Experimental study of scandium addition and precipitate formation on the mechanical and tribological properties of Ultrasonic-assisted stir-cast AA7075 hybrid metal matrix composites

ABSTRACT

Aluminium Alloy Hybrid Metal Matrix Composites (AAHMMCs) have been utilized in automotive, marine, military and aerospace applications due to their high strength-to-weight ratio, microhardness, tensile, impact, and compressive strength, along with superior tribological properties. This study focused on optimizing ultrasonic-assisted stir casting of AA7075/WC/SiC Hybrid Metal Matrix Composites (HMMCs) by varying the wt.% of WC and SiC (6, 8, and 10), melting temperature (700, 750, and 800°C), stirring time (5, 10, and 15 min), and stirring speed (200, 225, and 250 rpm), while keeping ultrasonic parameters constant. The inclusion of 0.25 and 0.5 wt.% Scandium increased microhardness by 18% due to the Hall-Petch effect and Al₃Sc precipitates, which strengthened the material. Ultrasonic assistance improved grain refinement and uniform particle distribution. The optimal process parameters were determined as a 750°C melting temperature, 250 rpm stirring speed, 5 min stirring time, and 8 wt.% WC+SiC. Aging at 300°C improved microhardness (by 15%), tensile strength (by 14.3%), compressive strength (by 12.4%), and wear resistance (by 51%) due to Mg₂Si and Mg₂Zn precipitates. Aging at 400°C increased impact strength by 59%, attributed to Al₂Cu precipitate.

Keywords:
Aluminium Alloy; Stir casting; Ageing temperature; Mechanical properties; Tribological properties

1. INTRODUCTION

Aluminium Metal Matrix Composites (AMMCs) are gaining widespread use in various engineering sectors, such as automotive, aerospace, and other industrial applications. These materials are well-suited for creating components that require a combination of strength, lightweight properties, and high performance. AMMCs are known for their exceptional electrical conductivity, low thermal expansion, impressive mechanical strength, and improved wear resistance, positioning them as a viable replacement for traditional aluminium alloys [¹]. Research in this field has expanded considerably over the last two to three decades. Functional and structural applications employ aluminum alloy-based matrix composites due to their high S/W ratio, enhanced stiffness, and improved corrosion and wear resistance [²,³,⁴]. Aluminum alloys possess very low density and excellent mechanical properties compared to other materials. However, the main demerits of alloys are their poor wear behavior and lower melting point [⁵, ⁶]. Various types of ceramics are used to reinforce aluminum alloys to address their wear resistance limitations. Ceramic reinforcements such as aluminum oxide, titanium carbide, titanium nitride, titanium diboride, silicon carbide, boron carbide, zirconium dioxide, and boron nitride are commonly employed to strengthen matrix materials [⁷,⁸,⁹] The properties of reinforced matrices are governed by factors such as particle shape, particle size, hardness, and modulus of elasticity [¹⁰, ¹¹]. The inclusion of ceramic reinforcements in aluminum or aluminum alloy matrices enhances hardness, making machining more challenging [¹²]. The machining difficulties of ceramic-reinforced composites can be mitigated by incorporating secondary reinforcements, which not only improve mechanical properties but also facilitate easier machining [¹³]. Stir casting is considered an appropriate technique due to its cost-effectiveness and simplicity [¹⁴]. The mechanical properties and wear resistance of aluminum reinforced with graphite and boron carbide are superior compared to aluminum alloy 6061, and Al-based composites are commonly manufactured using the liquid casting method [¹⁵]. Boron carbide and silicon carbide have been separately incorporated into aluminum matrices using the pressure infiltration technique, and their mechanical properties have been evaluated and compared. The strength variation between SiC and B₄Cseparately reinforced Al is found to be inversely proportional [¹⁶]. The influence of Al₂O₃ particle size on the mechanical properties of AA 2024/Al₂O₃ composites has been analyzed, and these composites have been fabricated using the vortex method [¹⁷]. Silicon carbide-reinforced aluminum alloy 6061 composites exhibit enhanced wear resistance and demonstrate the effects of temperature and applied load [¹⁸]. Hot pressure sintering and hot extrusion have been used to process these materials. The Sc addition of 0.2 wt% to the Al-Cu-Mg-Sc/ SiCp composite resulted in a YS of 600.6 MPa, an UTS of 681 MPa, and a 4.3% elongation in fracture. This addition improved both ductility and strength compared to the composite without Sc. The refinement and the formation of the intergranular θ contributed to a 126% improvement in fracture elongation by minimizing stress concentrations and facilitating better dislocation accumulation [¹⁹]. Research has shown that incorporating MgAl₂O₄ nanoparticles (~100 nm) into Al-2Mg-1Si using UCT enhances tensile properties without significantly compromising ductility. A detailed analysis for microstructure was conducted using polarized light microscopy,TEM and SEM/EDS. UCT effectively distributed MgAl₂O₄ nanoparticles homogeneously within the Al-2Mg-1Si matrix, refining the grain structure and increasing tensile strength while preserving ductility. Fractographic analysis of tensile-tested samples revealed ductile fracture behavior in its matrix and brittle nature in the magnesium aluminate reinforced composites [²⁰]. Numerous research have incorporated ultrasonic vibrations into traditional manufacturing methods to boost their efficiency. Key processes such as welding, casting, machining, forming laser cladding, and direct laser deposition have demonstrated enhanced performance when paired with ultrasonic vibrations. These vibrations can travel through various mediums, including solid and liquid phases, enabling ultrasonic-assisted manufacturing techniques to be categorized according to the medium through which the vibrations propagate [²¹]. The traditional stir casting route enables the dispersion of reinforcement into the melted matrix. However, once the stirring process stops, these particles often float to the surface and form clusters. In contrast, ultrasonic dispersion provides a different and more effective approach to distributing particles. A major advancement in this technique is the application of acoustic transient cavitation, which disrupts gas microbubbles near the clustered reinforcement particles, ensuring their even dispersion within the base material. Furthermore, this method removes the gas layer from the particle surfaces, greatly improving the wettability of the matrix material [²²]. Studies have demonstrated that ultrasonic-assisted cavitation is a highly effective approach for achieving uniform inclusion of reinforcement particles, including nanoparticles, inside the matrix, resulting in enhanced material properties. On the other hand, traditional mechanical stirring methods are limited to integrating a lower weight fraction of reinforcement. To address this limitation, an approach was developed to achieve a more even distribution of a higher proportion of reinforcement particles by integrating ultrasonic-assisted cavitation with the squeeze casting process. Furthermore, the research focused on improving outcomes by optimizing the influence of different process parameters [²³]. Producing nanocomposites is a complex task that faces numerous challenges, such as inconsistent distribution of reinforcement particles, inadequate wettability, porosity, and the development of clusters and agglomerates during the manufacturing process. Nevertheless, the ultrasonic-assisted stir casting method offers a promising solution to these problems by improving particle dispersion and enhancing wettability [²⁴]. Primary difficulties in stir casting is ensuring an homogeneous distribution of reinforcement particles inside the molten metal, which is frequently complicated by their high S/V ratio and poor wettability. Consequently, these particles tend to either sink to the bottom or rise to the surface, based on its density in contrast to the melted matrix. To rectify this problem, investigators typically employ huge impellers operating at higher RPM. However, this aspect often results in elevated porosity and increased incorporation content in the final cast material [²⁵].This study investigates the microstructural and mechanical properties of aircraft alloy reinforced with silicon carbide nanoparticles through the Friction Stir Processing (FSP) technique. Taguchi L9 OA was applied to optimize process parameters. Experimental results confirmed that the highest predicted mechanical performance was achieved using the identified optimal parameters. The key factors influencing aluminum metal matrix composites include axial load, traverse and rotational speed [²⁶, ²⁷]. The tensile strength, hardness, and electrical resistivity of AL7075 are enhanced by subjecting it to an aging process at varied temperatures. Aged AL7075 exhibits higher hardness than the unreinforced matrix material. The inclusion of silicon carbide at varying weight percentages results in higher tensile strength and lower ductility, outperforming unreinforced AL7075. Additionally, the incorporation of tungsten carbide into AL7075 further enhances its mechanical properties and wear resistance [²⁸]. Although nanocomposite coatings enhance surface hardness and reduce friction, they have certain drawbacks when compared to aged AA7075 composites reinforced with SiC and WC. One significant limitation is that the reinforcement is confined to the surface, lacking the overall structural strengthening provided by the bulk composite. As a result, these coatings are prone to delamination, wear-through, and failure under extreme conditions. Achieving a uniform, defect-free nanoscale coating is also challenging and often requires advanced and expensive deposition techniques. Additionally, weak adhesion between the coating and the substrate can accelerate wear and lead to failure in high-load environments. Due to these factors, nanocomposite coatings may have lower durability and reliability than bulk composites, particularly in demanding and long-term tribological applications [²⁹].

