Additive repair techniques on aircraft engines are going down an extremely exciting path of development and application on core components, such as turbines.
The machines that Harrison Ford was hunting down in Blade Runner were definitely dangerous: androids to be defeated. Likewise, the confidence with which GE engineers command machines made up of robots equipped with Cold Spray guns or Laser heads, to rebuild and repair aero engine parts, can conjure up an image of engineers 4.0. Such events could occur, if we observe with a bit of fantasy the most advanced GE Aviation sites for additive technology development: the Additive Technology Center (ATC) and the Advanced Service Repair Technologies (A.Se.R.T.) in the United States, and the Apulia Repair Development Center in Bari (Italy).
In each of these sites, 3D printing and design freedom enabled by additive manufacturing are ordinary practice: inside the Avio Aero laboratories in Bari, partnered with and hosted by the local polytechnic school, a team of engineers handles precisely these kinds of machines. "Machines are essentially stupid ... they think they’re executing commands with absolute precision, but they need human direction to do the job accurately. They are excellent arms, with quite advanced brains, but never like the human one." Gianluca Maggipinto is a young Researcher Engineer, one of the team led by the Test Engineer Leader in Bari, Giulio Longo. The team is made up of five other young engineers: Cesare, Marco, Nicola, Paolo and Vincenzo.
Gianluca has been working at the lab for almost two years. He works 70% of the time on the Laser Deposition technique; for the other 30%, he helps the rest of his team with the development of the other additive repair technique: Cold Spray. Indeed, Gianluca specialized in Additive Manufacturing and Rapid Prototyping (one of the main benefits of 3D printing), choosing these subjects when following in the footsteps of his professor who was a forerunner in additive. This professor, knowing about the Polytechnic of Bari-Avio Aero partnership and the upcoming additive technique laboratory, suggested to his students that they could follow this avant-garde path. "Prof. Galantucci has been studying these subjects since the end of the '80s, when they were just beginning ... in the earliest experimental stages. I met my current colleagues right in the same course, when the professor told us about the laboratory and what we could work on, we sat the entrance exam and here we are!”
Recently, the Apulia Repair Development Center made Cold Spray available to airlines, certifying the first additive repair technique developed for accessory transmissions of aircraft engines. Cold Spray is a revolutionary process, also called by specialists the 3D painting: it deposits metal powder flying at velocities of up to Mach 4 onto precise models to produce and repair jet engine’s components like gearboxes (currently being done now at the Avio Aero center) without resorting to machining or welding. The A.Se.R.T. team in the US is further developing this technology even for several other engine components (i.e. high pressure turbine nozzles, structural casings and frames and possibly compressor blisks). Gregorio Dimagli leads the Additive Technologies R&D for Avio Aero, since 2015 has been cooperating with the ATC and the A.Se.R.T. “We started leveraging on Avio Aero pioneering capabilities with additive to use them even for engine components repair and we opened our Lab in Bari” says Dimagli. “The teams at ATC and A.Se.R.T. in the US always gave an incredible support since the beginning, along with their know-how and active participation.” GE researchers in the US are currently working on ways to use Cold Spray to build new parts Justas well as fixing them, and have already used the experimental design to build an airfoil for a jet engine.
Anthony Matacia is the Executive Leader for the GE Aviation Advanced Services Engineering & Repair Technologies in Cincinnati. He closely collaborates with Dimagli: “GE is currently developing repairs using DMLM (Direct Metal Laser Melting, aka 3D printing for amateurs ed.) technologies,” he says, giving a preview on the next additive repairs frontier. “Demonstrations, including material and part component testing, are in process on actual engine parts to completely produce the repair process. It is planned to have DMLM production repair capability in GE service shops in the first half of 2019.” Matacia knows that DMLM, Cold Spray and Laser Deposition will provide significant improvements in repair cycle time and capabilities.
