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Modern repair procedures for jet engines
By Patrick Hoeveler
Materials now account for up to 60 percent of repair costs for a jet engine. The parts of a modern turbofan are becoming increasingly complex and thus more expensive. “That's why our philosophy is to repair rather than replace,” says Bernd Kriegel, head of engineering for the repair of civil aircraft at MTU Aero Engines Holding AG. However, it is becoming increasingly difficult to develop fast, high-quality repair procedures that offer good value for money because the design of the engine components is becoming more and more complex. “This complexity means that today's new engines can no longer be repaired by the same simple means as 20 years ago. Developments in the repair field are having to catch up in terms of technology. For example, we can now restore blades, whose materials used to be considered impossible to weld. Today, repairs are possible in cases where parts used to have to be replaced.”
For example, combined rotor disks and blades (known as blisks) are increasingly used instead of separate parts in compressors. Because they consist of a single piece, when damaged by a foreign body, theoretically the entire turbine wheel would have to be removed. But MTU has developed a procedure that can prevent this. The damaged or bent blade is detached and replaced by a prefabricated patch. A mill is then used to remove excess material. Pre-moulded parts cannot be used because they do not correspond to the more worn segments. Even an entire blade can be replaced, which represents a highly critical point.
This is all made possible by an inductive, high-frequency pressure welding procedure patented by. After the blade is detached, the replacement part is adapted to fit and an induction loop is placed around the weld. The current from the copper winding creates an extremely strong magnetic field that heats the material and makes it fusible. The new blade is then pressed onto the blisk and the excess material that is squeezed out is removed. “The procedure is contactless, and you can introduce targeted heat,” explains Kriegel. “It is therefore also suitable for parts that are difficult to access such as damaged blisk blades.”
Despite these high-tech methods it would, of course, be better if the blades didn't get damaged in the first place. In aggressive ambient conditions, as found in sandy regions or near the ocean, enormous damage can often be caused in the compressor. The engine sucks in particles of sand that badly erode the tips of the compressor blades. The front edges of the blades get thinned out and the rear edge becomes razor-sharp. Now, however, there is a new erosion-protection layer that will prevent this loss of material. The composition of ERCoatnt, which makes use of nanotechnology, is secret. This miracle coating contains different materials and consists of many nano-layers. The protective skin is around 30 µ thick. “It must be hard, but not too hard, to prevent it from chipping. The secret lies in the many layers, which have a cushioning effect,” explains Kriegel. The material is vaporised in a vacuum by means of an electric arc, and the part to be coated is held in the vapour. The titan nitride layers used up to now cannot be used with all materials, and they can only be used up to certain temperatures, according to Kriegel. “ERCoatnt is suitable for temperatures of up to 650 degrees Celsius and can thus also be used in the compressor delivery duct. The layer prolongs the life of the parts and contributes to more stable engine performance. Without a protective layer, there is a likelihood of wear by friction and thus cracking.”
This procedure is part of the “MTU plus repairs” brand launched last year, which focuses on high-tech repairs, including the repair of damaged turbine blades. After the remains of the heat insulation layer are removed, the blade is returned to its original shape by welding material onto it. This laser cladding process shoots metal powder into the laser beam, thus applying it to the surface of the blade. A milling and grinding machine is then used to trim the blade into the right shape. After cooling holes are bored, the part receives a new protective layer. The throughput time for the whole procedure, including analysis, is around 20 days, according to Kriegel. “The life of the blade afterwards is then almost as long as that of a new part.” The effort is worth it, because the repair of a V2500 high-pressure turbine blade, for example, costs 1000 dollars, whereas a new part costs 6000 dollars.
Nevertheless, there are clear limits for safety reasons. The remaining walls of each part must have the thickness specified by the original manufacturer. These thicknesses must not be violated. A process known as balance stripping comes into play at the beginning of the repair cycle. During the chemical removal of the old protective layer, the acid also attacks the exposed underlying material. However, the MTU process only removes the layer where necessary. The system scans the thickness of the layer and adjusts the amount of solution used accordingly. “In a fully automatic process, a uniform thickness of the remaining layer of the blade is achieved by means of individual stripping parameters,” explains Kriegel. “This allows us to subject the part to a further repair cycle and even repair it a third time. But our motto is still safety first.”
From page 82 of FLUG REVUE 4/2006
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