Cold Spray Technology Could Allow Air Force To Repair Aircraft Parts Quickly Instead Of Replacing Them

For decades, US Air Force aircraft maintenance crews have religiously followed the “component swap” model. When an aircraft part is worn out, replace it with a new one. It’s a model that works – if you have new parts, time, and money available. With three in short supply, Cold Spray (CS) additive technology could allow maintenance managers to renew worn or broken parts while remaining on an aircraft, quickly preparing it for the mission in the field.

The 28th Bomb Wing at Ellsworth Air Force Base in South Dakota is spearheading the USAF’s efforts to apply CS repairs to operational aircraft. Work with students and professors from the South Dakota School of Mining and Technology, maintenance group 28th additive manufacturing flight personnel repaired an over-wing slip seal on an operational B-1B bomber using CS in May.

The slip joint is basically a part that allows the wing of the B-1 to flex vertically. What is important to recognize is that the repair has been made in situ with the seal in place. The standard practice of swapping out a new slip joint would have required the costly and time-consuming removal of the wing from the B-1.

“It would normally have involved eight weeks of downtime,” says Brian James, a Ph.D. graduate student in Mines and chief additive manufacturing engineer for 28th Maintenance Group. “With cold spraying, we were able to do it in a few hours. “

The B-1B entered active service in 1986 and all 100 bombers were delivered in 1988, making parts difficult to obtain, often requiring the cannibalization of another aircraft. Combined with the required disassembly, replacing the slip joint would have resulted in a bill of over $ 500,000 to the Air Force. The cold spray repair of the original part only cost a small fraction of that.

With the CS slip joint repair, the B-1B flew again a week later. The process is fairly easy to understand, although it took years to perfect it.

Spray metal

Cold spraying is an additive manufacturing process a bit like 3D printing. Simply put, tiny metal particles are mixed with a carrier gas (usually nitrogen or helium) and accelerated to speeds of Mach 2-3 through a nozzle. For CS repairs, particles are sprayed directly onto a part, adding material to rebuild and strengthen it.

“Basically it’s the speed of a gun,” Robert Hrabe, CEO and co-founder of the South Dakota-based company. HRV metal systems Explain. “If you imagine shooting a rifle bullet through a thick metal plate, the bullet sinks into the surface, effectively bonding to it. We do the same thing only the particles are 5 to 15 microns [in size], about the consistency of baby powder.

The particles fuse together in a solid state, creating a high strength, structurally strong bond. Once applied, the new material is machined to match the original shape or tolerances of the part to be repaired. In this way, CS is both an additive and a subtractive process.

For the B-1 slip joint, the Ellsworth team used 6061 grade aluminum alloy particles to make the repair. Soft metals are generally used with the CS process, although recent advances allow the use of harder materials such as titanium and Inconel. And it’s not really a cold spray.

“Cold is a bit of a misnomer,” admits Hrabe. “It does not melt the material like flame spray or HVOF [High Velocity Oxygen Fuel], one of the other common welding or coating processes.

Instead, the additive materials are heated but only to less than 80% of their melting point, making them fungible. The process can be used to repair a variety of parts that may have experienced common fatigue or corrosion, from relatively simple wing skins to hinges, structural brackets, shafts, landing gear components and more.

“One of the advantages of [CS] the technology is the ability to cover large areas, ”says Hrabe. “There is no limit to the thickness [of additive material] that you can build. We even made whole parts additively by cold spraying.

Research into CS’s potential for the restoration of aircraft components began at the École des Mines in the mid-2000s. As it progressed, this led to spin-off companies, including VRC, which manufactures the equipment used to apply the CS.

The company manufactures two types of high pressure CS devices: a semi-portable system called Gen III which can be installed in a dedicated room or cabin in a repair center and a smaller, self-contained system called Raptor which can be used in the field for CS repairs.

