Non-equilibrium models for the arc in arc welding processes

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The project is aimed at the description of the interaction of arcs and electrodes in arc welding processes. In particular, the establishment of the arc roots and the sheath voltages as well as the resulting energy transfer to the electrodes should be determined by a model for the examples of a tungsten inert gas (TIG) process and a gas metal arc welding (GMAW) process. Beside the arc attachment at the tungsten cathode the anode attachment at the workpiece in the TIG process and at eh droplet depot in a pulsed GMAW process should be analysed in detail. Main emphasis will be put on the impact and the proper description of the metal evaporation at the electrodes.In contrast to previous approaches, a non-equilibrium model for argon-iron vapour mixtures should be worked out on the base of own groundwork and applied. Such a model enables a continuous description of the arc plasma from the centre where it is near to the local thermodynamic equilibrium (LTE) state to the regions in front of the electrodes where distinct deviations occur from the thermal, chemical and ionisation equilibrium. The additional consideration of a space charge region in front of each electrode assumed as collision less and of the particle fluxes across this region enables the determination of the sheath voltage. The whole model will be based on a magneto-hydrodynamic multi-fluid simulation of the plasma and the heat and current balances of the electrodes including the emission mechanisms at the electrode surfaces. The consideration of the sheath voltages in such a model finally enables the self-consistent description of the arc root structure and the electrode evaporation. In the first instance, the simulations should be limited to quasi-steady-state situations with given rotationally symmetric electrode geometries. Accompanying experiments on TIG and GMAW processes will support the choice of the electrode geometries and the validation particularly of the non-equilibrium model by means of optical emission spectroscopy.The results of the simulations should provide a significantly improved understanding of the arc root structures in TIG and GMAW processes, of the impact of metal evaporation on the arc roots and of the role of the electrode sheath regions for the power budget of the arc and the energy transfer to the electrodes. Beside others, the commutation of the arc across a falling droplet should be analysed also. Theoretically deduced values of the sheath voltages and the energy input into the electrodes for dc processes and selected states in pulsed GMAW processes will provide improved relations of arc length, current and total voltage which can be used for further studies among others for the welding process control. In addition, the results deliver the perspective to deduce and adapt simplified approaches for the description of the arc roots which can also improve established magneto-hydrodynamic simulations of welding processes based on the LTE assumption.