Numerical study of the effects and transport mechanisms of iron vapour in tungsten inert-gas welding in argon

Tungsten inert-gas (TIG) welding uses an electric arc between a tungsten cathode and a metal anode to partially melt the anode workpiece, forming a weld pool. Metal vapour emanating from the weld pool has important effects on the arc welding process. An axisymmetric computational model of the arc and weld pool is used to examine the transport and influence of iron vapour on an argon arc plasma. In contrast to previous studies that use approximate and incomplete treatments of diffusion, the present model incorporates the combined diffusion coefficient method, which takes into account all important driving forces. The influence of metal vapour is first examined for an arc current of 400 A. Metal vapour is predicted to be present in high concentrations above the anode and near the cathode tip, and in a lower concentration in the arc column. The presence of metal vapour in the arc is found to lead to a substantial reduction in arc temperature (up to 1600 K) and current density, resulting in a significant decrease in the weld pool depth and volume. It is shown that ordinary diffusion leads to iron vapour transport upward from the anode region along the arc fringes and into the recirculating convective flow, which carries the iron vapour to the cathode region. Here the upward diffusion driven by the electric field and temperature gradient traps the iron vapour below the cathode tip, leading to a high concentration in this region. The influence of arc current is investigated in the range from 150 to 400 A. The results obtained for standard welding currents of 150, 200 and 250 A also predict significant concentrations of iron vapour in the arc, with the concentration increasing with current in the arc column and near the anode. The concentration near the cathode tip is lower at 400 A because the temperature and electric field diffusion coefficients are lower at the higher temperatures present near the cathode. Spectroscopic measurements of atomic chromium emission for argon TIG welding of a chromium anode are presented and compared to predictions of the code. The measurements show the presence of metal vapour in both the cathode and anode regions, in agreement with the model.