The litrature survey shows that no work has been done on the fabrication and aging of AL7075/WC/SiC HMMC to analyze its metallurgical, mechanical and tribological properties. In the present study, the fabrication and aging of AL7075/WC/SiC HMMC are carried out to analyze its metallurgical, mechanical and tribological properties.

2. MATERIALS AND METHODS

2.1. Materials

The AA7075 was selected for this investigation due to its limited tribological properties. AA 7075 is broadly used in the manufacturing of shafts, missile parts, valve parts gears, aircraft fittings, and structural components [³⁰]. The aluminum alloy was purchased from M/s Trichy Metals, Trichy, Tamil Nadu, India, in the form of cylindrical round rods. The tribological properties of AA7075 were enhanced by reinforcing it with WC and SiC.SiC and WC reinforcements were chosen to address this limitation [³¹]. The carbide content in SiC and WC provides high hardness, while the presence of silicon and tungsten contributes to increased strength. The reinforcements were procured from Sigma Aldrich in powder form, with SiC and WC having mean particle sizes of 2 µm and 10 µm, respectively. Scandium was also purchased with an mean particle size of 2 µm and 99.2% purity for incorporation during the stir casting process. The inclusion of scandium raises production expenses because of its scarcity and high market price. These additional costs need to be justified by the performance gains, such as improved tensile strength and wear resistance, which can lead to longer product lifespans and lower maintenance requirements, especially in high-stress industries like aerospace and automotive. Additionally, the intricacies of processes like ultrasonic-assisted stir casting and aging treatments can further drive up manufacturing costs. As a result, the practicality of using scandium-enhanced AA7075 HMMCs depends on the intended application and the long-term advantages. Factors such as fewer part failures, enhanced operational efficiency, and potential reductions in maintenance expenses must be weighed against the higher upfront investment in these advanced materials.

The particle size of the ceramic reinforcements was analyzed using a PSA. The chemical composition of aluminum alloy 7075, WC, and SiC is presented in Table 1. The attributes of AA7075 and ceramic reinforcements are shown in Table 2. The EDAX and SEM test results of AA7075, WC, SiC, and Sc are displayed in Figures 1 and 2, respectively.

Table 1
Elements of AL7075, WC and SiC.

ELEMENTS OF AL7075
Elements		Si	Fe	Cu		Mg	Mn		Zn	Cr		Ti	Al
Wt.%		0.39	0.49	1.8		2.8	0.28		5.7	0.25		0.18	Rem.
ELEMENTS OF WC AND SIC
Elements	WC							SiC
	W				C			Si			C
Wt.%	75.63				24.06			66.4			33.6

Table 2
Description of matrix and reinforcement materials.

S.NO	DESCRIPTION	PROPERTIES
		AA7075	WC	SIC
1	Particle size	–	20 µm	50 µm
2	Purity	98.5%	99%	99.5%
3	Density	2.81 g/cm3	15.63 g/cm3	3.21 g/cm3
3	Melting point	635ºC	2730ºC	2870ºC
4	Thermal conductivity	130 W/(m-K)	110 W/(m-K)	490 W/(m-K)
5	Tensile strength	240 MPa	344 MPa	390 MPa
6	Hardness	94 HV	800 HV	2563 HV
7	Compressive strength	340 MPa	1404 MPa	2000 MPa
8	Youngs modulus	71.7 GPa	530 GPa	90 GPa

Figure 1
EDAX result of AA7075, WC, SiC and Sc.

Figure 2
SEM image of WC, SiC and Sc.

2.2. Fabrication and ageing of AL7075 HMMC

Ultrasonic-assisted SCP was used for manufacturing AA7075 HMMC to improve wettability between the matrix and reinforcement materials and to achieve homogeneous of reinforcements in the AA7075 material [³²]. The process parameters for SCP with ultrasonic assistance were optimized using the Taguchi technique. Stirring duration (5, 10, and 15 minutes), melting temperature (700, 725, and 775ºC), stirring speed (200, 225, and 250 rpm), and AA7075/x wt.% WC/x wt.% SiC (6, 8, and 10) were selected as input parameters for optimization, considering the inclusion of scandium at 0.25 and 0.5 wt.%. The reinforcements, WC and SiC, were weighed in equal proportions by wt.% employing an electronic weighing balance with an least count of 0.001 g. Table 3 presents the Taguchi-based L9 orthogonal array (OA) for optimization and displays the L9 OA with input parameters. The acquired AA7075 was cut into small fragments and put into a crucible furnace for melting.

Table 3
L9 Orthogonal array with parameters for input.

