Wire arc additively manufactured nitinol with excellent superelasticity for biomedical applications

Abstract

Despite some recent successes in the additive manufacturing of nitinol alloys, these parts exhibit poor superelasticity compared to wrought parts, limiting their applications in biomedical devices. In this work, wire arc additive manufacturing (WAAM) was used for processing superelastic biomedical grade nitinol wire as the feedstock material on a Ti-6Al-4V substrate. Nitinol prepared by WAAM was subjected to different controlled heat treatments to improve the superelasticity. Optical microscopy and scanning electron microscopy revealed microstructural anisotropy with columnar to equiaxed grains from the bottom to the top layer of the as-fabricated alloy, which did not alter with heat treatment. X-ray diffraction and transmission electron microscopy confirmed the presence of B2 austenite as the major phase along with NiTi₂, Ni₃Ti, and Ni₄Ti₃ precipitates. A_f temperatures lie between 20 and 30 °C and are favorable for biomedical applications. Due to 〈001〉 oriented grains revealed by electron backscatter diffraction, nitinol prepared by WAAM, when subjected to an optimized heat treatment schedule, demonstrated excellent superelastic recovery of 98%, which is remarkably higher than reported earlier and similar to wrought alloy. Additionally, the results of cell studies indicated that the nitinol surface better supported cell attachment following heat treatment and was comparable to that of as-cast nitinol. These findings have important implications in establishing WAAM as a viable fabrication route to prepare biomedical implants, wherein additively manufactured parts can be subjected to post-fabrication heat treatments to alleviate limitations of additive manufacturing and prepare implants of desired biomechanical performance.