Review of Wear and Mechanical Characteristics of Al-Si Alloy Matrix Composites Reinforced with Natural Minerals

Abstract

Al-Si alloys are vital in the aerospace and automotive industries due to their high strength-to-weight ratio, excellent ductility, and superior corrosion resistance. These properties, along with good thermal conductivity, low thermal expansion, and enhanced wear resistance due to silicon, make them ideal for lightweight, high-performance components like engine parts exposed to harsh conditions and thermal cycling. In recent years, the development of aluminium metal matrix composites using Al-Si alloys as the base material has gathered significant attention. These composites are engineered by integrating various reinforcing particles into the aluminium matrix, which results in remarkable improvements in the wear resistance, hardness, and overall mechanical performance of the material. The stir casting process, a well-established and cost-effective method, is frequently employed to ensure a uniform distribution of these reinforcing particles within the matrix. This review delves into the influence of different types of reinforcing particles on the properties of Al-Si alloy-based AMCs. The incorporation of these reinforcements has been shown to significantly enhance wear resistance, reduce friction, and improve the overall strength and toughness of the composites, making them ideal candidates for high-performance applications in the automotive and aerospace sectors. Moreover, this review highlights the challenges associated with the fabrication of these composites, such as achieving a homogeneous particle distribution and minimizing porosity. It also discusses the latest advancements in processing techniques aimed at overcoming these challenges. Additionally, this review addresses the potential environmental and economic benefits of using natural reinforcements, which not only reduce material costs but also contribute to sustainable manufacturing practices.

1. Background

Aluminium metal matrix composites (AMCs) are engineered materials that consist of an Al matrix with a reinforced secondary phase, typically a material with higher strength, stiffness, or wear resistance. Al alloys used in developing AMCs are driven by their appealing properties, such as light weight, superior corrosion resistance, low thermal expansion coefficient, high strength, recyclability, flame-retardant capabilities, and ease of casting for mass production. These attributes have positioned Al as a preferred material in the automotive and aerospace industries. This is particularly significant as the transportation industry faces challenges such as enhancing vehicle safety at a reasonable cost, reducing emissions, and improving fuel economy. However, the limited wear resistance, susceptibility to seizure, and galling of Al and its alloys restrict their broader application [1,2,3,4,5]. The integration of reinforcements into the Al matrix significantly enhances their mechanical properties, making them suitable for applications where high strength-to-weight ratios are critical. The history of AMCs can be traced back to the 1960s when initial research focused on incorporating ceramic particulates into Al to improve wear resistance. Over the decades, the field has evolved, with a growing interest in using natural reinforcements. Current trends in AMC research emphasize sustainability, cost-effectiveness, and the development of materials with tailored properties for specific applications. Figure 1 presents two bar charts illustrating the research work on AMCs based on the number of publications. Figure 1a shows the number of AMC-related publications by different countries or regions. It is evident from the chart that a few countries dominate the research landscape in this field. The leading country, represented by the tallest bar on the left, has published nearly 5000 papers on AMCs, followed by a gradual decline in publications by other countries as we move from left to right. This indicates that the research on AMCs is concentrated primarily in a handful of countries. Figure 1b illustrates the annual trend of publications on AMCs from 1980 to 2022. The figure shows the number of publications per year. The chart reveals a steady increase in research output over time, with a noticeable acceleration from the early 2000s onwards. The number of publications peaked between 2015 and 2022, reaching over 1200 in some years. This trend indicates a growing interest and research activity in AMCs, especially in recent decades, likely driven by the increasing demand for advanced materials in various industries.

2. Application

AMCs utilize natural and synthetic reinforcements to enhance their properties for various applications. Natural reinforcements offer eco-friendly and biodegradable options with high strength-to-weight ratios, making them suitable for automotive parts and sustainable building materials.

The diagram highlights the diverse applications of AMCs across several industries, demonstrating their critical role in enhancing performance and durability. In the aerospace sector, AMCs are used in components such as satellites, helicopter rotors, rockets, and fighter fins, where their lightweight yet strong properties are essential for high-performance aerospace and defense systems. In the aviation industry, AMCs are crucial for aero-engine components due to their ability to withstand high temperatures and improve engine efficiency. In the automotive and traffic sectors, AMCs find applications in brake systems and automobile engines, where they help reduce weight, enhance wear resistance, and improve overall vehicle performance. Beyond the aerospace and automotive industries, AMCs are also utilized in optical instruments and electronic packaging, where their high thermal conductivity and structural integrity make them ideal for managing heat and protecting sensitive electronics. In other fields, such as nuclear power and military applications (e.g., guided missiles), the exceptional strength and resistance of AMCs to extreme environments ensure safe and reliable operation under harsh conditions as shown in Figure 2.

Table 1. Applications of Al alloys reinforced with various materials in different industries.

Alloy	Reinforcement	Application	References
Al	Al2O3 & SiC	Armour, nozzle	[6]
Al	Al2O3	Engine block, brake disc, piston, fuel pipe, rack housing, suspension arm, brake calliper, pump case, flange, connecting rod	[7,8,9]
Al	GNPs	Piston engines, wheels, electric motor housing	[10]
AlSi12	2D-Cf	Electronic packaging	[11]
AlSi12	10% fly ash	Manifolds, cylinder heads, water pump housings	[12,13]
A356	Al–B–Mg	Automotive piston, sewerage pipes, brake rotors, paper mill rolls, textile mill rolls, nozzles, and liners for IC engines	[14]
AA7050	31% B4C	Automotive, aerospace, military, and nuclear industries	[15]
AA6061	fly ash, Gr	Worm gear production in the aerospace industry	[16]
Al7075	SiC	Wear-resistant components	[17]
Al356	TiB2	Automobile and aerospace industries	[18]
A380	Zircon	Automotive industry	[19]
LM13	20 wt.% graphite	Electronic industry, electronic, packaging, heat sinks, heat spreaders, base plates, coolers, discs, and rings	[20]

comprehensively overviews various Al alloys reinforced with different materials and their respective applications across industries. It lists specific Al alloys, reinforcement materials, and industrial applications. The table provides a summary of various aluminum alloys reinforced with different materials and their applications across industries. Reinforcements like Al₂O₃, SiC, GNPs, fly ash, and B₄C enhance the properties of these alloys for specific uses. For instance, graphene nanoplatelets (GNPs) improve thermal and mechanical properties, making the alloy suitable for pistons, wheels, and electric motor housings. Similarly, AlSi₁₂ reinforced with carbon fiber is utilized in electronic packaging, and A356 with Al-B-Mg is employed in components like automotive pistons and brake rotors. The reinforcements are crucial for enhancing wear resistance, thermal management, and mechanical strength, making these alloys applicable in sectors like automotive, aerospace, military, electronics, and industrial manufacturing. Each alloy–reinforcement combination is supported by corresponding references in the literature.

3. Case Studies and Challenges

Cho et al.’s [21] study investigates the effects of zircon particle size on friction stability, friction oscillation, and wear in brake linings. The results show that larger zircon particles (ZR75, ZR140) enhance friction stability by forming robust primary contact plateaus at the sliding interface, resulting in smooth surfaces and less destruction at elevated temperatures. Conversely, smaller particles (ZR1, ZR6) produce unstable, transient contacts, leading to increased friction oscillation and higher lining wear due to the rolling action of these particles. The wear patterns on brake linings and counter disks also varied with particle size; smaller zircon particles caused rapid lining wear but less disk abrasion, while larger particles caused significant disk wear through two-body abrasion. These findings suggest a trade-off between improving friction stability and reducing wear, with potential for optimizing brake lining performance by adjusting zircon particle size to balance friction characteristics and durability. The study highlights challenges in friction stability for Al-Si alloy brake linings reinforced with zircon particles. Smaller zircon particles lead to inconsistent friction stability and significant wear through three-body abrasion, while larger particles enhance stability but increase wear on counter disks. Thermal degradation is also a concern, as smaller particles lack reinforcement at high temperatures. The varying dynamics of contact areas, with larger particles forming stable plateaus and smaller ones causing transient contacts, further affect braking performance and noise levels. Future research should focus on optimizing particle sizes and improving thermal stability to balance friction stability and wear resistance. Sadagopan et al.’s [22] study on the SiC reinforced AMC brake rotor provides a comprehensive analysis of its mechanical, thermal, and wear characteristics, highlighting its advantages over conventional cast iron (CI) rotors. The study reports a maximum deflection of 0.277 × 10⁻⁴ m and stress distributions that are well within the yield strength limits, ensuring the safety of the design. Thermal analysis shows significantly lower operating temperatures for the AMC rotor, with a maximum temperature of 180.88 °C, compared to 497.35 °C for the gray cast iron rotor, thereby reducing thermal fatigue and brake fade. The microstructure analysis indicates a fine-grained structure with a uniform distribution of SiC particles, which contributes to improved hardness. The Brinell hardness tests reveal a remarkable increase from 77 BHN in the as-cast specimen to 132 BHN in the age-hardened specimen, enhancing wear resistance. Wear tests confirm that the AMC exhibits lower wear rates and frictional forces compared to CI, making it suitable for brake applications. Furthermore, dynamometer tests demonstrate that the AMC rotor has a shorter braking distance of 18.28 m at 50 km/h compared to 20.81 m for the CI rotor, showcasing its superior performance. Working with 20% silicon carbide-reinforced aluminum matrix composites (SiC AMCs) for brake rotor applications presents several challenges. A primary concern is achieving strong interfacial bonding between the silicon carbide and aluminum matrix, as weak bonds can compromise mechanical properties. Additionally, the differing thermal expansion rates between SiC and aluminum can lead to cracking during thermal cycling. Machining these composites is also challenging; the hardness of the SiC particles can cause excessive wear on cutting tools, necessitating more frequent tool changes and potentially increasing production costs. Uniform dispersion of the SiC particles within the aluminum matrix is crucial to ensure consistent mechanical properties, but achieving this can be complex. Optimizing heat treatment parameters is another hurdle, as improper conditions can adversely affect the composite’s performance. Furthermore, the high hardness and strength of SiC-AMCs may reduce ductility, complicating their ability to absorb impacts effectively. Standardizing testing methods for evaluating performance in real-world conditions is essential but adds complexity to the research and development process. Dyzia [23] examines the refining and modification of the AlSi7Mg2Sr0.03 aluminum matrix composite, emphasizing the benefits of adding magnesium (Mg) and strontium (Sr). The modifications enhance the mechanical properties and improve the wettability of the liquid metal, as evidenced by reduced solidification temperatures and better particle distribution. The study confirms the stability of the composite suspension, with uniform SiC particle distribution in cast pistons. It also highlights the challenges of machining these composites, demonstrating that optimal cutting parameters can mitigate wear on tools like polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN). Tribological tests further showcase the composite’s superior wear resistance compared to unmodified alloys, reinforcing its suitability for industrial applications, particularly in air compressors. While production costs are higher, the research lays a solid groundwork for future studies on the performance benefits of these composite pistons. The production of aluminum matrix composites, particularly those reinforced with SiC, presents several challenges that must be addressed to optimize their performance. One significant challenge is achieving the uniform distribution of reinforcement particles within the matrix, as inconsistent particle distribution can lead to weak points in the final product and affect mechanical properties. Additionally, the refining process, while effective at removing non-metallic inclusions, can introduce complexities such as maintaining optimal gas flow rates and refining times to ensure the complete removal of hydrogen content. Chemical modifications, including the addition of magnesium and strontium, require precise control of concentrations to avoid adverse effects on the alloy’s properties, such as brittleness or excessive fluidity during casting. Moreover, the machining of these composites poses challenges due to their hard machinability, which can lead to rapid tool wear and increased production costs. These factors necessitate careful consideration of process parameters and machining strategies to enhance the reliability and cost-effectiveness of aluminum matrix composites in industrial applications.

4. Fabrication Techniques

These fabrication methods highlight the diverse approaches available for processing AMCs, each suited to different applications and performance requirements. Techniques like powder metallurgy and stir casting are widely used for large-scale production, while laser-based additive manufacturing and electron beam melting offer precision for high-performance industries such as the aerospace and biomedical sectors. The properties of AMCs primarily depend on the processing method as shown in Figure 3, making the choice of production process critical for meeting industrial demands and achieving the desired functional properties [15,24,25,26]. Table 2 presents a comprehensive overview of various fabrication techniques used to produce AMCs. These techniques are categorized into solid-state, liquid-state, in situ, deposition, additive manufacturing, mechanical alloying, and liquid metal infiltration processes. Each category provides a detailed description of the method, its advantages, and disadvantages, with corresponding references. One of the main drawbacks of producing AMCs is the high cost of reinforcement materials, the potential for non-uniform distribution of reinforcement within the matrix, and the significant investment costs associated with specific methods. Therefore, developing cost-effective manufacturing techniques is crucial to broadening their applications [12,27,28]. The primary methods for fabricating bulk AMCs are represented in Table 3 [29]. A literature review, mainly focusing on metal matrix composites, indicates a gap in comprehensive reviews specifically addressing AMCs produced via stir casting. In the words of Kamyar et al. [30] of the articles published since 2000, 10 have dealt with manufacturing techniques. Kaczmar et al. [31] on the other hand, did not thoroughly review all liquid state processing techniques. They merely touched on manufacturing procedures in passing. Torralba et al. [32] concentrated on AMC production via powder metallurgy, while Miracle [33] focused primarily on the properties of AMCs that make them suitable for various applications, without detailed discussions on production methods. Similarly, Ye et al. [34] explored the manufacturing of metal matrix composites using metal injection molding, while Qu et al. [35] focused on metal matrix composites for thermal management applications. Kala et al. [27] provided insights into the mechanical and tribological properties of stir-cast Al-based composites.

