The Effects of Silicon Addition on the Microstructure and Mechanical Properties of a Mg-Al-Sn Alloy Produced by Vacuum Assisted High Pressure Die Casting

In this paper, the effects of a small addition of Si (about 0.4wtpct) on the microstructure and mechanical properties of a Mg-Al-Sn alloy produced with vacuum assisted high pressure die casting (HPDC) are reported. CALculation of PHase Diagrams-based modeling was used to design the heat treatment schedules to optimize the precipitation of Mg2Sn and Mg17Al12 in the Mg-7Al-2Sn-0.4Si alloy (in wtpct, designated as ATS). The HPDC test specimens were studied in the as-cast condition as well as in multiple heat-treated conditions to improve their tensile properties. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used to characterize the microstructure in each condition. The results suggest that the small addition of Si is effective in enhancing and accelerating the precipitation hardening of Mg-7Al-2Sn alloy AT72, due to enhanced precipitation of Mg17Al12 and Mg2Sn phases as well as the precipitation of Mg2Si phase. In as-cast conditions, the new ATS alloy has similar strengths but significantly improved ductility compared with AZ91D, and significantly improved strength but reduced ductility compared with AM60B. The ATS-T6 after a one-stage solution treatment provides further improvement of mechanical properties: yield strength 151MPa, ultimate tensile strength 253MPa, and elongation 4.9pct.