63rd Annual Assembly & International Conference of the International Institute of Welding 11-17 July 2010, Istanbul, Turkey

Numerical optimization of gas metal arc welding torches using ANSYS CFX

M. Dreher1,a, U. Füssel1 and M. Schnick1 1 Dresden University of Technology, Dresden, Germany

amichael.dreher@tu-dresden.de

Abstract

Gas metal arc (GMA) welding of aluminium, high alloyed steel or titanium requires a cover of shielding gas in order to provide a low PPM concentration of oxygen at the joint. Consequently, it is necessary to be able to describe and to analyse the flow of shielding gas. Next to the limited possibilities of flow measurement there is the option of numerical simulation. The paper presents numerical investigations in the shielding gas flow and atmosphere concentration. The model used is based on ANSYS CFX and contains effects of magneto hydrodynamic (MHD), turbulence and diffusion effects depending on temperature. The model is validated by Particle Image Velocimetry (PIV), Schlieren technique and gauging the oxygen. In the paper the formation of turbulences at the gas distributor and the influence of different gas temperatures on diffusion effects are demonstrated. Furthermore we show the dependency of the current profile (pulse welding) used on the shielding gas flow. The findings enable the development of new torch design features.

Keywords: GMA welding, Shielding gas, Oxygen, Numerical Simulation, Welding torch

1. Introduction

GMA welding is used as a semi-automatic or automatic arc welding process in many applications. The arc is burning between a continuous and consumable wire electrode and the work piece. The aim is a clean, defined and reproducible process. The shielding gas used has a lot of tasks, for example the cooling of the torch, the affecting of the arc and the protection of the melt from oxidation. Particularly the joining of aluminium, high alloyed steels or titanium requires an excellent cover of shielding gas to provide a low PPM concentration of oxygen. However, turbulent and transient flow characteristics cause often a mixture of shielding gas and atmosphere. The consequences are splatters, oxidation, soot or pores, which reduces the economic efficiency because of costly rework. In summary,

the shielding gas flow is so important, that it is essential to analyse it in detail and in the context with other process parameters and the geometry of the welding torch.

Until now, the design of current GMA welding torches and developments concerning their shielding gas flow are mostly investigated by welding experiments [1-3]. Besides this there are options to visualize the flow, e.q. the Particle Image Velocimetry [4, 5] and the Schlieren technique [6]. Both methods enable the characterisation of the flow field but they cannot quantify the atmosphere contamination caused by turbulences, eddy formations or diffusion processes. However, there is the limited possibility to measure the content of oxygen in the process region with the lambda-sensor principle. In this way quantitative information about the cover of shielding gas are possible, but the reason of a high PPM concentration is still unexplained. In summary, the conventional methods of flow field analyses and measurements cannot give comprehensive findings about the shielding gas flow. The poor visibility complicates the application of diagnostics caused by the covered areas inside the welding torch, temperatures up to 20,000 K inside the arc, the high radiation and the electromagnetic field.

The new approach of the numerical simulation of welding arcs enables an enhanced characterisation of shielding gas flow and diffusion effects. The modelling affords the description of complex physical processes with a high local and temporal resolution. Therefore it is possible to describe the shielding gas flow inside the welding torch, where no measurement is adaptive so far. Although recent simulation models of GMA welding processes include the arc [7-9] as well as the drip transition [10] or metal vapour [11], all of them assume to have a perfect cover of shielding gas within an atmosphere of 100 % argon. Otherwise there are numerical models, which describe the cover of shielding gas, but reduce the geometry of the welding torch [12] or simplify the physical arc effects [13]. This article introduces a numerical model for the analyse a welding torch relating to the shielding gas flow. Based on the results there are advices for construction and appliance.

2. Modelling

The software ANSYS CFX 12.0 was used for the numerical simulation of the shielding gas flow by arc welding. The analyses of the shielding gas flow include the regions inside the GMA welding torch and the free jet between the gas nozzle and the work piece. Two different geometrical models were used. An axially symmetrical 6°-model is used for transient type simulations of pulsed GMAW and for analyses of the influence of special predetermined profiles of the shielding gas flow in the lower nozzle region (gradient flow). This model is characterised by low numerical complexity and a short computing time but assumes an axially symmetrical flow. The 6°-model is deficient to represent the shielding gas flow in the complex geometry of actual welding torches. A 90° mirror symmetrical model was used for this reason. It comprehends the gas distribution in the upper part of the welding torch. It is possible to realize an extensive analyse of the shielding gas flow and of the appearance of turbulences caused by the gas distribution. Otherwise this model needs more computing time. A hexahedral mesh was used in both geometries. The mesh quality has been enhanced by a manual refinement according to the computed gradients.

