Surface nanocrystallization mechanism of additive manufactured 316L stainless steel via high-energy shot peening

High-energy shot peening (HESP) is an effective way to achieve surface nanocrystallization modification of additive manufactured metal parts, significantly improving their mechanical properties. Yet, there is still a lack of deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metals. For that, this study focuses on the surface nanocrystallization mechanism of selective laser melted 316L stainless steel treated by HESP treatment. The detailed microstructural characteristics are investigated via optical microscope (OM) and transmission electron microscopy (TEM). The experimental results show that the matrix layer, the low-strain matrix layer, the elongated ultrafine grain layer, the short rod-like ultrafine grain layer, and the equiaxed nanograin layer occurred in sequence from the matrix to the topmost surface. HESP treatment enables the gradient grain refinement from the matrix with a micrometer structure of about 120 mm to the topmost surface with an average grain size of about 25 nm. The evolution of surface nanocrystallization has resulted from the interaction of high-density dislocation, dislocation tangles (DTS), and twins. This study could provide a deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metal parts treated by HESP.