Numerical And Experimental Investigation On The Residual Stress In Thin Metallic Plate Induced By Laser Shock Processing

Laser shock processing(LSP)is a promising surface modification technology,which employs the laser with high power density and short pulse to induce the severely plastic deformation and high amplitude compressive residual stresses(CRS)in the material.The technology could significantly improve the properties of the metallic materials,such as fatigue resistance,corrosion resistance and so on.Consequently,LSP has extensively been applied foreground in aerospace industry,automobile manufacturing,biological medicine and other fields.In this paper,the numerical simulation and experiment were employed to investigate the effect of absorbing layer and boundary constraint conditions of the plate on the RS distribution induced by LSP.Moreover,the propagation and attenuation of laser-induced stress waves and RS distribution in the thin metallic plates induced by two sided laser shock processing(TSLSP)were systematically researched.The main research work and conclusions are as follows:Firstly,the experiment of LSP on 7075-T7351 aluminum alloy with and without absorbing layer were conducted and the surface morphologies,RS distribution and micro-hardness of the components were investigated respectively.The results show that for the case of without absorbing layer,there are remelting layer and ablation spots on the sample surface and the surface roughness is increased.The tensile residual stresses(TRS)are distributed on the surface layer while the CRS are distributed on the subsurface layer.Also,the micro-hardness of the surface material is decreased while that of the subsurface layer is increased.For the case with the absorbing layer,the smooth dent appears on the sample surface which is distributed with CRS,and the micro-hardness of the surface layer is increased.In thickness direction,the maximum magnitude of CRS and its impacting depth is higher than the previous case.Then,7075-T7351 aluminum alloy plates with boundary constraint conditions of bottom fully constrained and both ends clamped were respectively impacted by multiple LSP,and the plastic deformation and RS distribution were comparatively analyzed.The results show that micro-dents and CRS are generated in the impacted regions of both cases.When the bottom surface of the plate was fully constrained,the plate still keeps plat,and the RS distributed in thickness direction was “CRS-TRS”.For the condition of both ends clamped,there was overall convex deformation in the impacted region of the plate,and the pattern of RS distribution in thickness direction was “CRS-TRS-CRS”.Additionally,the maximum of CRS for the case of bottom fully constrained is higher than the other case,and the strengthening effect is better.Finally,the propagation and attenuation process of stress waves in surface and thickness direction and the RS field of the 7075-T7351 aluminum alloy plate impacted by TSLSP were investigated.In addition,the effect of laser spot diameter,pulse width,plate thickness,impact times and loading pattern on the RS distribution were systematically researched.The results show that the amplitude of first interaction between the stress waves and the material is the most serious,which determines the final RS field.The impacted region and its whole thickness section of the plate are distributed with CRS,while the TRS are distributed in the surrounding area distributed with CRS in the whole thickness section.Moreover,the CRS distributed in the thick plate and larger area in the whole thickness section could be realized by optimizing the laser parameters and loading pattern to enhance the effect of TSLSP.