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Technology + Science The repair mavens By Nicole Geffert While engineers elsewhere may resort to virgin parts to replace worn or damaged ones, the specialists at MTU Aero Engines prefer to repair, also when the parts involved are complex. They produce high-grade reconditioned parts that in terms of quality and reliability are as good as new. MTU’s customers appreciate the expertise involved: the company’s innovative high-tech repair processes indeed slash their maintenance costs and ensure the repaired parts perform flawlessly and the lives of their engines are extended. A technology leader and independent provider of engine maintenance (MRO) services, MTU vigorously presses ahead with the development of innovative repair techniques. “We aim to be and remain a technology leader in our market,” says Bernd Kessler, president and CEO commercial maintenance. MTU is clearly a step ahead of its competition. “We’re combining the knowhow of an engine manufacturer, one that develops materials and components, with our MRO background,” explains Bernd Kriegl, who supervises MRO repair engineering. “Our winning approach is we’re transitioning our design and manufacturing know-how to our repair activities, and vice versa. Typically, we’re world-class in blisk technology and so it stands to reason we pretty well know also how to repair these integrally bladed disks.” MTU experts have found a reliable repair solution even for heavily damaged blisk blades: patching. “We use an adaptive cutting process to remove the damaged portion of blade tip or edge by fully automated precision machining. That leaves the load-bearing structure of the blade intact,” explains Winfried Lauer, senior consultant, military engines. In a next step the blade is then restored to its original shape by welding. For the repair of the blisk’s titanium components, tungsten plasma arc welding has proved a trusted approach, because it produces a sound weld. The fully automated welding process is performed in an oxygenfree atmosphere to prevent oxidation. “The reconditioned component is 100 percent free Blisk repairs are among the most innovative practices in the industry: shown here are preparations for tungsten plasma arc welding at the Munich location. 10 REPORT REPORT 11
Technology + Science An EJ200 blisk being repaired using tungsten plasma arc welding. Following repair, detailed gaging makes sure the blisk is precisely within tolerances. Heat treatment, too, is a process step following the actual repair. of pores in the weld. It’s as good as new,” emphasizes Armin Eberlein from repair engineering, military engines. Before the weld is machined using an adaptive milling process to obtain an optimum contour, the repaired area is heat treated. “We use induction coils to generate a local temperature field,” Lauer explains. “That completely removes thermal stresses induced in the part during welding.” After the subsequent milling process, the part is compaction peened. This produces a compression stress condition to set off tensile stresses arising in the component during flight service. “It was on the EJ200 military program that we developed comprehensive capabilities in blisk repair, and now our commercial cus- MTU Plus Laser Powder Cladding is used to repair worn blade tips. tomers are benefiting from that as well,” says Bernd Stimper, senior manager, blisk repair. “We can transfer these sophisticated blisk technologies to commercial engines like the PW300, PW500, PW6000, GP7000 and GE90.” A challenging technique also is the high-temperature brazing process that MTU specialists have developed for the repair of the turbine center frame (TCF) in the GP7000 powering the A380 mega-transport. “We designed the component and know its stresses,” says Hans Banhirl from repair engineering, GP7000. The challenge was that the TCF uses a novel material for which so far there had been no suitable repair procedure. “Repair welding was no option, because of the cracks it would produce in the highly heat-resistant cast nickel alloy,” explains Karl-Heinz Manier from repair development. He led a team of experts that worked on a brazing material and a heat treat cycle for optimum strength. Successfully so. Says Banhirl: “Our new technique is now available for the GP7000 production launch.” “Repair beats replacement” is the mantra also of MTU Maintenance Hannover. This is where Dr. Frank Seidel is responsible for repair development: “MTU Plus Repair is our trade name for the innovative repair procedures we use to more flexibly respond to individual customer requirements. MTU Plus Repair satisfies demanding criteria like improved life and function.” Among these procedures is MTU Plus Balance Stripping, an electrochemical stripping process. What’s so special about it is that it strips each blade The plating shop at Hannover uses the balance stripping process, among others. separately. Its measuring robot, in use since 2006, gages each blade to determine the thickness of the coating to be removed and see how long it will have to dwell in the chemical solutions. “The reason is we want to strip only as much of the old coatings as necessary,” explains Seidel. “We don’t want to interfere with the original wall sections. It makes the blades repairable several times over, giving them a second, third or fourth lease on life. It goes easy on our customers’ pocketbooks without sacrificing any of the quality.” The same holds true for MTU Plus Laser Powder Cladding, where worn tips of highpressure turbine blades are repaired by welding, with a laser beam depositing powdered metal, which melts at 1,500 degrees centigrade, on the component. Remaining weld flash is removed by high-precision grinding. The procedure produces work of a quality impossible to achieve manually. The fully automated machine, developed inhouse, ensures process stability. Says Seidel: “We use continuous inspections like X-ray and others to ensure that the parts going into the engine are the highest quality.” Repair activities at the various MTU locations are coordinated to leverage synergies, as perhaps on LM2500 and LM6000 industrial gas turbines derived from CF6-6 and CF6- 80C2 aircraft engines, respectively. “Procedures we’re developing at MTU Maintenance Berlin-Brandenburg for the LM series can advantageously be used also on CF6 engines at Hannover,” says Christian Hornig, who works in repair development at MTU’s The five-axis milling machine—here shown at the Ludwigsfelde location—restores deformed blades to their prior contour in a fully automated process. Ludwigsfelde location. One of the location’s special forte is the fully automated adaptive restoration of the contours of deformed LM series blades. The need to perform elaborate manual grinding has been obviated by a fiveaxis milling machine controlled by sophisticated software. “The particular challenge was to cope with the individual geometries of blades pulled from current service, each sporting its own wear pattern,” Hornig elaborates. The machine gages the actual contour and matches it with the original shape. From the actual-versus-specified data it generates a fully automated milling program. According to Hornig, “automating the process saves time and money and so lets us offer repairs at affordable prices.” To further reduce customers’ maintenance costs by the flying hour, the various repair procedures remain under continuous development and improvement. “We’re hoping to expand our component repair business,” Kriegl says. “We want to outpace the market and accelerate the introduction of new repairs.” For additional information, contact Bernd Kriegl +49 89 1489-3315 To download images associated with this article, go to: http://www.mtu.de/107Repair 12 REPORT REPORT 13
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