Optimizing the Tribological Performance of Copper-Reinforced A356 Aluminum Alloy: Influence of Heat Treatment and Composition Variation

Abstract

Recent progress in metal matrix composites (MMCs) has led to significant research efforts aimed at refining reinforcement methods and processing techniques and enhancing material properties. Incorporating reinforcements has notably improved both mechanical strength and tribological performance while addressing issues such as porosity and particle agglomeration. This study investigates the impact of copper reinforcement (1–4 wt.%) on the tribological characteristics of A356 alloy under both as-cast and heat-treated conditions. The process of heat treatment involved age hardening, where the composites were solution heat treated (SHT) at 535 °C for 2 h, followed by rapid quenching and aging at 100 °C and 200 °C. The results demonstrate that increasing the copper content enhances the composite’s mechanical properties. Specifically, heat treatment promoted the redistribution of the Al₂Cu intermetallic phase during peak aging, leading to improved hardness and wear resistance. Wear testing demonstrated that heat-treated composites exhibited significantly better wear resistance than their as-cast counterparts, with improvements of 50–60% under lower loads and 80–90% under higher loads. Among the tested samples, A356 alloy reinforced with 4 wt.% copper showed the lowest wear rate across all the applied loads, along with a reduced coefficient of friction and enhanced load-bearing capacity, minimizing material deformation. Additionally, aging at 100 °C resulted in the greatest hardness and the lowest wear rate in comparison to untreated A356 alloy. These findings underscore the viability of copper-reinforced A356 composites for applications demanding enhanced mechanical characteristics and wear resistance.

1. Introduction

The rapid development of engineering materials and advancements in technology have created numerous opportunities for enhancing material properties, particularly in alloys and composites [1,2,3]. Among these materials, aluminum (Al) alloys and their composites are highly sought after because of their excellent strength-to-weight ratio, making them ideal for uses ranging from basic ductile foils to advanced engineering components [4,5]. However, a key challenge lies in achieving superior properties by modifying microstructure and tribological characteristics through cost-effective processing techniques.

Various fabrication procedures, like friction stir casting, powder metallurgy, stir casting, and infiltration, have been utilized to produce alloys and composites [6,7,8,9,10,11]. By adjusting manufacturing parameters, processing techniques, and reinforcement types, it is possible to tailor material properties without altering the base alloy composition [12]. This adaptability allows for customization based on specific application requirements. Compared to monolithic alloys, composites offer greater flexibility, making them a preferred research focus. Among different composite materials, MMCs stand out because of their enhanced mechanical as well as tribological characteristics. The incorporation of reinforcements such as SiC, Al₂O₃, graphite (Gr), B₄C, and TiC into metal matrices like aluminum, titanium, magnesium, and steel differentiates MMCs from other composite types [13,14,15,16,17]. SiC offers high hardness, excellent wear resistance, and thermal conductivity; Al₂O₃ contributes high strength, thermal stability, and chemical resistance; Gr functions as a solid lubricant, effectively lowering friction and enhancing wear performance; B₄C is valued for its exceptional hardness, low density, and neutron absorption properties; while TiC provides a high melting point, excellent hardness, and good wettability with aluminum matrices. These reinforcements are integral to composites used in critical applications across the aerospace, automotive, military, and electronics industries, including components such as brake rotors, pistons, bearings, and structural aerospace parts, due to their superior strength-to-weight ratio and tribological behavior [18,19]. The final properties of monolithic and hybrid composites depend on the type, quantity, and distribution of the reinforcements [20].

In aluminum-based composites, integrating hard reinforcement particles significantly improves wear resistance [21]. More than 80% of aluminum alloys are used in manufacturing rolled plates, foils, and various essential products [22,23]. Compared to wrought alloys, cast aluminum alloys offer a cost-effective alternative, albeit with lower tensile strength [24]. Among cast aluminum alloys, A356 is a eutectic alloy containing 6.5–7.5 wt.% silicon. Due to its lower density, thixotropic structure, and higher strength, A356 is extensively employed in the aerospace and automotive sectors [25,26]. Additionally, it exhibits superior tensile strength and rigidity compared to other aluminum grades, contributing to weight reduction and enhanced fuel efficiency. A356-based alloys are engineered to deliver excellent ductility, strength, elongation, hardness, and toughness in their as-cast state [27].