Table 2. Overview and categorization of different AMC fabrication techniques.

Production Method	Category	Method	Description	Benefits	Drawback	Refs.
Solid-State	Powder Metallurgy	Blending, compaction, sintering	Blending Al powder with reinforcement particles, compacting, and then sintering.	Good particle distribution, near-net shape	Limited to small or simple shapes	[36,37,38,39]
	Friction Stir Processing (FSP)	Friction stir welding	Localized, intense plastic deformation with frictional heat; reinforcement is stirred into the matrix.	Fine grain structure, minimal porosity	Limited to sheet or plate materials	[40,41,42]
Liquid-State	Stir Casting	Stirring reinforcement into melt	Reinforcements are stirred into molten Al, followed by casting.	Cost-effective, suitable for mass production	Possible agglomeration and poor wettability	[43,44,45,46]
	Squeeze Casting	High-pressure infiltration	Liquid metal is injected under pressure into a mold containing reinforcements.	Improved bonding and reduced porosity	High cost, complex equipment	[47,48,49,50]
	Compocasting	Semi-solid-state mixing	Reinforcement particles are mixed with semi-solid Al slurry before complete solidification.	Better reinforcement distribution than stir casting	Process control challenges	[16,51,52,53]
	Rheocasting	Semi-solid processing	It uses controlled cooling to create semi-solid slurries, which are then cast.	Improved mechanical properties, less segregation	Equipment and process control complexities	[54,55,56]
In Situ	Reaction Processing	Exothermic reaction	Reinforcement is formed in situ within the Al matrix by chemical reactions during fabrication.	Strong interface, good load transfer	Difficult to control reinforcement size	[57,58,59]
Deposition Techniques	Spray Deposition	Cold or thermal spray	Particles are sprayed onto a substrate and then consolidated	Good bonding, minimal thermal damage	Complex equipment, high operational cost	[60,61]
Additive Manufacturing	Laser-Based	Selective Laser Melting (SLM)	Powder bed fusion uses a laser to selectively melt areas to form the desired shape.	High precision, customizability	High cost, limited build size, and speed	[62,63]
	Electron Beam Melting (EBM)	Powder bed electron beam fusion	It is similar to SLM but uses an electron beam instead of a laser.	High precision, less residual stress	High cost, vacuum environment required	[64]
Mechanical Alloying	High-Energy Ball Milling	Mechanical impact and mixing	Reinforcement particles are mixed with Al powder using high-energy collisions in a ball mill.	Fine microstructure, homogeneous dispersion	High energy consumption, contamination risk	[65,66,67]
Liquid Metal Infiltration	Pressure Infiltration	Infiltration of porous preform	Liquid metal infiltrates a preformed porous network of reinforcement under pressure.	Good bonding, tailored reinforcement structure	Complex process, risk of voids	[68,69,70]
Ex Situ Methods	Hot Isostatic Pressing (HIP)	High-pressure, high-temperature process	Consolidation of powdered materials in a sealed environment at high pressure and temperature.	Reduces porosity, improves mechanical properties	High cost, complex equipment	[71]
	Spark Plasma Sintering (SPS)	Electric current assisted sintering	Powder is compacted and sintered with electric pulses, enhancing densification and reducing sintering time.	Rapid process, high density achieved	Limited to small batch sizes, high equipment cost	[6,72,73,74]

Table 2. Overview and categorization of different AMC fabrication techniques.

Production Method	Category	Method	Description	Benefits	Drawback	Refs.
Solid-State	Powder Metallurgy	Blending, compaction, sintering	Blending Al powder with reinforcement particles, compacting, and then sintering.	Good particle distribution, near-net shape	Limited to small or simple shapes	[36,37,38,39]
	Friction Stir Processing (FSP)	Friction stir welding	Localized, intense plastic deformation with frictional heat; reinforcement is stirred into the matrix.	Fine grain structure, minimal porosity	Limited to sheet or plate materials	[40,41,42]
Liquid-State	Stir Casting	Stirring reinforcement into melt	Reinforcements are stirred into molten Al, followed by casting.	Cost-effective, suitable for mass production	Possible agglomeration and poor wettability	[43,44,45,46]
	Squeeze Casting	High-pressure infiltration	Liquid metal is injected under pressure into a mold containing reinforcements.	Improved bonding and reduced porosity	High cost, complex equipment	[47,48,49,50]
	Compocasting	Semi-solid-state mixing	Reinforcement particles are mixed with semi-solid Al slurry before complete solidification.	Better reinforcement distribution than stir casting	Process control challenges	[16,51,52,53]
	Rheocasting	Semi-solid processing	It uses controlled cooling to create semi-solid slurries, which are then cast.	Improved mechanical properties, less segregation	Equipment and process control complexities	[54,55,56]
In Situ	Reaction Processing	Exothermic reaction	Reinforcement is formed in situ within the Al matrix by chemical reactions during fabrication.	Strong interface, good load transfer	Difficult to control reinforcement size	[57,58,59]
Deposition Techniques	Spray Deposition	Cold or thermal spray	Particles are sprayed onto a substrate and then consolidated	Good bonding, minimal thermal damage	Complex equipment, high operational cost	[60,61]
Additive Manufacturing	Laser-Based	Selective Laser Melting (SLM)	Powder bed fusion uses a laser to selectively melt areas to form the desired shape.	High precision, customizability	High cost, limited build size, and speed	[62,63]
	Electron Beam Melting (EBM)	Powder bed electron beam fusion	It is similar to SLM but uses an electron beam instead of a laser.	High precision, less residual stress	High cost, vacuum environment required	[64]
Mechanical Alloying	High-Energy Ball Milling	Mechanical impact and mixing	Reinforcement particles are mixed with Al powder using high-energy collisions in a ball mill.	Fine microstructure, homogeneous dispersion	High energy consumption, contamination risk	[65,66,67]
Liquid Metal Infiltration	Pressure Infiltration	Infiltration of porous preform	Liquid metal infiltrates a preformed porous network of reinforcement under pressure.	Good bonding, tailored reinforcement structure	Complex process, risk of voids	[68,69,70]
Ex Situ Methods	Hot Isostatic Pressing (HIP)	High-pressure, high-temperature process	Consolidation of powdered materials in a sealed environment at high pressure and temperature.	Reduces porosity, improves mechanical properties	High cost, complex equipment	[71]
	Spark Plasma Sintering (SPS)	Electric current assisted sintering	Powder is compacted and sintered with electric pulses, enhancing densification and reducing sintering time.	Rapid process, high density achieved	Limited to small batch sizes, high equipment cost	[6,72,73,74]

Figure 3. Illustration of the various methods used to process AMCs: Powder Metallurgy [75], Friction Stir Processing (FSP) [76], Stir Casting [77], Squeeze Casting [78], Compocasting [78], Rheocasting [79], Reaction Processing [80], Spray Deposition [78], Laser-Based Additive Manufacturing [81], Electron Beam Melting [82], High-Energy Ball Milling [83], Pressure Infiltration [10], Hot Isostatic Pressing (HIP) [84], and Spark Plasma Sintering (SPS) [85].

Figure 3. Illustration of the various methods used to process AMCs: Powder Metallurgy [75], Friction Stir Processing (FSP) [76], Stir Casting [77], Squeeze Casting [78], Compocasting [78], Rheocasting [79], Reaction Processing [80], Spray Deposition [78], Laser-Based Additive Manufacturing [81], Electron Beam Melting [82], High-Energy Ball Milling [83], Pressure Infiltration [10], Hot Isostatic Pressing (HIP) [84], and Spark Plasma Sintering (SPS) [85].

However, the literature clearly shows that few studies focus specifically on the stir casting process. Although Kumar and Menghani [86] discussed the challenges associated with stir casting, they did not address the use of natural reinforcements in stir casting design. Suthar and Patel [87] discussed processing issues in AMC production via stir casting machining, and applications, but their review differs from this one. The production and features of AMCs made by stir casting were briefly reviewed by Kumar et al. [88]. Shabani and Mazahery [89] established a novel technique dubbed the semisolid agitation process inside stir casting, which enhanced the mechanical properties of the composites. Similarly, Bhaskar et al. [90] reviewed the engineering and technical contests in producing metal matrix composites using the stir casting process. Several authors have reinforced alloys with zircon [91,92,93] and SiC [91,92]. For instance, some studies have explored the reinforcement of zircon particles and SiC particles in various alloys. Additionally, zircon particles have been used in different sizes and proportions, such as coarse particles, fine particles, and a combination of coarse and fine particles [94,95]. AMCs have been created using a variety of zircon particle combinations, including single and dual particle sizes. Stir casting was used by Das et al. [96] to incorporate zircon particles of different sizes and quantities into an Al-4.5 wt.% Cu alloy melt. They found that finer particles, measuring 15 and 65 μm, could only be scattered up to 20 wt.%, while coarser particles, measuring 90 and 135 μm, could be dispersed up to 30 wt.%. Remarkably, virtually no settling was seen when the composite was reinforced with 65 μm particles instead of coarser zircon particles. This was explained by the fact that finer particles might stay in suspension for longer than coarser ones. Several other writers have used this method to add zircon particles to Al-4.5 wt.% Cu alloy [97]. Al 6063 alloy and ZAS alloy were used as matrix to create a zircon-reinforced AMC, which were then produced following the fabrication method described by Banerji and his colleagues [97,98,99,100,101,102]. Al/zircon AMCs were studied by Pillai et al. [103] in both as-cast and forged states. After being immersed for 2 h at 475 °C, cylindrical castings were forged in a hydraulic press. The strength and fracture toughness of these AMCs were related to those of cast and forged Al/graphite AMCs, and ideal presentation zones were found.

Table 3. Summary of research studies on fabricating AMCs using the stir casting method.

Type of Reinforcement	Reinforcement	Fabrication Method	References
Synthetic	SiC	Stir casting	[104]
	Al2O3, TiO2	Stir casting	[105]
	B4C, Graphite	Stir casting	[106]
	Al2O3	Stir casting	[107]
	Y, Al-5Ti-1B	Stir casting	[108]
	SiC, Fly ash	Stir casting	[109]
	Ti, Sr	Stir casting	[110]
	SiC, B4C, Graphite	Stir casting	[111]
	Si, Al-20Fe, Al-10Mn	Stir casting	[112]
	B4C	Stir casting	[113]
Natural	Zircon (ZrSiO4)	Stir casting	[93,114,115,116]
	Rutile (TiO2)	Stir casting	[117,118,119,120]
	Ilmenite (FeTiO3)	Stir casting	[101,102,121,122,123,124,125]
	Sillimanite (Al2SiO5)	Stir casting	[126,127,128,129]
	Corundum (Al2O3)	Stir casting	[130,131]

Table 3. Summary of research studies on fabricating AMCs using the stir casting method.

Type of Reinforcement	Reinforcement	Fabrication Method	References
Synthetic	SiC	Stir casting	[104]
	Al2O3, TiO2	Stir casting	[105]
	B4C, Graphite	Stir casting	[106]
	Al2O3	Stir casting	[107]
	Y, Al-5Ti-1B	Stir casting	[108]
	SiC, Fly ash	Stir casting	[109]
	Ti, Sr	Stir casting	[110]
	SiC, B4C, Graphite	Stir casting	[111]
	Si, Al-20Fe, Al-10Mn	Stir casting	[112]
	B4C	Stir casting	[113]
Natural	Zircon (ZrSiO4)	Stir casting	[93,114,115,116]
	Rutile (TiO2)	Stir casting	[117,118,119,120]
	Ilmenite (FeTiO3)	Stir casting	[101,102,121,122,123,124,125]
	Sillimanite (Al2SiO5)	Stir casting	[126,127,128,129]
	Corundum (Al2O3)	Stir casting	[130,131]

5. Al-Si Alloy Matrix Composites

The strong castability, corrosion resistance, and good strength of Al-Si alloys make them widely employed. The matrix material’s characteristics are influenced by the silicon content, which affects properties like hardness and thermal stability. Rahvard et al. [132] showed that centrifugal casting of a commercial Al390 alloy, with magnesium (Mg) additions of 0, 6, and 12 wt.%, resulted in improved wear resistance. However, when silicon (Si) exceeds 20 wt.% in combination with Al, traditional solidification processes produce undesirable coarse and brittle silicon particles. Therefore, incorporating suitable reinforcements is essential for assessing AMCs’ mechanical, thermal, and tribological properties. AMCs can achieve a higher load-bearing capacity, improved wear resistance, and enhanced thermal stability by incorporating reinforcements. The type and morphology of the reinforcement, along with the interfacial bonding between the matrix and the reinforcement, are critical issues influencing the overall performance of the AMC. The choice of reinforcement depends on the desired properties of the final AMC. For instance, ceramic particulates are commonly used to enhance hardness and wear resistance, while carbon fibers are preferred for applications requiring high strength and stiffness.

