Welding of tantalum

Niobium is always found in nature associated with tantalum and it closely resembles tantalum in its chemical and mechanical properties. It is a soft ductile metal which, like tantalum, work hardens more slowly than most metals. It will in fact absorb over 90% cold work before annealing becomes necessary, and it is easily formed at room temperature. In addition, welds of high quality can be produced in the metal. In appearance the metal is somewhat similar to stainless steel it has a density slightly higher than stainless steel and a thermal conductivity similar to 1% carbon steel. 

We use commercial Ti02 crystals (Pi-Kent) cut and polished to within 0.3° of the (110) face and we prepare them further with cycles of Ar + bombardment and U H V annealing to approximately 950-1100 K, typically 5-10 min for each cycle. The samples are mounted onto tantalum back-plates via strips of tantalum spot-welded to the back-plate. Annealing is performed by high-energy electron bombardment of the back-plate from a hot filament. Temperatures are measured from optical pyrometers (Minolta) focused on the back-plate. The temperatures are not measured directly from the samples because they are translucent and get darker with each sputter/anneal cycle. 

Thorium metal and bismuth powders of nominal purities 99.8% and 99.999%, respectively, were used and the various alloys were prepared by induction melting the required amounts of thorium and bismuth in tantalum cmcibles. Handling the samples and welding of the cmcibles were conducted under argon atmosphere. 

Tantalum is a relatively high-cost heavy metal with a density more than twice that of steel. The physical properties of tantalum are similar to mild steel, except that tantalum has a much higher melting point (3000°C). The tensile strength is about 345 MPa, which can be approximately doubled by cold work. Tantalum is easy to fabricate. It is soft, ductile, and malleable and can be worked into intricate forms. It can be welded by a number of techniques but requires completely inert conditions during welding. 

Examination of equipment fabricated from tantalum that has been used in a wide variety of service conditions and environments generally shows that the weld, HAZ, and base metal display equal resistance to corrosion. This same resistance has also been demonstrated in laboratory corrosion tests conducted in a number of different acids and other environments. However, in applications for tantalum-lined equipment, contamination of the tantalum with iron from underlying backing material, usually carbon steel, can severely impair the corrosion resistance of tantalum. About the only known reagents that rapidly attack tantalum are fluorine, HP and acidic solutions containing fluoride, fuming sulfuric acid (H2SO4) (oleum), which contains free sulfur trioxide (SO3), and alkaline solutions. 

In this paper, the performanees of laser-ultrasound are estimated in order to identify lacks of weld penetration. The laser-ultrasonic technique is applied to cylindrical metallic strucmres (few mm thick) in a single-sided control. The results obtained for different materials (gold-nickel alloy and tantalum) are presented by B-sean views for which the control configuration is discussed with regard to the thermal effects at the laser impact. This testing is performed for different lacks of weld penetration (up to 0.5 mm for a thickness of 2 mm) even in the presence of the weld bead, which corresponds to an actual industrial problem. 

This paper deals with the control of weld depth penetration for cylinders in gold-nickel alloy and tantalum. After introducing the experimental set-up and the samples description, the study and the optimization of the testing are presented for single-sided measurements either in a pulse-echo configuration or when the pump and the probe laser beams are shifted (influence of a thermal phenomenon), and for different kind of laser impact (a line or a circular spot). First, the ultrasonic system is used to detect and to size a flat bottom hole in an aluminium plate. Indeed, when the width of the hole is reduced, its shape is nearly similar to the one of a slot. Then, the optimization is accomplished for... 

Fig. 2 Description of the weld depths penetration for gold-nickel alloy and tantalum cylinders.

Fig. 9 B-scan views of the lacks of weld penetration 1.5 mm (left image) and 0.5 mm (right image) in a split configuration for tantalum.

The weld depths penetration for gold-nickel alloy and tantalum cylinders have been well controlled by an entirely contactless ultrasound method. Nevertheless, the development of signal and image processing will allow to increase the resolution of the ultrasonic images. Moreover, in order to be able to size quite well the lacks of weld penetration, the simulation of the interaction beam-defect is presently developed in our laboratory. 

Joining Tantalum can be joined by riveting, brazing and welding however, due to the good properties of welded joints the former techniques are seldom used. 

Other Tantalum alloys It has been observed that the presence of a small amount of iron or nickel, for example, in a tantalum weld makes that site subject to about the same acid attack as would be experienced by iron or nickel alone. Galvanic action, as well as simple chemical attack, is undoubtedly involved. 

Figure 7.6. Experimental set up for temperature-programmed desorption in ultrahigh vacuum. The heat dissipated in the tantalum wires resistively heats the crystal the temperature is measured by a thermocouple spot-welded to the back of the crystal. A temperature programmer heats the crystal at a rate of typically 1-5 K s b Desorption of gases...

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