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)
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.
@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.
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: 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.
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.
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?
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?
@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.
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.
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