Natural Minerals

On the other hand, natural minerals commonly used in AMCs include sea beach minerals such as FeTiO₃, Al₂SiO₅, TiO₂, ZrSiO₄, and Al₂O₃, favored for their excellent mechanical and thermal properties.

Table 4. Physical and mechanical properties of natural reinforcement materials.

Reinforcement	Density (g/cm3)	Hardness (Moh’s scale)	References
Zircon (ZrSiO4)	4.61	7.5	[114]
Rutile (TiO2)	4.23	6.0	[139]
Ilmenite (FeTiO3)	4.72	5.9	[101]
Sillimanite (Al2SiO5)	3.24	6.0	[140]
Corundum (Al2O3)	4.02	9.0	[130]

describes the physical and mechanical properties of natural reinforcement materials, focusing on their density and hardness. Zircon stands out as a dense material with high hardness, making it suitable for applications requiring durability. Rutile and ilmenite offer similar density and moderate hardness, indicating their potential for use in applications that balance strength and weight. Sillimanite, being lighter, still provides sufficient hardness for reinforcement purposes. Corundum, though less dense, is exceptionally hard, making it highly resistant to wear. These properties suggest that these materials are valuable for enhancing the mechanical performance of composites in various industrial applications. These natural mineral sources have gained attention due to their low cost and environmental friendliness. They offer a more sustainable alternative and are gaining popularity in applications where environmental impact is a concern. The morphologies of some natural minerals are shown in Figure 4. During the past few years, there has been increasing attention paid to the use of natural minerals in Al-Si alloy matrix AMCs, driven by the need for cost-effective and environmentally friendly alternatives. These materials are abundant and renewable and offer a sustainable way to enhance the properties of metal matrix composites. Natural minerals generally have high hardness [101,130,133,134], which can result in improved AMC strength. However, the mechanical characteristics (tensile strength and hardness) may not match those of AMCs reinforced with synthetic materials. To address this, researchers have explored AMCs, where natural minerals are used to balance performance and sustainability. The table contrasts natural and synthesized reinforcements in composite materials. Natural reinforcements are abundant and lower in cost but can have variable properties due to their natural source. They are often used in eco-friendly applications but may need refinement for consistency. Synthesized reinforcements, on the other hand, offer uniformity and can be engineered for specific applications, though they are more expensive and have a higher environmental impact. While natural reinforcements are more sustainable, synthesized materials are favored for high-performance, precision-engineered uses. The major differences between the natural and synthetic mineral are shown in

Table 5. Comprehensive comparison of natural and synthesized reinforcements in composite materials.

	Natural Reinforcements	Synthesized Reinforcements
Source	Extracted from natural sea beach sand (e.g., garnet, sillimanite)	Manufactured under controlled conditions (e.g., SiC, B4C)
Availability	Readily available; abundant in nature	Limited by production capacity; requires specialized processes
Cost	Generally low-cost due to natural abundance	Typically higher due to complex manufacturing techniques
Consistency	Variable composition depending on natural source	Highly consistent with controlled properties across batches
Mechanical Properties	May exhibit variability; dependent on natural quality	Uniform mechanical properties, tailored for specific applications
Performance Customization	Limited ability to modify or enhance properties	Can be engineered for specific applications (e.g., strength, conductivity)
Environmental Impact	Lower environmental footprint; minimal processing required	Higher environmental impact due to energy-intensive production
Applications	Typically used in cost-sensitive or eco-friendly applications	Used in high-performance, precise engineering applications
Processing	May require additional steps for purification or refinement	Direct control over particle size, shape, and purity
Sustainability	More sustainable due to natural sourcing	Less sustainable; resource-intensive production

. Using natural minerals also contributes to developing green AMCs, which align with global sustainability goals. By utilizing natural materials, these AMCs reduce the environmental impact of raw material production and the components’ end-of-life disposal. Khoshhal et al. [135] investigated the creation of Al₃O₃/TiC-Fe AMCs using FeTiO₃, graphite, and Al. Terry et al. [136] focused on a single-step carbo-thermic reduction process with coal to produce Fe-Ti (O, C) AMCs from Fe-FeTiO₃ or Fe-TiO₂ mixtures. They achieved effective dispersion of Ti (O, C) within the iron matrix by heating the mixtures at 1300–1600 °C in an argon environment. The separation of Ti (O, C) from the iron was improved by using fluxes like group II halides or BaSO₄. Preliminary scale-up studies were also conducted. Manikandan’s [137] study details the mixture of Fe-TiN-Al₂O₃ AMC powder from FeTiO₃ using an alumino-thermic reaction in a plasma reactor. Razavi et al. [138] investigated the production of Fe-TiC nano-AMCs using FeTiO₃ concentrate, Al powder, and carbon black. High-energy ball milling activated the mixtures, which were then synthesized through microwave heating.

6. Importance of Material Characterization and Testing

Material characterization and testing are fundamental for understanding the performance, behavior, and properties of Al-Si alloy matrix composites, which find extensive use in high-demand industries like the automotive and aerospace industries due to their excellent mechanical properties and lightweight nature [141,142]. These composites undergo various mechanical tests, including hardness testing, which assesses their resistance to localized deformation, and tensile testing, which reveals their ability to withstand forces under tension and bending. These tests help determine how the composites will perform under real-world operating conditions, ensuring they can endure the mechanical stresses typically encountered in their applications [27]. Microstructural characterization techniques, including Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), provide detailed insights into the internal structure of Al-Si composites. This characterization is essential for understanding the distribution and bonding of reinforcement particles like silicon and ceramic phases, as uniform dispersion significantly enhances mechanical properties such as tensile strength, hardness, and wear resistance. SEM offers high-resolution images to study the morphology and size of reinforcement particles, while EDS provides elemental mapping to ensure compositional homogeneity [143,144]. Tribological studies are critical for evaluating the wear and friction characteristics of Al-Si alloy matrix composites, particularly in applications where surface interactions are significant, such as engine pistons, brake discs, and gears. The wear resistance of these composites can be significantly improved by incorporating ceramic reinforcements such as SiC and Al₂O₃, making them suitable for high-friction environments [145,146,147,148]. Studies indicate that increasing the content of hard reinforcements enhances wear resistance, which is crucial for automotive applications where components are subjected to high levels of friction and wear. The coefficient of friction (COF) is an essential tribological parameter that affects energy efficiency and component longevity. Tribological testing reveals that Al-Si composites reinforced with ceramics demonstrate lower COF values, which can reduce energy losses during operation [149,150]. Furthermore, wear mechanisms may vary depending on the reinforcement type, with hard ceramic particles often leading to abrasive wear, while softer materials may experience adhesive wear [151,152,153].

7. How Natural Reinforcements Interact with the Al Matrix

Natural reinforcements interact with the Al matrix in AMCs through various mechanisms that significantly influence the produced material. The primary role of reinforcements is to enhance mechanical strength by efficiently transferring applied loads from the metal matrix to the reinforcement particles. This load transfer relies on strong interfacial bonding, which can be achieved through mechanical interlocking or chemical bonding, ensuring the reinforcement can effectively bear stress [126,154,155,156]. However, differences in the thermal expansion coefficients of the matrix and reinforcement generate internal stresses when subjected to temperature changes, potentially leading to microcracking if not managed properly [95,157]. Additionally, uniform dispersion of reinforcements is critical, as clustering can create weak points that lead to stress concentration and early failure, while even distribution improves mechanical strength and hardness. Reinforcements can also act as barriers to dislocation movement, strengthening the matrix through dislocation pinning, a process that increases yield strength [101,123,158]. However, at elevated processing temperatures, the reinforcement and matrix may chemically react, forming brittle intermetallic phases that reduce toughness. Reinforcements, particularly ceramic particles, also enhance wear resistance by preventing material loss during friction, a key factor in applications such as in the automotive and aerospace industries. Finally, reinforcements promote grain refinement by serving as nucleation sites during solidification, further improving the material’s strength and toughness. Thus, the interaction between reinforcement and matrix is a complex interplay that determines the overall performance of composites.

8. Environmental and Economic Considerations

The environmental and economic considerations of using natural versus synthetic reinforcements in AMCs present a complex trade-off. Natural mineral particulates typically have a lower environmental impact due to their renewable nature, biodegradability, and less energy-intensive production processes. However, challenges such as moisture absorption and uniform dispersion need careful management. Economically, natural reinforcements are often less expensive and increasingly valued in markets focused on sustainability, although they may offer lower mechanical performance compared to synthetic alternatives.

9. Microstructures Study

Microstructural studies of AMCs are critical for understanding the distribution, morphology, and bonding of reinforcement elements that influence the mechanical and wear performance. Pillai et al. [103] found that cast Al/ZrSiO₄ AMCs displayed a dendritic system, while forged Al/ZrSiO₄ AMC had a microstructure characterized by a mix of tiny grains and fractured dendrites. In Al-Si alloy AMCs reinforced with ZrSiO₄, the microstructure revealed a well-dispersed distribution of ZrSiO₄ particles throughout the matrix. When Li et al. [159] examined the microstructure of the Zn-4Al-3Cu alloy; they found a small layer of eutectic phase at the ZrSiO₄-Zn-Al matrix contact and a hypoeutectic dendritic phase made up of zinc-rich primary η-phase dendrites. In the as-cast state, coarse acicular intermetallic elements were observed along the primary Al dendrite boundaries of Al 6061 alloy AMCs reinforced with graphite and ZrSiO₄. Furthermore, an excellent dispersion of the scattered ZrSiO₄ particles in the matrix alloy was reported by Ejiofo et al. [160]. Well-dispersed ZrSiO₄ particles within the Al matrix were seen in the SEM images of the strengthened Al matrix AMC, which also displayed internal porosity [38]. As the ZrSiO₄ content increased, particle agglomeration and clustering were observed, leading to increased porosity, which hindered the densification of the AMC. In a study involving the spray forming process, optical micrographs showed the as-cast LM13 alloy with dendritic growth of α-Al grains (25–50 µm) and a eutectic phase between the dendritic arms. Localized chilling effects caused by implanted ZrSiO₄ led to the formation of elongated silicon needles at distances farther from the interface. Near the ZrSiO₄, approximately 3–10 µm-sized globular silicon elements were observed [19]. This aligned with findings from other researchers [96,161], who noted the formation of a secondary phase (Al₂Cu) in interdendritic regions when ZrSiO₄ and SiC particles were incorporated into Al alloy. In the same way, the LM13 alloy reinforcements caused globular, finely dispersed eutectic Si to develop close to the reinforced particles [91,92]. The effect of reinforcing LM13 alloy with coarse and fine ZrSiO₄ particles has also been investigated. While uniform distribution was observed, AMCs with 15 wt.% fine ZrSiO₄ particles (20–32 μm) exhibited clustering and porosity, confirmed by micrographs and coarse dendrites and silicon presence between dendrite arms. The eutectic Si shape changed from acicular to dense globular formations near the ZrSiO₄ particles, indicating a homogeneous distribution of particles in AMCs with dual particle size reinforcement. Dual particle size reinforcement showed good bonding between ZrSiO₄ particles and the Al matrix, with some bunching of fine particles, similar to single particle size reinforcement AMCs, investigated by Kumar et al. [93]. A unique technique for introducing ZrSiO₄ and TiB₂ particles into A356.1 alloy was presented by Abdizadeh et al. [162]. The Al-5% TiB₂ AMC had superior wettability. However, there was an aggregation of reinforcement particles in the Al-5% ZrSiO₄ AMC. Figure 5 illustrates the strong bonding between the reinforcement and matrix in LM13 alloy AMCs reinforced by ZrSiO₄ reinforcement, as reported by Panwar et al. [94]. In the case of Al-4.5 wt.% Cu alloy AMCs, the microstructure showed a copper-rich secondary phase (CuAl₂) close to the particle–matrix interface, as well as a cellular pattern with a uniform distribution of ZrSiO₄ reinforcement. An investigation using an Electron Probe Micro-Analyzer (EPMA) validated the distinctive zone at the edges of the ZrSiO₄ particles where Mg and Si segregated at the particle–matrix interface in the microstructure of Al–11.8% Si alloy AMC made by stir casting [100]. As a result of the scattered ZrSiO₄ particles serving as nucleation sites, micrographs of the Al alloy and AMC containing 10, 30 wt.% ZrSiO₄ reinforcement showed a notable refinement in the eutectic-Si. A finer proeutectic Al structure and more recurrent eutectic Si nucleation around the ZrSiO₄ particles were the outcomes of increasing the ZrSiO₄ content. A homogeneous dispersion of reinforcement within the Al matrix was also demonstrated by a pressure die-cast AMC incorporating 60 wt.% ZrSiO₄ particles (40–100 µm).

