Heat input analysis of variable polarity arc welding of aluminum

Influence of direct current electrode positive (DCEP) duty ratio in variable polarity (VP) arc welding of aluminum has been investigated in the current work. VP arc adopts alternate current (AC) of DCEP and direct current electrode negative (DCEN) as power source, and the duty ratio indicates the polarity's durational portion in the current pulse. Experimental results showed proportional relationship between heat input and DCEP duty ratio although there was a fluctuation due to the arc instability increase caused by wearing of the tungsten electrode tip. This observation violates the conventional arc theory of anode heating, and therefore, a new theoretical approach combining the tunneling effect and random walk of cathode spot has been proposed. During DCEP duration, arc is concentrated on thinner oxide cathode spot, which further melts and breaks the oxide layer of the spot. This process is repeated at another cathode spot at a high frequency. Therefore, the higher the DECP duty ratio, the more cathode spots are generated and the longer was the arc concentration time. This in turn resulted in a higher heat input for longer DCEP polarity duration. The proposed mechanism combining the tunneling effect and random walk of cathode spot can not only explain the anomalous results of the current research but also could address the ambiguous result of deeper penetration in thicker oxide in the past research.