PSI - Issue 69

Carlo Alberto Biffi et al. / Procedia Structural Integrity 69 (2025) 121 – 126

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(SME) can be defined and fixed by such treatments [1]. However, both Ti rich and Ni rich NiTi SMAs are typically subjected to final annealing at a temperature in the interval 400-500°C for durations from few seconds up to some minutes [2-3]. The most common procedure for implementing the final shape setting regards the conventional heat treatments carried out in air furnaces, followed by air cooling or water quenching, depending on the size of the NiTi elements [4-7]. The heat treatment can tune the transformation temperatures of the NiTi SMA, and adjust the thermos mechanical response. In the last years, some unconventional solutions have been explored for implementing the shape setting on NiTi SMA elements, typically under the form of thin wires or tapes, with novel features, like short durations or with the possibility of locally tuning of the material performances. In the case of thin NiTi wires, fast shape setting was studied in literature using electrical pulses [7] or laser annealing [8-13]. The latter was explored with promising results, depending on the rapid heating/cooling and on the opportunity of promoting local functionalization on shaped elements [14]. In the present work laser annealing was explored for promoting optimal SME in thin Ti-rich NiTi wires, starting from a cold drawn condition which did not exhibit the martensitic transformation. The functional performances of the laser annealed NiTi wires were characterised evaluating the transformation temperatures of the martensitic transformation in correlation of different laser process parameters, such as laser power and scanning speed. 2. Experimental: 2.1 Material preparation: The study is based on the use of commercially available material, Ti-rich NiTi wires (mod. Smartflex from Fortwayne), having a diameter of 150 µ m in straight annealed condition. The wires were subjected to cold drawing for suppressing the martensitic transformation through the increase of the dislocations density. The final diameter of the cold worked wires, on which the laser annealing would be tested, was reduced down to 90 µ m. 2.2 Laser shape setting procedure: The laser shape setting treatment was implemented on thin Ti-rich NiTi wires in cold drawn condition, as described in the previous paragraph, using a continuous wave laser emission mode. The laser system, used in this work, is a home assembled equipment, based on: (i) quasi-continuous wave fiber laser (mod. 300/3000 QCW IPG Photonics), having 300 W maximum power; (ii) laser head (mod. Fiber Mini from Laser Mech) was used for the focalization of the beam; (iii) 2D motion stage (mod. PRO165LM from Aerotech). All details are reported elsewhere in previous works [11-12]. The laser scanning was performed by a single pass along the NiTi wire, kept straight for a length of 25 mm, without any shielding gas. Laser beam size of 1.3 mm was kept constant for irradiating the wire under almost uniform power distribution (see Figure 1). Table 1 reports the principal process parameters, which were investigated in this work, for exploring the promotion of the martensitic transformation of the cold worked wires.

Figure 1: Schematic of the laser irradiation on the NiTi thin wire (a), and the clamping system of the wire (b).