Effects of Arc Current and Electrode Size on Electrode Erosion in AC Plasma Torches

Abstract

Heavy erosion of the electrode is a problem in using AC plasma torches. In a series of our studies, the effects of electrode stem size (diameter and length) and tip shape on the electrode erosion were clarified at a low current of 200A. In this paper, on the basis of the 200A-data, in order to reduce the electrode erosion with the current increased, the effects of current and electrode size on electrode erosion were clarified experimentally and analytically at 100-550A. The electrode used was of solid type and the material was tungsten containing 2 wt% of lanthanum oxide. The plasma gas was argon. Main results are as follows.
(1) As the current increased, the electrode erosion increased linearly. In case of thin electrode(8mm φ), at 450-550A, the electrode tip was molten partially and the electrode erosion increased rapidly over 5mg/h. However, in case of thick electrode (13mm φ), the electrode erosion did not increase rapidly.
(2) The temperature distribution along the electrode axis was calculated. As a result, a good correlation was obtained between the calculated temperature of electrode tip and the measured electrode erosion. As the tip temperature decreased to 3, 000K (c. f. tungsten melting point: 3, 660K), the electrode erosion decreased below 5mg/h of the linearly increasing range.