Effects of electrode positive/negative ratio on microstructure and mechanical properties of Mg/Al dissimilar variable polarity cold metal transfer welded joints

Variable polarity cold metal transfer (VPCMT) welding process shows a large potential to join Mg/Al dissimilar alloys in the future manufacturing industry. In this study, Mg alloy AZ31B and Al alloy 6061 were joined by VPCMT process with ER4043 as filler metal, and the effects of the ratio of electrode positive/negative CMT cycles (EP/EN ratio) on the microstructure-strength relationships of welded joints were investigated. The results showed that hard brittle Mg-Al intermetallic compounds (IMCs) layers were formed in the weld interfaces and consisted of three intermediate layers: Mg2Al3 layer, Mg17Al12 layer, and Mg17Al12+α-Mg solid solution eutectic layer (very thin). With decreasing EP/EN ratio from 4:1 to 1:4, the thicknesses of the Mg2Al3 layer and Mg17Al12 layer were reduced from 80 µm to 10 µm and from 105 µm to 80 µm, respectively. The tensile strength, which was fairly dependent on the thicknesses of these intermediate layers, increased significantly by over 100 percent. The fractures of the joints with 4:1 and 1:4 EP/EN ratio occurred primarily within the Mg2Al3 layer and the Mg17Al12 layer, respectively. As a result, the more EN-CMT-cycles, the lower energy input, the thinner IMCs layer, and the higher joint strength.