M E Rouffet

M Wendt

G Goett

R Kozakov

H Schoepp
email: schoepp@inp-greifswald.de

K D Weltmann

D Uhrlandt

Spectroscopic investigation of the high-current phase of a pulsed GMAW process

Keywords: number: 52.70.Kz 52.80Mg 81.20.Vj Pulsed gas metal arc welding, GMAW, emission spectroscopy, Stark broadening Boltzmann plot

While metal vapours have an important impact on the efficiency of the pulsed gas metal arc welding process, only a few papers are focused on this effect. In this paper, methods based on emission spectroscopy are performed to improve the understanding of the physical phenomena occurring during the high-current pulse. Boltzmann plots applied to iron lines, the Stark broadening of the 696.5 nm argon line and composition calculations assuming local thermodynamic equilibrium are used to determine characteristic parameters of the plasma. It is observed that the central part of the arc is mainly composed of iron. The percentage of iron increases quickly at the beginning of the high-current pulse, and slowly decreases, when the central part broadens. During the high-current phase the temperature profile has a minimum value of around 8000 K at the axis of the arc while the argon envelope of the central part reaches temperatures of approximately 13.000 K. High percentage of iron and high radiation of the plasma at the centre can explain the measured shape of the temperature profile.

Introduction

Gas metal arc welding (GMAW) processes are widely used in industry to produce robust and high quality welded seams connecting metallic workpieces. GMAW processes use an electric arc in order to melt the welding wire so that metal droplets falling from the wire to the workpiece form the join. During the process the arc is enclosed in a shielding gas (typically argon) in order to control the current conduction path and protect the molten metal from oxidation by the atmospheric air. By using a pulsed current, it is possible to obtain a One Drop Per Pulse (ODPP) mode, which is characterized by a stable, periodical and controllable metal transfer to the workpiece [START_REF] Wang | Numerical analysis of metal transfer in gas metal arc welding under modified pulsed current conditions[END_REF]. The pulsed GMAW (GMAW-P) process is an effective way of achieving a controlled metal transfer to the workpiece over a wide range of heat and mass transfer rates [START_REF] Wang | Numerical analysis of metal transfer in gas metal arc welding under modified pulsed current conditions[END_REF]. Because of their good properties, ODPP GMAW-P processes are widely used in industry. They are intensively studied, in order to control, optimize and predict the quality of the welding seams.

The conditions needed to obtain an ODPP are well studied in literature [START_REF] Wang | Numerical analysis of metal transfer in gas metal arc welding under modified pulsed current conditions[END_REF][START_REF] Jacobsen | Monopulse investigation of drop detachment in pulsed gas metal arc welding[END_REF][START_REF] Arif | Force-displacement model for analysis of pulsed-GMAW[END_REF][START_REF] Zhang | metal transfer control by monitoring excited droplet oscillation[END_REF]. Parameters allowing improvement of the efficiency of such process were explored by Subramanian et al. [START_REF] Subramaniam | Experimental approach to selection of pulsing parameters in pulsed GMAW[END_REF]. Many works investigate the metal transfer and the electrical characteristics of the ODPP GMAW-P process [START_REF] Choi | The dynamic analysis of metal transfer in pulsed current gas metal arc welding[END_REF][START_REF] Praveen | Arc voltage behavior of one drop per pulse mode in GMAW-P[END_REF][START_REF] Hu | Effects of current on droplet generation and arc plasma in gas metal arc welding[END_REF].

Although the radiation emitted by the metal vapour contained in the arc is an important energy loss mechanism, few works are focused on the radiation of the arc plasma, in part because of the difficulty to measure a time-dependent spectroscopic signal [START_REF] Pellerin | Experimental investigations of the arc MIG-MAG welding[END_REF]. Zielinska et al. [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF] explore the characteristic of the arc with spectroscopic methods, in a study on the influence of the shielding gas on the plasma parameters in a GMAW spraying arc. Zielinska et al. [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF] use the Stark broadening of the argon line at 696.5 nm and the iron line at 538.3 nm in order to determine radial profiles of temperature and electron density. Ton et al. [START_REF] Ton | Physical properties of the plasma-MIG welding arc[END_REF] investigate a metal inert gas welding arc and observe a temperature minimum at the arc centre. They determine the electron temperature at the centre and in the outer region of the arc, by applying the Boltzmann plot method to iron lines and analyzing the ratio of the emission of Ar II and Ar I lines, respectively. A general overview on spectroscopic diagnostics of free burning arcs and available methods for determination of temperatures and electron densities is given in [START_REF] Wiese | Spectroscopic diagnostics of low temperature plasmas: techniques and required data[END_REF].

The objective of this paper is to analyze the evolution of the characteristic plasma parameters in the high-current phase of a GMAW-P process where the radiative losses are the most important. A better understanding of the radiative transfer during this phase will allow a better comprehension of the physical mechanisms occurring in the plasma, and will allow to improve the efficiency of the process. To this end a detailed, time and spatially resolved spectroscopic study of the arc during the high-current phase was performed. In this study the electron density profile were determined from the Stark broadening of the argon line at 696.5 nm. Radial temperature profiles were obtained from Boltzmann plots of iron lines at the arc centre, while they were deduced from the electron density and composition calculations in the outer region of the arc.

The text is divided into three parts. The first part presents the experimental setup, the spectrometer and the high speed camera, and describes the welding process. The diagnostic methods are described, and error estimations are performed. Results are presented in a second part, including temperature, electron number density and iron mol fraction profiles. These results are discussed in the third part.

Experimental apparatus and procedure

Experimental apparatus

We use a spectrometer to explore radiation emitted by the arc during the high-current phase of the GMAW-P process. Each spectroscopic measurement is completed by electrical signal and video records, to control the process. The experimental set-up (figure 1) is composed of 4 parts: the welding process, the electrical probes, the spectrometer and the high speed camera. These components are described in later paragraphs.

The welding process

A welding arc is generated at atmospheric pressure by a plasma torch, vertically oriented to a workpiece. Electrical power, gas flow and wire feed control were performed by a high-tech welding EWM machine Phoenix 521 Progress PULS ColdArc. The power source is operated with a positive polarity on the wire. The wire is made of steel G 3 Si 1 , and is 1.2 mm in diameter. It is fed at the velocity of 4 m/min. A 10 mm thick steel (S235) workpiece was employed. The chemical composition (percent by weight) of the workpiece and the wire are given in table 1. The main component is iron, chosen because of its hardness and versatility, wear-resistance and low cost. Shielding gas is pure argon 5.0 (purity up to 99.999 %), flowing at 12 l/min.

The welding wire is fed through a contact tube that is contacted to the power source. The end of the contact tube, the contact tip, is situated 6 mm within the nozzle and 20 mm above the workpiece. During the welding process, the distances between contact tip, nozzle and workpiece are kept constant in order to create arcs with reproducible plasma properties. The workpiece is moved in positive x-direction at 30 cm/min by means of a translation table. The electrical measurements are performed using current and differential voltage probes. The current is measured with a Hall-Effect transducer (LB 200-S, LEM) with a response time below 1 µs and an accuracy of 0.5 %. The voltage is measured with the high voltage differential probe (P5200, Tektronix) at the contact tip. Figure 2 shows current and voltage measured between the workpiece and the nozzle during one period, recorded by the data acquisition system (figure 1).

Figure 2 shows current and voltage curves for one duty cycle of the investigated GMAW-P. The duty cycle is divided into three phases P1, P2 and P3, during which the welding wire is molten, a molten drop propagates to the workpiece, and the plasma is sustained by a small holding current, respectively.

