Niobium to Tungsten Welding: Refractory Metal Joining in Atmospheric Conditions 

Tungsten melts at 3422°C. Niobium melts at 2477°C. Both are refractory metals — defined by melting points above 2000°C — and both oxidize rapidly at temperatures well below their melting points. Tungsten forms volatile WO3 above approximately 500°C, while niobium develops thick, non-protective oxide scales above 400°C.

Conventional welding of this pair normally requires electron beam (EB) welding in a hard vacuum, typically below 10^-4 torr. EB welding equipment represents a capital investment of hundreds of thousands of dollars, and the vacuum chamber limits part size and production throughput.

Percussion welding joins niobium to tungsten in open atmosphere, on benchtop equipment, with a cycle time measured in milliseconds.

The Energy Challenge

Bringing two refractory metal surfaces to their melting points simultaneously requires extraordinary energy concentration. Tungsten at 3422°C and niobium at 2477°C are both far above the operating range of most welding processes. The combined thermal mass, even for small-diameter wire, demands the highest energy settings in the percussion welding parameter library.

The forge voltage of 100V in these parameter sets is the highest in the entire database, delivering 28.10 joules of energy per weld. For comparison, typical dissimilar metal percussion welds operate in the 2-15 joule range. The Nb-W joint requires roughly double the energy of the next most demanding material pair.

Verified Weld Parameters: Niobium to Tungsten

Wire Dia.	Main V	Time (ms)	Cap	Forge V	Tip
0.026"	70V	1.50	Cap2	100V	Short
0.040"	70V	1.50	Cap2	100V	Short

Identical Parameters Across Diameters

A remarkable feature of the Nb-W parameter sets is that the voltage, time, capacitance, forge voltage, and tip selection are identical for both 0.026" and 0.040" wire diameters. This is unusual — most material pairs require significant parameter adjustments as diameter increases. The identical settings suggest that for this refractory pair, the arc energy is dominated by the extreme melting points rather than by the cross-sectional area of the wire.

At 70V main voltage with Cap2 capacitance, the arc delivers sufficient energy to bring both refractory surfaces to melting temperature. The 1.50ms weld time is remarkably short for such high-melting metals, which speaks to the efficiency of the concentrated percussion arc in delivering energy precisely to the interface.

The 100V Forge

The 100V forge voltage deserves special attention. Forge voltage controls the velocity and force with which the movable clamp drives the heated wire into the stationary wire. At 100V, the forging stroke is the most aggressive setting available on the percussion welder.

Both tungsten and niobium are extremely hard metals. Tungsten is particularly resistant to deformation, with a hardness of approximately 350 HV in annealed condition and considerably higher when work-hardened. The 100V forge is necessary to achieve sufficient upset (plastic deformation) at the interface to create complete metallic bonding across the full wire cross-section.

The Short tip selection concentrates the arc energy into a smaller, more intense zone. For refractory metals that require extreme temperatures at the interface, the concentrated Short-tip arc is more effective than the distributed Long-tip arc at achieving localized melting.

Oxidation Management

Both metals oxidize aggressively at welding temperatures, but the 1.50ms cycle provides the same atmospheric welding advantage as other percussion-welded refractory joints. The arc plasma actively strips surface oxides in the first microseconds of the discharge, and the forging stroke seals the joint before re-oxidation can occur.

For niobium in particular, the freshly exposed surface after arc cleaning is highly reactive. The time between arc termination and forge completion — measured in microseconds — is the critical window. At 100V forge, this interval is minimized, which is another reason for the extreme forge setting beyond simply achieving mechanical upset.

Applications

• Superconductor Leads: Niobium-based superconducting wire (NbTi or Nb3Sn) transitioned to tungsten current leads in high-field magnet systems. The joint must maintain conductivity at cryogenic temperatures and withstand thermal cycling between room temperature and liquid helium temperatures.
• Aerospace Propulsion: Refractory metal components in rocket nozzles, ion thrusters, and other extreme-temperature propulsion systems where both metals serve structural and thermal management roles.
• Nuclear Fuel Cladding: Niobium-alloy fuel cladding connected to tungsten structural elements in advanced nuclear reactor designs operating at temperatures exceeding the capability of conventional alloys.

Related Resources

View the complete Nb-W parameter database at Weld Schedules.