EX. NO	MELTING TEMPERATURE ºC	BLENDING SPEED REV/MIN	STIRRING DURATION MINUTES	WT.% OF WC AND SIC	WT.% OF SC
1	700	200	5	6	0.25 & 0.5
2	700	225	10	8
3	700	250	15	10
4	750	200	10	10
5	750	225	15	6
6	750	250	5	8
7	800	200	15	8
8	800	225	5	10
9	800	250	10	6

The aluminum alloy 7075 was heated for melting, and the temperature was maintained for a certain duration to reduce the density of the molten AA7075, which helped minimize defect formation. The WC and SiC reinforcements were preheated at 400ºC for 30 minutes to remove moisture and enhance wettability. The preheated WC and SiC reinforcements were then fed into the crucible furnace for mixing with the molten AA7075. To improve wettability between SiC, WC, and AA7075, 1 wt.% of Mg was added. Additionally, scandium was introduced at 0.25 and 0.5 wt.% into the molten AA7075 in the crucible furnace. The stirrer of the SCP setup was used to ensure uniform mixing of the reinforcements with the AA7075 matrix. The ultrasonic sonicator probe was dipped in the molten mixture, inducing high-frequency vibrations at 20 kHz. This cavitation effect, generated by high-frequency energy, facilitated homogeneous of the reinforcing particles. The melting process was conducted in an inert atmosphere to prevent oxidation. The die was preheated to 450°C to mitigate potential contract effects. The mixed AA7075/WC/SiC HMMC was injected into the cavity of mold for casting. The same steps was repeated to fabricate AA7075 HMMC according to the L9 OA combinations, incorporating 0.25 wt.% and 0.5 wt.% Sc. The optimized AA7075/SiC/WC HMMC, based on microhardness response, was further subjected to the aging process. In this process, the selected AA7075/SiC/WC HMMC was initially heated to 520°C for 2 hours and subsequently quenched in water. Additionally, the composite was aged at four different temperatures (250, 300, 350, and 400°C) for a fixed time of 3 hours and then cooled in a furnace environment. The ultrasonic-assisted SCP setup and the casting process flow are illustrated in Figures 3 and 4.

Figure 3
Stir casting setup.

Figure 4
Process flow for casting of AA7075 HMMC.

2.3. Testing of composites

The manufactured composites were cut into ASTM-standard testing dimensions using wire-cut EDM without causing dislocation. The produced AA7075 HMMCs, as per the L9 Orthogonal Array, and the optimized aged AA7075 HMMC were cleaned with acetone and further polished using abrasive sheets (400, 600, 800, and 1200 grit) to achieve a surface roughness of 1 µm. The roughness was measured using a BAKER SRT100 surface roughness tester. Keller’s reagent was applied to enhance contrast in the obtained results. The fabricated AA7075/SiC/WC HMMC specimens were subjected to EDAX (model JSM-6100) and SEM (CX-200TM) analysis to determine elemental composition and confirm the homogeneous of SiC and WC in the AA7075 matrix. The microhardness test was performed on AA7075 HMMCs, fabricated according to the L9 Orthogonal Array, as well as the optimized aged AA7075 HMMC, following ASTM E384. The Vickers microhardness test was conducted under a 300 g load for 15 seconds using a Vickers microhardness tester. Three trials were conducted to determine the microhardness of AA7075-based HMMCs with the addition of 0.25 and 0.5 wt.% scandium. The XRD bulk specimens with 10 x 10 x 10 mm were prepared from manufactured aged AA7075 Hmmcs by utilizing wire cut EDM (Mec Tech Machines & Tools, DK-7740). The Rigaku-Ultima IV X-ray diffraction from Tokyo, Japan apparatus with specifications of a 0.02°/s scanning rate, 40 kV, 30 mA, 20 to 100° Bragg angles, and a Cu target of 1.5406 Å Kα radiation. Standard diffraction patterns were used for comparison to identify and analyze the peaks, a process conducted with Malvern Panalytical’sX’PertHighScore Plus software (2009). The optimized aged AA7075/SiC/WC HMMC was examined using XRD equipment (Bruker’s) to identify present elements and their peak locations. Aged AA7075/8 wt.% SiC/8 wt.% WC HMMCs subjected to different aging temperatures (250, 300, 350, and 400ºC) were analyzed using an optical microscope (LABOMED Lx400) to confirm the formation of precipitates. The aged AA7075 HMMCs were further subjected to microhardness testing as per ASTM E384 [³³]. A tensile test was conducted on the aged AA7075-based HMMCs to determine tensile strength following ASTM E08 [³⁴]. The test was performed using a UTM (ATEUTM20T), and three trials were conducted to obtain an average tensile strength. SEM analysis was executed on the broken tensile specimens to examine the fracture surface. A compression test was conducted on aged AA7075-based HMMCs to determine compressive strength, following ASTM E09 [³⁵]. Three trials were conducted to obtain an average compressive strength. Impact testing was carried out for three times to determine the average impact strength of aged AA7075-based HMMC. The pin-on-disc (DUCOM) tribometer was used to evaluate the friction coefficient and wear loss of aged AA7075 HMMCs under varying applied loads (10, 20, 30, and 40 N) at a constant sliding velocity of 2 m/s and a sliding distance of 1500 m, following ASTM G99-05 [³⁶]. In this research, applied load was taken as variable because of within wear testing, the applied load exerts the most significant influence compared to lubrication and environmental factors.The load directly governs contact pressure, material deformation, and the operative wear mechanisms. Increased loads intensify stress on contact surfaces, accelerating wear through processes like plastic deformation, adhesion, and delamination. Although lubrication mitigates friction, and environmental conditions like temperature and humidity affect oxidation and corrosion, these factors primarily modulate wear behavior rather than establishing the fundamental wear rate. Conversely, the applied load directly dictates the intensity of surface interactions, often driving transitions from mild to severe wear regimes. Consequently, while lubrication and environmental conditions are relevant, the applied load remains the paramount factor in wear testing due to its direct control over the core mechanisms of wear. In wear testing, the applied load is the most critical factor when compared to sliding velocity and distance. The load directly controls contact stress, the active wear mechanisms, and the extent of material deformation. Higher loads increase surface pressure, leading to phenomena such as plastic deformation, material transfer, and frictional heating, all of which accelerate wear. Importantly, the applied load also determines the shift between mild and severe wear regimes, significantly influencing the wear rate. While sliding velocity can affect frictional heating and oxidation, and sliding distance impacts cumulative material loss, their effects are less immediate and significant compared to the applied load. The composite pin used for the wear test were finished with a surface roughness of 1 µm. The friction coefficient was calculated as the ratio of frictional force to the applied load, while wear loss was determined by measuring the weight difference of the wear test specimen before and after testing. The worn surface and wear debris were examined using EDAX and SEM analysis. Test specimens for this research are displayed in Figure 5.

Figure 5
Prepared test samples of non aged and aged AL7075/8wt.%WC/8wt.%SiC HMMC under 0.5wt. %Sc.

3. RESULTS AND DISCUSSION

3.1. Metallurgical examination for L9 OA based specimens

The EDAX test results of the L9 OA-based fabricated AA7075 HMMC are shown in Figure 6. The presence of the mixed wt.% of WC, SiC, and the elemental composition of the AA7075 matrix material was verified through the EDAX analysis. The SEM images of the L9 OA-based AA7075 HMMCs with 0.5 wt.% Sc inclusion confirm the incorporation of reinforcements in the AA7075 matrix. Moreover, increasing the melting temperature promotes a homogeneous of reinforcements within the composites, with a melting temperature of 750°C facilitating this uniform dispersion due to the lower density of molten AA7075 HMMCs [³⁷, ³⁸].

Figure 6
EDAX test result of L9 OA based AA7075 HMMCs (L₃, L6 and L₉).

