Regulative effect of Mg/Si ratio on microstructure evolution and mechanical properties of cast Al-Li-Mg-Si alloys

Abstract

The effect of the Mg/Si ratio of Al-2.5Li-1Cu-0.8Mg-0.8Si, Al-2.5Li-1Cu-1.6Mg-0.8Si, and Al-2.5Li-1Cu-2.4Mg-0.8Si alloys on the microstructure evolution and mechanical properties was investigated. The results show that the primary phases and their morphologies in the as-cast alloys are found to vary with the Mg/Si ratio. The improvement of Mg/Si ratio of as-cast alloys promotes the formation of Mg2Si primary phase at the expense of the AlLiSi primary phase. Moreover, a tiny amount of TB-Al7.5Cu4Li phase transforms into S-Al2CuMg phase with the increase of Mg content. In addition, the increase of Mg/Si ratio also causes the Cu-rich intergranular phase distributed along crystal boundary to Si-rich intergranular phase. After ageing treatment, the precipitation sequence as a function of Mg/Si ratio is as follows: δ/δ′+AlLiSi (Mg/Si is ∼1) → δ/δ′+β′-Mg2Si+AlLiSi (Mg/Si is ∼2) → δ/δ′+β′-Mg2Si (Mg/Si is ∼3). A good combination of strength and ductility can be obtained in Al-2.5Li-1Cu-2.4Mg-0.8Si alloy after solution and ageing. The rod-like β′-Mg2Si precipitate has a positive influence on the comprehensive mechanical properties of the alloy.