Effect Of Post-Weld Heat Treatment On Mechanical Characteristics Of AZ31 Magnesium Alloy And Its Welded Joints

As the lightest metal structural materials magnesium has the advantages of low density, high specific strength and high specific stiffness, good damping effect, high thermal conductivity, good electromagnetic shielding performance, good mechanical processing performance, parts dimension stability, easy recovery and other advantages, which result it in the aerospace, automotive electronics, home appliances and other industries been widely applied and has broad application prospect. But because of the magnesium alloy with low melting point, linear expansion coefficient and thermal conductivity is high, which the magnesium alloy in welding process is prone to oxidation, heat affected zone width, loss of alloy elements, making it difficult to obtain matched with the properties of the base metal of welded joints. Heat treatment can improve the processing and mechanical properties of the magnesium alloy, the microstructure of the heat affected zone of magnesium alloy welded joint is generally coarse and large, appropriate heat treatment can improve the microstructure and obtain good strength and toughness. Therefore, The study of heat treatment has certain theoretical significance and practical value for AZ31 magnesium alloy and the microstructure and mechanical properties of its welded joint.In this paper, we welded the AZ31 B magnesium alloy thin sheet through manual TIG welding and applied annealing and homogenization treatment on the welded joints and magnesium alloy base metal. We compared the change of the microstructure of the welded joints and base metal during heat treatment with the optical microscopy and scanning electron microscopy. We analysed the precipitated phases and composition changes of the base metal and welded joints before and after heat treatment by the help of the EDS and XRD. Through tensile tests of base metal and welded joints with universal tensile testing machine, we compared and analyzed the tensile strength and elongation of the base metal and welded joints in different heat treatment. We analyzed the microhardness variation of the base metal and welded joints before and after heat treatment.The results show that grains in the parent metal is coarse and uneven distributed under the observation of optical microscopy, and a large number of precipitates also been observed covered in the surface of the weld zone which resulted the boundaries blurred. Precipitates in the weld zone gradually dissolved in the base metal during the annealing and homogenization treatment process, and grains in the base metal and weld zone recrystallized in the same condition, the new generated small grains grew gradually engulfed large grains. The grains in base metal and weld zone becomes uniform and fine when annealed at 400℃for 1h, the cast structure in the weld zone formed in the process of welding is more uniform and fine than the structure in the parent metal. However, when the annealing temperature increase to 450℃, the grains in the base metal and weld area grow up quickly which result the coarse grain and uneven structure. By EDS and XRD analysis showed that the black precipitates in the weld zone under optical microscope is bright white coarse particulate under scanning electron microscope which is enriched by supersaturated metal atoms. There is no low melting point eutectic compound Mg17Al12 observed in the weld zone and base metal, along with the increase of annealing temperature, coarse metal atom enrichment particles gradually dissolve in the matrix.Tensile tests were performed on magnesium alloy base metal and welded joints, heat treatment has little effect on the properties of magnesium alloy, the tensile strength and elongation did not change significantly. The test shows that heat treatment can significantly improve the performance of welded joints. When the annealing temperature increased from 200℃ to 400℃, the tensile strength and elongation increase gradually, after annealing at 400℃ for 1h, tensile strength and elongation reach the maximum value. When the annealing temperature increase to 450℃, the tensile strength and elongation decreased sharply.The analysis of tensile fracture of welded joints show that there are small structure level on the tensile fracture section when the joints were annealed at 250℃ for 1h or homogenized at 250℃ for 10 h, and the fracture section is the typical brittle fracture surface which corresponding to its elongation. When the joints were annealed at 400℃ for 1h or homogenized at 400℃ for 10 h, various dimple appear on the tensile fracture surface.