Front. Mater. Sci. 2011, 5(2): 98–108 DOI 10.1007/s11706-011-0134-4 RESEARCH ARTICLE Numerical investigations of arc behaviour in gas metal arc welding using ANSYS CFX M. SCHNICK (✉)1, U. FUESSEL1, M. HERTEL1, A. SPILLE-KOHOFF2, and A. B. MURPHY3 1 Institute of Surface and Manufacturing Technology, Technische Universitat Dresden, Dresden, Germany 2 CFX Berlin Software GmBH, Berlin, Germany 3 CSIRO Materials Science and Engineering, Lindfield NSW 2070, Australia © Higher Education Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT: Current numerical models of gas metal arc welding (GMAW) are trying to combine magnetohydrodynamics (MHD) models of the arc and volume of fluid (VoF) models of metal transfer. They neglect vaporization and assume an argon atmosphere for the arc region, as it is common practice for models of gas tungsten arc welding. These models predict temperatures above 20 000 K and a temperature distribution similar to tungsten inert gas (TIG) arcs. However, current spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to TIG arcs they found a central local minimum of the radial temperature distribution. The paper presents a GMAW arc model that considers metal vapour and which is in a very good agreement with experimentally observed temperatures. Furthermore, the model is able to predict the local central minimum in the radial temperature and the radial electric current density distributions for the first time. The axially symmetric model of the welding torch, the work piece, the wire and the arc (fluid domain) implements MHD as well as turbulent mixing and thermal demixing of metal vapour in argon. The mass fraction of iron vapour obtained from the simulation shows an accumulation in the arc core and another accumulation on the fringes of the arc at 2000 to 5000 K. The demixing effects lead to very low concentrations of iron between these two regions. Sensitive analyses demonstrate the influence of the transport and radiation properties of metal vapour, and the evaporation rate relative to the wire feed. Finally the model predictions are compared with the measuring results of Zielińska et al. KEYWORDS: 1 arc welding, numerical simulation, GMAW, ANSYS CFX Introduction Gas metal arc welding (GMAW) is a long-established process that is used for joining metals. The arc is established between the work piece and a continuouslyfed wire. On the one hand, the arc flow and the arc Received November 20, 2010; accepted December 28, 2010 E-mail: schnick@mciron.mw.tu-dresden.de attachment at the anode wire have an important influence on the droplet formation and the heat transfer. On the other hand, the droplet geometry, surface temperatures and vaporization affect the fluid flow and the heat transfer inside the arc. A comprehensive understanding of the welding process and a detailed description of the physical effects are necessary to reduce the parameter space required for experiments and to provide direction to the development of welding techniques and equipment. Numerical simulation is a knowledge engineering tool for a better M. SCHNICK et al. Numerical investigations of arc behaviour in GMAW using ANSYS CFX visualisation and understanding of the complex cause and effect chains in arc welding. The GMAW models that have been used previously differ in the effects being studied and corresponding models [1]. The main focus of most of them is the prediction of droplet formation. State-of-the-art models include those that use volume of fluid (VoF) multiphase modelling of a free surface [2]. For the calculation of the electric current density, resistive heating and pinch force in the droplet, VoF-based models either use heat and electric current flux boundary conditions [3] or are combined with an arc model [4–6] that has been developed and tested for tungsten inert gas (TIG) arcs. They are based on magnetohydrodynamics (MHD) and assume a singlecomponent fluid of argon in a state of a local thermodynamic equilibrium (LTE) [7]. In Refs. [4–6], it is revealed that the arc attachment at the wire and the work piece are simplified by using an increased mesh size. Spille-Kohoff [8] neglected the droplet formation but used the unified sheath model of Lowke and Sansonnens [9–10] for modelling the transient behaviour of a GMAW process in argon. All of these arc models predict arc temperature distributions and plasma flows that have been measured at TIG arcs. The peak temperatures of 20 000–23 000 K were calculated direct below the wire. The arc temperature at the centreline of the arc is always higher than 16 000 K, except in the near-electrode