The specific limitations of monolithic material systems such as metallic and ceramic materials can be overcome by Metal Matrix Composites (MMC) through blending their typical property profiles. Aluminium alloy Metal Matrix Composites (AMMCs) are becoming as quite attractive materials for automobile, industrial and aerospace applications because of their unique properties, such as low density, high strength, and good structural rigidity. The AMCs are used in making drive shafts, fan blades, shrouds, springs, bumper, tires, brake shoes, clutch plates, etc. Because of its lightweight, corrosive resistance and constructive mechanical properties make the aluminium as a suitable matrix metal in MMC (Bodunrin et al., 2015). SiC and Aluminium Al₂O₃ are the most commonly used reinforcements. The variation in nature of constituents and their volume fractions influence the properties of the aluminium metal matrix composites.

In recent past, various methods have been used to introduce non-metallic materials into the aluminium based matrix. Lloyd (1994) discussed the different types of reinforcement material being used along with the alternative processing methods. The composition of matrix and reinforcement are independent to produce MMC through powder metallurgy process. Whereas in molten metal process, matrix and reinforcement are closely linked as reactions that occur between these materials at molten state.

Taha et al. (2008) conducted an upset test to study the workability of Al-SiC and Al₂O₃ reinforced metal matrix composites prepared through stir casting, squeezecasting, and powder metallurgy techniques. The study revealed that the volume fraction and particulate size have great effect on workability index. The workability for a material produced from stir casting was slightly more than that of the material produced through other techniques. The microstructure, Vickers hardness, tensile strength, and wear performance of SiC reinforced aluminium matrix composite were examined. The SiC content in aluminium matrix improved the hardness, wear resistance, and tensile strength, but resulted in clustering and nonhomogeneity in the composite (Rahman & Rashed, 2014).

Ram et al. (2017) prepared aluminium SiC particulate reinforced composite using stir casting method to study hardness and impact strength. The composite was prepared through stir casting method. The hardness and impact strength were increased with an increase in the proportion of reinforcement.

Singh and Chauhan (2016) investigated the feasibility and viability of developing low cost-high performance hybrid composites for automotive and aerospace applications. Further, the fabrication characteristics and mechanical behavior of Hybrid Aluminum Matrix Composites (HAMCs) fabricated by stir casting route have also been reviewed. From the review, it has been observed that the reinforcement combination and composition play a vital role in determining flexibility and reliability parameters while designing of hybrid composites.

Rajesh and Kaleemulla (2016) developed a hybrid metal matrix composite. Aluminium 7075 alloy as a base metal and SiC and Al₂O₃ (alumina) as reinforce metals. Naik et al. (2017) synthesized the 7075Al base metal reinforced with ceramic Al₄C₃ metal matrix composite. Metal matrix composites were developed using liquid metallurgy route, i.e., stir casting technique. The results revealed that the mechanical properties were found to be increased with the addition of reinforcement to the base metal. Parswajinan et al. (2018) studied the Aliminium metal matrix composite comprises of Aluminium and SiC in addition with Carbon Nanotubes was prepared using Stir casting technique. The specimens were tested for hardness, tensile strength, thermal conductivity, and melting point. Addition of SiC to aluminium matrix led to improvement in thermal conductivity, melting point, and hardness. Improvement in tensile strength has been reported due to the presence of Carbon Nanotube in the composite. Hynes et al. (2016) reported that liquid processing techniques and solid processing techniques are suitable to produce aluminium metal matrix composites. Due to simplicity and suitability for mass production, liquid processing techniques stand out.

In the present study, Al 6061 was used as matrix material and SiC and Al₂O₃ as reinforced materials and composite was prepared through stir casting route. The microstructural characterization of the composites was studied using metallurgical microscope. Properties, such as hardness, strength, density, electrical conductivity, and chemical analysis of the unreinforced Al and Al+15% SiC/ Al+15% Al₂O₃ / Al+10% SiC+5% Al₂O₃ (wt%) reinforced composites were examined.

1.1 Matrix Material-Al-6061

In this study, Al6061 alloy with theoretical density 2700 kg/m³ is used as matrix material. It is a medium to high strength alloy containing magnesium and silicon as principal alloying elements with heat treatable, weldable characteristics and with excellent corrosion resistance. It is one of the most popular alloys in its 6000 series. The Chemical Composition (in weight %) of aluminium alloy 6061 is shown in Table1.

Table 1. Chemical Composition of Al 6061 (in Weight %)

1.2 Reinforcement Materials

In this study, SiC and Al₂O₃ are used as reinforcement materials. SiC is generally known as carborundum. It is a compound of silicon and carbon. Al₂O₃ is the chemical formula of Aluminium Oxide. It is commonly referred to as alumina, or corundum in its crystalline form, reflecting its widespread occurrence in nature and industry.

