Investigation of the Optimal Heat Treatment of As-Cast Al-5.7Si-2Cu-0.3Mg Aluminium Alloys

Abstract

The aim of this work is to investigate the optimum heat treatment for Al-5.7Si-2Cu-0.3Mg aluminium alloys and study its effect on microstructure, phase transformations, and hardness. The test specimens were taken from the as-received alloy. Solution treatment was performed at 485°C and 500°C under various solution treatment times for 4, 8, 10, and 12h, and the samples were then hot water quenched at 60°C, followed by aged hardening at 150°C, 170°C and 190°C for 2,6,10, and 14h, and subsequently air-cooled. The hardness of the Al-5.7%Si-2Cu%-0.3%Mg alloys were determined using a Rockwell hardness tester. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to determine the microstructure of the samples, while X-ray diffraction (XRD) was used to identify the phase compositions. The resulting microstructures and hardness values were compared to the corresponding as-cast samples. It can be seen that the solution treatment at 485°C for 12 h and aging at 190°C for 10 h are the optimum T6 heat treatment conditions that would result in hardening precipitates over the as-cast alloy. OM and SEM morphologies show significant microstructure evaluation of improved distribution of the Si particles. After T6 treatment, the morphology of Si particles in the as-cast Al-5.7Si-2Cu-0.3Mg alloy changes from long and coarse plate-like grains to fine spherical shaped grain. The XRD plots confirmed the relatively high concentration of Al, Si, and Al2Cu in the heat treated Al-5.7%Si-2Cu%-0.3%Mg alloy relative to that of the as-cast alloy. The hardness of the T6 alloy also increased.