Microstructure and property optimization of thixomolding AZ91D magnesium alloy for automotive navigation housing applications

The effects of key thixomolding and T6 heat treatment on the microstructure and mechanical properties of AZ91D magnesium alloy for new energy vehicle navigation brackets were investigated. Optimized thixomolding conditions yielded fine and nearly spheroidized α-Mg grains with uniformly distributed secondary phases, facilitating a coordinated enhancement of both strength and ductility. The optimal performance was achieved at a slurry temperature of 580 °C, mold temperature of 250 °C, injection speed of 2.25 m/s, and die-casting pressure of 10 MPa, with a maximum ultimate tensile strength (UTS) of 263.9 MPa and an elongation of 6.6%. After solution treatment, the UTS further increased to 275.5 MPa and the elongation rose to 7.9% due to the dissolution of coarse brittle phases. Subsequent aging led to significant precipitation of β-Mg₁₇Al₁₂and Mg₁₆Al₁₂Zn, which increased the yield strength and UTS to 181.9 and 287.7 MPa, while slightly reducing the elongation to 4.8%. The coherent interfaces between the precipitates and the matrix, along with the localized strain accumulation, contributed to the strengthening of the alloy. Range analysis revealed the relative influence of each factor on mechanical properties as follows: injection speed > slurry temperature > mold temperature > die-casting pressure.