Parameters optimization for the generation of a keyhole weld pool during the start-up segment in variable-polarity plasma arc welding of aluminium alloys

Stable weld formation is still one of the goals pursued in variable-polarity plasma arc welding (VPPAW) of aluminium alloys. Basically, VPPAW is divided into three segments: the startup segment, during which the keyhole is generated, the main body segment, during which the keyhole is dynamically maintained, and the terminal segment, during which the keyhole collapses and the crater is ®lled. Very often, a smooth transition cannot be made from the start-up segment to the main body segment, and the weld easily fails because the keyhole weld pool is not properly generated. This paper explores four di€erent combinations of plasma gas and welding current for generating a smooth transition from the start-up segment to the main body segment in VPPAW. The keyhole weld pool is categorized as one of three types: heat conduction mode, strong force mode or heat and force mode. The heat and force mode of the keyhole weld pool can be generated by raising the gas ̄owrate, raising the current or by raising both the gas and current. An increase in the plasma gas ̄owrate accompanied with an increase in the welding current is the best combination for generating a smooth transition during the start-up segment, which results in only a few double arcs and no short circuiting between the nozzle and the workpiece. The quality of the initial weld bead can be guaranteed using this approach.