Modelling and measurements of gas tungsten arc welding in argon–helium mixtures with metal vapour

Argon–helium mixtures in gas tungsten arc welding of an iron workpiece are investigated using an axisymmetric computational model that includes the cathode, workpiece, and arc plasma in the computational domain. The three-gas combined diffusion coefficient method is used to treat diffusion of helium, argon, and iron vapour. Calculations for argon–helium mixtures without metal vapour are performed; good agreement with previous numerical results is found. A transition from a helium-like to an argon-like arc occurs when the argon mole fraction increases above about 0.3. Calculations for a wide range of argon–helium mixtures including iron vapour are then performed. Adding helium to argon alters the arc properties and affects the weld geometry. Iron vapour cools the arc for all argon–helium mixtures. Iron vapour is present above the workpiece, near the cathode and in the arc fringes for very low argon mole fractions. As the argon mole fraction increases, the iron vapour becomes increasingly confined to the region above the workpiece, with small amounts near the cathode tip. Emission spectroscopy measurements of arcs in argon–helium mixtures with water-cooled copper and uncooled iron workpieces were performed. The measured distributions of atomic helium and iron emission show good agreement with the predictions of the model.