The geometry used is shown in Fig. 1. A three-component fluid of argon, air and metal vapour was defined with a variable concentration of them. The density, specific heat capacity, dynamic viscosity and thermal conductivity of the components are described in dependence of temperature and the concentration of the components [14]. In addition to the conservation equation for mass, momentum and energy, the diffusion is solved by a transport equation and with a temperature depending kinematic diffusivity of argon-air-mixtures [15] and argon-metal vapour-mixtures. The equations for the arc model, the metal vaporization and the detailed boundary conditions are described in previous papers [11, 16].

Figure 1. Geometry of the axially symmetrical 6°-model

3. Results

3.1. Flow of shielding gas in the upper part of the welding torch and construction advices to avoid turbulences

The typical design of standard GMA welding torches leads to the assumption that the flow characteristics inside the torch evokes turbulent circumstances, which also affect the cover of shielding gas at the work piece. To approve this assumption the flow of shielding gas was analysed in numerical simulation with the two geometrical models which have been described above. Fig. 2 shows the comparison of a measurement and the numerical results.

Figure 2. Validation of the numerical models (6°-model and 90°-

model) by measurement without the arc and 10 l/min The results of the 6°-model demonstrate lower values of oxygen as the measured once in the outer areas. The contamination of the shielding gas by the atmosphere due to turbulences is not represented correctly. Thus it is proven that the geometrical complexity of the torch is simplified too much in the most numerical published investigations. The assumption of an axially symmetrical and steady state low turbulence shielding gas flow in the shielding gas nozzle is not suitable and strong moving eddy formations have to be taken into account.

To explain the high contents of oxygen in the measurements it is necessary to analyse the flow field inside the welding torch regarding the flow effects at the gas distributor. Fig. 2 shows the shielding gas flow in and near a borehole of the gas distributor. The numerical model predicts high velocities and the formation of several eddies in this area, in particular down stream the boreholes. The high velocities and the turbulences affect the flow field between the gas nozzle and the work piece. Thus there is a higher contamination of the shielding gas by the atmosphere in practise as it is predictable by an assumption of a steady and axially symmetrical model.

63rd Annual Assembly & International Conference of the International Institute of Welding 11-17 July 2010, Istanbul, Turkey

Figure 3. Gas distribution in the upper part of the welding torch (mass flow rate 10 l/min, without arc)

Figure 4. Simulation results of the shielding gas cover with different mass flow rates considering the flow above the gas distribution (90°-

model, without arc) Fig. 3 shows that the contamination increase due to higher mass flow rates. It is caused by the appearance of strong turbulence effects, which are transported downstream and affect the gas shield. Thus it is demonstrated, that a specific mass flow rate enables an optimal shielding gas cover for each torch design.

Based on these results some construction advices can be given. The function of the gas distributor is the guaranty of an equalised flow at the whole circumference of the gas nozzle. Thus the boreholes of the gas distributor are quite small in actual designs. However, it realizes an increased pressure above the boreholes and a compensation of inflow asymmetries. Therefore an equal flow at the whole circumference is realized but high velocities also induce turbulences and eddies. Otherwise, bigger boreholes support a steady flow field inside the lower part of the welding torch but the flow can be out of rotational symmetry. Thus the gas distribution should be realized above these boreholes.

In Fig. 4 two construction possibilities are shown. The first possibility shows some slots, which realize the equal symmetrical distribution of the shielding gas above the gas distributor. In the upper part of the slots the cross-sections are really small. Because of these small cross-sections and the resultant pressure the mass flow is nearly the same in every slot. The cross-sections in the slots should be designed gradually bigger in the lower regions. In this way the flow profile is equal and the boreholes could be bigger and more.

Figure 5. Construction advices to avoid turbulent circumstances in the shiedling gas flow

If the assembly of such slots is not possible and the inflow is unsymmetrical in the conventional way, there is a second suggestion for construction. A sinter-material-insert distributes the gas around the whole circumference. Turbulences and eddies are dissipated in the small spaces. Depending on the material or the thermal insulation of the sinter-insert there are options of cooling the welding torch with the shielding gas.

3.2. Shielding gas flow at the workpiece and influence of the current, the temperature and gradient flow profiles

The previous results according the appearance of turbulences are related to analyses without the arc and a mirror symmetrical 90°-modell, which includes the geometry of the gas distributor. Using the assumptions of a laminar and axially symmetrical flow analyses of the gas shield at the workpiece were made and are described following.

Fig. 7 shows the vector plot of the free jet of the shielding gas with and without a welding arc. Due to the arc effects, the gas is accelerated towards the axis of the arc, in particular near the arc attachment at the wire. Thus an axial plasma jet is caused which turns above the work piece outwards and streams parallel to the surface.

