Analyses of Oxygen Concentration on Anode Surface in Gas Tungsten Arc Welding Using CO₂ Gas

Gas tungsten arc welding is a non-consumable electrode-type arc welding process in which tungsten and inert gas are used as electrode and shielding gas, respectively. Since an inert gas is used as the shielding gas, a formed weld bead is highly clean and almost no slag is generated on the bead. Therefore, this welding process has advantages of producing a weld part with a good bead appearance and superior mechanical properties. On the other hand, there is a disadvantage that its welding efficiency is low in comparison with other welding processes. Hence, many previous studies have been conducted to improve the disadvantage of this welding process and to expand the range of application of it. As one of the methods for improving the welding efficiency, a gas tungsten arc welding with a double gas shielded system in which carbon dioxide gas can be used as the shielding gas was developed.In the double gas shielded system, a cylindrical nozzle was mounted between a shielding gas nozzle and an electrode in a welding torch. Because of this additional nozzle, a flow path of the shielding gas can be separated into an outer gas and an inner gas. It was clarified in the previous study that a penetration depth increased and the welding efficiency was improved compared with a conventional gas tungsten arc welding when inert gas and carbon dioxide gas were used as the inner gas and the outer gas, respectively. It was also shown that although carbon dioxide gas was used as the shielding gas, an amount of the electrode consumption was equivalent to that of the conventional gas tungsten arc welding. This was because the electrode was protected by the inner gas. However, the detailed mechanisms of the deep penetration have been still unclear. In this study, argon gas was used as the inner gas and carbon dioxide was used as the outer gas. Then, in order to clarify the behavior of carbon dioxide gas in the arc plasma, oxygen concentration measurement on the base metal surface and spectroscopic measurement of the arc emission were performed under these conditions. First, the oxygen concentration on the base metal surface was measured using different outer gas, welding current, arc length, and electrode extension. When argon gas was used as the outer gas, the oxygen concentration near the center of the arc did not increase. However, the oxygen concentration near the center of the arc increased when carbon dioxide gas was used as the outer gas. When the welding current increased, the oxygen concentration near the center of the arc increased. It was also clarified that the oxygen concentration near the center of the arc increased when the arc length and the extension became long, respectively. From these above oxygen concentration measurement results, it was shown that although argon gas was used as the inner gas, oxygen was supplied to the base metal surface near the center of the arc under these conditions. However, it was still unclear whether the oxygen detected in the oxygen concentration measurement was derived from the atmosphere or generated by the dissociation of carbon dioxide which was used as the outer gas. Then, the spectroscopic measurement was performed to investigate behaviors of species in the arc plasma. As a result, emissions from argon atom, and carbon ion were observed in the arc plasma. Because the emission from carbon ion dissociated and ionized from carbon dioxide was confirmed, it was clarified that the oxygen detected in the oxygen concentration measurement was derived from carbon dioxide gas used as the outer gas. In addition, it was suggested that carbon dioxide gas was more likely to be mixed into the center of the arc by increase of the welding current, the arc length, and the extension.