Welding process selection is determined by the physical and metallurgical properties of molybdenum.
Molybdenum has an extremely low room-temperature solubility limit (1 ppm or less) for oxygen, nitrogen, and carbon. Warm-working below the recrystallization temperature breaks up grain boundary films and produces a fibrous grain structure. This structure will have good ductility and strength in the direction of working, but not transverse to it.
Alloys. Some alloying is necessary to improve the high-temperature and room-temperature properties. Molybdenum is alloyed with small amounts of titanium, zirconium, and carbon to improve high-temperature and room-temperature strength properties. An alloy designated a TZM (Mo-0.5Ti-0.087Zr-O.015C) is produced commercially. Also, an addition of about 20 atomic percent of rhenium to molybdenum greatly improves ductility near room temperature.
Surface Preparation. Prior to welding, the surfaces must be clean and free of dirt, grease, oil, oxides and other foreign matter. The molybdenum components should be first degreased in a suitable, safe solvent; followed by a cleaning method recommended by the molybdenum supplier, rinsing in clean water, and air drying.
Welding. Fusion welding must be done in a pure inert gas atmosphere or in a high vacuum to prevent contamination by oxygen and nitrogen. Any fixtures used should provide minimum restraint on the weldment, especially when welding a complex structure.
The components should be preheated above the transition temperature of the metal. Weldments should be stress-relieved promptly at a temperature below the recrystallization temperature of the base metal.
For arc welding, molybdenum can be joined by the gas tungsten arc welding process using direct current electrode negative. Argon or helium may be used for shielding. Welds should be made using procedures that give a narrow heat-affected zone with minimum input of heat.
The electron beam welding process is well suited for joining molybdenum because of its high energy density. Narrow, deep welds can be produced by this process
using less energy than with arc welding. Since electron beam welding is done in a high vacuum, contamination of the weld metal with oxygen or nitrogen is prevented. See and CARBON STEEL and STAINLESS STEEL.