Additive manufacturing has been a boon to many types of industries, but certain industrial processes simply aren’t suited for it because either the process of the materials (or both) can’t be printed using existing technology. Shape memory alloys (SMAs) are one of those materials that haven’t found their way in AM to date. SMAs are materials that can be deformed at low temperatures, then recover their original shape upon heating. Additive manufacturing aside, overall applications for SMAs have been limited because they require such low temperatures to undergo transformation. For this same reason, additive manufacturing hasn’t been an option with SMAs, since the postprocessing requirements the materials require, such as cold work, eliminate 3D printing as a production method.
About 95% of commercial SMA involves nickel-titanium – also known as NiTi, TiNi or Nitinol — as the shape-memory material. When this material is worked by being drawn through dies or extruded, which would ordinarily make it a candidate for AM — it must be done at very low temperatures, and/or “trained” (a process of cycling the material repeatedly, usually thousands of times) before being put into applications. These processes drive the plastic strength (resistance to permanent deformations) above the operational stresses of the shape memory alloy devices, so the shape memory performances are reversible.
At the same time, these processes also impart permanent deformation to the material. The point of additive manufacturing is to print the material directly in the part geometry that you want to use in the application: you don’t want that geometry to change afterward. NiTi materials don’t have enough inherent strength to perform the same way repeatedly for thousands of cycles without some type of additional work that results in geometry changes. This is unfortunate because of the growth expectations for SMAs. While today’s commercial SMA market is dominated by medical implants and devices – mostly cardiovascular devices such as stents and artificial heart valves — the market is projected to grow by one to two orders of magnitude (from approximately $10 billion per year to as high as $100 billion per year) as the aerospace market and other applications mature. As a result, researchers have been looking to develop new SMAs that could be used in 3D printing processes.
The Alliance for the Development of Additive Processing Technologies (ADAPT), an industry-academia consortium headquartered at the Colorado School of Mines, has been conducting research on SMAs for metal additive manufacturing. In collaboration with NASA Glenn Research Center (GRC) and ADAPT member Confluent Medical Technologies, researchers recently announced they have developed 38 wrought NiTiHf SMAs for custom applications including high-temperature actuators, biomedical implants and ultra-dent-resistant bearing materials. According to Aaron Stebner, Executive Director ADAPT, the need for precipitation-strengthened SMAs is arising from the aerospace community.
“In making large structures that could, say, morph the wing of an airplane, it is also undesirable to cold work/train the material before using it,” he told Design News. “We’d like to just cut the parts we need straight from a melt, heat treat them, and have them ready. So, the strategy for using NiTiHf for AM is really born out of 10+ years of work led by the NASA Glenn Research Center to develop precipitate strengthened SMAs for aerospace. We’ve now developed chemistries that are also good for medical devices as well as bearings/wear components.”
Metal Parts Shape Memory
These new alloys, which use hafnium as a strengthening precipitate, offer the promise of requiring only heat treatment of between 30 minutes and a few hours to attain functional shape memory performance. This discovery opens the door to using additive manufacturing to create 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 super elasticity through the formation of H-phase precipitates without cold working, making them good candidates for additive manufacturing. The researchers examined the microstructures of the new alloys using a variety of advanced microscopy techniques, including 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. The tests are helping to reveal the responsible mechanisms affecting transformation temperature, super elasticity and plastic deformation in the new alloys.
The team expects that one of the first applications for the new alloys, which could be available in five years, will be 3D printing of SMA medical implants, especially orthopedic implants, but that hundreds of more applications will follow.
“One of the more exciting opportunities is to create multi-functional AM systems,” Stebner told Design News. “Since SMAs can provide so many functions — temperature sensing/switching, damping, medical implants, refrigeration, bearings, etc. – it’s feasible that we could 3D print entire systems of multiple functions if we can understand how the AM processing can be used to change the functional temperature of the SMAs locally.”
If history is any marker, the development of applications for AM-created SMAs will come fast and furiously once the processes become available. In the 1980s and 90s, there were more U.S. patents filed and awarded for SMA applications than any other alloy or metal.
“When someone sees a shape memory alloy work, it can be one of the most imagination-provoking experiences that an engineer or inventor experiences in their life,” said Stebner. “The combination of 3D printing, another one of those imagination-invoking technologies, together with SMAs, is undoubtedly going to lead to new ideas no one has yet dreamt.
Going forward, the research team at ADAPT will continue working with binary NiTi powders to develop the processing windows (additive manufacturing parameters) that will result in quality materials, knowing that it will give them a better starting point and practical experience to adopt the AM process to the new NiTiHf materials. In the longer term, there is opportunity to design new alloys that results in easier-to-additive manufacture SMAs.