AN INVESTIGATION ON TUNGSTEN CATHODE STABILITY IN PLASMA ARC WELDING

Experiments have been carried out to compare the burn-weight-loss and the deformation at the tips of 14 tungsten cathodes with different packed and cooling conditions. The test results revealed that:<1> Welding characteristics of the ceriated-tungsten electrode is almost the same as the thoriated electrode.<2> Bare electrode with insufficient cooling has the most burn-weight-loss due to tungsten oxidizing on the electrode cylindric surface and the consequent sublimation of the wolfram oxide. <3> Electrode damage by "flowering" at the cathode tip ("umbrella-like-halo" forming) and by "tumour-condense-forming" are also related to the sublimation prcess.<4> Electrod packed with copper,for example,or intensive cooling can improve the cathode tip steadiness without any harmful effect on the arc stability.<5> There is an important phenomenon that the melting of blunt tungsten cathode and the change of its chemical compositions are far more severe than those of the sharped one under the same conditions.It is considered to be a result of the cathode-spot-floating and increasing the ion-conducting-fraction in the cathode zone.Therefore,it is suggested that the selected cathode should have a satisfactory geometry at the tip. <6> Packed electrode is possible to take thin tungsten wire in place of the thick nonpacked one,which will increase the cathode stability and decrease the electrode grinding frequency.On the other hand,it will greatly minimize the contamination of surrounding environment by thorium radiation,if the thoriated electrode is selected.