High cycle fatigue behavior of 316L steel fabricated by laser powder bed fusion: Effects of surface defect and loading mode

• Torsion fatigue limit is found higher than bending fatigue limit for LPBF 316L. • Effect of lack-of-fusion defect dominates over the effect of stress gradient . • Skipping contouring introduces clustering defects without increasing surface roughness. • Clustering surface defects act synergistically as one to initiate the fatigue crack. The mechanical performances of additive manufactured (AM) material are highly dependent on the fabrication process which inevitably results in surface imperfection as well as porosity. In the present study, the high cycle fatigue (HCF) behavior of an AM stainless steel 316L is experimentally investigated to characterize and evaluate the effect of the inherent surface defects. Profilometry and Computed Tomography are used. A series of fatigue experiments is carried out under different loading modes including tension, bending, and torsion fatigue tests. For each loading condition, different surface preparations are used to investigate the effect of surface state. Fatigue tests reveal that surface treatment can improve fatigue performances, the improvements observed being higher under tension/bending loading than under torsion loading. The fractographic analysis is performed for all the available tested specimens to reveal the mechanism of fatigue crack initiation. Lack-of-fusion (LoF) defects play the predominant role in the fatigue performance of SS 316L fabricated by laser powder bed fusion (LPBF). The presence of multiple LoF defects at the surface or subsurface is detrimental to the endurance under cyclic loading. By using Murakami approach modeling the relationship between fatigue strength and defect size, it is found that the multiple clustering defects act synergistically as one large virtual crack to initiate the fatigue crack.