The pulse starts with a high-current phase (P1), where current and voltage are almost constant, at 430 A and about 13.2 V, respectively. During P1, a plasma arc with strong radiation is generated between the wire and the workpiece. The current density is estimated to 300 A/mm 2 . The heating of the wire leads to an accumulation of molten wire material at its tip. Phase P1 is chosen long enough so that electromagnetic pinching leads to the formation of a neck at the root of the molten wire material and drop detachment [START_REF] Choi | The dynamic analysis of metal transfer in pulsed current gas metal arc welding[END_REF]. The liquid metal in the droplet continues to fall down by inertia and is guided by the axial flow of argon [START_REF] Lowke | Theory of free-burning arc columns including the influence of the cathode[END_REF].

During a last phase (P3), current and voltage decreases further to values of 30 A and 7.5 V. The plasma intensity is lowered; no spectroscopic measurements are carried out.

Spectroscopy

The plasma radiation is focused onto a 20 μm wide entrance slit of a spectrometer (SpectraPro 750, Acton research) by a spherical mirror. The optical signal is recorded side on in one propagation direction perpendicular to the arc axis, i.e. parallel to the workpiece. The radiation emitted perpendicular to the arc axis and propagating in negative X-direction is recorded as a function of the lateral position Y (cf. figure 1). The characteristics of the optical set-up allow observing the plasma radiations for distances Y up to 5 mm from the centre of the arc. Spectra were recorded 4 mm above the workpiece whereas the distance between the workpiece and the nozzle was 14 mm. The spatial resolution of the imaging system is 0.026 mm/pixel. The spectrometer is equipped with two diffraction gratings, 1200 g/mm and 2400 g/mm giving a wavelength resolution of 0.013 and 0.0032 nm/pixel, respectively.

We explore all the high-current phase, making measurements at four times t 1 , t 2 , t 3 and t 4 . Each time t i is separated from the others by 270 µs as presented in figure 3.

Figure 3. Definition of the four times t 1 , t 2 , t 3 and t 4 chosen to explore the whole high-current phase.

The spectroscopic signals are analyzed in order to calculate the temperature and electron density profiles inside the plasma. Different parts of the spectrum are successively considered, focusing on iron lines (coming from the vaporized metal) and on argon lines (coming from the shielding gas). An absolute calibration to spectral radiances in W/(nm m 2 sr) is done by means of a tungsten ribbon lamp. This lamp is imaged on the entrance slit instead of the welding arc under same optical conditions. The radiance of the 532.4 nm and 696.5 nm lines from iron and argon, respectively, are presented in the figure 4

High speed camera

A high speed camera (MotionPro Y6) associated with a gray filter is used to obtain images of the plasma. The acquisition time is 4 μs, aperture is 32, and the rate is 20 MHz. The acquisition of current, spectral radiance and arc images are synchronized, and a delay is adjusted to explore the pulse at different times t i . Typical images are presented in figure 6. The arc consists of a bright central part, corresponding to the region where the iron vapour is present, and a wider, bell shaped part showing a lower brightness. This observed shape is typical of inert gas welding arcs as for example shown in [START_REF] Haidar | An analysis of the formation of metal droplets in arc welding[END_REF][START_REF] Lowke | Metal vapours in MIG arcs cause (1) minima in central arc temperatures and (2) increased arc voltages[END_REF]. During the high-current phase, the arc length is almost constant (equal to 8 mm), the diameter of the central region is increasing (from 1 mm to 2 mm), and the central part is rotationally symmetric in good approximate. Closer inspection of the high speed images reveals that the brightness of the arc core has a maximum near the wire tip and decreases towards the workpiece. Also, in the region with high brightness, the arc core diameter has a flat maximum and decreases slowly towards the workpiece. At distances to the workpiece below approximately 2 mm the stagnation zone of the flow leads to a strong increase of the core diameter. The axial position of the spectroscopic measurements at 4 mm is chosen well above the stagnation zone, and in a region with only a small axial variation of the arc core's brightness and diameter. The modelling results given in [START_REF] Lowke | Metal vapours in MIG arcs cause (1) minima in central arc temperatures and (2) increased arc voltages[END_REF] suggest that measurements closer to the wire tip would give higher plasma temperatures.

Diagnostic methods

Emission spectroscopy allows the determination of the plasma characteristics when a Partial Local Thermodynamic Equilibrium (PLTE) exists inside the plasma. The excitation temperature of the excited atomic levels is then equal to the electron temperature and their densities are related to the next ion's ground state population density by Saha-Boltzmann relations [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF][START_REF] Haidar | Departure from local thermodynamic equilibrium in high-current free burning arcs in argon[END_REF]. Following the Boltzmann plot method, we analyze iron lines to deduce the plasma temperature. The electron number density is determined using the Stark broadening of the 696.5 nm argon line. PLTE is not necessary for the latter method. The percentage of iron inside the plasma is deduced from both latter values and calculated plasma compositions. The applied methods are described in the next paragraphs.

Plasma temperature

In PLTE, the population of the atomic energy states adheres to a Boltzmann distribution, uniquely characterized by their temperature, T. This temperature may be obtained in applying the Boltzmann plot diagnostic method. This method is based on the fact that the emissivities of spectral lines belonging to the same atom are connected by the relation:

C T k E A g B j ji j ij ij +         - =         λ ε ln
Eq. 1

The left-hand term is usually called Boltzmann function. The quantities ε ij , λ ij and A ji are the emissivity (W/m 3 /sr), the wavelength (m), and the transition probability (s -1 ) of the line corresponding to the transition from upper level j to lower level i, respectively. E j and g j are the energy (eV) and the statistical weight of level j, respectively. k B is the Boltzmann constant. T is the temperature of the plasma (K), and C is a constant at fixed temperature. A linear fit of ln(ε ij λ ij / g j A ji ) on E j allows to deduce the plasma temperature. Seven iron lines are selected because of the large differences of their upper energy levels E j (table 2), and because of their close proximity so they can be measured simultaneously. Figure 7 shows an example of a Boltzmann plot obtained at the centre of the arc at t 3 . The linear fit is made by the least squares method [START_REF] Huel | The Total Least Squares Problem[END_REF]. Each point corresponds to the Boltzmann function value of one line. In this example, the slope is -1.50047 eV -1 , corresponding to the temperature T = 7734 K.

Table 2 : Characteristic of the iron lines selected for the Boltzmann diagnostic method [START_REF] Huel | The Total Least Squares Problem[END_REF].

Wavelength (nm)

A ji (10

Electron number densities

The electron number density in the arc is determined by analyzing the broadening of the neutral argon line at 695.5 nm. At atmospheric pressure and temperatures usually found in GMAW-P (around 10000 K) this line is mainly by electrons Stark broadened showing a Lorentz line profile with a full width at half maximum (FWHM) of approximately 0.1 nm. The FWHM of Doppler broadening width is approximately 0.001 nm. Additionally, the selected line shows low self-absorption, strong intensity and sufficient isolation from neighbouring lines.

Electron Stark broadening of spectral lines is caused by the interaction of the radiator with the electrons of the surrounding plasma leading to a perturbation of the lines upper and lower levels [START_REF] Dimitrijevic | Stark broadening of visible Ar I spectral lines[END_REF]. The corresponding line width is proportional to the electron density N e and has weak temperature dependence, so that measured widths can be used to deduce the electron density.

