Erosion mechanisms of hafnium cathodes at high current

Abstract

An investigation of cathode erosion has been conducted for conditions similar to those encountered in a plasma cutting process. A hafnium insert in a water-cooled copper sleeve serves as the cathode. Modifications to the torch allowed the observation of the cathode surface during operation and measurement of material loss from the cathode during different phases of an operating cycle. Erosion has been found to be predominantly due to ejection of molten material droplets. Most ejection events are associated with changes in the conditions of the plasma, e.g. during start-up, change of gas flow and shutdown. The ejections can be explained by imbalances of the forces acting on the molten surface, those associated with the arc current, those due to surface tension, and those associated with the fluid dynamics within the torch.

D.C. Plasma Spraying

“…The button typically consists of hafnium or zirconium, and the surface of the material is molten. However, the pressure exerted by the arc due to the ion flux to the cathode and due to the magnetic forces resulting from the self magnetic field and the change in current density in front of the cathode will keep the molten material in place [5].…”

Relating Spectroscopic Measurements in a Plasma Cutting Torch to Cutting Performance

“…One of the major sources of wear is erosion of the hafnium emitter. It has been shown that a significant portion of this erosion is in the form of particulates that are ejected from the molten hafnium pool [11]. These particulates are often deposited on the interior of the nozzle and also change the geometry of the cathode.…”

“…During operation the cutting torch cathode is eroded away by the arc. This erosion occurs in steady state operation due to fluid dynamic effects near the cathode surface [10], and under the transient conditions during arc ignition and extinction between cuts [11].…”

Cathode diameter and operating parameter effects on hafnium cathode evaporation for oxygen plasma cutting arc

“…During operation of plasma arc cutting torch, the molten material is ejected from surface of cathode insert [3]. A significant amount of the material deposits on the inside surface of the nozzle and also change the geometry of the cathode.…”