Effect of Al content on corrosion behavior and mechanical properties of mold casting and high-pressure die casting Mg alloys

The Al content plays a critical role in determining the corrosion and mechanical properties of AZ series Mg alloys. These properties have been widely investigated; however, the effects of high Al content above 9 wt.% on corrosion behavior, mechanical properties, and their relationship have not been explored. In this study, Mg–xAl–0.8Zn–0.1Mn–0.3Ca–0.2Y (x: 6, 9, 11, 13) alloys were prepared via mold casting (with a lower cooling rate) and high-pressure die casting (with a higher cooling rate) to investigate the effects of Al content on microstructure, corrosion behavior, and tensile properties. In particular, the tensile properties were evaluated before and after the salt spray test (SST). The corrosion rate was lower for the high-pressure die-casting alloy with a higher cooling rate than for the mold-casting alloy with a lower cooling rate. For mold-cast alloys, the corrosion rate increased significantly with increasing Al content. In contrast, for die-cast alloys, the corrosion rate initially increased and then decreased with higher Al content. The corrosion behavior was revealed to be related to the fraction and continuity of the β-Mg₁₇Al₁₂ phase and the Zn content within this phase through the EPMA and SKPFM analysis. Regarding mechanical properties, in both their initial state and their SST-exposed state, die-cast alloys exhibited improved strength with increasing Al content. Although the mechanical properties decreased after SST exposure, the extent of deterioration diminished with higher Al content.