Principle of Gas Tungsten Arc Welding is quite simple. Electron, which is emitted from Tungsten electrode, hits into the metal, and the heat from this impingement produce enough heat for fusion welding. However, actual behaviors are quite complicated. Electron emission is much influenced by physical condition of cathode and ambient circumstance. Considerable electron emits from unbelievable location of the cathode, especially at the ignition stage when the cathode is not enough hot. The anode spot, where main electrons hit into the metal, is also much influenced by surface condition and purity of shielding gas. The anode spot is usually appeared at the center of molten pool where the temperature seems to be maximum value. But, it sometimes moves irregularly or hits on the solid surface. This situation is mainly caused by oxidized layer on the metal or molten pool. Some minor elements on the metal act as a kind of catalytic agent. It sometimes drastically changes weld geometry. An amount of erosion on cathode surface is increased with increased oxygen contents. The penetration depth is also increased with oxygen contents. Plasma condition is also changed, because cathode and anode conditions are closely connected with plasma condition. Dynamic behavior of arc-cathode and arc-anode reactions was observed by ultra-high speed video system in a chamber from vacuum to high pressure, where contents of gas are controlled. Effects of oxygen and minor elements on anode and cathode reactions were considered.

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