Effects of minor Ca addition on microstructure and mechanical properties of the Mg–4.5Zn–4.5Sn–2Al-based alloy system

Abstract

Effects of minor Ca addition (0, 0.2, 0.4 and 0.6mass%) on microstructure and mechanical properties of the as-cast and the as-rolled Mg–4.5Zn–4.5Sn–2Al-based alloy system are investigated by OM, XRD, SEM and mechanical testing. The alloy castings are produced by water-cooling copper casting under pressure. Minor Ca addition can effectively refine both grains and grain-boundary compounds and this effect is more obvious with a higher Ca addition. However, a larger amount of grain-boundary compounds are detected with a higher Ca addition, resulting in the inferior roll forming ability. The as-cast alloy with 0.2% Ca addition exhibits a fairly good roll forming ability and the optimal combined mechanical properties, with the ultimate tensile strength and elongation of 243MPa and 13.9%, respectively. The 0.2% Ca addition plays important roles in refining twins and DRX grains, increasing twinning density and promoting precipitations in the as-rolled alloy, contributing to the enhanced strength. The as-rolled Mg–4.5Zn–4.5Sn–2Al–0.2Ca alloy exhibits the ultimate tensile strength of 406MPa and the yield strength of 285MPa, about 18% and 26% higher than the Ca-free alloy. However, the 0.2% Ca addition reduces the plasticity of the as-rolled alloy.