Prasad et al. [163] conducted an investigation on the influence of various chill ingredients on the microstructural evolution of Al alloy (LM13) AMCs reinforced with garnet and carbon particulates. The study highlights that the microstructure is highly dependent on the cooling rates during solidification. Faster cooling rates, achieved using materials with higher volumetric heat capacities like copper, lead to finer grain structures. In this research, adding 9 wt.% garnet and 3 wt.% carbon facilitated rapid solidification and a uniform dispersion of reinforcement particles. Copper-chilled AMCs showed a uniform dispersion of garnet and carbon particles with no voids or discontinuities, resulting in a finer microstructure and superior mechanical properties. In contrast, steel, iron, and silicon carbide chills resulted in relatively coarser grains and some accumulation of garnet particulates. Iron and silicon carbide chills produced the coarsest structures, suggesting slower cooling rates and less effective particle dispersion. Gupta et al. [102] focused on AMCs with reinforcement levels of 5 wt.% and 15 wt.% and mixing ratios of 1:3, 1:1, and 3:1. The authors reported that the dispersion of reinforcement particles contributed to the refinement of both eutectic silicon and primary Al. A higher reinforcement level was found to lead to greater refinement in the microstructure, with the AMC containing a higher concentration of B₄C particles demonstrating the most significant refinement. The research further explains that the density and thermal conductivity of the reinforced particles significantly influenced microstructural refinement. The lower density of B₄C particles compared to FeTiO₃ resulted in a higher number of reinforced particles, reducing inter-particle distance and restricting grain growth. This combined effect of lower thermal conductivity and higher particle concentration promoted a pinning effect, further refining the microstructure [125]. Sharma et al.’s [140] study highlights a significant reduction in primary silicon size across the AMCs, with sizes shrinking from 18.3 μm in the base alloy to 7.5 μm in the 15 wt.% reinforced samples. This refinement is attributed to both the stirrer action and the presence of Al₂SiO₅ particles, which act as heat sinks and create localized cooling effects that enhance solidification. In AMCs with 18 wt% Al₂SiO₅, the tendency for particle clustering increases, with finer particles showing a higher propensity to cluster compared to coarser ones. This clustering tendency significantly impacts the wear properties of the AMCs. Sharma et al. [164] presented a study on the reinforcement of LM30 alloy with Al₂SiO₅ particles, demonstrating that the high shearing action of the stirrer reduces particle settling, ensuring even dispersion, which is crucial for achieving superior wear resistance. The sample details are shown in

Table 6. Designation for various AMCs processed through the stir casting route [164].

Al2SiO5 wt.% in AMC	Designation of Dual Particle Size Reinforced AMCs with Al2SiO5 Size
	1:3	1:1	3:1
3	3DPS-1	3DPS-2	3DPS-3
6	6DPS-1	6DPS-2	6DPS-3
9	9DPS-1	9DPS-2	9DPS-3
12	12DPS-1	12DPS-2	12DPS-3
15	15DPS-1	15DPS-2	15DPS-3
18	18DPS-1	18DPS-2	18DPS-3

. The introduction of Al₂SiO₅ also affects the silicon morphology in the alloy; primary silicon undergoes refinement and changes from a needle-like to a globular form near the reinforcement particles. With their low thermal expansion coefficient, the ceramic particles act as heat sinks, promoting localized rapid cooling and solidification in the alloy matrix. This localized solidification leads to further refinement of primary silicon as the wt.% age of reinforcement rises from 6 wt.% to 15 wt.%, as shown in Figure 6a–h. However, the study also observed silicon-depleted zones due to restricted silicon diffusion in the matrix, which was accredited to the occurrence of Al₂SiO₅ particles. Additionally, with higher Al₂SiO₅ content, there is an increased tendency for particle clustering (as shown in Figure 6i,j), impacting the wear properties of the AMC. This work underscores the extensive impact of Al₂SiO₅ reinforcement on the solidification behaviour and microstructure of LM30 alloy, affecting its mechanical properties.

Elwan et al. [165] demonstrated that the accumulative roll bonding (ARB) process achieves a uniform dispersion of FeTiO₃ particles and fine microstructure. With increasing ARB cycles, the FeTiO₃ reinforcement is dispersed from the interfaces into the Al matrix, eliminating porosity and improving bonding. The study of Gupta et al. [102] revealed a dendritic structure with a reasonably uniform dispersion of TiO₂ reinforcement all over the Al matrix and no noticeable agglomeration. The addition of magnesium during fabrication creates a transient layer between the matrix and reinforcements, enhancing wettability and reducing surface tension, resulting in better dispersion of TiO₂ particles. A bi-stage addition method for the TiO₂ particles during stir casting helps control the viscosity of the molten matrix, preventing particle accumulation, a common defect in AMCs. AMCs with 1%, 2%, and 3% reinforcement exhibited that increasing the mass fraction of TiO₂ reduced the grain size of the matrix due to the pinning effect of the reinforcements at grain boundaries, which suppresses grain growth. Higher TiO₂ content resulted in finer grain structures and better mechanical properties. In the study by Abraham et al. [166], the macrostructural examination of AA6063/TiO₂ AMCs via FSP revealed a well-defined stir zone with no visible defects such as pinholes, tunnels, or cracks, which are common in FSP. The images confirm that the groove from the Al plate was uniform TiO₂ particle dispersion throughout the stir zone. SEM micrographs demonstrated effective TiO₂ distribution, with minimal agglomeration at higher contents, though more passes would be needed to further refine dispersion without inducing reactions that convert TiO₂ to other phases. The TiO₂ particles, mostly retained within the grain boundaries rather than aligning along them, underwent size reduction due to the high strain rate of FSP, enhancing mechanical properties through increased particle interaction. Optical and EBSD analyses of the AMC showed a considerable drop in grain size, attributed to dynamic recrystallization driven by FSP high deformation and temperature. TEM micrographs further revealed the presence of fine grains and dislocations, confirming practical grain refinement and the promotion of Zener pinning by TiO₂ particles. Gupta et al. [119] investigated the microstructural of Al₂SiO₅-reinforced AMC and TiO₂-reinforced AMCs. The solidification process affected the entrapment of mineral particles, which were influenced by thermodynamic and kinetic criteria at the solid–liquid interface. For the wavefront of the primary α-Al phase, Gibb’s free energy (ΔG) was reported to be greater than zero, causing the mineral particles to be pushed away from the primary α-Al phase and toward the Al-Si eutectic mixture region, as observed in the micrographs. In Al₂SiO₅-reinforced AMCs, some Al₂SiO₅ reinforcement was observed in the inter-dendritic areas of the primary α-Al phase, indicating that the particles had sufficient velocity to overcome the pushing forces of the solid wavefront and became entrapped in regions closer to the primary α-Al phase. In contrast, TiO₂ particles in AMCs were mainly present in the locality of the Al-Si eutectic combination, where they caused notable changes in silicon morphology. The presence of TiO₂ particles led to the transformation of silicon morphology from needle-like to a more rounded form, as well as the refinement of the silicon phase. This refinement restricted the grain growth of silicon and enhanced the AMC’s mechanical properties [133]. Kumar et al. [167] studied coarse garnet particles (5–15 wt.%) that are uniformly distributed within the matrix. Fine particles tend to agglomerate and form clusters as their concentration increases. The tendency for agglomeration and porosity, especially at higher reinforcement percentages, is attributed to sub-vortices forming during mechanical stirring and the entrapment of air bubbles. Garnet particles refine eutectic silicon. These particles increase the number of nucleation sites within the silicon. As a result, the dendrites become thinner. Due to their larger surface area, fine reinforcement provides more nucleation sites than coarse particles, leading to a refined microstructure but with increased clustering and porosity. Vamsi et al. [168] study reveals that preheating FeTiO₃ improves its bonding with the Al matrix, with optimal bonding observed at 800 °C. These findings highlight the impact of FeTiO₃ preheating on the AMC’s microstructural development and phase formation, leading to enhanced connection and material properties. Singhal et al. [101] revealed that the primary Si phase initially nucleates and raises into distinct morphologies, such as plates, stars, and needles. The introduction of FeTiO₃ particles as reinforcement creates nucleation sites that significantly refine the Si morphology, leading to a more excellent dispersion of reinforcement throughout the matrix. The authors note that the degree of refinement is directly proportional to the FeTiO₃ content, ranging from 5 wt.% to 15 wt.%. However, at higher reinforcement levels (20 wt.%), there is a noticeable increase in particle agglomeration, which may affect the AMC’s properties. Gupta et al. [134] effectively demonstrate the significance of thermal conductivity differences and inter-particle distances in determining the microstructure and grain refinement of AMCs. This study provides valuable insights into designing advanced AMC materials with enhanced act for numerous industrial applications. In the study by Singhal et al. [155], the microstructural effects of adding natural minerals, specifically Al₂SiO₅, and FeTiO₃, along with solid lubricants like tin (Sn) and graphite (Gr), to Al-Si alloy AMCs were examined. The addition of these natural minerals resulted in excellent homogeneity of the reinforcement within the Al matrix. This homogeneity was attributed to mechanical mixing, which exerted dynamic shear forces that prevented the dense ceramic particles from settling. The ceramic particles acted as nucleation sites for the silicon (Si) phase due to the substantial difference in thermal conductivity between the matrix and the reinforcement particles. This reduced the size of primary Si and the formation of finer, needle-shaped Si nucleates. Since refined Si is challenging, this refinement significantly enhances the mechanical properties of Al-Si alloys. The study further explored the impact of adding 1 wt.% Sn as a solid lubricant in an AMC with 10 wt.% natural minerals. Due to its lower solubility in Al and higher binding energy with vacancies, Sn was found to promote the diffusion and refinement of eutectic Si throughout the matrix. The lower melting point of Sn allowed it to encapsulate reinforcements in a liquid state, further refining the Si morphology. In another sample, the incorporation of 1 wt.% graphite (Gr) resulted in the refinement of primary Si morphology. Graphite acted as a solidification site for Si, and its poor wettability with LM30 alloy overcame the surface energy barriers during mechanical stirring, leading to a uniform distribution. AMC combining 0.5 wt.% Gr and 0.5 wt.% Sn with 10 wt.% Al₂SiO₅ and FeTiO₃ demonstrated further refinement of the primary faceted Si and needle-like eutectic Si phases, providing more nucleation sites and restricting the growth of the silicon primary phase. Singhal et al. [126] investigate the effects of various reinforcements on the microstructure of Al-Si AMCs. Their study reveals that incorporating 10 wt.% Al₂SiO₅ significantly refines the microstructure, transitioning eutectic silicon from acicular to rounded shapes and reducing primary silicon size by about 30%. Adding 1 wt.% tin further enhances refinement by filling surface pores and aiding in solidification. In contrast, 1 wt.% graphite contributes to wear resistance by forming a protective glaze layer despite its poor wettability and tendency to agglomerate. The combined effect of Al₂SiO₅ and graphite in the AMC results in the most significant alteration of primary silicon and improved mechanical properties. Maan et al. [130] investigated the fact that adding Al₂O₃ particles to LM30 refines both primary and eutectic Si morphologies. For instance, the average size of primary silicon is compact from 56 μm in the Al alloy to 14 μm in the 20 wt.% fine Al₂O₃ reinforced AMC, indicating a significant refinement, are shown in Figure 7. Additionally, silicon surrounding the Al₂O₃ forms complex Al silicate oxide, an intermetallic compound that improves the interfacial strength of the AMC. This increased interfacial strength improves the AMCs load-bearing capacity and wear resistance. Singhal et al. [154] conducted a detailed microstructural analysis incorporating 10 wt.% FeTiO₃ powder into the Al17Si alloy. This resulted in a more refined microstructure with uniformly distributed FeTiO₃ particles due to mechanical stirring that prevented particle settling. FeTiO₃ low thermal conductivity caused localized nucleation of Si on its surface, converting the acicular eutectic-Si to a more spherical form and reducing the size of the primary Si, thereby enhancing the AMC’s bulk hardness and wear resistance. Further, the accumulation of tin (Sn) improved the diffusion and refinement of eutectic Si within the matrix, owing to Sn high binding energy with defects. The AMC with Sn and FeTiO₃ showed further refinement and improved mechanical properties compared to the Al alloy. The inclusion of graphite (Gr) as reinforcement was also explored; graphite particles, despite poor wettability with the Al-Si alloy, provided nucleation sites for Si due to the thermal conductivity difference, forming a thin glaze layer that reduced metal-to-metal contact, thus enhancing wear resistance. The AMC containing Sn and Gr showed a more refined primary Si phase than the base alloy and other single-reinforced AMCs.

Singhal et al. [123] investigated the microstructure, revealing a uniform distribution of coarse and fine FeTiO₃ particles throughout the matrix. The study noted a transformation of secondary Si morphology from acicular to globular form due to the introduction of FeTiO₃ particles. FeTiO₃’s high melting temperature and low thermal conductivity compared to the Al alloy allows it to act as a nucleation site for molten Al-Si alloy, promoting silicon nucleation on the surface of FeTiO₃ particles. As a result, the intensity of the silicon phase increases around the FeTiO₃ particles, which creates obstacles for the diffusion process of Al and silicon after solidification. This phenomenon contributes to the refinement of primary silicon and eutectic Al-Si morphologies. The study also found that a higher concentration of tiny FeTiO₃ reinforcement produced more nucleation centers, leading to a more minor primary Si phase.

