN, and WC (for 10%(mass fraction) SiC p /Al) [99]. For example, while turning Al2124-SiC (45%(mass fraction)) MMCs, the PCD tool performed better than the CBN tool with lower flank wear and higher surface finish quality [100]. It was proposed that damage to the machined surface was related to the fracture and pluck out of SiC reinforcement by cutting tools [101]; specifically, the particles beneath the machined surface were fractured as subsurface damages because of squeezing by the flank face of the cutting edge [78]. Hence, the treated tool produces a  [78] better-machined surface of MMC material than the untreated tool [102], and lubrication will be helpful. In particular, kerosene with graphite powder yields better results on SR and surface hardness compared with other lubricants such as soluble oil, mineral oil, and pure kerosene [103]. In general, the peak residual stresses and residual stresses at most depths beneath the machined surface are higher for heat-treated samples than those for hot-rolled samples [104]. Concerning investigations conducted by Aurich et al. [105], the use of high feed rates decreased the residual stress in the surface of the workpiece in comparison to using low feed rates. However, the surface quality considerably deteriorated by using high feed rates. As depicted in Fig. 9, Sharma [89] studied the interaction effects of various factors and reported that an increase in nose radius improved the SR while the feed rate had a more severe effect on the SR, which increased with the increase in feed rate. Machining efficiency is an important factor in the machining operation of SiC p /Al composites. The operational cost of the machine is directly proportional to the square of the material removal rate (MRR) [56]. MRR is determined by the rate of change in volume [106]. In the turning process, the value of MRR (r MRR ) is calculated by the following formula: r MRR = V 9 F 9 D. Here, V is the cutting speed (m/min), F the feed rate (mm/r), and D the depth of cut (mm). Theoretically, increasing any of V, F, or D will significantly improve the machining efficiency. Fig. 9 Interaction effects of various factors on surface roughness (S: cutting speed, D: depth of cut, F: feed rate, and R: nose radius) a cutting speed versus depth of cut, b cutting speed versus nose radius, c depth of cut versus feed rate, and d feed rate versus nose radius [89] A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 287 However, the change in cutting parameters will produce non-negligible influences on other aspects, e.g., tool life, cutting force, energy consumption, and surface quality. Thus, it is necessary to optimize the machining parameters to achieve higher efficiency without causing severe tool wear, large energy consumption, etc. Generally, optimization methods, e.g., ANOVA and gray relational analysis [56,107,108], genetic algorithms (GAs) [109], Taguchi's optimization methodology [110][111][112][113][114], and response surface methodology (RSM) [115][116][117][118], have been adopted. Table 1 lists various recommended turning parameters for industry consideration based on optimization studies from Refs. [119][120][121][122][123][124].

Milling
There are several types of milling methods, e.g., end milling and face milling. From an overview of the literature, it can be found that most investigations of SiC p /Al composite machining are focused on end milling.

Tool wear
Uncoated cemented carbide inserts, nano TiAlN coated tools, and carbide-coated cutting tools can be adopted for the milling of SiC p /Al composites. Additionally, some ultrahard materials, such as CBN and PCD, are employed to avoid rapid tool wear [125]. Images of a milling cutter with an identical tool geometry are exhibited in Fig. 10 [126]. Shen et al. [127] demonstrated that the uncoated WC-Co milling tool sufferred the severest wear in its circumferential cutting edge, whereas the wear of the diamond-like carbon (DLC)-coated milling tool was slightly lower. Comparatively, the CVD diamond-coated milling tool exhibits a much stronger wear resistance. The wear on its circumferential cutting edge is less than 0.07 mm at the end of milling tests, which is only half of that of the DLCcoated milling tool. Huang et al. [128] conducted highspeed milling experiments of SiC p /Al composites with 20% (volume fraction) at dry and wet machining conditions. The results showed that the main tool wear was abrasion on the flank face, and the TiC-based cermet tool was not suitable for machining SiC p /Al composites with higher volume fractions and larger particles due to the heavy abrasive nature of the reinforcement. The diamond particle size has a great influence on the wear resistance of PCD tools. The larger the size of the diamond particle, the worse the wear resistance. However, when the tool wear goes into a stable wear state, the wear rate of PCD tools with different particle sizes is almost the same [129]. Wang et al. [130] showed that the wear pattern of PCD tools was the flank wear caused by abrasion of the SiC particles at relatively low cutting speeds. Since graphitization of PCD tools does not occur at low cutting temperatures, the wear mechanism of PCD tools will be abrasive and adhesive wear. Huang and Zhou [131] also reported that the flank wear was the dominant wear mode for the TiN-coated tool, cermet tool, and cemented carbide tool. The wear resistance was almost the same for the three different tool materials at both low and high speeds. In addition, the milling speed was the most influential machining parameter on tool wear. With increasing milling speed, the tool wear increased. The feed rate and depth of cut have slight influences on the tool wear. As shown in Fig. 11, Ge et al. [132] reported the tool flank wear under different working times during high-speed milling of a SiC/2009Al composite using a PCD tool; the available tool life could exceed 240 min when a 0.1 mm tool wear criterion was chosen. It was reported that the wear mechanism of diamond-coated micromills was adhesion, abrasion, oxidization, chipping, and tipping [133], and the volume fraction and size of SiC particles present in the aluminum alloy matrix had significant effects on the milling characteristics [134,135].

