Nanograined surface fabricated on the pure copper by ultrasonic shot peening and an energy-density based criterion for peening intensity quantification

A nanograined (NG) surface layer with the thickness of no less than 70 pm was successfully fabricated on the pure copper by ultrasonic shot peening (USP) and the mechanical performance of the fabricated NG surface was measured by a nanoindenter. The nanohardness of the NG surface at the peened surface is 1.526 GPa, which was increased by 30% compared to that of the coarse-grained pure copper. In addition, the thickness of the strengthened surface is approximately 400 gm, which will potentially enhance the mechanical performances of the entire copper components. To control the surface nanocrystallization of pure copper during USP, an energy density based criterion was proposed and formulated. High-speed camera was used to capture the motion behavior of a shot experimentally and the Lagrange description method was used to modelling the motion behavior of the shot mathematically. Coefficient of the restitution of the shot was calibrated based on the experimental data and embodied into the mathematical model. Research results indicated that the grain size and mechanical performance of the pure copper could be refined into nano-scaled regime and mechanical strengthened by means of USP efficiently. And the proposed energy-density based criterion for peening intensity quantification provided a reliable reference for the selection of the process parameters during the surface nanocrystallization of the metallic materials during USP.