Free-burning Arc Research Articles

Anode Melting From Free-Burning Argon Arcs

Predictions have been made of anode melting for free-burning arcs by making two-dimensional calculations of temperature profiles of the arc and the electrodes to make predictions of weld depth and weld shape in arc welding. Predicted properties at 150 A, for various arc lengths, are compared with experimental results of: 1) weld shape; 2) heat intensity as a function of radius; and 3) current density as a function of radius at the anode. The whole region of the arc system is modeled, including the tungsten cathode, the arc plasma and the solid and molten anode, including convection within the molten weld-pool. Theoretical and experimental results are also obtained for highand low-sulfur steel, which have markedly different surface tension properties of the molten liquid, resulting in weld depths that differ by a factor of three because of changed convective circulation properties in the weld pool. Although total power to the anode increases with increasing electrode separation, the current density becomes less, with the result that for electrode distances above 2 mm, for argon, the width and depth of the molten region become less, in agreement with experimental results that we have obtained.

Near-cathode region of a free-burning arc: a spectroscopic investigation

The cathode as an electron emitter possesses different modes of operation to maintain the current forced upon the discharge. We investigated spectroscopically the near-cathode region of a free-burning low-current argon arc operated at atmospheric pressure in order to measure the plasma parameters of that mode of operation that we call the blue-core mode. For better interpretation of the experimental data we applied a simple two-temperature model. As a result, we can describe the arc plasma up to 100 µm near the cathode surface.

Numerical Study of a Free-Burning Argon Arc with Anode Melting

Numerical modeling of free burning arcs and their electrodes is useful for clarifying the heat transfer phenomena in the welding process and to elucidate those effects which determine the weld penetration. This paper presents predictions for a stationary welding process by the free-burning argon arc. The whole region of the welding process, namely, tungsten cathode, arc plasma and stainless steel anode is treated in a unified numerical model to take into account the close interaction between the arc plasma and the molten anode. The time dependent development of two-dimensional distributions of temperature and velocity, in the whole region of the welding process, are predicted at a current of 150 A. The weld penetration geometry as a function of time is thus predicted. It is shown also that different surface tension properties can change the direction of re-circulatory flow in the molten anode and dramatically vary the weld penetration geometry.

2D display of tungsten impurity in a free-burning arcusing laser-induced fluorescence

We present laser-induced fluorescence (LIF) measurements, spatially and temporally resolved, of tungstenimpurities eroded from the cathode of a free-burning argon arc. Saturation,but more drastically quenching processes limit a quantitative evaluation ofthe measurements. Applying a special measuring technique, we were ableto determine relative densities and, via a Boltzmann plot, excitationtemperatures. A 2D display of the metal impurity can be obtained witha planar LIF arrangement. It shows the distribution of that part of metalcomponent being in a distinct mode of ionization and excitation. Asknown, the population distribution of the tungsten impurity isdominated by strong demixing effects.

Effect of Anode Heat Transfer on Melted Penetration in Welding Process by Free-burning Argon Arc.

In order to make clear the physical relation among the arc plasma, the anode heat transfer and the results of materials processing by thermal plasma, namely, free-burning arc, the materials processing was focused on the welding for simplification of discussion. The experimental results of temperature measurements of arc plasma, the distributions of current density and heat input density on the anode, and the weld penetration were presented. It was shown that the electron temperature above the anode and current and heat input density on the anode was dominated by the position of the tungsten cathode. Furthermore, it was also shown that electron temperature of arc plasma was dominated by the cathode shape. These results were related with the results of the welded penetration. As a result, it was concluded that the electron temperature above the anode and current density distribution on the anode decided the heat input density distribution on the anode and then the heat input density on the anode remarkably dominated the size of the weld penetration in the welding process by the free-burning argon arc. Furthermore, it was suggested that the cathode played the important role in the determination of the weld penetration.

The realization of Abel's inversion in the case of discharge with undetermined radius

A new method for the determination of the local plasma emissivity on the basis of measuring of the radiation intensity is presented. The method is related to the axial symmetric discharges with undefined radius (free burning arcs, etc.). In this method, the experimental profile of the radiative intensity is approximated by the linear combination of Gaussian functions, while the requested plasma emissivity is obtained in a similar oblique. The essence of the presented method lies in the procedure of determination of the coefficients in this linear combinations, as well as the parameters by which the exponents of Gaussian functions are expressed. The method was tested on a large number of examples which are related to almost all practically important cases of the profiles of local plasma emissivity. It was shown that the method works with high precision, which makes it a certain tool for an operative laboratorial application.

Diagnostics of discharge modes of a free-burning low-current argonarc

We investigated the cathode region of free-burning argon arcs at atmosphericpressure and at currents from 0.3 A up to 25 A. Apart from the well knowndiffuse mode of operation we also found a more constricted formwhich we call the blue-core mode. Different diagnostic methods aresimultaneously applied to these modes of operation, that is: emissionspectroscopy in conjunction with a partial LTE model for the density and temperaturemeasurements; Thomson scattering for the electron temperature and electrondensity determination; and Rayleigh scattering for a determination of thegas temperature. The large differences in appearance, densities,temperatures or current formation between the discharge modes are welldefined by the measurements.

