What is Tantalum?
Tantalum is inherently a soft, having forming properties, weldable metal that possesses a high melting temperature (5425°F or 2996°C) and excellent ductility at cryogenic temperatures. Its high melting point provides excellent potential for structural utilization at temperatures higher than any other common refractory metal except tungsten. However, tantalum is vastly superior to tungsten with respect to fabricability, ductility, and weldability. To develop this potential and provide competitive strength at temperatures greater than about 2000°F (1093°C), tantalum must be alloyed.
- Tantalum Ingot – ASTM B364
- Tantalum Rod and Wire – ASTM B365
- Tantalum Tubing – ASTM B521
- Tantalum Sheet, Plate, Strip and Foil – ASTM B708
- Medical Grade Tantalum – ASTM F560
Common Tantalum Alloys (UNS Codes)
- Unalloyed Tantalum (EB Melted) – R005200
- Unalloyed Tantalum (Powder Metallurgy Grade) – R05400
- Tantalum 2.5% Tungsten – R05252
- Tantalum 10% Tungsten – R05255
- Tantalum 7.5% Tungsten – Spring Grade
- Grain Stabilized – Furnace Grain
Commercial Grade ASTM Tantalum Purities
- 99.5% Pure 3N5
- 99.9% Pure 4N
- 99.995% Pure 4N5
- 99.999% Pure 5N
Due to Tantalum excellent fabricability and low-temperature ductility, with good strength at temperatures up to 3500°F ( 1926°C). Tantalum alloys can be classified into two main types:
(1) simple solid-solution strengthened alloys such as Ta-5W-2.5Mo, Ta-10W-2.5Mo, and Ta-8W-2Hf and
(2) Complex alloys system using solid-solution Strengthening and/or dispersion strengthening such asTa-9.6W-2.4Hf-0.01 C, and Ta-6.5W-3.0Re-1.0Hf-0.3Zr-0.025Y.
Mechanical properties of Tantalum B708
Weldability of Tantalum Alloys
Although tantalum is having good weldability, it need to be cleaned properly before use, and for best welding quality results, it should be welded in the inert gas chamber. Generally, Welding of Tantalum is influenced by below four factors:
(1) Interstitial impurities,
(2) Welding parameters, (3) Alloying elements, and (4) Weldment microstructure
Also, when annealed tantalum is welded, the cooling rate of tantalum-tungsten alloy is low, and tendency of heating crack is increased. Therefore, the grain size of the tantalum plate or other material for testing shall be taken into account in welding, and fine grains with size less than or equal to ASTM grade 5 should be used.
Welding electrode/ filler wire for TIG welding of Tantalum
Most of Tantalum welding in performed using Autogenous TIG which means welding without any filler wire. In case, filler wire need to be added, it can be cut & made from the same material which is being welded. Thin wires can be cut from Tantalum sheet with shearing as shearing will not produce any thermal damage to the material which can be a concern if using grinding.
Extreme care must be taken to cover with an inert gas all surfaces which are raised above 600°F by the welding heat-helium, argon, or a mixture of the two gases, creates an atmosphere which prevents embrittlement by absorption of oxygen, nitrogen or hydrogen into the heated metal. Where a pure, inert atmosphere is provided, the fusion and adjacent area will be ductile. Extremely high ductility can be obtained in a welding chamber which can be evacuated and purged with inert gas.
Resistance welding is another option for tantalum welding & can be done with conventional equipment.. Because its melting point is 2,700°F higher than that of SAE 1020 steel and its resistivity is only two-thirds that of SAE 1020 steel, tantalum requires a higher power input to accomplish a sound weld. The weld duration should be kept as short as possible in the range of one and ten cycles (60 CPS) to prevent excessive external heating. Where possible, the work should be flooded with water for cooling and reduction of oxidation
Shielding Gases for Tantalum Welding
Tantalum welding is performed with inert gas only. Argon & helium both are used in industries for Tantalum Welding. Helium gas shielding is better suited for Tantalum Welding as it offers improved penetration when welding without filler.
Where the use of a welding chamber is not practical, the heated surfaces can be protected by proper gas-backed fixturing. This usually serves three purposes:
(1) To hold the work in alignment, (2) To chill the work in order to limit the heat area, and (3) To act as a conduit for the inert gas and to exclude air from the heat area.