§1. Tying up shape casting processes [00:04]
I want to tie up a couple of loose ends. We had this thing on shape casting processes — a chart that lists the genealogy of a bunch of these things. They break it down into expendable molds, which would be sand casting, or metal molds, which would be high-pressure die casting. And we talked about the semi-solid stuff that's like your Slurpee or your soft ice cream, where you actually take out half the heat of fusion before you push it into a metal die. Mike Tenenbaum brought in the wooden molds for sand casting, and sand casting has been around for centuries if not millennia. There's green sand and so on, but one category here is expendable mold with expendable pattern casting.
We talked about investment casting, where you have an expendable pattern — the plastic part, the wax part that you mold. We didn't talk about lost foam; we did talk about the semi-solid process. Some of these — investment casting, bonded sand molding — go back thousands of years. Lost foam goes back to 1960 at MIT, but the interesting thing about this one is that it wasn't invented by a scientist. Professor Flemings had the Foundry — I think I told you about the Foundry, MIT had a real Foundry and they cast the gimbals for the space shots in the '60s. So this artist from the Boston area comes in — Anthony E. Arbino, or something like that, I can't quite remember the name. He wasn't a scientist or a foundryman, but he did sculpture. He had decided to sculpt something in styrofoam, pour the sand around the styrofoam, then pour the molten metal in and just let the styrofoam evaporate from the hot metal.
He was partially successful but not completely. He brings it into MIT and says, can you help me perfect this? I can't get consistent quality. Professor Flemings looks at it and says, hm, that's an interesting idea. You have to understand how the metal flows into the mold, where you should put the spout, where the pouring cavity should be — some of that fluid flow. MIT helped him perfect this. That is now called lost foam casting, and in fact most of your cast iron engine blocks for General Motors are made that way now. They make a styrofoam engine block, push it down the line, pour a bunch of sand on top of it. They don't have a cope and a drag now, where you put the two halves of the thing together to make a cavity. The styrofoam is the cavity, and when you pour the hot cast iron in it just vaporizes. You have to worry about gas vapors coming out and liquid metal coming in — you have to make sure you don't get a little explosion that spatters everything everywhere and blows up your mold. But it does work, and it's now a major industrial process. It came from an artist who came to Professor Flemings with a clever idea.
Flemings developed all of this. Investment casting — I passed around the turbine blade. That was really sort of perfected by a guy, Bud Schenck at Pratt & Whitney, but before he went to Pratt & Whitney he was a professor here at MIT. That was in the '60s, making some of these types of parts. The guy who made the ceramic inserts that you cast around is also an MIT ceramist grad, and he started a little firm to make those ceramic parts, which are very fragile. I actually have one in my office; maybe I'll remember to bring it. So MIT has an interesting history, and shape casting processes go back thousands of years or several tens of years depending on what you're looking at. I just wanted to give you a little idea of the significance of some MIT history in these shape casting processes.
§2. Jet engine economics and turbine blade value [05:03]
Want to talk a little bit about the economics of investment casting. Does anybody know what a jet engine costs roughly?
Student: A billion?
Not a billion. To design a new one is a billion dollars.
Student: A million?
It's between five and ten million. You get a small one on a small jet for a million. If you're talking about a 747 engine or a 737, they're about $5 million. The F-18 — a million bucks seems a little cheap to me. These blades, if you're really getting to active air cooling with these fancy passages, each blade's about $6,000. There are about a hundred of them that go around the rotor, and you typically have anywhere from two to five stages. You're talking about half a million dollars a stage just for the blades. So that's where half the value of that engine is, if it's a $5 million engine — and I suspect if it's a $1 million engine it may even be a little bit more than half.
There are all sizes of engines, so it's order of magnitude of a million, but for a big commercial jet it's $5 or $10 million. So if it costs $250 million to buy a 747 — at the Paris air show they just trotted out the 747-8, it's a four-engine jet — you've got $40 million worth of engines on there. It turns out that the turbine blade is not the most critical component, because if you lose a blade you only lose an engine, and losing one engine is not a big deal when you've got four. The old B-52s had eight engines, partly because you could afford redundancy in the 1950s. You can't afford the fuel to fly anymore. So you lose an engine, big deal, you've got another seven.
It turns out the 777, which came out in the early '90s for Boeing, was the first two-engine commercial aircraft qualified to fly across the ocean. By "across the ocean" they mean two hours from land. It used to be that if you wanted, you could fly up over Iceland and Greenland and down through Gander, Newfoundland, and you could make it from Europe to the United States in a two-engine plane. But the FAA wouldn't certify you for that until the early '90s. The reliability of engines went up, so they decided even if you lost one engine, the odds are you're not going to lose the second.
