Tungsten Electrodes Rim Phenomenon during TIG Welding

In TIG welding (tungsten inert gas welding), tungsten electrode under heavy current and poor argon protective is prone to form rim on the surface of the electrode tip. The rim will hinder the argon to flow and rare earth oxide on the electrode surface to migrate, exacerbating burning, and destabilizing electrode’s performance.

Under the same conditions, W-CeO2 is the earliest to form rim, followed by the W-Y2O3, and finally the W-La2O3. In addition, the argon gas flow, the arc length will have some influence on the rim forming.

Effect of argon gas flow rate: In the test electrode arc length is 3mm and electric current is 160A. Arcing voltage is 76V. By changed the flow rate of argon found that the smaller argon gas flow rate, the shorter the rim appears.

Effect of arc length: In the tests, argon gas flow rate is 3L / min and electric current is 160A. Arc voltage is 76V. By changing the length of the arc found, the longer arc length, the shorter the rim appears.

Observed electrode SEM photograph after welding found the electrode tip is divided into three areas. A area and C area is rough. B area is smooth. Besides, the rim usually appears between C area and B area. And this area is dendrite structure described rim growth method is based on dendritic.

In the arc burning process, under the joint action of the arc and the heat resistance, on one hand, the tungsten on the electrode surface will melt and evaporation. On the one hand, it will show recrystallization. In C area, tungsten recrystallization rate is greater than melting and evaporation rate, so there will form rim. In addition, during welding process although electrode has argon gas protection, but there is still a small part of the oxidizing gas impurities involved. Further, intruded by oxygen molecules in the air, the tungsten on the electrode surface will oxidize at a lower temperature C area to generate tungsten oxide. These oxides will decompose at high temperatures, deposited at recrystallization dendrite, forming the rim.

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