Investigation Of Residual Stress Field And Its Boundary Effect On Sheet Metal By Laser Shock Processing

Laser shock processing(LSP) is a new surface modification technology.In the process,the laser with high power density and burst pulse irradiates on metallic material,the coating on surface of material vaporized rapidlly,forming plasma,the plasma exploded to induce a high pressure shock wave,which can results in a high compressive residual stress in material superficial layer.So LSP can improve some mechanical properties of metallic materials,such as wear strength,corrosion resistance,fatigue resistance and so on.In this paper,based on the mechanism of laser shockwave inducing residual stress field,redidual stress field and its boundary effect on sheet metal by LSP are researched according simulation and experiment.The main contents are as follows:The mechanism of the laser-induced shockwave was analyzed theoretically,the propagation and attenuation of shock waves was probed.The mechanism of residual stress in the material by laser shockwave in laser shock processing processing was studied.The primary factors having effect on distribution and magnitude of residual stress were also analyzed.Modeling of entire process of laser shock processing was accomplished.With combination of explicit dynamics analysis and static equilibrium analysis,the numerical model of shockwave loading was established based on the ABAQUS soft,considering the characteristic of laser shock processing and the finite element theory.The solution time for explicit dynamics analysis was proposed based on modeling of laser shock processing.According to changing energy trend,the solution time was defined.The process of single impact was simulated.The results show that increasing the laser intensity could enhance the compressive residual stress field,but may induce "residual stress hole" on the impact center if laser intensity is high enough.Impact times have effects on single impact,the result shows that compressive residual stress gradually reached the saturated state with the increase of impacts in the same position.The proper impact times was about 2 to 3 times.In addition,the mechanism of "residual stress hole" was put forward:the release waves produced at the boundary of the impact region propagate to the impact center on the top surface, which will induce reverse lateral deformation to decrease the compressive residual stress on the impact center.Opposite shock processing and use of square spot or oval spot could strengthen residual stress in the center of impact region.The process of two points and multiple points overlapping LSP was simulated and residual stress field on overlapping region and at the boundary of impact region of specimen treated by different overlapping rates was researched.The results show that induced residual stress field was uniform on overlapping region and was stronger than that on single impact region.As to certain process parameters of LSP,overlapping region should be choosed reasonably and it’s not bigger always better.Because of boundary effect compressive residual stress at boundary of impact region was weak,but boundary effect could be weakened by improving overlapping rate.Residual stress field on metal surface induced by bosseyed and double-faced LSP was researched by experiment.The experimental results show that in comparision with bosseyed LSP, the distribution of residual stress produced by double-faced LSP was more uniform and the value of it was more higher.But there was even tensile residual stress on side face of metal because of edge effect.Fatigue test result shows that the overlapping LSP could improve fatigue life and crack was produced easily at the boundary of impact region due to boundary effect.