For decades, U.S. Air Force aircraft maintenance crews have religiously followed the “component swap” model. When an aircraft part is worn out, swap it for a new one. It’s a model which works – if you have new parts, time and money available. With all three in short supply, Cold Spray (CS) additive technology could allow maintainers to renew worn or broken parts while still on an airplane, getting it mission-ready quickly while in the field.
The 28th Bomb Wing at Ellsworth Air Force Base in South Dakota is spearheading USAF efforts to apply CS repairs to operational aircraft. Working with students and professors from the South Dakota School of Mines and Technology, personnel from the 28th Maintenance Group Additive Manufacturing Flight repaired an over-wing faring slip joint on an operational B-1B bomber using CS back in May.
The slip joint is basically a part which allows the B-1’s wing to flex vertically. What’s important to recognize is that the repair was done in situ with the joint in place. The standard practice of swapping in a new slip joint would have required the costly and time-consuming removal of the B-1’s wing.
This would have normally involved eight weeks of downtime,” says Brian James, a Ph.D. graduate student at Mines and the additive manufacturing chief engineer with the 28th Maintenance Group. “With cold spray we were able to do this in a couple of hours.“
The B-1B began active service in 1986 and all 100 bombers were delivered by 1988 so obtaining parts is difficult, often requiring cannibalization of another airplane. Combining this with the disassembly required, to replace the slip joint would have resulted in a $500,000-plus bill for the Air Force. Cold Spray repair of the original part cost a small fraction of that.
With the CS repair of the slip joint, the B-1B was flying again a week later. The process is pretty easy to understand, though it has taken years to perfect.
Cold Spray is an additive manufacturing process much like 3-D printing. In simple terms, tiny metal particles are mixed with a carrier gas (typically nitrogen or helium) and accelerated to Mach 2-3 speeds through a nozzle. For CS repairs, the particles are sprayed directly onto a part, adding material to build it back up and strengthen it.
“Basically it’s the speed of a rifle,” Robert Hrabe, CEO and co-founder of South Dakota-based VRC Metal Systems explains. “If you imagine shooting a rifle bullet into a thick metal plate, the bullet embeds into the surface, effectively welding itself there. We’re doing the same thing only the particles are 5 to 15 microns [in size], about the consistency of baby powder.”
The particles coalesce into a solid state, creating a high strength, structurally sound bond. Once applied, the new material is machined to match the original shape or tolerances of the part being repaired. In that way, CS is both an additive and subtractive process.
For the B-1 slip joint, the Ellsworth team used 6061-grade aluminum alloy particles to make the repair. Soft metals are typically used with the CS process though recent advances are allowing for the use of harder materials like titanium and Inconel. And it’s not really a cold spray.
“Cold is a bit of a misnomer,” Hrabe acknowledges. “It doesn’t melt the material like flame-spray or HVOF [High Velocity Oxygen Fuel], any of the other common welding or coating processes.”
Instead, additive materials are heated but only to less than 80% of their melting point, rendering them fungible. The process can be used to repair a variety of parts which may have suffered fatigue or common corrosion, from relatively simple wing skins to hinges, structural supports, shafts, landing gear components and more.
“One of the benefits of [CS] technology is the ability to cover large areas,” Hrabe says. “There’s no limit on the thickness [of additive material] that you can build up. We’ve even made whole parts additively using cold spray.”
Research into the potential of CS for restoring aircraft components began at the School of Mines in the mid-2000s. As it progressed, it led to spin-off companies including VRC, which makes the equipment used to apply CS.
The company makes two types of high pressure CS apparatus: a semi-portable system called Gen III that may be installed in a dedicated room or booth in a repair facility and a smaller, self-sufficient system called Raptor that can be used in the field for ColdSpray repairs.
