Mikio Minonishi

Experimental Study of Consumption of Hafnium Electrode in Oxygen Plasma Arc Cutting

An experimental study was conducted on the consumption of hafnium electrodes during oxygen plasma arc cutting (PAC). Hafnium (Hf) is used as a refractory cathode material for thermionic emission in a high-temperature region. The consumption of an Hf electrode was investigated by measuring the concavity depth on the Hf electrodes after operations. These measurements were performed under several operating conditions. The consumption properties are separated into two consumption rates: Hf consumption during transient periods, when the arc current is increasing or decreasing such as at start-up and shut-off; and Hf consumption during the stable arc phase, when the arc current is held at the operating current. The consumption rates are estimated from the experimental data. Spectroscopic measurements of the arc plasma jet coming from a nozzle were conducted to study the consumption behaviour of Hf electrodes. A line of Hf at λ = 340 nm was observed using a monochromator. Characteristic intermittent Hf radiations in addition to the background continuum were observed in transient periods when the arc current or gas pressure changed. The intermittent radiation is considered to arise from droplets of Hf electrode from the surface of a molten electrode. The ejection of droplets is thought to be caused by rapid consumption during transient periods, and to be induced by a disruption in the balance of the fluid dynamics on the melted electrode.