Functional characterization of laser annealed Nitinol wires

Shape memory alloys are functional materials, able to offer a unique mix of different properties: the shape memory effect and the pseudo-elasticity. Due to the reversible solid state martensitic transformation, these two properties can be found in this class of materials. These alloys require a specific annealing, called shape setting, in order to fix their principal properties; this treatment is usually done by using furnaces. This thesis work is based on the study of a new approach to perform the annealing on a pseudo-elastic NiTinol thin wire by using an Ytterbium active fiber laser source. The effect of laser power and process speed has been studied on the martensitic transformation and on the mechanical properties of the thin wires. The characterization of the wires were performed through differential scanning calorimetry, stress-strain tests, done at room temperature, and microstructural analysis. The obtained results were then compared to the performances of the commercially available wires, which were conventionally heat treated in oven. It was found that the wires laser annealed shown at least comparable properties of the commercial wires, obtained by conventional techniques. Due to rapid heating and cooling of laser treatment, the wires’ properties can be evidently improved by the generation of a microstructure, orientated along the wire length, more than the one obtained by furnace. Moreover, the modulation of the functional properties of the NiTinol can be done by changing the incident power. It can be concluded that the NiTinol pseudo-elastic wires, annealed by unconventional laser technology, can be proposed and potentially used for biomedical applications and the laser technology can be a real competitor of conventional thermal treatments for this materials.