Microstructure and Wear Resistance of Laser Surface Melting-Treated AZ31B Magnesium Alloy in Cryogenic Medium

Unlike the traditional air-cooling method, the AZ31B magnesium alloy surface was treated by laser surface melting in a liquid nitrogen cooling environment (LSM-LN). Microstructure, microhardness, and wear resistance tests were conducted to compare the substrate and laser-melted layer obtained in the liquid nitrogen cooling environment (LSM-LN layer). The grain size of the LSM-LN layer was much finer than that of the substrate, approximately 5.2 μm. The β-Mg₁₇Al₁₂ in the LSM-LN layer was far less than in the substrate. Under the comprehensive competitive factors of thermodynamics and kinetics, the mixed structure of nanocrystalline and amorphous appeared in some regions of the LSM-LN layer. Thermal stress was attributed to the uneven temperature change of the magnesium alloy in the laser heating and cooling process. Coupled with the magnesium alloy’s mechanical restraint stress, the LSM-LN layer’s dislocation density was greatly increased. Due to more remarkable grain refinement, solid solution strengthening, dislocation strengthening, and the amorphous structure, the microhardness and wear resistance properties of the magnesium alloy treated by LSM-LN were improved.