Cutting force
The cutting force and its impact factors in different milling investigations are generally not the same; however, the machining parameters and SiC particles play a key role. Jayakumar et al. [136] revealed that the depth of cut and size of SiC were the key impact factors of the cutting force. An increase in the volume fraction of SiC reinforcement over the matrix results in a higher tool-work interface temperature and requires a higher cutting force [137]. Vallavi et al. [138] observed that the cutting speed had negative effects on the cutting force while the axial depth of cut and the percentage of SiC showed positive effects on the cutting force. Huang et al. [139] also detected that the milling forces decreased with an increase in the milling speed, or increased with an increase in the feed rate and depth of milling. The influence of milling depth on the milling forces in the x and y directions is the most significant, while the influence of the feed rate on the z milling forces is the most significant. Babu et al. [140] demonstrated that the cutting force components were more sensitive in the high-speed and full immersion condition, and it was witnessed that the cutting force obtained additional undulations by both the unstable chip formation of composite material and randomly distributed reinforcement particles [141].
Ge et al. [142] performed high-speed milling tests on SiC p /2009Al composites by using PCD tools in the speed range of 600-1 200 m/min. The results showed that the peak value of the cutting force (in the tool radial direction) was in the range of 700-1 450 N. The maximum amplitude of the cutting force vibration in the tool radial direction can reach 700 N. Figure 12 illustrates the cutting forces and torque in high-speed milling of SiC p /Al composites with  A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 289 small particles and high-volume fraction by adopting PCD cutters with different grain sizes [143]. The cutting forces and torque of PCD tools of larger diamond grain sizes are less than those of smaller diamond grain sizes.

Surface integrity, machining efficiency, and optimization
The SiC reinforcement removal mode plays a decisive role in the formation of the machined workpiece surface [144]. The reinforcement enhances the machinability in terms of both SR and lower tendency to clog the cutting tool compared to a non-reinforced Al alloy using TiAlN-coated carbide end mill cutters [146]. Zhang et al. [147] reported that the SR of aluminum/SiC composites was smaller than that of the aluminum metal during an end milling experiment, which was due to the improvement in mechanical properties of the aluminum/SiC composite resulting from the addition of SiC particles. In the precision milling of the composites, the generation of the machined surface is a balance between the size effect of the Al matrix and the removal methods of SiC particles. When the feed per tooth is smaller than the minimum chip thickness of Al, the coating effect is dominant; when the feed per tooth is larger than the maximal advised value calculated by the method, the particle cracks dominate [148]. The SR mainly depends on the feed rate followed by the spindle speed, whereas the depth of cut has the least influence [149]. Thus, high cutting speeds, low feed rates, and low depths of cut are recommended for better surface finish [150]. Obtaining a very smooth surface for a high-volume fraction and large Fig. 12 Cutting force versus cutting distance a F x , b F y , c F z and d torque [143] SiC particle workpiece is very difficult; however, a mirrorlike surface with an SR (R a ) of approximately 0.1 lm can still be achieved by using diamond precision milling with small parameters in the range of a few micrometers [125].
Wang et al. [151] reported that the milled SR of 65% (volume fraction) SiC p /Al composites decreases gradually when the milling speed increases from 100 m/min to 250 m/min, and then the values remain stable. It has been demonstrated that using a CO 2 cryogenic coolant can improve the surface quality by reducing the SR value (in face milling) [152]. Figure 14 indicates the influence of SiC fraction on the SR [153]. When the machined SR enters into a relatively stable state, the SR of machined materials with a volume fraction of 56% is the highest, and the value is the lowest when the volume fraction of SiC particles is 15%. When the volume fractions are 25% and 30%, the values of the machined SR have little difference between each other. In general, the lower the volume fraction of SiC particles, the smaller the machined SR.
In terms of residual stress on the machined surface, the axial depth of cut has the highest influence, followed by the milling speed and feed rate, and the residual stress measured in the feed direction indicated that the conditions of the machined Al6063 surface were all tensile, while the conditions of Al/SiC/65p were compressive [154]. During milling, the matrix material was removed in a plastic way Fig. 14 Relation curves between cutting distance and machined surface roughness (using PCD tools) [153] A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 291 and presented a smooth machined surface. Most of the SiC reinforcements presented partial ductile removal with microfractures and cracks on the machined surface [125]. The material removal and tool wear mechanism in the milling of SiC p /Al composites are complex. Investigations aimed at achieving a higher MRR, lower tool wear, and higher surface quality have been conducted; thus, RSM [155][156][157], gray-fuzzy logic algorithm [158], and ANOVA [159] have been adopted. Based on the literature, the recommended milling parameters for industry application are listed in Table 2 [43, 135,136,160,161].

