SUMMARY

To date, ultimate tensile strength (UTS) within additively manufactured (AM) nickel-titanium (NiTi or Nitinol) has not been demonstrated at levels satisfactory for use in many commercial applications, especially medical and aerospace. AM NiTi parts have not yet reached UTS of 1000 MPa, but these industries desire 1200 MPa or higher. Building on the work demonstrating the mechanism by which this degradation of mechanical properties occurs will allow for progress towards more usable AM parts. Evidence suggests that one cause of this insufficient UTS could be oxygen based, ceramic inclusions, specifically Ti4Ni2O and/or TiO2. Other possible mechanisms are residual stress, surface defects, Ni3Ti intermetallics, over-coarsened Ni4Ti3 precipitates, solute segregation, porosity, unmolten powder, ceramic inclusions (carbides, oxides, etc), and control of these will allow testing the hypothesis that the major cause is oxide phases.

It is known that when oxygen forms ceramic phases, they limit the UTS of traditionally manufactured NiTi parts. Most powder for AM is exposed to oxygen after production, and this work attempts to demonstrate that this exposure, and the subsequent formation of a TiO2 passivation layer, leads to critical strength degradation in printed parts. This work attempts to be the first development of robust AM of NiTi using PBF- EB and will further the understanding of oxide versus other sources of heterogeneity. This work will also inform AM of other ELI (extra low inclusion) alloys, especially Ti alloys that form the same TiO2 layer. These methods will likely extend to materials which form passivation layers other than TiO2.