Hi Paul,
I agree with GRoberts that direct current, electrode negative is needed for the GTAW process. Welding is best done using argon or helium shielding in a gas tight welding chamber capable of maintaining a high purity atmosphere, or in a high vacuum to prevent contamination, because oxygen and nitrogen may be present in the metal, or they could be absorbed from the ambient atmosphere during welding.
Prior to welding, the surfaces must be clean and free of dirt, grease, oil, oxides, and other foreign matter, because any of these can inhibit wetting, flow, or fusion, and will also contaminate the metal. The components should first be degreased with a suitable and safe solvent. They should then be cleaned by any one of the following methods: immersion in 20% potassium hydroxide solution (boiling), electrolytic etching in 20% potassium hydroxide solution, chemical etching in 50 volume % HNO3 - 50 volume % HF solution, immersion in molten sodium hydroxide, or immersion in molten sodium hydride. In any case, the cleaning media should be thoroughly removed by rinsing with clean water, and then the component dried with hot air.
Fixturing should provide minimum restraint on a weldment, and components should be preheated to above the transition temperature of the metal to avoid cracking from the thermal stresses.
Welds should be made using procedures that produce good fusion and give a narrow, minimum width heat affected zone with a minimum input of heat. Runoff tabs are helpful when terminating a weld to avoid crater cracks in the joint. Proper grinding of the areas to be welded improves arc starting, reduces arc wandering and improves penetration. Grinding the weld surface to remove bead ripples, surface contamination, or both, may improve weld joint ductility.
Tungsten is sensitive to the rate of loading and to stress concentration. The ductile to brittle transition range is shifted upward by increasing the strain rate. Welded joints are very notch sensitive, and where possible, the weld surface should be finished smooth and faired gradually into the base metal. Notches at the weld root should be avoided.
Weldments should be stress relieved promptly at a temperature below the recrystallization temperature of the base metal. Incidently, there is no evidence that the mechanical properties of welds in tungsten are improved by heat treatment, however, heating to just below the recrystallization temperature of the base metal will relieve residual stresses, which could be beneficial by reducing the likelihood of cracking during subsequent handling.
Also, you may want to contact your local consumables rep. They should provide technical support that should assist you in resolving your problems.