Metal vapour emanating from the weld pool during tungsten-inert-gas (TIG) welding affects the arc welding process. To understand the transport mechanisms of metal vapour in a TIG arc, an axisymmetric computational model is developed that includes the tungsten cathode, stainless-steel anode workpiece and the arc plasma region self-consistently. The combined diffusion coefficient method, which calculates diffusion coefficients due to mole fraction gradients (ordinary diffusion), temperature gradients, pressure gradients and the electric field is used to treat iron–chromium–argon and iron–chromium–helium plasmas. It was found that in both cases, metal vapours can reach the cathode region. The effect of different diffusion coefficients on metal vapour transport was investigated. It was found that ordinary diffusion is the main driving force for upward metal vapour diffusion away from the anode workpiece in an argon arc. The diffusive transport carries the metal vapour into the recirculating convective flow, which then transports the metal vapour to the cathode region. Here the upward diffusion driven by the temperature gradient and electric field leads to the build of high concentrations of the metal vapours adjacent to the cathode. In the helium arc, in contrast, metal vapour is transported upwards from the workpiece by electric field diffusion, which is much stronger in this case. Spectroscopic measurements of atomic chromium emission show that metal vapour can reach the cathode region in an argon TIG arc, providing support for the predictions of the model. Only by taking into account all diffusion driving forces is it possible to predict the distribution of metal vapour in a TIG welding arc.
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