Drilling
Solid carbide drills, TiN-coated HSS twist drills, PCDcoated drills, and CVD diamond-coated carbide tools are widely used for the drilling process. Tosun and Muratoglu [162] advised that solid carbide drills were the most suitable tools for drilling of 17% (volume fraction) SiC p /Al composites, however, from an estimate of economic factors, the TiN-coated HSS drills were cheaper than the solid carbide tools. The best performance of the TiN-coated HSS twist drill was obtained with a lower cutting point, higher feed rate, and higher cutting speed [163]. Xiang et al. [41] suggested that when drilling high-volume fraction (e.g., 65%(volume fraction)) SiC p /Al composites, the CVD diamond-coated carbide tool should be preferred, owing to its stable cutting force, less tool wear, and its ability to produce acceptable machining quality. Monaghan and O'reilly [164] compared a series of drilling tests on a 25% (volume fraction) SiC/Al composite with different drilling tools (coated and uncoated HSS, carbide and PCD-tipped drills, and solid-carbide drills). The results indicated that the hardness of the tool material had a significant influence on the machining performance, and the presence of a ceramic coating on an HSS drill did not improve its performance appreciably compared to standard uncoated tools. The height of burrs produced during drilling was found to be greater with softer materials [165]. Moreover, burr dimensions were smaller at a lower feed rate, higher point angle, and higher concentration of reinforcements [166]. The experiment conducted by Babu et al. [167] showed that the point angle had a significant influence on the drilling performance. As the point angles of HSS and TiNcoated HSS drills increase, the damage zone increases. However, with increasing point angles of solid carbide drills, the damage zone decreases [168]. The temperature during the cutting process plays a major role in the tool wear evolution and wear mechanism [169,170]. The heat generation during machining is divided into plastic-deformation heat and friction-induced heat. The converted heat rate by plastic deformation leads to workpiece temperature variation in material forming and machining. Figure 15 shows the schematic of heat partitioning in the chip formation process.
Huang et al. [171] reported that the thrust force varied linearly with the feed rate, while the cutting speed had no significant effect on the thrust force when drilling SiC p /Al composites with high-volume fractions (55%-57% SiC) and large particle sizes. Hu et al. [172] developed a 3D finite element model for simulating the 3 mm diameter peck drilling behavior of SiC p /Al composites by using ABAQUS/Explicit. In the simulation, a J-C model was created for the SiC p /Al composites. A comparison of the simulation and experimental chip formation is shown in Fig. 16.
As displayed in Fig. 17, many uniform and close-packed abrasion marks on the chisel edge and flank face can be Al7075 alloy 5%, 10%, 15% Spindle speed 1000 r/min, feed 0.03 mm/r, depth of cut 1 mm and 5% SiC by weight The best combination PCD blade with carbide substrate [161] Al6063 aluminum 65% Cutting speed 300 m/min with a tool refreshment Surface R a less than 0.4 lm observed when drilling SiC p /Al composites with highvolume fractions and large SiC particle sizes using electroplated diamond drills [173]. It can be seen that the wear of the embedded diamond grit on the drill includes abrasive wear (see Fig. 17a), pullout (see Fig. 17b), cracks initiated around the particle (see Fig. 17c), and fracture (see Fig. 17d).
Tosun [174] observed that the most influential parameters on the workpiece SR were the drill type and feed rate, respectively. The spindle speed, drill point angle, and heat treatment have been determined to be insignificant factors on the SR. Barnes and Pashby [175] provided strong evidence that through-tool cooling led to a significant improvement in performance in terms of tool wear, cutting force, surface finish, and height of burrs produced. There is another drilling process called friction drilling, which has been adopted for SiC p /Al matrix composites; it is reported that the hole quality in terms of roundness is affected by the spindle speed, feed rate, and percentage of SiC in the workpiece [176,177]. Currently, optimization methods are available based on gray relational analysis [178], fuzzy logic and GAs [179,180], Taguchi's method [181], etc. The recommended drilling parameters for industry consideration are provided in Table 3 [168,174,178,179,[181][182][183].

Grinding
Grinding can be performed as surface, cylindrical, and ductile-regime grinding. Among them, cylindrical grinding has attracted most of the research interests in the grinding of SiC p /Al matrix composites.

Surface grinding
The material removal of SiC particles is primarily due to the failure of the interface between the reinforcement and matrix, and results from microcracks along the interface and many fractures or crushed SiC particles on the ground surface [184]. The chips can be divided into Al-matrix chips, SiC particle chips, and Al-SiC mixed chips, when diamond grinding SiC p /Al composites with higher volume fraction and larger particles [185]. The grindability is influenced by both the type of grinding wheel abrasive and the type of reinforcement of workpiece material [186]. Zhang et al. [187] compared the PCD compact (PDC) whisker with the CVD diamond whisker, and found that the PDC wheel had better edge evenness, which led to good machining quality. Xu et al. [188] suggested the potential of using SiC wheels for rough grinding of SiC p /Al composites in consideration of their economic advantages. Zhong [189] reported that there was almost no subsurface damage except for rare cracked particles when fine grinding 10% (volume fraction) SiC p /Al composites with a diamond wheel. Huang et al. [129] revealed that the normal grinding forces of SiC p /Al composites were always higher than the tangential grinding forces. With the increase in the grinding depth and table speed, both the normal and   [172] A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 293 tangential grinding forces of SiC p /Al composites increased evidently. Due to the high hardness of SiC p /Al composites, the thrust component of the grinding force showed a strongly increasing trend with wheel degradation [190]. Furthermore, with an increase in the grinding depth, both the normal grinding force and tangential grinding force increased evidently [191]. Among the different grinding wheels, the diamond wheel exhibits the lowest normal force followed by the CBN wheel. Surface damages such as debonding of reinforcement from the metal matrix cracked reinforcement, particle breakage, and cracks at the surface are the reason for the increased forces while grinding using the SiC wheel [192]. Considering the plastic deformation force of the matrix material, the friction force between grits and workpiece material, and the removal force of SiC particles, a grinding force model suitable for grinding holes of SiC p / Al composites with high-volume fractions was established by Lu et al. [193]. The effect of the grinding parameters on the grinding force, as shown in Fig. 18, was investigated by Xu et al. [188]. The results indicated that the grinding depth had a more significant effect on the grinding force than the feed speed; with increasing grinding depth and table feed speed, the grinding forces for both the tangential and normal components increased, and the increasing trend was more notable with a higher grinding depth.
The grinding temperature increases with an increase in the wheel velocity, workpiece velocity, feed rate, and depth of cut. High values of the grinding parameters result in high grinding temperatures due to the increase in the energy required to grind a unit volume of material [194]. When the grinding temperature exceeds 450°C, a black color appears on the ground surface due to the oxidation reaction, and the residual compressive stress of the burned surface layer is very high [195]. By adopting a triangular heat source model, the temperature distribution in the workpiece can be accurately and efficiently calculated during the precision grinding of SiC p /Al composites [196]. Du et al. [197] established a microgrinding model of SiC p / Al composites, which took into account the SiC-reinforced particle irregularity, as shown in Fig. 19, and the model was used to analyze the particle removal and surface formation processes in different machining conditions.
In the grinding of SiC p /Al composites, a common problem is the formation of voids and delamination on the machined surface, which is due to pulled-out reinforced particles and aluminum matrix adhesion on the machined surface. The surface feature of the workpiece varies with different grinding parameters. With a larger feeding   Fig. 18 Typical variation in grinding force with a grinding depth and b feed velocity [188] A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 295 velocity and grinding depth, more serious accumulation and adhesion are found [198]. Among many factors, a clear positive influence of the volume content of the hard phase on the surface finish is observed. Qualitative surface damage through particle fracture pullout appears to be common on most of the finish machined surfaces [199]. Zhu et al. [200] established a theoretical SR model of SiC p / Al composite grinding based on a combination of the theoretical SR model of aluminum alloy and SiC, as shown in Fig. 20. The exponential composition function proved to be the most suitable, and the coefficients of the function were fitted by the experimental SR. Pai et al. [201] claimed that the SR improved with an increase in SiC volume percentage and a decrease in depth of cut. This is because an increase in the volume percentage of SiC will increase the hardness of the specimen, which decreases ploughing of the wheel during grinding of a 35% (volume percentage) SiC/Al matrix composite. Hung et al. [202] insisted that a coarse-grit diamond wheel was appropriate for rough grinding, whereas a fine-grit diamond wheel was suitable for fine grinding to achieve the best MMC surface integrity. Nandakumar et al. [203] obtained the best performance by using cashew nut shell oil and nano TiO 2 -based minimum quantity lubrication (MQL), because the lubricant of an MQL system penetrated the workpiece and the wheel interface contact zone. Rough grinding with a SiC wheel followed by fine grinding with a fine-grit diamond wheel is recommended for SiC/Al MMCs [189].

