The Alliance for the Development of Additive Processing Technologies (ADAPT), an industry-academia consortium based out of Colorado School of Mines, USA, is conducting research on shape memory alloys (SMAs) for metal Additive Manufacturing. In collaboration with NASA Glenn Research Center (GRC) and ADAPT member Confluent Medical Technologies, the researchers developed thirty-eight wrought NiTiHf SMAs for custom functional performance.
According to the consortium, a limitation of SMas such as nickel–titanium (NiTi) is that their unique characteristics mean that they can only be used effectively with relatively simple, axially symmetric shapes like tubes and square stock. Conventional NiTi SMAs require cold work to achieve the proper superelastic behaviour; while Additive Manufacturing could enable more complex part geometries, post-processing requirements like cold work negate AM as a reasonable processing method.
The thirty-eight wrought SMAs produced by the researchers cover a range of functional performances, including high-temperature actuators, biomedical implants and ultra-dent resistant bearing materials. Funding for the work was provided by NASA GRC, Confluent and the National Science Foundation.
These alloys, which use hafnium as a strengthening precipitate, are said to require only heat treatment to attain functional shape memory performance. This opens the door to the use of AM to fabricate metal parts with shape memory characteristics and geometries that are far more complex than those made with conventional NiTi alloys.
Compared with the cold work required to add strength to NiTi, new NiTiHf alloys reach high strength and superelasticity through the formation of H-phase precipitates, without cold working. They might therefore be good candidates for Additive Manufacturing.
Dr Behnam Amin-Ahmadi collaborated with NASA and Confluent to apply different chemical compositions and heat treatments to NiTiHf alloys to achieve the desired mechanical properties and martensitic transformation temperatures.
The microstructures of these alloys were investigated in detail using advanced microscopy techniques, such as scanning electron microscopy (SEM), focused ion beam (FIB), electron backscatter diffraction SEM (EBSD-SEM), high-resolution transmission electron microscopy (TEM), scanning TEM (STEM), and STEM with energy-dispersive X-ray (EDX) analysis, among others. These fundamental studies are said to reveal the responsible mechanisms affecting transformation temperature, superelasticity and plastic deformation in these alloys.
The second part of Amin-Ahmadi’s project focused on optimising process parameters for AM NiTiHf and characterising AM parts in detail. Once ideal alloy compositions were identified, ADAPT member ATI Specialty Materials produced the compositions in bulk powder (with cost share from ATI and funding from the Colorado Office of Economic Development and International Trade), and ADAPT member Elementum additively manufactured test parts for characterisation work in the ADAPT lab.
According to the consortium, the use of AM with NiTi-X alloys removes the limitations of conventional NiTi SMAs and enables more complex parts, making available a new range of applications for SMAs in the biomedical field, aerospace, automotive and wind energy industries.