Tantalum shares many properties with refractory metals molybdenum and tungsten. It has an extremely high melting point of 2996°C, which is the melting point range of other refractory metals. This high melting point makes it an excellent choice for high temperature applications. There is, however, one caveat. Because it reacts with carbon, nitrogen, oxygen, and hydrogen, it cannot be used in environments where these elements or their compounds are present as gases or volatile substances. Therefore, its use at high temperatures is limited to vacuum or inert gas environments.
Successful processing of tantalum requires experience. High and low cooling rates generate heat that causes tantalum to react with carbon, oxygen and hydrogen to form extremely hard particles. These particles quickly blunt the tool and cause more heat to be generated. Tantalum is also a soft colloidal material that gels easily. To compensate for these characteristics, very slow rotational speeds should be used and large amounts of coolant are required. Water-soluble oils are suitable for most cooling; Fluorocarbon lubricants can provide additional benefits when needed. Table 1 shows some recommendations.
Tantalum is usually welded using a tungsten argon arc welding (TIG) process, using a tungsten electrode and argon shielding gas. Special care must be taken to keep air away from any heated surface. Welding should be carried out in a glove box filled with high purity argon or helium. Alternatively, argon or helium can be used with long trailing shields and backside shields (or backside purification). Precautions must be taken to prevent warm surfaces from being exposed to air, as welding embrittlement may occur. Tantalum and its common alloy containing 2.5% tungsten (Ta-2.5%W) can both be welded in this way. Since the melting temperatures of tantalum and tungsten are similar, the alloys do not segregate during solidification. Therefore, post-welding heat treatment is not necessary.
Compared with materials such as Group B and C nickel alloys, Ta-2.5%W maintains its resistance to forged metal corrosion in welding. Due to segregation, the corrosion properties of nickel alloys in welds are always significantly lower than the published corrosion data for forged alloys.
The cleanliness of tantalum surface will greatly affect the welding quality. Hydrocarbons (oils) react during welding to form fragile hydrides and carbides. The tantalum surface should be degreased before welding.
Other methods can also be used, such as electron beam welding (EBW) and plasma arc welding (PAW). The same precautions that prevent hot surfaces from being exposed to air apply.