1.2.1 Preparation of Aluminium Metal Matrix Composite

The metal matrix composites used in the present study were prepared by the stir casting process. For Al-SiC composite material, the SiC particles of uniform size in the weight fraction of 15% were added into aluminium molten metal. For Al-Al₂O₃ composite material, Al₂O₃ particles of uniform size in the weight fraction of 15% were added into aluminium molten metal. The SiC and Al₂O₃ particles of uniform size in the weight fraction 10% and 5%, respectively were added into aluminium molten metal to prepare Al-SiC-Al₂O₃ composite. In the processes of preparation of the composite specimens, Al was melted in furnace and when the temperature of the liquid Al reached to 760 °C, the heat treated reinforcement particles were added into molten metal through funnel. Before addition to aluminium, the reinforced particles were preheated to 450 °C for about 2 h to improve the wetness properties and remove the absorbed hydroxide and other gases from the surface. A furnace assembled with coupling gear-box-motor and aluminium coated mild steel blade stirrer was used for the stirring purpose. Finally the stirred liquid composites were poured in preheated metal moulds at 670 °C. The melt was allowed to solidify in the mould. The solidified cast material is shown in Figure 1. The composite block was then machined and cut into desired specimen test samples as seen in Figure 2.

Figure 1. Solidified Casting of Al-SiC-Al₂O₃ Composite Material

Figure 2. Test Specimens of Aluminium Metal Matrix Composites

2.1 Brinell Hardness Test

Resistance against permanent penetration indicates the hardness of a material. In hardness test, indenter is pressed against the material to be tested with a specific load for a definite time interval. Hardness measurements were carried out on both non-reinforced and reinforced samples by using standard Brinnel hardness test. In the present work, Brinell hardness tester with an indenter diameter 5 mm was used to determine the hardness of the specimens. A load of 500 kgf was applied for 30 s on the specimens and the Brinell Hardness Number (BHN) was calculated by dividing the load by the surface area of the indentation.

2.2 Metallurgical Microscopy

The distribution of the reinforcing particles in the matrix material was examined using metallurgical microscope. The standard metallographic procedure was followed for the preparation of composite test samples. The specimens were first belt grounded to remove deformations and scratches followed by polishing with different grades of emery papers. The subsequent polishing was done using alumina powder for obtaining mirror finish and etched by Keller's reagent for better contrast. These samples were observed at 200x and 500x magnifications using optical metallurgical microscope.

2.3 Chemical Analysis

Composition of both reinforced and non-reinforced materials was investigated using Emission Spectroscope (Spectromax). This investigation helps to know the composition of chemical compound in the material.

2.4 Electrical Conductivity

The electrical conductivity was determined using Techno four-Conductivity Meter Type 979, in % IACS with 1% accuracy. It works on the principle of eddy currents. A probe induces eddy currents at fixed frequency in the test part. These currents affect the electrical impedance of the test probe. The change impendence is proportional to the electrical conductivity of the test part. The composite metals are tested using this equipment in order to find the electrical conductivity. The instrument has eddy touch pointer (probe), which touches the surface of the metals.

2.5 Compression Test

The test specimens were prepared according to the ASTM standard and tests were conducted on Universal Testing Machine. During the test, the specimens were compressed between two hardened and ground flat plates, one being movable and the other being stationary, and specimen deformations at various loads were recorded. Values were tabulated and compared with each other.

2.6 Density Test

The samples were cut from casted material for porosity examinations. The bulk porosity of the fabricated MMCs was measured using the water displacement (Archimedean density) approach according to ASTM B962-08

3.1 Hardness Results

The results of hardness of aluminium alloy and various metal matrix composite specimens are given in Table 2.

Table 2. Hardness of Metal Matrix Composites of Different Compositions

The surface hardness of the metal matrix composites increase due to an increase in the ceramic phase of SiC particulates. The specimen containing 15% particulate SiC showed maximum hardness of 52.8 HBN. Figure 3 indicates that the addition of particulate SiC and Al₂O₃ to the base metal increases the hardness. Also, addition of SiC particles in Al matrix enhances the hardness of AMCs, in comparison with unreinforced Al and Al reinforced Al₂O₃. This might be attributed to,

• The addition of SiC and Al₂O₃ to aluminium lead to well bonding between the reinforcement and matrix and restricts the dislocation.
• High hardness of SiC and Al₂O₃ compared to the Al matrix.