Nagpal et al. [169] demonstrated a significant impact of TiO₂ particles on the surface morphology and structural characteristics of the AMCs. The study reveals that TiO₂ particles are pushed by the solid–liquid interface during solidification, leading to their occupation in the interdendritic regions. As the reinforcement content increases, the distance between reinforcement in the Al matrix decreases, which hinders dislocation movement and enhances the AMCs’ hardness, as represented in Figure 8. The increased particle content also causes finer eutectic silicon to form closer to the TiO₂ particles, transforming the matrix from dendritic to cellular due to interference at the solid–liquid interface. A more uniform distribution of particles is observed with a 12 wt.% increase in TiO₂ content. This addition increases the melt viscosity, reducing convection and slowing the diffusion of solutes. This changes silicon morphology from needle-like to blunted due to slower cooling rates and restricted diffusion. In the study by Pethuraj et al. [170], the microstructural analysis reveals a uniform distribution of both primary and secondary reinforcements throughout the Al matrix. Notably, in the hybrid AMCs, the primary and secondary reinforcements are well-integrated, with secondary reinforcements effectively positioned between the primary ones. This uniform distribution was achieved by carefully selecting the stirring speed and time, which are crucial to prevent the lighter reinforcements from settling due to density differences. The similar size, shape, and thermal conductivity of the reinforcements facilitated their even distribution. Wadhwa et al. [171] showed that hardness was 6% FeTiO₃, 400 rpm stirring speed, and 8 h aging, while for tensile strength, it was 4% FeTiO₃, 350 rpm stirring speed, and 8 h aging. Under these conditions, the AMC showed a ~43.33% rise in hardness and a ~82.46% increase in tensile strength related to tin Babbitt. Yadav et al. [172] studied the microstructure of LM25 alloy and its AMC reinforced with red mud and Al₂SiO₅ reinforcement. Micrographs demonstrate minimizing agglomeration and enhancing wear resistance. These particles have a low thermal expansion coefficient, leading to temperature gradients that serve as heat sinks and hasten cooling while encouraging the nucleation of eutectic silicon near the particles. Furthermore, the silicon crystallizes around the reinforcement particles because their low thermal conductivity inhibits the matrix’s cooling rate. This results in a refined microstructure that improves the mechanical and wear properties of the AMCs. Minimal accumulation and a somewhat uniform distribution of reinforcement throughout the Al alloy were observed by Yadav et al. [128] between the reinforcing material and the base alloy, a robust mechanical bond. This effect is attributed to the thermal incompatibility between the Al₂SiO₅ particles and the base alloy, which created a temperature gradient at the particle–matrix interface. The thermal gradient caused the reinforcement particles to extract heat from the surrounding matrix, promoting localized solidification around the particles. Since silicon has a higher melting temperature than Al alloy, this phenomenon led to increased nucleation of silicon near the reinforcement particles.

10. Mechanical Properties of Natural Reinforced AMCs

The mechanical qualities of AMCs, including their tensile strength, hardness, and impact strength, can be significantly improved by adding natural reinforcements. The quality, distribution, and interfacial bonding of the reinforcement particles inside the matrix all significantly impact the mechanical performance of these AMCs. Studies have shown that incorporating natural ceramic particles, such as red mud or Al₂SiO₅, into an Al matrix can increase the hardness of the AMC, thereby enhancing its wear resistance. This makes natural-reinforced AMCs suitable for structural applications in the automotive and aerospace industries. However, the performance of these AMCs is often limited by factors such as particle orientation, moisture absorption, and variability in the properties of the natural reinforcements. Addressing these challenges through improved processing techniques, such as better dispersion and uniform distribution of particles, and surface treatments to enhance interfacial bonding can further improve the mechanical performance of natural-reinforced AMCs. Pillai et al. [103] found that for Al/ZrSiO₄ AMCs, the crack tip opening displacement (CTOD) and toughness strength decrease with increasing ZrSiO₄ volume fraction. However, toughness improves with ZrSiO₄ content from 6 to 12 vol.% in both cast and forged AMCs. A reduction in toughness with coarse sizes was observed, but toughness increased with higher tensile strength in as-cast AMCs. The maximum fracture toughness values were 15.2 kJ/m² for as-cast AMC with 125–180 μm ZrSiO₄ particles and 6 vol.% ZrSiO₄, and 15.6 kJ/m² for forged AMCs with the same particle size but 12 vol.% ZrSiO₄. Forged graphite AMCs showed improved toughness than forged ZrSiO₄ AMCs, attributed to crack deflection by the particulate phase in the forged samples. For cast Al/ ZrSiO₄ AMCs with particle sizes ranging from 180–250 μm and 6% ZrSiO₄ addition, the highest values for yield strength (YS), tensile strength, % reduction in area, and % elongation are 70 MPa, 87 MPa, 9.5%, and 9.3%, respectively. Forged AMCs show improved values of 87 MPa for yield strength, 112 MPa for tensile strength, 10% for % reduction in area, and 9.9% for % elongation [159]. The maximum tensile strength is achieved at 750 °C for Al-ZrSiO₄ and at 950 °C for Al-5% TiB₂, based on the variation in tensile strength of Al 356.1 alloy reinforced with ZrSiO₄ and TiB₂ particles. The enhanced hardness and tensile strength after adding TiB₂ are attributed to its superior work hardening at low strain and better wettability with Al. Since TiB₂ has more excellent wettability than ZrSiO₄, a higher amount of TiB₂ particles can be incorporated into the matrix, resulting in improved hardness and tensile strength of the matrix. as shown in Figure 9 [162]. Sivakumar et al. [173] also noted enhanced tensile strength and load-carrying capacity with garnet additions, with a 32 MPa increase in tensile strength at 15 wt.% garnet. Ramnath et al. [174] examined the mechanical properties of AMCs with varying amounts of ZrSiO₄ sand and fly ash. The tensile test results showed that a AMC with 1% ZrSiO₄ and fly ash achieved the highest tensile strength due to optimal reinforcement distribution. Hardness tests confirmed that this sample also exhibited superior hardness compared to others, attributed to the minimal reinforcement percentage enhancing resistance to indentation. Additionally, the flexural test indicated that a sample with 2.5% ZrSiO₄ and fly ash had the best flexural strength, demonstrating improved resistance to bending loads, as shown in Figure 10. The study concludes that while lower reinforcement percentages enhance strength and hardness, higher percentages improve flexural performance but may increase brittleness. Elwan et al. [165] conducted a study on the production of Al1050-FeTiO₃ AMCs mechanical properties. The research reveals that the AMC’s hardness and tensile strength improved with more ARB cycles. Specifically, for the Al-8 vol.% FeTiO₃ AMC, hardness reached approximately 75 HV and tensile strength peaked at 169 MPa after seven cycles. Additionally, while the elongation of the primary annealed material decreased sharply from 37% to 2.12% after the first cycle, it increased to 3.52% for the Al–8 vol.% FeTiO₃ AMC after seven cycles. Prabhu et al. [117] considered the mechanical properties of AA6061 AMCs reinforced with TiO₂ particles. For instance, the yield stress (YS) and ultimate tensile strength (UTS) increase with the addition of TiO₂ up to 3 wt.%, with YS reaching 112 MPa and UTS reaching 165 MPa. However, the elongation percentage, which indicates ductility, decreases from 8.8% for the base alloy to 7.1% for the AMC with 4 wt.% TiO₂. In addition to promoting grain refinement during the stir casting process, the hard TiO₂ particles serve as barriers to dislocation movement inside the softer Al matrix, which is the primary cause of the hardness augmentation. The hardness of the AA6061 base matrix increases by up to 50% with the inclusion of TiO₂ particles, which have higher stiffness and hardness than the matrix itself. The brittle characteristic of TiO₂, which causes strain hardening during deformation, is why adding TiO₂ particles lowers the AMC ductility. The ductility of the AMC seen in Figure 11 is further decreased by the presence of porosities, particularly at the interface between the matrix and TiO₂ particles. The addition of TiO₂ reinforcement boosted microhardness dramatically, according to a study by Abraham et al. [166]. Microhardness increased noticeably from 62 Hv at 0 vol% to 142 Hv at 18 vol%, indicating a 129% enhancement. The homogeneous dispersion of TiO₂ particles, which prevented dislocation motion by Orowan strengthening, was credited with improving microhardness. The UTS also increased with the addition of TiO₂ particles, peaking at 325 MPa for 12 vol% TiO₂, a 46.4% improvement compared to the unreinforced alloy (222 MPa). However, a further rise in TiO₂ content to 18 vol% caused a decline in UTS to 288 MPa, likely due to particle clustering, which reduced the effective load-bearing area. However, the elongation percentage decreased from 25.5% for the base alloy to 12% at 18 vol% TiO₂ due to the contrasting deformation behaviors of Al (highly deformable) and TiO₂ particles (rigid).

Vamsi et al. [168] explore the effect of FeTiO₃ reinforcement on the hardness and tensile properties of AMCs. The Vickers hardness demonstration that incorporating 5% FeTiO₃ into the metal matrix composites significantly enhances its hardness, which is further increased by preheating the FeTiO₃ particles to higher temperatures before mixing. The base metal matrix composites have a hardness of 68 HV, which increases to 287 HV when 5% FeTiO₃ is preheated at 200 °C and 384 HV when preheated at 800 °C. This improvement in hardness is attributed to the formation of new phases and grain refinement in the AMC due to the addition of FeTiO₃. The stress–strain curves reveal that the YS and UTS of the AMC increase with the preheating temperature of FeTiO₃ up to a certain point, after which the tensile strength decreases at very high temperatures, as shown in Figure 12. However, the AMCs become more brittle with increasing preheating temperatures, as evidenced by the reduced ductility. The FeTiO₃ particles split and form bonds with the matrix, enhancing structural integrity and bonding during processing.

Kumar et al. [175] described that the microhardness of Al-garnet AMCs increases with higher garnet content. Specifically, a 15 wt.% Al-garnet AMC shows a microhardness increase of around 34% compared to the as-cast base matrix, attributed to the hard garnet particles, which impede dislocation movement and enhance the matrix load-carrying capacity. The addition of garnet restricts plastic deformation, thus increasing hardness. Al-garnet AMCs show significant improvements with increased garnet content and tensile strength. The UTS rises from 181 MPa for base alloy to 252 MPa for 15 wt.% Al-garnet AMC, a 40% increase. However, elongation decreases from 11.6% to 8% with higher garnet content, indicating reduced ductility. Manjunath et al. [176] study reveals that AMCs with a particulate reinforcement ratio of 9% SiC and 6% Al₂O₃ exhibit enhanced UTS, impact strength, hardness, and a lower coefficient of thermal expansion. The findings highlight that the optimal reinforcement level for achieving superior mechanical performance in AMCs is 15% total particulate reinforcement, as represented in Figure 13. Pethuraj et al. [170] said the Al₂SiO₅ and B₄C hardness reinforced AMC significantly better than the Al alloy. This increase in hardness is attributed to hard reinforcement particulates like Al₂SiO₅ and B₄C, which are much harder than the Al matrix. These particulates act as barriers to dislocation motion along slip planes, thereby resisting deformation under applied loads. Furthermore, the thermal stress induced by these reinforcements further restricts the local plastic deformation of the Al matrix. However, the hardness of AMCs was slightly lower than AMCs with 10 wt.% reinforcement, likely due to clustering effects between primary and secondary reinforcements. The tensile strength of the AMCs increased with the addition of reinforcements up to a certain limit. The homogeneous distribution of reinforcement particles throughout the Al matrix explains this increase. The strong interfacial bonding between the Al and reinforcement particles is essential for increasing tensile strength by preventing plastic deformation and offering resistance to applied loads. Additionally, the thermal stress generated due to the mismatch in thermal expansion coefficients between the Al matrix and the reinforcements contributes to strengthening the AMCs. This thermal stress acts as a barrier to dislocation motion, increasing the dislocation density and improving the overall strength of the AMCs, are shown in Figure 14.

Figure 11. (a) Mechanical properties of AA6061 − TiO₂ AMC with various mass fractions of TiO₂, (b) SEM images taken from fracture surfaces of AA6061 − TiO₂ AMC with different mass fractions of TiO₂ (1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%) [177].

Figure 11. (a) Mechanical properties of AA6061 − TiO₂ AMC with various mass fractions of TiO₂, (b) SEM images taken from fracture surfaces of AA6061 − TiO₂ AMC with different mass fractions of TiO₂ (1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%) [177].

Wadhwa et al. [171] studied that hardness was 6% FeTiO₃, 400 rpm stirring speed, and 8 h aging, while for tensile strength, it was 4% FeTiO₃, 350 rpm stirring speed, and 8 h aging. Under these conditions, the AMC showed a ~43.33% rise in hardness and ~82.46% increase in tensile strength compared to tin babbitt.

Figure 12. (a) Hardness at varying temperatures for preheating of FeTiO₃ and (b) stress–strain graph of AMC reinforced with FeTiO₃ [168].

Figure 12. (a) Hardness at varying temperatures for preheating of FeTiO₃ and (b) stress–strain graph of AMC reinforced with FeTiO₃ [168].

Figure 13. Variation of UTS, percentage elongation, impact energy, and hardness (HRB) of Al alloy 1350 with different percentages of SiC and Al₂O₃ reinforcements. The bar chart represents the UTS (in MPa), impact energy (in Joules), and hardness (HRB), while the line graph shows the percentage elongation for each sample composition. The data illustrate how the mechanical properties change with increasing reinforcement content, showing trends in the AMC material’s strength, ductility, toughness, and hardness [176].

Figure 13. Variation of UTS, percentage elongation, impact energy, and hardness (HRB) of Al alloy 1350 with different percentages of SiC and Al₂O₃ reinforcements. The bar chart represents the UTS (in MPa), impact energy (in Joules), and hardness (HRB), while the line graph shows the percentage elongation for each sample composition. The data illustrate how the mechanical properties change with increasing reinforcement content, showing trends in the AMC material’s strength, ductility, toughness, and hardness [176].

