Cryogenic Processing On The Microstructure And Properties Of Az31 Magnesium Alloy Tig Welded Joints

Magnesium alloy attracts much attention in wide fields, since it has the advantages of low density, high strength and can be recycled easily. Compared with the traditional engineering materials, magnesium alloy has wide application foreground. In practice, many structures need weld in the application process. Because of the low melting point, the large thermal expansion coefficient and the high heat conduction and so on,the weld performance of magnesium alloy is bad .The lower of mechanical properties generally exist in welding structures and the problem has become a key technology which restrained the wide application of magnesium alloy.Deep cryogenic treatment, as a new method of material modifying, now has been widely applied. Compared with the traditional method it has many advantages. But, the mechanism of nonferrous properties improved by deep cryogenic treatment is not clear for certain nonferrous materials.In this paper, the TIG welded joints of magnesium alloy AZ31 are studied, the variations of microstructures and mechanical properties before and after deep cryogenic treatment are observed; the mechanism of the magnesium alloy microstructures and mechanical properties changed by deep cryogenic treatment is analyzed. After the experiment, it is found that the grains were finer and the fine precipitation of second phase particles increased mean while the grain preferred orientation through the deep cryogenic treatment and there are certain relationships between mechanical properties and cryogenic treatment processing. The results showed that After-150℃cryogenic treatment for 4h,the hardness of weld joint has obviously improved, and with the longer of time, the better the results, when the temperature is -196℃if reaching the equal results needed shorter time. After-196℃cryogenic treatment for 10h,the tensile strength and yield strength reached the peak.Compared with the non-cryogenic treatment,they were respectively increased by 17.8%and 58.5%,but the improve of elongation is limited. Finally, based on the metal knowledge and experiment result, the impact of deep cryogenic treatment on the material performance and the micro mechanism of deep cryogenic treatment on the magnesium alloy were explored.