Control of surface finish and mechanical properties of nitinol stents fabricated via laser powder bed fusion

This study provides a comprehensive analysis of how laser powder bed fusion (L-PBF) processing parameters influence the functional properties of nitinol stents, leveraging a Box-Behnken design of experiments. The novelty of this work lies in its systematic investigation of the key attributes, including relative density, surface morphology (pre- and post-electropolishing), dimensional accuracy, composition, phase transformation behaviour, and mechanical performance, offering new insights into process-property relationships for L-PBF fabricated nitinol stents. An important observation is that L-PBF-produced stent strut diameters (148–201μm) exceed the CAD defined strut diameter of 100 μm, highlighting the challenges and effect of melt pool width on achieving required levels of dimensional accuracy. Electropolishing proved highly effective, reducing surface roughness by up to 87%, from an initial range of 11.8–22.8 μm to a minimum of 1.8 μm. Relative density varied from 85.2% to 99.5%, while in-situ thermal measurements revealed mean layer temperatures between 750 and 1150°C. Differential scanning calorimetry (DSC) analysis indicated varying austenitic finish temperatures of 23–29°C and martensitic start temperatures of 7.5–10.5°C, resulting from a 1–2% nickel loss during processing. By achieving high-precision fabrication with excellent surface quality and tailored functional properties, this research establishes L-PBF as a promising technique for manufacturing complex nitinol stents. This study demonstrates that precise control of L-PBF parameters is critical for optimizing porosity, phase transformation characteristics, and mechanical performance. Furthermore, for the first time, thermal gradient graphs for intricate structures such as stents have been captured and analysed using infrared (IR) pyrometry. This novel approach provides valuable insights into the thermal behaviour during L-PBF processing, offering a deeper understanding of heat distribution and its influence on the final properties of nitinol stents.