Physical Processes in Gas-Tungsten Arcs - IEEE Xplore

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14, NO. 4, AUGUST 1986

Physical Processes in

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Gas-Tungsten Arcs

G. N. HADDAD, A. J. D. FARMER, P. KOVITYA, AND L. E. CRAM Abstract-Experiments designed to validate a two-dimensional theoretical model of a gas-tungsten arc welding (GTAW) arc are described. The predicted temperature distributions agree well with the measured values in the body of the arc. Agreement between theory and experiment near the electrodes has been impoved by the new boundary conditions in the theory. Experimental determinations of the effects of gas flow rate, electrode stick-out distance, and nozzle diameter on the temperature of GTAW arcs are discussed. A theoretical investigation of an addition of 0.1-percent cerium to an argon arc shows that enhanced low-temperature conductivity and extra radiative cooling due to cerium can lead to marked changes in arc properties.

I. INTRODUCTION

TN gas-tungsten arc welding (GTAW) the arc plays many

ldifferent roles. It provides a mobile path for the flow of electric current. It heats and melts the workpiece and any filler material that may be present. The axial flow in the arc facilitates material transfer and weld penetration. The high temperature of the arc promotes important metallurgical reactions. This paper discusses recent work which aims to elucidate some of the physical processes occurring in GTAW arcs. It is convenient to identify three distinct regions in a GTAW arc: the cathode root or spot, the positive column, and the anode root or spot. Although recent work has offered some new insights into the behavior of cathode roots [1] and anode roots [2], the present paper treats only the positive column. In particular we describe a theoretical model of the positive column of a GTAW arc, and discuss the results of experiments undertaken to validate the model. We also show how the theoretical model may be used to predict and understand arc behavior in situations where experiments are difficult or impossible. This paper summarizes the results of work which has previously been reported, or is in preparation [3]-[9], and in addition presents calculations using the theory to predict the effects that would occur if a trace of cerium were present in the arc. II. THEORETICAL ARC MODEL

A preliminary description of the arc model has been provided by Kovitya and Lowke [3], and a more extensive account is given by them in [4]. Equations describing the conservation of mass, momentum, and energy are solved with Ohm's law and Maxwell's equation for the magnetic field in a cylindrical coordinate system. The material Manuscript received November 13, 1985; revised February 24, 1986. The authors are with the CSIRO Division of Applied Physics, Lindfield, NSW, Australia 2070. IEEE Log Number 8609491.

properties that appear in these equations are predicted by the application of basic thermodynamic principles and the application of classical transport theory, on the basis of the assumption of local thermodynamic equilibrium (LTE). The differences between the present arc model and the model of [4] lie in the a) dropping of the assumption of the axial electric field being constant radially; b) extension of the region of integration to include the zone up to 1 mm behind the cathode plane; c) change in boundary conditions at the cathode such that the cathode temperature is taken to be 3000 K except in the cathode spot where the temperature is assumed to be 20 000 K. This last modification approximately models the cathode fall, where the electrons are heated from 3000 to 20 000 K in a distance of the order of 0.2 mm [10]. These changes allow us to model the electrodes regions more precisely. An important component of the arc energy balance equation is the transfer of energy by radiation. Indeed, the current calculations show that about 40 percent of the electrical power dissipated in the positive column of a GTAW arc ultimately leaves the arc in the form of UV and visible light. One of the most challenging problems in providing realistic models of high-current electric arcs involves the treatment of this radiation loss, which is complicated by the presence of a great number of spectral lines and by the effects of self-absorption. An approximate method for representing radiation losses from thermal arc plasmas, known as statistical spectroscopy, has been developed by Cram [5] and applied to evaluate the net radiative emission coefficients used in the present calculations. The new argon radiation data are not significantly different from the values used in the calculations of [4] except at temperatures above 20 000 K, where the present values are significantly lower. The previous values were obtained by extrapolation from measurements at temperatures below 20 000 K [11]. A question of continuing interest in the study of welding arcs concerns the effect of traces of additives with low ionization potentials. To illustrate some of the effects that occur, we exhibit in Figs. 1 and 2 the result of calculations of the variation with temperature of two important material properties, the electrical conductivity and the net radiative emission coefficient. Curves are shown for pure argon and for argon containing 0.1-percent cerium vapor, at a pressure of 1 atm (101.3 kPa). Because cerium has a low ionization potential, it contributes a significant number of charge carriers at relatively low temperatures, leading to a marked increase in electrical conductivity. Moreover, the emission line spectrum of neutral and of ionized

0093-3813/86/0800-0333$01.00 © 1986 IEEE

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-14, NO. 4, AUGUST 1986

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argon and argon containing 0.1-percent cerium vapor are calculated assuming that UV lines are self-absorbed.

