Microstructure and mechanical properties of 2A12 aluminum alloy VPPA welded joint with Ar-He mixed shielding gas

The variable polarity plasma arc(VPPA) welding technology of binary mixed shielding gas was studied, and the joint of 2A12 aluminum alloy with 12 mm thickness was obtained. After welding, the weld formation, microstructure, interface element distribution and mechanical properties of the welded joint were analyzed by metallographic microscope, scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and universal hydraulic testing machine. Compared with argon arc, the effective cross-sectional area of argon-helium mixed gas arc was smaller, which made it have higher energy and more concentrated anode power density. The welding thermal cycle of each region of the welded joint was different, and the heat affected the microstructure characteristics, which in turn affected the mechanical properties. The results showed that compared with the one-dimensional shielding gas welded joint, the microstructure of the weld zone appeared more wheat-like dendrites, which were finer and denser. The EDS analysis from the base metal to the seam center showed that the Mg element in the heat-affected zone increased, and the Cu element in the weld center increased. The average tensile strength of the joint was 317 MPa, which was 75.8 % of that of the base metal, and the yield strength was 65 MPa lower than that of the base metal. The fracture showed typical dimple fracture; at the same time, the microhardness of the joint presented a “W” shape distribution, and the minimum value was 95.5 HV. Hybrid shielding gas VPPA welding technology had a good application prospect.