1. Introduction

Metal matrix composites (MMCs) are engineered materials composed of metal matrices reinforced with various types of fibers or particulate materials. These composites have drawn a lot of attention due to their exceptional mechanical and physical properties, such as high strength-to-weight ratio, improved wear resistance, and excellent thermal stability [1–4]. MMC’s versatility and suitability for numerous applications, including automotive, aerospace, military, and sports equipment, stem from the capacity to customize their properties through the selection of optimal matrix and reinforcement combinations [5]. MMCs provide notable difficulties during machining as a result of the use of tough reinforcement materials, resulting in severe tool erosion, elevated cutting pressures, and subpar surface quality. Traditional machining methods frequently confront challenges in meeting the exacting standards for accuracy and surface quality that are expected in contemporary engineering applications. Consequently, there has been an increasing interest in investigating sophisticated machining techniques to efficiently tackle these issues. Comprehending the complex interaction between various machining techniques and the behavior of MMCs is essential for optimizing production processes and improving the performance of the end products.

MMCs are advanced materials that combine the properties of metals with those of ceramics or other reinforcements to enhance their performance characteristics. MMCs consist of a metal or alloy matrix embedded with reinforcements such as ceramic particles, fibers, or whiskers. The metal matrix offers ductility and toughness, while the ceramic reinforcements impart high strength, stiffness, thermal stability, and wear resistance [6]. The improvement observed in the mechanical properties of developed MMCs is generally beyond projections [4]. Therefore, this resort to performance integrity, improved mechanical properties, and long life span in automobile, civil structure, aerospace, and space systems applications justify the high cost in relation to conventional alloys [7]. Common metal matrices used in MMC include aluminum, titanium, magnesium, and copper, while typical reinforcements include silicon carbide (SiC), aluminum oxide (Al₂O₃), titanium nitride (TiN), and carbon nanotubes (CNTs). The composition of MMCs can be tailored to achieve specific properties by varying the type, size, and volume fraction of the reinforcements. MMCs are classified into two main categories based on the distribution and orientation of the reinforcements: continuously reinforced MMCs and discontinuously reinforced MMCs. Continuously reinforced MMCs feature long fibers aligned in a particular direction, providing high strength and stiffness along that axis. Discontinuously reinforced MMCs, on the other hand, contain short fibers, whiskers, or particles that are randomly oriented, offering isotropic properties and improved machinability [8, 9].

Discontinuously reinforced (particulate) MMCs are of special interest relative to continuously reinforced because of the following benefits: They are cheaper, possess improved strength and modulus, and allow the feasibility of using conventional fabrication methods (casting and powder metallurgy) and secondary processes (forging). They also provide thermal stability enhancement, higher wear resistance, and exhibi better isotropic properties [9]. MMC’s exceptional combination of qualities makes them highly sought after for use in aerospace, automotive, marine, and structural industries. These materials can endure significant mechanical and thermal stresses while remaining lightweight. Although MMCs offer numerous benefits, their machining process presents notable difficulties because of the abrasive properties of the ceramic reinforcements. This can result in quick tool deterioration and problems with surface quality. Hence, comprehending the structure and characteristics of MMCs is essential for optimizing their machining procedures and attaining the appropriate performance in diverse applications.

Common types of MMCs comprise aluminum matrix composites (Al-MCs), titanium matrix composites (TMCs), magnesium matrix composites (Mg-MCs), and copper matrix composites (Cu-MCs). AMCs are extensively utilized and researched. They consist of an aluminum alloy matrix that is strengthened by materials such as boron carbide (B₄C), alumina (Al₂O₃), silicon carbide (SiC), or graphene. AMCs possess a favorable combination of high strength-to-weight ratio, excellent wear resistance, and enhanced thermal and electrical conductivity. These properties make them well-suited for use in the aerospace, automotive, and electronics sectors. TMCs are composed of a matrix made of titanium or a titanium alloy that is strengthened by including components like silicon carbide (SiC), boron (B), or carbon fibers. TMCs possess notable specific strength, exceptional corrosion resistance, and commendable thermal stability, rendering them appealing for aerospace, biomedical, and high-temperature applications. MMCs are strengthened by incorporating elements such as silicon carbide (SiC), alumina (Al2O3), or graphene. Mg-MCs possess a low density, high specific strength, and favorable damping properties, rendering them appropriate for utilization in the automotive, electronics, and sports equipment sectors.

