Single-crystal turbine blades (Pratt & Whitney / GE)

[Tom holds up a single-crystal turbine blade.] We have some single crystals that are structural materials. This is a turbine blade, and this is actually a single crystal because we need the mechanical properties — we don't want the grain boundaries. Actually we don't want the grain boundaries in any of these, but we go to large grains to get the fine grain size. When they solidify it at 2,400 degrees Fahrenheit they create these internal cooling passages, and then they laser or electron-beam drill holes in here for cooling channels. Each one of these is worth about $4,000, and every disk on the turbine has close to a hundred. So if you want to know why the engine costs five or ten million dollars, you've got a million dollars in one disk with the blades.

Sample passed around. Used to teach hot-section cooling design (boundary-layer cooling, ceramic core casting, seven-thousandths wall thickness limit).

[Tom hands a single-crystal blade sample around.] I also pass around single-crystal blades — I usually find some excuse to pass this around several times. This is from an old Pratt & Whitney engine from the 1980s. It's from the hot section — single-crystal blade. It has cooling ports, that's why I pass it around. It's been filleted; they took a wire EDM and sliced it. It was a single crystal, but it has an internal structure. They cast it for turbulent flow, so you're getting cooling of the hot gases coming through. Hot gases at three thousand degrees Fahrenheit, with a melting temperature of the blade material at twenty-four hundred degrees Fahrenheit.

Mert Shank (formerly MIT 3 faculty) drives directional solidification at Pratt & Whitney. Grain-boundary creep failure mitigated first by columnar growth, then by single crystals. Air-cooled internal channels then enable firing temperature above the alloy melting point (3000°F firing vs 2400°F alloy melt).

Then there started to be a big divergence. What happened in here? This turns out to have been metallurgy too. They went to directional solidification. It was a guy who had been a faculty member here, Mert Shank, who went down to Pratt & Whitney. There's a guy here, Tony Giamei, retired from Pratt & Whitney, who wrote a little article. Here's the turbofan engine. Over here on this page — Professor Flemings used to have these same three on a little plaque back in the mid-1970s when this stuff was all kind of new.

Student-driven aside on why we don't grow single-crystal structural metal. Tom explains creep resistance motivation, growth rate (~1-2 mm/hour), $7,000 per blade, Pratt & Whitney / GE Christmas-tree casting trees vs. Rolls-Royce's small-furnace approach.

Student: A lot of small companies are working on crystal technologies, where they grow one crystal that is one single grain — the point being it's stronger and has special properties for electronics.

For electronics, and for high temperature in engines.

Student: Is it a stupid question to say, what about trying to create that within the metal? What prevents you from creating a single grain crystal in any arbitrary large size of metal?

We do it. I'll bring in my single-crystal turbine blade if I can remember. We make single-crystal turbine blades if you want creep resistance at high temperature, so it doesn't flow like Silly Putty does at room temperature. If you don't want it changing shape at high temperature, you want a single crystal. Fine grain size at elevated temperatures is the kiss of death. They make single crystals in silicon because grain boundaries destroy the electronic properties. I just bought some outdoor solar-powered lights for my house, and you can see the solar cells have grains that are an inch across. You can see them just looking at it. It's like a piece of galvanized steel bucket — you see the huge grains. Wherever you have a grain boundary, you're losing electrons, and you're not getting the solar efficiency. So you'd love single-crystal silicon for solar cells, but that's too expensive in general.

Student: It's cost-prohibitive to make large-volume single-grain crystal metal?

To make single-crystal metal, we grow it in a furnace at about a millimeter an hour, maybe two. It's slow. But when you're growing a turbine blade for $7,000, and you're growing twenty-four of them at a time — Rolls-Royce grows two at a time, but they have lots of small furnaces, whereas Pratt & Whitney and General Electric use two or three dozen on one sprue. It's like a Christmas tree with turbine blades on the end, you just break them all off.

GE and Pratt cast 16–24 blades per investment sprue; Rolls-Royce uses 4. Ten-hour pull through a 1000° gradient, $3M furnace per ~16–32 blades/day. Cost per blade ~$6,000 but justified by life-of-engine fuel savings. Used to teach that the steep gradient suppresses constitutional supercooling and gives planar-front solidification.

I think I told you Rolls-Royce uses four blades on a little investment casting sprue. General Electric and Pratt, their casting shops use about sixteen or twenty-four on an investment casting sprue. They grow it as a single crystal in a gradient furnace. It takes about ten hours, because you have about a thousand-degree temperature differential over five or six inches in this furnace, and you slowly pull it through over about ten hours. You don't get dendrites because you suppress them with the steep gradient. You don't get constitutional supercooling that allows the instability for the cell to grow up into the liquid. You can get nice planar-front solidification — a very perfect structure. But it costs a lot of money. This gradient furnace is about three million bucks to grow sixteen blades a day or maybe thirty-two blades a day. You can start to see why these things start to cost $6,000 apiece. But if they allow you to use half the fuel over the life of the engine, it's worth it.

[Tom holds up a sectioned turbine blade.] We passed these around when Dr. Belmar was talking about growing single-crystal turbine blades. This turbine blade would go on either a 747 or 757 type engine. It's a single crystal, and they took it and they cut a fillet in it. Pratt Whitney gave me this. They take a wire EDM and they section it so you can see the inside, and the inside has got all kinds of turbulators because they're going to put cooling gas through here. They pump 1000-degree-Fahrenheit gas as the cooling gas. The outside of this blade is going to see an environment of about 3000-degree-Fahrenheit combustion gas, and it has to be cooled, because if it's not, the metal melts at 2400 degrees Fahrenheit. So if you didn't have cooling gas your engine would melt — not a good thing.

