Effect of TIG current on microstructural and mechanical properties of 6061-T6 aluminium alloy joints by TIG–CMT hybrid welding

In order to expand the direct current-cold metal transfer (DC-CMT) method to include large-thickness plate welding, a tungsten inert gas–CMT (TIG–CMT) hybrid welding process was employed in this work. Adding a stable TIG arc can increase the total heat input on the workpiece and the penetration capacity of the DC-CMT welding process. Al6061 plates 4 mm thick were welded successfully by this method. As the TIG current increased, the heat input of the welding process increased. The effect of TIG current on the microstructure and mechanical properties of the joints was investigated. The hardening phase in Al6061 was sensitive to the heat input provided by the welding process. Because of microstructural transformation, a decrease in the mechanical properties (softening phenomenon) invariably appeared in the heat-affected zone (HAZ). With increasing TIG current, the microstructures of the joints coarsened, and the width of the partially melted zone (PMZ) and HAZ of the joints broadened. A precipitation strengthening phase of Mg2Si appeared in HAZ. The hardness curve of the joint presented roughly W-type distribution. The hardness obtained in the softened zone was approximately 50% that of the base metal (BM). The joints demonstrated good deformation capability before fracture. As compared to the base material, the strength and the elongation of the joints decreased approximately 40% and 50%, respectively.