1

SIMULATION OF SHIELDING GAS FLOW

INSIDE THE TORCH AND IN THE PROCESS

REGION OF GMA WELDING

M. DREHER*, U. FÜSSEL* and M. SCHNICK* * Technical University of Dresden, Dresden, Germany

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. However, it is very difficult to observe the flow of shielding gas during arc welding processes. Responsible for this are the covered areas inside the blowpipe, temperatures up to 20,000 K, the high radiation of the arc and the electromagnetic field. Although recent simulation models of GMA welding processes include the arc as well as the drip transition, they assume a perfect cover of the shielding gas within an atmosphere of 100% argon. This paper introduces a model that describes the shielding gas flow and visualizes the causes and effects, which lead to atmosphere contaminations. The simulation model contains the effects of the arc apart from turbulence models and diffusion effects. It displays the flow conditions within the blowpipe as well as the resulting cover of shielding gas at the process part. The geometrical influences of various blowpipe components are simulated. The model was validated by the lambda sensor principle. The gas, which has to be analysed is taken directly from the arc.

INTRODUCTION

GMA welding is used as a semi-automatic or automatic arc welding process in many applications. In this process 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 definition 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 a mixture of shielding gas and atmosphere. The consequences are splatters, oxidation, soot or pores, which reduces the 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 context with other process components and the geometry of the welding torch.

The construction parameters of current GMA welding torches and developments associated with the shielding gas flow are mostly determined by welding experiments [1-3].

Besides this construction know how there are options to visualize the flow. In [4] the use of the Laser-Doppler-Anemometrie (LDA) and Particle Image Velocimetry (PIV) with GMA welding was tested. It is possible to analyse two-dimensional flow fields with high spatial and temporal resolution in this way. The measurement-setup is complex and relative costly. Statements concerning diffusion and the concentration of oxygen are not feasible. Furthermore solid, high melting particles are needed, which should be as small as possible. They must not interact with the welding process by producing chemical reactions or deflection due to the Lorentz force. PIV measurements were done at short arc, impulse arc and spray arc welding processes [4-5]. In addition to PIV the flow field can be described by the Schlieren technique [6]. Light beams are deflected caused by differences of density. The advantages are the low costs and the easy handling of the measurement setup. The method afford the qualitative appearance of turbulences, but can not quantitative display the complex three-dimensional shielding gas flow of GMA welding. According to the methods of visualisation there are limited possibilities of measurement. It is feasible to detect the concentration of oxygen in the arc by the lambda-sensor principle. The experimental determination of the oxygen content in the arc is based on the arc pressure measurement setup of [7]. Instead of a pressure sensor a small pump is used. A small partial flow is extracted which is subsequently analysed by a broadband lambda probe. In this way quantitative information about the cover of shielding gas are possible, but the reason of a high PPM concentration is furthermore unexplained. In summary, the conventional methods of flow field analyses and measurements cannot give comprehensive findings about the shielding gas flow and the poor visibility complicates the application of diagnostics. Reasons for the described problems are the covered areas inside the welding torch, temperatures up to 20,000 K inside the arc, the high radiation and the electromagnetic field.

These problems of diagnostics can be avoided by the multifaceted options of the numerical simulation. It 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 [8-10] as well as the drip transition [11] or metal vapour [12], 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 [13] or simplify the physical arc effects [14]. As an example a model was used in [14], which allows statements of the shielding efficiency and fume extraction of a GMA welding torch, but the arc is modelled as a frustum regarding only the thermal influence. Furthermore it is unclear, which kind of turbulence model was used in this simulation. SPEISEDER [15] compares PIV measurements with calculated flow fields in ANSYS CFX. The good accordance between the measurement and the model even with a simple arc and adiabatic electrodes model and without metal vaporisation can be shown. But only qualitative statements about turbulent and diffusive contamination of atmosphere have been possible. However, there are only few simulation models, which describe the flow and the cover shielding gas regarding to the complex geometry inside the welding torch, the physical principles of the arc and the transient turbulences during the process.

MODELLING

The commercial software ANSYS CFX 11.0 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. With a rotational symmetrical 6°-model the influence of special predetermined flow profiles (gradient flow) can be analysed. This type of geometry is characterised by low numerical complexity and a short computing time but assumes an axially symmetrical flow. The 6°-model is consisting of a mesh of 33,400 nodes for the fluid. To represent the shielding gas flow in the complex geometry of actual welding torches a 6°-model is deficient. For this reason a 90°-mirror-symmetric-model was used, which 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 because of the size of 823,500 nodes for the fluid. A hexahedral mesh was used in both geometries which was fined in relevant regions according to the computed gradients.