Mill grinding, cylindrical grinding, and ductileregime grinding
The mill grinding uses a grinding head (sintering or plating) that replaces the milling tool to remove the workpiece material with computer numerical control (CNC) milling Fig. 19 Machining surface simulation of SiC p /Al composites at different depths of cut [197] Fig. 20 Predicted and experimental surface roughness [200] machines. This process has integrated the characteristics embodied in a similar machining route/path as milling and multi-edge continuous cutting as grinding. The cut depth of mill grinding is generally larger to achieve a higher MRR [204]. There are four typical chip shapes, i.e., curved chip, huddled chip, schistose chip, and strip chip, among which, the curved and schistose chips are dominant. The chips generated in mill grinding of SiC p /Al composites are irregular and uneven under the same machining conditions. During the chip forming, SiC particles can greatly inhibit the deformation of aluminum matrix, and the different contact positions between the SiC particles and diamond grit cause the SiC particles to be fractured, pulled out, and/ or pulled into the surface of the chip [205]. The particle fracture and debonding force component in the mill grinding of SiC p /Al composites can be considered by developing a new force prediction model [206]. Yao et al. [207] recommended a resin-based diamond grinding wheel for 45% (volume fraction) SiC p /Al composites to achieve the best SR, whereas Li et al. [208] suggested HSS with a super-hard abrasive layer (diamond abrasive and binding agent) to increase the MRR. It is believed that appropriately increasing the feed rate and decreasing the millgrinding depth can obtain less SR [209]. Based on optimizations, the following parameters are recommended: for SiC/LM25Al (4% (volume fraction)) composites, wheel velocity of 43.9 m/s and workpiece velocity of 26.7 m/min with a feed of 0.056 m/min and depth of cut of 9.1 lm [210]; for 45%(volume fraction) SiC p /Al composites, wheel speed of 11.77 m/s, feed rate of 100 mm/min, and depth of cut of 0.8 mm [211]. Regarding cylindrical grinding, Thiagarajan et al. [212] suggested cylindrical grinding of 4% (volume fraction) SiC p /Al using a 60 grit Al 2 O 3 wheel at a cutting velocity of grinding wheel of 2639 m/min, cutting velocity of workpiece of 26.72 m/min, feed rate of 0.06 m/min, and depth of cut of 10 lm. The approach for the cylindrical grinding of Al/SiC composites can be extended with super-abrasive grinding wheels such as diamond and CBN.
For ductile-regime grinding, Huang et al. [213] revealed that the critical grinding depth of ductile-regime machining of SiC p /Al composites decreased with increasing volume fraction of SiC particles due to the decrease in the supporting function of the Al alloy matrix.