Figure 6. Shielding gas cover in dependency of the arc (6°-model, mass flow rate 10 l/min, arc 200A)

Figure 7. Content of oxygen at the workpiece in dependency of the current (6°-modell, mass flow rate 10 l/min)

Figure 8. Combined ordinary diffusion coefficients of mixtures of

argon and air [15] The plasma velocities rise clearly up to several hundreds meter per second. Thus the dead water region, which was obtained without an arc, becomes smaller and displaces below the contact tube. The calculation demonstrates that these results become more significant by increasing the welding current. Assuming a laminar flow in the welding torch a sufficient gas shield could be enhanced for high currents by increasing the gas flow rate. However, it is necessary to eliminate turbulences effects at the gas distributor using the design advised above. Diffusion effects in the outer regions of the arc decrease the gas shield quality especially in the area above the workpiece.

As shown in Fig. 8, the diffusion depends on local fluid temperature [15]. The increase of the fluid temperature of a few hundred Kelvin induces a multiplication of diffusion effects and consequently an increased atmosphere contamination of the shielding gas near the arc. Regarding these dependency in the physical model it can be shown that the temperature of the incoming shielding gas affects the concentration of oxygen at the workpiece only marginal. Due to the higher temperatures and constant mass flow rate the velocity of the shielding gas increases. This effect compensates the higher, temperature depending diffusion of oxygen and so the heating of the gas inside the torch is not extensive deteriorating the cover of shielding gas at the workpiece. Fig. 9 shows the marginal influence of high gas temperatures for low mass flow rates as well as for higher mass flow rates.

Figure 9. Content of oxygen at the workpiece in dependency of the gas temperature and the mass flow rate (6°-model, 200A)

Thus the usage of small gas cooled welding torches is not a big disadvantage for materials and processes which need low contents of oxygen, although the gas temperature is considerably higher as in water cooled torches.

These contaminations caused by high currents and temperatures are important for an improved understanding of the gas shield quality in pulsed GMAW. The shielding gas is accelerated towards the axis during and after the peak current. An eddy is induced during the upslope of the current and moves outwards, see Figure 10.

Figure 10. PIV measurement and simulation of a pulse process (6°-model, mass flow rate 10 l/min, vaporization 1% of wire material)

63rd Annual Assembly & International Conference of the International Institute of Welding 11-17 July 2010, Istanbul, Turkey

Figure 11. Content of oxygen (left side) and temperature (right side) during a 400 A pulse-process (6°-model, mass flow rate 10 l/min)

The described eddy is a hot mixture mainly consisting of argon and metal vapour at maximum of 15 mol %. After the current-peak this gas is still hot and the shielding gas is contaminated due to diffusion processes. In summary the shielding gas follows the pulse profile of the current and influence the welding process in a negative way. It is essential to develop methods to counteract these problems.

A new approach improving the gas shield quality is the use of special profile of the shielding gas flow between the gas nozzle and the contact tube. An advantage of the numerical simulation is to consider these as a boundary condition. Thus the suitability of these profiles can be proven in principle without experiments and expensive constructions of welding torches.

Fig. 12 shows the gas shield quality of three defined flow profiles and different currents. The maximum of the velocity is near the wall of the contact tube at first, the second case describes a symmetrical flow between the contact tube and the gas nozzle and the last case realise the maximum of the flow velocity near the wall of the gas nozzle. In all cases the best shielding gas cover was realised with a velocity maximum near the contact tube. Especially for low and middle welding currents there is the advantage of reducing the consumption of shielding gas. The appliance of special flow profiles with a pulse-process were analysed at time.

Figure 12. Content of oxygen at the workpiece in dependency of the gradient flow profile, the mass flow rate and the current (6°-model)

Furthermore the position of the gas nozzle relating to the contact tube is important for contamination processes with the atmosphere. It is proven that the concentration of oxygen at the workpiece increases with rising distances between the gas nozzle and the workpiece. This result was also validated with gauging the oxygen at the workpiece without an arc. With increasing currents the dependency of contamination processes by the gas nozzle position becomes lower. This result causes the conclusion that the distance between the workpiece and the gas nozzle could be enlarged without degrading the shielding gas cover significantly if the mass flow rate is adequate. But an important fact is that all of these analyses were done with the condition of a laminar inflow of the shielding gas.

Figure 13. Content of oxygen at the workpiece in dependency of the gas nozzle position, the mass flow rate and the current (6°-model)

4. Conclusion

The article describes the usage of a numerical model for the visualisation and the optimisation of the shielding gas flow inside the welding torch and in the free jet between the gas nozzle and the work piece. It is shown, that the flow of the shielding gas cannot be assumed to be axially symmetrical. Thus the atmosphere contamination of the shielding gas is caused by turbulences and eddies. These cannot be demonstrated completely using an axially symmetrical model. Thus a 90°-model was used. It was shown that high flow velocities at the gas distributor or rather the resulting turbulences affect the flow conditions at the work piece. Small boreholes of the gas distributor support this effect.

