Welding Refractory Metals

It is obvious that these metals must be perfectly clean prior to welding and that they must be welded in such a manner that air does not come into contact with the heated material. Cleaning is usually done with chemicals. A water rinse is necessary to remove all traces of chemicals from the surface. After the parts are cleaned they must be protected from reoxidation. This is best done by storing in an inert gas chamber or in a vacuum chamber.
Molybdenum is welded by the gas tungsten arc welding process and the electron beam process. The gas metal arc process can be used but sufficient thickness of molybdenum is rarely available to justify this process. Molybdenum has been welded by other arc processes but results are not too satisfactory. Welding with the gas shielding processes is accomplished in an inert gas chamber or dry box. This is a chamber that can be evacuated and purged with inert gas until all active gases are removed. Welding is done in the pure inert atmosphere with normally good results. The filler metal compositions should be the same as the base metal. The base metal in the heat-affected zone becomes embrittled by grain growth and recrystallization as a result of the welding temperatures. Recrystallization raises the transition temperature so that molybdenum welds tend to be brittle. Molybdenum is highly notch-sensitive, craters and notch effects such as undercutting must be avoided. Molybdenum can also be welded with the resistance welding processes and by diffusion welding.

Tungsten is welded in the same manner as molybdenum and has the same problems, only more intensely so. It has greater susceptibility to cracking because the ductile-to-brittle transition temperatures are higher. The preparation of tungsten for welding is more difficult. The gas tungsten arc welding process is used with direct current electrode (negative). Welding should be done slowly to avoid cracking. Preheating may assist in reducing cracking but must be done in the inert gas atmosphere.

Commercially pure tantalum is soft and ductile and does not seem to have a ductile-brittle transition. There are several alloys of tantalum commercially available. Even though the material is easier to weld, it should be well cleaned and for best results should be welded in the inert gas chamber. The gas tungsten arc welding process is recommended. Some tantalum products are produced by powder metallurgy technology and this may result in porosity in the weld. The arc cast product does not have porosity. Filler wire is normally not used when welding tantalum and for best results direct current electrode negative is used. High frequency should be used for initiating the arc. Helium is recommended for welding tantalum to provide for maximum penetration since joints are designed to avoid using filler metal.

There are several different alloys of columbium (niobium) available. Some are ductile and others brittle since the transition temperature is near room temperature. The gas tungsten arc welding process is used for the pure columbium and for the lower strength commercial alloys. In certain alloys the welding can be done outside of an inert gas chamber but special precautions should be taken to provide extremely good inert gas shielding coverage. For some of the alloys preheating is recommended to provide for a crack-free weld. Electron beam welding is used and columbium can also be resistance welded.