J Haidar

A simplified unified theory of arcs and their electrodes

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.

Predictions of metal droplet formation in arc welding

A two-dimensional time-dependent model has been developed for the prediction of droplet formation in gas metal arc welding. The model is a unified treatment of the arc, the welding wire, taken as the anode, and the workpiece, taken as a plane cathode. Predictions are made of the formation and shape of the welding droplets as a function of time, accounting for effects of surface tension, gravity, inertia and magnetic pinch forces. The wire feed rate and gas flow rates are also incorporated into the model. Calculations are made of current densities, electric potentials, temperatures, pressures and velocities in two dimensions both in the arc and also within the molten drop and solid electrodes. For an arc in argon with a mild steel wire of 1.6 mm diameter and a current of 325 A or more, we predict the formation of small drops of diameter 1.2 mm or less and large drop frequencies consistent with the spray transfer mode observed in welding. At currents of less than 275 A, we predict large drop sizes of about 3.8 mm in diameter or more, consistent with the globular transfer mode in welding. At a current of 300 A, in a transition zone between the two modes, we predict the presence of both small and large drops.

Prediction of properties of free burning arcs including effects of ambipolar diffusion

A method is described to predict the two-dimensional distributions of temperature, velocity and potential of free burning arcs and their electrodes for cathodes of tungsten and thoriated tungsten. The effects of non-equilibrium due to the ambipolar diffusion of charged particles are included for the calculation of the plasma electrical conductivity. The electron diffusion current is explicitly included in the solution of the current continuity equation. The plasma for the arc and the electrode sheath regions is treated as a continuum, so that the thickness of the non-equilibrium regions near the electrodes is determined within the model, depending upon the arc current and the arc and electrode configuration. This new treatment allows the calculation of the negative anode fall that may occur across the anode sheath when the electron diffusion current near the anode surface becomes larger than the total arc current. For a thoriated tungsten cathode we take the work function for cooling by electron emission to be that of tungsten, as, for small percentages of thoria in tungsten, cooling effects from electrons passing through the interfaces for tungsten-thoria and then thoria-plasma will add up to be that of a tungsten-plasma interface. Calculations have been made for arcs in argon at currents between 2.5 A and 200 A. For currents above 120 A, we calculate the anode fall voltage to be negative, being -2 V at 200 A. For currents less than 50 A, non-equilibrium effects in the plasma extend across the whole arc and electron number densities can be several orders of magnitude below the values for local thermodynamic equilibrium. Calculated arc voltages, arc temperatures and electrode temperatures are in agreement with experimental measurements to within 20%.

An analysis of the formation of metal droplets in arc welding

A dynamic two-dimensional arc model has been used to investigate the effects of the various forces acting on the droplet in gas metal arc welding (GMAW). The model is based on the equations of conservation of mass, energy, momentum and current, Ohms law and a Maxwell equation. The model treats the welding wire, the plasma and the workpiece. For molten metal droplets at the tip of the welding wire, we account for effects of inertia, gravity, surface tension, magnetic force, viscous drag force and arc pressure. Calculations are presented for a 1.6 mm diameter wire of mild steel for arcs in argon to determine the separate effects of these forces on droplet formation. It is found that, for arcs in pure argon at currents around the transition from the globular transfer mode to the spray transfer mode, viscous drag and arc pressure effects are approximately self-cancelling. It is also found that forces have a much larger effect than do forces on the transition from globular to spray modes of metal transfer.

Non-equilibrium modelling of transferred arcs

A two-temperature, variable-density, arc model has been developed for description of high-current free-burning arcs, including departures from thermodynamic and chemical equilibrium in the plasma. The treatment includes the arc, the anode and the cathode and considers the separate energy balance of the electrons and the heavy particles, together with the continuity equations for these species throughout the plasma. The output includes a two-dimensional distribution for the temperatures and densities both of the electrons and of the heavy particles, plasma velocity, current density and electrical potential throughout the arc. For a 200 A arc in pure argon at 1 atm, we calculate large differences between the temperatures of the electrons and the heavy particles in the plasma region near the cathode tip, together with large departures from local chemical plasma equilibrium. In the main body of the arc at high plasma temperatures, we predict minor differences between the temperatures of the electrons and the heavy particles, which are inconsistent with recent measurements using laser-scattering techniques showing differences of up to several thousand degrees. However, we find that, for the region in front of the cathode tip, the ground-state level of the neutral atoms is overpopulated relative to the corresponding populations under conditions of LTE, in agreement with experimental observations. These departures from LTE are caused by the injection of a large mass flow of cold gas into the arc core due to arc constriction at the tip of the cathode.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

The dynamic effects of metal vapour in gas metal arc welding

Numerical simulations for the dynamic effects of metal vapour in gas metal arc welding (GMAW) suggest that vapour from the welding droplet at the tip of the welding wire has a significant influence on the plasma properties. It is found that for the evaporation rates calculated for arcs in pure argon, the dynamic effects of metal vapour markedly cool down the plasma in the central region of the arc, leading to the formation of a low temperature zone centred on the arc axis, in agreement with experimental measurements in the literature. Radiation effects, omitted in this paper, may produce further cooling of the plasma gas. The results highlight major deficiencies in the common approach to modelling the GMAW process and suggest that accurate description of GMAW must include the influence of metal vapour on the plasma.

Effect of CO/sub 2/ shielding gas on metal droplet formation in arc welding

We have developed a unified arc electrode model that enables us to make predictions of the time development of molten drops from the welding wire in gas metal arc welding. The wire is taken as the positive electrode, and the effects of surface tension, magnetic pinch forces, and convection within the drop are taken into account to predict drop detachment for any given arc current. For pure argon, we have previously predicted the sharp transition that is observed experimentally at about 300 A between globular transfer at low current, when drop diameters are larger than the wire diameter, and spray transfer, for currents above 300 A, when drop diameters are smaller than the wire diameter. In this paper, we predict that addition of 25% of CO/sub 2/ to the argon leads to an increase in the transition current to more than 325 A, also in agreement with published experimental results. For pure CO/sub 2/, we find a significantly different drop behavior due to the more constricted arc. Both small and large drops are produced, with many very small drops being produced successively between each large drop.

Local thermodynamic equilibrium in the cathode region of a free burning arc in argon

Measurements of normalized plasma emission coefficients for the 696.55 nm Ar I line for free burning argon arcs at 1 atm are presented for various arc parameters. These coefficients show variations in the cathode region of the arc, depending upon the arc current, the cathode composition and the mode of operation of the arc, and suggest departures from local thermodynamic equilibrium in the plasma near the cathode tip. In this paper, these results are discussed and an explanation is proposed based on the overpopulation of the ground state level of the neutral atoms. This overpopulation is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.

Large effect of cathode shape on plasma temperature in high-current free-burning arcs

The temperatures of the cathode surface and of the plasma for an atmospheric-pressure high-current free-burning argon arc have been measured for a range of cone angles for cathodes of thoriated tungsten. These measurements have shown that both the surface temperature of the cathode and the temperature of the plasma depend strongly on the cathode shape. For cathodes with conical shape, plasma temperatures were found to be a maximum for a cathode cone angle of 60 degrees .