Figure 8 presents a summary of Stark widths given in [START_REF] Dimitrijevic | Stark broadening of visible Ar I spectral lines[END_REF]. The values are reported for plasma at atmospheric pressure and at a temperature of 10

Composition of the plasma

In PLTE, plasma composition can be deduced from both the plasma temperature and the electron number density, due to equilibrium equations. Calculations were made in solving a system of mass action laws together with equations for charge neutrality, given total pressure and iron content:

        + + + + + + + = + + + + + + 2 2 2 Ar Ar Ar Fe Fe Fe Fe Fe Fe Fe N N N N N N N N N f
Eq.3

Deviations from ideal gas behaviour due to the Coulomb interaction were taken into account following Ebeling et al. [START_REF] Ebeling | Theory of bound states and ionization equilibrium in Ergebnisse der Plasmaphysik und der Gaselektronik[END_REF]. These are contributions to the total pressure and to the chemical potentials of the species, where the latter lead to an effective reduction ionization potential of the species. Also following Ebeling et al., the Planck-Larkin partition function was used for the chemical potentials evaluation. The internal energy levels of the species were taken from Moore [START_REF] Moore | Atomic energy levels[END_REF] and NIST [START_REF] Ralchenko | NIST Atomic Spectra Database (version 3.1.3)[END_REF].

Because both N e and T are necessary to deduce the content of iron inside the plasma, and because T is only known at the centre (due to the too weak metal vapour radiation signal recorded at the edge), centre and edge are processed using two different methods.

At the centre, where both parameters N e and T are known, the percentage of iron atom (%Fe) is deduced from equilibrium equations.

At the edge of the welding arc, only N e is available. However, knowing %Fe at the centre, and knowing that %Fe is negligible at the edge of the plasma, it is possible to estimate the probable iron mol fraction profile at the boundary between the centre and the edge. It allows to know both parameters N e and %Fe. T is then deduced from equilibrium equations.

The table 3 summarizes the different tactics adopted to deduce the third parameter from both others.

Table 3. Summary describing the way to deduce the third parameter from both others. Position in the arc

Calculated values Probable values Deduced values

Centre (0 mm < r ≤ 1 mm) N e , T %Fe Edge (r > 1 mm) N e %Fe T

Error estimation

Iron and argon lines are not recorded simultaneously. Consequently, electron number density and plasma temperature are measured at two different current pulses. A first error source is to associate N e at T, whereas the state of the plasma is not exactly the same. To take into account this error, the variation of the parameters temperature, electron number density and percentage of iron at the same time in the period but over several pulses has been estimated. Such variations are lower than the error bars estimated below, whatever the parameter considered. The state of the plasma is almost the same whatever the current pulse considered. The error induced by variations of plasma state at various current pulses is then neglected. In the next paragraphs, estimation of other errors is made, focusing on each diagnostic method.

Boltzmann plot method. The error of the temperature resulting from the Boltzmann plot is:

        + ×                 - = ∑∑ == ji ji ij ij ll l l B A dA d E E T k T dT ε ε 2 / 1 7 1 2 7 1 2 7 1
Eq.4

The first term takes into account the upper energy level E l of the seven lines selected to apply the Boltzmann plot method. The left-hand term in the parenthesis corresponds to the error of emissivity values resulting here from limited measurement reproducibility. This error is 9 % at the centre and increases up to 15 % at a radial position of around 1 mm. The Abel inversion introduces an error on ε ij due to the smoothing of the radiances. This error is estimated to 15 %. The second term in the parentheses corresponds to the error generated by the uncertainty of the transition probability A ji taken from the NIST compilation [START_REF] Ralchenko | NIST Atomic Spectra Database (version 3.1.3)[END_REF] (see accuracy in table 2). The error of A ji is considered equal to 18 %, giving an upper limit of the error generated. Applying equation 4, a temperature error of 8 % at the centre of the plasma, and 9 % at 1 mm radius is obtained. Using more refined numerical methods for Abel inversion may reduce the total error of the temperature.

Stark broadening method. Equation 2 leads to an error estimation of the electron density of

( )         ∆ + ∆ ∆ ∆ = ∆ 0 0 002 . 1 S S S S e e N N λ λ λ λ .
Eq. 5

The first term (on the right hand side of equation 5) takes into account the uncertainty of the line width due to the fitting procedure and due to the random error caused by fluctuations of the arc plasma. It is estimated to 3.5 %. The second term is caused by the uncertainty of the scaling constant 0 S λ . Zielinska et al. [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF] use a scaling constant of 0.0739 nm, while Dimitrijevic et al. [START_REF] Dimitrijevic | Stark broadening of visible Ar I spectral lines[END_REF] give a Stark width of 0.0637 nm for a temperature of 10000 K and an electron density of 10 23 m -3 . Using these three values an uncertainty of 12 % can be estimated for 0 S λ . Zielinska et al. [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF] use an additional temperature dependence of

S λ ∆

proportional to T 0.3685 . This would give an additional term 0.3685 ∆T/T on the right hand side of eq. 5 and an additional error of 3.6 %. Thus, the resulting total error of the electron density is less than 20 %. While the error of the electron density is limited by the uncertainty of 0 S λ the uncertainty of S λ ∆ may be reduced by using spectrometers with a higher signal to noise ratio.

Deduction of the third parameter using equilibrium equations. The use of equilibrium equations is justified if the plasma is in LTE. Deviation from LTE is a source of error, especially at the edge of the plasma. The studie given in [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF] indicates that LTE is closely fulfilled in the GMAW process so that errors due to deviations from LTE are considered negligibly small.

At the arc centre, considering an error of 8 % on the temperature determined by Boltzmann plot method, and an error of 18 % on the electron number density deduced from Stark broadening, the error on the percentage of iron is estimated to 50 %.

At the edge of the arc core, considering an error of 18 % on the electron number density deduced from Stark broadening, and an error of 50 % on the percentage of iron, leads to an error of 8 % of the plasma temperature. The error of the temperature is relatively small due to the rapid increase of the electron density with rising temperature.

Results

Figure 10 presents temperature profiles of the plasma at different times t i of the high-current phase of the process. For all four times the temperature is lower at the centre of the plasma, as expected and observed in previous experiments [START_REF] Zielinska | Investigations of GMAW plasma by optical emission spectroscopy[END_REF][START_REF] Ton | Physical properties of the plasma-MIG welding arc[END_REF] and a numerical model [START_REF] Lowke | Metal vapours in MIG arcs cause (1) minima in central arc temperatures and (2) increased arc voltages[END_REF]. This inner temperature, around 8000 K, is stable all along the high-current phase. Temperature increases rapidly at around 1 mm, and then decreases slowly towards the edge of the arc. The radius of the inner (cold) region of the arc expands from 1.2 mm at t = 0 to around 1.7 mm at t = 810 µs. During this expansion both the temperature at the arc centre and the temperature profile in the outer region remain approximately constant.

Discussion

During the whole high-current phase, metal vapours coming from the wire are introduced into the plasma. Due to the flow of the shielding gas, these vapours are concentrated at the centre of the arc. The temperature profile of stationary arcs is determined by the energy balance:

      ∂ ∂ ∂ ∂ + - = ∂ ∂ r T r r r E z T v c N z P κ πε σ ρ 1 4 2 (6)
Here ρ , P c , ν z , N ε andκ are the mass density, specific heat capacity, the axial flow velocity, the net emission and the heat conductivity, respectively. Considering that the convective term on the left hand side of ( 6) is small with respect to the source terms, the pronounced minimum of the temperature profile at the arc axis can be understood following the arguments of Lowke et al. [START_REF] Lowke | Metal vapours in MIG arcs cause (1) minima in central arc temperatures and (2) increased arc voltages[END_REF]. In pure argon arcs where the net emission is small the ohmic heating term is balanced by the heat conduction term, so that heating leads to a negative curvature of the temperature profile. Consequently, the temperature profile has a maximum on the arc axis. For significant amounts of iron the net emission of the plasma is several orders of magnitude higher than for pure argon, e.g. for 8000 K the emission of iron is 7×10 10 W/m 3 /sr while it is only 1×10 7 W/m 3 for pure argon [START_REF] Menart | Net emission coefficients for argon-iron thermal plasmas[END_REF]. In this case the sum of heating and net emission can become negative, so that the curvature of the temperature gets positive, thus leading to a temperature minimum on the arc axis. At the radial position where the curvature of the temperature vanishes ohmic heating is balanced by the net emission. Comparison of figures 10 and 12 shows that this is the case for an iron content of approximately 20 %.

