Johns Hopkins APL Researchers Unfold the Power of Additive Manufacturing for Shape Memory Alloys

The panels unfold like an origami creation in reverse: once creased and packed tightly, they expand and return to their first life as a flat structure. When the hinges connecting the panels are exposed to heat, they transform — flattening the pile of squares into one plane.

Now imagine that happening in a remote environment where human intervention is impractical or impossible — like millions of miles from Earth, on a small satellite, where positioning must be precise and deployment must happen at a specific time and following a specific sequence.

That’s exactly what researchers at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, had in mind when they set out to demonstrate controlled, shape-morphing metallic components for deployable structures. In a paper recently published in the journal Materials & Design, a team from APL’s Research and Exploratory Development Department (REDD) describe how they leveraged additive manufacturing (AM) to design and fabricate shape memory alloy (SMA) components capable of precise, self-guided transformations when exposed to heat.

“The most interesting aspect of SMAs is their ability to recover shape when you apply heat, acting as compact and efficient actuators,” said Ian McCue, co-author on the paper and a materials scientist at APL. “But the temperature at which that process occurs is very sensitive to how the material is processed and is challenging to control.”

In this study, the team recognized the potential of metal AM, including its flexibility in producing complex parts, to control the temperature at which SMAs transform. The material used, nickel titanium (NiTi), can take on different transformation temperatures — anywhere from 68 to 212 degrees Fahrenheit — depending on the ratio of nickel to titanium, specific processing method and presence of any additional elements.

After discovering how to control transformation temperature, the team explored the functional use of SMA as an actuator. “We created unique SMA hinges with self-regulating features that prevent the hinges from overstraining, while still maintaining compact packing sizes that can expand more than five times their stowed area,” added Gianna Valentino, a materials scientist in REDD and co-author on the paper. “Our diverse team of material scientists, mechanical engineers and process engineers created plywood-style SMA hinges with tuned transformation rates that could deploy in a prescribed sequence.”

“If you have a complex series of hinges, they can’t all release at once: They have to do so in sequential order. That requires tailoring the thermal response,” added Steve Storck, a mechanical engineer in REDD and the corresponding author on the paper. “You could do that with the hinge geometry — thicker versus thinner structures, as a base example — or the transformation temperature. But the cool thing about these materials is that this shape memory behavior is an intrinsic property.”

When the process works correctly, as the team demonstrated, the NiTi pieces won’t respond to every stimulus (such as the heat from a spacecraft launch) but rather only those to which they’ve been calibrated, producing a desired domino effect.

While the team’s published work focuses on space applications, the impact of this technology could be farther-reaching. The unique processing of complex-shaped objects with AM, combined with the tailorable functional behavior of the NiTi material, opens up a world of possibilities.

“Looking forward, we are working to achieve two-way shape memory response so that these structures can fold and unfold repeatedly,” said Morgan Trexler, who manages APL’s Science of Extreme and Multifunctional Materials program. “We are also exploring SMAs that can be activated magnetically, which would provide enhanced precision in the deployed shape and would be beneficial for optics applications. The ability to fabricate adaptive materials into complex structures has the potential to unlock novel future capabilities.”

“Developing the capabilities for complex shape-morphing kinematics, without the need and use for specialized and heavy external motors, is ideal for any scenario where payload volume and weight are critical,” the authors conclude. “The designed panel arrays demonstrate future scalability and set the stage for other deployable structures that take advantage of AM SMA hinges, such as an antenna or a solar sail…. The results presented in this study lay the groundwork for future research in SMA-enabled complex deployable structures in space or other remote environments.”