Low density materials not only enable a lighter engine and, therefore, less noise, fuel burn and emissions on take-off for aero-engines but they also reduce the centrifugal forces and stresses experienced by rotating components allowing faster core speeds and reductions in disc and shaft weights. Two low density materials used in Rolls-Royce products include titanium alloys and titanium aluminides.
Titanium alloys offer high strength to intermediate temperatures at a density almost half that of steel and nickel-based superalloys. As a result, they have been adopted widely in the fan and compressor stages of the gas turbine for both disc and blade applications.
The Rolls-Royce wide chord fan blade is perhaps the best example of the application of titanium, coupled with advanced processing techniques, to give a significant service advantage. Modern blades are manufactured from three sheets of titanium representing the two outer skins and the internal corrugated structure. An inhibitor is applied, to define the internal structure, and then the three pieces are bonded in a high temperature pressure vessel. The blade is twisted and the cavity inflated at very high temperature using an inert gas in a shaped die to yield its final aerofoil shape.
The total process results in bonds with properties equivalent to the parent material and an internal stiffening structure which bears its share of the centrifugal load. Compared to the original solid clapper design this gives a fan module which is 24 per cent lighter, an overall engine weight benefit of 7 per cent, with a significant increase in foreign object damage (FOD) resistance over competitor designs.
Current interest in titanium alloys is focused on weight and cost savings. This focus has resulted in the recent introduction of integrally bladed discs, or blisks, in which up to 50 per cent weight savings have been achieved by the elimination of blade fixing features.
The gamma titanium aluminides are at present the most advanced of the successor materials to the conventional nickel-based and titanium alloys. Their potential to offer cost competitive improvements in performance through significant density reductions has long been recognised. The problem, in common with all intermetallics, is low temperature brittleness. Selective additives coupled with microstructural control have gone some way toward addressing this issue and ductilities of up to 3 per cent have been achieved in some current alloys.
Manufacturing routes for both forging and centrifugal casting have been established and specimen and component test results have been very promising. A current temperature capability of approximately 750°C, limited by high temperature degradation, which in future development may be extended to 850°C, makes them suitable for back end compressor and turbine applications. The first application of forged materials will be in static and rotating compressor aerofoils while the biggest prize is the application of centrifugally cast gamma titanium aluminide to the low pressure turbine blades of the next-generation Trent XWB. Significant weight reduction can be achieved through reduced blade weight and the subsequent reductions in disc and shaft weights. Once engineering confidence is gained in running this class of materials in production engines, it is likely that other applications will emerge.
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