Advanced materials have been a part of aircraft engines since the Wright Brothers’ first flight, where due to weight they substituted an aluminum engine for one made of steel because of weight. That trend continues to this day, with engine weight still an important factor. But equally important are temperature resistance, corrosion resistance, and durability in today’s modern engines with higher internal temperatures and pressure ratios. A panel discussion at Pratt and Whitney’s media day highlighted where engine technology, and the materials required to deliver that technology, are headed.
The next generation of military engines will be adaptive, moving from high bypass ratios to low bypass ratios to provide additional performance when necessary, but also optimize performance in cruising modes. These new adaptive requirements require new investments in advanced materials and manufacturing technologies.
The future for civil engines will also require improvements to both propulsive and thermal efficiencies. Propulsive efficiencies require optimizing fan performance, and the debate between metallic and composite blades. On the GTF, for example, the ability to shape a thinner fan blade using a titanium-aluminum bi-metallic approach outperformed a composite fan, albeit that performance might reverse for a larger fan size for a wide-body engine. Inside the engine core, a variety of materials are available to withstand higher temperatures and pressures to improve thermal efficiencies. The portfolio of materials include powdered metal alloys that provide better bore and creep resistance, single crystal and fine casting of passages to reduce cooling air, and high temperature ceramic matrix composites. While CMCs are not required for the GTF, which has few stages and a small low pressure turbine, the next generation of materials include advanced alloys including Cobalt, Molybdenum, monolithic ceramics and ceramic matrix composites with higher temperature fibers – all in a portfolio of choices that will be examined for the various trade-offs in engine decisions.
Automation is taking a greater role in manufacturing at PW, as well as materials, as the company looks to optimize the manufacturing process. Automated coating, finishing, inspection, and other functions are being incorporated into new assembly lines, and process modeling techniques are being used to determine how to optimize manufacturing of parts. Adaptive grinding, electro-chemical machining and additive manufacturing have joined the portfolio of manufacturing options that, when blended with the choice of material, can optimize manufacturing processes for higher quality, higher throughput, and lower unit costs.
Additive manufacturing has been used at PW since they were a pioneer in the use of stereolithography in the 1980s, but candidate parts must pass a rigorous cost-evaluation process. While additive manufacturing can enable new and innovative designs, it must also cost-justify itself, and is not considered a panacea. PW will be delivering the first “3D printed” parts on the GTF engine, which will enter service later this year.
Ceramic Matrix Composites are a material class that entails different characteristics based on its composition. The US government is investing in this class of materials for operation at 2,700 degree temperatures. Coatings exist to enable matrices of these temperatures and beyond today, but fibers are the limiting issue for this technology. The focus of current research is to find fiber capabilities for higher temperature operations.
PW is not convinced that CMCs are best for static parts, due to low thermal conductivity versus other materials that may require less cooling air. As with most decisions, this is an engineering trade off of various parameters. In military engines, the focus is on thermal efficiency and high temperatures for additional performance, while for commercial engines the focus is on reducing temperatures for increased reliability. The less cooling air required, the better, and at PW the focus is on technologies, whether metal or ceramic, to eliminate film cooling.
While the industry consensus believes that the future is CMC, PW is more cautious in their outlook, citing the 1980s when the future was seen as ceramic engines. Today’s metal materials can surpass the performance of ceramics of that time, and PW is hedging their bets looking at all options.
Similarly, although PW is introducing their first additive manufactured parts on the PW1000G GTF engine later this year, they are concerned over the “hype” additive manufacturing has received in the media. They see another 20 years of development to get the properties right for large scale and cost-effective widespread additive manufacturing. At the same time, however, PW has heavily invested in and is a leader in powdered metal since its inception, manufacturers its own powder, and has a strong research program in additive manufacturing in addition to its production parts.
The Bottom Line:
The future will continue to see different and diverging requirements for the military and commercial sectors. Variable mission and adaptive capabilities will drive military programs, while time on wing and efficiency will continue to be the drivers for commercial aircraft. Material choices will differ with the technological and engineering trade-offs within each sector, and the portfolio of materials for aircraft engines will continue to grow with additional research and development.