Study On Laser Surface Modification Of AM50 Magnesium Alloy

Magnesium (Mg) alloys are regarded as the ideal engineering materials in the 21st century because of their rich reserves, low density, high specific strength and rigidity, good heat and electrical conductivity, and other properties. However, the poor corrosion resistance, wear resistance and mechanical properties of Mg alloys limit the advantages of their performance. Therefore, by taking the surface modification technology, improving the chemical and mechanical properties of Mg alloys has important practical significance. In this dissertation, laser surface melting and laser surface shock processing were carried out to improve the surface properties of AM50 Mg alloy. The microstructure and properties of the laser surface modified layer and its modification mechanism were studied systematically through theory and experiments.Mg alloy was surface melted using a continuous wave CO₂ laser. The technical method and parameters of laser melting, the change of the microstructure and the properties of the laser melted layer with the parameters were studied, the results laid a foundation for popularization and application of laser surface melting Mg alloys.The experimental results show that the microstructure of the laser melted layer was refined highly, element Al in solid solution was increased, the composition and structure were more uniform, the P-phase was increased and distributed uniformly. Theoretical analysis show that the improvement of strength and hardness in the laser melted layer could be due to microstructure refinement, the increase of element Al in solid solution and the uniform distribution ofβ-phase.The experimental results show that the wear and corrosion resistance of the laser melted Mg alloy were improved significantly. The main wear mechanism is abrasive wear, in addition, there is oxidative wear. The superior wear behavior of laser surface melted Mg alloy is attributed to improved micro-hardness in the surface layer. The better corrosion performance of the laser melted layer is ascribed to the decrease size of grains and diffuse distribution ofβ-phase, as well as the increase Al and impurity elements in solid solution.In this dissertation, we first studied the technology of laser shock processing Mg alloy with a high intensity pulsed laser (Nd: Glass). The technical method and parameters of laser shock processing, the microstructure and properties of the laser modified layer were studied. The results show that plastic deformation take place in the surface layer and result in large compressive stress, surface roughness was increased but the microscopic surface finish was improved. The surface was protected from laser ablation by the laser absorbent sacrificial coating, surface chemistry was almost unchanged and no new phase appeared in the surface layer after laser shock processing. Extensive slip lines and twins appeared in the microstructure, this shows that plastic deformation depends on the interaction of dislocations and twins, the increase of dislocations and twins improves the strength and hardness of the material.A finite element model was established to simulate the residual stress field in Mg alloy under laser induced shock loading, some key programs such as the material constitutive model under high strain rate, the exertion of load and the finite element analysis process were discussed. On the base of theory analysis of the temporal and spatial distribution of laser induced shock wave, the residual stress field under different laser shock pressure was analyzed by using finite element analysis, the computed and the testing results were good agreement, the analytical results provide scientific basis for choosing the optimal parameters and predicting the residual stress field induced by laser shock processing.The experimental results show that the wear resistance of laser shocked Mg alloy is not improved but decreased slightly, while the corrosion resistance of laser shocked Mg alloy is improved significantly. The main wear mechanism was abrasive wear, in addition, there were adhesive and oxidative wear. The plough effect of abrasive wear is severe cold-work hardening, therefore, the improved hardness of laser shocked Mg alloy based on cold-work hardening can't improve the wear resistance of Mg alloy, on the other hand, the increased roughness of surface lead to the slight decrease of wear resistance of Mg alloy. The superior corrosion behavior of laser shocked Mg alloy is attributed to densification of material, as well as the residual compressive stress in the surface laser and the improved surface finish.Laser surface melting and laser surface shock processing are advanced surface modification techniques, having high potential in surface treatment of Mg alloys. It provide new methods and technologies for studying the characteristics of the material microstructure and properties in the condition of rapid solidification, and the behavior characteristics of material under the condition of high pressure and high strain, therefore, it has important value for scientific research.