Microstructure and mechanical performance of AZ31/6061 lap joints welded by laser-TIG hybrid welding with Zn-Al alloy filler metal

A series of Zn-xAl (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 MgZn2 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 Mg32(Al,Zn)49 ternary compounds, leading to the deterioration of joint performance.