Figure 14. Mechanical properties of Al-based AMCs with varying wt.% of reinforcement. (a) Hardness (HV) is a function of reinforcement percentage, showing an initial increase of 5% reinforcement and a subsequent slight decrease of 10% for hybrid AMCs. (b) Tensile strength (MPa) plotted against the wt. %age of reinforcement, demonstrating a peak at 5% reinforcement and a significant drop after that. (c) Impact strength (J) versus wt. %age of reinforcement, indicating a similar trend with maximum strength at 5% reinforcement and reduced impact strength at higher reinforcement percentages and in hybrid AMCs [170].

Figure 14. Mechanical properties of Al-based AMCs with varying wt.% of reinforcement. (a) Hardness (HV) is a function of reinforcement percentage, showing an initial increase of 5% reinforcement and a subsequent slight decrease of 10% for hybrid AMCs. (b) Tensile strength (MPa) plotted against the wt. %age of reinforcement, demonstrating a peak at 5% reinforcement and a significant drop after that. (c) Impact strength (J) versus wt. %age of reinforcement, indicating a similar trend with maximum strength at 5% reinforcement and reduced impact strength at higher reinforcement percentages and in hybrid AMCs [170].

11. Room Temperature Wear and Friction

The tribological properties of AMCs, such as wear resistance and friction coefficient, are critical for applications involving sliding or abrasive contact. Natural reinforcements can improve the tribological performance of AMCs, although their effectiveness varies depending on the type of reinforcement and the operating conditions. The friction coefficient of natural-reinforced AMCs is generally lower than that of unreinforced Al, which can be attributed to hard reinforcement particles that reduce direct metal-to-metal contact. However, the effectiveness of natural reinforcements in improving tribological properties depends on factors such as the reinforcement’s distribution within the matrix and the quality of the interfacial bonding. The wear and friction properties of Al-Si alloy matrix AMCs are critical factors in determining their suitability for various applications. Wear resistance, in particular, is essential in components subjected to friction, such as engine parts, bearings, and gears. Including minerals in the Al-Si matrix helps reduce wear by providing a barrier to material loss and improving load-bearing capacity. Studies on these AMC’s wear and friction properties have revealed that optimal performance is often achieved through careful selection and combination of minerals and control of processing parameters such as particle size, distribution, and the interfacial bonding between the mineral and the matrix.

Metal matrix materials, like Al alloys, are prone to wear and seizing during sliding, limiting their use in tribological applications. Ceramic reinforcements such as fibers, whiskers, and particles can be added to the matrix to improve wear resistance [177]. AMC in LM13 alloy with 3 wt.% tiny ZrSiO₄ particles (20–32 μm) and 12 wt.% coarser ZrSiO₄ particles (106–125 μm) showed a greater wear rate, according to Kumar et al. [92,178]. According to Archard’s rule of adhesive wear, a roughly linear relationship between wear rate and increasing stress was seen in AMC materials. Similar trends were noted by Sucitharan et al. [97] for Al 6063 alloy strengthened with ZrSiO₄ particles. The mechanical properties of LM 13 alloy reinforced with ZrSiO₄ [93] have been investigated. It was discovered that as the amount of ZrSiO₄ increased [94,95] the wear resistance of the LM13 alloy increased as finer ZrSiO₄ particles dispersed.

Kumar et al.’s [179] experiments showed that the friction coefficient of the AMCs improved with a rise in TiO₂ amount. AMCs with 12% TiO₂ exhibited a higher friction coefficient than the Al matrix. When reinforcement particles act as second-body abrasives during sliding wear, they generate significant plastic strain, increasing friction. Conversely, AMC with 4% TiO₂ showed a lower friction coefficient than those with 8% and 12% TiO₂. This drop is brought about by the oxide layer on the worn surface, reducing the friction coefficient and the contact area between the sliding surfaces. The impact of TiO₂ content on the wear loss of the AMCs, indicating a significant decrease in wear loss with an increase in TiO₂ content. The hardness restricts plastic deformation during sliding wear, reducing wear loss, are shown in Figure 15. Furthermore, the more robust connection between the Al alloy and the reinforcement increases the shear strain required to commence plastic flow, contributing to enhanced wear resistance. The friction coefficient rises with the sliding velocity at a fixed sliding distance. AMC reinforced with TiO₂ have a more significant friction coefficient than Al-15% SiC and base matrix. The frictional energy of sliding wear oxidizes the surface, generating a defensive oxide film. However, at more incredible sliding speeds, the reinforcement phase’s micro-machining impact tears down this protective layer, resulting in direct metal-to-metal contact and a rise in the coefficient of friction (COF).

Sharma et al. [140] examined the wear behaviour of AMCs with varying particle sizes up to a sliding distance of 3000 m. Their findings illustrated AMC with 20 wt.% fine particles demonstrated a 33% improvement in wear resistance compared to the unreinforced AMC. This enhancement is accredited to the reduced inter-particle spacing in AMCs with fine particles, which improves load transfer and wear resistance. Additionally, frictional heat generated during sliding can diminish the grip of coarse particles, leading to increased wear. The study also notes that wear resistance improves with increasing load due to the formation of protective oxide films, which can be worn away under continuous sliding. Under high loads, the AMC shows greater wear strength than the Al-alloy, likely due to the work hardening of the wear surface, as shown in Figure 16. Sharma et al. [164] investigated the run-in-wear zone. Increased wear rate was due to micro-cutting and micro ploughing from asperity contact and plastic deformation of pins, while the subsequent decrease was due to debris alignment and compaction, reducing abrasive action. During the steady-state-wear zone, a mechanically mixed layer (MML) forms, balancing its growth and removal, thus stabilizing the wear rate. Incorporating Al₂SiO₅ reinforcement enhanced the wear resistance, with 15 wt.% dual particles size AMCs showing superior performance by achieving a steady state at a shorter sliding distance and exhibiting reduced wear rates compared to the Al. Comparatively, the 15 wt.% dual particles size AMC demonstrated a similar wear rate to cast iron but was significantly lighter, making it a promising material for industrial applications. The wear rate of 15 wt.% dual particles size was also lower than dual particles size ZrSiO₄ and TiO₂ reinforced AMCs, indicating better wear resistance. Additionally, the wear rate increased linearly with contact pressure, with higher reinforcement levels and finer particle sizes contributing to better wear resistance by buffering the applied pressure and preventing particle fracture. Shivakumar et al. [180] examine LM24 AMCs reinforced with garnet wear properties. The study found that at low loads, the AMCs mainly experienced abrasion wear, while at higher loads, delamination wear became the dominant mechanism. A critical load threshold was identified, beyond which the wear rate increased sharply, signifying a transition to a more severe wear regime. Garnet particles in the AMC were found to delay this transition, enhancing wear resistance under higher stress conditions. Gupta et al. [119] accumulation of natural reinforcement to the Al alloy significantly boosted its wear properties due to the higher hardness of these particles than the Al alloy. Bonding reinforced particles with the matrix is also crucial; strong interfacial bonding delays particle removal, allowing them to bear the load longer. Conversely, weak bonding leads to rapid particle removal and increased wear due to direct contact between the matrix and the steel disc. Al₂SiO₅ AMC demonstrated higher wear resistance than TiO₂ AMC due to higher hardness and better interfacial bonding, which restricted crack propagation. In contrast, TiO₂ AMC exhibited more crack formation and propagation, along with a greater increase in temperature during sliding motion. This led to the softening of the matrix and a higher wear rate. The wear rate increased with the applied load for the Al alloy and its AMC. At lower loads, the sliding surfaces experience point contact, which causes a plowing action that generates cracks. These cracks eventually merge, resulting in material removal from the surface of the pin. Under low loads, the MML remains more stable, leading to a reduced wear rate. However, intense plastic deformation and elevated frictional forces cause a significant temperature rise at higher loads. This increase in temperature softens and oxidizes the matrix, destabilizing the MML and exposing the pin surface to the steel disc. Regarding the COF, the base alloy (LM27) demonstrated the highest COF across various applied loads. This is attributed to the absence of hard mineral particles within its matrix. In AMCs, the COF is influenced by factors such as the hardness of the reinforcement, the formation of a transfer film, and the temperature increase during wear testing. AMC reinforced with TiO₂ showed a greater temperature increase compared to those reinforced with Al₂SiO₅. This difference is due to the lower thermal conductivity of TiO₂, which leads to more softening of the matrix and lower COF values for TiO₂-reinforced AMCs. Conversely, the higher hardness of Al₂SiO₅ particles results in increased surface-to-surface friction, leading to a higher COF for Al₂SiO₅ AMC compared to TiO₂ ones. Kumar et al. [167] explored the effect of varying amounts (5 to 15 wt.%) of reinforcement particle sizes on AMCs’ wear resistance. The study found that AMC reinforced with fine particles exhibited better wear resistance compared to those with coarse particles for the same wt.%. This is due to the reduced interparticle distance in fine particle AMCs, which increases hardness. However, porosity in fine particle-reinforced AMCs leads to only slight variations in wear rate among the AMCs studied. Additionally, the wear rate reduced with a rise in garnet amount as reinforcement, with the lowest wear rate observed in AMC with 15 wt.% fine garnet reinforcement. This shows that AMC with fine reinforcement and a uniform distribution in the Al alloy tends to display more wear resistance, which aligns with findings from other researchers. Besides the higher reinforcement content, good interfacial bonding prevents particle pull-out from the matrix during sliding, as shown in Figure 17.

Gupta et al. [133] found that incorporating mineral particles like TiO₂ and Al₂SiO₅ significantly improved the wear resistance of Al alloy. This reinforcement, along with their uniform distribution and robust bonding at the interface with the Al, effectively impeded crack propagation and prevented direct contact of the Al to the sliding disc, enhancing the load-bearing capacity. The 15 wt.% dual particles size AMC, in particular, achieved a maximum wear rate reduction of 52% compared to the unreinforced material, demonstrating superior wear performance. When tested under an applied pressure of 1 N/mm², the 15 wt.% dual particle size AMC demonstrated a wear rate of only 11% higher than a commercially available brake rotor material, indicating its potential as a competitive alternative for such applications. Singhal et al. [101] conducted wear tests under loads ranging from 1 to 7 kg. They identified high wear rates caused by the interaction and deformation of pin asperities and the steady-state zone, where the formation of oxide layers and MML leads to stabilizing the wear rate. As the load increased, the wear rate rose linearly due to increased friction and plastic deformation on the pin surface, which resulted in debris formation. However, including more rigid FeTiO₃ particles in the AMCs contributed to a lower wear rate by distributing the load more effectively and protecting the matrix. The COF for the AMCs decreased as the size of the FeTiO₃ particles decreased; the AMC showed a 48% and 29% lesser COF than the base Al at loads of 1 kg and 7 kg, respectively, owing to the protective effect of the FeTiO₃ particles, which shield the matrix from abrasive wear. Gupta et al. [134] focus on the influence of sliding distance and reinforcement particles. As the sliding distance increases, the wear rate reduces, attributed to the entrapped wear debris and strain hardening, which form a protective MML that reduces further wear. The study also explored the influence of applied load, finding that wear rates increase with higher loads due to delayed work hardening and more significant oxidation and instability of MML at higher temperatures. The study highlighted the benefits of reinforcing particles in reducing wear rates, showing that AMCs B₄C and FeTiO₃-reinforced AMCs exhibited lower wear rates compared to the base alloy under various conditions. The study examined the COF for various formulation AMCs. The COF decreased with the accumulation of reinforcement due to reduced contact area and lower energy required for sliding. Still, it increased with applied load because of greater plastic deformation of the sliding surfaces. FeTiO₃ reinforced AMCs showed lower COF values compared to B₄C reinforced AMC, attributed to the lubricity effect of FeTiO₃ particles and their tendency for early oxidation owing to lower thermal conductivity as present in Figure 18 (

Table 7. Reinforcement levels and designations for B₄C AMC and FeTiO₃ AMCs [134].

