Mechanisms driving high-cycle fatigue life of as-built Inconel 718 processed by laser powder bed fusion

Abstract

This study investigates the relationships among the high-cycle fatigue life, surface roughness, and additive manufacturing processing parameters in laser powder bed fusion Inconel 718 in the as-built condition. Standardized fatigue specimens were manufactured using 25 different sets of processing parameters by varying laser power, scan speed, layer thickness, and build orientation, with three repeat specimens per parameter set. Surface roughness measurements were conducted using white light interferometry; high-cycle fatigue life was measured; and fractography analysis was performed using scanning electron microscopy. Two processing-parameter metrics were observed to dominate high-cycle fatigue life: build orientation and laser-energy density. Build orientation affected fatigue life due to the relationship between build orientation and surface roughness. Increasing surface roughness decreased the fatigue life due to increasing number of surface-crack initiation sites. For a fixed build orientation, the laser-energy density, outside of the optimal range, decreased the fatigue life due to sub-surface defects. Specifically, fractography analysis showed that sub-surface defects consisted of lack-of-fusion pores at low laser-energy densities and secondary cracking and pores (possibly related to keyholing) at high laser-energy densities. While variability in residual stresses among the specimens could also play a role, this work focuses on geometrical surface and sub-surface defects caused by different processing parameters and their corresponding impact on total fatigue life. Based on these findings, guidelines are offered to improve fatigue life of additively manufactured Inconel 718 in the as-built, non-heat-treated condition.