Effect of wire composition on microstructure and properties of 6063 aluminium alloy hybrid synchronous pulse CMT welded joints

Hybrid synchronous pulse cold metal transfer (CMT) welding on a 3.0-mm-thick 6063 aluminium alloy sheet with different welding wires (ER4047 and ER5183) was implemented. The effect of the welding wire composition on the solidification process of the molten pool and the microstructure and mechanical properties of 6063 aluminium alloy hybrid synchronous pulse CMT welded joints was studied. The results showed that the thermal conductivity of the molten pool in the ER4047 welded joint was higher than that in the ER5183 welded joint, and the difference was about 15.7 W/(m· K) at 540 °C. The cooling speed of the ER4047 welded joint molten pool increased so that the pores did not overflow readily, which caused the porosity in the fusion zone in the ER4047 welded joint (0.21%) to be significantly higher than that in the ER5183 welded joint (0.01%). Large pores (278.1 μm) appeared near the upper surface of the fusion zone in the ER4047 welded joint. The surface tension of the molten pool in the ER4047 welded joint was lower than that in the ER5183 welded joint (the difference value was approximately 33.2 mN/m), which resulted in the width of the upper and middle part of the fusion zone (8168.0 μm and 6353.4 μm, respectively) being significantly higher than that in the ER5183 welded joint (7985.0 μm and 5009.9 μm, respectively). The centre microstructure of both fusion zones comprised a large number of equiaxed grains. The average grain size of the fusion zone in the ER4047 welded joint (78.5 μm) was smaller than in the ER5183 welded joint (103.2 μm). There were different degrees of element segregation at the grain boundaries of the fusion zone. The microstructure of the fusion zone in the ER4047 welded joint mainly α-Al and Al–Si eutectic, while the microstructure of the fusion zone in the ER5183 welded joint mainly contained α-Al and Al–Mg eutectic. The presence of pores (especially large pores) in the fusion zone in the ER4047 welded joint severely reduced the effective load bearing area of the fusion zone, which resulted in stress concentrations around the pores that finally caused fracture during stress deformation. The tensile strength of the fusion zone in the ER4047 welded joint (68.1%) was significantly lower than that in the ER5183 welded joint (85.7%). [Display omitted] •The porosity and microstructure models of CMT welded joints were established.•The molten pool solidification and FZ formation mechanism with different welding wire compositions were studi