Influence of build rate and post-AM surface treatments on fatigue life of powder bed fusion – laser beam 316L stainless steel

This study investigates the influence of increased build rates on the high-cycle fatigue (HCF) life of PBF-LB manufactured 316L stainless steel by detailed analysis of pore characteristics using X-ray Computed Tomography (XCT) and surface roughness. Furthermore, the effects of post-processing surface treatments by chemical mechanical processing (CMP) and Hirtisation® on surface improvement and fatigue life were studied as well. Although increased build rates led to reduced fatigue life compared to the reference condition, this was related to distinct pore morphologies revealed by XCT. Specifically, the impact of porosity generated by increased hatch distance (PA) exhibited more uniform lack of fusion (LoF) pores, while porosity generated by higher scan speed (PB), displayed larger, more randomly distributed defects, leading to larger scatter in fatigue life. Examination of the fracture surfaces confirmed that crack initiation sites were associated with surface defects in the reference samples, LoF along the hatch lines in the PA samples, and larger, randomly distributed LoF pores in the PB samples. Both CMP and Hirtisation® surface treatments approximately doubled the fatigue life of the reference condition. However, limited material removal in CMP exposed subsurface porosity, acting as initiation sites. In contrast, the material removal during Hirtisation® effectively mitigated surface and near-surface defects. These findings highlight the critical role of build strategy in controlling defect morphology and the effectiveness of surface treatments in enhancing the fatigue performance of additively manufactured stainless steel, crucial for expanding their industrial applications.