No abstract

When alternating current (AC) is used in tungsten inert gas (TIG) welding, the shape of weld penetration is known to be largely changed depending on electrode positive (EP) polarity ratio which is defined as a ratio of EP period occupied in one cycle of current waveform. This fact implies that heat transport processes between electrode negative (EN) and EP polarity are significantly different to lead to different temperature distribution of weld pool. However, these processes have not been understood clearly, because they are thought to be directly linked to the complex cathode spot behavior. This study aims to discuss difference in heat transport processes between EN and EP polarity in AC TIG welding of aluminum A1050 by comparing temperature distributions in both polarities measured with two colour pyrometry taking into account the cathode spot behavior. As a result, it was found that the temperatures at the center in EN were greater than that in EP and decreased gradually toward the edge of the weld pool. Moreover, the temperature in 30% EP polarity ratio was higher than that in 10% EP polarity ratio. Consequently, the heat flux produced by cathode spots is suggested to significantly affect the surface temperature distribution of weld pool as well as the weld shape. The heat flux distribution in EP polarity is considered to become not Gaussian but ring-shaped. Furthermore, the reason to achieve higher average temperature in 30% EP polarity ratio is explained by longer heating through cathode spots.