Enhancement of the hardness of lotus-type porous copper by shot peening

Shot peening process was done on the surfaces of lotus copper plates, which were cut from the lotus copper slabs (with porosity of 37 and 48%) so that the thickness direction was parallel to the pore growth direction and the pores appeared on the surface. Some pores were closed by plastic deformation of the copper near the surface during the shot peening. The number of closed pores increased as peening time increased. Almost all the pores were closed after peening time of 40 s in the specimens of the both porosity of 37 and 48%. The closed pores were covered with a newly formed nonporous skin layer with about 200 μm in thickness. Moreover, the microstructure was fine- grained to the submicron scale in the surface layer with thickness of about 40 μm. The mechanical properties of lotus copper such as Vickers hardness, tensile strength, and bending strength were improved after shot peening. The increment of the mechanical properties was larger than that of nonporous copper. From these results, it became clear that shot peening process can form a work hardened and fine-grained nonporous skin layer on the surface of lotus metal plate. Therefore, shot peening process is considered to be an effective method for the enhancement of mechanical properties of lotus metals with maintaining porous structure.