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Effects of Shielding Gas and Magnetic Field on Characteristics of AZ31 Magnesium Alloy by TIG Welding

The effects of shielding gas and external longitudinal magnetic field on arc shape and weld quality of AZ31+1%Ce+1%Sb magnesium alloy TIG welding were investigated. The results show that a weld joint with a good appearance, few defects and enough penetration can be obtained under the protection of helium-argon gas mixture, especially when the helium ratio is about 50%. However, the weld quality is significantly decreased by adding additional CO2 or N2 in He-Ar gas mixture. The weld seam consisted of fine equiaxed grains which mostly grow in a certain direction were formed when a longitudinal magnetic field is added, due to its electromagnetic stirring in weld pool. The grains of weld seam formed under the alternating current longitudinal magnetic field are much finer than those under the direct current magnetic field. A weld joint with good formability and finer microstructure is obtained when the exciting voltage of AC longitudinal magnetic field is 20 V. The average microhardness of the corresponding weld joint is 68.5 HV0.1, and its average tensile strength is 220 MPa which is up to 88% of that of the base plate.