| AVS 65th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Friday Sessions |
| Session PS-FrM |
| Session: | Plasma Modeling |
| Presenter: | Anthony Murphy, CSIRO, Australia |
| Authors: | A.B. Murphy, CSIRO, Australia J. Xiang, CSIRO, Australia H. Park, CSIRO, Australia F.F. Chen, CSIRO, Australia |
| Correspondent: | Click to Email |
Arc welding is very widely used in manufacturing industry to join metals. The process relies on the intense heat flux from the arc plasma to partially melt metal. This also produces metal vapour, which can be transported into the arc by diffusion and convection. Metal vapour plasmas emit radiation much more strongly than those in standard welding gases such as argon and helium. The presence of metal vapour therefore leads to increased radiative cooling of the arc, which decreases the heat flux to the workpiece and leads to shallower welds.
It is well known that metal vapour dominates the arc plasma in the case of in metal inert gas / metal active gas (MIG/MAG) welding, in which the upper electrode is a metal wire whose tip melts to form droplets. Large amounts of metal vapour are produced from the wire tip, and the strong downward convective flow in the arc ensures that the central region of the arc contains around 50% metal vapour.
In contrast, the upper electrode in tungsten inert gas (TIG) welding is tungsten, which does not melt or vaporize. Metal vapour is produced only from the weld pool (the molten region of the workpiece). Computational models have predicted that the strong downward convective flow in the arc confines the metal vapour close to the workpiece. The models therefore predicted that the strong radiative cooling of the arc that is observed in MIG/MAG welding does not occur in TIG welding.
We have developed a computational model of TIG welding that treats the diffusion of the metal vapour in the arc plasma accurately for the first time. Previous treatments only considered ordinary diffusion (driven by concentration gradients); we now also take into account diffusion driven by temperature gradients and the applied electric field (cataphoresis). Our results demonstrate that cataphoresis causes upward diffusion of the metal vapour into the centre of the arc, despite the strong downward convective flow, leading to substantial radiative cooling of the arc.
We also report intriguing results obtained for TIG welding of stainless steel, in which we treat the diffusion of iron and chromium vapours separately. Our results show that the iron and chromium vapours have different trajectories through the arc, explaining the surprising measurements of Tanaka and Tsujimura (Quart. J. Japan Weld. Soc. 30 164, 2012), which found that iron vapour reached only as far as the tungsten electrode tip, whereas chromium vapour was deposited on the tungsten well above its tip.
Finally, we examine the substantial influence of the choice of welding gas, arc current and other parameters on the influence of metal vapour on the arc and the weld.