III. TEMPERATURE MEASUREMENTS Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in pure argon at a pressure of 1 atm. Fig. 3 is a schematic illustration of the experimental setup, which is described in detail by Haddad and Farmer [6]. The temperature distribution in the arc is obtained from the measured intensity distribution in the Arn spectral line at 696.5 nm, using an Abel inversion to obtain the volume emission coefficients and the Fowler-Milne method [12] to normalize the relative emission coefficients. The spectroscopic temperature determinations rely on the assumption of LTE. While the experiments have themselves revealed clear evidence of departures from LTE [7], theoretical studies of a nonequilibrium collisional-radiative model of the behavior of argon in an arc plasma [8] imply that the adopted spectroscopic diagnos20 15 10 5 0 tic methods nevertheless yield reasonably accurate values Temperature 1000K) Fig. 1. Electrical conductivity as a function of temperature for pure argon of the kinetic temperature in the body of the arc. (solid curve) and argon containing 0.1-percent cerium (dashed curve). Fig. 4 compares the predicted and measured isotherms of a 200-A 5-mm argon arc at the pressure of 1 atm. The 5 measured values are obtained from Haddad and Farmer 10 [6]. Away from the electrodes, there is close agreement between theory and experiment, with both giving axial temperatures of order 14 000-18 000 K. The agreement is poorer near the electrodes, where the spectroscopic techniques will be most adversely affected by any nonX 10 equilibrium processes that may be present [7]. However, the modified model gives better agreement with the exresults than the order model [4], particularly / perimental 3 3o' _ ~ ~~// near the electrodes. 10 The experiments reported above have been conducted / in a chamber filled with pure argon. Since GTAW usually / I takes place with an argon-shielded welding torch in an O. environment of normal air, it is important to determine whether or not there are significant differences between E the chamber experiments and situations that correspond w~~~~ more closely to actual GTAW practice. To this end, spectroscopic measurements of temperatures in arcs burning between a GTAW torch and a water-cooled anode in open air have been conducted. 10 As reported by Haddad and Farmer [9], these experiments show that there is very little difference (less than 500 K at any given point) between the temperature distributions in the two cases. Moreover, Haddad and Farmer [9] show that, within values corresponding to normal 30 25 15 20 10 5 GTAW practice, the effects on the temperature distribuTemperature OOOK) tions of variations in gas flow rate, electrode stick-out Fig. 2. Net radiative emission coefficients as a function of temperature for distance, and nozzle diameter are also small. Typical papure argon (solid curve) and argon containing 0.1-percent cerium (dashed rameters for welding arcs are: a) gas flow rate of 10 curve). 1/min; b) nozzle internal diameter of 10 mm; and c) stickdistance, defined as the length of protrusion of the out in lines known 25 000 cerium is particularly rich (over each species) and consequently the presence of cerium electrode tip from the face of the nozzle, of 2 mm. By leads to a strong augmentation of the net radiative emis- contrast, the electrode tip shape is quite important in desion coefficient. The net radiative emission coefficients of termining the temperature distribution. Experiments have 0

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Fig. 3. Experimental apparatus for free-burning arcs in a chamber environment. For experiments on GTAW torches the chamber is removed and the torch burns to an anode in an "open" environment.

IV. RARE-EARTH CONTAMINATION IN ARGON ARCS CATHODE

Additions of rare-earth (RE) minerals have been used successfully to control the detrimental effects of sulphur in pipeline steels, but there is evidence suggesting that under certain conditions the RE additives can impair the metal active gas (MAG-CO2) weldability of the steel, e.g., [13]-[15]. In a first attempt to understand how traces of RE might influence the arc welding process, we have undertaken the theoretical study of the effect of a 0.1-percent cerium additive to a pure argon arc at 1 atm. As shown in Figs. 1 and 2, the addition of 0.1-percent 16/ cerium significantly alters the electrical conductivity at relatively low temperatures, and the net emission coeffiZRmm) cient at all temperatures. The predicted consequences of this trace of cerium on a 200-A 5-mm argon arc at 1-atm 3/1 are illustrated in Figs. 5 and 6. With the exception of a region close to the cathode, the introduction of cerium leads to a decrease in the arc temperature. For example, the axial temperature near the midpoint of the arc is reduced by about 2000 K. Moreover, the arc radius, defined ~ ~ (say) by the position of the 10 000 K isotherm, is reduced from about 3 to 1 mm. The cooling and reduction in radius are principally the result of the enhanced radiative-cooling due to the cerium. The presence of cerium also leads to a decrease of axial flow velocity, from a value of about 220 to about 180 ms 1 This decrease reflects RADIUS(mm) both the enhanced viscous stresses that occur in the narFig. 4. Measured (dashed curves) and predicted (solid curves) isotherms of a 200-A 5-mm-long argon arc at a pressure of atm. rower arc, and the increase in gas density resulting from the lower temperatures. The addition of cerium results in also been conducted in which the anode is permitted to an increase in the arc voltage from 9.8 to 15.9 V. melt. In this case metal vapor can be detected by spectroThese calculations suggest that even a trace of an elescopic means near the anode of the arc, with concentra- ment such as cerium may exert a marked influence on the tions of the order of 2000 ppm (parts per million). How- structure of an argon arc. Whether or not the reported detever, it is found that the temperature distribution is not rimental effects of RE additions on the CO2 weldability significantly altered by metal vapor. of pipeline steels is due to a modification of the MAG 2

