T6 age hardening treatment of conventionally cast and semi-solid processed A356 aluminum alloy

摘要

In this work, the effect of T6 heat treatment on microstructure and hardness of Al-7.0Si-0.3Mg alloy (A356) produced in metal moulds by conventional casting and by semi-solid metal casting using a sloping copper cooling plate technique was investigated. The T6 age hardening treatment consisted of solution treatment at 540℃ for 6 hours followed by aging at 140, 160, 180℃ for 6-54 hours. The microstructures were examined by XRD, OM and SEM. Rockwell macro-hardness was measured. It was found that the as-cast microstructure in the conventionally cast alloy consisted of α-Al dendrites with eutectic Si and interdendritic Al5FeSi phases. In the semi-solid cast alloy, the primary a-Al phase changed to a globular shape and the eutectic structure was finer compared to the conventional casting alloy. The microstructures after solution treatment and T6 aging are similar to those of the as-cast condition, except that some spheroidisation of the eutectic Si occurred and some intermetallic phases have been taken into solution. The area fraction of eutectic constituents was decreased from the as-cast condition. Peak hardness of both alloys was obtained after aging at 160℃ for 30 hour. The hardness of semi-solid cast alloy after T6 heat treatment was higher than in the equivalent conventionally cast alloy.