Abstract:Abstract.Thoriated tungsten cathodes operating in an open-air plasma torch at current intensities between 30 and 200 A were experimentally studied. The morphology and composition of the cathode tip after arcing were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Three relevant zones were found on the cathode tip (spot zone, thorium-depleted zone and thorium-enriched zone), and their dimensions were measured. For each current intensity, the spot temperature T during arcin… Show more
“…3 (10 mm and 50 A) in comparison with case a (6 mm and 50 A) in spite of the small changes in the heavy particle temperature near the cathode surface. These results are in good agreement with pyrometric measurements [18] which are reported, however, for slightly different parameters (tungsten electrode with 2 % ThO2 of 2.4-mm diameter and a cone tip of 68°). Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A).…”
Section: Resultssupporting
confidence: 91%
“…Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A). The tip temperature decreases to about 3,000 K at 50 A and 2,900 K at 30 A [18].…”
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.
“…3 (10 mm and 50 A) in comparison with case a (6 mm and 50 A) in spite of the small changes in the heavy particle temperature near the cathode surface. These results are in good agreement with pyrometric measurements [18] which are reported, however, for slightly different parameters (tungsten electrode with 2 % ThO2 of 2.4-mm diameter and a cone tip of 68°). Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A).…”
Section: Resultssupporting
confidence: 91%
“…Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A). The tip temperature decreases to about 3,000 K at 50 A and 2,900 K at 30 A [18].…”
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.
“…In accordance to the results of Fig. The effective work function of the metal would introduce different electron thermoionic emission departing from the classical Richarson-Dushman expression with a constant W f [13]. In addition, the operation temperatures of the emissive Langmuir probes are close to the melting point of the metallic wires.…”
The measurement of the plasma potential of unmagnetized Maxwellian plasmas by using the floating potential of emissive Langmuir probes is discussed. The temperature of the probe was monitored in order to estimate the emitted thermoionic electron current and to determine the limits of the strong electron emission regime. Under these ideal conditions, the measurements of the plasma potential of the emissive probe are cross checked against those of collecting Langmuir probes. In agreement with previous works, the current voltage curves of emissive probes show the temperature dependent electron saturation currents. An empirical expression is suggested for the dependence of this saturation current with the probe temperature which recovers the response of collecting probes when the probe is cold. The experimental data indicate that the floating potential of the emissive probe is close to the local plasma potential only when the emitted thermoionic electron current is similar to the electron saturation current. Larger electron emission currents lead to small increments the floating potential over the local plasma potential. Our results suggest that the electron saturation current of a hot emissive probe is composed by the thermal electrons from the plasma and a returned fraction of the thermoionic emitted electrons.
“…This phenomenon, discovered by Langmuir with tungsten filaments [44], was further investigated via studies on e.g. electrode erosion [18,35,[45][46][47]. It is known that the rare earth oxides do not all behave in the same way during their diffusion.…”
Section: Cathode-modelling Assumptionsmentioning
confidence: 98%
“…The effect of the rare earth oxides constituting the cathode surface layer in physical state (II) on the cathode sheath remains to be investigated. Also, after some operating time more physical states are known to be formed [18,35,[45][46][47]. Their modelling has not been addressed yet.…”
A recent review pointed out that the existing models for gas tungsten arc coupling the electrode (a cathode) and the plasma are not yet complete enough. Their strength is to predict with good accuracy either the electric potential or the temperature field in the region delimited by the electrode and the workpiece. Their weakness is their poor ability to predict with good accuracy these two fields at once. However, both of these fields are important since they govern the heat flux to the workpiece through current density and temperature gradient. New developments have been made since then. They mainly concern the approaches addressing the electrode sheath (or space charge layer) that suffered from an underestimation of the arc temperature. These new developments are summarized and discussed, the modelling assumptions are examined, and important modelling issues that remain unexplored are underlined.
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