Heat transfer and fluid flow in a high-intensity free-burning arc: an improved modeling approach

The high-intensity free-burning arc is modeled using a computational domain including the arc itself and the arc cathode and anode. It is shown that the energy equation with the temperature, instead of the specific enthalpy, as the dependent variable has to be used in order to obtain physically reasonable heat fluxes from the plasma to the electrodes if a SIMPLE-like algorithm is employed in the modeling. New difficulty encountered in the numerical solution of the energy equation is discussed in some detail and has been overcome successfully by use of a `pseudo-density' method and a deferred-correction discretization scheme. A more realistic boundary condition is adopted at the rear face of the anode plate for the solution of the potential equation in order to reveal the effect of electrical collection at the anode on the current density and temperature distributions within the anode plate. Special treatments at the plasma–electrode interfaces are also discussed and adopted in the modeling.

An analysis of volumetric radiation heat flux and experimental comparison with arc light sensing in GTA welding process

The volumetric radiation heat flux was analyzed from a model of the entire free burning arc of GTA welding and its results were compared with measurements of the arc light intensity to find a relationship between them. Experiments were carried out for various welding currents using electrodes of different vertex angles. The results showed similar behavior for both the volumetric radiation heat flux and the light intensity of GTA welding arc. Both presented a linear relationship with the logarithmic value of the arc length. Consequently the arc light intensity could be assumed to have a linear relationship to its radiation heat in the range of normal arc length.

Vaporization and condensation of some refractory oxides in plasmas derived initially from argon in expanded dc arcs

Experiments are described in which aluminium(III), silicon(IV) and titanium(IV) oxides were vaporized in and re–condensed from plasmas derived initially from argon. These were carried out in a rotating-wall, dc-arc plasma reactor. Conditions were found under which the oxides vaporized completely and at high rates. The crystalline structure and morphology of materials which condensed were examined by optical microscopy, electron microscopy, and X-ray diffraction, and were found to depend upon the location in the reactor from which they were collected. Qualitative mechanisms are proposed by which crystalline materials may be expected to nucleate and grow from cooling plasma vapours. It is suggested that the crystalline modification and morphology of solid condensates may depend upon the degree of supersaturation of vapours fromwhich they form. It is shown that the properties of the materials recovered during these studies, and themetal oxide smokes recovered by earlier workers from the plasmas of free-burning arcs and plasmatorches, may be accounted for by the mechanisms put forward here. It is also shown that, by suitableselection of conditions, the rotating-wall, dc-arc plasma reactor may be used to condense a polymorphicmetal oxide predominantly in a crystalline modification characterized by either a large or a small primitiveunit cell. Consequences of the nucleation mechanism proposed here are supported by experimentalobservations, and some of these predictions and observations are at variance with predictions ofthe Ostwald Rule.

Comparison between a two- and a three-dimensional arc plasmaconfiguration

A three-dimensional (3D) argon arc plasma at atmospheric pressure ispresented. The model is developed using the commercial code Fluent. Two arcplasma configurations are studied: a free burning arc and a transferred arc.In the free burning arc configuration, the 3D results are compared with atwo-dimensional (2D) configuration. The natural axis of symmetry of thesystem, given by the physical operating conditions and by the cathode tipgeometry, leads to a good agreement between the 2D and 3D results. The modelis validated by comparisons with experimental and theoretical temperaturefields. The results show a small difference between the 2D and 3D models, asdoes the other literature. In the transferred arc configuration model theinjection is given by three injectors. A rigorous modelling of the injectioncannot be treated using a symmetric plane or a symmetric axis, so the resultsare presented in a real 3D configuration and compared with results obtained ina 2D configuration. Indeed, in a 2D configuration, due to the symmetricalcondition on the axis, the main assumption consists of representation of thereal injection geometry by an equivalent surface injection. The imposed massflow rate is then distributed along a ring and differences can appear in theresults.

A comparison of measurements and calculations of demixing infree-burning arcs

Spectroscopic measurements of the composition and temperatureof free-burning arcs in mixtures of argon with helium, nitrogenand hydrogen are compared with the predictions of a two-dimensionalnumerical model. The measurements have been performed by twodifferent techniques, each requiring the measurement of the emissions fromtwo lines, and the measurement of emission from a pure argon arc.Both the measurements and the calculationsshow that significant demixing occurs in arcs in all gas mixtures, with the lighter gas being concentratednear the arc axis. Generally good agreement is foundbetween the measurements and the calculations. Discrepancies aretentatively explained as resulting from departures from localthermodynamic equilibrium.

