Functional Behaviour of Cold-Worked and Straight-Annealed NiTi Elements Processed with Ultrashort Laser Cutting

Abstract

Among functional materials, quasiequiatomic NiTi alloys are one of the most promising and diffused for some sectors, from the biomedical to aerospace ones. Their peculiar performance, namely, shape memory effect and pseudoelasticity, is induced via a thermomechanical treatment called shape setting or with the heat treatment of annealing. This heat treatment is carried out in cold-worked conditions. The present work studies the effect of the material conditions of straight annealing and cold working on the functional performance of diamond-shaped NiTi microdevices realised through ultrashort laser cutting. In detail, experiments were carried out aimed at studying the effect of laser power, scanning speed, and number of passes on the kerf width with the focus on defining the most suitable process condition on both straight-annealed and cold-worked sheets of 100 µm in thickness. After the process parameters had been defined, the transformation temperatures and superelastic behaviour were analysed though differential scanning calorimetry and force–displacement testing. The femtosecond cutting of straight-annealed NiTi did not change the characteristic temperatures of the base material, while the same process induced a soft martensitic transformation with respect to the cold-worked material due to a heat accumulation effect.