Observations on the influence of process and corrosion related defects on the fatigue strength of 316L stainless steel manufactured by Laser Powder Bed Fusion (L-PBF)

Abstract

Corrosive environments are known to be detrimental to the mechanical strength of metallic alloys. In the case of 316L stainless steel, the main corrosion mechanism observed is pitting, which leads to localized rough defects. As for other materials submitted to cyclic loadings and prone to pit, corrosion defects tend to be at the core of crack initiation leading to failure. The work here-by presented will thus focus on the relationship between the fatigue performance and the presence of process or corrosion related defects for a 316L stainless steel manufactured by Laser Powder Bed Fusion (L-PBF). To do so, cylindrical samples were produced vertically, then machined to fatigue specimens in as-built state (no heat treatment). Specimens were polished in order to only characterize the bulk material and not its raw surface. The fatigue responses of three batches corresponding to three configurations of surface integrity (polished, pre-corroded and with an Electric Discharge Machined (EDM) defect) were investigated. For the polished batch, fracture surface observations showed that initiation started from a lack of fusion (LoF) surface pore in all specimens. The pre-corroded batch was prepared under potentiodynamic anodic polarization conditions in a neutral NaCl solution. On those samples, some cracks initiated on corrosion pits, depending on the severity of the corrosion applied to each sample and its initial population of defects. For the EDM batch, a hemispherical defect, at the source of all failures, was machined in the middle of the gauge length. A good correlation between Murakami’s square root of the area parameter and the fatigue strength was observed on a Kitagawa-Takahashi diagram for all tested specimens. This correlation showed an independence of the defect type (LoF, pit, machined defect) on the specimens fatigue strength. Defect morphology doesn’t seem to be an important driving force for crack initiation as ellipsoidal corrosion pits, hemispherical EDM defects and flat lacks of fusion, although very different in terms of shape, are just as harmful at equivalent sizes.