Effect of multiple thermal cycles on the microstructure evolution of GA151K alloy fabricated by laser-directed energy deposition

The microstructure and mechanical properties of high-performance Mg-RE (Magnesium rare earth) alloys prepared through laser-directed energy deposition (LDED) are worthy of an in-depth study, due to their excellent application perspectives in the aviation industry. In this work, a nominal Mg-15Gd-1Al-0.4Zr wt% (GA151K) alloy was fabricated by LDED, and the solid phase transformation induced by the application of multiple thermal cycles was investigated in detail. Moreover, in the last deposited layer, fine α-Mg grains, particle-like secondary phases, and intergranular island-like phases were observed. In addition, in the second layer from the top, α-Mg grains grew slightly by 1.4 ± 1.0 µm and the fraction of the intergranular island-like phase was significantly decreased by 9.3 ± 2.1%. As the deposition process progressed, the pre-solidified layers experienced additional thermal cycles, and two solid phase transformations occurred. One of the recorded transformations was that of the intergranular phase from Mg3Gd to Mg5Gd, while the other one was the precipitation of β′-Mg7Gd within the α-Mg matrix from the supersaturated solid solution. Here, the thermal cycles acted as an incomplete solution and aging treatment, leading to the unique morphology of the as-built GA151K alloy. The microhardness was improved by 6.9 ± 5.0 HV0.2 with the application of consecutive thermal cycles. This work established a simplified relationship between the process parameters, the microstructure, and the mechanical properties of LDED Mg-Gd alloys, which is expected to promote the development and application of LDED high-performance Mg-RE alloys.