Figure 10 shows a radial expansion of the arc core with a velocity of approximately 0.6 m/s. This expansion may be due to radial diffusion or an increase of the evaporation region on the welding wire.

The decrease of the iron content towards the end of the high-current phase can be caused by the expansion of the arc core but also by a decrease of the evaporation rate of the welding wire.

Haidar investigated the population of the argon ground state in a TIG welding arc using a numerical non-equilibrium model [START_REF] Haidar | Departure from local thermodynamic equilibrium in high-current free burning arcs in argon[END_REF]. For distances from the tungsten cathode bigger than 2.5 mm the deviation from LTE was small. The deviation in the model is caused by the divergence of plasma flow. In regions where the flow is only in axial direction and its variations only in radial direction this term equals zero and LTE is assumed. At the position of the measurements made here the arc appears homogeneous in z-direction so that we do not expect deviations from LTE due to the flow. However, near the wire tip the divergence of the flow may lead to deviations from LTE. In argon arcs the overpopulation of the ground state is enabled by a large energy separation of the first excited state from the ground state (11.55 eV). For iron this separation is much smaller (0.86 eV for the lowest level not in the a 5 D Term) so that an overpopulation of the ground state is less likely.

Very recently, results of fluid simulations [START_REF] Haidar | The dynamic effects of metal vapour in gas metal arc welding[END_REF][START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF] for GMAW processes similar to the object of this work have been reported. A temperature minimum in the arc axis has been found in the both theoretical studies too. In [START_REF] Haidar | The dynamic effects of metal vapour in gas metal arc welding[END_REF] the influence of a different extent of metal evaporation has been studied, however transport data for pure argon have been applied for this part of the study. Material properties of the mixture of argon and iron vapour including diffusion effects have been used in the simulations in [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF]. In addition, comparative calculations applying each one property of iron in the work [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF] have been revealed that the increased power loss due to radiation represents the main reason for the low temperature of the iron vapour core. The absolute values of the temperature in these studies can not be compared with the measured one because the considered processes are not exactly the same. A direct comparison is planned for future work.

Conclusion

A spectroscopic study was performed at the high-current phase of a GMAW-P process. Plasma characteristics have been determined utilizing Boltzmann plot analysis of iron line radiation, Stark broadening analysis of an argon line and plasma composition calculations.

The diagnostics allowed a sufficiently high accurate determination of the plasma temperature at least in the arc core as well as estimations of the radial and temporal behaviour of electron density and iron mol fraction. The centre of the arc is mainly composed of iron, coming from the wire, where the plasma temperature is around 8000 K over the whole high-current phase. The percentage of iron increases considerably at the beginning of the current pulse. Later on in the high-current phase it slowly decreases together with a radial expansion of the region dominated by the iron vapour. The metal vapour core shows a sharp edge where the temperature increases radially up to 13000 K to compensate the lower ionization and as a result of the lower radiative losses of the shielding gas argon.

Beside the first discussion of the main impact of iron vapour in the welding arc, this work should be the base of further studies like the impact of shielding gas kind and flow or the analysis of arc regions near the wire and the workpiece. In addition, the use of the experimental results for validations of corresponding arc models is aimed in future studies.

Bibliography

Figure 1 .

 1 Figure 1. Experimental set-up.

Figure 2 .

 2 Figure 2. Current and voltage measured during one period.

at t 3 .

 3 The values are shown as functions of the lateral Y position, as defined in the figure1. In this figure the radiance is the integrated value of the separated line and represents the side-on distribution. Assuming cylindrical symmetry a polynomial fit is made before the Abel inversion could be executed. This procedure is necessary to obtain the local emissivity (in W/m 3 /sr) of the plasma at each radial position. A typical graph obtained is given in the figure5, where the local values correspond to the radiance values of the figure4. We note that the iron radiation is concentrated to the centre of the welding arc. The argon emissivity extends over the whole measured range of Y and reaches a maximum at approximately 1 mm out of axis.

Figure 4 .

 4 Figure 4. Radiance of the iron line at 532.4 nm and the argon line at 696.5 nm at t 3 = 540 µs, as functions of the lateral Y position. Solid lines correspond to the experimental values, and dashed lines correspond to the polynomial fit values.

Figure 5 .

 5 Figure 5. Emissivity of the iron line at 532.4 nm and the argon line at 696.5 nm at t 3 = 540 µs, as functions of the radial position r, obtained by Abel inversion of the radiances presented in figure 4.

Figure 6 .

 6 Figure 6. Pictures of the plasma at different times t 1 , t 2 , t 3 and t 4 .

Figure 7 .

 7 Figure 7. Example of a Boltzmann plot allowing the deduction of the plasma temperature, obtains at t 3 = 540 µs, at the centre of the welding arc.

Sλ

  4 K. A fit of the FWHM Δλ S on the electron number density N e in m -3 gives the relation = 0.0803 nm. This equation is used to determine electron number densities from measured line widths. For each lateral position Y the line width Δλ s is determined by fitting a Lorentz function to the measured spectral radiance. An example for Y = 0, giving a width of Δλ s = 0.11594 nm which corresponds to N e = 1.08×10 23 m -3 is shown in figure9.

Figure 8 .

 8 Figure 8. FWHM of the 696.5 nm argon line as function of electron number density. Circlescorrespond to literature values[START_REF] Dimitrijevic | Stark broadening of visible Ar I spectral lines[END_REF], and solid line corresponds to the linear fit.

Figure 9 .

 9 Figure 9. Lorentz line fit of the 696.5 nm argon line at t = 540 µs and lateral position Y = 0. Solid line corresponds to the experimental data, and dash line corresponds to the Lorentz profile fit.

Figure 10 .

 10 Figure 10. Temperature profiles of the plasma at different time t i of the high-current phase.

Figure 11

 11 Figure11shows the electron density of the welding arc in dependence of the radial position, for the four times t i of the high-current phase. The variation over time is low in comparison with the error bars estimated. Nevertheless, first conclusions on the spatial and temporal behaviours can be given. The electron density is weakly lower at the centre of the plasma than at the radius of 0.6 mm.

Figure 11 .

 11 Figure 11. Evolution of the electron density at different times t i of the high-current pulse.

Figure 12

 12 Figure12presents the evolution of the iron atom percentage in the plasma, in function of the radial position, at the four different times of the high-current phase. The iron vapour density achieves a maximum value at the centre of the arc, and decreases strongly at around 1 mm radius. For radial positions larger than 3 mm, the plasma can be considered as pure argon plasma. Percentage of iron at the centre of the arc increases from 50 % to 70 % at the beginning of the high-current phase. Then it continuously decreases until the end of the high-current phase.

Figure 12 .

 12 Figure 12. Iron mol fraction at different times t i of the high-current phase.

Table 1 .

 1 Chemical composition of the workpiece and the wire.

	Fe	C	Mn	S	P	Al	Si Ni Cr Mo	V	Cu Other
	S235 > 98.3 ≤ 0.17 ≤ 1.4	≤ 0.045	≤ 0.045	≥ 0.02					N 2 : ≤ 0.009
	G 3 Si 1 > 96.2 ≤ 0.14 ≤ 1.6	0.025	0.025	0.02	≤ 1.0 0.15	0.15	0.15	0.03	0.35	Ti+Zr : 0.15

Table 4 .