The processing method used in composite fabrication is essential in shaping the ultimate properties. Modifying eutectic Si and α-Al phases in the matrix alloy can improve composite performance by reducing porosity and enhancing phase distribution. Research by Manojsingle et al. (2009) [28] demonstrated the effectiveness of a two-step mixing process in MMC fabrication. The semi-solid state of the matrix increased viscosity, preventing excessive movement of denser reinforcement particles, which tended to settle rather than float. To achieve uniform particle distribution, a second mixing phase involved heating the slurry above its liquidus temperature and stirring at a higher speed (approximately 600 RPM).

Similarly, Fan and Juang (2016) [29] observed reduced porosity and enhanced hardness in Al-based composites by increasing the reaction time during fabrication. The interfacial bond between the matrix and reinforcement particulates were improved with the addition of a small amount of hexafluorotitanate [30]. Liquid stir casting is commonly used as a cost-effective composite fabrication technique. In a study by Lal et al. (2020) [31], A356 (0–10 wt.%)–B₄C composites were produced using a vacuum-assisted electromagnetic hybrid stir-casting technique. The vacuum primarily served to minimize porosity in the matrix material. During solidification, continuous stirring resulted in a non-dendritic microstructure. The use of a vacuum reduced porosity but slightly affected ductility.

Ohmori et al. (2002) [32] reported that the artificial aging of Al-Si alloy between 77 °C and 152 °C enhanced hardness attributed to the development of solute atom clusters, whereas softening occurred at 177 °C. Similarly, Li et al. (2004) [33] studied the precipitation hardening behavior of A356 alloy and observed multiple hardness peaks at higher aging temperatures, with the first peak exceeding the second in magnitude. However, Al-Si-Cu and Al-Si-Mg alloys exhibited a single peak plateau. Moller et al. (2009) [34] found that the amount of intermetallic compounds and hardness values were significantly higher in T6-treated samples, whereas T4-treated samples displayed superior ductility [35].

Research by Shabestari et al. (2004) [36] indicated that adding copper to A356 resulted in a finer, well-distributed eutectic Si phase, optimizing mechanical properties within a specific copper content range. The cooling rate was also a critical factor in achieving optimal properties. Yadav et al. (2015) [37] examined the effects of high-solubility Cu addition in pure Al fabricated through friction stir casting. Their findings confirmed that friction stir processing effectively embedded Cu particles into the Al matrix, enhancing strength and ductility while maintaining thermal stability above 300 °C. Wear resistance improved with increased load and sliding speed, with the wear ratio decreasing under higher loads but increasing at greater sliding speeds. Compared to pure Al, Cu-reinforced composites exhibited higher density and toughness, with microstructural changes influencing wear and friction behavior [38]. The present work highlights the development and evaluation of copper-reinforced A356 aluminum matrix composites, focusing on their mechanical and tribological behavior under both as-cast and heat-treated conditions, incorporating varying copper contents (1–4 wt.%) and applying a systematic heat treatment process, including a solution heat treatment at 535 °C followed by aging at 100 °C and 200 °C. The study also aims to optimize the distribution of intermetallic phases and enhance overall tribological performance with the addition of copper reinforcements.

2. Methodology

2.1. Fabrication

The composites were fabricated using A356 aluminum alloy as the matrix, incorporating copper reinforcements at varying weight percentages (wt.%). A356 alloy has good castability and corrosion resistance and favorable mechanical properties.

Table 1. Chemical composition of A356 alloy.

Element	Si	Mg	Fe	Cu	Zn	Ti	Al
wt.%	7.2	0.39	0.18	0.15	0.08	0.16	Balance

shows the chemical composition of A356 alloy. After acquiring the base alloy and reinforcement materials, a liquid-phase two-step stir-casting method was utilized to produce A356 (1–4 wt.%)–Cu composites. In this process, the A356 alloy was melted in a furnace and heated to 700 °C. Once the alloy reached a fully liquid state, the temperature of the furnace was lowered to 600 °C. This reduction in the temperature facilitated the transition to a semi-solid state, which improved the wettability of the reinforcements within the matrix.

At 600 °C, a motorized stirrer was placed into the crucible, operating at a rotational speed of 200 RPM. Once a vortex formed, a predetermined amount of copper reinforcement was gradually introduced into the melt. To prevent oxidation and slag formation during casting, small amounts of alkaline powder and solid hexafluorotitanate were added. After the complete addition of the reinforcements, the speed of stirring was elevated to 400 RPM and maintained for five minutes before the molten mixture was moved into a mold and left to cool to room temperature. This approach ensured a homogeneous spreading of copper particulates within the A356 alloy (matrix).

