Microstructure and fatigue behavior of laser-powder bed fusion austenitic stainless steel

The microstructures and stress-controlled fatigue behavior of austenitic stainless steel (AISI 316 L stainless steel) fabricated via laser-powder bed fusion (L-PBF) technique were investigated. For L-PBF process, zigzag laser scanning strategy (scan rotation between successive layer was 0°, ZZ sample) and cross-hatching layer scanning strategy (scan rotation between successive layer was 67°, CH sample) were employed. By inducing different thermal history, it is found that the scan strategies of laser beam have a significant impact on grain size and morphology. Fatigue cracks generally initiated from persistent slip bands (PSBs) or grain boundaries (GBs). It is observed that PSBs could transfer the melt pool boundaries (MPBs) continuously. The MPBs have better strain compatibility compared with grain boundaries (GBs), thus MPBs would not be the initiation site of fatigue cracks. A higher fatigue limit strength could be achieved by employing a crosshatching scanning strategy. For the CH sample, fatigue cracks also initiated from GBs and PSBs. However, fatigue crack initiated from process-induced defects were observed in ZZ sample in high-cycle regions. Solidification microstructures and defects characteristics are important factors affecting the fatigue performance of L-PBF 316 L stainless. Process-induced defects originated from fluid instability can be effectively reduced by adjusting the laser scan strategy.