Microstructure evolution of laser powder bed fusion ZK60 Mg alloy after different heat treatment

The laser powder bed fusion (LPBF) was employed to process the ZK60 Mg alloy. The effect of heat treatment on the microstructure and hardness of LPBF ZK60 Mg alloy was investigated. The results showed that the grain boundary of LPBF ZK60 Mg alloy was composed of a network Mg₇Zn₃ phase. Three types of precipitated phases were distributed inside the grains: the type-Ⅰ is Zn-rich precipitated phase with Zr-rich core, the type-Ⅱ is Zn-rich precipitated phase without Zr, the type-Ⅲ is Zr-rich particle. When the solution treatment was used, the Mg₇Zn₃ phase at the grain boundary melted and merged into the matrix. The precipitated phase in the grain mainly evolved into Zn-rich phase with Zn₂Zr core. Continue with the aging heat treatment, the rod-shaped β₁′-MgZn precipitated phase separated out from the α-Mg matrix. The length of the rod-shaped precipitated phase undergone double aging is longer and the quantity is more than that of the single aging. The rod-shaped β₁′-MgZn precipitated phase significantly improved the mechanical properties of LPBF ZK60 Mg alloy. The average hardness increased from 0.88 ± 0.025 GPa to 1.01 ± 0.010 GPa and the ultimate tensile strength increased to 287 ± 4 MPa after the solution treatment plus double aging heat treatment.