Electron Beam Welding delivers perfect repair
Engine Parts and Accessories Repair (EPAR)
Outer case patch repair
Lufthansa Technik has received full approval from GE Aircraft Engines for an outer case patch repair on the CF6-80C2 turbine rear frame. The repair contains welding in new material using electron beam welding.
Among other things the Turbine Rear Frame (TRF) on the CF6-80 serves as one of the structural elements linking the engine to the pylon. After several thousand cycles, this casting made of Inconel 718 is susceptible to stress cracking due to aging and high thermal loading. Per regular weld repair it can only be overhauled up to a point, and frequently it then sustains cracks again and must be scrapped.
Good reason indeed for Lufthansa Technik's engine shop to develop a better repair technique – which turned out to be a real challenge for Wolfgang Ay, the responsible engineer. At the beginning of the process, the component is heat-treated to prepare it for welding. The damaged area is then removed using a CNC-controlled milling machine, following a "racetrack" patch (named after its oval shape) which is manufactured out of Inconel 718 and fitted into the cutout. This requires extreme precision, as the weld is performed without any filler material.
The electron beam welding (EBW) is carried out in a vacuum chamber. Quality is ensured by extensive non-destructive testing, including an x-ray inspection. The surface of the patch is then carefully machined to the original shape and dimensions of the TRF. It is particularly important to reconstruct the stiffener ribs. This is followed by further heat treatment, a dimension inspection, metallurgical examination and shot peening of the repaired area.
The repair developed by Lufthansa Technik was submitted to the manufacturer for approval. GE Engineering then performed in-depth testing on it and a number of necessary corrective actions were identified. EPAR is very positive about the experience of working with GE to perfect the technique, not at least the positive response with which the new repair was greeted. This was important to the success of the project.
Meanwhile, Lufthansa Technik in Hamburg is the only company worldwide which has received full approval from GE to perform this repair. As this repair has been tried and tested in practice, it is now available to Lufthansa Technik customers. Compared with the cost of a new part – the TRF has a price tag of over $400,000 – this repair is extremely economical.