Experimental investigation of dominant factors for droplet ejection from electrode during AC TIG welding

The droplet ejection from an electrode during an alternative current tungsten inert gas (AC TIG) welding process was observed using a visualization system composed by a high-speed camera with a band-pass filter to clarify dominant factors of the droplet ejection. Different welding currents, electrode diameters and electrode positive (EP) ratios were set. The numbers of droplets ejected from the electrode tip were measured for the different conditions. The timings of droplet ejections from the electrode were also determined in one AC cycle. The results indicated that droplets were likely to be ejected when the welding current was high, when the electrode diameter was small, when the EP ratio was large, and in the latter half of the EP term. Because the electrode temperature under these welding conditions was higher, the high electrode temperature was considered to be a dominant factor for the droplet ejection. However, immediately after the start of the electrode negative (EN) term, the number of droplets decreased even though the electrode temperature was the highest in one AC cycle. Therefore, it was suggested that other factors affected the electrode ejection. Moreover, raised portions were formed on the surface of the molten electrode right before droplet ejections. It was considered that the formation of the raised part might be suppressed by the collision of positive ions in the arc plasma with the electrode at EN term. Estimated pressure due to the ion collision acting on the ridge was larger than that due to the surface tension and the electromagnetic force, respectively. Therefore, it was suggested that the collision of positive ions at EN term suppressed the droplet ejection.