 4 Summary concerning the uncertainties of diagnostic methods.

	Parameters	Diagnostic method	Accuracy
	T	Boltzmann plot	8 % at r = 0 mm 25 % at r = 1.5 mm
	N e	Stark broadening	18 %
	%Fe	Composition computation	50 %


M Schnick

U Fuessel

M Hertel

M Haessler

A Spille-Kohoff

A B Murphy

MODELLING OF GAS-METAL ARC WELDING TAKING INTO ACCOUNT METAL VAPOUR

The most advanced numerical models of gas-metal arc welding (GMAW) neglect vaporization of metal, and assume an argon atmosphere for the arc region, as is also common practice for models of gas-tungsten arc welding (GTAW). These models predict temperatures above 20 000 K and a temperature distribution similar to GTAW arcs. However, spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to measurements of GTAW arcs, they have shown the presence of a central local minimum of the radial temperature distribution. This paper presents a GMAW model that takes into account metal vapour and that is able to predict the local central minimum in the radial distributions of temperature and electric current density. The influence of different values for the net radiative emission coefficient of iron vapour, which vary by up to a factor of hundred, is examined. It is shown that these net emission coefficients cause differences in the magnitudes, but not in the overall trends, of the radial distribution of temperature and current density. Further, the influence of the metal vaporization rate is investigated. We present evidence that, for higher vaporization rates, the central flow velocity inside the arc is decreased and can even change direction so that it is directed from the workpiece towards the wire, although the outer plasma flow is still directed towards the workpiece. In support of this thesis, we have attempted to reproduce the measurements of Zielinska et al for spray-transfer mode GMAW numerically, and have obtained reasonable agreement.

INTRODUCTION

Gas-metal arc welding (GMAW), also known as metal inert-gas (MIG) and metal active-gas (MAG) welding, is a long-established process that is used for joining metals. The arc is established between the workpiece and a continuously-fed wire anode. The plasma flow and the arc attachment at the wire have an important influence on the droplet formation and the heat transfer. Conversely, 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 the physical effects involved are necessary to reduce the number of experimental parameter studies required and to advance the development of welding techniques and equipment. By developing reliable numerical simulations of arc welding, we can better understand of the complex cause-and-effect-chains in involved in the process; further, the simulations can be applied as an engineering tool for better process design and visualization.

The GMAW models that have been developed previously differ in the effects being studied and the corresponding models [START_REF] Radaj | Schweißprozesssimulation: Grundlagen und Anwendung[END_REF]. The main focus of most models has been the prediction of droplet formation. State-of-the-art models include volume-of-fluid (VoF) multiphase modelling of a free surface [START_REF] Hirt | Volume of fluid method for the dynamics of free boundaries[END_REF]. 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 [START_REF] Wang | Modeling of the effects of surface-active elements on flow patterns and weld penetration[END_REF] or are combined with an arc model [START_REF] Haidar | An analysis of the formation of metal droplets in arc welding[END_REF][START_REF] Hu | Heat and mass transfer in gas metal arc welding. Part I: The arc[END_REF][START_REF] Hu | 'Heat and mass transfer in gas metal arc welding. Part II: The metal[END_REF] that has been developed and tested for gas-tungsten arc welding (GTAW) arcs. The arc models are based on magnetohydrodynamics (MHD) and assume a single-component fluid , usually argon, in a state of a local thermodynamic equilibrium (LTE). In [START_REF] Haidar | An analysis of the formation of metal droplets in arc welding[END_REF][START_REF] Hu | Heat and mass transfer in gas metal arc welding. Part I: The arc[END_REF][START_REF] Hu | 'Heat and mass transfer in gas metal arc welding. Part II: The metal[END_REF], the arc attachment at the wire and the workpiece is simplified by using an increased mesh size. Spille-Kohoff [START_REF] Spille-Kohoff | 'Numerische Simulation des ChopArc-Schweißprozesses[END_REF] neglected the droplet formation but used the unified sheath model developed by Lowke et al [START_REF] Lowke | HAIDAR: 'A simplified unified theory of arcs and their electrodes[END_REF] and Sansonnens et al [START_REF] Sansonnens | 'Prediction of properties of free burning arcs including effects of ambipolar diffusion[END_REF] for modelling the transient behaviour of a GMAW process in argon. All of these arc models predict arc temperature distributions and plasma flows similar to those that have been measured for GTAW arcs.

Peak temperatures of 20 000 to 23 000 K were calculated directly below the wire. The arc temperature at the centre line of the arc was always higher than 16 000 K, except in the near-electrode regions.

However, spectroscopic measurements of the plasma temperature in GMAW arcs contradict the predicted arc temperatures. The unpublished papers of Metzke and Schöpp [START_REF] Metzke | Spektralanalyse Metall-Lichtbogenplasma[END_REF] and Goecke [START_REF] Goecke | Auswirkungen von Aktivgaszumischungen im vpm-Bereich zu Argon auf das MIG-Impulsschweißen von Aluminium[END_REF] contain optical emission spectroscopy (OES) analyses of pulsed GMAW processes using copper and aluminium wires. The results indicated high local concentrations of metal vapour during the pulse. The metal vapour was not evenly distributed; a high concentration occurred in the arc core, while argon dominated in the fringes of the arc. The measured radial temperature distribution had a maximum at 13 000 K, which is located at the transition region between the regions in which metal vapour and argon dominated. In contrast to GTAW arcs, a central temperature minimum of 8000 K was observed. The recent measurements of Zielinska et al [START_REF] Zielińska | [END_REF][START_REF] Briand | Experimental investigations of the arc in MIG-MAG welding[END_REF] were performed for GMAW with a mild steel wire in spray transfer mode, and confirmed the results of [START_REF] Metzke | Spektralanalyse Metall-Lichtbogenplasma[END_REF][START_REF] Goecke | Auswirkungen von Aktivgaszumischungen im vpm-Bereich zu Argon auf das MIG-Impulsschweißen von Aluminium[END_REF]. The measured temperatures were compared with numerical predictions of [START_REF] Haidar | An analysis of the formation of metal droplets in arc welding[END_REF], and the large discrepancies were traced back to the influence of the metal vapour, which was neglected in the model.

The influence of the metal vapour on the arc behaviour has been analysed numerically mainly for GMAW [START_REF] Murphy | Modelling of thermal plasmas for arc welding: the role of shielding gas properties and of metal vapourJ[END_REF]. Tashiro et al [START_REF] Tashiro | Plasma properties of helium gas tungsten arc with metal vapour[END_REF] modelled a GTAW arc in helium, assuming a uniform concentration of metal vapour. The metal vapour was found to lead to higher temperatures and greater arc constriction. The GTAW models of Yamamoto et al [START_REF] Yamamoto | Numerical simulation of diffusion of multiple metal vapors in a TIG arc plasma for welding of stainless steel[END_REF][START_REF] Yamamoto | Metal vapour behaviour in thermal plasma of gas tungsten arcs during welding[END_REF] and Lago et al [START_REF] Lago | A numerical modelling of an electric arc and its interaction with the anode: Part I The two dimensional model[END_REF] included vaporization at the workpiece surface, and considered the mixing of metal vapour and argon. However, the simple treatments of diffusion used are likely to be inaccurate [START_REF] Murphy | A comparison of treatments of diffusion in thermal plasmas[END_REF], and did not allow the effects of subsequent demixing [START_REF] Murphy | Demixing in free-burning arcs[END_REF][START_REF] Murphy | Thermal plasmas in gas mixtures[END_REF] to be taken into account.

In contrast to GTAW, the metal vapour in GMAW results mainly from evaporation at the wire and droplet surfaces. To take this into account, we developed an MHD model of GMAW assuming a defined distribution of metal vapour at the lower side of the wire, which then streams into the arc [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF]. We used a vaporization rate of 1% of the wire feed rate, and presented results for arc currents between 100 A and 400 A. The workpiece was assumed to be flat, and the transient behaviour of droplet formation was neglected. A sophisticated model of diffusion and demixing was incorporated to ensure that the metal vapour transport was accurately treated. The calculations predicted a central temperature minimum, in accordance with the measurements described above.

