| High specific strength and abundant nature recourses have made magnesium alloys an ideal candidate for automobile, aerospace, navigation and electronic etc. It is known as "the most potential and prospective green structure material in the 21st century ".However, due to quantity of intrinsic properties including higher reactivity, lower melting point, bigger thermal conductivity and expansion coefficient etc, it's very difficult for magnesium alloy to precede hot working, which limits its application greatly. Compared with traditional heat process, laser processing is called as "a common processing technique in the future", which that is characterized due to its high energy density and efficiency, short process, energy conservation and flexible manufacturing etc. Therefore, it has great significance to study the pulsed laser behavior of magnesium alloy.A solid Nd:YAG pulsed laser were designed and manufactured for laser processing of magnesium alloy materials. In this paper, laser processing behaviors of AZ31B magnesium alloy material was investigated including laser cutting, laser welding, laser surface modification etc. The behavior of nanocrystallization and amorphization of magnesium alloy surface was explored by laser heating-liquid nitrogen cooling. Some conclusions were drawn as follows:The solid Nd:YAG pulsed laser with a single pump cavity was designed and manufactured successfully with an average output power of 500W, wavelength of 1064nm, frequency of 1-2000Hz, peak power of 9200W, single of pulse energy 83.8J. Its beam parameter product Kf was 16.5mm-mrad and its minimum beam spot diameter was 12.7μm. The numerical control processing system corresponding to the laser was designed and manufactured.The laser-cutting mechanism and factors were discovered and the microstructure of pulsed laser cutting surface was observed. The results showed that it made the laser cutting possible because a micro-district was melted coordinated with the action of vaporization, burning, and gas flow. The single pulse energy, peak power, pulse frequency, pulse width, defocusing amount, and assistant gas type were the main factors that determined the cutting quality. The heat affected zone was almost indiscernible between the remelted layer and the base material. A 6mm in thickness of laser cutting could be reached with the optimized parameters.The weld metallurgy and defect behaviors of laser welding magnesium alloy were also studied. The element burning, evaporation, splash, cracks, porosity, and inclusion were the main metallurgy problems of pulsed laser welding magnesium alloy. Especially, the hot cracking was the main crack formed in the weld. Although the content of hydrogen in weld metal was higher, there was no delayed crack observed. The microhardness in the weld was improved to 72HV0.05 because of the grain refinement and dispersive distribution ofβ-Mg17Al12 particles. The crystal size was refined from 10～30μm to 3～10μm due to fast heating and quick cooling. The refined crystalline grain had positive effect on the fracture properties of the weld metal, which was characterized by ductile fracture in partial.Laser surface melting was carried out on AZ31B magnesium alloy by the pulsed Nd:YAG laser. The results showed that the microstructure of melted layer was refined obviously with the grain size of about 2～10μm, and a lot nanophase Mg17Al12 was dispersed uniformly. Microhardness of the melted layer was twice of that of the as-received magnesium alloy. The results of electrochemical corrosion showed that the corrosion resistance of laser surface melted layer had been improved due to the grain refinement, dispersion reinforcement of the second phase particle, solid solution of the impurity element and the increase of the content of Al.To improve the wear resistance and corrosion resistance of magnesium alloys, the composite coating of Al-Si and Al2O3-TiO2 on AZ31B magnesium alloy surface was prepared successfully by laser cladding. When the content of Al2O3-TiO2 was lower than 15%, the composite coating had good technological performance. The laser cladding was bonded metallurgically with the magnesium alloy substrate. Those ceramic particles were embedded by granular "particle" in the composite coating layer. Simultaneity, there were a lot of Mg-Al intermetallic and Mg2Si generated. In addition, it was found that little unmelted ceramic particles distributed in the composite coating. The average microhardness of the composite coatings was significantly improved to 225HVo.o5 compared to that of the Al-Si coating, which was 125HV0.05. The results showed that the wear resistance and corrosion resistance of the composite layer were considerably improved compared to the substrate because of the presence of Mg-Al metallic compounds, ceramic particles and grain refinement.Based on the above, a rapid melting method of laser heating and liquid nitrogen cooling was proposed. The hybrid microsturcture of nanocrystalline and amorphous on the surface of magnesium alloy were prepared successfully. The relevant mechanism was discussed in the aspects of solidification characterization and grain growth in thermodynamics. The maximum microhardness of the liquid nitrogen cooling layer reached to 148 HV0.05, which was about three times of that of the base material. The results of electrochemical corrosion showed that the corrosion potential of the melted layer cooled by liquid nitrogen was -1439mV. It shifted 26mV higher than the sample cooled in the air and 124mV higher than primitive magnesium alloys. Improved corrosion resistance can be achieved by expanding the solid solubility, forming amorphous phase, and prohibiting grain growth.
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