An increase in stirring speed also aids in achieving uniform reinforcement dispersion, with a stirring speed of 750 rpm proving effective. Conversely, lower melting temperatures and excessively high stirring speeds can disrupt the uniform distribution of reinforcements, leading to density irregularities. SEM images reveal that as the stirring speed increases, the distribution of reinforcements becomes more uniform. This uniformity is further enhanced by raising the melting temperature from 700 to 750°C; however, reinforcement buildup is observed at 800°C [³⁹]. When key parameters such as a higher stirring speed, moderate melting temperature, and an optimal wt.% of WC and SiC are combined, the denser reinforcements are evenly dispersed. The moderate melting temperature also helps prevent porosity by maintaining optimal density. A stirring speed of 750 rpm ensures high density and uniform dispersion of reinforcements within the AA7075 matrix.

From the SEM image of SEM image of L9 OA based AA7075 HMMCs under 0.5 wt.% Sc with 20 µm magnification, it can be concluded that more number of grains are formed and these grains are evenly distributed and it causes to improve the properties of mechanical and tribological of the aged AA7075 HMMCs under 0.5 wt.% Sc and the SEM images of aged AA7075 HMMCs are displayed in Figure 7.

Figure 7
SEM image of L9 OA based AA7075 HMMCs under 0.5 wt.% Sc with 20 µm magnification.

3.2. Optimization on ultrasonic assisted stir casting process parameter of AA775 HMMCs under Scandium addition

The optimization of the ultrasonic-assisted SCP parameters for AA7075 HMMCs with Scandium addition was performed using the Taguchi approach, based on their hardness. Minitab 19 software was employed to optimize the SCP parameters under 0.25 wt.% and 0.5 wt.% Sc. The L9 OA with input and response parameters is displayed in Table 4. The microhardness comparison of the L9 OA-based AA7075 HMMCs with Scandium addition is revealed in Figure 8. The obtained microhardness results reveal that the incorporation of Scandium, in weight percentages from 0.25 to 0.5, significantly enhances the microhardness of AA7075/WC/SiC HMMCs through different strengthening mechanisms. Scandium forms fine, uniformly dispersed Al3Sc precipitates, which effectively act as barriers to dislocation movement during deformation, thereby increasing hardness. These Al3Sc precipitates create a coherent interface with the aluminum matrix, enhancing material strength by limiting dislocation motion without causing substantial lattice distortion. Additionally, Scandium promotes grain refinement in AA7075 by facilitating nucleation during solidification, resulting in smaller grain sizes, a process known as grain boundary strengthening or the Hall-Petch effect. As the grain size decreases, hardness increases due to the presence of a higher number of boundaries for grain, which hinder the movement for dislocation [⁴⁰].

Table 4
L9 OA based parameters for input and response for optimization on AA7075 HMMCs.

EX. NO	STIRRING SPEED RPM	MELTING TEMPERATURE ºC	STIRRING DURATION MINUTES	WT.% OF WC AND SIC	WT.% OF SC
					0.25	0.5
1	200	700	5	6	98	118
2	225	700	10	8	106	124
3	250	700	15	10	108	120
4	200	750	10	10	102	120
5	225	750	15	6	105	122
6	250	750	5	8	119	138
7	200	800	15	8	107	118
8	225	800	5	10	112	120
9	250	800	10	6	105	123

Figure 8
Comparison of micro hardness of L9 OA based AA7075 HMMCs under Scandium addition.

Additionally, incorporating tungsten carbide (WC) and silicon carbide (SiC) reinforcements alongside scandium enhances hardening through load transfer and particle strengthening mechanisms. In this process, the hard ceramic particles absorb part of the applied load, reducing the stress on the softer aluminum matrix. Therefore, the combined effects of precipitation hardening, grain refinement, and load sharing from the reinforcements lead to a significant increase in microhardness with higher scandium content.

3.2.1. Effect of input parameters for ultrasonic assisted SCP produced AA7075 HMMCs on micro hardness

The effect of input parameters for ultrasonic-assisted SCP on the microhardness of AA7075 HMMCs is analyzed using the Taguchi approach. Table 5 presents the response table for the means of microhardness. The response table shows that the optimal combination of input process parameters for fabricating AA7075.

Table 5
Table for response of means of micro hardness.

LEVEL	MELTING TEMPERATURE (ºC)	STIRRING SPEED (RPM)	STIRRING DURATION (MINUTES)	WT.% OF WC AND SiC
1	120.7	118.7	123.7	121.0
2	125.0	122.0	122.3	125.0
3	120.3	125.3	120.0	120.0
Delta	4.7	6.7	3.7	5.0
Rank	3	1	4	2

HMMC under 0.5 wt.% Sc inclusion consists of a higher level of stirring speed (250 rpm), a medium level of temperature of melting (750ºC), a medium level of wt.% of WC with SiC, and a lower level of stirring duration (5 minutes).

The sequence of dominant of input process parameters on the microhardness of AA7075 HMMCs under 0.5 wt.% Sc inclusion is as follows: stirring speed (rpm), wt.% of WC and SiC, melting temperature (ºC), and stirring duration (minutes). Figure 9 illustrates the main effect plot for the microhardness of AA7075 HMMCs. The plot reveals that increasing the melting temperature from 700 to 750ºC improves the microhardness, whereas the microhardness decreases when the temperature is raised from 750 to 800ºC. The microhardness of AA7075/WC/SiC composites improves as the temperature increases from 700 to 750ºC, largely due to better wettability, improved particle distribution, and refined grain structure, all of which contribute to enhanced load transfer and material strength. However, increasing the temperature beyond 750ºC leads to excessive grain growth, particle clustering, and the formation of brittle intermetallic compounds, which negatively impact mechanical properties. Additionally, thermal stress and the uneven distribution of reinforcement particles cause a reduction in microhardness by weakening the matrix-reinforcement bond. Increasing the speed of stirring from 200 to 250 rpm enhances the microhardness. Raising the stirring speed from 200 to 250 rpm improves the microhardness of AA7075/WC/SiC composites by facilitating better dispersion of WC and SiC within the AA7075 matrix. Increased stirring rates lead to more efficient mixing, achieving a more even distribution of particles and reducing the risk of clustering. This uniformity improves the transfer of load between the matrix and the WC with SiC reinforcements, thereby increasing mechanical properties such as hardness. Additionally, a higher stirring speed refines the grain size of the matrix, further boosting microhardness. Increasing the sliding duration from 5 to 15 minutes decreases the microhardness of AA7075 HMMCs. Extending the duration for sliding from 5 to 15 minutes leads to a decrease in the microhardness of AA7075/WC/SiC composites due to wear mechanisms and thermal effects. Longer sliding times generate increased frictional heat, which can soften the matrix material and compromise the bond between the reinforcing particles (WC and SiC) and the matrix. Additionally, extended sliding may wear down the reinforcing particles, leading to their loss or fragmentation, which diminishes the overall effectiveness of the reinforcements. The combined effects of thermal softening and wear contribute to a reduction in microhardness as the structural integrity of the composite is progressively weakened. The wt.% of WC and SiC enhances microhardness from 6 to 8 wt.% and decreases from 8 to 10 wt.% WC and SiC. The improvement in microhardness of AA7075/WC/SiC composites from 6 to 8 wt.% of WC and SiC is primarily attributed to the effective reinforcement provided by these particles, which enhances load transfer and strengthens the matrix. Within this range, the uniform distribution of particles improves the overall structural integrity and resistance to deformation. However, when the weight percentage rises from 8 to 10%, a decline in microhardness occurs due to potential particle agglomeration and reduced interaction between the matrix and the dispersed particles, causing to clustering. This clustering can create weak spots in the composite, ultimately diminishing its hardness and overall mechanical properties [⁴¹].

