Tailoring the mechanical and corrosion properties of direct metal deposited 316L stainless steel by underwater ultrasonic impact treatment

Direct metal deposition (DMD) holds significant promise for repairing damaged components located in underwater environments. However, the uncontrolled microstructure, tensile residual stress and defects formed by DMD significantly restrict its application. In this study, underwater ultrasonic impact treatment (UUIT) is employed to improve the surface quality, mechanical properties and corrosion resistance of the DMD 316L stainless steel. The results demonstrate that while UUIT is capable of closing the defects that are fully distributed within the surface plastic flow region (similar to 75 mu m), it is unable to affect those that are distributed beyond this region. The high-frequency impact of the needle on the surface is the primary factor contributing to the formation of a severely deformed layer. Conversely, the role of the bubble collapse near the needle tip is minor. The micron-sized cellular structures (similar to 5.4 mu m) on the top surface are refined into nano-sized grains (similar to 195 nm) by UUIT. Moreover, UUIT transforms tensile residual stresses into compressive residual stresses (61-99 MPa). UUIT increases the microhardness of the surface region by 35 %. Additionally, the tensile strength of the DMD 316L is significantly improved by UUIT, which is due to the combined effects of grain refinement and elevated dislocation density. However, the work-hardened surface layer restricts the movement of dislocations, thereby markedly reducing ductility. Furthermore, the DMD-UUIT 316L exhibits an enhanced corrosion resistance compared to the DMD 316L. Nevertheless, the beneficial effects of grain refinement and microstructure homogeneity are partially offset by the presence of dislocations and alpha ' martensite.