Improving surface finish and wear resistance of additive manufactured nickel-titanium by ultrasonic nano-crystal surface modification

Nickel-titanium (NiTi) alloys have great potential to be used as biomedical implants or devices due to their unique functional properties (i.e., shape memory properties and superelastic behavior). The machining difficulty associated with NiTi alloys is impeding their wide application. Additive manufacturing (AM), however, provides an alternative method to manufacture NiTi structures. One major concern associated with NiTi devices fabricated in this route is the potential for the release of toxic Ni ions due to the poor surface finish as well as high surface porosity. In this study, NiTi samples were produced using selective laser melting, the most common AM techniques. Then, an innovative surface processing technique, ultrasonic nano-crystal surface modification (UNSM), was used to mitigate the potential for the Ni ions release. By simultaneous ultrasonic striking and burnishing, UNSM can significantly improve surface finish and decrease surface porosity. In addition, UNSM induces plastic strain which in turn hardens the surface layer. The synergistic effect of better surface finish, lower subsurface porosity, and a hardened surface layer resulted in higher wear and corrosion resistance. It is therefore expected that UNSM can be potentially used to treat biomedical devices.