Influence of cooling condition after solidification on T5 heat treatment response of hypoeutectic Al-7Si-0.4Mg casting alloy

Abstract

The effect of the cooling conditions after solidification of a hypoeutectic Al-7Si-0.4Mg alloy on the precipitation behavior during T5 heat treatment (artificial aging immediately after casting) was investigated. Al-7Si-0.4Mg alloys with two different cooling conditions, mold cooling and water quenching, were respectively fabricated by holding the sample in a permanent mold throughout solidification and by placing the sample in room temperature water just after solidification was completed. TEM analysis of the as-cast alloys confirmed that mold cooling after solidification led to the formation of a small amount of clusters, Si and B ' in the mold which may have pre-precipitated during cooling, increasing the hardness and strength of the as-cast alloy. However, the hardenability of the alloy with mold cooling during T5 heat treatment is less pronounced than the alloy with water quenching. This suggests that the pre-precipitation in the alloy with mold cooling suppresses the aging hardening response, reducing the overall strength of the alloy with T5 heat treatment compared to that of the alloy with no pre-precipitation. Analysis of precipitation kinetics and number densities of precipitates shows that the pre-precipitation can interfere with the precipitation of beta", otherwise promoting the precipitation of coarse beta', U2 and Si, which reduced overall T5 precipitation hardenability. Therefore, for the T5 heat treatment condition, we suggest that the cooling conditions after solidification are a key factor affecting the best performance of precipitation hardening.(c) 2022 Published by Elsevier B.V.