Experimental analysis of the behaviour of high current electrodes in plasma arc cutting during first cycles

Abstract

The behaviour of hafnium (Hf) cathodes at the beginning of their service life when operating at high current levels (250 A) in the plasma arc cutting (PAC) process has been experimentally investigated with the final aim of describing the phenomena that take place during those initial cutting cycles (CCs), and optimizing (with respect to expected service life) the initial shape of the electrode emissive surface. The experimental tests were carried out under realistic operative conditions for cutting mild steel plates with oxygen/air as the plasma/shield gas. Starting with an electrode with a new planar emissive surface, subsequent CCs were completed. With each cycle the dimensions of the growing concave recess naturally created on the insert, and the amount of Hf oxide (HfO2) collected on the nozzle surface and on the flat surface around the edge of the recess was recorded. Morphological and compositional analysis of the tested electrodes and nozzles led to a detailed description of Hf erosion mechanisms, modifications of the Hf insert morphological structure, and effects of HfO2 deposits on the nozzle inner surface. High-speed imaging during early stages of the transferred arc mode has been used to highlight the possible presence of non-destructive double-arcing phenomena taking place during the first few CCs as a consequence of deposition of HfO2 on the nozzle. Conclusions can be drawn concerning the optimization of the dimensions of the initial recess of the Hf insert fit to avoid the massive deposition of material on the nozzle inner surface that would cause a strong reduction of electrode and nozzle service life and a rapid degrading of cut quality.