Subsurface microstructure versus surface topography: Impact on the fatigue strength of stress-relieved Laser Powder Bed Fusion (L-PBF) 316L parts

The Laser Powder Bed Fusion (L-PBF) process allows to manufacture parts with both a complex geometry and a high mechanical performance. The as-built and net-shape L-PBF 316L stainless steel parts - i.e. without any heat nor surface treatment, have tensile residual stresses, subsurface pores, a rough surface and a contour-core microstructure that synergistically lead to a low fatigue strength. Residual stresses can be relieved with a heat-treatment to enhance the fatigue properties, but the best performance is obtained after machining and polishing. The differences between the polished and the heat-treated net-shape conditions lie in the subsurface microstructure, the surface topography, and the population of subsurface pores. This study aims to quantify the impact of each of these subsurface parameters on the fatigue behaviour. To do so, the subsurface microstructure of the net-shape condition is characterized. Then, uni-axial fatigue tests are carried out on stress-relieved specimens with the following surface conditions: net-shape, partially polished, and “machined and polished”. The Kitagawa-Takahashi representation shows that for defects smaller than 200 μ m , the subsurface microstructure is the most influential parameter on the fatigue strength. Conversely, the surface topography has a limited influence. After examining the microstructure surrounding killer defects of both net-shape and polished specimens, the grain size under the surface of all surface conditions is considered in the Kitagawa-Takahashi diagram relatively to the killer defect size: this allows to align the various batches’ results.