Effects of non-equilibrium microstructures on microstructure evolution and mechanical properties of laser powder bed fusion IN625 Ni-based superalloy during long-term thermal exposure at 700 °C and 750 °C

Abstract

The effect of non-equilibrium microstructures on microstructure evolution and mechanical properties of laser powder bed fusion (LPBF) IN625 alloy during long-term thermal exposure at 700 °C and 750 °C was studied in this study. The as-built (AB) LPBF IN625 alloy had a typical non-equilibrium microstructure consisting of fine dendritic and cellular microstructures, Laves phase and high-density tangled dislocations. The non-equilibrium microstructure transformed into uniform recrystallized microstructure after solution treatment (ST). During long-term thermal exposure, δ phase in AB sample preferentially nucleated in inter-dendritic regions with high-density dislocations, while δ phase in ST specimen firstly nucleated at grain boundaries (GBs) and then grew into grains. The coarsening rate of δ phase in AB sample was lower and the size of δ phase was smaller than those in ST sample. The coarsening of Laves phase at GBs led to the depleted-δ zone in AB sample, which was not found in ST sample. For AB and ST samples, the ultimate tensile strength (UTS) and yield strength (YS) increased with the increase of thermal exposure time, while the elongation (EI) decreased. The tensile strength of AB sample was higher than that of ST sample during the thermal exposure at 700 °C, while their tensile strength was close during the thermal exposure at 750 °C. In addition, the anisotropy of mechanical properties of AB LPBF IN625 alloy was eliminated gradually during long-term thermal exposure. The microstructure evolution of LPBF IN625 alloy during long-term thermal exposure was explained by a newly proposed mold.