Mechanical behaviours of the hierarchical microstructure of PBF-LB/M 316L SS during high cycle fatigue

A thorough understanding of the relations between the processing parameters, microstructures, and fatigue performances is essential for applying Laser Powder Bed Fusion (PBF-LB/M) 316L stainless steel (SS). The close study of the evolution of its hierarchical microstructure during high cycle fatigue enables an understanding of the fatigue mechanisms. This work investigates the evolution of the microstructures of two sets of PBF-LB/M 316L SS specimens produced with two preheating temperatures of the building platform, namely 200°C and 400°C, during high cycle fatigue (HCF) tests. The fatigue tests were performed in a quasi-in-situ way to facilitate the observation of the local plasticity down to the level of dislocation cells. It was found that the lack of fusion defects dominate the fatigue crack initiation, while plasticity is constrained in the interior of the subgrains. The stable dislocation cells are also confirmed to resist plastic deformation and contribute to the strength of the material. • Higher preheating temperature of the PBF-LB/M building platform results in higher porosity, larger dislocation cells, and lower local misorientation. • The deformation during HCF loading is mainly constrained at the subgrain level. • The stable dislocation cells resist dislocation slip throughout the HCF tests. • Plasticity was dominated by dislocation slip with limited activation of twinning close to the main fatigue crack.