Plasma arc cutting: Microstructural modifications of hafnium cathodes during first cycles

In the present work, the microstructural modifications of the Hf insert in plasma arc cutting (PAC) electrodes operating at 250 A were experimentally investigated during first cycles, in order to understand those phenomena occurring on and under the Hf emissive surface and involved in the electrode erosion process. Standard electrodes were subjected to an increasing number of cutting cycles (CCs) on mild steel plates in realistic operative conditions, with oxygen/air as plasma/shield gas. Microstructural analysis was performed for each electrode at different erosion stages by means of scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and Raman spectroscopy. Electrodes cross sections were also observed by means of optical microscopy (both in bright field and in reflected polarised light) after chemical etching. In the insert, three typical zones were found after cutting: monoclinic HfO2 layer; thermally-modified transition zone with O2–Hf solid solution; unmodified Hf. The erosion cavity and the oxide layer thickness increase with the number of cutting cycles. Macrocracking was observed in the oxide layer, while microcracking and grain growth were detected in the remelted Hf. Moreover, detachment was found at the Hf/Cu interface. Based on thermodynamics and kinetics of the Hf high temperature oxidation, conclusions can be drawn on the erosion mechanism involved.

► Hf microstructural modifications in cathodes after plasma arc cutting cycles investigated.
► 3 zones identified after cutting: HfO2 layer; remelted zone with O2–Hf solid solution; unmodified Hf.
► Hf-based ejections both in arc-on and arc-off phases; erosion cavity deepens with cutting cycles.
►  Detachment at the Hf/Cu interfaces, worsening heat dissipation and oxidation/erosion phenomena.
► The use of a different current ramp at subsequent erosion stages is suggested.