Enhancement of unique heat treatment process on microstructure and mechanical properties of thixomolded complex lightweight AZ91D magnesium alloy laptop shell with ultra-thin wall thickness

In this research, thixomolding was conducted to thixoform complex AZ91D magnesium alloy laptop shell with ultra-thin wall thickness of 0.6 mm and lightweight of 192 g. Subsequently, unique heat treatment process (i.e., low temperature and short time solution and artificial aging) was proposed to achieve collaborative enhancement of strength and ductility. Thixomolding process guaranteed exceptional surface quality, preeminent dimensional accuracy, dense microstructure, excellent grain refinement and adequate filling of laptop shell due to high pressure and sub-rapid solidification. The optimal parameter including 380 °C-0.5 h solution and 180 °C-4 h aging was obtained via single variable experiments. Consequently, the distinguished mechanical properties of 351 MPa (ultimate tensile strength), 201 MPa (yield strength) and 7.2 % (elongation) were achieved, realizing comprehensive improvement of 24.0 %, 8.6 % and 157 %, respectively, compared to without heat treatment. In thixomolding process, the second phases caused mixed fracture dominated by brittle fracture. With optimal solution treatment, the second phases dissolved and realized completely ductile fracture. After aging treatment, the nano precipitates were dispersed uniformly in the α-Mg matrix and transformed into mixed fracture dominated by ductile fracture. Nanoscale phases of β-Mg₁₇Al₁₂, Mg₁₆Al₁₂Zn and Al₈Mn₅ dispersed precipitation within the matrix after unique heat treatment, cooperated with dislocations and twins to jointly strengthen the laptop shell. The calculated contributions of precipitate phases strengthening, grain refining strengthening, dislocation strengthening, and texture strengthening were 41.3 %, 34.4 %, 16.3 % and 8.0 %, respectively. This paper provided new alternative for ultra-thin-walled lightweight magnesium alloy products, achieving the breakthrough for AZ91D alloy in mechanical properties.