Both consist of a base unit (a generator or welder-sized box) that heats the carrier gas and precisely controls the feed of powder into the gas stream using proprietary software. A hose and a variety of different nozzles for different spray velocity and patterns join the main controller. VRC also has a modularized system that can fit down the hatch of a submarine, be assembled to effect repairs then disassembled and removed or stowed.
VRC equipment has been used to repair parts on a V-22 as well as the B-1, joining CS repair work undertaken by the Ellsworth team on F-15 parts and Navy shipboard systems. Given the prevalence of metal hardware across the military, Cold Spray repairs could become ubiquitous for fixing parts for air, land and sea systems.
Ellsworth AFB is the center of cold spray expertise. James goes further, calling it the “focal point for field level additive manufacturing in the Air Force.”
The 28th Bomb Wing established the Air Force’s initial field-level additive manufacturing flight in 2019, working from a 15,000-square-foot Additive Manufacturing Facility on the base. The facility can not only make CS repairs, it can validate the strength of any repair within hours using a full lab with scanning electron microscope, hardness testers, tinsel testing, and other equipment.
“What we are doing here is taking technology that’s been tested and proven in the lab and infusing it right at the combat level,” James stresses. The facility at Ellsworth is the first off its kind but it’s a model James would like to see replicated at selected Air Force installations and a font of ColdSpray expertise he would like to see propagate to forward to deployed units along with portable CS equipment.
Just The Fix
Unlike the myriad technical and process initiatives that routinely rise and quickly fade into obscurity inside the Air Force, the CS idea and the Ellsworth team seem to have sparked widespread interest.
The Air Force’s(RSO) has been a leader in fostering investigation of CS by the service. The portable Raptor CS machine used to repair the B-1B was acquired with funding provided by the RSO for the Ellsworth team. RSO has also facilitated CS component repairs for additional aircraft including the A-10, B-52 (TF33 engine), C-5, E-3, and F-16.
Support for CS repair experimentation and development has come from around Air Force Materiel Command Brian James says with collaboration from the Air Force Research Laboratory, the Air Force Sustainment Center at Tinker AFB and the Oklahoma City Air Logistics Complex. The same group of players is working with the Ellsworth/SDSMT team to evaluate the potential of CS battle damage repair, 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 be successful,” James adds. “When we did this [B-1] repair we let it be known we had some physical obstacles to overcome. All we had to do was ask and we had experimental nozzles being over-nighted to us.”
VRC Metal Systems exemplifies the rarity of CS industrial partners. There are just five Cold Spray equipment manufacturers worldwide ( in Japan, Germany, Russia, Canada and the U.S.) according to Rob Hrabe. VRC is one of only three which specialize in more advanced high-pressure Cold Spray equipment that is used for structural component repair.
His company looks to be successfully crossing the “valley of death” that often ensnares other defense technology startups between initial development and government/commercial acceptance.
“Over the last several years, the military has really begun to adopt the technology,” Hrabe says. “It’s growing rapidly there and across aerospace. There are many expensive parts that now get thrown away for a minor amount of corrosion or wear that can easily be repaired with this technology.
CS may help with the maintenance backlogs that have plagued the Air Force since its shift to a two-level maintenance (flightline/depot) program that relies more on commercial depots for repair. While it has created some efficiencies, sending aircraft and parts to depots takes time, often extended by union rules which limit how long depot maintainers can work.
The predictive “conditions-based maintenance” approach adopted by the Air Force could jive well with CS repair which enhances the ability of service maintainers to decide when and where the maintenance should be performed and shortens supply lines.
Hrabe estimates that for typical aerospace components CS repair yields a “10 to 20:1 return on investment ratio,” potentially stemming the Air Force’s perpetual drive to replace aircraft parts. James who has deployed seven times with Air Force units to provide engineering support sees the benefit in terms of quick restoration of mission availability wherever units deploy.
“If you can permanently or even temporarily repair an existing [component], it saves so much downtime. It takes more time to bring the Cold Spray equipment out and get it running than it does to apply the spray.”