On the influence of direct aging on the superelastic effect of NiTi manufactured by laser powder bed fusion

In the recent years, the possibility to produce Nitinol (NiTi) parts through additive manufacturing (AM) has gained a lot of interest, mainly for biomedical components. Among all the existing techniques, laser powder bed fusion (L-PBF) is the preferred method. Nevertheless, manufacturing NiTi via L-PBF technology is challenging, as presents some unsolved issues mainly related to the control of the final properties. In particular, the obtainment of the superelasticity is crucial for NiTi endovascular surgery devices.

Heat treatments are commonly used to tune the properties of NiTi parts, as they affect the transformation temperatures and the microstructure. However, since AM techniques involve a different thermal history compared to conventional methods, carefully tuning the relative heat treatments is necessary. One of the current trends is performing direct aging. The advantage is to exploit the supersaturated condition (reached due to the high cooling rates typical of L-PBF process) through a precipitation treatment without solution treatment, thus maintaining the unique microstructure deriving from the process.

In the present work, NiTi samples were produced through L-PBF and subjected to short-time direct aging heat treatments. After microstructural characterization, cyclic compression tests were carried out both at 6% of constant strain and incremental strain up to 10%, to provide a comprehensive analysis on the superelastic behavior. Compression tests up to failure and Vickers microhardness were also performed. Results were correlated to the microstructure with the aim to deeply investigate the effects of direct aging on L-PBF NiTi components.