Laser Deposition, the other technique available at the Bari lab, is currently under development and applied to real engine components. The way it works is fascinating too: metallic powder is deposited on the engine part to be repaired, or on the portion to be rebuilt, by a high-precision laser. Laser deposition is suitable for very thin engine components, which are small in size and difficult to weld or restore, such as stator or rotor blades of the turbines. “Powder fed laser deposition technologies continue offer unique repair capabilities for aircraft engine parts,” continues Matacia. “GE currently repairs some CF6 engine HPT blades using powder fed laser technology. Other applications include various static seals, retainers and structures parts, or TiAl low pressure turbine blades that are now processed in Bari.”
Watching inside the Laser Deposition machine, overseen by Gianluca and the team at the Avio Aero lab, a large robotic arm stands out at the center of a cabin about three meters wide by three meters deep. The robot has a laser head that moves on a five-axis system; the laser is directed towards a bench on which the part to be repaired is positioned, which then slides on a carriage allowing it to exit or enter the cabin through front or rear doors. The laser processing can only take place when the cabin inside has a protected atmosphere saturated with argon, an inert gas that eliminates oxygen and prevents oxidation during the process. "After that, the laser deposits the molten metal on the part to be repaired, which can take 20 to 50 minutes. Next, the turbine blade, in this case, is worked to restore the original shape, removing the residue of the metal added to the first missing part. We say it is: “New as out of the shop!"
Using reverse engineering, Gianluca can detect any defect or small missing portion on the component, and view the component reproduced in 3D on his screen. This process can be performed thanks to a laser scanner with a maximum error tolerance of 65micron, almost half the width of a human hair. At that point, "I precisely position the part on the PC, because the computer software I use to program, operate and execute the deposition must be perfectly aligned with the positioning and with the actual dimension inside the machine. And then I start to work on the defect."
The way the scanner works is stunning: in fact, there is a secondary laser in the machine that operates on the bench and captures a points cloud. Gianluca's computer software performs a triangulation using this myriad of points (i.e. positioning, size, thickness, surface, and depth data) and finally reproduces an object on-screen, to which Gianluca then applies actual measurements and controls in order to work on it in real terms. "I provide the laser with the sintering strategy as well as the commands: the machine would make inaccurate approximations, so an engineer is needed to indicate with mathematical and geometric certainty how to perform the metal deposition on the blades." This laser technique is suitable for a wide range of metals or alloys; however, these will have to undergo the necessary experimentation and development times. The Italian team is now testing TiAl (titanium and aluminum alloy), featuring the low-pressure turbine blades of the new GE9X engine: "A highly complex superalloy, due to its adaptability and malleability properties. We are welding an alloy, something that was unthinkable 10 years ago!"
It is highly probable that the Laser Deposition technique development process will follow that of Cold Spray: testing and certification by the air transport authorities in order to become a new resource for the service and repair. GE Aviation teams in Europe and the US are applying the brightest GE Store collaboration model among labs, research centers and factories. Along with the Avio Aero Pomigliano repair station (where Cold Spray is starting application for specific worn-out on the GE90 engine gearbox), the GE Aviation Wales repair station is also drawn in by this technology. “It will bring cost savings to our customers as there was no repair to salvage those parts until Avio Aero developed the Cold Spray repair.” says Martin Zobole, Component Repair Wales Engineering Leader.
GE Aviation in Wales employees approximately 1,300 people, and offers overhaul capability for the GE90, GP7200 and CFM56 engine models. It also provides component repair services for GE engines as well as for third party customers. “For me the most exciting bit is yet to come,” continues Zobole. “Wales is scheduled to receive a cold spray unit later this year. I am looking forward to collaborating with the Avio Aero team as we follow their path on our own cold spray journey!”
Everything around these GE centers and factories foreshadows a thrilling future where not just engineers, but every worker dealing with jet engines will run such amazing machines. Quite like Harrison Ford in Blade Runner, but luckily there are no two-meter high androids to be destroyed – only engine components to be repaired.
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