Composite specimens were designated based on the copper content, with 1, 2, 3, and 4 wt.% Cu reinforcements labeled as C1, C2, C3, and C4, respectively. Figure 1 presents the bar specimens and composite samples utilized in this study.

2.2. Microstructure

Microstructural analysis was performed on the fabricated composites to assess the spreading and uniformity of the reinforcement particulates within the A356 alloy (matrix). Prior to analysis, each composite specimen underwent sequential polishing using abrasive papers of varying grit sizes, ranging from 120 to 2000. A mirror-like surface finish was then achieved by further polishing with diamond paste with a 0.2- and 0.1-micron grit size. To enhance grain boundary visibility and remove surface impurities, the mirror-polished specimens were etched using Keller’s reagent.

A Scanning Electron Microscope (Model: JSM-IT200) was employed to perform SEM analysis, enabling the observation of phase transformations during the solidification process. Additionally, X-ray Diffraction (XRD) analysis verified the successful integration of reinforcements into the matrix alloy. Additionally, special attention was given to detecting porosity and reinforcement agglomeration, as these factors significantly influence the overall properties of the composite.

2.3. Hardness Test

The resistance offered by the material to permanent deformation or indentation is evaluated using the hardness test. To evaluate the micro hardness value, a Vickers hardness tester (Model: BV-250-S) was used with an applied load of 5 kgf for a dwell time of 20 s. Each sample was prepared according to the ASTM E92 standard. To obtain precise values, the hardness values were noted in six consecutive regions at a distance of 2 mm. The hardness samples were polished before each test to eliminate any surface defects. The hardness values obtained for each sample (C1–C4) were noted and compared with the matrix A356 alloy.

2.4. Age Hardening

Age hardening, also known as precipitation hardening, is a heat treatment technique commonly employed to improve the strength and hardness of alloys, particularly those containing aluminum, titanium, copper, and nickel. This process enhances mechanical properties by forming fine precipitates that impede dislocation movement. The treatment consists of three primary stages:

Solutionizing: The alloy is heated beyond the solvus temperature to dissolve existing precipitates, forming a homogeneous single-phase solid solution.

Quenching: The material is rapidly cooled, typically using water or oil, to retain the alloying elements in a supersaturated solid solution. After quenching, the alloy remains relatively soft and ductile.

Aging: The alloy is reheated to a temperature between the solvus and solidus lines, allowing the gradual precipitation of intermetallic phases over time, which enhances hardness and strength.

In this study, A356 and its composite specimens underwent solution treatment at 535 °C, followed by an isothermal hold for two hours. Immediately after solutionizing, the specimens were quenched to room temperature to preserve the single-phase solid solution. The aging process was then carried out at 100 °C and 200 °C separately. The peak hardness values and corresponding peak aging times for each composite sample (C1–C4) were determined.

2.5. Wear

To assess how adding copper affects the tribological characteristics of both as-cast and heat-treated A356 composites, wear tests were executed utilizing a wear testing device. The experiment was conducted to measure the wear rate and corresponding frictional values. Figure 2a shows the pin-on-disc apparatus used for the wear test. A specimen with a diameter of 6 mm and height of 30 mm was used as the wear specimen. Figure 2b shows the pin specimen of the C1, C2, C3, and C4 composites. The load that was applied was adjusted between 20 and 60 N while maintaining a sliding distance and speed constant at 1500 mm and 200 RPM, respectively, under dry conditions. Additionally, the frictional force was recorded for all the composite samples to determine the coefficient of friction.

3. Results and Discussion

3.1. Microstructure

The microstructural analysis identified various matrix and reinforcement phases present in the fabricated composites. To examine the distribution of reinforcement particles, the C4 composite specimen, which contained the highest copper content, was chosen for SEM analysis. Figure 3 presents the SEM images (backscattered electron imaging mode) of the A356 alloy and the C4 composite. The results indicated that the introduction of Cu reinforcements facilitated grain refinement in A356. The matrix primarily consisted of a dendritic α-Al phase surrounded by eutectic Si, which serves as the major alloying element. Additionally, Fe-rich phases were seen at the grain borders. However, an increase in Cu content showed minimal influence on further refining the matrix phase.

