Fatigue behaviour of notched laser powder bed fusion AlSi10Mg after thermal and mechanical surface post-processing

Laser powder bed fusion (LPBF) as an additive manufacturing technology offers high potential to fabricate parts with complex geometries layer-by-layer. However, these parts have inhomogeneous microstructure and very poor surface quality in their as-built condition. The presence of high surface irregularities especially in the downskin surfaces is a challenging issue that can directly influence their mechanical performance especially under fatigue loading conditions. Hence, applying post-treatments to modulate these imperfections can play a critical role. In this study, the individual and hybrid effects of different post-treatments including T6 heat treatment and shot peening on microstructure, mechanical properties and fatigue behaviour of LPBF V-notched AlSi10Mg specimens were investigated. Two different shot peening processes were applied on both as-built and heat treated specimens using steel and ceramic shots with different Almen intensity, shot diameter and shot hardness. The specimens were comprehensively characterized in terms of microstructural features, surface morphology and surface roughness. Mechanical properties including microhardness and residual stresses were measured and fatigue behaviour of the specimens was determined using a stair-case method; fracture surfaces were also critically analyzed. The results of the analysis performed both on the smooth section and the notched section (including notch root, up and down skin areas) indicated the importance of the choice of shot peening parameters with respect to the target geometry and its material properties. In this case, the shot peening treatment with smaller media and lower intensity was more efficient in terms of surface modification and homogenization especially in the downskin surfaces leading to higher fatigue strength. The significant finding of this study is that by pairing the heat treatment and shot peening, the effect of the presence of the notch can be masked obtaining almost the same fatigue strength for the notched specimens as the un-notched counterparts.