Effect of Pre-Ageing on the Precipitation Behaviors and Mechanical Properties of Al-7Si-Mg Alloys

Abstract

For alloys that are quenched to 25˚C room temperature, there are Mg-Mg, Si-Si and Mg-Si clusters shown by exothermic peaks in a A357 alloy. However, there are no clear Mg-Mg, Si-Si and Mg-Si cluster exothermic peaks in a A356 alloy. Hence, the low Mg content in a A356 alloy has less impact with natural ageing. The natural ageing impact on the mechanical properties of a A357 alloy is higher than a A356 alloy due to the precipitation of Mg-Si clusters for which the nucleation size does not reach the critical size. The 90˚C pre-ageing process could promote Mg-Si clusters to a critical size to become the nucleation site for of β''. This then increases the artificial ageing strength and mitigates the impact of natural ageing.