regions. However, spectroscopic plasma temperature measurements contradict the predicted arc temperatures. Metzke et al. [11] analyzed a pulsed GMAW process with a copper wire and argon as the process gas. By using a narrow bandpass filter and spectroscopy he showed high local concentrations of metal vapour during the pulse. The metal vapour is not evenly distributed. A high concentration occurs for the arc core, while argon dominates in the fringes of the arc. Furthermore spectroscopic measurements of the arc temperatures were done. The radial temperature distribution demonstrates a maximum of 13 000 K, which can be located at the transition region between the metal vapour and the argon. In contrast to TIG arcs, a central temperature minimum of 8000 K was observed. Goecke [12] performed spectroscopic investigations at GMAW of aluminium in argon. He used different spectroscopic methods but he always obtained the same characteristic temperature distribution as in Ref. [11]. The recent measurements of Zielińska et al. [13–14] were done for GMAW of mild steel in spray transfer mode and confirm the results of [11–12]. They compared the 99 measured temperatures with the predictions of Ref. [4] and suggested the influence of the metal vapour, which was neglected in the model, as the cause of the lower measured temperatures. The influence of metal vapour on arc behaviour has so far been analysed numerically only for TIG arc welding. Tashiro et al. [15] modelled a TIG arc in helium considering a fixed fraction of metal vapour. The results predict higher temperatures and arc constriction. The TIG models of Yamamoto et al. [16–17] and Lago et al. [18] include vaporization at the work piece surface. They both consider mixing of metal vapour and argon but their simple treatments of diffusion did not allow the effects of demixing [19] to be considered. In this paper, a stationary axially symmetric model is used for numerical investigations of the influence of metal vapour in GMAW. In the model a flux of metal vapour is defined at the lower side of the wire and the flow and diffusion of metal vapour in the arc is considered. The transient behaviour of droplet formation is neglected initially. 2 Arc model The commercial simulation software ANSYS CFX is used for a rotationally-symmetric steady-state model of a gas metal arc. An MHD model was used for the implementation of the electromagnetic effects in the fluid and the solids. It is based on the conservation equation of electric current and Ohm’s law ! j ¼ – gradΦ (1) and the calculation of the magnetic vector potential ! ! Δ A ¼ – 0 $ j (2) for the calculation of the magnetic field, the resistive heating and the Lorentz force, which appear in the conservation equations of energy, mass and momentum. The effects of the sheaths are simplified by using a mesh size of 0.1 mm at the electrodes. The computational domain includes the welding torch, the wire, the arc (fluid domain) and the workpiece. The diameter of the wire is 1.2 mm, and the stick out and the arc length are 5 mm. The shielding gas flows around the gas nozzle and contact tube. We defined an argon fraction of 100% at the shielding gas inlet and at the opening. An interface mass 100 Front. Mater. Sci. 2011, 5(2): 98–108 source of iron vapour at 3023 K was modelled at the lower side of the wire. The total mass flow of iron vapour corresponds to 0.01%, 0.1%, or 1% of the 10 m/min wire speed. The configuration of the model and the distribution of the iron vapour mass source are shown in Fig. 1 [20]. Fig. 1 Configuration of the model and the iron vapour mass source. (Reproduced with permission from Ref. [20], Copyright 2010 IOP Publishing Ltd.) The plasma is assumed to be in a LTE. Density, enthalpy, thermal and electric conductivity and viscosity are calculated as functions of argon mole fraction and temperature, which is equal for heavy particles and electrons, as shown in Fig. 2 [21]. The thermodynamic and transport properties, including the combined diffusion coefficients, were calculated using the Chapman-Enskog method for a temperature range of 300–30 000 K [18]. Radiation is treated using the net emission coefficient (NEC) model with the published data of Menart et al. [22]. An SST turbulence model is used and demixing effects are taken into account. The combined diffusion coefficient model [19,23] is used to treat diffusion of metal vapour relative to argon. It is based on a conservation equation for the mass fraction yA of component A:   ! ∂ðyA Þ ! (3) þ div  u yA þ J A ¼ 0 ∂t with the diffusion mass flux JA: ! JA ¼  ! n2 mA mB DxAB gradxB þ DpAB grad