Research from the European Space Agency (ESA) has resulted in an aircraft-grade titanium-aluminum alloy that has properties similar to nickel superalloys but weighs half as much. The research was conducted by participants in the Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS) project, which the ESA manages.
It's not news that titanium and titanium-aluminum alloys can be lighter and at least as strong as the nickel superalloys used in conventional jet engines, but casting them in complex shapes such as turbine blades has not been easy. The researchers estimated that in the next eight years, manufacturers will produce more than a million jet turbine blades. Using titanium aluminide could reduce their weight by 45 percent over components made of traditional materials.
Research from the European Space Agency has helped to develop an aircraft-grade titanium-aluminum alloy that's half the weight of conventional nickel superalloys but has similar properties. This alloy could make jet turbine blades (such as this one shown in flight) 45 percent lighter. (Source: Creative Commons–A. Rueda)
IMPRESS, a pan-European multi-disciplinary research project in applied material science, consists of 40 research groups and companies. Its primary goal is understanding the links among material processing, the resulting processed material's microstructure, and the final properties of new intermetallic alloys. Topics studied include heat transfer, solidification, mechanical properties, catalysis, circular motion, and microgravity. Besides looking at alloys and solidification processes, the project's team has examined centrifugal casting methods. Applications range from aerospace components to power generation systems. The project is co-funded by the European Commission.
The researchers looked at how changes in gravity affect the behavior of metals during their solidification process. They heated aluminum samples in a small furnace carried by a sounding rocket. After the rocket was launched in Kiruna, Sweden, during six minutes of free fall, or microgravity states, the samples were heated to more than 700C and monitored via X-ray as they cooled.
After viewing the results, the research team decided to try melting and solidifying the metals under the very different conditions of hypergravity. They used the ESA's Large Diameter Centrifuge, located at the European Space Research and Technology Centre, to cast the metals in a centrifuge at up to 20 times normal gravity. This helped ensure that the liquid metals could fill every part of the mold -- even molds created to produce complex component shapes. The result was a perfectly cast alloy that can withstand temperatures of up to 800C.
We've reported before on titanium blades used in jet engines by Pratt & Whitney. Last spring, the company began flight testing its PurePower PW1200G engine family under its PurePower Geared Turbofan program. The fan blades used in this program are made of a proprietary hybrid metallic substance that includes titanium and other metals. Pratt & Whitney concluded that the metallic materials demonstrate better impact resistance for smaller engines (such as the ones in this class) than either pure titanium or molded composites.
The cost of titanium isn't as astronomical as it used to be, based on information I've heard from several manufacturers that make fasteners, engine parts, and other components for the aerospace industry. Prices have been declining for a decade or so, as new sources have become available. Combining it with aluminum, as in this alloy, has helped to bring that cost down even more.
@Ann: Thanks for posting this. Metals and metalcasting are two of my favorite topics.
Nickel aluminide has been around for decades, but it is difficult to cast, so it is only in the past few years that it has started to be taken seriously as an alternative to superalloys in turbine applications. General Electric uses investment-cast nickel aluminide for the low-pressure turbine blades in its GEnx engines, and also uses a nickel-aluminide coating on the high-pressure turbine blades. NASA did a lot of work to support this.
It will be interesting to see whether casting in this type of centrifuge is a practical manufacturing method.
Interesting article but there are some practical realities to overcome.
There are three types of blades in a modern gas turbine engine--fan, compressor, and turbine. The fan blades form the large fan at the front of the engine that essentially acts like a propeller (or bird shredder) and operates at low temperatures. FAA engine certification rules designate the fan blade a critical part as failure can result in conditions hazardous to continued flight.
Compressor and turbine blades are not considered critical in this respect as the thinking is even if a failure occurs and the engine is shut down as a result, the fragments exit the tailpipe with no hazard to continued safe flight of the aircraft. One may argue that an engine shutdown is unsafe although the aircraft is certified to fly on one engine. Blades are extremely high integrity parts.
Maybe this is a leap in metallurgical technology however we are a long way from replacing reliable and durable single crystal nickel superalloys with any titanium alloy in the hot section of gas turbine engines. Current designs are running turbine inlet gas temperatures above the melting point of the nickel blade and vane materials--cooling is key here and this design requirement demands internally cooled blade and vane castings that are difficult to produce.
The cold (compressor) section may be more suitable for the material although again current designs are pushing temperatures at the compressor exit above 1200F. For fan blades applications, integrity will have to be demonstrated rigorously. Composites are currently used in this application with great success to reduce weight.
Also any practical use must consider the fabrication cost of the material which appears astronomical.
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