Figure 9
Main effect plot for micro hardness.

3.2.2. Confirmation test

From the obtained optimization results, it can be confirmed that a higher level of stirring speed (250 rpm), a medium level of temperature of melting (750ºC), a medium level of wt.% of WC and SiC, and a lower level of stirring duration (5 minutes) are the optimum process parameters for the ultrasonic-assisted SCP in manufacturing AA7075 HMMC under 0.5 wt.% Sc inclusion. Hence, the better combination of AA7075 HMMCs is AA7075/8 wt.% WC/8 wt.% SiC HMMCs, which is fabricated under a speed of stirring 250 rpm, a melting temperature of 750ºC, with a stirring duration of 5 minutes. The better combination of AA7075 HMMCs is fabricated under the inclusion of 0.25 wt.% Sc and 0.5 wt.% Sc, and they are subjected to microhardness tests. The obtained microhardness for 0.25 wt.% Sc inclusion and for 0.5 wt.% Sc inclusion are 119 VHN and 138 VHN, respectively. The better combination of AA7075 HMMCs is subjected to optical microscope tests to confirm grain refinement. Figure 10 displays the macrostructure of AA7075/8 wt.% WC/8 wt.% SiC HMMC under 0.25 and 0.5 wt.% Sc. The grain size is smaller with 0.5 wt.% addition when compared to 0.25 wt.% Sc.

Figure 10
OM image of AA7075/8wt.%WC/8wt.%SiC under 0.25wt.%Sc and 0.5wt.%Sc.

The EDAX test results reveal that aluminum, zinc, silicon, tungsten, The grain size of the AA7075/8 wt.% WC/8 wt.% SiC composite with 0.5 wt.% Sc is typically smaller compared to the 0.25 wt.% Sc when produced using ultrasonic-assisted stir casting. Scandium promotes grain refinement by forming Al₃Sc particles, which act as nucleation sites during solidification. A higher scandium content (0.5 wt.%) increases the number of these nucleation points, resulting in finer grains. Additionally, the ultrasonic-assisted method enhances grain refinement through cavitation, which helps to evenly distribute the reinforcement particles (WC and SiC) and disrupt the dendritic structure.

The combined effects of more Sc and ultrasonic processing lead to finer grains, while the lower Sc content (0.25 wt.%) results in fewer nucleation sites and consequently larger grains. The EDAX test was conducted on the manufactured better combination of AL7075 HMMC under 0.5 wt.% Sc to verify the mixed wt.% of elements. The result of the EDAX test is shown in Figure 11. and carbide elements are present in the manufactured 8 wt.% SiC and WC reinforced AL7075 HMMC. The microstructure of the manufactured AL7075/8 wt.% SiC/8 wt.% WC was analyzed using an SEM test, and the SEM image of the manufactured 8 wt.% SiC and WC reinforced AL7075 HMMC with the inclusion of 0.5 wt.% Sc is displayed in Figure 12. The SEM image of the aged AL7075/8 wt.% SiC/8 wt.% WC HMMC confirms the uniform dispersion of SiC and WC reinforcements in the matrix of AL7075 material. Oxide formation occurred on the surface of the manufactured 8 wt.% SiC and WC reinforced AL7075 HMMC during the atmospheric cooling process. The optical microscope examination was conducted on the aged AL7075-based HMMC at different aging temperatures (250, 300, 350, and 400ºC) to confirm the formation of precipitates after the aging process. Figure 13a-e depicts the OM images of the aged AL7075/8 wt.% SiC/8 wt.% WC HMMC.

Figure 11
EDAX image of AL7075/8wt. % SiC/8wt. %WC under 0.5wt. % Sc.

Figure 12
SEM image of AL7075/8wt. % SiC/8wt. %WC under 0.5wt. %Sc.

Figure 13
a–e: Optical Microscope image of aged AL7075/8wt.%WC/8wt.%SiC HMMC under 0.5wt. %Sc.

Figure 13a displays the OM image of the non-aged AL7075-based HMMC. The dispersion of SiC and WC in the AL7075 matrix material is confirmed by the OM image of the non-aged AL7075 HMMC. Figure 12a-e shows the uniform dispersion of SiC and WC in the base AL7075 material, as well as the formed precipitates at different aging temperatures. Furthermore, the number and volume of precipitates formed increase with an aging temperature of 400ºC. Figure 14a-e displays the XRD results of non-aged and aged 8 wt.% SiC and WC reinforced AL7075 HMMC. The XRD of the non-aged AL7075 HMMC confirms the presence of the matrix, reinforcement materials, and MgZn₂ (magnesium–zinc) phase [⁴²]. Increasing the aging temperature from 250ºC to 300ºC enhances the volume of precipitate formation. However, the number and volume of precipitates formed at an aging temperature of 350ºC are lower. The base material Al – (98-006–2688) – 001 and 110 and reinforcements WC - (03-1096) – 001 and 110 and SiC - 42-1360 – 111-220 peaks are matched with the JCPDS no and hkl planes for confirming the presence [⁴³,⁴⁴,⁴⁵].

Figure 14
a: Non aged AL7075 HMMC. b: Aged AL7075 HMMC at 250ºC. c: Aged AL7075 HMMC at 300ºC. d: Aged AL7075 HMMC at 350ºC. e: Aged AL7075 HMMC at 400ºC.

Aging of the AL7075 HMMC at 250ºC confirms the presence of reinforcements, the matrix, and precipitates such as MgZn₂ (magnesium–zinc) and Mg₂Si (magnesium–silicon)..The formed intermetallics phases MgZn₂ – (34-0457) – 1 1 2 – 0 2 1, Mg₂Si – (35-0773) – 111 are matched and confirmed with literaure [⁴⁶, ⁴⁷]. The matrix AL7075, reinforcements (SiC and WC), and large volumes of formed precipitates such as MgZn₂ and Mg₂Si are confirmed in the aged AL7075 HMMC at 300ºC. The Al₂Cu (aluminum–copper) precipitate forms along with the matrix and reinforcements during aging of the AL7075 HMMC at 350ºC. The matrix AL7075, reinforcements (SiC and WC), and formed precipitates such as Al₂Cu and Mg₂Si are confirmed in the aged AL7075 HMMC at 400ºC. The formed intermetallics phase Al2Cu – (25-0012) – 110 – 220 are matched and confirmed the presence [⁴⁸].

