Effects of aging and shot peening on surface quality and fatigue properties of material extrusion additive manufactured 17-4PH stainless steel

Material extrusion additive manufacturing (MEX) gains increasing interest due to its cost-effectiveness. However, concerns arise regarding inherent MEX characteristics potentially affecting mechanical properties. This study investigates the effects of shot peening and heat treatment on MEX 17-4PH stainless steel. After shot peening, surface nanocrystalline and compressive residual stress are induced. Aging results in higher hardness and tensile strength with lower elongation, while shot peening results in higher surface hardness with minimal change in elongation. For fatigue properties as-sintered specimens, Al2O3 particle contamination dominates at high cycle regimes, while other MEX surface inhomogeneities dominate at low cycle regimes. Furthermore, as-aged properties are dominated by all surface inhomogeneities, resulting in the non-improvement of fatigue performance compared to the as-sintered specimens. However, shot peening on the as-sintered specimens significantly enhances fatigue properties through the formation of surface nanocrystalline structures and the induction of compressive residual stress, albeit with the occurrence of wrapped corners acting as fatigue crack initiation sites. Remarkably, the combination of aging and shot peening yields the most improvement in fatigue performance, exhibiting behaviours akin to as-sintered specimens with shot peening but without the generation of wrapped corners. Instead, fatigue crack initiation sites shift to subsurface region's inhomogeneities.