As well as giving a more detailed description of the model than was possible in the earlier short communication, the current paper presents three important extensions. First, we investigate the influence of the net emission coefficient on the predictions of the model. This is a significant question, since published datasets of the net emission coefficient of iron vapour differ at least an order of magnitude. Second, we previously presented results for only one metal vaporization rate. This rate was deliberately chosen to be on the low side of those estimated in the literature so as to demonstrate that large changes could occur for relatively low metal vapour concentrations. However, higher rates are estimated for the GMAW spray transfer mode, and we find that increasing the vaporization rate has a major effect on the arc properties. Finally, we present a thorough comparison of our results with both the high-speed video images, and the spectroscopic measurements, that were presented by Zielinska et al [START_REF] Zielińska | [END_REF].

We begin the paper with an extended description of the model in section 2, and present some representative results in section 3. In sections 4 and 5, we present our calculations of the influence of the net emission coefficient for iron vapour and the influence of the vaporization rate, respectively. The comparison of our calculations with the measurements of Zielinska et al, and a discussion of the ramifications, are presented in section 6. Our conclusions are given in section 7.

ARC MODEL

The commercial simulation software ANSYS CFX is used for an axisymmetric steady-state model of an electric arc. We use the standard equations of computational fluid dynamics (conservation of mass, momentum and energy) and add the electromagnetic and radiative phenomena that occur in thermal plasmas [START_REF] Murphy | Demixing in free-burning arcs[END_REF]. The changes required are the addition of ohmic heating and radiative loss terms to the energy equation and a magnetic pinch term ( j×B) to the momentum equation; j is the current density and B is the magnetic field. Two further equations are required: an equation for current continuity

∇ •j = -∇ •(σ ∇ φ)=0
, where σ is the electrical conductivity and φ is the electric potential, and an expression for the magnetic field B = ∇ ×A, where the magnetic vector potential A is given by ∇ 2 A = -µ 0 j, µ 0 being the permittivity of free space. The effects of the sheaths are simplified by using a mesh size of 0.1 mm at the electrodes, as recommended by Lowke and Tanaka [START_REF] Lowke | LTE-diffusion' approximation for arc calculations[END_REF]. The pressure is equal to atmospheric pressure.

The configuration of the model and the distribution of the iron vapour mass source are shown in figure 1. The computational domain includes the contact tube BQRC, the wire ANOPB, the gas nozzle DEFG, the cathodic workpiece JKLM and the arc region. The arc region is the entire fluid domain, since we do not model droplet formation or flow in the weld pool. The diameter of the wire is 1.2 mm and the length of wire that sticks out beyond the contact tube and the arc length are each 5 mm. We define an argon fraction of 100% at the shielding gas inlet CD and at the opening GHIJ. The shielding gas inflow rate is 12 l/min. A boundary mass source of iron vapour at 3023 K, distributed as shown in figure 1, was included at the lower side of the wire to simulate the vaporization of the wire anode. The total mass flow of iron vapour is determined relative to the wire feed rate, which is chosen as 10 m/min. The plasma is assumed to be in local thermodynamic equilibrium (LTE), as is usual for models of welding arcs [START_REF] Murphy | Demixing in free-burning arcs[END_REF]. The thermodynamic properties were calculated for a chemical equilibrium composition, obtained using minimization of the Gibbs energy. The following species were considered: Ar, Ar + , Ar 2+ , Ar 3+ , Ar 4+ , Fe, Fe + , Fe 2+ , Fe 3+ , Fe 4+ and e -. The thermodynamic properties of the species were calculated using the atomic lines listed by Moore [24,25]. The transport properties, including the combined diffusion coefficients, were calculated using the Chapman-Enskog method for a temperature range of 300 -30 000 K [START_REF] Murphy | A comparison of treatments of diffusion in thermal plasmas[END_REF][START_REF] Murphy | Transport coefficients of argon, nitrogen, oxygen, argonnitrogen, and argon-oxygen plasmas Plasma[END_REF]. The required collision integrals for interactions between argon species were calculated using the methods given by Murphy and Arundell [START_REF] Murphy | Transport coefficients of argon, nitrogen, oxygen, argonnitrogen, and argon-oxygen plasmas Plasma[END_REF]. The collision integrals for Fe-Fe interactions were obtained using the Lennard-Jones [START_REF] Zielińska | [END_REF][START_REF] Hu | 'Heat and mass transfer in gas metal arc welding. Part II: The metal[END_REF] potential, those for Ar-Fe by averaging the Ar-Ar and Fe-Fe collision integrals, those for Ar-Fe + and Fe-Ar + interactions using the polarization potential, and for those for Fe-Fe + interactions using a combination of the charge exchange parameters determined from the work of Rapp and Francis [START_REF] Rapp | Charge exchange between gaseous ions and atoms[END_REF] and the polarization potential. All collision integrals for interactions between charged species were determined using the shielded Coulomb potential. Relevant details and references are given in [START_REF] Murphy | Transport coefficients of argon, nitrogen, oxygen, argonnitrogen, and argon-oxygen plasmas Plasma[END_REF]. The e --Fe collision integrals were determined by integrating the momentum transfer cross-section, which was obtained using the effective radius approximation for low collision energies, and the classical approximations for high collision energies [START_REF] Cherny | Physical and Chemical Processes in Gas Dynamics: Cross Sections and Rate Constants[END_REF].

The influence of the addition of iron vapour to argon on the molar weight, specific heat at constant pressure, viscosity, and the thermal and electrical conductivity is shown in Fig. 2. The addition of just 1% iron vapour has a very significant effect on the electrical conductivity at temperatures up to around 12 000 K. This is because iron atoms ionize at a much lower temperature than argon atoms. All the other properties are only weakly affected for 1% iron vapour, although changes become significant for 25% iron vapour.

-0 % Fe, ---1 % Fe ---25 % Fe, -• -• 50 % Fe, -•• -•• 75 % Fe, •••••••• 100 % Fe.
Radiation is treated using the net emission coefficient (NEC) model. 'Net' means the difference between emitted and absorbed radiation defined for a sphere with a uniform temperature. This simple radiation model allows accurate determination of the energy loss due to the radiation emission in the hottest arc regions, although it neglects the radiation transport and absorption in the colder gas [START_REF] Murphy | Advanced thermal plasma modelling High Temp[END_REF]. Net emission coefficients have been calculated by Menart and Malik [START_REF] Menart | Net emission coefficients for argon-iron thermal plasmas[END_REF], Aubrecht [31] and Cram [START_REF] Cram | Statistical evaluation of radiative power losses from thermal plasmas due to spectral lines[END_REF]. The coefficients are given as function of plasma temperature and pressure, for certain sphere radii. A selection of the data is shown in figure 3. The datasets of Aubrecht and Menart and Malik are in a good agreement. The values of Cram exceed both, especially for temperatures above 15 000 K. The values for a 1 mm sphere are almost an order of a magnitude lower than the values for 0 mm diameter, which neglect absorption. This is because of the large ultraviolet component, which is strongly self-absorbed. For our standard calculations, we have used the 1 mm sphere data of Menart and Malik, since we also assumed a 1 mm sphere for the NEC of argon [START_REF] Evans | Measurement of emission and absorption of radiation by an argon plasma[END_REF], and since this represents approximately the size of the high temperature region. For mixtures of argon and iron vapour, we calculate the net emission coefficient as a mole-fraction weighted average, as recommended by Cressault et al [START_REF] Cressault | Thermal plasma properties in gas or gas-vapour mixtures[END_REF]. Results are shown in figure 2. Clearly even a small amount of iron vapour greatly increases the net emission coefficient.

