Although the defense industry has been the biggest customer for hypersonic aircraft materials, there is growing interest from the commercial sector as well. Companies like SpaceX and Virgin Galactic are developing technologies that will slash the time it currently takes to transport both passengers and cargo around the world. Composite materials will play a key role not only in the development of these hypersonic aircraft, but also in the production of many other innovative future aircraft.
3D-Printed Aircraft Components
Additive manufacturing (AM) of composites enables quick production of components of almost unlimited geometries with very little waste. But most AM work today is done with thermoplastic materials, which don’t have the properties necessary for aircrafts’ structural components. Thermoset composites made with carbon fiber can provide the required strength and durability, but incorporating carbon fibers into the printing mixture has been problematic. When the carbon fiber count gets too high, the fibers clump together and clog the nozzle of the 3D printer so that it can’t print.
Emrah Celik, assistant professor of mechanical and aerospace engineering at the University of Miami, has developed a process that solves this problem. He found that vibrating the carbon fibers at the printer’s nozzle prevents them from clumping and allows the printing process to proceed.
In the course of working on AM projects for NASA and the Air Force, Celik was able to customize off-the-shelf, 3D extrusion printers to produce carbon fiber thermoset parts. In 2019, Celik and his Air Force research partners published a paper describing how they had been able to manufacture a thermoset part with about 6% carbon fiber by volume. “That was great at the time, and we exceeded what was state-of-the-art practice,” he says.
But Celik wanted to add more fiber to produce even stronger composite components. With his graduate assistant, Nashat Nawafleh, he explored several variables, including the optimal length of the carbon fibers, the surface treatment of those fibers and the impact of different levels of vibration on the thermoset material.
With vibration, for example, “You don’t want to vibrate too much, because you don’t want to reduce your resolution or change the dimensional accuracy,” Celik says. “But the vibration has to be the right amount to separate the fibers from each other so they don’t clog the nozzle.”
After experimenting with Kevlar® and glass fibers, Celik focused on carbon fibers because of the strength they provide to the composite material. He initially worked with chopped carbon fibers about 6 millimeters in length, but eventually chose milled carbon fibers approximately 50 microns long. After testing a variety of surface chemistries for these shorter fibers, Celik found that sized materials provide the best linkage between the resin and the carbon fibers.