Tig Welded Joints Of Az31b Magnesium Alloy Cryogenic Strengthening Mechanism

Magnesium alloy is the metal structural material for saving energy andenvironmental protection. The welding technology is one of the keytechnologies which decide the magnesium alloy will be widely used or not. Oneof the welding problems for magnesium alloy is the softening of welded joints.There are some limitations whether to take measures in welding procedure or totake heat treatment measures after welding. Deep cryogenic treatment is putforward to improve the strengthening of welded joints by TIG of AZ31Bmagnesium alloy,and welding test and deep cryogenic treatment test are alsotaken in this paper. The author studies the microstructure and mechanicalperformance of welded joints by TIG in the different parameters of deepcryogenic treatment, and investigates the mechanism to improve theperformance of the welded joints by TIG, and explores the new ways to improvethe softening of the welded joints. The major research results are as follows:Mechanical properties of welded joint by TIG of AZ31B magnesium alloywere improved by deep cryogenic treatment. The Vickers hardness value of thewelded joints for each region tends to be homogenizing after deep cryogenictreatment for4h at-185℃. The tensile strength and yield strength of the weldedjoint by TIG of AZ31B magnesium alloy improves as the holding time increasesand treatment temperature decreases. When the deep cryogenic temperature is-185℃, the holding time is shortened for achieving the same the purpose ofstrengthen. The tensile strength and yield strength reaches its maximum whenthe deep cryogenic treatment for12h at-185℃,16.1%and6.51%respectively,but elongation improves limited.The tensile fracture surface of the welded joints is observed by SEM. Thesample before cryogenic treatment breaks by way of brittle fracture, and thelocal area after cryogenic treatment shows the characteristics of ductile fracture.There is45°oblique fracture with the tensile axis direction on the fracturesurface. The form of fracture is shear fracture, with the fracture flush, and novoid exists. There is a torn edge and small dimple in the local area, showing the characteristics of the compound fracture.The microstructure before and after deep cryogenic treatment of weldedjoint by TIG is observed by using optical microscope, X-ray diffraction (XRD),and transmission electron microscope (TEM). The cryogenic strengtheningmechanism of the welded joints of AZ31B magnesium alloy is analyzed by thedislocation theory. The subgrain structure was formed in the welded joint ofAZ31B by deep cryogenic treatment. The Mg17Al12particles precipitated, whichimproved the continuity. The number of second phase particles increased, whichrefined the microstructure of welded joint by TIG of AZ31B magnesium alloyand strengthened mechanism. The analysis by XRD shows that the orientation ofgrain crystal of welded joint of AZ31B changes, resulting in the grain rotationafter deep cryogenic treatment. During the deep cryogenic treatment, thedislocation loop and twins are formed by TEM in the welded joints.BP neural network was used to simulate and predict for the tensile strengthof welded joint by TIG of AZ31B magnesium alloy. The reasonable parametersof deep cryogenic treatment are as follows: temperature is-135℃, duration timeis more than150minutes. The predicting results of BP neural network arerelatively accord with the experimental test results.Based on the FEM analysis, the distribution and the law of the temperatureand stress field of the welded joints of magnesium alloy are studied during thecryogenic treatment process. These results are consistent with the resultsobtained from experiments of effect of welding.