Printed headline: Cold Is Hot
Additive metal technologies have received plenty of notice in recent years as processes to transform how aircraft parts are created, but repair methods also can benefit from these techniques.
Enter cold spray, an additive surface-repair technique that allows replacement parts to be eliminated without the pitfalls of using traditional metal-repair processes such as welding or thermal spray.
Cold spray takes metal particles and accelerates them at supersonic speeds within a jet of expanded gas toward a solid surface, where enough energy is generated for the particles to bond with the surface. As its name implies, cold spray has application temperatures that are much lower than for other thermal-spray and welding processes, which means the distortion and stresses associated with those repair techniques are avoided.
The repair technique has been around for well over a decade, but much of its research and development, as well as its usage for airframe component repairs, up until recently has been limited to military applications. The U.S. Army Research Laboratory (ARL) began working on applications for aerospace in 2000 and established its Center for Cold Spray Research and Development in 2001. The center has implemented cold-spray repairs on aircraft such as the B-1 bomber, F/A-18 fighter, and Black Hawk and Seahawk helicopters. According to Victor Champagne, team leader at the ARL Cold Spray Center in Aberdeen, Maryland, the center has been spearheading implementation and development of the technology and works with all of the major companies that use cold-spray repairs. Champagne says many of the military applications have now been spun off to the commercial side.
Moog, which has worked with the ARL on the development and qualification of cold spray for civilian and military MRO, has been developing its cold-spray repair techniques since 2007. Its solutions use titanium, stainless steel, copper and other feedstock powders for repairs on a wide variety of components such as wheels or body panels. Moog says its most common cold-spray application is for restoring material lost to wear or corrosion—often on magnesium and aluminum substrates. Transmission housings and gearboxes of all types have been popular items.
“Although the technology is robust, we are still early in its lifespan,” a company spokesman notes. “As more and more repairs are developed, more aviation maintenance professionals will see those repairs and want to use cold spray for their own needs.”
One company following suit is GE Aviation subsidiary Avio Aero. The company began researching cold spray in 2010 in partnership with various universities and research centers in Europe. Right now, Avio Aero’s research is focused mainly on aluminum components, but the company is looking into increasing the range of materials and components it uses for cold spray. It also hopes to replace legacy repair techniques.
StandardAero has been conducting in-depth testing of cold spray for a variety of components and metals since 2016. According to Keegan Smith, a special process engineer there, the company has been looking at the viability of cold-spray repairs for various parts within turbine engines. “Because the process is still new and unproven, the substantiation has required StandardAero to continuously work with various OEMs in order to investigate repairs,” he explains. “StandardAero hopes to turn more and more of these repair investigations into standard repairs that can be located in any engine manual for use across the industry.”
Smith adds that as standard practice manuals evolve along with the technology, this evolution could drive the industry to accept both softer metals such as aluminum and copper and more complex materials such as nickel-based alloys.
Another company that has become heavily involved in cold-spray repairs is Honeywell. According to Daniel Greving, Honeywell Aerospace’s director for global repair and overhaul engineering, the company began investigating the technique as early as 2004. “Over the course of four to five years, we developed a better understanding of the process, and more capable equipment became available. Much of the development was done with capable suppliers who possessed the most modern cold-spray equipment,” says Greving.
Honeywell put its first cold-spray repairs into service in 2009 and now has approximately 70 parts that are repaired that way. These include aluminum and magnesium static structures such as gearboxes, pump housings and valve bodies for turbine engines—all of which are dimensional and not structural in nature. “High-pressure cold-spray technology will be the breakthrough that enables structural applications,” notes Greving.
One project that incorporates research into making cold-spray repairs for structural applications a reality is underway at the University of Akron’s National Center for Education and Research on Corrosion and Materials Performance. Through funding from the State of Ohio, the university is partnered with Airborne Maintenance and Engineering Services (AMES) and SAFE Inc. to obtain FAA approval for cold-spray repair of corroded and worn parts on commercial aircraft.
According to Greg Smith, director of engineering and manufacturing at AMES, approved cold-spray repaired aircraft parts have been limited to accessory components such as gearboxes or housings. “What we’re trying to prove is that it can be used for actual airframe components. The premise is that we can restore those to an airworthy condition—to meet or exceed the original design strength of each item,” he says.
For the first phase of the project, secondary structural items with a lower impact on airworthiness were chosen, among them panels, an air load rib and a Boeing 767 wheel.
“The other piece that made this somewhat unique is that most all previous applications of cold spray were done using helium, whereas the application we are seeking approval for uses nitrogen,” notes Smith. He says the project is using nitrogen because of its lower cost and greater environmental availability.
Although it has been used for cold spray in military applications, SAFE Inc. President Scott Fawaz says this project to their knowledge represents the first commercial cold-spray application of nitrogen. According to Rex Ramsier, the University of Akron’s executive vice president and chief administrative officer, nitrogen’s lower cost also fits with the project’s goal of receiving FAA approval and then helping to create jobs and grow the local economy.
With all research and testing completed, the project’s repair specifications have been submitted, and the team is awaiting the FAA certification office’s final review. Smith says they hope the review will be completed soon so cold spray can be added to AMES’ repair-station capability list.
“This is a first step. The next step will be taking it into primary structural elements,” says Smith. “That will make it more attractive because those are typically the more expensive components on an aircraft.”
Once cold spray moves into primary structural elements, Smith says, it may become more commonplace within the industry—especially for aging aircraft that are expected to be in service for longer periods.
As a believer in cold spray’s structural applications, Honeywell sees the technology as having more substantial use in the future, including development of environmental coatings for hot-section protection.
“Honeywell will continue to aggressively develop cold-spray repair technology,” says Greving. “Cold spray is considered a primary repair technology and a focus area for our development efforts.” He adds that the company this year plans to install a high-pressure system at its Phoenix repair station to enable structural repairs and allow for continued development of new opportunities.