Heat-treatment induced microstructural refinement and mechanical properties optimization of laser powder bed fused superalloy

Aiming to investigate the effect of heat treatment on microstructure evolution and mechanical properties of nickel-based superalloy fabricated by laser powder bed fusion (LPBF), the microstructures of the samples in as-built, solid solution, and solution aging states are examined by optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Monotonic tensile tests are conducted on specimens in all three states at 25 °C and 650 °C. The results show that the heat treatment eliminates the laser scan tracks prevalent in the LPBF process and relieves the residual stresses. The grain morphology and orientation are not significantly altered. The solid solution treatment dissolves the Laves phase and precipitates needle-like or short rod-like δ phases, while the double-step aging treatment refines the δ phase and precipitates γ′ and γ'' phases. The specimens in all three states exhibit good resistance to elevated temperature in the elastic tensile stage at 650 °C. Solution aging significantly enhances the yield strength and tensile strength of LPBF nickel-based superalloy, increasing them by 66.3 % and 33.2 % at 25 °C, and by 54.1 % and 29.8 % at 650 °C, respectively. The tensile fracture modes of the specimens in all three states at 25 °C and 650 °C are ductile fractures.