Cathode fall voltage of TIG arcs from a non-equilibrium arc model

Abstract

This work presents modelling results concerning a tungsten inert gas (TIG) welding arc. The model provides a consistent description of the free burning arc, the arc attachment and the electrodes. Thermal and chemical nonequilibrium is considered in the whole arc area, and a detailed model of the cathode space-charge sheath is included. The mechanisms in the cathode pre-sheath are treated in the framework of a non-equilibrium approach which is based on a twofluid description of electrons and heavy particles and a simplified plasma chemistry of argon. A consistent determination of the electrode fall voltages and temperature distributions is achieved. The model is applied to arcs in pure argon at currents up to 250 A, whereby welding of a workpiece made of mild steel with a fixed burner is considered. Arc voltages in the range from 12 to 17 V are obtained at 50 at 250 A, respectively. The space-charge sheath voltage is found to be about 7 V and almost independent of the current. The corresponding temperatures of the cathode tip are in the range from 3,000 K to about 3,800 K. The results obtained are in a good agreement with measurements.

Gas tungsten arc models including the physics of the cathode layer: remaining issues

“…The number of collisions is small since λ D < λ c ; this sub-layer is thus at the transition between being collisional and collisionless [49], and a continuum approach does not apply. The cathode sheath model combined with either a LTE-plasma in [25,26], or a FNE-plasma in [11][12][13][14], is based on the following assumptions:…”

“…Electric work is also gained by the charged particles as they move through the pre-sheath potential drop U i . In the FNE-plasma approach [11][12][13][14], the closure relation is instead the electron energy conservation through the sheath; it involves the heat fluxes and the electric work sketched in Fig. 15.…”

“…Here, assumption (A16) simply implies that each plasma component (molecule, atom, ion) is thermally perfect and thus obeys the ideal equation of state. Assumptions (A16)-(A17) were also made in the FNE-plasma approach [2,[11][12][13][14] (see [53]). …”

“…Restricting this area allowed reducing the temperature discrepancy in the plasma column, but the drawback was the introduction of an adjustable parameter. In fact, the model of [13] does not verify current continuity and energy conservation at the cathode/plasma interface. Recently, the model was significantly improved to match current continuity and energy conservation of the plasma column with the cathode [14].…”

“…Using a FNE-plasma model the cathode sheath is treated as a boundary layer while the pre-sheath is combined with the plasma column. The underestimation of the arc temperature went hand in hand with an overestimation of the arc attachment area [13]. Restricting this area allowed reducing the temperature discrepancy in the plasma column, but the drawback was the introduction of an adjustable parameter.…”