Enhancement and underlying fatigue mechanisms of laser powder bed fusion additive-manufactured 316L stainless steel

Abstract

In this study, the enhancement of additively manufactured (AM) 316L, by annealing, to the fully reversed tension-compression fatigue performance, in terms of fatigue life and fatigue damage, were investigated under two conditions: as-built (AB) and heat-treated (HT) at 900 °C. The underlying fatigue mechanisms were comprehensively characterised through intensive microstructural observations of cyclic-strained microstructures and fracture surfaces using laser confocal scanning microscopy (LCSM) and secondary electron imaging using scanning electron microscopy (SEM). The experimental results showed that the fatigue resistance of HT 316L was significantly enhanced by 100% as the fatigue limit was increased from 75 to 150 MPa for AB and HT 316L, respectively. The fatigue cracking mechanism in AB 316L is mainly related to two imperfections of the AM-induced microstructural components: residual stresses, which cause highly localised deformation, and dendritic cellular structures, which possess a weak link in their grain boundaries against crack propagation. Upon heat treatment at 900 °C, the residual stresses and dendritic structure were effectively reduced. Consequently, the fatigue life of AM 316L was significantly enhanced by promoting the formation of high-angle boundaries. More precisely, the cyclic deformation processes in fatigued HT 316L involve persistent slip bands and strain hardening.