3.3. Testing on mechanical properties

The tensile strength, microhardness, compressive strength, and impact strength of the non-aged and aged AL7075 HMMCs were determined using Vickers hardness equipment, a UTM, and an impact testing machine. The microhardness of the aged AL7075 HMMC at various temperatures is displayed in Table 6. Figure 15 illustrates the effect of aging temperature on AL7075 HMMC. Increasing the aging temperature from 250°C to 300°C improves the microhardness of the AL7075 HMMC. At an aging temperature of 350°C, the microhardness value decreases, but it increases again at an aging temperature of 400°C. The MgZn2 and Mg2Si precipitates, with their high volume fraction, enhance the microhardness of the aged AL7075 HMMC, while the Al2Cu precipitate reduces the microhardness at 350°C. The MgZn₂ and Mg₂Si precipitates in the aged AL7075 hybrid metal matrix composites (HMMCs) enhance microhardness due to their large volume fraction and strong interaction with dislocations. The microhardness of Al7075/8wt.%SiC/8wt.%WC hybrid metal matrix composites (HMMCs) is significantly influenced by aging temperature due to the complex precipitation dynamics of strengthening phases. Increasing the aging temperature from 250°C to 300°C enhances microhardness by accelerating the formation and ensuring a uniform distribution of fine, coherent precipitates such as η (MgZn₂) and Al₃Sc within the Al7075 matrix. These precipitates hinder dislocation motion, thereby strengthening the material. At 350°C, over-aging results in the coarsening of precipitates and a loss of coherence with the matrix, diminishing their reinforcing effect and reducing microhardness. However, when the temperature is raised to 400°C, microhardness increases again, likely due to the dissolution of larger precipitates, the re-precipitation of more stable phases like Al₂Cu,and improved interfacial bonding between the Al7075 matrix and the SiC and WC reinforcements. This enhanced bonding enables better load transfer, contributing to the observed rise in hardness. The interplay between precipitate formation, coarsening, dissolution, and interfacial bonding dictates the microhardness variations with aging temperature, highlighting the importance of precise control over the aging process to optimize the material’s mechanical performance.

Table 6
Micro hardness of AL7075 HMMC.

S.NO	2 HRS AGING DURATION	VICKERS MICRO HARDNESS
		TRIAL 1	TRIAL 2	TRIAL 3	AVG
1	Non aged	138	139	137	138
2	Aged at 250°C temperature	149.2	148.8	149.4	149.2
3	Aged at 300°C temperature	160	159	158	159
4	Aged at 350°C temperature	146	145.6	145.2	145.6
5	Aged at 400°C temperature	152.4	152	152.8	152.4

Figure 15
Variation of micro hardness of aged AL7075 HMMC.

These precipitates act as effective barriers to dislocation movement, enhancing resistance to deformation and consequently improving hardness. While Al2Cu precipitates also contribute to strengthening, they are less effective in hindering dislocations than MgZn2 and Mg2Si. Additionally, Al2Cu precipitates may reduce microhardness over time due to over-aging or softening from extended heat exposure. Therefore, the presence and volume of MgZn2 and Mg2Si result in greater hardness compared to Al2Cu. Moreover, the microhardness of the aged AL7075 HMMC is enhanced at 400°C due to the formation of the hard Mg2Si precipitates. The silicon element in the Mg2Si precipitate further extends the microhardness enhancement [⁴⁹]. The hard Mg2Si precipitates prevent dislocation movement with higher internal resistance, while Al2Cu precipitates reduce microhardness due to their softer characteristics. The tensile strength of the non-aged and aged AL7075 HMMC is displayed in Table 7. The variation in tensile strength of the non-aged and aged AL7075 HMMC at various temperatures is shown in Figure 16. The tensile strength of Al7075 hybrid metal matrix composites (HMMCs) is significantly influenced by aging temperature, mainly through the process of precipitation hardening.

Table 7
Tensile strength of AL7075 HMMC.

S.NO	2 HRS AGING DURATION	TENSILE STRENGTH (MPa)
		TRIAL 1	TRIAL 2	TRIAL 3	AVG
1	Non aged	286	286.5	287	286.5
2	Aged at 250°C temperature	299.2	298	298.6	298.6
3	Aged at 300°C temperature	326.4	327.2	328.4	327.3
4	Aged at 350°C temperature	315.2	315	316.2	315.5
5	Aged at 400°C temperature	320.1	320	320.6	320.2

Figure 16
Variation of tensile strength of aged AL7075 HMMC.

Increasing the aging temperature from 250°C to 300°C enhances tensile strength by promoting the formation and even distribution of fine, coherent precipitates like MgZn₂ and Mg₂Si, which obstruct dislocation motion the primary cause of plastic deformation. These precipitates also form strong bonds with the SiC/WC reinforcements and the Al7075 matrix, improving load transfer efficiency and achieving maximum tensile strength, typically around 300°C. At 350°C, over-aging occurs, causing precipitates to grow and lose their coherence with the matrix, weakening their strengthening effect and possibly leading to the formation of less effective phases, such as Al₂Cu, which reduces tensile strength. A rise in temperature to 400°C can restore tensile strength due to the dissolution of coarser precipitates, the re-precipitation of finer, more stable phases, and possibly enhanced bonding between the matrix and reinforcements. Although the precipitates at 400°C are less effective than at 300°C, the combination of improved bonding and a more favorable distribution of remaining precipitates can still result in increased tensile strength compared to the over-aged condition. Thus, the changes in tensile strength highlight the intricate balance between precipitate formation, coarsening, dissolution, and matrix-reinforcement interactions, with 300°C being the optimal temperature for achieving the highest tensile strength. The high load-bearing capacity of the reinforcements enhances the tensile strength of the aged AL7075 HMMC. The increase in aging temperature of AL7075 HMMC enhances the tensile strength from 250°C to 300°C.

A decrease in tensile strength of AL7075 HMMC is observed at the aging temperature of 350°C. Further, the tensile strength of AL7075 HMMC increases at 400°C. The tensile strength is improved due to the hard characteristics of MgZn2 and Mg2Si precipitates, which have better bonding with the reinforcement and matrix material. The highest tensile strength is achieved at the aging temperature of 300°C. The soft characteristics of Al2Cu reduce the tensile strength of aged AL7075 HMMC at 350°C [⁵⁰].

The tensile strength of the aged AL7075 HMMC is further improved due to the precipitation strengthening effect, where hard MgZn2 and Mg2Si precipitates form during aging and hinder dislocation movementThese precipitates establish a robust interface with the matrix and reinforcement, effectively blocking dislocations and enhancing the resistance of material to stress-induced deformation. Their high volume fraction and strong bonding with the matrix also contribute to better load transfer, further boosting tensile strength. At an aging temperature of 300°C, these precipitates achieve optimal size and distribution, maximizing the strengthening effect. In contrast, the softer Al₂Cu precipitates are less effective at restricting dislocations and may coarsen with prolonged aging, leading to over-aging and a reduction in tensile strength due to material softening. Figure 17 displays the SEM image of the fractured surface of the optimized combination of aged and non-aged AA7075 HMMCs with the inclusion of 0.5 wt.% Sc.

Figure 17
Displays the fractured surface SEM image of better combination of aged and non aged AA7075 HMMCs under the inclusion of 0.5wt.%Sc with 100 µm scale.

The compressive strength of both non-aged and aged AL7075 HMMCs is displayed in Table 8, and its variation is depicted in Figure 18. The compressive strength of the aged AL7075 HMMC increases from 250°C to 350°C and from 350°C to 450°C due to the formation of MgZn₂ and Mg₂Si precipitates. The hard nature of the silicon element offers high load-bearing capacity, and hence the aged AL7075 HMMC at 300°C aging temperature exhibits higher compressive strength. The large volume of hard-formed precipitates contributes to the enhanced strength. These high-hardness precipitates provide greater resistance to fracture, and dislocation movement is hindered by the formation of the hard Mg₂Si precipitates [⁵¹].