The existence of copper reinforcement was verified using EDS analysis, as depicted in Figure 4. At this stage, the uniformity of reinforcement dispersion and the presence of porosity or agglomeration were evaluated. The findings revealed a uniform distribution of reinforcements with no detectable porosity or agglomeration on the surface of the composite. Figure 5 exhibits the XRD results of the C4 composite after heat treatment, confirming the development of hard intermetallic compounds because of aging.

The XRD pattern of the peak-aged C4 composite (aged at 100 °C) displayed distinct peaks corresponding to the crystallographic planes of different phases within the material. These peaks appeared at specific 2θ angles, representing the aluminum matrix (α-Al) and intermetallic compounds (Si and Al₂Cu).

The dominant XRD peaks were associated with the aluminum matrix, which forms the primary phase in A356 alloy. The characteristic peaks of aluminum’s face-centered cubic (FCC) structure, specifically from the (111) and (200) planes, were observed around 38° and 66° in the 2θ range. The incorporation of copper facilitated the formation of intermetallic phases, particularly Al₂Cu (θ-phase), which contributed to enhanced mechanical properties. Peaks related to Al₂Cu were typically detected at higher 2θ angles, around 44° and 70°. Furthermore, the inherent presence of silicon in A356 resulted in eutectic Si particles, which were evident in the XRD spectrum within the 25° to 55° range, reinforcing the composite’s strength and wear resistance.

The broadening of peaks observed in the XRD pattern indicates the presence of fine grains and microstrains, likely resulting from the stir-casting process and the incorporation of copper reinforcements. Peak intensity in the XRD spectrum corresponds to the relative abundance of various phases, with higher-intensity peaks representing the aluminum matrix and lower-intensity peaks signifying secondary phases like Si and Al₂Cu. The XRD analysis clearly shows the existence of an aluminum matrix along with intermetallic Al₂Cu phases and silicon particles in the C4 composite. This characterization provides essential insights into the composite’s microstructure, which plays a key factor in determining its overall mechanical and tribological effectiveness.

3.2. Hardness Test

3.2.1. As-Cast Condition

The hardness test results for A356 and its composites with varying copper content were recorded in the form of bar graphs, as shown in Figure 6. It is critical to note that changes in hardness values are directly linked to the overall properties of the composites. The average hardness values obtained from three different regions for A356, C1, C2, C3, and C4 were 66, 74, 85, 93, and 102, respectively. The percentage increase in hardness compared to A356 was 10.79% for C1, 22.4% for C2, 29.3% for C3, and 35.8% for C4. The improvement in hardness with increasing Cu reinforcement is attributed to the enhanced surface area of reinforcements within the matrix. A larger surface area facilitates a more efficient transfer of load between the reinforcement and matrix, leading to a higher hardness value. Additionally, grain refinement in the matrix phase contributed to greater resistance to plastic deformation. A finer grain structure increases the grain boundary area, thereby restricting dislocation movement. The combined effect of eutectic Si particle size and shape, intermetallic phase distribution, and reinforcement presence played a crucial role in the overall enhancement of hardness properties in the composites.

3.2.2. Age-Hardened Condition

An artificial age-hardening treatment (T6) was applied to all the composite samples (C1, C2, C3, and C4) to determine their peak hardness values and corresponding aging times. The results demonstrated a significant improvement in hardness at peak aging, with a further increase observed as the Cu content increased. This enhancement suggests that adding Cu to the A356 alloy contributed to solid solution strengthening. However, once the peak aging time was reached, prolonged isothermal holding resulted in a decline in hardness, indicating an over-aged condition. The obtained hardness trends over aging time were consistent with previous studies involving minor Cu additions to Al-Si base alloys. Figure 7 and Figure 8 illustrate the hardness variations for all the composite samples aged at 100 °C and 200 °C. Additionally, greater Cu content shortened the period to peak aging, most likely because aging promotes the production of Al₂Cu intermetallic phases.

The Al₂Cu intermetallic phase plays a critical role in the microstructural strengthening of A356-Cu composites. During solidification and subsequent aging, copper atoms diffuse and combine with aluminum to form finely distributed Al₂Cu precipitates. This occurs due to the limited solubility of Cu in aluminum, especially at lower temperatures, leading to precipitation hardening. These Al₂Cu phases nucleate at grain boundaries and within the grains, acting as barriers to dislocation movement, which significantly enhances the hardness and wear resistance of the material.