ln ptot þ DEAB E  – DTAB grad ln T 3 Results Figure 4 shows calculated results for a 250 A arc and a vaporization rate of 1% relative to the wire feed rate [20]. The calculated arc temperatures are in good agreement with experimental data [11–13]. The highest arc temperature of 18 000 K is predicted at the centre line, directly below the wire. Closer to the workpiece, a local minimum in the radial temperature distribution is predicted at the centre of the arc. At a height of 1.5 mm above the workpiece, the central temperature is 10 000 K whereas the highest temperature of 12 500 K occurs off centre. The calculated mass fraction of the iron vapour shows an accumulation in the arc core. This arises from the iron vapour boundary source at the tip of the wire. Another accumulation is visible at the arc edge between 2000 and 5000 K. Demixing effects lead to low concentrations of iron vapour between these two accumulations. The calculated mass fraction distribution is consistent with the observed sharp optical separation of the arc core, mainly dominated by the metal vapour, and the outer arc regions. Steep gradients are predicted at the border of the arc core. The vapour accumulation in the outer regions is correlated with the well-known (especially for GMAW with magnesium-alloy aluminium filler) optically-bright arc fringes. The highest temperatures and current densities occur at the boundary of the arc core. Analyses of the sensitivity of the four driving forces of diffusion demonstrate that the temperature and the mole fraction gradients are the strongest drivers of demixing. The effects of the electric field and the pressure gradient are very small and could be neglected. In principle, the diffusion leads to an iron vapour flux from regions with higher temperatures to lower temperature regions. The offcentre radial maximum of the temperature leads to iron vapour diffusion fluxes both radially outwards and inwards. However, a calculation without demixing effects demonstrates that the plasma jet is the primary reason for the high iron vapour concentration in the arc core and the low fraction in the arc fringes. (4) where the terms respectively describe diffusion due to the gradient in the mass fraction of component B, the gradient in the total pressure, the electric field of the arc, and the temperature gradient. mk are the average masses of the heavy species in the two components, and DIAB are combined diffusion coefficients, which are shown in Fig. 3 [21]. 4 Influence of vaporization and iron vapour properties Different mechanisms have been suggested as responsible for the radial minimum in the temperature distribution. The vapour is about 3000 K at the wire surface, and heat is required to raise its temperature. Further, the electric M. SCHNICK et al. Numerical investigations of arc behaviour in GMAW using ANSYS CFX 101 Fig. 2 Influence of iron vapour on plasma properties of argon. (Reproduced with permission from Ref. [21], Copyright 2010 IOP Publishing Ltd.) conductivity and the net radiative emission coefficient of iron vapour are much higher than that of argon at the same temperature, especially below 10 000 K. To investigate this, calculations were done with and without vaporization and with a vapour that was assumed to have the properties of argon; see Fig. 5. The results make clear that the thermophysical properties of the metal vapour cause the low plasma temperatures at the centre of the arc. Subsequent calculations were done for evaluation of the influence of each physical property of the iron vapour, see Fig. 6. The minimum in the radial temperature distribution can 102 Front. Mater. Sci. 2011, 5(2): 98–108 Fig. 3 Diffusion coefficients of iron vapour in argon. (Reproduced with permission from Ref. [21], Copyright 2010 IOP Publishing Ltd.) Fig. 4 Calculated values of the temperature (left), the Fe mass fraction (right), and the flow (vectors) in a 250 A arc with a vaporization rate of 1% relative to the 10 m/min wire feed rate. (Reproduced with permission from Ref. [20], Copyright 2010 IOP Publishing Ltd.) Fig. 5 Influence of the vaporization and the physical properties of vapour on the radial temperature distribution at a position 1.5 mm above the workpiece (parameters as in Fig. 4) M. SCHNICK et al. Numerical investigations of arc behaviour in GMAW using ANSYS CFX 103 Fig. 6 Influence of the physical properties of the vapour on the radial distributions of temperature, electric current density, axial velocity and arc voltage drop above the workpiece (parameters as in Fig. 4). only be found if the radiative properties (NEC) of the