The influence of the arc on the flow field and for the gas shield quality was analysed for constant and pulsed current GMAW processes. The arc stabilizes the shielding gas flow but also induces a contamination of the shielding gas if the flow rate of the shielding gas is too low or also with high currents. It is proven that diffusion affects the shielding gas cover and has to be compensated due to a higher gas flow rate. In practice this is eminently important for small gas cooled welding torches. To avoid eddies and turbulences in succession of higher mass flow rates the construction of such torches should include the advices given in the first part of this paper. Furthermore it is shown that the current influences the shielding immediately and intense in pulse current GMAW.

Generally it is advisable to use water cooled welding torches with preferably cold shielding gas for materials and processes which need low contents of oxygen. A new approach to counteract the contamination processes is the appliance of gradient flow profiles. It was proven that high flow velocities near the contact tube enhance the shielding gas cover at the workpiece.

The measurement methods and the numerical model will be improved for the use of GMA welding in the further progress.

Acknowledgement

This paper was supported by the AiF (KF 15.871). The described results are part of the AiF-DFG-cluster project: arc welding – physics and tool, application development.

References

[1] Bürkner C., "MIG/MAG Schweißbrenner", Puplication DE 103 14 278 A1.

[2] Bürkner G., "Robotic welding torch", Patent EP 0 850 717 B1, 07.03.2001.

[3] Kusch M., "MSG-Schweißen mit gepulster Schutzgaszufuhr", DVS-Berichte, Vol. 240(2006), DVS-Verlag, Düsseldorf, 2006.

[4] Zschetzsche, J., "Diagnostik von Schutzgasschweißprozessen", Dissertation, Technische Universität Dresden, Verlag der Wissenschaften, Dresden, 2007.

[5] Füssel, U., Zschetzsche J., Zosel, J. and Guth, U., "Strömungsmessung in Lichtbogen- und Plasmaprozessen", Stiftung Industrieforschung, Forschungsvorhaben S 559, Abschlussbericht, 2005.

[6] Settles, G. S., "Schlieren and Shadowgraph Techniques", Springer Verlag, 2006.

[7] Lowke, J.J., Morrow, R. and Haidar, J., "A simplified unified theory of arcs and their electrodes", Journal of Physics D: Applied Physics, Vol. 30(1997), pp. 2033-2042, 1997.

[8] Wendelstorf, J., "Ab initio modeling of thermal plasma gas discharges (electric arcs)", Dissertation Technische Universität Carolo-Wilhelmina zu Braunschweig, Braunschweig, 2000.

[9] Gleize, A., Gonzales, J.J. and Freton, P.: "Thermal plasma modeling (Topical Review)", Journal of Physics D: Applied Physics, Vol. 38(2005), pp. R153-R183, 2005.

63rd Annual Assembly & International Conference of the International Institute of Welding 11-17 July 2010, Istanbul, Turkey

[10] Wang, F., Hou, W.K., Hu, S.J., Kannatey-Asibu, E., Schultz, W.W. and Wang, P.C., "Modelling and analysis of metal transfer in gas metal arc welding", Journal of Physics D: Applied Physics, Vol. 36(2003), pp. 1143-1152, 2003.

[11] Schnick, M., Hertel, M., Füssel, U., Hässler, M., Spille-Kohoff, A. and Murphy, A.B., "Numerical Investigations of the influence of Metal Vapour in GMA Welding", Schweißtechnik – Faszination in Forschung und Fertigung. Dresdner Fügetechnisches Kolloquium 2009, Technische Universität Dresden, Dresden, 2009.

[12] Füssel, U., Schnick, M. and Zschetzsche, J., "Simulation and measurement of plasma and gas flows in plasma arc welding and cutting", Mathematical Modeling of Weld Phenomena 8, Verlag der Technischen Universität Graz, pp. 37-48, 2007.

[13] Cooper, P., Godbole, A. and Norrish, J., "Modeling and Simulation of Gas Flows in Arc Welding – Implications for Shielding Efficiency and Fume Extraction", Proc. on the 60th Annual Assembly of the International Institute of Welding, Dubrovnik (Croatia), 2007.

[14] Murphy, A.B., "Thermal plasmas in gas mixtures", Journal of Physics D: Applied Physics, Vol. 34(2001), pp. R151-R173, 2001.

[15] Murphy, A.B., "Transport Coefficients of Air, Argon-Air, Nitrogen-Air, and Oxygen-Air Plasmas", Plasma Chemistry and Plasma Processing, Vol. 15, No. 2, pp. 279-307, 1995.

[16] Dreher, M., Füssel, U., Schnick, M., "Simulation of shielding gas flow inside the torch and in the process region of GMA welding", Mathematical Modelling of Weld Phenomena 9, Verlag der Technischen Universität Graz, Graz, 2009