Study Of TIG Welding Process And Stress Corros--ion Behavior On Joint Of AZ31 Magnesium Alloy

In this study, the welding parameters of TIG is optimized with 2mm thick AZ31 magnesium alloy. The influence of slow strain rate on mechanical behaviour of the welded joints is investigated. And the corrosion resistance of welded joints and corrosion sensitivity is researched under different media and stress.The results show that the AZ31 magnesium alloy welded joint consists of base metal(BM), heat affected zone(HAZ), The weld(WZ), and the microstructure of the base metal and heat affected zone are made up of α-Mg single phase solid solution and the microstructure of weld consists of α-Mg solid solution and β-Mg17Al12 intermetallic compounds. With the current enhancing, the welding zone β-Mg17Al12 intermetallic compounds increased, the grains in heat affected zone became larger and the tensile strength increased first and then decreased. When the current is 90 A, the mechanical properties of the welded joint is best. The tensile strength is 223 MPa, which is the 89.2% of base material and the fracture whose mode was the ductile brittle mixed fracture occurred in the heat affected zone.Under different strain rates, AZ31 magnesium alloy welded joint exhibit positive strain rate. With the strain rate decreasing, the strength decreased, the elongation increased and the ability of plastic deformation enhanced. The main plastic deformation mode of AZ31 magnesium alloy is twins. With the decrease of strain rate, the twins of welding joint increased, the grain size became smaller. Furthermore, the grain orientation on joint changed which preferred the orientation in {00 0 2} crystal surface. With different strain rates, the necking of joint fracture is smaller than the base material. The fracture mode was ductile brittle mixed fracture and the toughness characteristics is more obvious with the decrease of strain rate.The corrosion resistance of AZ31 magnesium alloy at weld is better than the base material and heat affect zone which is the poorest. In the solution of 3.5wt.%NaCl, 3wt.% NaCl+0.5wt.%H2O2 and 3.5wt.%NaCl+2wt.%KCrO4, the failure parts of welding joints are at heat affect zone where there is no obvious plastic deformation and the tensile strength is only half compared with the surrounding medium. The fracture is flat and there are a number of main crack with bifurcation existed on the section. The stress corrosion exponent ISSRT of welding joints in this three kinds of solution is larger than the value of base material, which the largest appeared in 3.5wt.%NaCl+2wt.%KCrO4 solution with the vaule of 0.61.