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Welding-nitinol,a baby alloy, 40 years old.SOLUTIONS with Effective, Practical Advice
Welding-nitinol is a critical processing step frequently needed to manufacture medical and general use devices. The alloy Nitinol is probably the most successful representative of Shape Memory Alloys. Its name is an acronym standing for Nickel Titanium Naval Ordnance Laboratory. The alloy is relatively new, although its origins go back to the sixties. Its composition includes almost equal proportions of Nickel and of Titanium. Nitinol must be properly prepared by melting in vacuum, refined by Vacuum induction melting (VIM) and/or Vacuum arc remelting (VAR). It is further forged and rolled into wrought products like plate, sheet, bar or wire. The amount of cold work the material had prior to its shape set anneal establishes the ultimate strength of the material. The Nitinol alloy has to be shape set annealed or straightened under controlled time, temperature, and pressure conditions. It then displays two remarkable properties or characteristics: superelasticity and shape memory effect. Above its transformation (Austenite Finish) temperature, Nitinol is superelastic. It has the capability to withstand extensive elastic deformation under load, of up to eight percent strain. It will return to its original shape without permanent plastic deformation when the load is removed. This deformation causes a stress-induced phase transformation from Austenite to Martensite. The stress-induced Martensite is unstable at temperatures above Af (the Austenite Finish transformation temperature). When the stress is removed the material will immediately spring back to the Austenite phase and its pre-stressed position. Below its transformation temperature, it displays the shape memory effect. The material, if cold deformed in its Martensitic (low temperature) phase, will remain in that shape. When heated above the Af (Austenite Finish) temperature it changes back to Austenite and the deformation is lost, as the material returns to its pre-deformed, original shape. In other words, the shape memory effect permits to the material to recover fully a given plastic deformation upon being heated to a certain temperature.
The force behind the amazing behavior of shape memory alloys is a reversible, solid phase change known as martensitic transformation. The alloy has a variable structure. It is capable of changing reversibly from one form called martensitic phase at low temperature (weaker state) to another called austenitic phase at high temperature (stronger state). This transformation, promoted by heat energy, permits to perform actual mechanical work, e.g. in devices called actuators. Welding-nitinol must take into account the above transformations. Furthermore Nitinol is biocompatible and corrosion resistant. These are most important qualities for medical devices, having demonstrated in numerous occasions to be absolutely safe for use in contact with living tissues. Welding-nitinol was not simple to develop and implement. It is now routinely employed for joining the material to itself, provided a suitable protective atmosphere is employed. The processes used are laser beam, gas tungsten arc welding, electron beam and friction stir welding. Obviously the procedures employed for Welding-nitinol must not unduly reduce the mechanical properties of the joints. Other joining methods like brazing, soldering, and mechanical means like crimping and swaging have been used. However fusion Welding-nitinol to stainless steels or more generally to ferrous alloys cannot be performed. The development of brittle phases or intermetallic compounds, like TiFe and TiFe2 reduces joint strength to nothing. A suitable way to overcome this hindrance was found by introducing in the joint an additional material, compatible with both nitinol from one side and stainless steel from the other. The in between added material is called transitional in that it permits transition from nitinol to stainless. Tantalum and Niobium proved adequate to permit successful Welding-nitinol. New applications besides medical implants, are developed for every conceivable consumable and industrial product. Therefore suitable methods for Welding-nitinol will have to be found and perfected. It is going to be interesting to remain updated of future progress. An Article on Biocompatible Materials was published (11) in Issue 82 of Practical Welding Letter for June 2010. To read every issue of PWL as it is published, please subscribe.
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