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positive column in an analogous manner is a problem that requires further investigation. V. CONCLUSION Predicted temperature distributions in a 200-A 5-mm argon arc at atmospheric pressure agree reasonably well with measured values, except within 1 mm of the electrodes. Improvements in the boundary conditions of the theoretical model have been introduced, and better agreement with experimental results near the electrodes has been obtained. Temperature measurements on GTAW arcs burning in the open air have shown minimal effects on the arc temperature of changes in flow rate, electrode stickout distance, and nozzle diameter, although electrode shape does have an important effect. A theoretical study has shown how enhanced electrical conductivity and increased radiation losses due to a 0.1-percent cerium addition to argon can markedly change the arc properties.

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Fig. 5. Predicted isotherms for a 100-A 5-mm-long arc at a pressure of 1 atm burning in pure argon (solid curves) and argon containing 0.1-percent cerium (dashed curves).

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Predicted axial velocities as a function of radius in a 100-A 5-mm-

long arc at a pressure of 1 atm burning in pure argon (solid curves) and argon containing 0.1-percent cerium (dashed curves).

[15]

L. E. Cram, "A model of the cathode of a thermionic arc," J. Phys. D: Appl. Phys., vol. 16, no. 9, pp. 1643-1650, Sept. 1983. D. M. Chen and E. Pfender, "Two-temperature modeling of the anode contraction region of high-intensity arcs," IEEE Trans. Plasma Sci., vol. PS-9, no. 4, pp. 265-274, Dec. 1981. P. Kovitya and J. J. Lowke, "Two-dimensional calculations of welding arcs in argon," in Proc. 30th Nat. Conf Australian Welding Inst. (Hobart, Australia), 1982, pp. 293-298. P. Kovitya and J. J. Lowke, "Two-dimensional analysis of free-burning arcs in argon," J. Phys. D: Appl. Phys., vol. 18, no. 1, pp. 5370, Jan. 1985. L. E. Cram, "Statistical evaluation of radiative power losses from thermal plasmas due to spectral lines," J. Phys. D: Appl. Phys., vol. 18, no. 3, pp. 401-411, Mar. 1985. G. N. Haddad and A. J. D. Farmer, "Temperature determinations in a free-burning arc: I. Experimental techniques and results in argon," J. Phys. D: Appl. Phys., vol. 17, no. 1, pp. 1189-1196, July 1984. A. J. D. Farmer and G. N. Haddad, "Local thermodynamic equilibrium in free-burning arcs in argon," Appl. Phys. Lett., vol. 45, no. 6, pp. 24-25, June 1984. L. E. Cram, L. Poladian, and G. Roumeliotis, "Departures from equilibrium in a free-burning arc," in preparation. G. N. Haddad and A. J. D. Farmer, "Temperature measurements in gas-tungsten arcs," Weld. J., vol. 64, no. 12, pp. 339s-342s, 1985. K. C. Hsu and E. Pfender, "Analysis of the cathode region of a freeburning high intensity argon arc," J. Appl. Phys., vol. 54, no. 7, pp. 3818-3824, July 1983. D. L. Evans and R. S. Tankin, "Measurement of emission and absorption of radiation by an argon plasma," Phys. Fluids, vol. 10, no. 6, pp. 1137-1144, June 1967. R. H. Fowler and E. A. Milne, "Intensities of absorption lines in stellar spectra," Mon. Notic. Roy. Astron. Soc., vol. 83, pp. 403424, May 1923. H. Sasaki, K. Akahide, and J. Tsuboi, "CO2 short arc weldability of rare earth treated pipeline steels in circumferential welds," Trans. Japan Weld. Soc., vol. 7, no. 2, pp. 100-106, Sept. 1976. E. E. Banks and K. W. Gunn, "Australian experience in the welding of cerium-treated C/Mn/Nb steels for structural and pipeline usage," BHP Tech. Bull., vol. 21, no. 1, pp. 7-16, May 1977. B. R. Keville, "The effect of rare earth metal additions on the weldability of controlled rolled pipe plate," Teeside Laboratories, British Steel Corp., North Yorkshire, England, Rep. T/PROD/176/27/C, 1976.