Experimental Study of Dominant Factors for Droplet Ejection from Electrode During AC TIG Welding

Alternative current tungsten inert gas (AC TIG) welding is one of arc welding processes in which a tungsten is used as an electrode and the polarity of the electrode is periodically switched. This welding process is applied to the welding of aluminum alloys and the like. However, it has been reported that the tip of the electrode is easily melted in the AC TIG welding, and the molten electrode is consumed by detaching a part of the tip of the molten electrode. When the tip of the electrode is scattered, tungsten, which is the main component of the electrode, may be mixed into the welded part on the surface of a base metal. The contamination of the tungsten deteriorates the welding quality. Therefore, prevention of the droplet ejection from an electrode tip is required to obtain high quality welded part with no defects in an AC TIG welding. However, there were few guidelines for preventing the droplet ejection because it has not been clarified under what conditions the droplet ejection is likely to occur until now. In this study, the droplet ejection from an electrode tip during an AC TIG welding process was observed using a high-speed camera with a band-pass filter (938 nm ± 38 nm) to clarify dominant factors of the droplet ejection. Different welding currents, electrode diameters and electrode positive (EP) ratios were set as welding conditions. Using the results, the numbers of droplets ejected from the electrode tip were counted. Moreover, the timings of droplet ejections from the tip of electrode were also obtained in one AC cycle. As a result, 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. This was because the electrode temperature under these welding conditions was higher than that in other conditions. Hence, the high electrode temperature was considered to be a dominant factor for the droplet ejection. On the other hand, immediately after the start of the electrode negative (EN) term, the number of droplets decreased even though the electrode temperature at the timing was higher than at other timing 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 during EN term. In order to verify whether the pressure due to the collision of ions can suppress the scattering, the ion collision force, the surface tension force, and the electromagnetic force acting on the surface of the molten electrode during EN term were estimated as pressures. As a result, the pressure due to the collision of ions was 4.4 kPa, which was the largest of the three pressures. Therefore, it was suggested that the collision of positive ions during EN term suppressed the droplet ejection. In conclusion, it was clarified that the droplet ejection from the tip of the electrode was reduced not only by selecting the welding conditions that keep the electrode temperature low, but also by decreasing the ratio of EP polarity.