Engine makers engaged in quest for material grail
Snecma and GE Aviation are developing new materials to make future engines lighter and improve their efficiency. In the works are alloys using exotic metals such as niobium, and composites using organic, ceramic or metal matrices. The two companies will employ these technologies for the Leap-X engine they are developing under their CFM joint venture (Hall 4 Stand B13) and possibly for other projects.
Current engines are made using titanium alloys in their cold section (that is, the fan, booster and compressor). “Titanium, which has a relatively low density, can be used up to 500 to 550 degrees Celsius,” Jean-Yves Guédou, a Snecma expert in metals, told AIN. Steel, composites and aluminum also can be found in the cold section.
For the hot section, engine makers prefer to use nickel-based and, to a lesser extent, cobalt-based materials called superalloys, he said. This means the metallurgic process has given them higher resistance to heat and mechanical stress. “Cobalt is heavier and more expensive than nickel but is slightly better at very high temperatures, around 1,100 degrees Celsius. It is also more resistant to corrosion,” Guédou said.
In the hot section, GE has traditionally used cobalt alloys, but change is coming. About a decade ago, Japanese researchers identified cobalt-based, precipitation-strengthened superalloys as showing greater high-temperature strength. In 2006, researchers said these superalloys–made of cobalt, iridium, aluminum and tungsten–were very promising as candidates for next-generation high-temperature materials.
This is only the beginning of looming change, experts say. There are two motives driving a quest for new alloys and composites. First, as engineers try to make engines less fuel-thirsty, they tend to increase the bypass ratio. This increases the fan diameter and, in turn, makes the turbofan heavier–a condition that cries out for lighter materials.
A second way to cut fuel consumption is to improve the engine’s thermal efficiency. “If we strongly increase the compression ratio and raise combustion temperatures by around 200 degrees Celsius, we have a potential 5- to 10-percent gain in fuel efficiency,” said Vincent Garnier, Snecma’s research and technology director. This calls for higher operating temperatures for parts and therefore suitable materials, he said.
GE is already using titanium aluminide (TiAl) in its GEnx engine, which is powering both the Boeing 747-8 and 787 in test flights. A so-called intermetallic compound, TiAl features an ordered structure with strong interatomic bondings, which provide high strength at lower ductility than metal alloys. In other words, TiAl’s behavior is close to that of ceramics and, therefore, is relatively brittle. This drawback can be countered by the addition of other elements, such as niobium and chromium.
TiAl’s main feature is its ability to withstand heat up to 800 degrees Celsius, which is more than aluminum or titanium separately. Yet, its density is about half (3.9 versus 8) that of more typical nickel alloys. It also can operate in severe conditions such as high-corrosion or high- oxidation environments.
“There is a challenge in processability, which is why development has taken so long,” said Bob Schafrik, general manager for materials and process engineering department at GE Aviation. Still, the GEnx’s low-pressure turbine won’t likely be the last engine component that benefits from the use of TiAl. “We are studying other intermetallic compounds,” Guédou added.
Engineers seem to place renewed hopes in ceramics, which had been much touted in the 1980s, as they can withstand 1,300- to 1,500-degree Celsius temperatures. However, this family of materials is still in an early research stage.
Closer to production are ceramic-matrix composites, which can work at 1,100 degrees Celsius. They can used for turbine blades and nozzles, for example, Guédou said. “Ceramic-matrix composites are already tested on several of our engines,” added Schafrik, who sees applications in the next five to 10 years for powerplants such as a later version of the Leap-X.
New Metal Matrix
Another group of composites with a metal matrix also looks promising. They can be made of a titanium alloy matrix around a silicon carbide reinforcement, for example. Thanks to the strength of the silicon carbide fibers they have higher resistance, so parts can be made smaller.
“Such materials can help in designing parts submitted to high centrifugal forces and cycle fatigue,” Garnier said. This makes metal matrix composites suitable for parts in the booster and compressor, such as disks.
However, Shafrik only partly agrees with this approach. In his view, metal matrix composites are a niche material because of their high cost. “Titanium matrix composites, thanks to their strength and stiffness, are suited to certain sorts of link parts, such as long cylinders. But we usually find cheaper solutions,” he said.
Among materials used for lower temperatures, polymer resin matrix composites compete with titanium and aluminum. They can withstand temperatures of 200 degrees Celsius, or even a bit beyond, depending how long they are exposed to the temperature.
Snecma has begun full-scale endurance tests with three-dimensional-woven fan blades for the Leap-X1C–the first version of the engine, for the Chinese Comac C919 airliner. The resin transfer molding (RTM) process wraps the woven carbon fibers into a resin designed for crack resistance. The development schedule calls for the new fan blades to undergo certification tests in 2014.
The design uses the same material for the fan case. In total, Snecma expects to save 1,000 pounds in weight on the airplane, thanks to the new fan section and knock-on benefits such as the fact that a lighter pylon will be sufficient for the lighter engine. The RTM fan blade, as Snecma calls it, will be ready for the 2016 entry into service of the C919.
In the high-pressure turbine range, it is hoped that a silicon-niobium intermetallic compound will help in manufacturing more heat-resistant blades–up to 1,300 degrees Celsius. The challenge is in protecting the compound against oxidation. Tin and aluminum can be added to the mix, but while this improves the situation, it is not a complete solution so an effective coating also is needed. “We are now working on a metal-based coating,” Guédou explained. Engines equipped with silicon-niobium turbine blades will not fly until 2020, he said.
These new alloys, superalloys and compounds are less ductile–more brittle–than today’s engine materials, and engineers have to take these properties into account right from the design stage. “Manufacturing is more challenging,” Guédou said.|
Steel and Alloys
GE also wants to continue improving steel. This high-strength metal is used in bearings, shafts and gears, where strength is critical. “For a shaft, the higher the strength, the smaller the diameter, which greatly influences the configuration of the engine,” Schafrik explained.
In future engines, there is room for further improved nickel superalloys, he said. “We desire higher temperature alloys, with lower thermal expansion for some applications. Also, we have learned that protective coatings should be developed concurrently with new superalloys.”
Titanium components could be formed from “meltless” titanium powder that is derived from a vapor or liquid. “This meltless titanium technology can substantially decrease the number of major processing steps and provide large improvements in product yield, energy use and emissions,” researcher Eric Ott said in a recent paper. However, meltless titanium is still at an early stage of research and development.
Some rare materials, like rhenium, will be less commonly used mainly because there is concern about their long-term availability. “That’s a change from the past, when conventional wisdom was that such materials would always be readily available,” Schafrik said.