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Vacuum Heat Treatments of Laser-Powder Bed Fusion Nitinol Stents

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Nitinol (NiTi) is a nickel-titanium alloy that, thanks to a low temperature phase transformation, shows two peculiar properties: the shape memory effect, i.e. the ability to reset its previous geometry upon heating, and the superelasticity, i.e. the ability to recover large deformations. The latter of these properties, coupled with good biocompatibility, makes Nitinol the most used material for manufacturing endovascular surgery devices, such as stents.

Laser Powder Bed Fusion (L-PBF) process of NiTi could open up new possibilities in the production of biomedical implants, thanks to the freedom of design offered and to its typical flexibility. In this regard, manufacturing NiTi stents through L-PBF would be a promising alternative, as it easily allows production of complex-shaped parts customized to each patient with short lead times. However, the influence of the process parameters on the final component is a critical aspect that still needs to be fully understood.

Moreover, heat treatments are commonly applied to Nitinol components to improve thermo- mechanical properties, as they can sensibly affect the final behavior. Nevertheless, it is crucial to accurately select the correct heat treatment, tailoring the parameters based on the microstructure of the as-built samples.

The present study aims at determining the effect of heat treatments on the microstructure, transformation temperatures and mechanical properties of Nitinol stents manufactured by L-PBF. Heat treatments were performed in vacuum furnace at annealing temperatures of respectively 600°C and 1100°C. The microstructural characterization of the samples was performed using both optical and scanning electron microscopy, while transformation temperatures were determined through differential scanning calorimetry during heating and cooling cycles with a rate of 10°C/min. Finally, the mechanical behavior was investigated through compression tests in plate-to-plate configuration according to the ISO 25539-2 standard at 37°C, to mimic the behavior of the stents inside the human body.

Learning Objectives:

  • Upon completion, participant will be able to understand how additive manufacturing could optimize the production of Nitinol endovascular surgery devices
  • Upon completion, participant will be able to understand what are the critical aspects related to L- PBF manufacturing of Nitinol endovascular surgery devices
  • Upon completion, participant will be able to understand how post-process heat treatments affect the properties of L-PBF manufactured Nitinol endovascular surgery devices