Microstructure and Mechanical Properties of High-Pressure Die-Casting Mg-Al-RE Alloys with Minor Ca Addition.

With the increasing demand for magnesium (Mg) alloys with high strength and good ductility, this study explores high-pressure die-cast (HPDC) Mg-6Al-2RE (AE62), Mg-8Al-2RE (AE82) and Mg-8Al-2RE-0.2Ca (AEX820) alloys (wt. %). Their microstructures and mechanical properties are investigated under both as-cast and T5-aged (direct artificial aging after casting) conditions. HPDC alloys consist of outer fine-grain regions and inner coarse α-Mg grains with abundant eutectic phases. The increasing addition of Al has an insignificant effect on the refinement of the grain size of α-Mg, but it significantly influences the morphology and area fraction of the second phases. The average grain sizes of α-Mg in HPDC AE62, AE82 and AEX820 alloys are approximately 4.0 μm, 3.9 μm and 3.7 μm in the edge regions and about 13.9 μm, 12.8 μm and 12.1 μm in the core regions, respectively. When aged at 200 °C, β-MgAl precipitates are predominantly formed in the studied alloys. Increasing the Al and Ca concentrations effectively refines the microstructures and enhances the aging hardening response and the strength, albeit at the expense of considerably reduced ductility. The peak-aged AE62 alloys demonstrate balanced tensile properties, with ultimate tensile strength (UTS), yield strength (YS), and elongation at fracture (E) at room temperature of ~241 MPa, ~141 MPa and ~7.1%, respectively, and values of ~129 MPa, ~96 MPa and ~19.8%, respectively, at 175 °C. Compared to peak-aged AE62 alloy, the UTS and YS of peak-aged AEX820 alloys are improved by ~6.7% and ~14.2%, respectively, at RT and ~8.5% and ~12.5% at 175 °C, while E is decreased by 35.2% at RT and 33.3% at 175 °C, primarily due to the high area fraction of secondary phases.