The U.S. Army Combat Capabilities Development Command Army Research Laboratory (CCDC-ARL) has awarded $25 million to Worcester Polytechnic Institute (WPI) to advance a 3D printing technique called cold spray that is used to repair vehicles and other critical technology in the field.
So, what is “Cold Spray Technique”?
“The Army is interested in cold spray 3D printing as a repair technique,” said Danielle Cote, assistant professor of materials science and engineering and director of WPI’s Center for Materials Processing Data, who is the principal investigator for the ARL project.
Researchers explain that, in cold spray, spray powder is injected into a gas jet which accelerates the material to velocities above a unique temperature-dependent threshold that enables metallurgical and mechanical bonding to the substrate at relatively low temperatures. Operators can deliver un-limited thickness in a controlled way which makes it an ideal candidate for additive manufacturing.
“It’s cheaper to repair a part than to replace it, and you get the equipment back in service faster. The Army’s primary interest is unit readiness. If you’re on a mission and need to move quickly to a safer place, and a critical part on your vehicle breaks, you’re stuck unless you can repair it quickly. That’s where cold spray comes in”, explains Danielle Cote.
WPI’s main goal will be to develop, characterize and test new alloys optimized for use in cold spray. Cote said the characteristics of the metal powders used in cold spray are critically important since the metal is not melted before being sprayed onto a part that needs repair, nor is it heat treated after application. “With most manufacturing methods, metal alloys are alerted by first being melted, and then often heat treated to strengthen or otherwise improve their properties. With cold spray, what you end up with in the repair is exactly what you start with, so the characteristics of the powders are quite important.”
WPI will develop and study powders using a variety of state-of-the-art equipment, including instruments acquired as part of the new ARL award. These include tools to study the chemical and structural properties of the powders at the scale of nanometers, such as a SEM/EDS (scanning electron microscope and energy dispersive spectroscopy) unit, a synchronous laser diffraction and dynamic image particle analyzer to determine powder morphologies, and nanoindenters to measure nano-scaled mechanical properties.
Among the modifications that will be made to the powders are unique thermal processing treatments, a technique WPI pioneered, Cote said. Unlike metals used in other metal manufacturing processing, including casting and forging, the alloys used in cold spray do not have to be capable of being heat treated, which gives the WPI researchers access to a wide range of potential materials. However, the properties of cold spray powders can be fine-tuned with the careful application of heat. “This expertise is part of the reason the ARL continues to support WPI,” Cote said. “We have discovered that the properties of metal powders can be significantly enhanced with thermal processing, and that is what we are looking to do with this new award.”
How custom designed cold spray powders could help improve military readiness
Several examples can illustrate the importance of cold spray powders for military readiness. Helicopter gearboxes for instance need frequent repairs. “If you need to replace a part like that, it can take months or even years, and the cost is significant—assuming that the part is available or even still being made. To repair a gearbox with cold spray, you need alloys with high strength, toughness, and ductility. Our methodology will enable us to develop powders that can be used to effectively repair or even manufacture parts like that and get helicopters back in the air quickly”, explains Cote.
While the primary focus of the ARL award will be alloys for repairs, Cote noted that “cold spray is a foundational technology with a wide variety of applications, in the military and beyond.” As part of the research program, a team of co-principal investigators from multiple disciplines at WPI will explore some promising new applications, including the use of cold spray to apply copper coatings to give equipment antibacterial properties.
Researchers in WPI’s robotics engineering program will explore the use of multi-axes robots to automate cold spray. “The Army is especially interested in portable cold spray systems, but the technology can also be used on a larger scale—in industry, for example—and it will be exciting to see how robots can help expand the use of this and other additive manufacturing processes.
“I think there is much potential for this technique. With the work we will be doing with powder development, in robotics, and in a number of other areas, I think we are going to go a long way with cold spray. There really are endless possibilities.”
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