Directed Energy Deposition of Shape Memory Alloys for Aerospace Sector (Funding Agency: North Dakota NASA EPSCoR, Research Collaborator: AddiTec Inc.)

The nickel-titanium (NiTi) alloy has two distinct properties: shape memory and superelasticity. The shape memory effect of NiTi alloy has been widely studied for applications such as biomedical; on the other hand, superelastic NiTi alloy has recently been studied for tribological applications. Superelasticity is known to enhance the tribological behavior of the materials. NiTi alloy is not only superelastic but also can be tailored to be harder than conventional bearing materials such as M-50 and 440C steel with less density. However, because of the challenges associated in producing NiTi alloys using widely used manufacturing processes such as casting, hot isostatic pressing (HIP), and vacuum induction melting (VIM), recent studies have concentrated on additively manufacturing NiTi alloy. Our research studies the mechanical properties and tribo-mechanical behavior of NiTi alloys produced via wire and powder laser directed energy deposition (DED) and cold rolling. Using XRD, SEM/EBSD, and compression/tensile testing, we investigate effect of manufacturing processes and used feeding media on grain/precipitation formation and mechanical properties including superelasticity, hardness, and tensile strength of NiTi alloy. Using the temperature-controlled ball-on-plate reciprocating dry sliding testing, white light interferometer, and SEM/EDS, we investigate tribological behavior of specimens at room temperature and elevated temperatures. This includes investigating wear mechanisms, wear track morphology, wear volume, and friction coefficients. We recently have characterized the tribo-mechanical behavior of superelastic cold rolled NiTi. Our results demonstrate that at elevated testing temperatures, the wear mechanism changes from abrasive to adhesive on superelastic cold rolled NiTi surfaces.