The peak hardness of C1 was recorded as 110 VHN at 10 h for composites aged at 100 °C. Similarly, peak hardness values for C2, C3, and C4 were measured at 125, 142, and 156 VHN at 9, 8, and 7 h, respectively. In comparison to the as-cast A356 alloy, the C4 composites exhibited a 60% enhancement in hardness, primarily accredited to the uniform allocation of hard Al₂Cu intermetallic phases. This contributed to an enhanced age-hardening response, accelerating the hardening rate. The increased Cu content led to a more pronounced formation of intermetallic phases, resulting in a 9–30% improvement in hardness for C4 in comparison to C1, C2, and C3. Additionally, the development of solute-enriched zones from the supersaturated solid solution played a key role in hardness enhancement during aging.

Aging at 100 °C yielded higher hardness values compared to 200 °C; however, peak hardness was achieved more quickly at 200 °C due to the accelerated precipitation process. Hardness improvements of 19.8%, 17.9%, 11.3%, and 14.2% were observed for C1, C2, C3, and C4, respectively, at 100 °C compared to 200 °C. This indicates that a lower aging temperature promotes finer and more effective precipitate formation, leading to greater hardness. In contrast, higher aging temperatures facilitated faster precipitation kinetics, resulting in the formation of coarser precipitates that were less effective in strengthening the composites, ultimately leading to reduced hardness values [39].

The decline in hardness at 200 °C is primarily due to precipitate coarsening and over-aging effects. At lower temperatures (100 °C), the formation of fine, uniformly distributed Al₂Cu precipitates effectively impedes dislocation movement, enhancing precipitation hardening and overall composite hardness. However, at higher aging temperatures (200 °C), the accelerated diffusion of Cu atoms leads to the growth of larger precipitates. This transformation from fine, coherent Al₂Cu phases to coarser, semi-coherent or incoherent phases reduces their effectiveness in strengthening the material. Larger precipitates offer fewer obstacles to dislocation motion, thereby lowering hardness. Additionally, prolonged exposure to high temperatures promotes precipitate coalescence, which relaxes lattice strain and diminishes hardening effects.

The coefficient of thermal expansion (CTE) mismatch between the A356 matrix and the Cu reinforcements further influences aging kinetics. At elevated temperatures, increased diffusion and thermal mismatch contribute to stress relaxation, promoting the formation of coarser precipitates, which weakens the composite structure. Excessive growth of intermetallic zones at greater temperatures also reduces the overall hardness of the material.

Thus, aging at 100 °C results in superior hardness to that at 200 °C, as it facilitates the formation of fine, well-dispersed Al₂Cu precipitates, maximizing precipitation hardening while preventing over-aging effects.

3.3. Wear Test

3.3.1. Dry-Sliding Wear Test in As-Cast Conditions

The wear test results for as-cast A356 composites, as presented in Figure 9, demonstrate substantial enhancements in resistance to wear through the inclusion of reinforcement particulates. The wear rate increased with higher applied loads for all the tested samples, which aligns with Archard’s wear law [40]. The wear depth (µm) was measured as the linear thickness of material removed perpendicular to the sliding direction, which directly correlates with material loss.

A356 alloy with only Cu alloying (without particulate reinforcement) exhibited higher wear rates than the A356 Cu-reinforced composites. This increase in wear can be attributed to the absence of dispersed reinforcement particles, which act as load-bearing constituents and wear barriers. In the alloyed A356, copper exists mostly in solid solution or as coarse intermetallics, which are less effective in impeding plastic deformation and abrasive wear during sliding. However, the Cu-reinforced composites contain uniformly distributed Cu particulates and intermetallic phases such as Al₂Cu, which enhance surface hardness, improve load distribution, and reduce localized deformation, thereby significantly improving wear resistance under all load conditions.

Under lower load conditions (up to 40 N), the C2 and C4 composites exhibited similar wear behavior to those of other samples. At 20 N, the wear difference between C2 and C4 was minimal; however, at 40 N, C4 exhibited lower wear. At the highest employed load of 60 N, the wear depth in the C2 composite increased to 358 µm, whereas the C4 composite demonstrated the least wear among all the samples. These results across different load conditions indicate that the incorporation of copper reinforcement significantly enriched the resistance to wear of the A356 composites, with the 4 wt.% Cu composite (C4) exhibiting the best performance under varying applied loads.

