## 2 Materials and Methods
### Materials  
The base material employed for this work was Al6061 alloy steel. The base material was supplied by M/s Coimbatore Metal Mart, Coimbatore, Tamil Nadu, India. The reinforcement used for the improvement of mechanical properties was silicon carbide (SiC). The reinforcement material was supplied by M/s Carborundum Universal, Kochi, Kerala, India. The percentage of SiC added were 1%, 3%, 5% and 9%. The composite with 1% SiC was used for the validation of the model. The manufacturing process employed for the composite was stir casting (Figure 1.). The methodology adopted was as follows:  
* The casting die was preheated to 400 \({}^{\circ}\)C;
* An Al6061 cylindrical rod of 25 mm diameter, cut to a length of 100 mm, was added to the crucible and kept in the furnace. The base material was heated to the temperature of 850 \({}^{\circ}\)C for taking it above the liquidous state. * The molten metal was stirred using a stirrer at 700 rpm and allowed to cool down slowly. * The reinforcement (SiC) powder was slowly added to the molten metal without stopping the stirring action. * 2% magnesium was added to the molten composite to improve its wettability. * The molten mixture was poured into the preheated rectangular die of size 200 mm \(\times\) 150 mm \(\times\) 30 mm to obtain the final composite. * The poured composite was allowed to cool down in the die to obtain the final specimen in the solid condition. The fabricated composite material is examined via Scanning Electron Microscope (SEM), optical microscope and Energy Dispersive X-ray Analysis (EDAX) for microstructural characterisation. The optical microscope employed for the analysis was QS-17 AT manufactured by M/s QS Metrology (New Delhi, India). The magnifications available in the microscope were 100, 200 and 400\(\times\) (10\(\times\) at the eyepiece and 10, 20, and 40\(\times\) at the achromatic objective). It is understood that the uniform distribution reinforced particles in the matrix. The SEM, optical microscope and the EDAX images of the specimen are provided in Figures 2 and 3, respectively. The SEM images of the sample clearly indicate the presence of reinforcement in the grain boundaries, refining the grain boundaries and improving the material properties. The sample preparation for the SEM and microscopic analysis consists of the following steps:  
* A piece with a square cross section with length 80 mm and 25 mm side was cut from the specimen;
* The sample to be tested was first polished manually using a series of emery papers 1/0, 2/0, 3/0 and 4/0;
* The hand-polished specimen was repolished by using a mechanically rotating wheel covered with polishing cloth, and simultaneously, alumina powder mixed in water was poured on the wheel area where polishing was carried out;
* For mirror-type surface finish, diamond paste was used on the clean surface;
* The sample was cleaned using flowing water and Kellars etchant, which is a mixture of nitric, hydrochloric and hydrofluoric acid applied on the surface to reveal the microstructure. * The sample is dried using a hand drier and carefully covered and preserved for microstructure analysis without any contact with the polished surface. Figure 1: Casting die. Figure 3: Material composition analysis of the fabricated composite: (**a**) EDAX and (**b**) Elemental mapping. Figure 2: Microstructure of the composite (**a**) SEM image; (**b**) optical microscope image. The EDAX image of the composite shows that the material composition in the final composite is satisfactory. The elements which are part of the Al6061 alloy as well as the reinforcement can be observed in the EDAX image. Therefore, it can be clearly identified that the reinforcement is uniformly distributed in the specimen. The proper reinforcement distribution in the base matrix, especially at the grain boundaries, plays an important role in determining the machinability of the fabricated composite. The deposit of the reinforcements in grain boundaries provides substantial improvement to the material's properties of the base matrix material.