Surface modification and fatigue mechanisms of laser powder bed fused components subjected to shot peening with varying shot diameters

Shot peening is a promising post-processing technology that enhances the surface integrity and fatigue reliability of additively manufactured components by introducing compressive residual stress and surface hardening. This study utilized tempered martensitic steel shots with varying diameters (ASH 110, ASH 330, and ASH 550) to treat the laser-based powder bed fused (PBF-LB) 304L steel, systematically investigating their essential effects on surface quality, subsurface quality, and fatigue performance. Fatigue deformation behavior and hardening mechanisms were elucidated through self-heating effects, microstructural evolution, and fatigue fractography. Results indicate that medium shot diameters (ASH 330) approached optimal shot peening conditions, achieving a balance of surface roughness and gradient structures that enhance fatigue crack initiation resistance, support progressive strain hardening, and improve overall fatigue performance. Although larger shot diameters provided greater penetration depth and contact area, they led to increased surface imperfections and decreased structural heterogeneity, heightening stress concentrations and reducing dislocation hardening. Additionally, cyclic loading facilitated the recovery of pre-existing dislocations and twin boundaries, potentially restricting new twin boundary formation and reducing performance strengthening. These insights into fatigue damage mechanisms provide valuable guidance for optimizing shot peening in additively manufactured components.