Nitinol is highly attractive for biomedical applications because of its unique shape memory and superelastic properties as well as acceptable biocompatibility. Additive manufacturing (AM) is getting significant attention in making complex and patient customizable nitinol devices. However, due to its high microstructural and compositional sensitivities, it is still challenging to fabricate functional NiTi devices via AM. It has been widely reported that evaporation of Ni, oxidation of Ti and formation of precipitation phases during fabrication significantly diverts the expected functional properties. To this date, laser powder bed fusion (LPBF) was the choice among many AM techniques to fabricate NiTi devices but successfully fabricated only on a NiTi substrate because of its poor bonding to other substrates (i.e., steel and Ti). In this work, a multi-step printing approach was systematically developed, which enabled printing NiTi on a Ti substrate using a very low laser energy density of 35 J/mm3 without any visible defect such as delamination and crack. This printing method reduced the high warping issue due to the process induced residual stress, avoided the Ni evaporation issue as well as formation of undesirable precipitation phase during printing. It was also found that a higher oxygen level in the printing chamber reduced the austenite finish (Af) temperature and negatively affected the printability. These results showed the feasibility of LPBF in printing NiTi on a substrate other than nitinol, providing a possible route to reduce the cost of NiTi fabrication via AM. The as-printed NiTi sample exhibited distinct one-step phase transformation with the Af temperature of 2.1°C. To increase the Af temperature to 30.2°C (within the recommended range of Af temperature for biomedical applications), a heat treatment protocol was developed, which includes a solution cycle (at 900 °C for 1 hour) followed by an aging cycle (at 450°C for 30 minutes). The heat treatment protocol enabled to attain the homogenized microstructure while creating ultrafine metastable Ni-rich precipitate, Ni4Ti3, which facilitated the desirable phase transformation behavior with the increased Af temperature. The heat-treated sample showed narrower and sharper two-step martensitic phase transformation with the formation of intermediate R-phase. The presence of both Ni4Ti3 and the R-phase was confirmed by the transmission electron microscopic (TEM) analysis. In the superelasticity test at the body temperature, these samples, starting from the 2nd cycle, demonstrated a recovery ratio of more than 90% and a recoverable strain of more than 6.5%. After 10th cycles, the stable recoverable strain was 6.52% with a recovery ratio of 96%, which is the highest superelasticity reported for the LPBF processed NiTi to the best of our knowledge. After the initial deformation process, we expect these samples to attain near full superelasticity during service. The micro-hardness study showed that the hardness of the heat-treated samples is less affected by the cyclic loading. Nitinol stent is attractive since they are self-expandable and behave superelastically when deployed inside the body. In contrast to the multi-step conventional manufacturing route, AM is attractive in making nitinol stent since it provides one-step processing as well as wide option for customizable design. However, the individual strut of a stent is less than 150 μm which is very challenging to fabricate by LPBF with structural accuracy, mechanical integrity and maintaining proper superelasticity. In this work, the LPBF processing parameter, as well as the post heat treatment and surface finish has been systematically developed to minimize the porosity, avoid structural failure during deformation and maximize the superelastic property at body temperature. Finally, the processed thin strut showed the Af temperature of 33 °C (which is less than the body temperature) and demonstrated complete superelasticity with 100% strain recovery ratio with 4.5% recoverable strain at body temperature, which is first time to report complete susperelasticity in AM processed NiTi thin sample that is suitable for the stent application. The work presents an important roadmap in making NiTi devices by AM while maintaining excellent functional properties of NiTi for biomedical applications.
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