Welding. Tungsten possesses only moderate welding properties. Welding must be carried out under controlled weld atmospheres, preferably in a dry box, since any contamination by oxygen will reduce the ductility of the joint. Before welding, the metal has to be degreased and pickled, commonly with a mixture of nitric acid and hydrofluoric acid (90/10 vol%). The weld seams have a coarse-grained structure in the hot-fusion zone, owing to recrystallization, and can therefore withstand only low mechanical stresses. Tungsten can be welded by tungsten inert gas (TIG) welding, laser beam welding (for thin parts), plasma welding, friction welding and electron beam welding, the latter being the preferred fusion-welding method. Tungsten can be welded to W-Re and Mo-Re alloys. W-Re alloys (in particular W-26Re) are recommended as filler metal for welding tungsten. Tungsten can be diffusion welded to tungsten and other metals. Despite the high melting point of tungsten, temperatures of 1300-2000℃ and pressures of 2—20N•mm-2 give satisfactory joints. Intermediate layers of Ni, Pt, Rh, Ru, and Pd accelerate the diffusion process. Tungsten wires can be spot-welded to other metals under a protective gas blanket.
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