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Next Steps For Using Additive Manufacturing For Repairs

How additive manufacturing will open new avenues for aviation aftermarket providers.

Printed headline: A Quick Fix

 

Amid all the excitement about the potential of additive manufacturing (AM) in the aftermarket, today’s applications are often quite prosaic. Plastic air vents, window breather pipes and video-monitor shrouds are understandable starting points for MRO companies familiarizing themselves with the technology, but they are unlikely to persuade that 3D printing will transform the industry anytime soon.

Of course, much bigger developments are underway at aircraft and engine manufacturers, which are investing billions in AM and have already begun producing some metal components. Examples include fuel nozzles for the CFM Leap engine and a 1.5-m-wide (4.9-ft.) front-bearing housing for the Rolls-Royce Trent XWB engine.

In principle, the metal printing process is very similar to that used by MROs to produce plastic cabin parts, but metallic AM components can exhibit lower static and fatigue strengths than rolled billets of metal. Overcoming such challenges requires considerable investment and testing, which may keep the production of more advanced components outside the reach of airlines and MRO providers.

“The AM manufacturing methods, and to some degree the materials, lack the same degree of industry standardization that we now take for granted with metallic and composite laminated parts,” notes Victor Ho, AAR vice president for engineering. “AAR has found part-to-part variability during structural tests of AM articles that were printed between similar machines.”

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Instead of complex structural and metal components, the MRO community is more likely to focus its AM efforts on parts and tooling that are simpler to prototype, produce and certify. The other avenue for them to explore is additive repairs.

Depending on how one defines AM repairs, the technique is either in its infancy in aerospace or is building on decades of prior experience. Certain types of welding are a form of additive manufacturing, although the techniques commonly associated with fabricating components—such as fusing or melting metal or plastic powders—are still under development in the repair context.

“Welding is one form—and there are many others—of AM that has been around for decades, and in many situations it is a great way to restore parent material that has been lost to corrosion or wear,” says Travis Guenther, aerospace product engineer for Lucideon, an engineering and consultancy company for materials technologies.

Given that, he says, “AM is just as important to the repair of aircraft components as it is to rapid prototyping of mock parts or manufacturing fixtures for parts.”

Added Advantages

Some industry experts think the first regulatory approval for a nonwelding-based AM repair could come this year, opening the door to a new way of thinking about component support.

“The sky is the limit for AM repair,” says Ho. “Particularly for composite components, AAR foresees the use of imaging, CNC [computer numeric control] machining and AM technologies as a fully automated repair process.”

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Indeed, AM is set to be a crucial part of automated, end-to-end repair processes that encompass inspection, repair and testing, but the technology also has intrinsic advantages when it comes to repairing metals, composites and other plastics.

Combined with optical scanning technologies, AM allows for the repair of complex geometries, either to treat wear, restore design shape or both. Another advantage is that the amount of post-processing in powder-bed-based repairs is reduced when compared with many welding techniques, which require excess material to be machined off afterward. Repair of complex geometries is also served by the lower heat input of printed repairs, which avoids the thermal distortion that can occur with welding. The application of less material and less energy to the repair should also lower costs.

“Additive manufacturing offers the possibility to rebuild the worn material such that the repaired component is in a near net-shape condition,” says Aenne Koester, head of Lufthansa Technik’s additive manufacturing center in Hamburg.

In other cases, AM will allow repairs that, while already technically feasible, were economically unsound due to the cost of labor. “It will be a faster response as soon as the qualifications and precertifications have been done and validated,” says Frederic Becel, additive manufacturing leader in the aircraft modification unit of Air France Industries KLM Engineering & Maintenance (AFI KLM E&M).

Ho agrees. “Performing repairs on highly contoured parts with large damage may be more economically and efficiently performed, where previously the parts had to be replaced,” he says.

With more repairs on the table, MRO companies and airlines will become less subject to long lead times and price inflation for replacement parts. Long waits for replacement parts have bedeviled the engine overhaul sector recently, but this might ease as AM repairs are introduced, with turbine blade tips an early candidate for the technology. There is also a strong business case for airframe structures, particularly those that make extensive use of carbon fiber.

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“As aircraft flight and fuel performance increase and external components take on more complex contours, the ability to repair and maintain previously repaired wind-swept surfaces within engineering tolerances will become more important to our customers to maintain fuel efficiency over the aircraft’s life cycle,” says Ho.

He says AAR aims to reverse-engineer complex contours by recreating external mold surfaces of large damaged areas where aerodynamic surface shape would normally be lost after repair. The company has a head start in this respect as a result of work with an OEM, which involved the fabrication of molds exceeding 60 in. in length for composite layup. “This developing knowledge base will lay the foundation for applying AM to the AAR’s MRO operations in the future,” says Ho.

Tooling

Another application of AM, and one that is already in use, is to build and repair tooling. Until recently, Estonia-based Magnetic MRO had mostly used AM for prototyping, but it is now exploring whether it could speed up certain repair processes with custom tools. One example is a drilling jig for surfaces with complex curvatures.

“Such a process would involve 3D-scanning the surface, building a necessary CAD file that would match the curvature of the scanned surface and then printing and preparing the final jig,” says Partel-Peeter Kruuv, interior project manager for Magnetic MRO.

