A multi-scale experimental investigation for fatigue limit and fatigue crack initiation behavior of powder bed fusion-laser beam 316L stainless steel

In this study, the fatigue limit of powder bed fusion-laser beam (PBF-LB) 316L prepared by various printing power intensities was studied. After finding the PBF-LB printing parameters giving the best fatigue limit, the fatigue crack initiation (FCI) behavior of dense PBF-LB 316L manufactured by the parameters corresponding to the best fatigue limit was investigated. To this end, the fatigue limit was evaluated by self-heating experiments, and a novel method based on the acoustic emission (AE) technique was suggested to assess the FCI stage during the whole fatigue process. In addition, the AE source mechanism was studied to validate the proposed method, and the fatigue behavior and the fatigue cracking mechanism were deeply clarified through detailed characterizations of microstructures and metallurgical defects. The results indicate that the fatigue limit of dense PBFLB 316L is affected by a combined effect of the subgrain size and gas pores. However, for the defective PBF-LB 316L, the LOF (lack of fusion) defects induced by low printing power play a crucial role in deteriorating the fatigue limit mainly by promoting FCI. Besides, the printing parameter corresponding to the best fatigue limit is found to be 60 J/mm3 in terms of the volume energy density (VED). Furthermore, as for the FCI behavior, it is shown that the percentage of the FCI stage of dense PBF-LB 316L ranges from 12.4% to 45.3% during the whole fatigue process, and the fatigue crack scale after FCI is on a micro level (from 3.8 to 46.6 mu m). Moreover, areas around the fine grains (FGs) and coarse grains (CGs) regions and the junctions between ferrite and austenite are the sites where fatigue cracks are prone to initiate.