Effect of surface burnishing process with different strain paths on the copper microstructure

Abstract

The single strain path (SSP) and the reciprocating strain path (RSP) machining are performed on the sample surface with a multi-ball surface burnishing tool to study the effect of different deformation modes on the microstructure. The samples are characterized mechanically and metallographically via a microhardness tester, an optical microscope, and a backscattered scanning electron microscope. The effect of the two strain paths on the plastic strain in the material is explored by developing a numerical simulation model. The characterization and simulation results show that the deformation layer thicknesses are 79 μm, 208 μm, and 290 μm for the SSP treated samples and 108 μm, 191 μm, and 305 μm for the SSP models at penetration depths Dp of 30 μm, 60 μm, and 90 μm, and meantime, 195 μm, 249 μm and, 390 μm for the RSP treated samples and 146 μm, 248 μm, and 396 μm for the RSP models. Moreover, the RSP treated samples are harder than the SSP treated samples at the same penetration depth Dp, attributed to the smaller grain size and longer total length of grain boundaries of the RSP treated samples. The simulation results show that the RSP machining, with the assistance of the Bauschinger effect, further increases the strain degree and the machining influence depth in the sample compared with the SSP machining. The RSP machining disintegrates the dislocation cells in collaboration with the Bausinger effect, which causes the shielding effect of long-range stress to be significantly reduced, further transferring machining stress to the sample interior. It is the main reason why the RSP machining leads to deeper affecting depth. Furthermore, the grain boundaries in the fine-grain layer of the RSP and SSP treated samples are mainly dominated by high angle grain boundaries, supplemented by low angle grain boundaries, with a smaller percentage of twin boundaries, while in the coarse-grain layer are mainly twin boundaries, supplemented by high angle grain boundaries, with a smaller percentage of low angle grain boundaries. The micro-textures of the samples machined by the two strain paths are minor differences, but the grains in the subsurface layers produced different grain rotations.