Abstract: Variable polarity plasma arc (VPPA) welding has shown its unique advantages in the welding of thick aluminum alloy plate. Pilot arc and its power source have great influences on arc shape, mechanical properties and static characteristic of arc in variable polarity plasma arc. By means of high-speed photograph, pressure sensor and signal detection system based on industrial computer, the rule of influences of pilot arc on arc shape, arc force and welding process are systematically analyzed. It is found that during the reversed polarity period of VPPA, the arc force is decreasing due to the shunting action of power source of pilot arc, which can easily cause such phenomena as double arc and decrease of arc voltage. The experimental results have important theoretical guiding significance to the stability of plasma welding process and to the welding process control.

Key words: Arc force, Arc shape, Variable polarity plasma arc welding