1.0E+00 1.0E+02 1.0E+04 1.0E+06 1.0E+08 1.0E+10 1.0E+12 1.0E+14 0 10000 20000 30000 temperature [K] radiation [W m -3 ] Aubrecht Fe (0mm) Aubrecht Fe (1mm) Menart Fe (0mm) Menart Fe (1mm) Cram Fe (0mm)
Evans Ar (1mm) Fig. 3 Different values of net emission coefficients or iron vapour calculated by Aubrecht [31], Menart and Malik [START_REF] Menart | Net emission coefficients for argon-iron thermal plasmas[END_REF] and Cram [START_REF] Cram | Statistical evaluation of radiative power losses from thermal plasmas due to spectral lines[END_REF] and the argon values of Evans and Tankin [START_REF] Evans | Measurement of emission and absorption of radiation by an argon plasma[END_REF] for comparison. Results are given for emission without absorption (0 mm) and for a 1 mm radius absorbing sphere.

The model implements mixing of argon and metal vapour due to turbulence and diffusion effects. Turbulence was included using the shear stress transport turbulence model [START_REF] Menter | Two-equation eddy-viscosity turbulence models for engineering epplications[END_REF]. This approach combines the K-ω model in regions near walls and the K-ε model in regions away from the walls. It has been rated the most accurate turbulence model (compared with two other two-equation models and a one-equation model) for a wide range of flows [START_REF] Bardina | Turbulence Modeling Validation, Testing, and Development[END_REF]. It was found that the inclusion of turbulence in fact has a negligible influence on the predicted arc properties. In particular, the turbulent diffusion coefficient, also known as eddy diffusion, was negligible compared with the ordinary diffusion coefficient, calculated as described below. The turbulent diffusion coefficient D t is given by D t = η t /(ρ Sc), where Sc is the turbulent Schmidt number, set to 0.9, and η t = ρK/ω is the turbulent viscosity (eddy viscosity). The spatial distributions of the turbulent kinetic energy K and the turbulent frequency ω are determined by solving the equations of the turbulence model.

The combined diffusion coefficient model [START_REF] Murphy | Thermal plasmas in gas mixtures[END_REF][START_REF] Murphy | Diffusion in equilibrium mixtures of ionized gas[END_REF] is used to treat diffusion of metal vapour relative to argon. It is based on a conservation equation for iron vapour mass fraction y Fe (defined as the sum of the mass fractions of the component species Fe, Fe + , Fe 2+ , Fe 3+ and Fe 4+ ):

0 ) ( div ) ( = + + ∂ ∂ Fe Fe Fe J y u t y r r ρ ρ (3)
where ρ is the mass density, u is the flow velocity, and the iron vapour diffusion mass flux is

T D E D p D x D m m n J
- + + = r r ρ (4)
The terms describe diffusion due to the gradient in the mole fraction of argon, due to the gradient in the total pressure, due to the electric field of the arc and due to the temperature gradient, respectively. The m Fe and m Ar are respectively the average molar masses of the argon and iron vapour heavy species and n is the number density. The D I FeAr are the combined diffusion coefficients, respectively due to mole fraction gradients, gradients in the total pressure, the arc electric field, and the temperature gradient. They are shown in figure 4, and are all clearly strongly affected by the proportion of iron vapour present.

RESULTS

Figure 5 shows predicted temperature, iron vapour mass fraction and velocity distributions for a 250 A arc and a vaporization rate of 1% relative to the wire feed rate. For the wire feed rate of 10 m/min and wire diameter of 1.2 mm, this corresponds to 0.015 g/s. The highest arc temperature of 18 000 K is predicted on 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 axis. These results are in qualitative agreement with those of the measurements referred to in section 1 [START_REF] Metzke | Spektralanalyse Metall-Lichtbogenplasma[END_REF][START_REF] Goecke | Auswirkungen von Aktivgaszumischungen im vpm-Bereich zu Argon auf das MIG-Impulsschweißen von Aluminium[END_REF][START_REF] Zielińska | [END_REF], but differ strongly from those predicted by previous models of GMAW, which neglect the effects of metal vapour [START_REF] Haidar | An analysis of the formation of metal droplets in arc welding[END_REF][START_REF] Hu | Heat and mass transfer in gas metal arc welding. Part I: The arc[END_REF][START_REF] Hu | 'Heat and mass transfer in gas metal arc welding. Part II: The metal[END_REF]. The calculated mass fraction of the iron vapour shows an accumulation in the arc core. This arises from the position of the iron vapour boundary source at the tip of the wire, and the strong downward plasma flow, driven by the magnetic pinch force. Another accumulation is visible at the arc edge between 2000 and 5000 K. Demixing effects lead to low mass fractions of iron vapour (y Fe < 1.E-3) between these two accumulations, as discussed below.

The calculated mass fraction distribution is consistent with the observed sharp optical separation of the arc core, which 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 wire) optically-bright arc fringes. The highest temperatures and current densities occur at the boundary of the arc core.

Results given previously [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF] showed that the separation between metal vapour in the central region and argon in the edge regions is stronger for higher arc currents. This is because the magnetic pinch force, which drives the downward convective flow from the wire electrode, increases with arc current. The central temperature minimum extends further downwards for higher currents, and is very weak for a current of 100 A.

Analyses of the sensitivity of the four driving forces of diffusion demonstrated that the temperature and the mole fraction gradients are the strongest sources of demixing. The effects of the electric field and the pressure gradient were very small and could be neglected. Diffusion due to mole fraction gradients leads to a flux of the chemical element with the lower ionization energy from higher-to lowertemperature regions. Diffusion due to temperature gradients drives a flux of the lighter chemical element in the same direction [START_REF] Murphy | Demixing in free-burning arcs[END_REF][START_REF] Murphy | Thermal plasmas in gas mixtures[END_REF]. In both cases, it is the iron vapour that will flow from higher-to lowertemperature regions. The off-centre radial maximum of the temperature leads to iron vapour diffusion fluxes both radially outwards and inwards, leading to the minimum in iron vapour concentration at a similar radius. However, a calculation performed neglecting the effects of demixing demonstrates that the high axial plasma flow velocity is the primary reason for the high iron vapour concentration in the arc core and the low fraction in the arc fringes.

We have previously investigated the relative importance of electric conductivity, strong radiative emission of metal vapour and the direct cooling of the arc due to vapour's relatively-low initial temperature of 3000 K [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF]. Calculations were performed with the iron vapour replaced by the same mass flux of argon at 3000 K, and with the same mass flux of an imaginary vapour with argon properties, but with the radiative emission coefficient or the electrical conductivity of iron vapour. Results are shown in figure 6. It was thereby demonstrated that the main mechanism for the decrease in temperature near the arc axis is the strong radiative emission of iron vapour. The direct cooling caused by the influx of vapour is relatively small, accounting for a decrease in the central temperature of only about 1000 K. The influence of the much higher electrical conductivity of iron vapour is very small, too. The influence of all other thermophysical properties on the central temperature was also tested and found to be similarly small. Finally, it was noted that the power required to vaporize the iron is negligible. It is less than 5% compared with the total radiated power.

INFLUENCE OF NEC DATA IN MODELLING METAL VAPOUR RADIATION

As noted in section 3, the main mechanism for the decrease in temperature near the arc axis is the strong radiative emission from the iron vapour. We also showed (see figure 3) that there are significant differences between the NEC data calculated by different researchers, and also according to the absorbing sphere radius. To investigate how important these differences are, we have performed calculations for different sets of NEC values. The results are shown in figure 7.

