Low cycle fatigue of additively manufactured thin-walled stainless steel 316L

To ensure the robust design freedom of metallic additive manufacturing, the fatigue properties and the dimensional limitation of as-built components by laser powder bed fusion (PBF-LB) are investigated. Fully reversed and strain-controlled fatigue tests were carried out on tubular specimens with different wall thicknesses, 1 mm and 2 mm, for the purpose of studying the thin-wall effect without having risk of buckling problem during compression. Two wrought conditions are also enclosed as a comparison, which are the cold worked (CW) and solution annealed condition (SA). In the as-built PBF-LB tubular specimens, deformed microstructure and deformation twins are discovered close to the surface region, together with a higher roughness of the inner surface due to the heat accumulation. The surface roughness is evaluated as micro-notches, and a higher fatigue notch factor, K-f, at lower applied strain range is revealed. The factors influencing K-f include, the non-conductive inclusions serving as crack initiation sites at the surface region, and the deformation twins formed by the local stress concentration. The strain-life of PBF-LB samples is comparable with the wrought samples. However, the fatigue strength of the responding mid-life stress shows greater difference and is in the following order, CW wrought > PBF-LB > SA wrought. Secondary cyclic hardening owing to deformation induced martensitic transformation is found in both of the wrought samples. Yet, only cyclic softening exhibits in the PBF-LB samples, which is the result of the suppressed martensitic transformation and the dislocation unpinning from the cell boundaries.