Experiment And Numerical Simulation On Warm Laser Shock Forming Of AZ31 Magnesium Alloy Sheet

Magnesium element is one of the most abundant elements on earth. And magnesium alloys have been widely applied in aerospace, automotive and other fields owing to its low density, high specific stiffness and specific strength, good heat dissipating capability and recoverability. However, the disadvantages of insufficient strength and difficulty in plastic processing have limited its development and application. Therefore, it’s meaningful for the popularization and application of magnesium alloys to improve the plastic deformation capacity and related mechanical properties by the technology of laser shock forming(LSF) and warm laser shock forming(WLSF).In this paper, the LSF and WLSF experiments have been carried out on the AZ31 magnesium alloy. The effects of power density and temperature on the surface residual stress, surface morphology, microstructure and micro-hardness of the formed samples were analyzed. Finally, the model of WLSF was established to simulate the surface residual stress and deformation of the formed sheets using ABAQUS simulation software. The results of simulation and experimental were compared as well. The main research contents and results are as follows:(1) The experiment results showed that AZ31 magnesium alloy sheet exhibited excellent laser shock forming ability with high temperature and high strain rate. High amplitude compressive residual stresses were detected on the surface of the samples after LSF and WLSF. With the increase of temperature, the distribution of compressive residual stress tends to be uniform. But, due to the greater relaxation of the residual stress at high temperature, the magnitude of the compressive residual stress decreased. The power density also has a great influence on the residual stress. The maximum compressive residual stress reached 143 MPa after WLSF at 150℃ when the power density was lower. When the power density was higher, the maximum compressive residual stresses of sheets after WLSF were less than those after LSF. Therefore, the influence of temperature and laser power density should be considered comprehensively in practical applications.(2) The surface roughness on the forming region was higher than that on target material due to the large plastic deformation produced by laser shock. The samples after WLSF have lower surface roughness than those after LSF. But, the surface of samples prone to ablation at too high temperature(250℃~300℃).(3) The microstructure of magnesium alloys changed obviously in the process of laser shock. High density dislocations were formed in the grain and grain boundary after LSF and WLSF. A large number of twins generated after laser shock at lower temperature. With the increase of temperature, the dynamic recrystallization deformation mechanism of magnesium alloy was gradually activated, and the twins disappeared as recrystallization grains generated. The grains of specimens become smaller after WLSF. With the temperature increasing, the size of grain decreased firstly and then increased. The nanocrystals were observed in the specimen after WLSF at 100℃. The hardness of targets was improved effectively after LSF and WLSF, and the distribution of hardness was more uniform after WLSF.(4) Numerical simulation results showed that the whole spot of samples reached high compressive residual stress after WLSF. The maximum forming depth increased with the increase of laser power density at the same temperature. However, the temperature has little effect on forming depth with the same laser power density. In the temperature range of 200℃~300℃, the sheets were easy to break for the thickness decreased rapidly during WLSF.(5) According to the experimental and simulated results, the optimum technological parameters of WLSF of magnesium alloy was 100℃/150℃, and the laser power density was 1.28GW/cm2.