Numerical Simulation Of Laser Shock Peening On Fatigue Crack Growth Of Magnesium Alloy Joint

As a representative of the lightweight metal, magnesium alloy has attracted widespread attention because of its superior mechanical properties. At present, in some industries that have higher quality requirements such as aviation, aerospace, automobile and other industries, there is an increasing demand for magnesium alloys. However, some disadvantages of magnesium alloy itself such as poorer anti-fatigue performance, weldability and corrosion resistance, limit the possibility of wide application. Therefore, it is of great significance to improve the mechanical performance of magnesium alloy. Currently, most researches on magnesium alloy are related to fatigue crack propagation after welding or fatigue crack propagation after laser shock peening(LSP), and there are few studies on the combination of welding, LSP processing and fatigue crack propagation analysis. Therefore, it is of great significance to study in this respect.In this paper, based on the finite element analysis software ABAQUS 6.12-1/Explicit, multi-pass welding of butt joint and T-joint of thick plate is simulated. The shape of the heat source and moving load are implemented into SUBROUTINE DFLUX using Fortran language, and life and death unit technology is used to realize the finite element simulation of multi-pass welding, and C3D8T unit (thermal-displacement coupling unit) is used to calculate the temperature field and residual stress field at the same time. Simulation results show that the residual tensile stress in weld zone and heat affected zone of the welding joint are relatively larger, and the residual stress can seriously influence the service life of the welded joint.The finite element simulation of LSP of welded joint is further carried out. Because LSP is conducted at the high strain rate, the constitutive relations of quasi static will be no longer applicable in this case. So the Johnson-Cook(J-C) dynamic constitutive model is used in the LSP simulation, and constitutive model under quasi static and room temperature conditions is used when rebound occurs. According to the impact analysis for processing parameters of LSP such as power density, spot size, impact frequency and overlap rate, a set of relatively complete laser shock processing parameters are optimized. Simulation results show that the ideal residual stress field can be obtained under the condition of 1.5 GW/cm2 of laser power density (5GPa of peak pressure),2 mm of spot size,75% of overlap rate and 2-4 times of shock time along the welding line.Finally, the residual stresses induced by welding and LSP are induced into MSC Fatigue software to simulate fatigue crack propagation. By comparing the results it can be found whether the fatigue performance of the welded joint is improved or not. The results show that the fatigue crack propagation life of the specimen treated by LSP is increased significantly, and the fatigue crack length is also increases at the same time. Moreover, the simulation results are compared with experimental data in other literatures and the final calculation results are reasonable and accurate.