iron vapour are used. The influence of the electric conductivity is very small and causes just a minor decrease of the electric current density in the arc core. In fact the electrical conductivity of iron vapour is significantly larger that that of argon only at lower temperatures. For temperatures above 15 000 K, that of argon is larger. The higher density of the iron vapour results in a higher specific enthalpy and leads to a wider arc. The central minimum in the radial temperature distribution is correlated with a minimum in the current density distribution. The highest electric conductivity is located at the edge of the metal-vapour-dominated arc core. This has been predicted by Goecke [12], who measured the highest charge density at the intersection of the metal vapour core and the outer argon arc. High vaporization rates lead to a higher arc voltage. This is mainly caused by the higher radiative losses and the higher specific enthalpy. Further, it seems that the arc attachment at the wire is less constricted because of the vaporization. However, the model does not allow predictions about the attachment mode and charge transfer at the surfaces of the wire and the workpiece. 5 Influence of vaporization rate The influence of the welding current (from 100 to 400 A) on arc temperatures and flow field has been examined previously. GMAW at a current of 400 A with argon is characterized by the spray mode of droplet transfer; this means that the wire melts so that its tip forms a cone of liquid and a large number of small detaching droplets is produced. Observations of a spray mode arc at these parameters show a differently-shaped metal vapour 104 Front. Mater. Sci. 2011, 5(2): 98–108 distribution in the arc core; the strongly-radiating metal vapour region looks more like a cone than a cylinder. Numerical analyses demonstrate that the shape of the metal-vapour-dominated region is mainly affected by the vaporization rate. The calculations presented above were all done for vaporisation rates of 1% of wire feed. However, vaporization rates of 5%–10% were measured for GMAW of aluminium alloys and a rate of 5% was measured for GMA brazing with copper filler [7]. Figure 7(a) shows the calculated metal vapour mass fraction, arc temperature and flow vectors of 250 A arcs depending on the vaporization rate relative to a wire feed rate of 10 m/min. For a low vaporization rate of 0.1%, the model predicts a narrow metal-vapour region as well as a flow field and arc temperatures very similar to those found in TIG arcs. The influence of the metal vapour is almost negligible. Vaporization rates above about 3% cause a dramatic change in the arc flow field and the metal vapour distribution. As it can be seen in the results for the vaporization rate of 5%, the region with the metal vapour mass fraction above 0.5 (the red region) becomes conical. A similar conical region of low temperature is formed. The results indicate that the formation of the conical metal vapour distribution is accompanied by a reversal of the direction of plasma flow in the centre of the arc. In contrast to all previous results, the central plasma flow is directed axially from the workpiece towards the wire. In the fringes of the arc the direction of flow is radially inward and axially downward. This outer region is characterized by the highest temperatures and a high flow velocity. Especially in the upper arc region there are high gradients of the metal vapour content between the iron vapour and argon plasma regions. Figure 7(b) shows the radial distributions of temperature, current density and the axial flow velocity 1.5 mm above the workpiece. The radial position of the maximum in temperature and current density shifts radially outwards as the vaporization rate increases. For vaporization rates above about 2%, the current density in the arc core vanishes because of the low temperature. In such cases, the electric current flows through the high-temperature region between the metal vapour and argon plasma regions. An interesting property of the arc in the case of 5% vaporization rate is that the velocity at radii less than 2 mm is directed upwards. This behaviour is very unusual; it is generally found, in both measurements and simulations, that the flow velocity is downwards in free-burning arcs such as those used in gas tungsten arc welding (GTAW) and GMAW. Upwards flow can be obtained when the attachment to the lower electrode is forced to be constricted Fig. 7 Influence of the vaporization rate, relative to a wire feed rate of 10 m/min, in a 250 A arc using the 1 mm NECs of Menart and