Abstract: 7075Al alloy is an important industrial material due to its excellent mechanical properties and corrosion resistance. But its high alloy element content results in poor weldability, which limits its wide application in complex connection scenarios. Therefore, the effective welding of 7075Al alloy is of great significance for its further application. The metal transfer behaviors and weld formations in AC-CMT arc welding of 7075 Al alloy with ER7075 and ER5356 wire were comparatively investigated using a synchronized acquisition system for process parameter and image. In the EN (electrode negative) half cycle, the droplets at the ends of both types of welding wires are in a regular elliptical shape, and the transfer processes are both stable without obvious spatter. In EP (electrode positive) half cycle, the molten droplet at the end of ER5356 welding wire takes a pear shape, and is transferred into the weld pool with slight spatter. The droplet at the end of ER7075 welding wire is irregular and elongated, and transferred into the weld pool with increased spatter. The elongated droplet vibrates up and down, which is prone to cause premature contact with the vibrating molten pool surface, resulting in abnormal short circuit. The intense and ununiform Zn evaporation on the droplet enhanced by the anode spots inherently occurring on the metal evaporation sites is the main reason for the increased spatter in EP half cycle. On the other hand, the Zn evaporation causes noticeable arc contraction, reducing the weld fusion width and increasing the penetration capacity. The weld made with ER7075 wire is quite good-looking with fine fish scale patterns, although it is covered a dark oxide film and with much more tiny spatter particles scattered nearby. The tensile strength of the T6 heat-treated joint welded with ER7075 wire is 117.8% higher than that welded with ER5356 wire.

Key words: CMT arc welding, EP half cycle, EN half cycle, spatter