DN Staff

May 26, 2011

6 Min Read
How Advanced Materials Improve Aerospace Engines

To achieve greater enginefuel efficiencies, engines are running at higher temperatures and must becooled with more intricate cooling schemes, requiring the casting of complexcooling passages. Stronger metal alloys are being used in the casting process,and the core material must be able to withstand the extremely high temperaturesused to pour these alloys.

How Advanced Materials Improve Aerospace Engines

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As an example, consider the gas turbine, the efficiency of whichis largely determined by turbine temperature since the less cooling air used,the more air is available for propulsion. Increasing temperature capability ofthe turbine is therefore key to improving such engines. Since engines runhotter as processing temperature is increased, there is a need for demandingmaterials to put together the engines.

Seeking ways to lower cost and emissions while increasing fueleconomy and performance, engine designers have been turning to advancedceramics and high-temperature metal materials. The ability of these materialsto withstand heat is key to making engine improvements.

The Ancient and Modern Art of Brazing

Brazing alloys are usedfor metal-to-metal bonding in engine MRO (maintenance repair and overhaul),assembly of aerospace components and repair of micro-cracks. They are also usedfor ceramic-to-metal assemblies requiring joining by metallizing ceramic surfaceand brazing of components, including pressure and temperature sensors,thermocouple housings and fire-detection feed-thrus.

Brazing is a term used for high-temperature joining attemperatures above 600C. In a general sense, brazing is a joining process thatrelies on the wetting flow and solidification of a brazing filler material toform a metallurgical bond, a strong structural bond, or both between materials.The process is unique in that this metallurgical bond is formed by melting thebrazing filler only; the components being joined do not melt.

Research into the development of advanced brazing materials foraerospace engine component repair has given rise to both precious andnon-precious alloys. Precious alloys (for example, gold, silver, platinum andpalladium) are used mainly in original equipment manufacturers' assemblies forvanes, nozzles, sensors and igniters. Non-precious alloys are used in MRO andare constantly evolving as better and more heat-efficient alloys are developed.

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Click here for larger image.

As shown in the table, a number of new brazing alloys areavailable for use in aerospace engine repair and reassembly. For example,Morgan Technical Ceramics' Wesgo Metals business (MTC-Wesgo Metals) suppliesNioro, a low-erosion alloy that allows the base material to retain itsproperties and is a good choice for repairing fuel systems and compressors.

Another example of the superalloys availablefor high-temperature braze repair applications are pre-sintered preforms(PSPs), a customized blend of the superalloy base and a low melting braze alloypowder in either a plate form, specific shape, paste or paint. PSPs are usedextensively for reconditioning, crack repair and dimensional restoration ofsuch aerospace engine components as turbine blades and vanes. Thin areas andcrack healing is done with paste and paints, while preforms are used fordimensional restoration.

With turbine temperatures reaching up to 1,300C (2,350F) and thepresence of hot corrosive gases, aerospace engine components experienceconsiderable erosion and wear. The pre-sintered preforms are customized to fitthe shape of the component and then tack-welded into place and brazed. PSPs areoffered in various compositions and shapes, including curved, tapered andcylindrical, as well as paste and paint. They save time and money and extendthe life of engine components by up to 300 percent, making it a more reliableand cost-effective method than traditional welding, which requires post-brazemachining or grinding. Brazing allows whole components to be heated in a vacuumfurnace, reducing distortions and increasing consistency, resulting in ahigh-quality repair process.

PSP plate thicknesses range from 0.010 inch (0.3 mm) to 0.200 inch(5 mm). In addition to plates, Morgan Technical Ceramics-Wesgo Metals suppliesPSPs in pastes for filling oxidation corrosion fatigue cracks, and paints, whichare best suited for deep, narrow micro-cracks.

Advanced ceramics are ideally suited for aerospace applicationsthat provide a physical interface between different components, due to theirability to withstand the high temperatures, vibration and mechanical shocktypically found in aircraft engines. For example, Morgan TechnicalCeramics-Alberox business provides aerospace engine pressure and temperaturemonitoring sensors, thermocoupling housings, and fire detection feed-thrusconstructed from a variety of metal components and high-purity alumina ceramic.Ceramic-to-metal components are sealed to metals by the high-performancebrazing alloys, providing a reliable seal.

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Investment Casting

Investment casting is akey process used in the production of aerospace engine blades; high-qualityceramic cores have emerged as the material of choice for use in the investmentcasting process. Investment casting of new super engine alloy materials enablesthe development of more intricate designs that perform better in engines.Operating temperatures have increased, from about 400 to 1,100C, and along withthat change has been an evolution in materials that meet the demand forsurviving these higher temperatures.

Fused silica ceramic cores are used in investment airfoil castingof blades and vanes for rotating and static parts of aerospace engines. Theprocess is used primarily with chrome-bearing steel alloys. Advanced ceramicswith controlled material properties allow component designers to make specialcooling channels that keep engines from overheating. These ceramic cores arecapable of producing thin cross sections and holding tight tolerances, whichhelp produce accurate internal passageways. The ceramic cores are strong enoughto withstand the wax injection step in the investment casting process. Whilethe casting is poured, the ceramic core remains stable, yet is readily leachedusing standard foundry practices once the casting has cooled.

For example, Morgan Technical Ceramics' Certech business(MTC-Certech) has developed a ceramic core with its proprietary P52 material,which exhibits greater dimensional accuracy while maintaining tight toleranceswithout distortion. The cores remain stable at high temperatures and do notprematurely deform, which is important, given the extremely high temperaturesrequired for engine component production. The cores can be chemically dissolvedafter the casting has cooled, leaving the clean air passage replica needed intoday's efficient turbine engines.

While dimensionally strong, the P52 core material also exhibitsimproved crushability during solidification. This means that it remains rigidand stable through the casting process but is crushable when it needs to beduring the metal solidification process. This is particularly useful for alloysthat are prone to hot-tearing (those that exhibit lower core temperature inequiax castings) and/or recrystallization (castings that are involved indirectionally solidified or single-crystal castings).

FredKimock is vice president, technology, for Morgan Technical Ceramics. For moreinformation on Morgan Technical Ceramics, go to http://www.morgantechnicalceramics.com/.

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