Reinforcement (wt.%)	Designation
	B4C AMC (BRC)	FeTiO3 AMC (IRC)
5	BRC-5	IRC-5
10	BRC-10	IRC-10
15	BRC-15	IRC-15

). Singhal et al. [155] concluded a reduction in wear loss, with decreases of up to 37% and 29% during the run-in phase and 26% and 23% during steady-state wear at lower and higher loads, respectively. This improvement is linked to the inclusion of ceramic phases within the matrix, which absorb a significant applied load and reinforce the matrix, thus protecting it from deformation and further enhancing wear resistance. Additionally, the study assesses the COF at various loads, showing that COF initially increases with load due to frictional heating but later decreases as a stable lubricating film forms. Singhal et al. [126] investigated the effect of Al₂SiO₅ particles significantly enhancing the wear resistance of the AMCs compared to the base alloy. This improvement is attributed to the strong bonding between Al₂SiO₅ particles and the Al matrix, which helps carry the applied load and form a protective oxide layer that reduces wear. The wear rate of 10 wt.% Al₂SiO₅ AMC was 30% and 25% less than the Al alloy at loads of 9.81 N and 68.67 N, respectively. Incorporating solid lubricants like Sn and Gr further reduced wear rates, with Sn showing superior performance due to its low melting point and creating a lubricating oxide layer. AMCs with both Sn and Gr exhibited the lowest wear rates, achieving up to 79% and 57% reductions at different sliding distances and loads. Additionally, the wear rate was reduced with increased solid lubricant content and reinforcement level. The Al₂SiO₅ with both Sn and Gr AMC also showed comparable wear performance to cast iron, highlighting its potential for use in industrial applications as a lightweight alternative. The COF for the AMCs was lower than that of the base alloy, with Al₂SiO₅ with both Sn and Gr AMC again demonstrating the best performance due to adequate lubrication and the development of an MML. Moreover, using solid lubricants further enhances wear strength, with the combination of Tin and Graphite (both at 3 wt.%) yielding the most substantial improvement. The study highlights that dual solid lubricants offer superior wear resistance, with reductions in material loss of up to 71% at lower loads (9.81 N) and 49% at higher loads (68.67 N) [101]. Maan et al. [130] investigated the wear behaviour of base alloys and corundum-reinforced AMCs at varying sliding distances and contact pressures. They found that wear rates initially increased due to abrasive interactions but decreased and stabilized as sliding continued. AMCs with 20 wt.% Al₂O₃, especially the 20 wt.% Al₂O₃-reinforced AMC variant with fine particles, showed significantly lower wear rates and could handle higher contact pressures better than the base alloy. The 20 wt.% Al₂O₃ reinforced AMC also demonstrated a lower COF, reducing wear further. These results highlight the superior wear resistance and friction performance of Al₂O₃-reinforced AMCs, making them suitable for demanding industrial applications, as shown in Figure 19. Maan et al. [181] analyzed Al₂O₃ particles in AMCs significantly reduced wear rates by shielding the matrix, restricting crack propagation, and creating a protecting oxide layer under high contact temperatures. The wear resistance of AMCs improved with increased Al₂O₃ concentration and a higher ratio of fine particles. The 20 wt.% reinforced AMC showed a wear rate 64% lower than the base alloy at 1.4 MPa and was 13% lower than a similar AMC with fine particles. The COF increased with contact pressure but decreased with finer particle content, with the 20 wt.% reinforced AMC, achieving a COF 51% lower than the Al alloy. Gupta et al. [125] study revealed that the wear rate enhanced with applied load due to higher frictional force and plastic deformation between the relative motion of the AMC sample and the disc, resulting in frictional heating and surface softening. Incorporating reinforcement particles, particularly B₄C and FeTiO₃, significantly improved the wear resistance of the AMCs. Increased reinforcement levels resulted in lower wear rates, with the 15 wt.% B₄C: FeTiO₃ particle (3:1) AMC showing the lowest wear rate, 67.74% less than the Al alloy. This improvement is attributed to the synergistic effects of FeTiO₃ and B₄C. FeTiO₃ particles formed strong interfacial bonds, facilitating effective load transfer and minimizing shear stress while promoting early oxidation of the moving surfaces due to their lower thermal conductivity. Friction coefficient analysis showed a decreasing trend with higher reinforcement levels, particularly for the 15 wt.% B₄C: FeTiO₃ particle (3:1) AMC, which exhibited the lowest friction coefficient across all load conditions due to reduced contact area and minimized plastic deformation. However, the friction coefficient increased with applied load due to enhanced plastic deformation and the introduction of the Al alloy to the counter disc. The study by Singhal et al. [154] investigates the wear behavior of an LM30 alloy and AMCs under different normal loads (9.81–68.67 N) for sliding distances up to 2500 m. The wear rate of both the Al alloy and synthesized AMCs improved initially due to asperity contact between the surfaces, which caused cutting and ploughing actions that resulted in abrasive wear. The study highlights the influence of adding solid lubricants like tin (Sn) and graphite (Gr), which summarize wear loss and COF under various loading conditions by forming lubricating films. The AMC 10 wt.% FeTiO₃ with Gr/Sn exhibited superior wear resistance and lower COF compared to the base material. The findings suggest that the integration of ceramic reinforcements and solid lubricants synergistically enhances the load-carrying capacity and wear resistance of the AMCs, providing insights into optimizing material design for high-performance applications. Singhal et al. [123] investigated the wear behaviour of AMCs with dual size range FeTiO₃ particles. The study highlights that incorporating FeTiO₃ particles significantly improves wear resistance, particularly as the ratio of fine to coarse particles increases. The hard FeTiO₃ particles, which bear most of the contact pressure and protect the matrix from wear, are responsible for this improvement. When uniformly distributed fine reinforcement increases the load-bearing points, effectively decreasing the distance between particles and preventing their segregation. Coarse particles maintain this uniform distribution as load-bearing elements during sliding action. These particles, which become fragmented under shear force, contribute to three-body wear as they are stuck between the sample and counter surface. Bypassing the shear stress through finer particles to the matrix, this process further guards against wear on the pin. Additionally, FeTiO₃ reinforcement reduces the contact temperature between the interacting surfaces, which minimizes plastic deformation and material loss. Among the AMCs studied, the 15 wt.% FeTiO₃ reinforcement (4:1; fine: coarse ratio) AMC demonstrated the highest wear resistance, showing a 57% improvement over the LM30 base alloy at a contact pressure of 1.4 MPa.

Morampudi et al. [182] conducted pin-on-disc wear tests on AA6061 base metal and AMC with varying wt.% of FeTiO₃. Their findings demonstrate that the wear rate decreases with increased reinforcement, with the lowermost wear rate detected at 15% FeTiO₃. This drop in wear is attributed to the enhanced bonding strength of the reinforcement particles, which provides better protection to the matrix material. Additionally, COF is minimized in AMCs with the highest FeTiO₃ content, indicating improved frictional performance. Pethuraj et al. [170] study on AMC with varying particulate reinforcements reveals that adding reinforcement typically decreases the specific wear rate. Their results indicate that AMCs, particularly with 5 wt.% and 10 wt.% reinforcements, show the lowest wear rates, especially at a relative motion of 0.75 m/s. However, increased normal load and sliding velocity significantly raise the specific wear rate by promoting surface roughness and the breakdown of stable oxide layers. At higher sliding velocities, wear is further intensified due to metal softening and increased friction coefficients. The friction coefficient trends closely follow wear behavior, with pure Al exhibiting higher coefficients compared to reinforced AMCs. Reinforced AMCs benefit from a strong mechanical mixed layer and oxide formation, which reduce friction and enhance wear resistance. Yet, higher loads and velocities eventually compromise these benefits by tearing oxide layers and increasing friction. Nagpal et al. [169] examined the wear properties of LM27 reinforced with TiO₂ particles. The study showed that increasing the reinforcement content from 6 to 12 wt.% significantly reduced the wear rate of the AMCs. This was attributed to the hard TiO₂ particle role in cultivating wear resistance by bearing the applied load and resisting material removal. Hard particles also facilitate the formation of a smoother wear surface, reducing sudden declines in wear loss and promoting a more stable wear state. The study demonstrated that AMCs with fine TiO₂ reinforcement (50–75 µm) exhibited higher hardness and better wear resistance compared to those with coarser particles (106–125 µm). Due to their larger surface area and ability to embed more effectively into the matrix under load, fine particles provide better wear protection. In contrast, coarser particles are more prone to protrusion and may lead to accelerated wear due to increased material removal by indenting and ploughing actions. It was found that the wear rate of AMCs increased significantly with an increase in load. Under high load conditions, the protective oxide film on the metal surface breaks down, exposing the substrate and causing plastic deformation, which leads to severe material removal. The study also highlighted that AMCs reinforced with fine particles exhibit weaker interfacial bonding compared to those with coarse particles under high load, which negatively influences wear resistance. Yadav et al. [172] directed a study on the tribological behavior of LM25 Al alloy reinforced with Al₂SiO₅ and red mud particles. The research focused on optimizing the wear and friction characteristics of the AMC by varying the reinforcement levels (5, 10, and 15 wt.%) and analyzing their effects using the Box-Behnken Design technique. The microstructural analysis revealed a uniform distribution of reinforcement particles within the matrix, which is crucial for improving the mechanical properties of the AMC. The study identified an optimal reinforcement percentage of 13.48 wt.% that significantly enhanced the wear resistance and friction performance of the LM25 alloy. Wear tests presented that the AMCs were more prone to wear under higher loads due to increased surface deformation. This highlights the importance of managing load conditions in practical applications to prevent excessive wear. Gupta et al. [122] present a comprehensive study on the wear characteristics of AMCs reinforced with FeTiO₃ particles implanted in an A390. The FeTiO₃ reinforcements bear the common of the applied load, protecting the softer matrix from deformation while sliding a steel disc. The authors highlight that smaller-sized FeTiO₃ particles, with uniform distribution and reduced inter-particle distance, enhance load-bearing points and improve wear resistance. The addition of FeTiO₃ also increases the hardness of AMCs, with sample 15 wt.% dual reinforced AMC with Gr/Sn showing the highest hardness due to a higher content of finer particles, which increase dislocation density and restrict movement, enhancing the material’s hardness. The study also introduces coarse particles to avoid an accumulation of fine particles, further optimizing wear resistance. Moreover, the authors incorporate a solid lubricant mixture (1 wt.% Gr + Sn) into the AMCs, substantially improving wear resistance. Tin (Sn) solidifies at the Al grain boundaries, while graphite (Gr), with its hexagonal-layered structure, forms a lubricating film during wear testing, reducing the material wear rate. Additionally, the study notes that the COF of the synthesized AMCs is significantly lower than that of the A390 alloy. The inclusion of solid lubricants further decreases the COF by up to 55% at lower applied loads. This comprehensive research underlines the effectiveness of FeTiO₃ reinforcement and solid lubricants in enhancing the wear resistance and hardness of AMCs, positioning them as viable materials for applications such as braking rotors, where weight reduction and durability are crucial.

12. High-Temperature Wear and Friction

High-temperature wear and friction are crucial considerations in materials used in high thermal environments, such as automotive engines and aerospace components. At elevated temperatures, materials can experience thermal softening, oxidation, and phase transformations, affecting their wear resistance and frictional properties. Oxidative wear is expected at high temperatures, where oxide layers form and can reduce wear but may fail under severe conditions. For AMCs, reinforcement particles improve wear resistance by enhancing thermal stability and reducing thermal expansion. However, challenges like matrix softening and debonding at high temperatures require advanced materials design, such as optimized microstructures and AMCs, to enhance performance. The wear behavior of AMCs is significantly influenced by temperature. Generally, when temperatures rise, all metallic components become more vulnerable to wear damage, intensifying the wear and perhaps causing the matrix material to flex plastically, as seen in Figure 20 [94]. characteristics. It was discovered that employing a 1:4 ratio of finer to coarser particles increases wear resistance, increases hardness, and guarantees more efficient load transfer from the matrix to the reinforcing elements. In AMCs with 15–20 wt.% of ZrSiO₄ particles, the wear rate remained consistent even though it generally increased with higher applied stresses for both the matrix and the AMC. When compared to other dual-size reinforced AMCs, AMCs with a 10 wt.% ratio of coarse (106–125 µm) and fine (20–32 µm) particles showed improved wear resistance under all situations among the various compositions examined. The wear rate of the AMCs increased as the temperature was raised from 50 to 150 °C. Particle wedging in the softer matrix, which offered superior wear resistance in both stress scenarios, was blamed for this. All dual-size reinforced AMCs saw a decrease in wear rate at 200 °C when an oxide layer formed on the surface because it prevented direct contact between the specimen’s sliding surfaces and the steel disk. To evaluate the AMCs, wear behaviour, additional tests were conducted on them at 200 °C and 250 °C with a 49 N load. When the worn surfaces were topographically analysed, all AMCs showed improved wear resistance at higher loads and temperatures. The wear properties of AMCs supplemented with different wt.%ages of ZrSiO₄ and SiC particles were investigated by Kumar et al. [91]. They looked at how temperature affects the wear rate of reinforced AMCs, both single-size and dual-size. Both AMCs showed a decrease in wear rate at modest loads of 1 kg as the temperature rose from 50 °C to 200 °C. The wear rate did, however, increase above the critical temperature of 200 °C, which was linked to thermally induced deformation processes that soften the material close to the contact surfaces. The wear rate for both dual-size and single-size reinforced AMCs showed a modest rise from 50 °C to 150 °C at first but then dropped at 200 °C, probably due to strain hardening, under a load of 5 kg. An oxide layer formed on the pin surface was shown to be responsible for the lower wear rate between 150 and 200 degrees Celsius. Delamination caused the oxide layer to shatter over 200 °C, causing a shift in wear from mild to severe. It was discovered that ZrSiO₄ and SiC particles combined in a 1:3 ratio offered superior wear resistance at all loads and temperatures. Panwar et al. [95] looked at the ZrSiO₄-reinforced LM13 alloy’s high-temperature wear.

