The Effects Of Mechanical Property And Structural Changes In The Surface Material Induced By Laser Shock Processing On AZ91 Magnesium

Magnesium alloys absorb great attention because of their rich reserves, low density, high specific strength and rigidity, good heat and electrical conductivity, and other properties. However, the poor wear resistance and mechanical properties of Magnesium alloys limit its application. Therefore, by taking the surface modification technology to improve the mechanical properties of Magnesium alloys has important practical significance.In this dissertation, two experiments, which took AZ91 magnesium as research object. were carried out to study the effects of mechanical and surface structural changes, respectively. Neodymium glass laser was used to shock the magnesium alloy. The mechanical properties of nano-hardness and residual stress on the surface of magnesium alloy that induced by laser shock was studied after laser shock. And the software ABAQUS was employed for the simulation study of residual stress generated by laser shock. Then magnesium alloys were shocked by different pulses energy. The changes of surface morphology, cross-sectional microstructure and material composition of the shocked region were studied under different energy states.The studies of mechanical property changes on magnesium alloy show that the nano-hardness of the laser shocked region, as well as some other nano-mechanical properties, was improved. A plastic deformation was induced by the laser shock wave in the surface material, so was a high residual stress distribution. And the simulation result was in good agreement with the experimental result. We can see that the strength and hardness of the surface material were all improved.The studies of magnesium alloy surface structure changes show that periodic ripple microstructure appeared on the surface material after laser shock. And the distribution and characteristics of the surface wave were impacted by the energy of the laser. According to the distribution of the cross-section microstructure, we can find grain refinement and twinning phenomena in the grains. But there was no change to the composition in the shocked region. Finally, the characteristic changes of wave-like periodical structure surface induced by different pulse energy were studied. Based on the effects of the plasma to the specimen and the grating effects caused by the mechanism of coherent stimulated light scattering within plasma, the process of generating thermal micro-disturbance on the sample surface by coupling the thermal conductivity, the thermal radiation and the laser irradiation was discussed, and then the formation mechanism of the surface ripples caused by the non-equilibrium state of the surface thermal micro-disturbance was discussed.Laser shock processing is an advanced surface modification technology with many advantages. It is expected to play a significant role in surface modification and application of magnesium alloys. Therefore it has a significant value for scientific research.