Nonconventional machining of SiC p /Al matrix composites 4.1 EDM
EDM is a common nonconventional machining method, which has been widely used in the aerospace, mold, and automobile industries. During machining, a discharge channel is created, where the temperature reaches approximately 12 000°C, removing material by evaporation and melting from both the electrode and workpiece A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 297 [214]. The MRR and SR are regarded as two indicators of the EDM process, which can evaluate the time of completing the material volume removal and the quality of finished surface, respectively [215]. Additionally, the tool wear ratio (TWR) is also very important for EDM. The percentage and size of the SiC in SiC/Al MMCs generally have a negative influence on machinability. Karthikeyan et al. [216] revealed that an increase in the volume fraction of SiC decreased the MRR and increased the TWR as well as SR when performing EDM of 6%-20% (volume fraction) SiC/Al composites. Dev et al. [217] reported that an increase in weight percentage of SiC, as well as particle size, had resulted in a decrease in MRR and an increase in TWR and SR. Besides the SiC particles, electrical parameters are the key factors that affect MRR, TWR, and SR. Singh et al. [218] machined an A6061/10% SiC composite and found that with an increase in pulse on time, the MRR, TWR, and SR increase, and the SR increases with an increase in gap voltage. Seo et al. [219] conducted experiments on 15%-35%(volume fraction) SiC p /Al composites and revealed that the MRR increased with increasing product of peak current and pulse on time   [246] up to an optimal value and then decreased drastically; the combination of low pulse on time and high peak current led to a larger tool wear, higher energy, and rougher surface. It was found that a high current resulted in higher thermal loading on both electrodes (tool and workpiece) leading to a higher amount of material being removed from either electrodes [220]. Surface integrity effects of EDM include roughening of the surface by deposition of a recast layer and pitting of the surface by spark penetration and particulate pullout, as well as surface microcracks [221]. As can be observed from Fig. 21, craters and erosion are evident; metal loss, erosion, and crater formation depend on the intensity of the spark. The high energy of the arc consumed during machining will increase the crater diameter, surface irregularity, and heat-affected zone (HAZ), and the surface will have more ridges and grooves. When adopting a rotating tube electrode, an increase in the rotational speed of the tube electrode can produce a higher MRR and better SR [223]. For instance, Yu et al. [224] machined microholes on a SiC/2024Al workpiece with a cylinder electrode and tube electrode under the same machining conditions. The MRR of EDM with the tube electrode was significantly greater than that of the cylinder electrode. Moreover, the accuracy of EDM holes can be improved by using a tube electrode (rotating speed 800 r/min), as shown in Fig. 22. However, the TWR of a rotating tube electrode tends to be higher and can even be increased by 11.79% compared to that of a cylinder electrode [225]. Regarding the flushing adopted in the EDM of Al/SiC composites, a higher flushing pressure hinders the formation of ionized bridges across the gap and results in a higher ignition delay and decreased discharge energy, thereby decreasing the MRR; however, the SR was found to reduce with an increase in flushing pressure under a certain range [223]. Singh et al. [226] showed that more than 40% reduction in TWR and more than 28% increase in MRR could be achieved by adopting compressed air for the EDM of Al/15% SiC p ceramic composite.

Powder mixed EDM (PMEDM)
PMEDM is a process variant of EDM, which is performed by adding powder into a dielectric fluid [242]. It has a different machining mechanism from conventional EDM processes. It can improve the SR and is now applied in the finishing stage [243]. The powder particles in the dielectric fluid increase the gap between the tool and the workpiece A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 299 while providing a bridging effect between the electrodes for an even distribution of spark energy, making the process more stable [244]. Kansal [245] declared that there was a discernible improvement in the SR of work surfaces after suspending the aluminum powder when machining 10% (volume fraction) SiC p /Al composites. Hu et al. [246] compared the microsurfaces machined by using EDM and PMEDM, as shown in Fig. 23, and the SR of PMEDM decreased by approximately 31.5%. Compared to conventional EDM, the presence of tungsten powder in PMEDM resulted in a 48.43% enhancement of MRR in the machining of AA6061/10%SiC composite [247] and 42.85% reduction in the recast layer of the machined surface [248]. The thickness of white recast layer also reduced, whereas the surface hardness was increased with tungsten PMEDM [249]. Besides tungsten powder, carbon nanotubes (CNTs) [250] and multi-walled CNTs [251] are also added in the dielectric to obtain excellent performances in PMEDM of Al/SiC MMCs. Vishwakarma et al. [252] revealed that the PMEDM process provided a better MRR at higher values of peak current, lower concentration of powder, mid-value of gap control, and lower value of duty cycle [253]. Optimization of machining of SiC p /Al MMCs with PMEDM can be achieved by using the RSM [254], Taguchi and gray analysis [255], ANOVA [256], etc. Kumar and Davim [257] suggested an optimum set of parameters to obtain the highest MRR: powder concentration 4 g/L, pulse duration 100 ls, peak current 9 A, and supply voltage 50 V; for the lowest SR: powder concentration 4 g/L, pulse duration 100 ms, peak current 3 A, and supply voltage 50 V.

Wire EDM (WEDM)
WEDM differs from conventional EDM, as the electrodes are in the form of a thin wire with a diameter of 0.05-0.3 mm [258]. WEDM is also known as wire electric discharge cutting. The schematic of the WEDM process is presented in Fig. 24 [259,260].
The electrical conductivity and thermal conductivity of MMCs are lower than those of unreinforced matrix alloys, which decrease the MRR of WEDM [261]. With an increase in the percentage of SiC particles, the machinability of WEDM decreases [262]. An increase of 10% in ceramic reinforcements may lead to an almost 12% reduction in machining efficiency [263]. However, SiC p /Al composites with high-SiC fractions can still be machined using WEDM [260,262,264]. Yang et al. [265] reported the WEDM of a 65% (volume fraction) SiC/2024Al composite and proposed that the machining mechanism was a combination of melting of the Al matrix and decomposition of SiC particles. Figure 25 illustrates the microstructure of the residual SiC particles on the surface after the WEDM process. Figure 26 shows a cross-sectional microstructure of the WEDM of the 65% (volume fraction) SiC/2024Al.  Pramanik [266] observed a significant variation in the wire diameter during machining of SiC particles reinforced with 6061 aluminum alloy. The variation was mainly caused by the presence or absence of the matrix material coating on the wire, which might cause uncontrolled spark and variation in the ability of electrolytes. Wire breakage is a limitation on the MRR, which can be observed when machining Al/SiC composites. However, wire breakages can be reduced by employing higher flushing pressures, higher pulse off times, and suitable values of servo reference voltage. In general, it was suggested that large pulse on time, high flushing pressure, appropriate wire speed and wire tension, large pulse off time, and appropriate pulse current should be used to obtain optimum machining performance [267]. Figure 27 displays the effect of discharge energy on SR and MRR during the WEDM of 45% (volume fraction) SiC p /Al [268]. It can be observed that the discharge energy presents a strong relationship with machinability by affecting the SiC thermal status.
Different from the conventional WEDM, the dry WEDM was adopted as an environmentally friendly modification of the oil WEDM process, in which the liquid dielectric is replaced by a gaseous medium. An Al 6061C 25% SiC workpiece has been machined with dry WEDM by Fard et al. [228]. Moreover, WEDM was modified to machine a SiC/Al7075 MMC using a wire electrical discharge turning (WEDT) process. WEDT was found to have advantages over the conventional turning process [269]. Many optimizations have been conducted to predict the machining performance or improve the machinability of   [281] SiC/Al MMCs, e.g., ANN-RSM [270], RSM [271][272][273][274], Taguchi's approach [275], Taguchi-based hybrid grayfuzzy grade approach [276], particle swarm optimization [277], AHP-TOPSIS (a hybrid approach obtained by integrating the AHP with TOPSIS technique) [278], and nondominated sorting genetic algorithm [279].

