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Tantalum thin-walled pipe welding joint grain refinement

Tantalum welding

In materials science field, control material freezing process and Grain refinement has been important method to improve traditional material property and develop new structure material. Tantalum is kind of refractory metal with good plasticity and easy to manufacture. It has Strong corrosion resistance and good mechanical property, has been widely used in aviation, space navigation, atomic energy and chemical industry. It is an important strategic materials. While, in conventional welding conditions, there will be big columnar crystals texture in tantalum welding line, it seriously influenced the joint function. The main factors influenced these textures :

In Heat Affected Zone the grain growth dimension is influenced by welding thermal cycle. Tantalum has no phase transformation in high temperature, the crystal grain will grow once the temperature reaches threshold value. The grown crystal grain will never transform even under through cooling process.
Welding zone tissue mainly influenced by the weld pool shape and cooling speed in solidification process. During weld shaping process, temperature and pressure are important factors affect crystal strain dimension, the temperature have effect on strength of welded joint. For carbon steel, tantalum alloy and aluminum alloy, there are relevant files indicate stress has influence to grain refinement.
So there is important mechanical significance and practical value to control tantalum crystal grain in welding process.
Tantalum thin-walled tube¡¯s forming needs a series of dynamic welding process and dynamic solidifying includes extrusion, rough rolling and precision rolling,Roller extrusion and arcing. During tantalum tub welding process, point of weld has long distance with idler wheel, which is difficult for heat dissipation, so small heat filling can form weld pool and get good welding formation. Also, because the high welding speed, it will touch the idler wheel before welding crystal get solidification. In this time, heat dissipation speed of tantalum tube increased, temperature difference increased, it also under through extrusion process. So adjust the welding speed and  Roller spacing can help to control the solidification of weld cool.
To get fine property tantalum welding joint and control tantalum strain size, we have taken a sample of tantalum plate and tantalum tube welding joint, after grinding, polishing and corrosion, observed by metalloscope, we found tantalum crystal grain will grow up in temperature 1150 to 1300¡æ. When heated below 1150¡æ, tantalum crystal grain have no obvious variation, grain size almost keep the same as manufactured. When tantalum thin-walled tube formed by rolling, welding joint is obviously thinner, the biggest crystal grain size is almost half size of Plate butt welding conditions. Tantalum thin-walled tube suffered idler wheel stress in welding process, which make Weld crystallization nucleation number and nucleation rate increased and forms large amount of dislocation inner tantalum. Proliferation and interaction of dislocation lowered Grain boundary surface energy and broke the growing crystal grain, which refined weld joint and In Heat Affected Zone grains.

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