Molten pool behaviour and weld forming mechanism of tandem narrow gap vertical GMAW

A tandem gas metal arc welding system was utilised for narrow gap welding in vertical down position. In the welding process, the two pulses are asynchronous and two arcs worked in the same molten pool. The molten pool forming process was investigated, and the results suggest that an asymmetric pool was generated, which resulted in the asymmetry of the weld bead. This condition could be improved by decreasing the distance between the two wires. The forces acting on the molten pool and the droplet were analysed. It is pointed out that, in vertical welding with low welding speed, more fusion metal flows down to the head of the molten pool due to the gravity, which leads to the reduction of the penetration depth. Based on the results, a multilayer welding was conducted with a high welding speed and a close distance between two wires, and a high quality weld was acquired.