Materials for Thermionically Emitting Electrodes in Arc Welding

Nonconsumable electrodes are made of tungsten and its alloys, for arc welding processes, and of graphite, for arc gouging. However, current technical literature does not give detailed explanations for not using other materials as thermionic emitters. Taking the challenge of contributing to the fulfillment of this knowledge gap, this study attempts to justify the unviable use of pure iron as a nonconsumable electrode material and to demonstrate the thermionic emission capacity of pure and thoriated tungsten and graphite electrodes. Initially, the emission areas, and the respective electrodes diameters, for welding at 100 and 300 A at several temperatures were theoretically determined. Then, the maximum temperatures reached by different diameters electrodes due to Joule effect were calculated. The outcomes validation was carried out by confronting them to the phenomena observed in practice. Nonconsumable iron electrodes showed to be unviable, because they would require extremely large diameters to emit in usual current levels below their melting temperatures. Besides, they would not be heated by Joule effect up to the emission temperature before arc extinction. On the other hand, it has been demonstrated that pure and alloyed tungsten and graphite electrodes emit with small diameters approaching their melting/sublimation temperatures and they promptly reach the emission temperatures by Joule effect. Despite the adopted simplifications, the electrodes sharpening need for arc welding was also justified, as well as it was demonstrated that the W electrode doping with lower work-function oxides limit the heating of the electrode preventing its superficial melting.