TIG welding occurs when an electric arc between a non-consumable tungsten electrode and the workpiece provides the heat needed for the welding operation. The tungsten electrode is not melted and any filler metal needed to build up the weld profile is added separately. The molten metal in the weld pool, the tip of the filler wire and the hot electrode need to be shielded by an inert gas to protect from atmospheric contamination.
Usually the gas is argon. However, helium, or a helium-argon mixture, may be used for special applications. Argon-hydrogen mixtures can be used for welding austenitic stainless steel.