ADM
To some extent, ADM is similar to EDM, but ADM adopts arc discharge whereas EDM utilizes spark discharge. Generally, the machining efficiency of ADM is much higher than that of EDM. Blasting erosion arc machining (BEAM) was one type of ADM, which was developed recently by Zhao et al. [11]. BEAM has been adopted in the processing of SiC p /Al composites to improve the machining efficiency [280]. A flushing system is necessary to conduct BEAM. Figure 28 depicts a flushing device that can be fixed on a standard tool holder [281].
Gu et al. [282] machined a 20% (volume fraction) SiC p / Al composite and achieved a high MRR of 8276 mm 3 /min (peak current of 500 A) with a specific MRR of 16.4 mm 3 / (AÁmin). Compared to the EDM MRR of 140 mm 3 /min (peak current of 100 A) with a specific MRR 1.4 mm 3 / (AÁmin) [219], the efficiency of BEAM is much higher. Chen et al. [283] also conducted experiments on the machining of 50% (volume fraction) SiC p /Al. The results revealed that even for the high-SiC fraction SiC p /Al composites, BEAM still could be used and the obtained MRR was as high as 7 500 mm 3 /min. It was reported that BEAM could also be used for other difficult-to-machine materials, such as titanium alloys [284] and nickel-based superalloys [285]. As shown in Fig. 29, both positive and negative polarity machining can be adopted in BEAM; however, the machined surface qualities are generally not the same. Generally, positive BEAM tends to obtain a better surface but a lower efficiency and higher TWR. The side effect of BEAM is a rough surface, but fortunately, this problem can be solved by adopting combined machining of CNC, as reported by Chen et al. [281].

ECM
ECM is based on a controlled anodic electrochemical dissolution process of the workpiece with the tool as the cathode in an electrolytic cell [286].
By analyzing the influence of the current density in the ECM of 10% SiC/Al MMC, it was found that feed velocity could be approached by a linear function beginning in the origin of ordinates, which led to an active dissolution of the workpiece material, at a low current density of 4 A/cm 2 , an SR of 0.65 lm was achieved. The roughness was decreased to 0.2 lm at 10 A/cm 2 [287]. Kumar and Sivasubramanian [288] compared the ECM of an A356 aluminum alloy reinforced with 5%, 10%, and 15% (mass fraction) SiC particles. They found that the maximum MRR was obtained by applying the least voltage and least SiC content, a moderate value of electrode feed rate, and the highest electrolyte concentration. Senthilkumar et al. [289] illustrated that an increase in the applied voltage, flow rate, and electrolyte concentration resulted in a higher MRR. The optimized parameters for the ECM of LM25 Al/10% SiC were as follows: electrolyte concentration 12.53 g/L, electrolyte flow 7.51 L/min, applied voltage 13.5 V, feed rate 1 mm/min. The corresponding MRR was 0.877 3 g/min and the SR was 6.566 7 lm. An optimal machining parametric combination for the ECM of LM25-25% (volume fraction) SiC, i.e., electrolyte concentration 22.74 g/L, electrolyte flow rate 7.57 L/min, applied voltage 14.8 V, and tool feed rate 0.902 mm/min, was found out to achieve a maximum MRR of 0.051 3 g/min and minimum R a of 7.013 8 lm [290]. Another group of optimal parameters for the ECM of 10%(mass fraction) SiC/Al matrix composites was obtained by Dharmalingam et al. [291]. The optimal values for maximum MRR were machining voltage 7 V, electrolyte concentration 24 g/L, and frequency 50 Hz. The optimal values for minimum overcut were machining voltage 9 V, electrolyte concentration 18 g/L, and frequency 50 Hz. Lehnert et al. [292] adopted an electrochemical precision machining process for complex geometries. A voltage of 16 V and a feed rate of 0.25 mm/ min to generate the geometry with the smallest extent were suggested.

