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A series of Zn- x Al (x = 0‒35 wt.%) alloy filler metals were designed to join AZ31 Mg alloy to 6061 Al alloy by laser-TIG hybrid welding. The effect of Al content on the wettability of filler metals, microstructure evolution and strength of joint was investigated. The results indicated that the strength of joints was improved with the increase of Al content in filler metals. When Zn-15Al filler was used, the ultimate fracture load reached the maximum of 1475.3 N/cm, which was increased by 28% than that with pure Zn filler. The reason is that the Al element acts as a "reaction depressant" in filler metal, which contributes to inhibiting the dissolution of Mg base metal and the Mg-Zn reaction. The addition of appropriate quantity of Al element promoted the precipitation of Al-rich solid solution instead of Zn solid solution. The MgZn 2 IMCs have lower lattice mismatch with Al solid solution than Zn solid solution, thus the strength of joints is improved. However, the excessive addition of Al caused the formation of brittle Mg 32 (Al,Zn) 49 ternary compounds, leading to the deterioration of joint performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • This work proposed a laser-offset-TIG hybrid welding method to join Mg-Al dissimilar metals, the joint strength is 1.3 times higher than that with normal laser-TIG hybrid welding. • The laser-offset-TIG hybrid heat source can not only promote the wettability of filler metal, but also inhibit the formation of brittle Mg-Zn IMCs. • The attraction effect of the laser to arc is conducive to reducing the heat input on Mg base metal. This work proposed a laser-offset-TIG hybrid welding method to join AZ31 Mg alloy to 6061 Al alloy with Zn filler in lap configuration. Through the lateral offset of laser and arc, the wettability of filler metal was improved, and the amount of intermetallic compounds (IMCs) was well controlled. The spread width of filler metal increases from 2.13 mm to 3.49 mm and the reaction depth decreases from 0.91 mm to 0.66 mm as the offset distance increases from 0 mm to 2 mm. The major phases formed in joint are Al solid solution, MgZn 2 IMCs, Zn solid solution and Mg 32 (Al,Zn) 49 ternary IMCs. At the appropriate offset distance of 2 mm, the coarse MgZn 2 grains in fusion zone are refined to (Al)+(Zn)+MgZn 2 eutectic, and the brittle Mg 32 (Al,Zn) 49 ternary IMCs in weld corner are eliminated. The maximum joint strength is 1190 N/cm, which is 1.3 times higher than that with normal laser-TIG hybrid welding. [ABSTRACT FROM AUTHOR]

A non-axisymmetric laser-tungsten inert gas (TIG) heat source was designed to join Mg–Al dissimilar metals with pure Zn filler at a series of welding speeds (500–900 mm/min). Laser and TIG heat sources respectively acted on Al and Mg base metals to precisely control their dissolution into the welding pool. The solidification rate of liquid metal was controlled by adjusting the welding speed, then the reaction process of Mg, Al and Zn could be accurately regulated. The results indicated that various microstructures including Al solid solution, Zn solid solution, Mg–Zn intermetallic compounds (IMCs) and eutectic structure formed in the joint produced at different speeds. Lower welding speed (500 mm/min) caused the microstructure coarsening and higher welding speed (900 mm/min) would lead to the enrichment of MgZn2 intermetallic compounds. At the optimal welding speed of 800 mm/min in particular, fine MgZn2 IMCs grains uniformly distributed in the Al and Zn solid solution. The tensile-shear load reached a maximum of 1052.5 N/cm and the joint fractured at the fusion zone near the Al base metal. [ABSTRACT FROM AUTHOR]