Laser Straight Annealing of HT-SMA Thin Wires

In recent years, the interest in shape memory alloys with high transformation temperatures (HT-SMAs) has significantly increased not only in the academic sphere, but also due to a market-push, especially from aerospace, automotive, and robotics.

Among several systems, Ni-Ti-Hf-Nb emerged as particular promising due to superior mechanical properties of its alloys compared to those of the ternary Ni-Ti-Hf system. In particular, the ductility of Ni-Ti-Hf increases significantly by adding Nb, allowing for formation of a soft eutectic network composed of shape memory matrix phase and a not transforming β-Nb phase at the grain boundaries. Recently, it was demonstrated that this alloying approach allows to obtain thin wires using conventional drawing techniques, increasing the opportunities for a successful industrial commercialization.

In this work, laser technology was explored for straight annealing of Ni-Ti-Hf-Nb thin wires, in cold worked condition, for promoting shape memory behaviour.

The functional performances of laser treated wires were assessed through differential scanning calorimetry, tensile tests and strain recovery under constant load. The results were compared to the ones of furnace-annealed wires.

The feasibility map, including the effect of the laser power and the scanning speed, was determined for obtaining optimal functional properties in terms of high temperature range of the martensitic transformation and suitable values of strain recovery.