Fig. 1 Geometries of computing grids

The stick out of the wire is 10 mm and the distance between its tip and the work piece is

5 mm. The diameter of the wire is 1.2 mm. A two-component fluid of argon and air was defined with a variable concentration of them. The density, specific heat capacity, dynamic viscosity and thermal conductivity of argon are described in dependence of temperature and the concentration of the components [16]. In addition to the conservation equation for mass, momentum and energy, the diffusion is solved by a transport equation (1) and with a temperature depending kinematic diffusivity of argon-air-mixtures [17].

Diffusion

YonargYonargonarg

S+)YgradDρ(div=)Yuρ(div+t∂

)Yρ(∂ v (1)

ρ Density of the mixture depending on the calculated concentration

onargY Mass fraction of argon

t Time

ur

Velocity

YD Binary diffusion coefficient depending on temperature

YS source-term (unattended)

The applicability of different two-equation turbulence-models (k-Epsilon, k-Omega and SST) was analysed. The equations of these turbulence models are named in [18]. The SST model was used for the present results, because of a good agreement with the results of validation measurements by gauging the oxygen at the work piece. The arc was modelled with an additional MHD model for the arc column and a LTE assumption for the near electrodes regions [19]. The MHD model combines the equations of the fluid mechanics (Navier-Stokes equations) with the Maxwell’s equations of the electro-magnetic [20]. The following governing equations of the electro-magnetic are solved:

Charge conservation

0=jdivr

(2)

Ohm’s law

Φgradσ=jr

(3)

Magnetic potential

=)Agrad(divr

- jµ0

r (4)

Magnetic field

Arot=Brr

(5)

The influence of the Lorentz force on the velocity is considered by a source term in the

momentum equations. The resistive heating is regarded by a source term in the energy conservation equation [20].

Lorentz force

B×j=f L

rr (6)

Resistive heating

σ

j=S

2

RH (7)

jr

Electric current density

σ Electric conductivity

Φ Electric potential

Ar

Magnetic vector potential

0µ Magnetic permeability

Br

Electro-magnetic field

Table 1 shows the boundary conditions used for the 6°-model. The boundary conditions

of the 90°-model are limited of the flow conditions at time. All Interfaces between the Fluid and the Solids are defined as no-slip-walls with conservative interface flux boundary conditions.

Table 1 Boundary conditions 6°-model with MHD

RESULTS

For these analyses the numerical model was validated by the Schlieren technique and by gauging the oxygen. The comparison of the Schlieren pictures and the numerical simulation show good agreement. The shadows of the Schlieren picture indicate the bell-shaped form of the arc and can be used to localize the isothermal line of circa 6000 K.

Fig. 2 Comparison of density gradients with Schlieren-technique and numerical simulation.

Fig. 3 shows the comparison between the measured oxygen content and the results of

simulation of a GMA welding torch without the arc. It was shown that the gas distributor is an important component of the shielding gas flow and it has to be analysed deepened.

Fig. 3 Validation of the numerical models (6°-model and 90°-model) by measurement without the arc

State of the art is to assume, that the flow of shielding gas is axial symmetrical. However,

the typical construction of standard GMA welding torches leads to the assumption that the flow characteristics inside the torch evokes turbulent circumstances in the borehole, which also affect the cover of shielding gas at the work piece. To approve this assumption the flow of shielding gas was analysed with two different geometrical models in numerical

simulation. The results of the 6°-model show lower values of oxygen as the measured contents in the outer areas (Fig. 3). The reason of this effect seems to be the unavailable feasibility to represent three-dimensional flow fields and eddies by using a rotational symmetrical grid without regarding to the influence of the gas distribution. The contamination of the shielding gas by the atmosphere due to turbulences cannot be completely demonstrated. To explain the higher contents of oxygen in the measurements it is necessary to analyse the flow field inside the welding torch regarding the oncoming flow over the gas distributor. That is why a 90°-model was used for these analyses. Fig. 4 shows the shielding gas flow in and about a borehole of the gas distributor. The results of the numerical simulations predict high velocities and the formation of several eddies in the regions below these boreholes. The different velocities and turbulences affect the flow field between the gas nozzle and the work piece. In consequence there is a higher contamination of the shielding gas by the atmosphere as with the assumption of a steady and axial symmetrical process. The 90°-model show a good agreement between the measured content of oxygen and the result of numerical simulation because of regarding these effects in the upper part of the welding torch.

