Pure Argon Arc Research Articles

Numerical study on thermal non-equilibrium of arc plasmas in TIG welding processes using a two-temperature model

A two-temperature (2-T) model for tungsten inert gas (TIG) welding process is developed to investigate the arc phenomena of the pure argon and helium plasmas. The model considers the energy conservations of the heavy particles and the electrons separately. Compared with the 1-T model, the 2-T model obtains the plasma shapes more similar to the arc appearances. Furthermore, the heavy particle temperature of the 2-T model shows good agreement with the experimental results. For a pure helium arc, the electron temperature is much higher than the heavy particle temperature, whereas both temperatures are almost identical for a pure argon arc. Thermal non-equilibrium of a pure helium arc is discussed in terms of the energy exchange between heavy particles and electrons. It is found that ions and atoms of a pure helium arc cannot exchange their energy sufficiently with electrons because the plasma has a small number of electrons and consequently the collision rate between plasma species is relatively low. The simulation results show that when a welding current is lower, thermal non-equilibrium of an arc plasma is stronger. In a low welding current condition, not only the pure helium arc but also the pure argon arc shows thermal non-equilibrium.

Thomson scattering diagnostics of steady state and pulsed welding processes without and with metal vapor

Thomson scattering is applied to measure temperature and density of electrons in the arc plasma of the direct current gas tungsten arc welding (GTAW) process and pulsed gas metal arc welding (GMAW) process. This diagnostic technique allows to determine these plasma parameters independent from the gas composition and heavy particles temperature. The experimental setup is adapted to perform measurements on stationary as well as transient processes. Spatial and temporal electron temperature and density profiles of a pure argon arc in the case of the GTAW process and argon arc with the presence of aluminum metal vapor in the case of the GMAW process were obtained. Additionally the data is used to estimate the concentration of the metal vapor in the GMAW plasma.

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

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 non-equilibrium 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 two-fluid 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.

Temperature's optical tomography diagnosis of arc plasma jet flowing into air

An argon arc jet plasma flowing into air is chosen as a practical example to study the multiple species jet plasma's optical computerized tomography (OCT) diagnosis. The refractive index models of the pure argon and the multiple species arc plasmas are supplied. On the basis of which, the temperature reconstruction model of the multiple species arc plasma is further derived. By theoretical calculation, the effect of mixed air on the refractive index is given. For the sake of better proving the effect of directly omitting the mixed air on flow field's temperature reconstruction from the refractive index, a simulated experiment is supplied. Finally, the condition, which can be adopted to estimate that whether the pure argon arc plasma refractive index model still can be used as the temperature reconstruction model of the argon arc plasma jet flowing into air, is proposed.

Metal Vapour in MIG Arcs Can Cause (1) Minima in Central Arc Temperatures and (2) Increased Arc Voltages

Radiation emission coefficients of plasmas from high temperature metal iron vapour are about a factor of a hundred larger than emission coefficients of high temperature argon plasmas as occur in argon welding arcs. Iron vapour can be easily introduced into the central region of such a welding arc due to melting of an electrode containing iron. The high radiation emission can produce several effects contrary to the usual behaviour of arcs in pure argon. Firstly, a central cylindrical core of iron vapour will be highly radiating and calculations show that the central core can consequently be at a lower temperature than the surrounding argon plasma. Because this central core has higher radiation emission it can give the appearance of having a higher central temperature. Secondly, it is predicted that radiation cooling of the arc due to iron vapour reduces the arc temperature and has the effect of increasing the arc voltage. The effect of increased radiation in the main arc column outweighs the effect of iron vapour increasing the electrical conductivity of the plasma at low temperatures which has the effect of reducing the arc voltage. This latter effect is limited to a small region near the electrodes.

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.

A Power Interruption Technique for Determining the Difference Between Electron and Gas Temperatures in the Argon d.c. Arc Supplied with Aqueous Aerosol

The difference between electron and gas temperatures in a low current U‐shaped d.c. arc with aqueous aerosol supply was studied using a power interruption technique. Monitoring of the temporal evolution of recombination continuum after power switch‐off revealed that it was necessary to do a correction of the measured values due to a change in electron concentration. It was shown that the plasma studied was a two temperature type and that the temperature difference ranges from 1850 K to 600 K for arc currents from 3 to 10.5 A, respectively. A comparison with literature results for pure argon arc plasma shows that aerosol introduction reduces the temperature difference.