Research by Arora et al. [183] on wear properties at temperatures from 50 °C to 300 °C indicated that AMCs with fine-sized TiO₂ particles offered nearly double the wear resistance compared to those with coarse particles. A mild to severe wear transition was observed above 200 °C, and the reduced particle size led to increased hardness and improved wear resistance due to the refined silicon morphology within the matrix. The formation of a compacted oxide glazing layer at higher temperatures delayed the transition from mild to severe wear. Fine-sized particles consistently resulted in lower wear rates than coarse-sized ones in the mild wear regime. In another study by Arora et al. [118] conducted at 200 °C, AMCs with fine-sized particle reinforcements demonstrated significantly lower wear rates compared to those with coarse-sized particles under higher loads. A critical shift from mild to severe wear was observed for all AMCs at temperatures above 150 °C. The morphological analysis of the wear track and debris suggested that surface damage was mainly due to crack nucleation around voids at the particle–matrix interface. Below the critical transition temperature (150 °C), the formation of a MML was identified as a vital factor in lowering wear rates. However, above this temperature, the removal of MML, followed by plastic deformation, was found to be the primary cause of increased wear rates.

Gupta et al. [157] had lower wear rates but higher COF, whereas FeTiO₃ AMC had a lower COF due to the lubricating effect of FeTiO₃ particles. Wear mechanisms shifted from abrasive to delamination with increasing load at room temperature and combined with oxidation wear at elevated temperatures. The study concluded that while B4C reinforcement improves hardness and wear resistance, FeTiO₃ offers a lubricating effect that reduces COF, with both reinforcements enhancing wear properties under different conditions. Sharma et al. [184] conducted a study to develop and investigate the effect of temperature-wear-resistant LM30-Al₂SiO₅ AMC for automotive applications. The AMCs were synthesized using dual particle size Al₂SiO₅ minerals, mixed in different fine-to-coarse weight ratios (3:1, 1:1, and 1:3) and reinforced into the Al alloy. The study covered operating temperatures between 50 to 300 °C. The best results were observed in the AMCs containing 15 wt% Al₂SiO₅ with a fine-to-coarse particle ratio of 3:1 (15 wt.% dual size reinforced AMCs). At a temperature of 200 °C and a contact pressure of 1 MPa, this AMC exhibited a 15% increase in hardness, an 80% drop-in wear rate, a 48% reduction in the COF, and a 28% lower thermal expansion coefficient compared to the base alloy. The wear mechanism behind the wear loss is represented in Figure 21. In their study, Singhal et al. [124] studied the high-temperature wear behaviour of AMCs reinforced with FeTiO₃ particles, conducting wear tests at temperatures ranging from 50 °C to 300 °C and loads between 9.81 and 68.67 N. They compared the wear performance of AMCs with that of a base Al alloy and observed that the wear resistance of the Al alloy declines with growing temperature and load due to thermal softening and grain growth. The study found that the wear rate increases significantly beyond 200 °C, where the oxide layer on the surface becomes uneven, leading to a transition from mild to severe wear. The incorporation of FeTiO₃ particles into the LM30 alloy improved the wear resistance of the AMCs. FeTiO₃ particles acted as barriers, preventing direct contact between sliding surfaces and reducing the apparent contact area, enhancing the overall load-carrying capacity. The presence of FeTiO₃ particles also resulted in lower thermal expansion, reducing plastic deformation and improving wear resistance. The study showed that fine-sized FeTiO₃ particles (32–50 μm) provided more uniform distribution within the matrix, minimizing stress concentrations and improving wear performance compared to coarser particles (75–106 μm). The wear rate of the AMCs decreased with increasing FeTiO₃ content. At 15 wt% FeTiO₃, the AMCs demonstrated superior wear resistance and retained a finer grain structure at high temperatures, enhancing their performance. However, beyond the critical temperature of 200 °C, a rapid increase in the wear rate was noted for all AMCs due to softening of the contact surface and significant plastic deformation, as represented in Figure 22. Sharma et al. [185] study explored the effects of different operating temperatures (50 °C–300 °C) and pressures (0.2–1.0 MPa) on the wear and friction properties of the AMCs. Wear analysis showed that the AMCs kept good wear strength up to 200 °C, after which the wear rate increased significantly. The study also revealed that abrasive and adhesive wear mechanisms dominated at high operating temperatures and pressures, with adhesive wear becoming more prominent at temperatures above 250 °C and pressures above 0.6 MPa. Gupta et al. [186] examined the COF values and wear-rate behaviour of AMC reinforced with covalently bonded discontinuous particles (CDP) and natural discontinuous particles (NDP) under varying reinforcement levels, applied loads, and operating temperatures. The COF and average steady-state wear-rate of the Al alloy and AMC were analysed across different temperatures, considering the wear-rates in the sliding distance range of 1500–3000 m to determine the average steady-state wear-rate. It was experimental that for the LM13 alloy, both COF and wear-rate increased with higher operating temperatures, particularly beyond 100 °C, indicating a transition from mild to severe wear. This was due to oxide layer rupture and thermal softening of the AMC pin. Conversely, AMCs demonstrated lower average steady-state wear-rates compared to the LM13 alloy under similar conditions, attributed to the condensed CTE and enhanced thermal stability from the reinforcement particles. Moreover, AMCs exhibited a higher mild-to-severe transition temperature (200 °C) compared to the base alloy (100 °C). Among the AMCs, CDP and NDP showed maximum wear-rate decreases of 61% and 52% over the Al alloy at 9.8 N load. This improvement was linked to grain refinement and lower CTE of the AMCs, which increased the grain boundary area and restricted crack propagation, reducing material removal rates. The lower thermal conductivity of reinforced particles also promoted oxide layer formation, preventing direct metal-to-metal contact and further reducing wear rates.

13. Study the Impact of Heat Treatment on Natural Mineral Reinforced AMCs

The investigation of heat treatment’s impact on natural mineral-reinforced AMCs is crucial in advancing the field of lightweight, high-performance materials. AMCs, which incorporate natural minerals such as sillimanite, zircon and ilmenite leverage the inherent advantages of these reinforcements, including improved mechanical properties and thermal stability. The interaction between the Al matrix and the natural minerals during heat treatment can significantly influence the microstructural evolution, which in turn affects the mechanical performance. Heat treatment processes, such as solution heat treatment (SHT), aging, and annealing, induce critical changes in the microstructure, including the dissolution of alloying elements, precipitation of strengthening phases, and modification of grain boundaries. Sharma et al. [187] examines the mechanical properties and wear behavior of LM13/Zr composites reinforced with zircon particles. It highlights the influence of T4 and T6 heat treatments on hardness and wear resistance. During T4 heat treatment, a peak hardness of 201 ± 5 BHN was observed after 20 days, while T6 treatment achieved a maximum hardness of 212 ± 6 BHN after 4 h at 180 °C. Fine particle composites showed greater improvement in hardness and wear resistance compared to coarse particle composites due to a higher number of reinforcement particles, dislocation concentration, and increased stress localization. The wear analysis revealed that the T6 heat-treated composites exhibited superior wear resistance, especially at higher contact pressures. This was attributed to the formation of hard intermetallic phases like Al₂Cu and Mg₂Si, and the fine distribution of particles which reduced particle-to-particle distance and increased load-bearing capacity. T6-10Zr-F composites, in particular, showed the best wear resistance, with a reduction in wear rate by 59% compared to the LM13 base alloy. XRD analysis of the wear track confirmed the presence of protective oxide layers, which played a critical role in reducing wear rates at extended sliding distances. These findings suggest that T6 heat-treated LM13/Zr composites are suitable candidates for applications such as brake rotors, offering significant weight savings over cast iron.

Sharma et al. [188] investigates the effects of heat treatment on the microstructure, hardness, wear resistance, and phase formation AMCs reinforced with fine sillimanite particles. XRD analysis reveals that heat treatment promotes the formation of hard phases like Mg₂Si and Al₂Cu, which enhance the hardness and wear resistance of the composites. The microstructure analysis shows that heat treatment, particularly T6, leads to spheroidization of eutectic silicon and refinement of primary silicon, improving the uniform distribution of reinforcing particles. Hardness testing demonstrates that artificial aging at 180 °C for 4 h results in a peak hardness of 216 BHN as seen in Figure 23. Wear analysis confirms that the addition of sillimanite and silicon refinement reduces the wear rate significantly, making the T6-treated AMCs a potential replacement for cast iron in automotive brake rotors due to their superior wear resistance and lower weight. Singhal et al. [156] investigates the effect of tempering on the wear performance and mechanical properties of an ilmenite-reinforced LM30 aluminum composite (ITG10), containing 10 wt.% ilmenite with 0.5% tin and 0.5% graphite. The study compares the performance of non-heat-treated, T4 (natural aging), and T6 (artificial aging) heat-treated samples. Optical microscopy revealed that the addition of ilmenite resulted in a homogeneous distribution of reinforcement particles and refined silicon phases. Heat treatment significantly improved the microstructure, leading to globular eutectic silicon and improved hardness due to the formation of intermetallic phases such as Mg₂Si and Al₂Cu. The highest hardness was achieved in T6 samples after artificial aging at 180 °C and 250 °C. Wear testing indicated that heat-treated ITG10 samples, especially the T6-treated composite, exhibited superior wear resistance compared to non-treated samples, with the formation of hard intermetallics reducing material loss. The coefficient of friction (COF) also decreased significantly in the T6-treated samples due to the formation of a stable tribolayer and the presence of solid lubricants.

14. Challenges and Limitations of Natural Reinforcement AMCs

AMC faces several challenges and limitations when reinforced with either natural material. Natural reinforcements exhibit variability in material properties due to differences in their sources, leading to inconsistencies in AMC performance. These natural reinforcements are also hygroscopic, absorbing moisture that can cause dimensional instability and weaken the AMC over time. Poor interfacial bonding with the Al matrix at high processing temperatures further complicates their use, while their generally inferior mechanical properties limit their application in high-stress environments. The natural mineral has some common limitations. Achieving uniform distribution of the reinforcement within the Al matrix and ensuring strong interfacial bonding are critical yet challenging tasks. There is often a trade-off between enhancing strength and maintaining ductility, and while AMCs are valued for their lightweight properties, adding reinforcements can sometimes increase the AMC density. Application-specific limitations also arise, such as reduced performance at elevated temperatures and potential negative impacts on corrosion resistance. These challenges highlight the complexity of selecting appropriate reinforcements for AMCs, where the balance between mechanical performance, cost, environmental impact, and processing feasibility must be carefully managed.

15. Conclusions

This review has comprehensively examined the wear and mechanical characteristics of AMCs reinforced with natural minerals. Al-Si alloys, known for their excellent mechanical properties, have been enhanced further by incorporating natural reinforcements such as ZrSiO₄, TiO₂, FeTiO₃, Al₂SiO₅, and Al₂O₃. The use of natural minerals not only improves the wear resistance and mechanical properties of AMCs but also offers environmental and economic advantages by providing sustainable and cost-effective alternatives to synthetic reinforcements. The stir-casting process has proven effective and economical for the homogeneous distribution of reinforcement particles within the matrix. However, challenges such as particle agglomeration and porosity persist. This review highlights the significant impact of reinforcement type, size, and distribution on the tribological and mechanical properties of AMCs. Fine particles provide better wear resistance due to improved load transfer capability and reduced inter-particle spacing. At the same time, dual-size reinforcements exhibit enhanced properties by preventing particle clustering. The findings suggest that optimizing reinforcement content, particle size, and processing conditions can lead to the development of high-performance AMCs for automotive and aerospace applications. Future research should focus on overcoming the challenges of achieving uniform dispersion and, minimizing porosity, and exploring novel reinforcement combinations to enhance the properties of AMCs further. Heat treatment significantly improves the mechanical properties and wear resistance of natural mineral AMCs. Processes like T4 and T6 promote the formation of hard intermetallic phases and refine microstructures, leading to enhanced hardness and wear resistance. T6-treated composites, in particular, show superior performance, making them promising candidates for lightweight applications, such as automotive brake rotors, where weight reduction and durability are crucial.

16. Future Directions

Despite the promising properties of Al-Si alloy matrix AMCs reinforced with synthetic and natural minerals, several challenges persist. Key issues include achieving uniform dispersion of minerals within the matrix, ensuring strong interfacial bonding, and balancing trade-offs between different mechanical properties. Future research is expected to address these challenges through advanced manufacturing techniques such as powder metallurgy, additive manufacturing, and friction stir processing. Additionally, there is growing interest in developing smart AMCs with self-healing capabilities, where minerals play a crucial role in the material’s ability to repair itself after damage. Exploration of new natural minerals and developing hybrid AMCs combining synthetic and natural materials will also be critical. The goal is to create materials that meet the mechanical and wear requirements of modern engineering applications and contribute to environmental sustainability.

Research is increasingly focused on enhancing AMCs with nanomaterials like carbon nanotubes and graphene, which can significantly improve properties such as tensile strength and thermal conductivity. Hybrid reinforcements blending natural reinforcement with synthetic materials like silicon carbide and alumina are being developed to create sustainable and high-performing AMCs. Additive manufacturing (3D printing) is being explored for producing complex geometries and customized components, while advanced welding methods aim to ensure strong, reliable bonds and minimize thermal distortion. Sustainability efforts are focused on making production processes more eco-friendly and improving recycling methods for end-of-life AMCs. Innovations in AMCs are advancing high-performance applications in the aerospace, defense, and automotive industries. Sophisticated computational modeling and simulation tools enhance the design and optimization of AMC properties. Additionally, integrating smart technologies, such as embedded sensors for real-time monitoring, promises to improve the safety, reliability, and performance of AMCs in critical applications. These advancements are set to expand the versatility, efficiency, and sustainability of AMCs across various industries.