Fig. 4 Gas distribution in the upper part of the welding torch (argon 10 l/min, without arc)

The content of oxygen increases clearly in the outer areas. It is proved that the complexity

of the shielding gas flow is oversimplified in the most numerical investigations because it is assumed that the gas distribution with GMA welding processes is nearly axial symmetrical and turbulences are not taken into account in this way. Based on this knowledge it is necessary to analyse the appearance and the characteristics of the turbulences depending on the mass flow rates and the geometry of the welding torch. Analyses with increased mass flow rates show that the turbulences increase clearly. The result is a decrease of the shielding gas cover, although the mass of the shielding gas increase. Sensitive analyses show that turbulences inside the welding torch caused by high mass flow rates of shielding gas can be reduced by bigger diameters of the borehole. The influence of the borehole diameter decrease with the distance to the gas distributor. The difference between maximum and

averaged velocities at the outlet of the gas nozzle becomes apparently only with high mass flow rates. Selected parameters are displayed in Fig. 5. The Fig. 6 shows the streamlines inside the welding torch and the cover of shielding gas at the work piece depending on the mass flow rate and the diameter of the boreholes of the gas distributor.

Fig. 5 Influence of different mass flow rates and diameters of the gas distributor boreholes depending on the selected flow parameters

Fig. 6 Streamlines inside the welding torch and cover of shielding gas at the work piece with different mass flow rate and diameters of the borehole

Based on these results some construction advices can be given. The aim of the gas distributor is an equal flow at the whole circumference of the gas nozzle. To reach this aim the boreholes of the gas distributor are quite small in actual designs. In this way an increased pressure is realized, which compensates the unsymmetrical inflow. Therefore an equal flow at the whole circumference is realized, but there are also high velocities in the boreholes of the gas distributor, which effect turbulences and eddies in the down flow of the shielding gas. Bigger boreholes support a steady flow field inside the lower part of the welding torch but the flow has to be constant for a rotational symmetry. That is why the gas distribution should be done above these boreholes. With regard to the turbulence there is a limit of the mass flow rate, where the flow field change into a turbulent streaming. Turbulences, which cannot be avoided by bigger boreholes, must be steady by a longer distance between the gas distributor and the outlet of the gas nozzle.

The results of describing the shielding gas flow inside the welding torch were done neglecting the influence of the arc so far. At first the arc model was integrated in the 6°-model. Fig. 7 shows the velocities of the flow field and the concentration of oxygen at the work piece. Maximum velocities of 2-5 m/s were predicted in the free jet of shielding gas without the arc. Furthermore a big dead water region exists between the work piece and the contact tube.

After implementation of the arc model, the gas is accelerated towards the axis of the arc and following it streams parallel to the work piece. The velocities rise clearly up to several hundreds meter per second in the arc. The dead water region below the contact tube and the gas shield becomes smaller. With high currents or stick out length these effects become more significant. A high increase of the oxygen content was predicted with low levels of shielding gas. The supply of shielding gas is unsatisfactory and so there are contaminations because of diffusion processes, which are caused by the high diffusion coefficient at the high temperatures inside the arc.

Fig. 7 Cover of shielding gas in dependency of the arc and argon mass flow

CONCLUSION

The article introduces a numerical model for the visualisation and optimisation of the shielding gas flow inside the welding torch and the free jet between the gas nozzle and the work piece. The arc model includes a MHD model and a LTE assumption for the arc column and the near electrode regions. The model allows statements about atmosphere turbulences within the shielding gas flow regarding diffusion processes, which are caused by the high diffusion coefficient at the high temperatures inside the arc. The concentration of oxygen at the work piece was calculated and partly validated by gauging the oxygen using the lambda sensor principle. Furthermore the results were validated with the Schlieren technique. It was proved that the flow of the shielding gas cannot be assumed to be axial symmetrically. From this it follows that the contamination of the shielding gas by the atmosphere due to turbulences and eddies cannot be completely demonstrated by using a rotational symmetrical 6°-model. Therefore a 90°-model was used. It was shown that high flow velocities at the gas distributor or rather the resulting turbulences affect significantly the flow conditions at the work piece. Small boreholes of the gas distributor support this effect. Because of that construction advices can be given. The influence of the arc and the consequences for the flow field and for the cover of shielding gas were analysed. 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. In the further progress the measurement methods and the numerical model will be improved for the use of GMA welding. By using this results a GMA welding torch will be developed, which regards all the comprehensive statements, acquired by the appliance of the numerical simulation.

ACKNOWLEGDEMENT

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.

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