Research on the Microstructural Evolution of Injection Molded AZ91 and Ultralight LAZ561Ca Magnesium Alloys during Solidification and Heat Treatment: Multiscale Characterizations and Multiphase Field Modeling

AZ91 magnesium injection molding is suitable for manufacturing complex-shaped electronic product frames or thin plates. However, the strengthening effect of the Mg 17 Al 12 precipitate in AZ91 is limited, and it tends to dissolve during heat treatment, leading to a lack of particles that can pin grain boundaries and prevent grain growth. To address these challenges, the LAZ561Ca alloy has been developed, offering a reduced density (83% of AZ91), AlLi nanoprecipitates with strong strengthening capabilities, and thermally stable Ca-bearing intermetallics that effectively pin grain boundaries, maintaining a fine-grained structure (~8 μm) even after heat treatment. Experimental results demonstrate that AZ91 undergoes abnormal grain growth after solution treatment at 400°C due to a significant reduction in Zener pinning forces. In contrast, the LAZ561Ca alloy, with stable Al 2 Ca precipitates, resists such growth during two-stage heat treatment at 370°C – 400°C. Through the coupling between Thermo-Calc and MICRESS software, multiphase field modeling reasonably reproduced the microstructure evolution during injection molding and heat treatment processes, highlighting its value in establishing digital physical metallurgy models. This study reveals the microstructural mechanisms of magnesium alloys, confirming the critical role of Ca-bearing precipitates in grain growth suppression. It provides a foundation for further optimization of alloy compositions and heat treatment conditions, paving the way for advanced magnesium alloys with enhanced performance in injection molding applications.