The $6,000 single-crystal blade as the showcase application of investment casting. Tom walks the full chain: ceramic-shell mold → polycrystalline cast → temperature-gradient furnace → pigtail seed-crystal selection → CAT-scan wall-thickness inspection. Used to teach how casting can produce the most thermodynamically demanding part in modern engineering.

[Tom holds up a single-crystal turbine blade, cut in section.] And that's how we start to make these $6,000 turbine blades. When they do it, they have a little ceramic mold the shape of the inside of this thing — this one's been cut in two so you can see inside. When Pratt and Whitney sold this, it would have been a solid part. They make it by investment casting and they end up with a polycrystalline grain.

Physical sample. Used to illustrate the $6-7K value of an aerospace component, electron-beam-drilled cooling passages, and the firing-temperature chart.

[Tom passes around a turbine blade cross-section.] This is about a thirty-year-old turbine blade — it's been split, they cut filleting just for display. Pratt & Whitney gave me this. It has internal cooling passages. This is a single crystal. If this was a good blade it'd be worth six or seven thousand dollars. It's not much good when you cut it open. It's an electron-beam hole drilled at different angles, and we'll talk a little bit more about that.

Physical demonstration. A 20-year-old single-crystal turbine blade ("no grain boundaries"), worth $6,000–$7,000, ~100 per wheel. Used to motivate the grain-boundary discussion that frames Backofen vs. Grant's research programs.

What was happening: Professor Backofen in the '60s was looking at the effects of fine grain size. Professor Grant was looking at the effects of no grain size, or very large grains, because he was a high-temperature materials person. He was the person looking at things like turbine blades for jet engines. [Tom holds up a turbine blade.] This turbine blade goes back 20 years. It's a single crystal, no grain boundaries. Grain boundaries destroy the creep high-temperature properties. The grain boundaries are like butter and the thing just slides and deforms. If this was a good blade it would be worth about $6,000 or $7,000, and there's a hundred of them on every wheel of every turbine engine. That's why the engines cost five or ten million bucks. Grant was looking at how to get rid of grain boundaries in structural materials for high-temperature properties. Backofen was going in the opposite direction, looking at how to get very fine grain size, and how things deformed. We'll talk about that.

The physical blade Tom holds up as exemplar of precision-ground "tree" mechanical attachment, Wood's-metal fixturing for the $10M grinding machine. Returns in §8 as the artifact Tom distributed to high-school visitors with the corner-cut / notch quality-control markings to prevent reuse.

I wanted to mention three other types of friction welding. One is linear friction welding. This is circular friction welding — it's nice and easy to build a machine that goes in circles, but they would love to do linear friction welding. If you want to do some friction welding, you can do it with your hands — you feel your hand warming up. They would love to be able to make turbine discs for jet engines, where they weld the turbine blade to the substrate. Here's a regular turbine blade. The jointing is a mechanical joint, and they call this the tree — turn it upside down, it looks like a tree. This is ground very precisely. This is the most expensive part of the turbine, I think — $10 million grinding machine to grind these flats. You mount the whole thing in Wood's metal, a low-melting metal, to fixture it, because you can't clamp this precisely enough to get the tolerances they want. It has to fit into the mechanical sleeve on the turbine disc within a few tenths of a thousandth of an inch, otherwise at the speed it's going you'll get vibration and fretting wear. You have to be careful what temperature you assemble these at — a few degrees of temperature, you basically have to assemble these in a controlled temperature environment.

$6,000/blade × 100 blades/disc = $600K replacement; engine companies make 40% of profit on 20% of volume from blades/vanes.

They don't oxidize. When I was 19 years old, between my freshman and sophomore years, I got a job working in the Naval Air Rework Facility in Norfolk, Virginia rebuilding engines as an engineering student that summer. The TF30 engines were coming back from Vietnam, and we had to rebuild them and send them back. The engines had 500 hours on them — that was the lifetime of an engine, 500 hours, and you had to rebuild it every 500 hours. Today, 30,000 hours on a commercial engine before you have to rebuild it. So it's not just operating temperature, it's operating lifetime that has improved over this time, in terms of oxidation resistance and other things. From 1972 to 1990 or 1995, you're up to 30,000 hours. We went from better and better alloys to single crystals — single-crystal turbine blades that cost six thousand dollars a blade. You've got a hundred blades on a disc, so six hundred thousand dollars for a replacement set of blades on one disc in one engine. That's why the engines cost five or ten million dollars — it's the blades. Engine companies make maybe twenty percent of their volume on the blades, but forty percent of the profit is on the blades. The vanes are the most valuable part.

Development arc from uncooled solid blades → nickel superalloys → directionally solidified → single crystal by early 1990s, led at Pratt & Whitney by an MIT-affiliated metallurgist (name garbled in captioning).

People started working on better and better materials. These were uncooled solid materials, and they started going to nickel-based superalloys in the 50s and 60s and 70s. As they went to the nickel-based superalloys, they learned you didn't like finer grain sizes — at high temperature that meant creep; the metal would deform. So they went to directionally solidified turbine blades, and they went to single crystal blades by the early 1990s. The guy who really led that at Pratt & Whitney was Gell [unclear — captioner garble; likely a Pratt & Whitney metallurgist with MIT faculty ties], who had been a faculty member here at MIT in both mechanical and materials. He went to Pratt & Whitney and led all that effort in the 60s.