In the old days when reliability wasn't quite as good, you had four engines on a 747. The military — the F-15, that was two engines. That doubles the cost. The F-16 was supposed to be the low-cost one, make a bunch of them — single engine. If you lose your engine, hopefully the pilot ejects, and if he doesn't, funerals aren't that expensive, I guess. There's an economic analysis to all this, whether it's nice or not, but it's real.
Turbine blades constitute 20% of the revenue of one of the big engine manufacturers, General Electric — but 40% of the profits. Not just anybody can go out and make one of these things. There's a lot of technology, a lot of know-how, and if it's commercial you have to get FAA approval. So the OEMs — original equipment manufacturers — the Pratt, the GE, the Rolls-Royce, they guard this viciously, because this is 40% of their profit. This is the core of their business: to make these castings. The American Institute of Physics came up with the twenty greatest achievements in physics of the 20th century, and they included the metallurgy of making these things as a triumph of physics. The physicists couldn't even spell "metal," but anyway.
Just to give you an idea of the cost of these things, the CFM56 engine, which is on the 737 and a bunch of Airbuses, is the most widely used commercial engine in the world. It has two stages — they've designed it to have few stages, because the fewer stages the better off you are. Now the critical components on an engine, the ones they really worry about, are things like the big rotors that those things go on, or the shafts. You lose a shaft or you lose a rotor, you lose the engine, because when one of those things breaks there's enough energy in one of those big heavy things that it'll cut the plane right in half. That's the Sioux City Iowa problem — they had a rotor failure, it went through and cut all the control cables, and the pilot had to basically fly without any of his flaps. He had to land without any regular control surfaces. So the FAA spends lots of time on defects and fatigue crack growth in rotors and shafts, because these things weigh hundreds of pounds.
But the blade frankly is not a critical component. You lose one blade and you might lose the engine, because one of these things going through your engine tends to wipe out a lot of the other blades. But if you have enough reliability in your other engines, you keep on flying. If you're a two-engine plane like a 777, you only need two engines when you take off — you can fly just fine with one. I saw the statistics from General Electric once: reliability expressed as a percentage of uptime per hour of flight was like 99.75 — one quarter of a percent — which means once every 400 hours you lose an engine in flight. When you say "lose an engine," it doesn't blow up; it just shuts down and you have to turn it off. Or you have a fire, or you lose a blade and it's destroyed. How many of you have ever flown 400 hours on commercial aircraft? I know I've flown more than 400 hours. So the odds are they were shutting down engines but they didn't come over the loudspeaker to tell us, did they? "Oh, we're going to shut down an engine because it's not working" — can you imagine. You'd have a few upset people, no doubt.
§3. Properzi, Southwire, and Hazelett continuous casting [11:28]
I did leave them in my office — the Southwire and the Hazelett process that I tried to describe. This is the Properzi casting machine. This is only 2 m, so about a 6-foot casting wheel, steel, with a steel belt. You pour metal in at 12:00, it goes around and solidifies — you have a water spray on the outside of the steel belt — and you end up with a cast bar that comes out the top. The Southwire process is not a lot different: you pour it in, it goes around and comes out the top. This is 2.4 m in diameter, a steel wheel, you have a spray band, and you end up with a bar. Here's the actual cross-section of the bar, here's the spray. This is the steel cross-section of the steel wheel. Here's the as-cast rod, and you're going to break that down and turn it all into wire. You can do this because what you're casting is copper, which has very good thermal conductivity, and you can get something about 2 or 3 inches in diameter — 75 mm × 52, about 2 × 3 inches, 35 square centimeters of mold for the Southwire process.
It works partly because what you're casting has good thermal conductivity, it melts at just under 2,000 °F, and the steel band doesn't melt until 2,600. If you keep water spray on it, and it's a thin steel band, the water spray keeps it cool enough that it doesn't fail. If the steel band does fail, you get a few hundred pounds of molten copper on the floor — you're cash-shopping and you just clean it up.
§4. Copperweld and the 200-ton railroad-track casting [15:13]
Which reminds me of a story from when I was an undergraduate student. I never saw this myself but one of my classmates went off for his summer internship — I think it was Copperweld Steel, which probably doesn't exist anymore, and when you hear the story you'll understand why. We talked about ingot casting in a steel mill, and there is a moral to the story.
They were supposed to be doing ingot casting. Even back in 1970 or whenever this was, the operators were often in an air-conditioned pulpit — it gets pretty hot in the mill. The guy running all the controls is just sitting there looking at gauges; it's like a nuclear reactor control room. You're just in a room with a bunch of gauges and a little desk in an air-conditioned room. The guys running the melt shop are union guys, back then in the steel industry, Steel Workers of America. They got paid a lot of money — probably more than a lot of the managers — because they were responsible for hundreds of thousands of dollars an hour worth of steel and making sure it's cast properly.
