Comparative Study on Mechanical Properties of SiC/Gr &Al2O3/Gr reinforced AL6061 hybrid metal matrix composites

Silicon carbide / Graphite and Alumina / Graphite reinforced AL6061 Hybrid metal matrix composites are fabricated by stir casting (liquid metallurgy) route. Four samples A,B,C,D with varying proportions in both matrix and reinforcements by fixing graphite proportion (5%) constant for all sampled are prepared. Mechanical properties of all the samples are compared with matrix material (AL6061). Scanning electron microscope is used to examine the microstructural characteristics of the composite samples. Mechanical test results exhibit 3.5% (sample C) increase in the hardness number than the base matrix. But, yield and ultimate tensile strength are reduced with all the reinforcements. Microstructural characterisation clearly depicts the presence of cracks, Agglomeration of reinforcements, cast defects on the surface of prepared composites which leads to poor yield and ultimate tensile strength.


Introduction
In recent times, the engineering world is in requisites of newer materials to meet their demands. Such newer materials are also called as composite materials, their usage in military, automobile, aviation, defence sectors, keeps on increasing owing to their less weight, high thermal resistance, high strength to weight ratio, wear resistance, high hardness, and stiffness when compared to conventional engineering materials. In this way it is witnessed that hybrid metal matrix composites have overcome the limitations of other composite materials from recent researches. A hybrid metal matrix composites consist of a minimum of two distinct phases for reinforcement with the matrix phase. Aluminium based hybrid metal matrix composites find way in recent times to replace older materials. Researchers fabricated different grades of aluminium (AL6061, AL7075, AA6351, AL6082, AL2024) based hybrid MMCs with ceramic reinforcements like Alumina (Al 2 O 3 ), Silicon Carbide (SiC), Titanium Boride (TiB 2 ), Titanium Oxide (TiO 2 ), Boron Carbide (B4C), etc. These reinforcements enhance the mechanical properties of composites to meet industrial demands. B.Jayendra,D.Sumanth,G.Dinesh, Dr. M.Venkateswara Rao had observed that reinforcement of B4C and Graphite in AL7075 enhanced the hardness, impact, and tensile strength of the composites considerably [1]. M.Satheesh, M.Pugazhvadivu found 8% reinforcement of  [2]. V.Jaya Prasad, K.Narasimha Rao, N.Kishore Babu R reinforced ceramics (TiB 2 /SiC) and observed the addition of ceramics in aluminium increases the mechanical properties of the composites [3]. Abhishek Sharma,Vyas mani sharma,Jinu paul has found reinforcement of graphene and carbon nanotubes (CNT) in AL6061-SiC matrix increases the nano hardness of composites by 27% and microhardness values by 36% than alone AL6061 [4]. V. Anirudh, M.Vigneshwaran,E.Vijay,R.Pramod, GB Veeresh Kumar studied the TiB2 and graphitereinforced AL6061 alloy and concluded increasing the proportion of reinforcements increases the hardness and UTS of composites than alone AL6061 [5]. V.Mohanavel, K.Rajan, P.V.Senthil, S.Arul in another study informed after the dispersion of Al 2 O 3 and Graphite in AA6351 alloy increased the mechanical properties of composites than pure AA6351 [6]. B.Ramgopal Reddy, C.Srinivas has studied the reinforcement of SiC and fly ash in AL6082 matrix as a base and found a considerable enhancement in UTS, Hardness, and wear resistance in the composites [7]. Cheng-jin Hu, Hong-ge YAN, Ji-hva CHEN.Bin SU reinforced Graphite and SiC in AL2024 matrix and concluded tensile strength and elongation of composites were reduced with reinforcements [8,9]. From this literature survey it was observed that aluminium based hybrid metal matrix composites are prepared by various processes such as Squeeze casting, Vacuum hot pressing, Friction stir processing, Stir casting, and Powder metallurgy. Among the above methods stir casting is mostly preferred owing to some advantages over other methods, one of the important phenomena where stir casting stands prior to other methods is the preparation of a wide range of shapes with larger sizes is possible and this process is economically suited. There is a lack of research concentration in AL6061 based hybrid metal matrix composites. So, this work is aimed to fabricate a hybrid metal matrix composite of AL6061 reinforced with SiC/ Al 2 O 3 (varying proportions) with constant proportion (5%) of Graphite using stir casting process and to compare their mechanical properties with the base matrix (AL6061).

Material Selection
AL6061 was chosen as matrix material because of its cast ability and being utilized in wide range of applications such as in aircraft wings, fuselages etc., table 1 and table 2 shows the chemical composition and mechanical properties of AL6061.   3. Stir casting has been selected for fabrication of composites as the reinforcements having uniform distribution in the matrix phase in this process. Al6061(CP) was taken in form of ingots and melted in the graphite crucible (750°C) and then followed by mixing SiC with stirring speed 350 rpm for 15 minutes and graphite is mixed to the molten mixture to prepare samples A&B. Then above procedure is followed by replacing SiC with Al 2 O 3 to prepare samples C&D respectively. SiC and Al 2 O 3 were used in the form of powders with 20µm particle size. Mechanical type stirrer is used in the stir casting setup. Stir casting setup and work flow chart is shown in the figures 1 and 2 as a pictorial illustration. Molten mixture was poured into the die to get circular specimen of 225mm length and 20mm diameter.

Hardness test
Rockwell hardness test machine (B-Scale) was used to measure hardness values in the fabricated samples. Hardness test specimens were prepared as per ASTM standards and the test results are shown in the below chart 1. Pre-treatment was given to the test specimens; its surfaces were degreased and polished to get even surface. Load of 450kgs was applied for 10 seconds. Sample "C" exhibits higher RHN value than the base matrix.

Tensile test
As per ASTM SA370 standard tensile test specimens were prepared like the design shown in the figure 3. Tests were carried on the prepared specimens in universal testing machine. The ultimate tensile strength (UTS) and yield strength values are obtained for all samples. The values are given in the below charts 2 & 3. Test results reveals that the yield strength and the ultimate tensile strength of all samples are decreased when compared with the AL6061 base matrix.

Micro structural characterization
Scanning electron microscope was used to observe micro structural changes on the stir casted specimen surfaces with high magnification. Micro structural images of different samples were given in below figures 4,5,6&7. Image results show the distributed reinforcement particles in the AL6061 matrix phase with higher level of agglomeration of reinforcements and cracks in the surface of the composites. For the sake of getting good quality images pre-treatments like mirror polishing and degreasing were done on the surface each sample. The cracks and cast defect on the surface of the composites fails to improve the yield and ultimate tensile strength when compared with as cast AL6061 base matrix.

Conclusion
AL6061 based hybrid metal matrix composites reinforced with Al 2 O 3 /Graphite and SiC/Graphite samples with varying proportions with fixed proportion of graphite (5%) were fabricated using stir casting process. From the experimental results the following comparison statements can be drawn. There is an increase in the hardness value with increasing the proportion of the reinforcements, Sample "C" exhibits 3.5% increment in RHN number then the base matrix when compared with other samples. Micro structural characterization images of all the samples A, B, C, D clearly shows the distribution of reinforcement particulates in the matrix phase with clusters of reinforcements in some places. Also, cast defects and surface cracks are present in the samples, which reduce the mechanical properties of the composites drastically. Cast defects are the major limitations found in the study. This work will be extended by reducing the clustering of reinforcement particles in the stir casted composite by performing further heat treatment process. Secondly, optimization of process parameters could be done with optimization tools.