Leading role of the subsurface microstructure over the surface topography on the fatigue strength of stress-relieved L-PBF 316L parts
Résumé
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 parts - i.e. without any heat nor surface treatment, have tensile residual stresses, sub-surfaces pores, a rough surface and a contour-core microstructure that 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 sub-surface microstructure, the surface topography, and the population of sub-surface pores. This study aims to quantify the impact of each of these sub-surface parameters on the fatigue behaviour. To do so, the sub-surface 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. For defects smaller than 200 μm, the sub-surface microstructure is the most influential parameter on the fatigue strength. The grain size under the surface is considered in the Kitagawa-Takahashi diagram relatively to the killer defect size and allows to align all the batches.
