Small punch creep test reveals the differences of high-temperature creep behaviours for laser powder bed fusion and Rolled Inconel 718 alloys

Abstract

The laser powder bed fusion (LPBF) additive super alloy is with the manufacturing advantages in high temperature equipment, but their creep behaviour especially for the effective heat treatment is not well understood. In this paper, small punch creep test (SPCT) is used to compare the creep properties of Inconel 718 fabricated by LPBF and the counterpart processed via rolling, considering different heat treatments. After aging heat treatment, both creep deformation and life of LPBF Inconel 718 are much lower than those of rolled nickel-base alloy. In contrast, after solution and aging heat treatment, the creep deformation capacity and life of LPBF Inconel 718 are significantly improved, and the difference of creep resistance between them is greatly reduced. Based on the Chakrabarty membrane stretching model, the equivalent creep strain and equivalent creep stress are estimated. The equivalent creep strain-time curve proves that the solution and aging heat treatment effectively improves the creep resistance of LPBF Inconel 718. Then, based on Norton creep model, Larson Miller model and Monkman Grant model, the creep life equations of LPBF Inconel 718 and Rolled Inconel 718 under two heat treatment conditions are obtained. Finally, the fracture mechanism of Rolled Inconel 718 under aging heat treatment is the creep induced brittle intergranular cracking, while LPBF Inconel 718 is dominated by the creep induced layered brittle cracking with serrated propagation along columnar grains. After solution and aging heat treatment, intergranular brittle cracking caused by microcracks at grain boundaries occurs in Rolled Inconel 718, accompanied by the appearance of creep micro-voids. In comparison, the fracture mode in LPBF Inconel 718 is the zigzag cracking along columnar grain with the elongation and stagger of long columnar grains.