Optimization of Stir Casting Method of Aluminum Matrix Composite (AMC) for the Hardness Properties by Using Taguchi Method

Aluminum matrix composite (AMC) was fabricated using stir casting with fly ash and SiC as reinforcing materials. In this work, Taguchi optimization technique was utilized to analyze the significant contributions of stir casting parameters on the hardness properties of AMC. For this reason, stir casting was carried out by utilizing the combination of process parameters based on three-level of L9 Taguchi. The signal-to-noise (S/N) and the analysis of variance (ANOVA) were used to find the optimum levels and to indicate the impact of the process parameters on the hardness properties. The results show that some of process parameters have significant effect on the hardness, by comparing with the other three sintering factors, the composition of reinforcement materials gave the most significant effect on the hardness.


INTRODUCTION
Reducing weight and cost are some of main issues in the automotive industry.Intensive efforts have been conducted by the automakers to reduce the car weight.Meanwhile the consumers demand for improved safety, interior comfort, navigation and entertainments (Macke, Schultz & Rohatgi 2012).To answer these challenges automotive manufacturers of automotive are turning to light-weight metals as solution.
Light-metal such as aluminum (Al) is the exact choice to replace steel in automotive components due to its low density compared to steel.Alternator housings, transmission housings, valve covers, and intake manifolds are the potential automotive components to that can be replaced with Al.Although Al is capable in reducing the weight of automotive components mechanical properties of aluminum such as hardness, strength and impact properties should be improved.Several studies have been performed to meet these challenges in terms of the processing route, design and material modification (Krishna & Xavior 2014;Latif, Sajuri & Syarif 2014;Sun, Lyu, Jiang & Zhao 2014;Vogiatzis, Tsouknidas, Kountouras & Skolianos 2015).
Issues of fly ash waste and high cost of reinforcement materials result to the main idea of combining fly ash and common reinforcement materials such as SiC and Al 2 O 3 .By mixing aluminum alloy with fly ash using stir casting, high dislocation density of such composites can be created and enhance their mechanical properties (Anilkumar, Hebbar & Ravishankar 2011).Anilkumar et al. (2011) also reported that smaller size of fly ash weakens hardness, tensile and compressive strength the composite.Investigations on the mechanical and physical properties of AMC that involve fly ash mixed with various common reinforcement materials have also reported (Alaneme & Sanusi 2015;Krishnaraj, Divinesh & Mohaideen 2016;Kumar, Srinivas, Ramachandra, Mahendra & Nagara 2015;Lin, Li, Hou & Li 2015;Visa, Andronic & Duta 2015).The addition of SiC into aluminum -fly ash composite plays an important role in increasing micro and macrohardness (David Raja Selvam, Robinson Smart & Dinaharan 2013).It was found that the increasing weight proportion of SiC increased the micro and macrohardness of such composite.The same trend was also reported where the hardness and tensile stress are improved as the SiC and fly ash contents increased (Krishnaraj et al. 2016).In this work, a hybrid composite of Al (SiC+Fly ash) was prepared, and the microstructure analysis and the optimization of mechanical properties by using Taguchi method, for such composite were presented.

METHODOLOGY
In this work, alumunium alloy is a Al-Si type, while SiC and fly ash are the reinforcement materials.Al-Si alloys was melted in a steel crucible at 700°C furthermore.The SiC and fly ash were than added.During this melting process, stirring was added by a steel blade which generated by an electric motor.
Aluminum matrix composite fabrication via stir casting involves many process parameters such as mold, composition and furnace.Figure 1 shows the parameters from each parts (mould, furnace, matrix alloy and stirrer) that may influence the hardness of AMC during the stir casting process is performed.
Composition of AMC, stirring speed, stirring time and molten temperature were selected according to Taguchi experimental design methodology where each of the parameters have three levels as shown in the Table 1. Figure 2 shows stir casting process that has been done to melt the AMC using alumunium alloys, mixing blade that is generated by the electric motor with controllable speed.

RESULTS AND DISCUSSION
Figure 3 shows the hardness values of AMC based on Brinell Hardness Number (BHN) with a steel ball as the indenter.Meanwhile, Table 3 shows the reading for each experimental condition.ANOVA was utilized to identify the factors that contribute significantly to the hardness of AMC.Table 4 shows the hardness average and S/N ratio.To achieve the optimum hardness value of AMC, "Larger the better" was utilized.
Figure 4 shows a response graph of mean hardness for each factors.An optimum conditon of hardness value was achieved based on the conditions of; A3, B2, C3 and Based on ANOVA results as shown in Table 5. it is known that Factor A and Factor B improve the hardness.Factor C and Factor D gave less effect on the hardness of AMC.However, Factor D is an important source and can not be eliminated during the manufacturing process of AMC.
Table 5 also shows that the contribution error of 8.49% was obtained where such percentage means that all significant factors affect the average value; therefore, it is enough to be involved in the experiment.This is due to the requirement by Taguchi method that the message contribution must be ≤ 50%.The 90% confidence interval (Cl mean ) for the expected yield from the verification experiment can be determined by Equation ( 2 Cl mean = ±1.30 The confidence interval for the optimum mean surface hardness value is: 59.257 ± 1.30 kgf/mm 2 , which equivalent to = 57.957 to 60.557 kgf/mm 2 .
From the analysis of experimental result using Taguchi method, the addition of fly ash plays an important role to the hardness of AMC.The maximum hardness was given by the biggest weight percentage of fly ash composition.The second factor that contributes the highest value of hardness is the stirring speed of 350 rpm.It was found that the hardness deteriorated as the stirring speed increase than 350 rpm.It is believed that porosity was formed in the AMC due to the stirring blade as the speed increased.

CONCLUSIONS
Fabrication of AMC using the stir casting was succesfully conducted.The hardness of AMC was optimised by Taguchi method.ANOVA showed that two stir casting factors which are composition and stirring speed affect the AMC hardness significantly.The optimal level was found at A3 and B2.Based on ANOVA, the composition of AMC (Factor A) and strirring speed (Factor B) influence the hardness significantly by 74.19% and 10.23%, respectively.Meanwhile, the molten temperature of AMC (Factor D) gave no significant effect on the hardness of AMC.

TABLE 2 .
) as Experimental layout and factors distribution of L9 OA

TABLE 4 .
Hardness and S/N response values

TABLE 5 .
ANOVA result for aluminum matrix composite hardness