Both consist of a base unit (a generator or welder-sized box) that heats the carrier gas and precisely controls the powder feed into the gas stream using a proprietary software. A hose and a variety of different nozzles for different spray speeds and patterns join the main controller. VRC also has a modular system that can fit into a submarine’s hatch, be assembled for repairs, then disassembled and removed or stowed away.

VRC equipment was used to repair parts on a V-22 as well as the B-1, joining the CS repair work undertaken by Team Ellsworth on F-15 parts and shipboard systems of the Navy. Given the prevalence of metallic hardware in the military, cold spray repairs could become ubiquitous for fastening parts for air, land and sea systems.

Ellsworth AFB is the center of expertise for cold spraying. James goes further, calling him the “focal point for field-level additive manufacturing in the Air Force.”

The 28th Bomb Wing established the Air Force’s first additive manufacturing field flight in 2019, working from a 15,000 square foot additive manufacturing facility on the base. The facility can not only perform CS repairs, it can validate the strength of any repair within hours using a full lab with scanning electron microscope, hardness testers, string tests and other equipment.

“What we’re doing here is taking technology that has been tested and proven in the lab and integrating it right into the combat level,” says James. The Ellsworth facility is the first of its kind, but it is a model that James would like to see replicated in some Air Force facilities and a source of CS expertise that he would like to see spread to pass on to deployed units. with portable CS equipment.

Just the solution

Unlike the myriad of technical and process initiatives that develop steadily and quickly fade into obscurity within the Air Force, the idea of ​​CS and Ellsworth’s team seems to have garnered widespread interest.

that of the air force Quick support desk (RSO) has been a leader in promoting CS investigations by the service. The portable Raptor CS machine used to repair the B-1B was acquired with funding provided by RSO for the Ellsworth team. RSO has also facilitated CS component repairs for other aircraft including the A-10, B-52 (TF33 engine), C-5, E-3 and F-16.

Support for CS repair experimentation and development came from Air Force Materiel Command, said Brian James, with support from the Air Force Research Laboratory, Tinker’s Air Force Sustainment Center. AFB and the Oklahoma City Air Logistics Complex. The same group of players are working with the Ellsworth / SDSMT team to assess the potential for repairing combat damage CS, an arena in which the speed, portability, and cost-effectiveness of the process could prove invaluable.

“Without the support of [South Dakota School of Mines], the Army Research Laboratory and our industrial partners, we couldn’t do it, ”adds James. “When we did that [B-1] repair we made it known that we had physical obstacles to overcome. All we had to do was ask and we made ourselves spend the night with experimental buzzards. “

VRC Metal Systems illustrates the scarcity of CS’s industrial partners. There are only five manufacturers of cold spray equipment in the world (in Japan, Germany, Russia, Canada and the United States) according to Rob Hrabe. VRC is one of only three to specialize in more advanced high pressure CS equipment used for the repair of structural components.

His company appears to be successfully crossing the “valley of death” that often traps other defense tech startups between initial development and government / commercial acceptance.

“In the last few years, the military has really started to embrace the technology,” Hrabe said. “It is growing rapidly there and in the aerospace industry. There are many expensive parts that are now discarded for a minor amount of corrosion or wear that can easily be repaired with this technology.

CS can help with the maintenance backlogs that plague the Air Force since moving to a two-tier (flight line / depot) maintenance program that relies more on commercial depots for repairs. While this has improved efficiency, sending planes and parts to depots takes time, often prolonged by union rules that limit the working time of depot maintenance workers.

The predictive ‘condition-based maintenance’ approach adopted by the Air Force may well adapt to CS repair, improving the ability of maintenance managers to decide when and where maintenance should be performed and shortening supply lines.

Hrabe estimates that for typical aerospace components, the CS repair yields a “10 to 20: 1 payback ratio,” which could end the Air Force’s perpetual drive to replace aircraft parts. James, who has deployed seven times with Air Force units to provide technical support, sees the advantage in quickly restoring mission readiness wherever units deploy.

“If you can permanently or even temporarily repair a [component], this saves a lot of downtime. It takes longer to get the cold spray equipment out and operate it than to apply the spray.

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