Improvement of fatigue performance of laser powder bed fusion fabricated IN625 and IN718 superalloys via shot peening

Abstract

This study aims at investigating the possibility of fabricating IN625 and IN718 Ni-based superalloys with acceptable room temperature fatigue lives using the laser powder bed fusion (LPBF) process. For this purpose, several test coupons were printed and compared in terms of (i) density, (ii) tensile strength, (iii) surface roughness, and (iv) residual stress to find the optimum sets of process parameters utilized to print fatigue test specimens. Fatigue test was performed on the as-built and shot-peened specimens at multiple stress levels and compared to their wrought counterparts. The fatigue results were scrutinized by discussing the contributing factors, such as (i) surface roughness, (ii) residual stress state and magnitude, (iii) grain and sub-grain structure, and (iv) non-equilibrium phases in the microstructure. The fatigue life of both superalloys in as-built condition was inferior to their unnotched wrought counterparts, though exceeding the notched ones. After shot peening, the maximum stress level at infinite life for IN625 was the same as its unnotched annealed wrought counterpart. However, for shot-peened IN718, the fatigue life was still lower than that of the solution-annealed and aged wrought specimen in unnotched condition due to the absence of γ′ and γ′′ strengthening agents. This study suggests that the LPBF fabricated IN625 in the shot-peened state can be reliably used as a substitute for annealed and wrought IN625 counterparts without the need for surface smoothening or heat treatments. Regarding the shot-peened IN718, its lower fatigue life has to be considered in the design stage upon skipping the heat treatment.