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Both AFI KLM E&M and Lufthansa Technik also use AM for rapid tooling, with production time at the former estimated at roughly 1/10th of the time needed to have a new tool delivered.

A separate issue regarding tooling is the extent to which MRO providers can use their existing AM machines—mostly used for fabricating parts—for repairs.

“It is quite likely that equipment used for AM of new components could be suitable for repairs, but it will depend on the technology used,” says Guenther. “For example, powder-bed fusion [machines] may work for new-part manufacture but likely won’t work well for repair.”

There is also the question of how well software designed for 3D-printing fabrication works for repair functions.

AFI KLM E&M’s Becel says the company’s existing additive-layer manufacturing tooling is not well-suited for  repairs. “The specifics of the repair need to be better managed by the software,” he says, noting that simulation of the repair process and of the mechanical properties of a component post-repair are key challenges. “We are looking for equipment and software that will better allow us to do this kind of work,” he adds.

Choice of Techniques

There are numerous modes of 3D printing, each suited to certain jobs. Stereolithography (SLA) is often used for prototyping plastic parts and works using lasers or light to cure a liquid plastic resin to build a structure top-down, layer by layer. Selective laser sintering (SLS) works in a similar way, but instead of a liquid resin, powdered material is fused together with high-powered lasers. As a result, many different materials can be used, including metals, glass and ceramics.

Fused-deposition modeling (FDM), in contrast, builds from the ground up. A machine extrudes a plastic filament that is melted by the printing nozzle and then hardens after deposition.

Selective laser melting (SLM) fully melts the metal powder rather than just fusing it together, as occurs with SLS. This technology creates dense components but is currently restricted to certain metals. Electron-beam melting (EBM) works in a similar way.

A form of additive manufacturing already used for engine repairs is laser metal deposition (LMD). Also known as laser cladding, this process uses a laser to generate a weld pool on the component surface. Material is then added to the melt pool as a powder or wire. The melted particles fuse and solidify while the nozzle is manipulated to add the desired structure to the component.

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AFI KLM E&M and its subsidiary CRMA have performed laser-cladding repairs for many years and believe this provides a strong foundation for future repairs using AM. “At first, we will look to use new technologies on our existing use cases of surface reconstruction in order to see if we can improve results, costs and times of repair. Other cases can be for tooling, with which we can restore initial dimensions after several uses,” says Becel.

Lufthansa Technik offers laser cladding as well but also is pursuing powder-bed-based AM repairs, an approach that “opens up completely new possibilities in the overhaul and repair of aircraft engines,” says Koester.

Nonetheless, powder-bed repairs are difficult, mainly because powder must be applied to an existing component, rather than being fused or melted inside a standard AM manufacturing platform. As a result, specific fixtures must be developed for each component to be repaired. Even so, she is confident Lufthansa Technik will overcome such difficulties and plans to have its first powder-bed repair certified for 2020.

Gaining Approval

“It is clear that there is strong interest in adding AM to the daily activities of MROs, but regulation in its current form is very unclear about printed parts or repairs,” says Magnetic MRO’s Kruuv.

Although certain directed-energy deposition processes such as laser cladding are already approved, as are the manufacture of certain components via powder-bed fusion, there is uncertainty about how quickly regulators will approve the latter form of AM for repairs.

“AAR expects some hurdles in showing certification compliance, especially in the areas where the repair requires equivalent strength to components previously manufactured using conventional manufacturing methods,” says Ho.

AFI KLM E&M’s Becel concurs: “Yes, it will be more difficult to gain approval due to the lack of general experience in our industry and the lack of feedback on this type of repair process,” he says. “Those technologies have only an experience of a very few years, and it is difficult for an authority or a regulator to be very confident without sufficient experience.”

That said, others point out that once-novel AM repairs such as LMD and electron-beam welding gained certification many years ago, so regulators can have reasonable confidence about applying their approval methodologies to new techniques.

“It won’t be any more difficult to gain regulatory approval for AM repairs than any other new process that has been introduced to maintenance, repair and overhaul over the last 50-70 years,” says Guenther, adding that electron-beam and laser welding “are very specialized AM processes that required significant testing to prove out, but it has been done.”

Gaining Traction

Once new additive repairs are approved, the door opens to their incorporation into automated and semi-automated component overhaul processes. “Many of the technologies to do this currently exist, and it’s just a matter of integrating software and systems,” observes Ho.

Lufthansa Technik is developing an automated end-to-end process with its AutoInspect and AutoRepair robots, which are designed to inspect and repair cracks in certain combustor components, and this experience will prove useful if it seeks to integrate automated AM repairs.

“It won’t be long before parts will be inspected by a machine, conditions identified and parts repaired through AM and subtractive manufacturing—all on one machine,” says Guenther.

In time, AM repairs will extend to electronic and structural components, Becel believes, but as the technology matures it will bring new challenges. For example, today’s certified and in-development AM repairs focus on conventionally manufactured parts but might not be suitable for 3D-printed parts, which have different internal structures.

Overall, AM is set to open new avenues for MRO providers to improve the speed, cost and scope of repairs. In the long term, Ho speculates that AM might even require “some alternative basis of certification or authority” if it leads to a surge in demand for quick-reaction MRO services such as AM-based just-in-time parts replacement and repair.

“We are at the ground level of application of AM technologies for manufacturing and repair and are excited for the future,” he concludes.

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