The Effect of Dual Particle Size SIC Reinforcements and Heat Treatment on Microstructure, Mechanical and Tribological Properties of A357 Composites

Abstract

The demand for light-weight materials is increasing in the automobile industry due to the increasing cost of fuel. In particular, there is a huge demand for high-strength and wear-resistant materials for engine cylinder applications. Al-Si-Mg series alloys such as A357 alloy would be an ideal choice for such applications owing to their low density, excellent castability, high strength and wear resistance. Enhancing the strength of any material can be achieved by work hardening, heat treatment, or reinforcing with a hard phase. The present work focused on development of A357 composites wherein the A357 alloy was reinforced with dual-size SiC particles. In the current work, two different sizes of SiC particles (coarse (140 ± 10μm)) and (fine (30 ± 5 μm)) were used to reinforce A357 alloy. Stir casting was used to develop A357 composites, with different weight ratios of the two sizes of SiC powders, keeping the total weight fraction at 6%. Three composites were cast in finger moulds; DPS1 (coarse: fine;1:1), DPS2 (coarse: fine;2:1), and DPS3 (coarse: fine; 1:2). The cast A357 alloy as well as the composites were subjected to heat treatment as per T6 temper conditions. The effect of varying solution temperature (500ºC to 540ºC in steps of 20°C for 9h and keeping aging temperature constant at 150 ºC for 6 h) and aging temperature (160°C to 200°C in steps of 20°C for 6h and keeping solution temperature constant at 540 ºC for 9h) were studied for both A357 alloy and the developed composites. Both A357 alloy and dual-size SiC reinforced composites were subjected to microstructural analysis using optical, scanning, and transmission electron microscopy techniques. Hardness and tensile testing were carried on the A357 alloy and its DPS composites before and after heat treatment. Tribological properties namely wear rate was assessed by conducting dry-sliding wear test using a pin-on-disc machine. In the wear test, the effect of varying load on wear rate was studied by keeping sliding velocity and sliding distance constant. Worn surface analysis was carried out using SEM to study the wear mechanisms operating in both untreated and heat-treated alloy and composites. Mechanical testing results showed improved hardness, yield, and tensile strength values for DPS composites when compared with that of A357 alloy. The strengthening of A357 composites is based on the addition of hard phase like SiC particles to the A357 alloy. The strengthening mechanisms that contributed to improvement in properties were effective load transfer, precipitation hardening and dislocation strengthening due to thermal mismatch. Precipitation hardening occurs for the A357 alloy and its composites because of T6 heat treatment. Formation of βʺ phase and Mg2Si precipitates were primarily responsible for strengthening after heat treatment. Wear rate of composites was found to be less than that of A357 alloy. Prohibition of direct contact between the two mating surfaces by presence of dual-size SiC particles was one of the primary reasons for low wear rate in composites. The key conclusions from this work include: Among the three developed composites, hardness, and wear resistance of DPS2 composite before and after heat treatment was found to be significantly higher than the other two composites (DPS1 and DPS3). Also, the tensile and yield strength values of DPS3 composite before and after heat treatment was found to be significantly higher when compared to the other two composites (DPS1 and DPS2). Lastly, the ratio of coarse particles to fine particles has an effect on the mechanical and tribological properties. Presence of more fine particles was found to be good for strength and ductility whereas more coarse particles were found to be good for hardness and wear resistance.