Table 8
Compressive strength of AL7075 HMMC.

S.NO	2 HRS AGING DURATION	COMPRESSIVE STRENGTH (MPA)
		TRIAL 1	TRIAL 2	TRIAL 3	AVG
1	Non aged	378	377	378	377.7
2	Aged at 250°C temperature	396	398	397	397.0
3	Aged at 300°C temperature	423	424	423	423.3
4	Aged at 350°C temperature	411	412	411	411.3
5	Aged at 400°C temperature	416	418	417	417.0

Figure 18
Variation of Compressive strength of aged AL7075 HMMC.

Additionally, higher bonding strength is achieved by the hard WC and SiC reinforcements. The load-bearing capacity of the AL7075 HMMC is also enhanced by these reinforcements. The formation of the soft Al₂Cu precipitate, however, reduces the compressive strength of the aged AL7075 HMMC. The main mechanism behind the improved compressive strength of the aged AL7075 HMMC as the temperature increases from 250°C to 450°C is precipitation strengthening. At elevated temperatures, MgZn₂ and Mg₂Si precipitates form and grow, leading to a more refined and uniform distribution within the matrix. These hard precipitates act as barriers to dislocation movement, strengthening the material by restricting dislocation motion under compressive stress. Additionally, the stronger bonding between the precipitates and the matrix enhances load transfer, improving the composite’s load-bearing capability. The refinement and uniform dispersion of precipitates at these temperatures help maintain a stable microstructure, significantly boosting resistance to compressive deformation. Aging temperature plays a crucial role in determining the compressive strength of Al7075 hybrid metal matrix composites (HMMCs) due to its influence on precipitation hardening. Increasing the aging temperature from 250°C to 350°C, and further to 450°C, generally enhances compressive strength. This improvement stems primarily from the formation and distribution of strengthening precipitates, such as MgZn₂ and Mg₂Si, within the Al7075 matrix. These precipitates hinder dislocation motion, the primary mechanism of plastic deformation, thus increasing the material’s resistance to compression. The inherent hardness of the silicon present in the SiC reinforcement also contributes to the composite’s ability to withstand compressive loads. The synergistic effect of these precipitates and the reinforcement enhances the material’s overall compressive strength. An optimal balance of fine, coherent precipitates, maximizing strengthening, is often achieved around 300°C, leading to peak compressive strength at this temperature.

The impact strength of both non-aged and aged AL7075 HMMC is displayed in Table 6. The comparison of impact energy for non-aged and aged AL7075 HMMC at various temperatures is shown in Figure 9. The highest impact energy is achieved at an aging temperature of 400°C. The combination of MgZn₂, Mg₂Si, and Al₂Cu precipitates provides high shock resistance capability. The impact strength of the aged AL7075 HMMC is displayed in Table 9. The comparison of impact strength for aged AL7075 HMMC at various temperatures is presented in Figure 19. The impact energy of the non-aged and aged AL7075 HMMC increases as the aging temperature rises from 250°C to 300°C, reaching the highest impact energy at 400°C. However, the impact energy decreases at 350°C. The formation of MgZn₂ and Mg₂Si precipitates at aging temperatures of 250°C to 300°C improves impact energy due to their high load-bearing capability. The impact energy decreases at 350°C due to the formation of the very soft Al₂Cu precipitate. The bonding strength between the matrix and reinforcements, with the high-volume hard precipitate Mg₂Si and the soft Al₂Cu precipitate, leads to higher bonding strength, which results in the highest impact energy attainment [⁵²].

Table 9
Impactenergy of AL7075 HMMC.

S.NO	2 HRS AGING DURATION	IMPACT STRENGTH (J)
		TRIAL 1	TRIAL 2	TRIAL 3	AVG
1	Non aged	2.47	2.49	2.47	2.48
2	Aged at 250°C temperature	2.88	2.86	2.88	2.87
3	Aged at 300°C temperature	3.62	3.66	3.64	3.64
4	Aged at 350°C temperature	3.42	3.44	3.46	3.44
5	Aged at 400°C temperature	3.96	3.98	3.94	3.96

Figure 19
Variation of Impact strength of aged AL7075 HMMC.

The impact energy of both untreated and treated AL7075 HMMC samples generally rises as the aging temperature increases from 250°C to 300°C. This enhancement in toughness is linked to the development of MgZn₂ and Mg₂Si precipitates, which act as barriers to crack propagation and improve the material’s load-bearing capacity. The fine and uniform distribution of these precipitates strengthens the matrix by restricting dislocation motion while preserving sufficient ductility. As a result, the highest impact energy is typically achieved at 400°C. However, a decline in impact energy is observed at 350°C, primarily due to the formation of softer Al₂Cu precipitates. These precipitates encourage localized deformation and crack formation, reducing the composite’s ability to absorb impact energy and diminishing its overall toughnes.

3.4. Tribological properties

The tribological properties, such as the coefficient of friction (CoF) and wear loss (WL), were determined for non-aged and aged AL7075 HMMC. A pin-on-disc setup was used to determine the WL and CoF of the aged AL7075 HMMCs under wear test conditions with varied applied loads (10, 20, 30, and 40 N), a constant sliding velocity of 2 m/s, and a sliding distance of 1500 m [⁵³, ⁵⁴]. The observed wear loss is displayed in Table 10.

Table 10
Wear loss for varying applied of non aged and aged AL7075 HMMC under 0.5wt.%Sc.

APPLIED LOAD (N)	WEAR LOSS (mg)
	NON AGED	250°C	300°C	350°C	450°C
10	33	25	16	28	18
20	39	31	24	34	27
30	46	38	30	42	32
40	52	47	39	50	43

The comparison of non-aged and aged AL7075 HMMC under different applied load conditions is shown in Figure 20. The increase in aging temperature enhances wear resistance by reducing wear loss under the different applied load conditions.

Figure 20
Wear loss for applied load at different ageing temperature.

AA7075 reinforced with silicon carbide (SiC) and tungsten carbide (WC) in presence of Scandium were aged composites, exhibit superior tribological properties compared to conventional nanocomposite coatings. The presence of scandium enhances grain refinement, leading to a more robust and wear-resistant microstructure. Additionally, the inclusion of SiC and WC improves hardness, load-bearing capacity, and resistance to wear and abrasion. The aging process further stabilizes the microstructure, reducing the likelihood of micro-cracking under stress and ensuring long-term durability. As a bulk material, this composite provides uniform mechanical and tribological performance, making it highly suitable for demanding aerospace and automotive applications. The increase in applied load raises the wear loss due to the enhancement of detaching force for particle detachment. The breaking of bonds between the precipitates, reinforcements, and matrix is promoted with the increase in applied load [⁵⁵, ⁵⁶]. The formed precipitates resist particle detachment as the applied load increases. Wear resistance improves from 250°C to 300°C, but it decreases at 350°C. Further, wear loss decreases at an aging temperature of 400°C due to the high volume of formed precipitates. The highest wear resistance is observed at an aging temperature of 300°C due to the MgZn₂ and Mg₂Si precipitates with high volume. The stronger bonding and higher load-resisting properties of the formed precipitates improve wear resistance [⁵⁷]. The relationship between the CoF, aging temperature, and applied load is shown in Table 11. The comparison of CoF with aging temperature and applied load is depicted in Figure 21. The SEM images of wear debris and the worn surface of aged AL7075 HMMC at 300°C under an applied load of 40 N are shown in Figure 22c.

