Microstructure, surface quality, residual stress, fatigue behavior and damage mechanisms of selective laser melted 304L stainless steel considering building direction

Abstract This study reports the comprehensive effects of building direction and scanning speed on the fatigue performance of 304L austenitic stainless steel (SS) fabricated by selective laser melting (SLM) through a series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), acoustic emission (AE), and thermal imager (TI). The fatigue tests were performed at a single stress level (90% of yield strength). The correlation analyses between fatigue life and the factors of surface roughness, porosity, and residual stress were performed to elucidate the comprehensive influence of these factors on fatigue lifetime. The combination of fractography analysis, AE signals, and TI signals were used to reveal the underlying microstructural mechanisms during cyclic deformation. Direct evidence is offered to show that the process of fatigue crack initiation promoted by cyclic deformation occupied most of the fatigue history. The synergistic effects of part densification, residual stress, and microstructure variable dominate the fatigue performance of SLM 304L SS. Clear evidence is shown that the higher density of high angle grain boundaries (HAGBs) on the top surface can hinder dislocation flow rate, promoting the dislocation piling-up at HAGBs, thus weakening the fatigue performance. In addition, the stronger texture on the top surface can improve the twinning behavior, strengthening the fatigue crack initiation resistance.