For thin sheet metal work, TIG welding can be used without a
filler metal.For thicker items or when joining different materials,
a filler metal is used in the form of a handheld rod or a wire
supplied by a separate feed device, generally currentless.In standard
TIG welding the arc is free, but a variant known as plasma welding
uses a secondary nozzle to constrict the arc.
The melt, the hot tungsten electrode and the molten end of the
filler metal are protected from the atmosphere by an envelope
of inert gas. Argon is generally used, although there are quality
and productivity advantages in using a mixture of either argon
and helium or argon and reduced hydrogen.