Influence of carbon dioxide on the radial temperature distribution in an argon arc

The temperature profiles in wall-stabilized atmospheric-pressure arcs in pure argon and in argon–carbon dioxide mixtures have been obtained at currents of 5–40 A and for molar percentages of CO2 ranging from 0.9% to 25% using spectroscopic techniques. With the addition of CO2 to a pure-Ar arc, we show the formation of a core of higher temperatures with a larger gradient of the radial temperature distribution at the arc centre. These investigations show that the addition of even a few percent of CO2 to Ar has a marked effect both upon extinction of the arc according to current values and upon the temperature profiles. Few measurements have been done in Ar–CO2 mixtures in a wall-stabilized arc.

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.

Theoretical radiative transport results for a free-burning arc using a line-by-line technique

Many thermal radiation results for a two-dimensional, axisymmetric, atmospheric, free-burning arc for several different wavelength intervals, as well as the entire thermal spectrum, are presented. Quantities such as the emission, average intensity, radiative source term and radial flux are calculated from a theoretically rigorous technique for handling radiative transport in a medium such as a plasma in which the absorption coefficient is a strong function of the wavelength. An S-N discrete ordinates method is used on a line-by-line basis to perform this task. Most of the results presented are for a pure-argon arc; however, results indicating the effect of copper contamination on the radiative quantities are also given. Comparisons between the pure-argon and copper-contaminated arcs show that a small amount of copper greatly affects the radiative characteristics of the plasma.

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, Ohm's 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.

Prediction of gas tungsten arc welding properties in mixtures of argon and hydrogen

A theory of gas tungsten arc welding (GTAW) arcs that treats the tungsten electrode, the arc, and the workpiece as a unified system has been applied to make predictions in two dimensions of the temperature distributions in the arc, the tungsten cathode, and the workpiece, for any given arc current and gas mixture. Predictions of arc temperatures, radii, and voltages are compared for argon and mixtures of argon and hydrogen. It is found that arcs in gas mixtures containing hydrogen are more constricted and have a higher maximum temperature and arc voltage than arcs in pure argon. The addition of hydrogen also significantly increases the volume of molten material in the weld pool due to the higher thermal conductivity of argon-hydrogen mixtures at temperatures at which molecules of hydrogen dissociate. Predictions are also compared for workpieces of steel and aluminum. The volume of molten material is very much less for aluminum, despite its lower melting point, because of the higher thermal conductivity of aluminum. Predicted arc voltages as a function of current for a mixture of 10% hydrogen in argon are in good agreement with experimental results.

Spectroscopic study of a magnetically rotating arc between opper electrodes in contaminated argon

The effects of N2 and CO contaminants in atmospheric-pressure argon on an arc rotating between two concentric copper electrodes has been studied using optical spectroscopy of copper lines. The axial temperature of the magnetically driven arc in Ar + %N2 was determined to be around 10,000 K for arc currents of SO to 200 A. The diffusion process of the copper vapor from the cathode was also studied. A copper density maximum 1 mm from the cathode along the arc column was found in Ar + %N2. Removal of the contaminated cathode surface layers by the arc when contaminant injection in the plasma gas was stopped was found to be a slow process with a time scale depending on the type of the gas contaminant. The presence of gas contaminant in the electrode material controls the cathode erosion mechanism and the overall arc behavior in the transition between a contaminated to a pure argon arc.

Arc Measurement of Some Argon Transition Probabilities.

Transition probabilities and line shape parameters for three argon lines have been measured in a wall-stablilized argon arc containing a trace of hydrogen. The H β line shape measurement and the assumption of local thermodynamic equilibrium provided the determination of the argon level populations. From these three lines ten other argon line transition probabilities have been obtained by relative measurements in a pure argon arc. The results are compared with those of other investigators and sources of error are discussed.