Abrasive waterjet (AWJ) cutting
AWJ machining has many advantages compared to other machining technologies. In contrast to thermal machining processes (laser and EDM), AWJ does not induce high temperatures, and thus, there is no HAZ [293]. In the AWJ machining process, the workpiece material is removed by the action of a high-velocity jet of water mixed with abrasive particles based on the principle of erosion of the material upon which the waterjet hits [294,295]. It is believed that the AWJ machining can be a real competitor of the current techniques employed for cutting superabrasive materials [296]. Early in the 1990s, AWJ had been used for the cutting of a 30%(volume fraction) SiC particulate/6061 matrix composite plate with a thickness of 5.08 mm. The MMC plate was easily machined and good surface finish was produced [297]. Srinivas and Babu [298] observed the cut surfaces with SEM, as shown in Fig. 30, and proposed a possible mechanism of material removal, which was the fracturing and ploughing of SiC and the ductile fracturing of the matrix material.
Based on experiments performed on SiC/Al matrix composites with different SiC mass fractions (5%-20%), Srinivas and Babu [299] suggested that appropriate choices of abrasive mass flow rates and jet traverse speeds were of considerable importance over other parameters such as waterjet pressure. Patel and Srinivas [300] employed an AWJ to perform similar turning of an aluminum-SiC MMC and showed that AWJ could be suitable for turning MMCs without the problems encountered in conventional turning such as tool wear. In addition, it was found that the traverse rate and nozzle angle influence the SR and MRR more than the SiC contents.

Laser machining (cutting)
Laser machining offers significant advantages for rough cut-off applications. Laser is very suitable for machining at high feed rates (up to 3 000 mm/min) and can produce a cut with a narrow kerf width (0.4 mm). However, the quality of the laser-cut surface is relatively poor, e.g., striation patterns on the cut surface, burrs at the exit of the laser, and significant thermally induced microstructural changes can be observed [293]. Sharma and Kumar [301] reported that the most prominent input parameters of laser cutting of AA5052/SiC were cutting speed, reinforced SiC particles, and arc radius. The formations of a recast layer and new phase Al 4 C 3 were detected respectively. When the reinforced SiC particle quantity was fixed at 20% (mass fraction) and the nozzle standoff distance was decreased from 2 mm to 1 mm, the dross height increased from 0.373 mm to 0.481 mm [302]. Figure 31 displays a group of SEM images of surfaces cut by a laser beam process. Unburned SiC particles (marked in circular dashed line) and restricted flow of molten material into a downward direction can be observed.
The laser beam can also be utilized as a cutter or driller to conduct turning or drilling. For example, Biffi et al. [303] used a short-duration laser beam as a tool and cut a thread in an A359-20% SiC composite material (although with limited removal rates). Padhee et al. [304] employed a laser beam and drilled holes on 15% (mass fraction)SiC/Al matrix composites (limited to microhole drilling).

Jet-ECM
Jet-ECM is a technology for quickly and flexibly generating microstructures and microgeometries in metallic parts regardless of the material hardness and without any thermal or mechanical impact [305,306]. As indicated in Fig. 32, the electrolytic liquid is pumped through a small nozzle and ejected with a mean velocity of approximately 20 m/s to form a free jet [307]. By using a pulsation-free pump, a continuous supply of fresh electrolyte with constant pressure is assured to generate a well-defined geometrical shape [305].
The dissolution characteristic in the machining of SiC/ Al MMCs utilizing Jet-ECM varies with the electrolyte used. When using NaNO 3 , the depth and width were hardly affected by the particle fraction, however, in the case of NaCl and NaBr, the particles significantly influenced both the width and depth [308]. Figure 33 shows that the aqueous electrolytes of NaNO 3 and NaCl produce different electrochemical dissolution characteristics [309]. While the diameters of the calottes created with both electrolytes are similar, the use of NaCl electrolyte results in significantly deeper calottes for machining times of approximately 1.5-2 s.

Conventional and nonconventional hybrid
machining of SiC p /Al matrix composites

Laser-assisted machining (LAM)
Compared with the conventional cutting process, LAM [310][311][312][313][314] heats the workpiece with a laser beam to change the microstructure or locally harden the material near the cutting tool. To date, most investigations regarding LAM A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 303 of SiC p /Al matrix composites are focused on laser-assisted turning. Figure 34 presents a schematic of the laser-assisted turning. The LAM process demonstrates a considerable improvement in the machining of MMCs through a lower tool wear and thus increased tool life, as well as reduction in cutting time [315].
LAM provides a higher MRR under the same SR compared to conventional machining. LAM reduced the machining time of Al/SiC p /45% MMCs by 45% due to fewer tool changes, high MRR, and longer tool life compared to conventional machining, the shorter machining time and longer tool life provide a 40%-50% cost saving per part, but with the additional cost of a graphite coating and diode laser [316]. Figure 35 illustrates a comparison of tool (uncoated and coated) life for conventional machining and LAM.
Kawalec et al. [317] found a decrease in cutting force during LAM of aluminum matrix composites compared to conventional turning. Kong et al. [318] explained that abrasive tool wear was the most dominant wear mechanism for three different WC tools in the LAM of SiC p /45% composites. The adhesion wear and diffusion wear were accelerated to some extent with increasing temperature.

