Effect Of Cryogenic Treatment On Microstructure And Properties Of AZ31 Magnesium Alloy CMT Welded Joint

Magnesium alloys have high specific strength and specific stiffness and other excellent performance, coupled with China has rich reserves of magnesium, therefore its on a very broad prospects in the industrial. Magnesium alloy structural member must involve in the connection problems in the course of using,some large-scale equipment usually applied to welding technology, but magnesium alloy structural parts suffered a softening of welded joint problems: welded joints easy generate welding deformations, cracking, inclusions, porosity, etc. These weld defects seriously degrade the performance of magnesium alloy structure, and therefore an urgent need to improve or solve the soften joints.Cryogenic processing techniques have demonstrated their improved material microstructure, to improve the mechanical properties in a lot of ferrous and non-ferrous metals copper, aluminum, etc,as well as convenient operation, environmental protection. As soon as the mechanism of cryogenic treatment to magnesium alloys research breakthrough, it will bring enormous economic and social value.In this paper, 7mm thick rolled state AZ31 magnesium alloy CMT welding joints are studied, which requires less heat input in the welding, to study the effect of microstructure and mechanical properties of welding joints by deep cryogenic treatment. Study found that cryogenic treatment improves the microstructure of the weld zone, the grain refinement, β-Mg17Al12 second phase particles at the grain boundaries and within the grains dispersed and dense,and the XRD results showed that the grain occurs rotation after cryogenic.The welded joints’ hardness, tensile strength and elongation are also increased in different degree, there are certain relationships between the degree of change and the cryogenic process parameters. The average Vickers hardness of welding joints are highest after deep cryogenic treatment at-180℃ for 4 hours,Vickers hardness value at the trough is 51 HV, than those non- cryogenic increased by 15.9%. At this temperature, with the extension of the deep cryogenic of time, the hardness does not change significantly. Changes of tensile strength consistent with yield strength, yield ratio essentially unchanged. The tensile strength of welding joints are highest after deep cryogenic treatment at-180℃ for 4 hours, which increased by 15.9% compared to non-cryogenic treatment and joint efficiency reach to 94%. Elongation is also increased with decreasing temperature. Along with the deep cryogenic temperature reduced, the number of dimples gradually increased and the dimples became uniform and densification. It improved the strength and ductility of welding joints significantly. Experiments show that the strength and hardness of the joints to improve significantly at low cryogenic temperatures(-180 ℃), but a short period treatment can better improve the softening problem which is better process parameter. Finally, based on the experiment result and metal knowledge, the cryogenic treatment strengthen the microscopic mechanism of magnesium alloy welded joints were explored.