Effect of Ca Contents on the Microstructure and Properties of Friction Stir Processed Mg-2.5Si-4Zn-xCa Alloys

The study investigates the influence of Ca addition (0, 0.5, 0.7, and 1 wt.%) on the microstructure and mechanical properties of friction stir-processed (FSPed) Mg-2.5Si-4Zn-xCa alloys. The microstructures of the processed alloys were characterized using OM, SEM, EBSD, and TEM. The results indicated that Ca addition combined with FSP can synergistically refine and homogenize the Mg2Si phase, and with increasing Ca content, the size of both primary and eutectic Mg2Si phases first decreases and then increases, reaching an optimum refinement at 0.7 wt.% Ca. In this composition, the Mg2Si phases were uniformly dispersed, and the stir zone exhibited significantly refined recrystallized grains compared to its Ca-free counterpart. Under identical FSP conditions, the Mg2Si phases in the Ca-containing alloys underwent a higher degree of fragmentation. The addition of Ca promoted the formation of the CaMgSi phase by enriching Ca atoms at or near the Mg2Si phases during FSP, which further assisted in fragmenting the Mg2Si phases. Consequently, the alloy with 0.7 wt.% Ca demonstrated the best mechanical properties at both room and elevated temperatures, exhibiting tensile strength and elongation of 276.57 MPa and 11.60% at room temperature, 190.13 MPa and 12.17% at 150 °C, and 133.43 MPa and 15.96% at 200 °C, respectively.