Tungsten Electrode Chemical Composition Affected Arc Performance

Pure tungsten electrode has higher electron work function, which is not good for electron current  emission, so that the electrode can not work under high load conditions. In order to enhance electrode’s welding performance and to increase applications fields, the manufacturers developed various types of tungsten electrodes, including thorium tungsten electrode, zirconium tungsten electrode, lanthanum tungsten electrode and yttrium tungsten electrode and so on. According to electric current density formula J_e = AT²exp (-eØ / kT) (A is a constant, k is Boltzmann's constant, T is the temperature of the cathode, e is the electronic charge, Ø is work function) can know, when the temperature is the same, the smaller the value eØ, the greater the value Je. Therefore, in order to reduce eØ, manufacturers will add a small amount of rare earth oxide in the tungsten electrode material. Doped with different amount of rare earth oxides, the electrode has specific Je value, namely having specific arc performance.

In the same rare earth tungsten electrode, the electrode having high rare earth oxide content has better arc performance. In addition, uniformity distribution and diffusion and migration behavior of rare earth oxides also affect electrodes’ arc performance. The arc performance deterioration is mainly due to the rare earth oxides evaporation and decomposition causing electron emission performance degradation. Form this we can know tungsten electrode chemical composition’s type, content, distribution and temperature characteristics determines arc performance.

During the welding process, the electrode will appear Rim phenomenon, having nodules appeared in the tip of electrode and this phenomenon can well explain the impact of the chemical composition on electrode’s arc performance. Rim phenomenon occurs mainly because the rare earth oxide content of the electrode surface is higher than the central portion, thus facilitating to arc in a short time, but in the arc process will mix a small amount of O₂ or welding parameters adjusted overload in the short term are likely to experience Rim phenomenon. This is the result of the tungsten electrode chemical composition changes at high temperatures. This phenomenon is in favor of arcing at the beginning, but after a long time will lead to the concentration of rare earth oxide on surface layer depleted, thereby reducing the arc performance.

• < Prev
• Next >