Malik: (a) the temperature distribution and flow vectors (right) and metal vapour mass fraction distribution (left), images are scaled and sized; (b) radial distributions of temperature, current density and downward velocity at a position of 1.5 mm above the workpiece). M. SCHNICK et al. Numerical investigations of arc behaviour in GMAW using ANSYS CFX [24]; however the attachment here is diffuse. We attribute the flow reversal to the strong radiative cooling of the central regions of the arc because of the high metal vapour concentration. This decreases the electrical conductivity in this region, so the current is forced to flow at higher radii. The magnetic pinch force then drives the downward plasma flow at these higher radii. Mass continuity then leads to the upward plasma flow in the central regions. 6 Comparison with experimental results of Zielińska et al. We have mentioned three recent publications containing results of arc temperature distributions in GMAW. The results revealed in Refs. [11–12] are related to pulsed GMAW processes and aluminium or copper wires. The measurements of Zielińska et al. [13–14] are most relevant to the calculations presented here, because they are for spray-transfer mode GMAW, which can be approximated by a steady-state calculation, and an iron wire was used. The arc temperature measurements were performed for rather academic welding parameters, with a much longer arc than used industrially. A measured arc temperature distribution was presented for spray-transfer mode GMAW with an arc current of 330 A, a wire feed rate of 9 m/min and argon as shielding gas. We obtained best agreement with the measured radial temperature distributions using a vaporization rate of 3% relative to the wire feed rate. For the purposes of comparison of our numerical predictions with the experimental results of spray-transfer mode GMAW, the idealized wire tip shape and the assumption of a flat workpiece without weld pool 105 depression are not longer suitable. Thus the geometrical boundary conditions of the partially liquid wire were determined using the high-speed video images also presented by Zielińska et al. [14]. The shape of the depressed weld pool can not be extracted from these pictures, because it is obscured by the workpiece. We assumed a weld pool depression with a depth of 2 mm and a diameter of 7 mm. The presence of detached droplets between wire and workpiece was neglected. To allow a comparison between simulations and highspeed video images, the calculated distribution of the net radiative emission for an axisymmetric plasma was used to determine the intensity distribution of radiation from the arc, viewed from side on. The axisymmetric calculation gives the local radiation intensity rad(z, r) where z is the vertical and r the radial position. The side-on view radiation (as recorded by the camera) Rad(z, x), with horizontal distance x from arc axis, was calculated by an integral of the local radiation intensity over a line of sight perpendicular to axial direction z, according to  x  Radðh, xÞ ¼ rad h, $xð1 þ tan2 αÞdα (5) cosα ! where α is the angle between the axial direction and the radial direction to the integration point on the line of sight. Figure 8 shows a high-speed video image [14] and the calculated radiation intensity. The radiative contributions of argon and metal vapour were calculated separately; the metal vapour radiation contains mainly lines at wavelengths below 650 nm, and is scaled transparent-to-blue, and the argon radiation contains mainly lines above 650 nm and is scaled transparent-to-red. The figure demonstrates good agreement, especially for Fig. 8 Comparison of a high-speed video image (left) with reconstructions of calculated radiation intensity for spray-transfer mode GMAW with arc current of 330 A; the middle figure shows the argon (scaled transparent to red) and iron vapour radiation (scaled transparent to blue) separately; the right-hand figure shows them combined. (Reproduced with permission from Ref. [21], Copyright 2010 IOP Publishing Ltd.) 106 Front. Mater. Sci. 2011, 5(2): 98–108 the shape of the metal vapour radiation. The highest metal vapour and argon radiation intensities were calculated at the edge of the conical arc core. There is a lower-intensity region inside the metal vapour zone of the arc in both measured and calculated figures; this is due to the much lower temperatures in the lower central region of the arc. The predicted extent of the argon radiative emission is much smaller than in the measured image. This is expected, since it has been shown that absorption and