CMCs consist of a copper or copper alloy framework that is strengthened by including elements such as silicon carbide (SiC), CNTs, or graphene. CMCs possess exceptional thermal and electrical conductivity, impressive strength, and commendable wear resistance. Consequently, they find practical applications in heat sink systems, electrical components, and thermal management systems. Furthermore, MMCs can be fabricated by employing matrices composed of alternative metals, such as nickel, iron, or cobalt, which are strengthened with diverse ceramic or metallic elements to attain distinct properties tailored for certain purposes.

Due to numerous benefits, there has been a growing need for machined MMC parts in the aerospace and automobile industries. Therefore, it is crucial to investigate the concept of MMC machining to meet these increasing demands. Machining refers to a wide range of procedures that involve using power-driven equipment to remove materials from a workpiece in order to obtain a desired form or shape. Machining operations encompass cutting, boring, drilling, grinding, milling, and turning. Multiple reports have confirmed the challenges associated with machining MMCs [3, 10, 11]. The issues arise from the heterogeneous nature of MMC, which refers to the variations in the chemical and physical behavior between the matrix and reinforcement(s). Additionally, the thermal conductivity of the MMCs’ workpiece is diminished as a result of the reinforcing phase, leading to an elevation in the temperature of the tool surface [12]. The challenges have led to the excessive erosion of machining tools, resulting in various forms of damage such as delamination and debonding of the interface between the matrix and reinforcement, particle fracture, and fiber pull-out. Recently, there have been significant advancements in fabrication technologies for MMC parts, allowing them to be produced in a nearly perfect shape. Nevertheless, secondary procedures such as machining are inevitable. The primary criteria influencing the machinability of produced MMCs are (i) the kind of reinforcement, (ii) the orientation of the reinforcement, (iii) the ratio of matrix to reinforcement, (iv) the type of tool, (v) the geometry of the tool, and (vi) the parameters of the machining process [13–15].

The research on machining of MMCs has experienced a significant surge in recent years. According to the Web of Science™ database, a search for “ALL = (Machining of Metal Matrix Composites)” from 2015 to 2024 yielded 2326 articles, with a notable increase of around 300 publications per year over the past five years and approximately 155 articles in the first half of 2024. This marked upward trend demonstrates the growing interest in this field. Narrowing the search to “ALL = (Hybrid Machining of Metal Matrix Composites)” produced 490 articles. Interestingly, there were fewer than 30 review articles. However, an exact search for this review article title using “ALL = (Innovative Machining Strategies for Metal Matrix Composites: Trends and Future Prospects)” yielded no results. This highlights the need for a comprehensive review manuscript that focuses on advances in the machining processes of MMCs. Such a review would address the existing gap in the literature and provide valuable insights for researchers and industry practitioners working in this important and rapidly evolving field. Figure 1 shows the analysis of the search from Web of Science™.

