MTU (Motoren und Turbinen Union), working with Darmstadt Technical University has run a fiber-reinforced plastic (FRP)-bladed compressor rotor on a transonic test stand. MTU claims it as the first FRP compressor rotor ever known to have been tested at 20,000 rpm and 400 m/s tip speed. The compressor run was monitored via an optical blade vibration measuring system in which the rotor was taken to 20,000 rpm for several test runs. MTU believes the FRP composite will be a significant element in the quest for higher pressure ratios and efficiencies.
The composite benefits particularly from its low weight, which at the same strength is only 1/3 that of the titanium conventionally used in this application. This enables designers to give the compressor blades "daringly twisted geometries that even with high-strength titanium would be unthinkable considering the high centrifugal service loads."
MTU has also revealed that it is using a novel repair procedure to salvage low-pressure turbine casings. After about 5000 engine cycles in large commercial aircraft, wear often occurs, particularly on the inner walls of the low-pressure turbine casing at the aft end of the engine. The wear manifests as alternating notches and projecting metal spikes, each about 0.8 mm in size - the maximum allowable at the 2-mm wall section. The casing uses superalloys (mostly chrome and nickel). Although highly wear resistant, eventually vibration-induced damage could occur and the casing would then be "retired." However, research and development work by the company into novel welding techniques has led to the use of a laser powder cladding method that allows worn webs on a low-pressure casing to be repaired and returned to service.
Machinists mill a 4 x 60 x 1-mm piece out of the defective area on the casing. They then deposit pulverized metal made of the same alloy as the casing on the gap that has been cut, and weld it to the casing wall using a 5-axis laser deposition welding facility before mechanically blending the weld area. They inspect the material with the fluorescent penetrant method to visualize potential cracks in the repaired area, which is followed by X-raying the weld. If all is well, they then treat the casing in a vacuum furnace, heating it to 980°C annealing temperature to relieve stresses in the metal and restore the original metal structure. The process takes about two weeks.
According to MTU, the technique is being applied to General Electric's CF6-80C2 powerplant. GE has sanctioned the work, which results in an enormous cost saving compared to a new casing.
Stuart Birch
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Aerospace Engineering September 2000