reemission of radiation by the cool gas surrounding argon arcs leads to radiative emission from regions up to 10 mm or more from the arc core [25–26]. Such radiative transfer effects are of course not taken into account in the NEC approach, and therefore cannot be reproduced by the model. The left-hand side of Fig. 9 [21] shows a quantitative comparison between the measured and numericallypredicted radial temperature distributions at different heights above the workpiece. The distributions of the iron vapour content and temperature as well as the flow velocity vectors are shown on the right-hand side. The numerical predictions are in reasonable agreement with the measurements (note that the temperature scale starts at 6000 K, which exaggerates the discrepancies). In particular, the radial positions at which the maximum temperatures occur agree closely. The discrepancies in temperature are around 2000 K, with the calculated temperatures generally exceeding the measured temperatures, especially in the outer arc regions. We emphasize that there are many uncertainties inherent in the comparison. In particular, the rate of vaporization of the wire, the distribution of the metal vapour source at the edge of the wire, and the shape of the weld pool depression are not known and are estimated values are used in the model. Further, we have not taken into account the influence of droplets falling through the arc. Nevertheless, the reasonable agreement between the predictions and the measurements suggests that the most important physical processes are adequately taken into account in the model. In particular, the flow reversal found for high vaporization rates in Section 5 is again present. Our simulations indicate that the conical region of high metal vapour concentration in the arc centre is always associated with the flow reversal. As discussed in Section 5, this is a consequence of the strong radiative cooling in the central region of the arc, and the consequent effects on the current density and flow distributions. The fact that we are able to predict the observed temperature profile and the radiative emission distribution is evidence that the flow reversal is indeed occurring. 7 Conclusions An MHD model of the GMAW process has been used for numerical studies of the influence of iron vapour on the arc temperature and the current transfer. An iron vapour mass source was defined at the lower side of the wire. The effects of turbulent mixing, and laminar diffusion and demixing, have been taken into account. Fig. 9 Comparison between predicted and measured radial temperature distributions at different heights above the workpiece (left), and calculated metal vapour mass fraction and temperature distributions and flow velocity vectors (right) of a spray-transfer mode GMAW process for an arc current of 330 A. (Reproduced with permission from Ref. [21], Copyright 2010 IOP Publishing Ltd.) M. SCHNICK et al. Numerical investigations of arc behaviour in GMAW using ANSYS CFX The model predicts a central minimum in the radial temperature distribution. The calculated temperature distributions are in accordance with measurements. A high concentration of metal vapour is predicted in the arc core, with a second accumulation in regions with temperatures between 2000 and 5000 K. Between these accumulation regions, the Fe mass fraction is very small. The high concentration of iron vapour leads to the decreased temperatures near the arc axis. The main mechanism is the strong radioactive emission from the iron vapour. The role of the higher electric conductivity of metal vapour below temperatures of 15 000 K is found to be much smaller than previously suggested. The rate of vaporization of the wire has a strong influence on the arc temperature and current density distributions, affecting both the maximum and minimum temperatures and the position of the minimum. The model also predicts a significant influence on the arc flow field. For high vaporization rates of 3%–5% or more relative to the wire feed rate, a reversal of the plasma flow direction for the inner arc region was predicted. Downward flow occurs in the outer regions of the arc, and an upward flow occurs in the central regions. Such a flow reversal has not been previously observed in welding arcs. However, it appears to be associated with a change in the shape of metal vapour distribution; the shape of the central region with high-metal vapour concentration was predicted to change from cylindrical at low vaporization rates to conical at high vaporization rates. 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