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In this work, stainless steel 316L obtained by Laser Powder Bed Fusion (L‐PBF) has been produced and characterized. The experimental campaign focuses on the samples in the as‐built state under cyclic tension‐compression loadings. Low cycle fatigue (LCF) and high cycle fatigue (HCF) tests are carried out. Microstructure observations are performed before and after the loadings. As‐built L‐PBF 316L has a good LCF performance despite the presence of surface defects but a low fatigue limit in the HCF regime. Removing the surface roughness has a beneficial effect but does not improve the fatigue strength to that of machined wrought 316L. Observations on the microstructure of fatigue samples indicate that LCF is dominated by the local plastic deformation, which is mostly achieved by the slip in the studied material and is not sensitive to the defect. The lack‐of‐fusion defect impairs the fatigue resistance in the HCF regime. Highlights: Monotonic tensile and LCF behavior of L‐PBF 316L are not sensitive to the surface defects.HCF behavior of L‐PBF 316L is dominated by the surface or subsurface defects.Slip planes produced during cyclic deformation are not affected by the melt pool boundaries. [ABSTRACT FROM AUTHOR]

• Microstructure-sensitive framework has been extended and applied to SLM 316L. • As-built surface roughness has been considered explicitly in the finite element model. • Textured microstructure of SLM 316L has been evaluated concerning fatigue strength. • Competitive effects of surface defect and microstructure have been distinguished. This paper aims to study the individual and competitive effects of surface defects and microstructural components in an AM 316L by numerical methods. Based on the microstructure sensitive modeling framework, numerical simulations of models in different surface states with virtual quasi-realistic explicit microstructures are performed. Problems encountered during the establishment of the numerical polycrystalline aggregate model are discussed, including the grain morphology, the strongly textured microstructure with preferential crystallographic directions, roughness and surface defect. A statistical method based on generalized extreme value distribution along with a non-local method is used to describe the stochasticity of microstructural attributes. Simulations of differently configured explicit microstructure models are analyzed and discussed. [ABSTRACT FROM AUTHOR]

• Experimental torsion and bending fatigue strengths of 90°, 45°, and 0° LPBF building direction were determined. • Kitagawa diagrams were provided in bending and torsion loading at different building directions. • Bending fatigue behavior is very sensitive to building direction compared to torsion. • Torsion fatigue behavior of LPBF 316L is not sensitive to lack-of-fusion defects. The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested. Stress-relieving heat treatment was carried out to reduce the residual stresses. The 90°, 45°, and 0° built near-net-shape and polished specimens have fatigue strengths between 140 and 250 MPa under bending loading and 150 and 170 MPa under torsion loading. This difference illustrates a more pronounced defect sensitivity in bending compared to torsion. Fractographical analyses revealed the fatigue failure mechanisms. As the building direction inclines from vertical to horizontal, the effective areas of inherent defects diminish contributing to improving fatigue strength under bending loading whilst the differences from building directions in torsional fatigue strengths are minor. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]