Comparative Study of Fatigue Behavior and Microstructural Evolution in As-Built and Heat-Treated Additively Manufactured 316L Stainless Steel

This study investigates the influence of heat treatment (HT) at 900 °C on the fatigue resistance of 316L stainless steel fabricated through selective laser melting (SLM). Fully reversed, force-controlled fatigue tests were conducted on both as-built (AB) and HTed specimens to assess their cyclic deformation behavior and fatigue life. The fatigue fracture mechanisms were analyzed through detailed microstructural characterization using secondary electron imaging in a scanning electron microscope (SEM) and laser scanning confocal microscope LSCM. Results show that the HT 316L exhibited improved fatigue resistance and a longer fatigue life compared to the AB 316L. Fatigue cracking along dendritic columnar grains and the formation of slip bands were identified as key microstructural features in both AB and HT materials. In the AB material, the columnar dendritic grains and cellular substructure appear to create weak points at grain boundaries, facilitating fatigue crack initiation due to localized strain in persistent slip bands. However, HT at 900 °C effectively reduced the cellular substructure, promoting the formation of high-angle grain boundaries, which significantly enhanced the fatigue resistance of HT 316L.