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.
Russia isn't the only country supplying titanium: it also comes from South Africa, currently the second largest supplier, and elsewhere: http://www.designnews.com/author.asp?section_id=1392&doc_id=251754
One problem I have with this whole thing is that Russia controls titanium. We could gear up for a new titanium world and Russia could cut us off. Look at the threats to the E.U. over natural gas? I would hate to be beholdin' to a belligerent government for my supplies. Remember tantalum just a few years ago?
Dave, thanks for weighing in on the discussion with your expertise in metals and casting processes. That's interesting to hear that the acceleration levels used in the ESA hypergravity research aren't actually as high as what's already being done in typical centrifugal casting.
@TJ McDermott: Casting in microgravity eliminates convective flows that normally occur due to density gradients in the solidifying metal. This could eliminate macrosegregation. On the other hand, I wouldn't expect macrosegregation to be a serious problem in ordered intermetallics like titanium aluminide.
Most of the microgravity casting research I've heard about is focused more on understanding the solidification process, rather than improving the microstructure per se. By eliminating convection currents, it's easier to observe the effects of other variables, such as magnetic fields, on solidification.
20-g acceleration is actually a lot less than is typically used in centrifugal casting. A typical centrifugal casting machine might produce something on the order of 75-g acceleration. On the other hand, ESA's Large Diameter Centrifuge is very different from a typical centrifugal casting machine.
For the forseeable future, I would expect titanium aluminide parts to be made by either investment casting, forging, or powder metallurgy.
TJ, I'm not sure I understand your question. Previous research on casting metals in space was apparently done only with microgravity, and definitely aimed at improving the microstructure. But this research, with a similar aim, uses hypergravity and results in a much lighter alloy. Given this context, can you clarify your question?
The hypergravity casting process sounds like spin casting writ large. But wouldn't one expect the molten alloy to begin separating by density? I thought that was one reason for doing metallurgy in space, that eliminating gravity from the equation made for a better microstructure?
I agree, Dave, about the relative costs. I also agree with you and Lou: the big hurdle here is figuring out how an economical manufacturing method can be devised using that big centrifuge, or somehow duplicating its effect.
@Ann: The cost of raw titanium and raw aluminum are probably only a small part of the cost of making a turbine blade out of titanium aluminide. As your article points out, titanium aluminide is difficult to process, and these processing difficulties are a big cost driver.
Centrifugal casting is a common way of casting hollow cylinders (such as pipes), but it sounds like ESA's Large Diameter Centrifuge is very different. It would be interesting to know whether an economically feasible manufacturing method could be developed based on this work.
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