Nanostructure Formation Mechanism And Properties Of AZ91D Magnesium Alloy By Laser Shock Processing

Surface nano-crystallization induced by laser shock processing(LSP), as a new field of LSP, is an extension of LSP technology. It improves the mechanical properties of metal materials effectively, such as strength, hardness, and corrosion resistance, especially fatigue fracture resistance. Due to the close relationship between mechanic properties and microstructure of metal materials, it’s essential to study the formation mechanism of surface nano-crystallization. The AZ91 D magnesium alloy was taken as a study object in this paper, and the paper was to make some foundational researches on mechanical properties, surface topography, the formation process and mechanism of surface nano-crystallization, thermal stability and corrosion resistance. Some important conclusions of this paper could be made as follows.Firstly, the process of laser shock processing was analyzed. Grain refined, evolution of grain boundary and dislocation were illustrated during LSP. The parameters used in LSP were determined.Secondly, under the condition of LSP, the micro-hardness, surface topography and roughness were analyzed. The mechanism of surface nano-crystallization induced by LSP was discussed as follows. Mechanical twining only played a coordinating role in the initial deformation, which was induced by the concentration of stress. Then, dislocation cells and dislocation tangles were formed to a network in refined grains and sub-grains. Sub-grain boundary with a small angle and orientation divides coarse grains into sub-grains, which was transformed by dislocation tangles. Ultimately,nano-crystallines formed after sub-grain boundaries with equiaxed shape and random orientation, and continued to absorb new dislocations to become a great angle grain boundaries.Thirdly, thermal stability of surface nano-crystalline layer in AZ91 D magnesium alloy induced by LSP was studied. Surface nano-crystallines of AZ91 D magnesium alloy exhibit excellent thermal stability below 300°C. Crystal defect is the main reason of keeping thermal stability. Besides, intrinsic structure of nano-crystalline, alloying elements, impurities and work hardening rate are another reasons.Finally, corrosion resistance behavior of surface nano-crystalline layer in AZ91 D magnesium alloy induced by LSP was studied. Electrochemical impedance spectroscopy curves reported the increase of reaction resistance, which confirmed the improving of corrosion resistance property. Corrosion morphology of AZ91 D magnesium alloy after LSP in 3.5% NaCl solution at different temperatures was tested. Influence on surface nano-crystalline of AZ91 D magnesium alloy by temperature was analyzed.