Welding-nitinol,a baby alloy, 40 years old.SOLUTIONS with Effective, Practical Advice
The relatively new alloy itself, (although its origins go back to the sixties) called Nitinol (which is an acronym standing for Nickel Titanium Naval Ordnance Laboratory) is probably the most successful representative of Shape Memory Alloys. Its composition includes almost equal proportions of Nickel and of Titanium. (Sponsored Links) Nitinol must be properly prepared by melting in vacuum, refined by Vacuum induction melting (VIM) and/or Vacuum arc remelting (VAR), 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. Once the Nitinol alloy has been shape set annealed or straightened under controlled time, temperature, and pressure conditions, it 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, and to 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, so that 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, 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, 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, but is now routinely employed for joining the material to itself, provided a suitable protective atmosphere is employed, by such processes as 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 without the development of brittle phases or intermetallic compounds, like TiFe and TiFe2 that reduce the 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. Tantalum and niobium proved adequate to permit successful Welding-nitinol. New applications besides medical implants, are developed for every conceivable consumable and industrial product, and therefore suitable methods for Welding-nitinol will have to be found and perfected. It is going to be interesting to be updated in 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. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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