99 / 2021-07-10 16:09:21

Effect of pulse energy on femtosecond laser shock peening of pure copper

micro-scale laser shock peening,femtosecond laser,surface morphology,microhardness,residual stress,copper

The fatigue life, hardness, wear resistance and other properties of metal micro-components can be improved by micro-scale laser shock peening, which provides a feasible method to solve the failure problem of key bearing structures in  Micro-Electro-Mechanical Systems (MEMS). In this paper, pure copper, which is commonly used to fabricate key bearing structures in MEMS, is chosen as experimental materials. Pure copper sheets with thickness of 300 μm are shock peened by a femtosecond pulse laser with wavelength of 1030 nm without protective and confining medium. The effects of micro-scale laser shock peening on the surface morphology, microhardness and residual stress of pure copper are investigated when the pulse energy is 0, 12, 28, 51 and 67 μJ. The results show that the dents are regularly arranged on the peened surface of pure copper, and laser-induced periodic surface structures (LIPSS) begin to appear at 51 μJ. The surface roughness increases with the increase of pulse energy, and increases from 0.014±0.005 μm to 0.271±0.016 μm before and after treatment at 51 μJ. With the increase of energy, the microhardness first increases from the unpeened 113 HV to the maximum 127 HV at 28 μJ by 12.4% and then decreases to 121 HV. All samples before and after treatment show residual compressive stress which increases with the increase of energy. It increases from the unpeened 23 MPa to the maximum 108 MPa at 51 μJ, and the increase of residual compressive stress becomes insignificant after the energy reaches 51 μJ. At low pulse energy, the surface morphology and high roughness of pure copper induced by laser shock peening may contribute to the improvement of microhardness. However, at a high level of laser energy, laser ablation leads to oxidation and the formation of LIPSS on the surface of the material, which inhibits the enhancement of microhardness and residual compressive stress.