This review comprehensively analyzes and consolidates current knowledge on the machining of MMCs, offering an integrated examination of both conventional and nonconventional machining techniques. Beyond covering traditional processes such as turning, milling, drilling, and grinding, this paper also explores emerging and advanced methods including abrasive water jet machining (AWJM), electrical discharge machining (EDM), ultrasonic machining (USM), and electrochemical machining (ECM). Recent technological developments—such as hybrid machining strategies, nano- and micromachining, additive manufacturing (AM) integration, and the transformative application of artificial intelligence (AI) and machine learning (ML)—are critically reviewed. Unlike previous works, this review emphasizes sustainability and green machining practices as critical factors for future manufacturing environments. The paper discusses the fundamental principles, advantages, limitations, and practical applications of each machining technique, identifies persistent challenges in machining MMCs, and proposes potential solutions and research directions. By bridging conventional methods, advanced innovations, and sustainable practices, this review provides a holistic, forward-looking resource for researchers and practitioners aiming to develop more efficient, precise, and environmentally responsible machining strategies for MMCs.

2. Machining Process

Machinability refers to the ease with which a material can be shaped or cut during the machining process of MMC, while also achieving a desired surface finish. Therefore, a material with lower power consumption and minimal observable deformation on both the workpiece and tool is considered to have superior machinability compared to a material that requires more power and causes greater deformation during the machining process. Li and Laghari [16] and Bejjani et al. [17] have elucidated that the machinability of MMCs is influenced by both extrinsic and intrinsic characteristics, as outlined in Figure 2. The extrinsic factors include the qualities of the metal matrix and reinforcement material, the weight % ratio of matrix and reinforcements, and the bonding between the matrix and reinforcement. The intrinsic parameters refer to those related to the machine and cutting tools. The factors that affect machine stability and vibration levels, as well as performance, include tool characteristics, tool geometry, and machining parameters such as cutting speed, feed rate, and depth of cut.

Markopoulos et al. [18] categorized the parameters that affect the machinability of MMCs into tool life and wear mechanism, workpiece surface quality, chip formation behavior, cutting pressures, machining power, and the likelihood of built-up edge (BUE) development. The heterogeneous and abrasive characteristics of MMCs can result in accelerated tool deterioration, subpar surface quality, higher machining forces, and potential harm to the machine operator. Consequently, specific machining methods have been created to meet the distinct demands of MMC materials. Sections 2.1 and 2.2 will examine both traditional and alternative machining techniques that have been modified and improved to efficiently shape and finish MMC components. Figure 3 illustrates the categorization of the machining process of MMC.

2.1. Conventional Machining Process of MMCs

Extensive research has been carried out to comprehend the operational mechanisms of traditional machining methods used for shaping MMCs. Researchers have explored techniques such as turning, milling, drilling, and grinding to attain the appropriate geometry. The machining performance is significantly affected by factors such as the workpiece material, feed rate, statistical fluctuations, and the choice of cutting tool. Notwithstanding these progressions, the accurate machining of MMCs remains difficult due to their distinctive mechanical features. Modern composite materials present considerable challenges in traditional machining due to their high strength, low thermal conductivity, and the presence of a ductile matrix combined with hard, abrasive reinforcements [19]. The high cost of the MMC machining technique is attributed to short tool life, significant tool wear, poor quality surface, and the occurrence of microcracks [20]. The fatigue resistance of the workpieces is compromised as fracture initiation sites become more effective due to surface imperfections [21, 22]. This is crucial in high-quality performance applications of MMCs. Nevertheless, researchers persist in directing their attention toward exploiting the constraints of MMCs, given their significance in several industrial sectors such as car, aerospace, thermal management, and structural industries [16, 23, 24]. The objective is to precisely determine the optimal machining tools and settings for MMCs.

