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In the arc welding field, it is not easy to achieve a high-quality joint of aluminum alloy due to several reasons. The coefficient of thermal conductivity of aluminum is six times larger than that of steel. Therefore, the heat source for welding aluminum needs to be further intense and concentrated than that for steel. Furthermore, the formation of oxide layer on the aluminum surface and its physical characteristics are also important reasons. The low electrical conductivity of the oxide layer prevents to maintain the arc current. Besides, the difference in melting points between aluminum (660°C) and its oxide (2,060°C) causes other problems; aluminum oxide tenacious and rapid-forming oxide hinders the heat transfer process and releasing process of dissolved gas in the weld pool. Due to these reasons, aluminum weld bead generally tends to have low penetration and contain fusion defects, oxide inclusions, and porosity. At present, there are new welding processes developed to joint aluminum such as electron beam welding, laser welding and friction stir welding. Each process has advantages and also limitations. The choice of the welding process depends on properties of the specific aluminum alloy, joint configuration, strength requirement, appearance, and cost. However, some conventional processes, such as Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) welding processes, are still in use widely because these are flexible and require a low capital cost. Moreover, TIG welding process has the ability to produce a higher quality joint than that of MIG welding process. TIG welding is generally used in Direct Current Electrode Negative (DCEN) mode because this mode could provide a stable arc, large heat flux to the base metal leading to deep penetration, and high welding speeds. However, in case of aluminum welding, due to the need to clean the oxide on the aluminum surface, Alternating Current (AC) mode, which can utilize cathode spots to clean the oxide in Electrode Positive (EP) polarity, must be used. On the other hand, use of EP polarity of AC mode could result in the consumption of tungsten electrode and low penetration. The consumption of the electrode could reduce the lifetime of the electrode. Therefore, in aluminum welding, there are still some issues needed to be resolved such as oxide cleaning, electrode consumption, and penetration. In AC mode, the EP polarity duration is generally adjusted around 30% of a cycle to balance these issues. However, if the cleaning efficiency of the oxide is improved more, the EP polarity duration can be reduced to decrease the electrode consumption and increase penetration. The commonly used shielding gas for the AC TIG welding of aluminum is argon. The oxide tends to be cleaned over a wide range during welding with argon. Another advantage of using argon is that the arc ignites easily at polarity reversal in AC TIG welding. Another option of shielding gas in AC TIG welding

Behaviors of Cathode Spot in Alternative Current Helium TIG Welding of Aluminum