Table 11
CoF for varying applied of non aged and aged AL7075 HMMC.

APPLIED LOAD (N)	CoF (µ)
	NON AGED	250°C	300°C	350°C	450°C
10	0.36	0.41	0.45	0.39	0.42
20	0.39	0.46	0.51	0.43	0.46
30	0.44	0.51	0.56	0.47	0.51
40	0.49	0.57	0.62	0.54	0.59

Figure 21
CoF for applied load at different ageing temperature.

Figure 22
Worn surfaces of aged AL7075 HMMC at 300°C for various load conditions.

The mass loss per sliding distance correlated directly with the applied normal load, peaking at the highest load and progressively decreasing as the load was reduced. At 10 N, minimal wear occurred, primarily characterized by plastic deformation where the surface experienced strain without substantial material removal. Increasing the load to 20 N shifted the dominant wear mechanism to abrasion, with the hard reinforcement particles (SiC and WC) causing material loss through micro-cutting and scratching. At 30 N, a mixed regime of abrasion and delamination emerged, as cyclic stress induced subsurface cracks, resulting in partial detachment of surface layers. At 40 N, delamination became the primary wear mechanism, with extensive subsurface crack propagation leading to the separation of larger material fragments and a corresponding acceleration of surface degradation [⁵⁸].

Scanning electron microscopy (SEM) analysis corroborated plastic deformation, abrasion, and delamination as the primary wear mechanisms. Parallel grooves, likely resulting from interactions with hard asperities on the counterface or detached particles, were observed on most sample surfaces. Abrasion dominated at lower loads, with the grooves widening and deepening with increased wear. While abrasion remained prevalent at 20 N and 30 N due to the presence of the hard reinforcing particles (acting as load-bearing constituents), at 40 N, SEM images revealed a fractured surface with characteristics of broken particles, indicative of high shear forces contributing to material failure [⁵⁹]. This fracture, likely induced by the increased normal load, was accompanied by noticeable plastic deformation in the worn surface. AA7075 reinforced with silicon carbide (SiC) and tungsten carbide (WC) in presence of Scandium were aged composites, exhibit superior tribological properties compared to conventional nanocomposite coatings. The presence of scandium enhances grain refinement, leading to a more robust and wear-resistant microstructure. Additionally, the inclusion of SiC and WC improves hardness, load-bearing capacity, and resistance to wear and abrasion. The aging process further stabilizes the microstructure, reducing the likelihood of micro-cracking under stress and ensuring long-term durability. As a bulk material, this composite provides uniform mechanical and tribological performance, making it highly suitable for demanding aerospace and automotive applications.

Adding SiC and WC to an aluminum matrix poses challenges such as poor wettability, leading to porosity and voids that weaken the material and cause cracks under stress. Particle clustering creates stress concentrations, resulting in uneven wear and surface deterioration. Weak interfacial bonding can cause particle pull-out, increasing abrasive wear and accelerating degradation. While these reinforcements enhance hardness, they also increase brittleness, making the composite prone to micro-cracking and fracture under high loads [⁶⁰]. The hard ceramic phases further raise friction, causing excessive heat and faster wear. Incorporating scandium (Sc) during stir casting, followed by aging treatment, addresses these issues. Sc improves wettability, reducing porosity and voids for a denser composite. It promotes uniform particle dispersion, minimizing clustering and stress concentrations. Sc-rich precipitates strengthen interfacial bonding, preventing particle pull-out and wear. Grain refinement enhances toughness while reducing brittleness, preventing fractures. Aging treatment further optimizes hardness and wear resistance while maintaining toughness.

The increase in aging temperature raises the CoF from 250°C to 300°C and decreases at 350°C. Additionally, the CoF increases at the aging temperature of 400°C. The highest wear resistance is attained at an aging temperature of 300°C due to the formation of hard precipitates, which offer resistance to motion. The soft nature of the Al2Cu precipitate reduces the CoF at all varying applied loads [⁶¹, ⁶²]. Al7075 HMMC wear resistance is strongly influenced by aging temperature and its effect on precipitation hardening. Increasing the aging temperature from 250°C to 300°C improves wear resistance by promoting the formation of fine, coherent MgZn₂ and Mg₂Si precipitates. These hard, well-bonded precipitates hinder dislocation movement, increasing resistance to surface deformation and material loss. Peak wear resistance at 300°C reflects the optimal precipitate size, distribution, and coherency. At 350°C, over-aging occurs, coarsening precipitates and reducing their effectiveness, thus decreasing wear resistance. While raising the temperature to 400°C may not restore peak wear resistance, it can still reduce wear loss due to a higher volume fraction of precipitates, which, despite being larger, still resist plastic deformation and material removal. Stronger precipitate-matrix bonding and inherent hardness further contribute to improved load-bearing capacity and wear resistance.

4. CONCLUSION

Effect of Scandium Addition and Precipitate Formation on the Mechanical and Tribological Properties of Ultrasonic-Assisted Stir-Cast AA7075 Hybrid Metal Matrix Composites has been investigated and the following conclusions were made,

1. The higher level of stirring speed (250 rpm), medium level of melting temperature (750ºC), medium level of wt.% of WC and SiC (8 wt.%), and lower level of stirring duration (5 minutes) were identified as the optimum combination of input process parameters for the ultrasonic-assisted stir casting process used to fabricate AA7075 HMMC with 0.5 wt.% Sc inclusion.

2. The manufacturing of the optimal combination of AA7075/8 wt.% WC/8 wt.% SiC HMMC with 0.5 wt.% Sc addition was accomplished using the ultrasonic-assisted stir casting method.

3. The fabricated AA7075 HMMC was subjected to an aging process at various aging temperatures (250, 300, 350, and 400°C).

4. The metallurgical properties of the non-aged AL7075 HMMC were analyzed using EDAX and SEM equipment.

5. Additionally, the metallurgical properties of both non-aged and aged AL7075 HMMC were analyzed using OM and XRD equipment.

6. The presence of elements was confirmed using EDAX, and the uniform dispersion of reinforcements was confirmed using SEM.

7. The formed precipitates were identified through XRD and OM analysis. The tensile strength, microhardness, impact strength, and compressive strength were highest at an aging temperature of 300°C due to the formation of MgZn₂ and Mg₂Si precipitates.

8. The highest wear resistance was achieved at an aging temperature of 300°C due to the high volume of MgZn₂ and Mg₂Si precipitates, and the highest CoF was attained in the aged AA7075 HMMC at an aging temperature of 300°C under the applied load of 40 N.

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Publication Dates

• Publication in this collection
  07 Apr 2025
• Date of issue
  2025

History

• Received
  15 Oct 2024
• Accepted
  17 Feb 2025