Revealing the relationship between the shot type and fatigue performance of laser powder bed fused 304L steel following shot peening

Abstract

Shot peening (SP) is gaining popularity as a post-processing method for improving fatigue properties of laser powder bed fused (LPBF) metallic materials. This work investigated the influence of shot type on the surface strengthening and life improvement of LPBF 304L stainless steel (SS) with a rough as-built surface using ASH 110 steel shot, AZB 300 ceramic shot, and CW 35 strengthened steel shot. The shot characteristics (size, shape factor, and hardness), SP surface characteristics (morphology, roughness, residual stress, and hardness), and SP subsurface characteristics (grain refinement, deformation twinning, martensitic transformation, and defect distribution) were fully characterized and quantified. Particularly, the foldings with rolled-up edges that affected surface finish appeared on the SP surface, and their formation was ascribed to the as-built surface morphology containing incompletely melted powder particles and agglomerations, as well as the shot properties such as size, hardness, and sharpness. The irregular LoF defects between the contour region and infill region could not be entirely eliminated/healed utilizing SP procedures, and these defects were always the fatigue crack initiation sites for AB and SP samples. Therefore, the critical role of “penetration depth” of the kinetic energy on irregular LoF defects and fatigue performance was highlighted. A larger penetration depth can be achieved by employing small-sized ceramic shots with high hardness, deformation resistance, and kinetic energy, inducing refined structures and compressive residual stress around the irregular LoF defects, thereby constricting the fatigue crack initiation and propagation. This work provides a valuable reference for SP process design and optimization of LPBF metallic materials.