Laser Shape Setting for Inducing Shape Memory Effect in Thin NiTi Wires: Microstructure and Functional Performance

Abstract Shape setting is a crucial step in the production of NiTi shape memory alloys (SMAs) elements for tuning their functional performances. The laser technology can be successfully implemented for fast shape setting of thin, cold-worked NiTi wires, promoting optimal thermo-mechanical response. In this work, Ti-rich NiTi wires were subjected to laser scanning at different power values for inducing shape memory effect (SME). Differential scanning calorimetry and strain recovery testing allowed to characterize the transformation temperatures and the functional response of the laser-annealed wires. Moreover, transmission electron microscopy was carried out for evaluating the precipitation and the evolution of the microstructure at varying the laser process conditions. Reference wires, in straight-annealed condition, from which cold-worked wires were manufactured, were compared for selecting the most suitable laser condition. It was found that optimal functional properties can be achieved by selecting the correct laser power; the corresponding TEM observations indicate the presence of both equiaxial and elongated grains, containing a high degree of defects, due to the previous plastic deformation. These characteristics give the laser-treated wire functional properties that are comparable to, or even superior to, those of commercially available wires. These achievements can drive novel applications, in which regions offering different performances of SME can be tuned depending on the incident laser power.