For exerted loads of 20 N, 40 N, and 60 N, an overall increase in wear rate was noted for all the samples, with wear loss being most pronounced at the highest load of 60 N. This increase in wear at greater loads can be linked to elevated contact pressure, which intensifies material deformation and accelerates wear. Additionally, the wear behavior shifted from mild to severe as the exerted load was raised from 40 N to 60 N.

Figure 10 illustrates the deviation of the coefficient of friction (µ) with applied load for as-cast composites. For the C1 sample, µ begins at 0.375 under a 20 N load. As the exerted load increases to 40 N and 60 N, the coefficient of friction rises to 0.485 and 0.52, respectively. This trend indicates that with a lower copper content, the material lacks sufficient reinforcement to counter surface deformation, leading to increased material interaction and friction at higher loads.

In the case of the C2 sample, a substantial enhancement in the coefficient of friction is observed at lower loads. At 20 N, µ is recorded at 0.46, increasing to 0.554 at 40 N. For higher loads (60 N), the coefficient of friction stabilizes at 0.6. This behavior suggests that incorporating 2 wt.% copper enhances the load-carrying capacity and reduces friction at lower loads. However, as the applied load increases, plastic deformation in the material causes µ to stabilize.

For the C3 sample, the coefficient of friction remains relatively stable across different loads, exhibiting minimal variation. At 20 N, µ is 0.65, slightly decreasing to 0.545 at 40 N and increasing slightly to 0.6 at 60 N. This limited fluctuation indicates that the inclusion of 3 wt.% copper significantly improves the composite’s ability to withstand wear and surface damage. The reinforcement enhances load-bearing capacity, minimizes surface deformation, and reduces the fluctuation in friction even under higher loads.

Similarly, the C4 sample shows the lowest variation in the coefficient of friction, maintaining stability across all loads. At 20 N, µ is recorded at 0.65, decreasing marginally to 0.6375 at 40 N and 0.61 at 60 N. This behavior highlights the beneficial impact of 4 wt.% copper in enhancing wear resistance and surface durability. The increased copper content strengthens the composite structure, thereby improving its load-carrying capability, reducing surface deformation, and stabilizing the coefficient of friction under increasing loads.

3.3.2. Dry-Sliding Wear Test in Heat-Treated Conditions

The wear test results for heat-treated A356 composites, as depicted in Figure 11, demonstrate notable developments in wear resistance with the inclusion of reinforcement particulates. Under heat-treated conditions, wear increased with higher applied loads for all the tested samples, following a trend consistent with previous wear behavior studies of aluminum matrix composites (AMCs) [41]. However, the wear observed remained mild, with no evidence of severe wear at either low or high loads. The heat-treated A356 alloy without reinforcement continued to exhibit higher wear rates than those of the Cu-reinforced composites. Despite the improvements in hardness and strength due to aging, the absence of discrete, well-dispersed reinforcement particles limited the ability of the alloy to resist material loss under applied loads. The heat-treated Cu-reinforced composites benefited from both precipitation hardening (due to the formation of fine Al₂Cu phases) and the mechanical interlocking effect of the reinforcement particles. This dual strengthening mechanism enhanced the resistance to plastic deformation and surface damage, resulting in superior wear performance across all load conditions.

Analyzing the wear results for C1, wear decreased from 709 to 96 µm after heat treatment, showing a remarkable 86.5% improvement compared to the as-cast condition. This significant enhancement indicated that the aging process increased the wear resistance of the composite, likely by refining the microstructure and enhanced hardness value. The C2 composite showed wear reduction from 358 to 112 µm, corresponding to a 68.7% improvement after undergoing heat treatment. This indicated that even with moderate copper content, aging effectively enhanced the wear performance. In C3, wear dropped from 500 to 170 µm, yielding a 66.0% improvement. The benefit, although slightly less than in C2, is still substantial and reflects the combined effects of a higher Cu content and age-hardening treatment. However, the C4 composite showed the lowest wear in both as-cast and heat-treated conditions. Wear decreased from 214 to 79 µm after heat treatment, resulting in a 63.1% improvement in wear. Although the percentage gain is the lowest among all the samples, C4 consistently showed the best wear resistance, confirming that a higher copper content provides excellent baseline wear behavior even before heat treatment.

Compared to the as-cast composites, the heat-treated samples exhibited a 50–60% enhancement in wear resistance under lower load conditions. At higher loads, wear resistance improvements ranged between 80 and 95%, indicating a substantial benefit from heat treatment. The combination of copper reinforcement and subsequent age-hardening treatment extensively enhanced the wear resistance of A356 composites, with the C4 composite (4 wt.% Cu) delivering the best wear resistance across all the applied loads.