Ultrasonic assisted machining (UAM)
UAM or ultrasonic vibration machining is a hybrid process. It can reduce the influence of tearing, plastic deformation, and BUE in cutting and can restrain flutter, making the cutting process more stable [319]. By employing an ultrasonic-vibration source, conventional cutting processes can be modified as ultrasonic vibration-assisted processes. Typical UAM processes are ultrasonic assisted turning [320,321], ultrasonic assisted milling [322,323], ultrasonic assisted drilling [324,325], and ultrasonic assisted grinding [326][327][328].
Ultrasonic assisted turning shows improvement in both cutting force and surface topography compared to conventional turning [321]. Zhong and Lin [320] reported that the roughness of an MMC A359/SiC/20p surface turned with vibration was better than that turned without vibrations. In ultrasonic milling, the SiC particle removal form can be classified into type of cut, pulled, pressed, and crack penetration; increasing the number of SiC particle cut type results in better surface smoothness [323]. Xiang et al. [322] reported that a superior roughness of ultrasonic assisted milling of 65% (volume fraction) SiC/Al composites could be obtained at a cutting speed 160 m/min, feed rate 0.02 mm/z, and depth of cut 0.2 mm. During the ultrasonic vibration drilling, the SiC particle in the composites tended to break along the crystal connection boundary or suffer ductile fracture under the dynamic ultrasonic impulse, in which the cutting resistance could be reduced and the tool edge could be protected. Thereby, the drilling location precision and hole surface quality were enhanced; the wear of the drill chisel edge was effectively improved, and the drilling torque was reduced by approximately 30% [324]. Ultrasonic vibration produces a smaller burr height and width in the drilling of Al/SiC MMC. The burr height and width in UAM are respectively 83% and 24% lower than those in conventional drilling [325]. In ultrasonic grinding, the grinding force and SR were found   [307] lower than those in ordinary grinding for the same grinding parameters [326,327]. The reduction in cutting force and SR can reach 13.86% and 11.53%, respectively [329]. Zheng et al. [328] showed some optimum conditions for the grinding of 45% SiC p /Al2024 composites using ultrasonic vibration. For a minimum value of SR, the parameters were as follows: spindle speed 15 000 r/min, vibration amplitude 5 lm, cutting depth 15 lm, and feed rate 5 mm/min.

Other hybrid machining technologies
The electrolytic in-process dressing (ELID) technique applies an electric current during the conventional grinding process. Shanawaz et al. [330] employed ELID for the machining of low fraction SiC p /Al composites and found that a smoother surface could be obtained at a high current duty ratio. Yu et al. [331] obtained a high-integrity machined surface for a high-SiC fraction (56%(volume fraction)) SiC p /Al composite. On the workpiece surface, most of the SiC particles were removed in ductile mode, and the brittle fracture of SiC particles was reduced substantially.
Surface-electrical discharge diamond grinding consists of diamond grinding and EDM with a rotating disk, which can enhance the MRR and produce a better surface finish [332]. Agrawal and Yadava [333] found the best combination of processing 10% (mass fraction)Al/SiC, which was as follows: wheel speed 1 400 r/min, table speed 4 mm/s, in feed 20 m, current 24 A, pulse on time 50 ls, and duty factor 0.817.
The waterjet-guided (WJG) laser process uses a pressurized microwaterjet as a laser beam guide. Marimuthu et al. [334] conducted an experiment on the WJG laser drilling of 40% (volume fraction) SiC reinforced aluminum MMCs. The advantages found include high levels of hole circularity, no HAZ, no recast layer, and no changes in microstructure.
Electrochemical discharge machining (ECDM) combines the actions of EDM and ECM. Liu et al. [335] employed ECDM to machine 20% (volume fraction) SiC/ Al matrix composites and revealed that smaller median and maximal debris sizes were found in the ECDM process, which indicated that the arc energy of ECDM was likely to   A review on conventional and nonconventional machining of SiC particle-reinforced aluminium… 305 be smaller than that of the EDM process (which could be explained from the aspect of total energy).

Conclusions
This review has summarized the aspects regarding the machinability of SiC p /Al composites with conventional machining, i.e., turning, milling, drilling, and grinding, and nonconventional machining, i.e., EDM, PMEDM, WEDM, ECM, AWJ, Jet-ECM, and newly developed high-efficiency machining technologies. Machining efficiency, surface quality, and tool wear need to be first considered regardless of the machining method. With conventional machining methods, the machining efficiency tends to be enhanced by increasing machining parameters such as machining speed, cutting depth, and feed rate; however, the increased parameters can easily intensify tool wear and shorten tool life. Besides, different SiC fractions of SiC p /Al composites also present different degrees of influence on the machining mechanism, tool wear mechanism, chip formation, and even the machined surface integrity. Higher percentages of SiC particles are more likely to result in a lower machining efficiency and higher tool wear. Hence, various optimization methods, i.e., ANOVA and gray relational analysis, regression models, ANN models, and response surface methodology can be employed to find the most suitable machining condition. For the nonconventional machining of SiC p /Al, i.e., EDM, PMEDM, WEDM, ECM, Jet-ECM, and AWJ, it is believed that the SiC particles can interfere with the electrical discharges during the EDM of SiC p /Al. Hence, the MRR, TWR, and surface quality are strongly related to the electrical parameters, i.e., gap voltage, peak current, pulse on time, and pulse off time. Moreover, non-electrical parameters such as flushing can affect machinability, e.g., a higher flushing pressure can decrease the discharge energy and reduce the MRR. One of the main problems encountered with the nonconventional machining of SiC p /Al is the relatively low machining efficiency. However, this problem can be partly solved by adopting newly developed high-efficiency arc discharge technologies, e.g., BEAM, where the achieved MRR can be hundreds times higher than that of the conventional EDM. The drawback of the arc discharge is the rough machined surface, but fortunately, this can be eliminated by a combination of conventional cutting processes. Hence, employing of arc discharge to obtain a high MRR and the combination of conventional cutting to achieve a fine surface quality may be an efficient and economical way of machining SiC p /Al composites.
In recent years, an increasing number of SiC p /Al composites with high-SiC fraction, e.g., 50%, 55%, and 65% (volume fraction), have attracted the attention of investigators. For these high-SiC fraction SiC p /Al composites, turning and milling processes are generally adopted, and nonconventional processes such as EDM, BEAM, and Jet-ECM are also preferred by researchers. It is concluded that there will be more machining methods and investigations regarding high-SiC fraction SiC p /Al composites in the future.
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