2.1.1. Turning of MMCs

Turning is a fundamental machining process used to create rotational parts by cutting away material from a workpiece as it rotates. In the context of MMCs, turning is frequently employed to achieve desired dimensions and surface finishes. Basically, turning operations achieve the machining of a surface (external) with the workpiece revolving or a “single-point” cutting tool as shown in Figure 4. Nevertheless, the process of turning MMCs poses distinct difficulties because of the abrasive properties of the ceramic reinforcements. These properties might result in heightened tool erosion and make it more challenging to achieve superior surface finishes. Tool wear is a significant issue when machining MMCs due to the rapid deterioration of the cutting tool induced by the presence of tough ceramic particles in the composite material [25, 26]. These particles can exhibit a much higher hardness than the material of the tool, resulting in accelerated wear and the need for frequent tool replacements. The wear mechanism in turning MMCs usually includes abrasive wear, adhesion, and occasionally diffusion wear, which varies depending on the tool material and cutting conditions. Diverse works have been done on different aspects of the MMC turning process to carefully study the quality of several cutting tool types like coated carbide, cemented carbide, uncoated carbide, and diamond [27]. Studies have revealed that the material choice in the machining process contributes to the effect of turning MMCs [28]. Vasu et al. [29] found that higher B₄C content and cutting speeds in AZ91/B₄C composites reduced tool life, with CVD diamond-coated inserts outperforming uncoated ones. Siddeshkumar et al. [30] demonstrated that adding MoS₂ to Al2219/n-B₄C MMCs reduced tool wear, with TiN-coated tools showing better performance. Bains et al. [10] reviewed the fabrication and machining of MMCs, emphasizing the superior wear resistance of polycrystalline diamond (PCD) and diamond-coated tools. Anandakrishnan and Mahamani [31] revealed that increasing TiB₂ content in Al-6061 MMCs led to higher tool wear and surface roughness (SR) but reduced cutting forces, highlighting the unique machinability of in situ MMCs. Li and Laghari [16] also reported that the volume of reinforcing particulates which brings about the hardenability property of MMCs determines the effect of tool wear.

The presence of hard reinforcements in MMCs also results in higher cutting forces compared to conventional metals. These increased forces can cause tool deflection, affect dimensional accuracy, and increase the overall power consumption of the machining process. It is crucial to optimize cutting parameters such as cutting speed, feed rate, and depth of cut to minimize these forces and improve machining efficiency [32]. The study by Siddeshkumar et al. [30] found that at lower cutting speeds and higher feed rates, BUE formation occurred due to high temperature and pressure at the tool–work interface. As cutting speed increased, tool wear also increased due to elevated temperatures and thermal softening, which reduced adhesion characteristics and prevented BUE formation. SEM analysis revealed significant flank wear as seen in Figure 5, with results consistent with previous studies by Bhushan [33] and Pugazhenthi et al. [34]. However, the flank wear experienced in the TiN carbide tool is lesser compared to that of the carbide tool. Attaining a superior surface finish in the process of turning MMCs can be difficult since the presence of hard particles often leads to surface imperfections and microcracks. The selection of cutting tool material and geometry, in addition to the application of appropriate cutting fluids or lubricants, has a substantial impact on improving surface quality. Diamond and cubic boron nitride (CBN) tools are frequently chosen due to their exceptional hardness and resistance to wear. To address the challenges of turning MMCs, extensive research has been conducted to optimize cutting parameters and develop advanced tool materials and coatings. Studies have shown that using lower cutting speeds and feed rates can reduce tool wear and improve surface finish, albeit at the cost of increased machining time.