We find that the results for each set of NEC values are in general agreement. In particular, the radial temperature distribution shows an off-axis maximum for all the sets. However, there are major differences in the temperature values on the arc axis, especially directly below the wire. Calculations for the NEC values with a 1 mm absorbing sphere give a maximum temperature directly below the wire of about 16 000 K. The temperature on the arc centre line decreases gradually in the direction of the workpiece.

In contrast, the 0 mm NEC values give central arc temperatures below 7000 K and the maximum arc temperatures always occur off axis. The NEC values of Cram [START_REF] Cram | Statistical evaluation of radiative power losses from thermal plasmas due to spectral lines[END_REF] are higher than the others, particularly for temperatures above 15 000 K. However, since the temperature of the arc core is relatively low, the arc temperature, current density and axial velocity distributions are very similar to those obtained using the 0 mm NEC values of Menart and Malik [START_REF] Menart | Net emission coefficients for argon-iron thermal plasmas[END_REF] and Aubrecht [31].

As a consequence of the much lower central arc temperature for the 0 mm NEC values, the model predicts a near-zero current density in the arc core and on the lower side of the wire. This means the arc attachment is situated towards the sides of the wire. As a consequence, the current density and therefore the magnetic pinch force is lower, so the velocity is smaller. It is likely that the altered attachment will also affect the heat transfer to the wire, and therefore the rate and distribution of the vaporization. Unfortunately due to the simple sheath model we use, and the assumption of constant evaporation rate, we cannot model these effects. Nevertheless, they are of academic interest only, since the 1 mm NEC values are more realistic, since in reality there will be significant self-absorption of the emitted radiation.

INFLUENCE OF VAPORIZATION RATE

As discussed in section 3, the influence of the welding current (100 to 400 A) on arc temperatures and flow field has been examined previously [START_REF] Schnick | Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission[END_REF]. 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 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 [START_REF] Metzke | Spektralanalyse Metall-Lichtbogenplasma[END_REF].

Figure 8 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 close to negligible. Vaporization rates above about 3% cause a dramatic change in the arc flow field and the metal vapour distribution. As 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. The lower half of figure 8 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 GTAW and GMAW. Upwards flow can be obtained when the attachment to the lower electrode is forced to be constricted [START_REF] Heberlein | The anode region of electric arcs: a survey[END_REF]; 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. COMPARISION WITH EXPERIMENTAL RESULTS OF ZIELINSKA ET AL.

We have mentioned three recent publications containing results of arc temperature distributions in GMAW. The results of [START_REF] Metzke | Spektralanalyse Metall-Lichtbogenplasma[END_REF][START_REF] Goecke | Auswirkungen von Aktivgaszumischungen im vpm-Bereich zu Argon auf das MIG-Impulsschweißen von Aluminium[END_REF] are related to pulsed GMAW processes and aluminium or copper wires. The measurements of Zielinska et al [START_REF] Zielińska | [END_REF] 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 spraytransfer mode GMAW, the idealized wire tip shape and the assumption of a flat workpiece without weld pool 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 Zielinska et al [START_REF] Zielińska | [END_REF]. 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 high-speed 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

( ) ∫ + ⋅ ⋅ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = α α α d x tan 1 cos x h, rad x) Rad(h, 2 (3)
Here α is the angle between the axial direction and the radial direction to the integration point on the line of sight. Figure 9 shows a high-speed video image [START_REF] Zielińska | [END_REF] and the calculated radiation intensity. The radiative contributions of argon and metal vapour were calculated separately; the metal vapour radiation mainly contains lines at wavelengths below 650 nm, and is scaled transparent-to-blue, and the argon radiation mainly contains lines above 650 nm and is scaled transparent-to-red.

The figure demonstrates good agreement, especially for 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 [START_REF] Farmer | Rayleigh scattering measurements in a free-burning argon arc[END_REF][START_REF] Murphy | Laser-scattering measurements of temperature profiles of a free-burning arc[END_REF]. 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 figure 10 shows a quantitative comparison between the measured and numerically-predicted 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 conequence 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.

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. The model predicts a central minimum in the radial distributions of temperature and current density, which is caused by a high concentration of metal vapour in the arc core and its high radiative emission.

Since the radiative emission is critical in determining the arc properties, and since published net radiation emission coefficients differ by a large factor, it was important to investigate the sensititivity of the model's predictions to the dataset used. We found that simulations performed for the different datasets gave qualitatively similar results. However, net emission coefficients calculated without taking into account self-absorption gave much lower temperatures and current densities near the arc centre line. The strong ultraviolet component of the radiation from metal vapour means that this is unrealistic, and it is important to consider the presence of an absorbing sphere.

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.

While there is no direct measurement of the flow reversal, its existence has been given indirect support through the comparison of our modelling results with the arc temperature measurements and high-speed video images of Zielinska et al [START_REF] Zielińska | [END_REF]. The images showed a conical metal vapour region, in agreement with the predictions of our model. Further, good agreement was obtained with the position of the offcentre temperature maximum and reasonable agreement with the temperature values. These results suggest that our simulations are taking into account the main physical processes, and therefore that the prediction of the inner flow reversal is realistic.

Fig. 1

 1 Fig. 1 Configuration of the computational domain (left) and the iron vapour mass source (right)

Fig. 2

 2 Fig. 2 Influence of iron vapour mole percentage on the average molecular weight, specific heat, viscosity, thermal conductivity, electrical conductivity and net radiative emission coefficient of mixtures of argon and iron vapour: --0 % Fe, ---1 % Fe ---25 % Fe, -• -• 50 % Fe, -•• -•• 75 % Fe, •••••••• 100 % Fe.

Fig. 4

 4 Fig. 4 Combined diffusion coefficients of iron vapour in argon for different iron vapour mole percentages: --1 % Fe, ---25 % Fe, -• -• 50 % Fe, -•• -•• 75 % Fe, •••••••• 99 % Fe

Fig. 5

 5 Fig. 5 Calculated values of temperature (right), iron vapour mass fraction (left), and the gas flow (vectors) in a 250 A arc with a vaporization rate of 1% relative to the 10 m/min wire feed rate.

Fig. 6

 6 Fig. 6 Influence on the temperature and current density of iron vapour, the same mass flux of argon and the same mass flux of imaginary gases with the properties of argon except for the radiative emission coefficient or the electrical conductivity of iron vapour. Results are for a position 1.5 mm above the workpiece in a 250 A arc [22].

Fig. 7

 7 Fig. 7 Influence of different net emission coefficient data in a 250 A arc with 1% vaporization rate relative to the wire feed rate of 10 m/min (top: temperature and flow velocity vectors (right) and metal vapour mass fraction (left) and bottom: radial distributions of temperature, current density and velocity at a position 1.5 mm above the workpiece).

Fig. 8

 8 Fig.[START_REF] Lowke | HAIDAR: 'A simplified unified theory of arcs and their electrodes[END_REF] Influence of the vaporization rate, relative to wire feed rate of 10 m/min, in a 250 A arc using the 1 mm NECs of Menart and Malik (top: the temperature distribution and flow vectors (right) and metal vapour mass fraction distribution (left), images are scaled and sized as in figure7and bottom: radial distributions of temperature, current density and downward velocity at a position of 1.5 mm above the workpiece).

Fig. 9

 9 Fig. 9 Comparison of a high-speed video images (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.

Fig. 10

 10 Fig. 10 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.

ACKNOWLEDGEMENTS

This work was partially supported by DFG (FK: FU 307/5-1), which is gratefully acknowledged. Many thanks to Dr. J. J. Lowke of CSIRO Materials Science and Engineering, Australia, Prof. M. Tanaka of Joining and Welding Research Institute, Osaka University, Japan and Dr. D. Uhrlandt of Leibniz-Institut für Plasmaforschung und Technologie, Germany for the discussions of these results.