How To Weld Tantalum?

1. What is tantalum?

Tantalum (Ta) is very hard, ductile, has high corrosion resistance, high electrical and thermal conductivity, and can be welded. Tantalum has a very high melting temperature of 5463°F or 3017°C. The microstructure of tantalum is body-centered cubic (BCC). Grinding tantalum is not practical due to its high hardness. Naturally, tantalum is soft in its pure form.

But it is usually alloyed for higher mechanical properties. Tantalum has very high toughness at room temperature and low temperature, as reported in various literatures. Tantalum is a good refractory material due to its high melting point, but its cost is a key factor.

Due to its high melting point, toughness and strength, tantalum is used in aircraft engines, electrical equipment, surgical implants, and in the handling of corrosive substances in laboratories.

2.Material Specifications

Standard Specifications: ASTM B364, B365, B521, B708 and F560

Standard tantalum materials and UNS numbers: R005200, R05400 (pure tantalum), R05252 (alloyed 2.5% tungsten), R05255 (10% tungsten).

3. Due to its excellent machinability and low temperature ductility, tantalum has good strength at high temperatures up to 3500°F (1926°C), making it an ideal alloy for aerospace components. Tantalum alloys can be divided into two categories:

1) Ta-10W-2.5Moly, Ta-5W-2.5Moly, & Ta-8W-2Hf (Solid solution strengthened alloy)

(2) Ta-9.6W-2.4Hf-0.01C, Ta-6.5W-3.0Re-1.0Hf-0.3Zr-0.025Y and other complex alloy systems using solid solution strengthening and/or dispersion strengthening.

Tab.lchemical composition of the ASTM B708 RO5252 tantalum sheet,%

4. Welding rod/wire for tantalum TIG welding

Most tantalum welding is performed using autogenous TIG, which means that no filler wire is required for welding. If filler wire needs to be added, it can be cut and fabricated from the same material that is being welded. Thin wires can be cut from the tantalum sheet by shearing, as shearing does not cause any thermal damage to the material, which can be a problem if grinding is used.

As a highly reactive metal, tantalum requires a high degree of care during welding. Once the tantalum surface reaches above 600°F, a layer of inert gas is required to prevent embrittlement of the material. Use argon or helium or a mixture of both for shielding to avoid this embrittlement. Any embrittlement due to oxygen or hydrogen absorption results in a loss of material ductility. Therefore, tantalum must be welded in an inert gas chamber for maximum ductility welds.

5. Solderability of tantalum alloys

Although tantalum has good weldability, it needs to be properly cleaned before use, and for best weld quality it should be welded in an inert gas chamber. Generally speaking, the welding of tantalum is affected by the following four factors:

(1) Interstitial impurities,

(2) Welding Parameters, (3) Alloying Elements, and (4) Weldment Microstructure

Resistance welding is an alternative to tantalum welding and can be done using conventional equipment. Due to the higher melting temperature of tantalum compared to carbon steel or stainless steel alloys, higher welding currents are required during resistance welding. The dwell time for resistance welding should be kept low to avoid overheating of the outer surface of the welded joint.