Figure 12 depicts the impact of load exerted on the coefficient of friction for heat-treated composites. The results indicate minimal variation in frictional behavior across the heat-treated samples. As the load increased, the coefficient of friction displayed a decreasing trend. This reduction can be attributed to the development of a stable wear-resistant layer at higher loads, which minimizes direct metal-to-metal contact and reduces adhesion.

Furthermore, at elevated loads, increased contact pressure facilitates more efficient material transfer, potentially smoothing surface asperities and thereby lowering the coefficient of friction. In heat-treated composites, microstructural modifications, such as enhanced hardness and refined grain structure, likely contribute to maintaining stable frictional characteristics despite variations in the applied load.

The obtained wear results for the A356-Cu composites in this study demonstrate significantly enhanced wear resistance, particularly after aging at 100 °C, and align well with findings from previous researchers. Donthamsetty and Babu [42] observed a wear depth of approximately 130 µm at 60 N for A356 reinforced with 5 wt.% Cu after aging, which is notably higher than the 79 µm recorded for the C4 (4 wt.% Cu) composite in this study under similar loading. Kumar M [43] reported a wear depth of about 160–180 µm for a 3 wt.% Cu-reinforced A356 composite after T6 treatment, whereas the C3 aged sample in this work achieved comparable performance with a slightly lower wear of 170 µm. Cam et al. [44] introduced the hybrid reinforcements A356+ (Cu + TiB₂), and wear was observed to be around 140–160 µm under high loads. Similarly, Chandramoorthy et al. [45] reported a wear depth of around 130 µm for graphite and Cu A356 hybrid composites. Overall, the 63–86% improvement in wear resistance post-aging observed in this study surpasses that of earlier works, highlighting the effectiveness of using copper reinforcement alone, combined with low-temperature aging.

4. Conclusions

This study investigated the impact of copper reinforcement (1–4 wt.%) on the microstructure and tribological performance of A356 aluminum alloy. The results demonstrated that copper addition led to significant grain refinement, which was instrumental in enhancing the composite’s overall properties. The refined microstructure effectively restricted dislocation movement, contributing to enhanced mechanical characteristics such as increased hardness and wear resistance.

The fabricated composites exhibited no signs of voids, porosity, or agglomeration, confirming the efficiency of the stir-casting method for incorporating copper reinforcements into the A356 matrix. XRD analysis verified the presence of α-Al, eutectic Si, and Al₂Cu intermetallic compounds in the composites. The key findings from the study conducted are as follows:

• The introduction of Cu reinforcements significantly improved the hardness of A356 composites. The as-cast composites showed hardness enhancements of 10.79%, 22.4%, 29.3%, and 35.8% for C1, C2, C3, and C4, respectively, in comparison to the base A356 alloy. The greatest hardness improvement of 58% was observed in the 4 wt.% Cu composites, primarily due to the development of Al₂Cu intermetallic phases, which contributed to solute hardening and promoted age-hardening effects.
• Aging treatment at 100 °C resulted in superior hardness to that at 200 °C due to the development of fine, well-dispersed Al₂Cu precipitates. Peak hardness values were achieved at different time intervals, increasing with higher Cu content.
• The addition of 4 wt.% copper extensively enhanced the wear resistance of A356 composites, exhibiting the lowest wear rate under all the applied loads. Higher Cu content also reduced the coefficient of friction and improved the composite’s load-bearing capacity, minimizing material deformation during wear testing. Heat-treated A356 composites demonstrated substantial improvements in wear resistance, with Cu reinforcement leading to a 50–60% increase at lower loads and 80–90% at higher loads compared to the as-cast samples. The C4 composite (4 wt.% Cu) exhibited the highest wear resistance across all test conditions, highlighting the advantages of copper addition and age-hardening treatment.
• The improvement in wear resistance properties was due to the incorporation of Cu reinforcements with optimized processing through two-step stir casting and heat treatment, which led to a refined and homogeneous microstructure with minimal porosity or agglomeration. This uniformity contributed to better stress distribution during wear testing. The fine, evenly dispersed intermetallic phases formed during the aging treatment also acted as a barrier to wear propagation, confirming the suitability of the composite with 4 wt.% Cu addition under high wear conditions.