Figure 3. Hardness of Metal Matrix Composites of Different Compositions

3.2 Microstructure Study

The microstructure and interface characteristics between the matrix and reinforcement influence the properties of the composites. Figures 4 - 7 show the optical microstructures of unreinforced Al and Al+15% SiC/ Al+15% Al₂O₃ and Al+10% SiC+5% Al₂O₃ (wt%) reinforced composites at 200x and 500x magnification. In Figure 4, the microstructure of aluminium Al 6061 alloy is shown, with no reinforcement particles. Figure 5-7 show the microstructure of alluminium alloy reinforced with SiC and Al₂O₃. The reinforcing particles were not identified in some places. The missing reinforcements at some places, clustering and agglomeration could be the result of poor stirring of the particles into the metal matrix during the fabrication process. The variation of contact time between reinforced particles and molten Al during composites processing, high surface tension and poor wetting behaviour of reinforced particles in the liquid Al results clustering and agglomeration/non-homogeneous (rich and depleted regions of reinforcement) distribution of reinforced particles in Al Matrix. As the reinforced particles were added in the melt during casting, the entrapped air between the particles introduced air in the melt that results porosity in the composite.

Figure 4. Aluminium Alloy (a) 200x Magnification (b) 500x Magnification

Figure 5. Al +15% SiC (a) 200x Magnification (b) 500x Magnification

Figure 6. Al +15% Al₂O₃ (a) 200x Magnification 2 3 (b) 500x Magnification

Figure 7. Al +10% SiC +5% Al₂O₃ (a) 200x Magnification (b) 500x Magnification

3.3 Chemical Analysis

Table 3 shows the chemical composition of AMM composite samples. The reinforcement of SiC particles in aluminum matrix causes an increase in the silicon content due to the presence of silica in SiC. Whereas the content of silica is less in aluminium comparatively.

Table 3. Chemical Composition of MMC Samples

3.4 Electrical Conductivity

The experimentally determined electrical conductivity in % IACS of metal matrix composite specimens reinforced with SiC and Al₂O₃ particulates is presented in Table 4. Among the composite samples, aluminium reinforced with SiC and Al₂O₃ showed lower electrical conductivity than aluminium alloy. It has been observed that electrical conductivity of the Al₂O₃ reinforced composite is lower than SiC reinforced metal matrix composite. Pure aluminium is a good conductor of electricity, but when reinforced by SiC and Al₂O₃, the flow of current is less because the reinforced materials are bad conductors.

Table 4. Electrical Conductivity in % IACS

3.5 Compressive Strength

The compressive strength of aluminium alloy and various metal matrix composite specimens are recorded in Table 5. The compressive strenth of composite material is improved with SiC reinforcement. Maximum compressive strength was observed in Al with 15% SiC particulates. The SiC reinforced aluminium composite exhibited more comressive strength than that of Al₂O₃ reinforced composite. The matrix reinforced with Al₂O₃ exhibited strong compression than Al alloy. The presence of SiC and Al₂O₃ increases the strengthening effect of Aluminium Matrix. The increase in hardness in SiC and Al₂O₃ reinforced aluminium MMCs may be due to the strong interfacial bonding between the matrix and reinforcement interfaces. The presence of harder and well bonded SiC particles in Al matrix that impede the movement of dislocations increases the strength and hardness compared to the other samples used in the study.

Table 5. Maximum Compressive Strength

3.6 Density Test Results

The particulate reinforced composites shown lower density compared to that of Al alloy as a result of void density formed due to the reinforcement materials. Al-SiC composite showed lower density among other composites (Table 6).

Table 6. Density

The aluminium metal matrix composites reinforced with SiC and Al₂O₃ were prepared using stir casting method. The standard shape composites were tested for estimating density, hardness, electrical conductivity, chemical composition, microstructure analysis, and compressive strength.

The investigation led to following conclusions: The Brinell hardness number and compressive strength of Al-SiC composite is higher than that of Al alloy and also Al-Al₂O₃ and Al-SiC-Al₂O₃ metal matrix composites. The presence of harder and well bonded SiC particles in Al matrix that impede the movement of dislocations increases the strength and hardness compared to the other samples used in the study.

The clustering and agglomeration dispersion of reinforcement particles in Al matrix are observed in the microstructures. Porosity noted in the microstructures could be due to poor stirring of the particles into the matrix during the fabrication process. The density of Al alloy reinforced with particles (SiC and Al₂O₃) has lower density than that of Al alloy. SiC particle reinforced Aluminium metal matrix composite has lower density than the other composites used in the study. The electrical conductivity of the composites with reinforcement decreased. The chemical composition and microstructure of the composites have also changed due to the addition of the reinforcements, as compared to the structure of base aluminium alloy.