Engineers at John Hopkins explore additive manufacturing of shape memory alloys
Using 3D printing and shape memory alloys, researchers at the Johns Hopkins Applied Physics Laboratory (APL) have created an antenna that can change its shape based on its temperature.
Electrical engineer Jennifer Hollenbeck said she got the idea from “The Expanse” series, where alien technology is organic and shape-changing. “I have spent my career working with antennas and wrestling with the constraints imposed by their fixed shape,” she said in a press release. “I knew APL had the expertise to create something different.”
APL mechanical engineer and materials scientist Andy Lennon had used nitinol — a shape memory alloy of nickel and titanium — to create coils that would extend down through a person’s esophagus to assist with heart imaging. The challenge is that nitinol and other shape memory alloys conventionally require extensive mechanical processing — known as cold work — to achieve the shape memory effect, and as a result they are typically only available as wire or in thin sheets.
“Doing an extreme amount of cold work would defeat the whole point,” Lennon said in the same release. “If you take that complex shape and pass it through a die to stretch it out, you’re back to a wire.”
The APL team initially conducted research to tackle the fundamental challenges associated with scalable additive manufacturing of nitinol components, later applying these techniques to create shape-changing structures that could be deployed in space applications. After extensive experimentation toward the antenna application, the team altered the ratio of nickel and titanium, but the first attempt to create a shape-shifting horn antenna using 3D-printed nitinol fell short. While the antenna did technically expand and contract and change its frequency, it was also rigid and difficult to expand.
“It turned out to be a really complicated design, and it didn’t work as well as I would have liked,” Hollenbeck said.
Their next attempt was based on success Lennon’s team had seen in 3D printing nitinol using two-way shape memory, in which the alloy can be heated and cooled to alternate between two remembered shapes. Hollenbeck’s team developed an antenna that was shaped like a flat spiral disk when cool but became a cone spiral when heated.
Heating the spiral proved to be a challenge. The team had to determine how to heat the metal of the antenna enough for it to change shape, but without interfering with the RF properties or burning out the structure. To solve the problem, the team, led by RF and microwave design engineer Michael Sherburne, had to invent a new form of power line.
“For peak heating, the power line has to handle a lot of current,” Sherburne said. “We had to go back to fundamentals to make this work.”
The final piece of the puzzle was working out how to 3D-print the antenna in a consistent, repeatable fashion. Lennon’s modified nitinol, with its higher concentration of nickel, made it challenging to print at scale.
“We have a lot of experience optimizing processing parameters and designs for alloys, but this was a step beyond,” explained additive manufacturing engineer Samuel Gonzalez. “There aren’t many people out there, if anyone, printing this material, so there’s no recipe for how to process it.”
“We made shrapnel in the printer a few times because the antenna is trying to change shape as you’re printing it, due to the heat,” added colleague Mary Daffron. “It wants to peel apart.”
Typically, the team can process an alloy in less than four days, but Daffron and Gonzalez said this particular material took two to four weeks of build time. Now that they have optimized the processing parameters, they’re looking for ways to build on their initial success.
“We want to optimize the parameters to work on multiple different machines, to make this more widely applicable, and we know we’ll need to optimize for different variations of the material that might actuate at different temperatures,” Daffron said.
APL is pursuing a full patent on behalf of the team for the shape-adaptive antenna technology. The Lab has also provisionally decided to pursue patents for the novel power line for heating the spiral, a method for controlling the antenna, and a method and process for using shape memory alloys to create a phased array antenna.
The research is published in the journal ACS Applied Engineering Materials.