Experiment and modeling of an atmospheric pressure arc in an applied oscillating magnetic field

A set of experiments are carried out to measure and understand the response of a free-burning atmospheric pressure carbon arc to applied transverse dc and ac magnetic fields. The arc is found to deflect parabolically for the dc field and assumes a growing sinusoidal structure for the ac field. A simple analytic two-parameter fluid model of the arc dynamics is derived, in which the arc response is governed by the arc jet originating at the cathode, with the applied J×B force balanced by inertia. Time variation of the applied field allows evaluation of the parameters individually. A fit of the model to the experimental data gives a value for the average jet speed an order of magnitude below Maecker’s estimate of the maximum jet speed [H. Maecker, Z. Phys. 141, 198 (1955)]. An example industrial application of the model is considered.

Experimental and theoretical investigation of an enclosed free burning arc in SF6

The behaviour of a free burning SF6 arc in a two-volume cylindrical chamber, which simulates the initial arcing in a modern high voltage auto-expansion circuit breaker, has been investigated both experimentally and theoretically. Arc voltage and pressure variations at four strategically chosen locations in the chamber were measured for electrode gap lengths of 30, 40 and 50 mm in the peak current range from 5 to 20 kA of a positive half-cycle alternating current at 50 Hz. A simplified model for free burning arcs in an enclosure was developed by radially integrating the governing equations under the assumptions of a uniform radial temperature profile and a variable radial profile of the axial velocity (equation (1)) in the arc. The effects of Lorentz force, Ohmic heating, radiative energy transfer, and turbulence enhanced momentum exchange are taken into account. The computed arc voltage and pressure variation in the chamber agree with the experimental results within the estimated range of uncertainty.

Coupled radiative, flow and temperature-field analysis of a free-burning arc

A computer simulation of a thermal plasma that utilizes a detailed line-by-line radiative analysis coupled to a flow and temperature-field analysis has been written. The radiative transport portion of the model uses the S-N discrete ordinates method and the required spectral radiative properties are calculated from basic atomic data. The flow and temperature-field portion of the calculation is handled using the finite-volume method. The thermal plasma configuration studied using this coupled model is a two-dimensional, axisymmetric, 200 A, 1 cm long, free-burning arc located in a one-atmosphere argon environment. Results from this analysis are compared with those from an uncoupled, line-by-line radiative analysis of this same arc and from a flow and temperature-field analysis that utilizes net emission coefficients to model the radiative source term. As thought, noticeable differences are found between results from the coupled and uncoupled analyses. This work proves that it is feasible to conduct two-dimensional, coupled, line-by-line calculations in a thermal plasma, but the computational times are large.

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%.

A Non‐equilibrium Treatment for Free Burning High Current Arcs.

ABSTRACT: A two‐temperature variable‐density treatment 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 and the electrodes, and considers the separate energy balance of the electrons and the heavy particles, together with the continuity equations for these species throughout the plasma. 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. 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. However, we predict large departures from local chemical equilibrium throughout the plasma, due to overpopulation of the ground state level of the neutral atoms caused by the injection into the arc core of a large mass flow of cold gas, resulting from the arc constriction at the tip of the cathode.

Circuit simulation for gas metal arc welding

An electrical circuit was developed to model the behaviour of gas metal arc welding. The model incorporates results from a one-dimensional free burning arc model to describe the arc and a second order differential equation to describe one-dimensional energy conservation in the welding anode. Commercial circuit analy sis software was used to solve the circuit model. Predictions have been compared with experimental data for short circuit transfer mode as well as spray. Predictions agree well with experimental results. The model provides a fast and useful technique to simulate the gas metal arc welding process. In short circuit transfer mode, the model predicts that chaotic phenomena are occurring.

Observations of the anode boundary layer in free-burning argon arcs

In order to make clear the physical grounds of the potential drop in front of the anode, namely, the anode fall in atmospheric free-burning argon arcs, the results of experimental measurements of the laser-scattering method and Langmuir-probe method are presented. The experimental results show that the anode boundary layer at low arc currents such as 50 A remarkably deviates from local thermodynamic equilibrium (LTE), whereas the boundary layer at higher arc currents such as 150 A preserves a similar state to the LTE. The Langmuir-probe measurements also show that the anode fall for 50 A is positive, whereas that for 150 A is negative. From these results, an assumption regarding the physical state of the anode boundary layer in the free-burning argon arcs is presented synthetically and it is also concluded that the sign and magnitude of the anode fall in the arcs relate vary closely to the thermal state of the anode boundary layer and that the thermal state should be influenced strongly by the arc current density, namely, the electron number density.

Numerical Study of a Free-Burning Argon Arc with Copper Contamination from the Anode

The present modeling of a free-burning argon arc accounts for copper vapor contamination from the anode. Simulations are made for an atmospheric arc that has a length of 10 mm and an electric current of 200 amps. Predicted results for two different anode evaporation rates are compared to those from a pure argon arc with no copper vapor contamination. Copper vapor concentration, temperature, electric potential, and current density profiles are presented. Included in this analysis are radiation losses from both the argon and copper by using recently calculated net emission coefficients. It was found that evaporation of copper from the anode results in a cooling of the arc in a region close to the anode, but has an insignificant influence on the arc close to the cathode. Due to the arc flow characteristics most of the copper vapor tends to be confined to the anode region.