Although the high‐intensity, free‐burning argon arc has been the object of many studies, modeling of the entire arc has been precluded because of complexities due to the interaction of electric, magnetic, fluid dynamic, and thermal effects, and the associated lack of realistic boundary conditions, in particular, close to the cathode. For establishing the most crucial boundary condition, which is the current density in the vicinity of the cathode, the maximum current density has been determined experimentally by measuring the size of the molten cathode tip (thoriated tungsten) for a given arc current. Calculated temperature profiles for a 100‐ and 200‐ A atmospheric pressure argon arc (electrode gap of 1 cm) are in good agreement with spectrometric measurements based on absolute line and continuum intensities. The arc current and arc current distribution are not only responsible for the temperature distribution in the arc, but also for the magnetohydrodynamics (MHD) pumping action in the cathode region, i.e., the arc behavior is mainly controlled by the current. In contrast to the sensitivity of the current density boundary condition on the results, the calculations show that variations of the boundary condition for the flow field are insignificant.

Study of the free‐burning high‐intensity argon arc

This paper gives a review of recent papers which have led to the capability of the prediction of weld depths for gas tungsten arc welding, for any given arc current, electrode shape or separation and welding gas. The methodology is given for deriving plasma composition as a function of temperature and pressure from basic atomic and molecular properties. Transport coefficients of density, specific heat, enthalpy, electrical conductivity, thermal conductivity, viscosity and radiation emission coefficients can then be derived as a function of temperature. The conservation equations of fluid dynamics are then used to derive weld profiles for stainless steel for welding gases such as argon, helium, carbon dioxide and a 10% mixture of hydrogen in argon. The markedly different weld depths which are obtained are related to basic material functions such as specific heat, electrical and thermal conductivity. The temperature dependence of the surface tension coefficient has a marked effect on weld depth and profiles because it can influence the direction of circulatory flow in the weld pool. Electric arcs in helium and carbon dioxide are more constricted than arcs in argon and as a consequence the magnetic pinch pressure of the arc, transmitted to the weld pool, can force strong downward flows in the weld pool and thus lead to a deep weld. It is found that because of the interactions of the arc and the weld pool through effects such as viscous drag forces of the plasma on the weld pool, it is necessary to treat the arc, the electrode and the weld pool in a unified system.

https://iopscience.iop.org/article/10.1088/0022-3727/40/1/R01/pdf

Predictions of weld pool profiles using plasma physics

A recently developed unified theory of arcs and their electrodes, with cathodes which are thermionic emitters, has been simplified so that there is a reduction in computation times by approximately a factor of 100. Electrode and arc regions are treated together and points at the surface of the electrodes are treated in a special way to account for electrode effects; no assumptions are made concerning the current density at the cathode surface. The theory is used to make predictions of arc and electrode temperatures and arc voltages for arcs in argon as a function of current in the range 50 - 400 A. The maximum temperatures of the arc and the cathode and also the current - voltage characteristics are in reasonable agreement with experimental results for cathodes with a included angle. For a cathode with a included angle, we predict a maximum in the temperature several millimetres from the tip of the electrode, in approximate agreement with experiment. Temperatures of the cathode tip are predicted to be much higher for tungsten cathodes than for thoriated tungsten cathodes and are in reasonable agreement with experimental results. Tungsten electrodes are hotter than thoriated tungsten electrodes, partly due to increased ion heating, but largely due to greater heat conduction from the arc to the electrode due to the arc plasma covering a greater area of the electrode surface for tungsten.

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A simplified unified theory of arcs and their electrodes

The calculation and measurement of the properties of thermal plasmas in mixtures of different gases are reviewed. The calculation of composition, thermodynamic properties and transport coefficients is described. Particular attention is given to the calculation of diffusion coefficients, which is a significant problem in mixed-gas plasmas. The combined diffusion coefficient formulation is shown to be a useful method for the treatment of diffusion. Computational fluid dynamic modelling of thermal plasmas in gas mixtures is considered, using the examples of demixing in welding arcs, the turbulent mixing of atmospheric air into a plasma jet and a plasma waste destruction process. Diagnostic techniques for mixed-gas plasmas, in particular emission spectroscopy, laser scattering and laser-induced fluorescence, are discussed.

http://iopscience.iop.org/article/10.1088/0022-3727/34/20/201/pdf

Thermal plasmas in gas mixtures

Previous predictions of the properties of free-burning arcs have usually required assumptions to be made of conditions at the cathode, in particular, the current density. Numerical calculations presented show that the current density at the cathode, for a 200 A arc in argon with a tungsten cathode having a 60 degrees conical tip, is determined to a good approximation by the arc column and cathode shape. The cold non-equilibrium sheath next to the cathode is very thin ( approximately 0.02 mm) and can be neglected. Predictions are made of arc column temperatures, velocities, pressure and potential by solving in two dimensions the conservation equations of mass, momentum and energy. Predictions are in fair agreement with experimental results with maximum arc temperatures of approximately 22000 K. Current densities depend strongly on the electrode radius at the cathode tip. For a radius of 0.1 mm the current density is approximately 20000 A cm-2.

https://iopscience.iop.org/article/10.1088/0022-3727/25/11/006/pdf

Theory of free-burning arc columns including the influence of the cathode

Volumetric radiative loss measurements, correlated with temperature in the range of 10 000 to 26 000°K, have been made on an argon plasma. Pressures of 0.5, 1.0, and 2.0 atm have been used. The 1.0‐atm measurements have been corrected for both absorption and ultraviolet emission and the results agree with those of Emmons in the common temperature range. The 6965 Ar I line has also been studied yielding lineshifts, halfwidths, absorption and emission coefficients. The line shift and halfwidth results are below theoretical predictions. Transition probabilities determined from both emission and absorption studies are found to be in reasonable agreement.

R. S. Tankin

Papers

Measurement of Emission and Absorption of Radiation by an Argon Plasma