Additionally, the application of coolants and lubricants can help dissipate heat and reduce friction, further enhancing tool life and surface quality. Several studies focused on investigating alternative lubricants and coolants to improve the machining performance of MMCs. Josyula et al. [35] explored the use of liquid nitrogen (LN₂) as an eco-friendly coolant and found that it significantly reduced SR, tool wear, and cutting temperatures compared to conventional flood cooling methods. Asgari et al. [36] evaluated the effects of a 5% cutting fluid emulsion and determined that it enhanced the machinability of an MMC by reducing adhesion on the tool flank face, leading to improved surface quality. Shetty et al. [37] developed a novel steam-based cooling and lubricating system that outperformed traditional coolants in terms of cutting force, friction, SR, and temperature generation during MMC machining. Muthuraman and Pradeepkumar [38] demonstrated that cryogenic LN₂ cooling led to substantial reductions in cutting force and temperature, as well as improved surface finish, compared to wet machining of Al/SiC MMCs. The microscopic analysis of flank wear during the machining of discontinuously reinforced aluminum composite provided insights into the wear mechanisms under different cooling conditions. As shown in Figure 6 from Josyula et al. [35], the worn tool surface exhibited a similar two-body wear mechanism under dry and wet machining conditions. However, the application of cryogenic cooling with LN₂ and compressed cold air (CCA) was found to reduce the three-body abrasion wear. This reduction was attributed to the effective flushing away of chips and abrasive particles by the cutting fluids, which prevented the recutting of these abrasive elements by the tool. The absence of cracks and pits on the tool surface under cryogenic and CCA conditions further corroborates the benefits of these alternative cooling approaches in mitigating tool wear during the machining of MMCs. These studies collectively highlight the potential of using specialized lubricants and coolants to address the difficulties related to MMCs’ machining, including high tool wear and poor surface integrity, and improve the overall efficiency and sustainability of the manufacturing process.

The use of advanced tool materials such as PCD cutting tools, diamond-coated carbide, mono-crystalline diamond (MCD), tungsten carbide (WC), and CBN has shown promise in improving the turning performance of MMCs [35, 39]. These materials offer high hardness and thermal stability, making them suitable for machining hard and abrasive composites. Coated carbide tools, with coatings such as TiN, TiC, or Al2O3, also provide enhanced wear resistance and tool life. The usage of a predictive model has been also effective in optimizing turning parameters. Srinivasan et al. [27] utilized response surface methodology (RSM) to model and optimize the machining performance of a particulate-reinforced aluminum MMC during turning operations. The study investigated the combined effects of cutting speed, feed rate, and depth of cut on tool wear, SR, and cutting force. The results showed that increasing cutting speed led to higher tool wear but lower SR, as the formation of a BUE was suppressed at higher speeds. Cutting force increased with feed rate and was higher at lower cutting speeds. The optimal machining conditions were identified as a cutting speed of 100 m/min, feed rate of 0.1 mm/rev, and depth of cut of 0.1 mm, which minimized both SR and cutting force. While turning is a critical process for machining MMCs, it requires careful consideration of tool materials, cutting parameters, and cooling strategies to overcome the challenges posed by the abrasive nature of the composites. Ongoing research and development in tool technology and process optimization continue to enhance the efficiency and quality of turning operations for MMCs.

2.1.2. Milling

In the production of MMC, a common machining process that comes after turning is milling [18]. The procedure is relatively close to turning. The milling operation entails driving a rotating milling cutter into a workpiece for the removal material to achieve the required size/shape. Milling is generally a two-step procedure: (i) roughing, where the machining parameters are set high, and (ii) finishing, where the parameters are rather of lower values, and finishing is done in fewer cycles. The main challenges in milling MMCs are tool wear and SR, as also discussed for the turning process. The main methods of milling are face milling and end milling as shown in Figure 7, whereby most of the studies on the milling of MMC concentrated on end milling. Workpiece flat surfaces are made by employing face milling while slotting/profiling of workpieces is made using end milling. Based on the rotation of the cutter used for milling operation to the direction of feed of the workpiece, milling methods could be up milling and down milling as shown in Figure 8 The movement of the cutter’s rotation and the workpiece are opposite directions in up milling while the cutter’s rotation coincides with the workpiece in down milling. Figure 9 shows the geometry of a milling cutter. It is well known that the various milling techniques listed do not provide the same cutting forces (also thermal gradients) [41]. As a result, there would be differences in the machined surface integrity [43, 44], the wear rate of the cutter [45], and the milling process dynamic stability [46]. Milling operations on MMCs typically involve high cutting forces and temperatures, leading to accelerated tool wear. The abrasive nature of reinforcements like silicon carbide (SiC) and aluminum oxide (Al₂O₃) contributes to flank wear, crater wear, and even tool fracture in severe cases.