A quick fix – 3D printing

Jul, 2019

Amid all the excitement about the potential of additive manufacturing (AM) in the aftermarket, its 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 are unlikely to convince anyone that 3D printing will transform the industry any time soon.

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

In principal, 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, vice-president engineering for US MRO company AAR.

He adds: “AAR has found part-to-part variability during structural tests of AM articles which were printed between similar machines.”

Instead of complex structural components, the MRO community is more likely 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. Technically, welding could be seen as a form of additive manufacturing, although conventional welding wouldn’t be described as 3D printing, which is often synonymous with AM in the modern context.

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

He adds: “Thus, 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 Advantage

This article will focus on AM in the context of 3D printing, a type of repair that at the time of writing is still to be certified. Nonetheless the first regulatory approval could arrive before the end of 2019, opening the door to a new way of thinking about component support

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

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 has also intrinsic advantages when it comes to repairs of metals, composites and other plastics.

Combined with optical scanning technologies, AM allows the repair of complex geometries, either to treat wear, restore design shape – or both. Furthermore, the amount of post-processing is reduced when compared with many welding techniques, which require excess material to be machined off afterwards. 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.

“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 (AM) Center in Hamburg.

In other cases, AM will allow repairs that, while already technically feasible, were economically unsound due to the man-hour cost. “It will be a faster response as soon as the qualifications and pre-certifications have been done and validated,” says…,    for Air France Industries KLM Engineering & Maintenance.

With more repairs on the table, MRO companies and airlines will become less beholden to long lead times and price inflation for replacement parts. Long waits for replacement parts have bedeviled the engine overhaul sector in recent years, 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.

“As aircraft flight and fuel performance increases 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 lifecycle,” says Ho.

Ho 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.

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 is now exploring whether is 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 Pärtel-Peeter Kruuv, interior project manager for Magnetic MRO.

Both AFI KLM E&M and Lufthansa Technik also use AM for rapid tooling, with production time estimated at roughly a tenth of the time if would take to have a new tool delivered.

Choice of Print

There are numerous modes of 3D printing, each suited to certain types of job. Stereolithography (SLA) is often used for prototyping plastic parts and works with lasers or light to cure a liquid plastic resin and built 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 via a machine that extrudes a plastic filament that is melted by the printing nozzle and then hardens after deposition.

Selective laser melting (SLM), meanwhile, 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 and the melted particles fuse and solidify while the nozzle is manipulated to add the desired structure to the component.

AFI KLM E&M and subsidiary CRMA have performed laser cladding repairs for many years and believe this provides a strong foundation for future AM repairs. „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 on which we can restore initial dimensions after several uses,” says …

Lufthansa Technik also has experience of laser cladding 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.

However, Guenther says that “powder bed fusion may work for new part manufacture, but likely won’t work well for repair”, although he also acknowledges that the choice of AM repair depends on the type of parts.

Koester agrees that 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. Nonetheless, she is confident that Lufthansa Technik will overcome such difficulties, and plans to have its first powder-bed repair certified for 2020.

Sticking to powder-bed-based AM technology, the AM repair differs above all from the AM manufacturing process in that you have to specifically apply powder to an existing component and not to an "empty" build platform. For this purpose, it is mandatory that appropriate fixtures developed for each specific application are used in the installation space. Especially when talking about adaptive hybrid repair, it is also necessary to combine the information about the individual geometries due to the wear of the components with the design strategy of the AM system on the software side.