One of the engineers overseeing the melt shop comes in, looks at the gauges, and says, okay, tap the heat — which means cast it into the molds. The operator says, "but —" The engineer says, no, go ahead and tap the heat, and walks out. He comes back about fifteen minutes later, looks up at the gauges, and says, I told you to tap the heat. The operator says, "but —" The engineer says, I told you to tap it, now tap it. So he did. The reason he hadn't tapped is because the railroad car that had all the casting molds had not come up to the casting station. When he was ordered to cast it, he cast 200 tons of steel right on top of the railroad tracks. He did what he was told.
The moral is: if you're the manager, you don't just order people around — you actually listen to what they're trying to tell you. And if you're the laborer, you don't suspend common sense just to get back at a god. They both lost their jobs. Except when you cast a couple hundred tons of steel on the railroad tracks, it takes about a week to clean up.
How is an interesting question itself. You go in there with big steel lances, because steel is easy. If you walk through one of these shops, the floor is dirt — usually about 2 feet of sand, fallout. You're just casting in a big sand mold, like the old Saugus Iron Works, the sow bar. You're just pouring it into a sand floor. It may be 2 or 3 inches thick as it spreads out and solidifies as a pancake — or a drop biscuit, I guess you think of it as a drop biscuit, but it weighs a couple hundred tons; this is a heavy biscuit. You go in with an oxyacetylene torch — you may have 100 psi of oxygen behind it, big tips, and you can cut through two feet of steel. You may only go an inch a minute, but you can cut through — I've seen 3 feet of steel cut. So it's easy if it's steel.
§5. The Alabama aluminum foundry freeze-up [18:49]
The first year I was on the faculty — you know the little short door, 8-137, as you go around the corner? That was my first office as a faculty member. I'm sitting in the office and I get a phone call from some guy in Alabama who worked in an aluminum smelting shop — actually it's an aluminum foundry — and they had a big holding pot for the aluminum. He calls up and says, I'm trying to get some advice on how to cut aluminum. I said, well, you can saw it, it's not hard to saw. He says, no, that's not going to work. I said, you can plasma cut it. He says, no, I've got something pretty thick. I said, how thick is it? He says, about 2 feet.
What happened is he had an electric furnace and they lost power for a day, and the big aluminum pot — about 2 feet deep, 4 feet long, 6 feet wide — solidified. Now he has a furnace with 2 feet of solid aluminum in it, and he can't pour it out because it's solid. I said, you do have a problem, because you can't flame cut aluminum — it has a very protective oxide. I'll explain why later, but you can't flame cut it. So I didn't know the solution. I was only 26 years old.
About ten or fifteen years later, I was having dinner with Peter O'Brien, who at the time was senior executive VP of Alcoa. I remembered this and said, Peter, you have these big aluminum smelting pots where you make aluminum, and you must have freeze-ups from time to time — what do you do? He says, well, there's this guy in Pittsburgh — of course Pittsburgh is the home of Alcoa, where the Pittsburgh Reduction Company, which was the beginnings of Alcoa, got its name because they were reducing aluminum. There's this guy in Pittsburgh, and he will come and he will blow it for you. He'll drill some holes, put some charges in, and blow it up. And if he's good, he won't blow up your furnace too. He can break it up with an explosive charge — if he's lucky, but it doesn't always work. This is sort of like blowing out the oil wells, when you have a wildcat well and Red Adair would come in with his crane full of explosives and try to blow it out like a candle. This is the guy who blows up the solid aluminum masses. It's good work if you can get it, but the world probably doesn't need too many of those people.
§6. The Hazelett process and water-in-molten-metal explosions [21:45]
This is the Hazelett process, which again is casting copper — big copper sheets a couple of inches thick. You have two steel bands. Here's your molten copper coming in, big steel bands; this whole thing can be 6 feet wide and maybe 10 or 12 feet long. You're going to cast it 1 inch thick — 1.25 cm, well, that's a half-inch, but anyway. Water spray to take the heat out, remember you have to extract the heat of fusion when you're casting. Here's another picture of it, the lower belt coming up. They basically pour the copper in as the stuff is spinning, and just by friction and the viscosity of the liquid, you pull it in there, it solidifies, and you get big sheets of copper, which you then put through an electrolytic process to make it pure. It's amazing what people would do. Who would ever think you're going to start pouring molten copper out into a steel band with water spray?
If you ever lost that — do you know what happens in a molten metal shop when water comes in contact with molten metal? It's called an explosion. You generate steam very quickly, and as that steam expands it'll blow molten metal all over everywhere. I've had a number of times where someone calls up and says, we had an explosion in our casting facility. I say, where's the water? Because you have water-cooled molds, and you get a leak in one of those, you've got a problem.
Most recently — a year or two ago — they had a big electric arc furnace. We talked about the big electric arc furnaces, almost the size of this room, with these big 3-foot to 3-and-a-half-foot diameter carbon electrodes. The walls — they used to make about 40 or 50 tons of steel in one of those vessels, but to increase productivity over the last 25 years they've done all kinds of things. They put in new transformers and got to 150 megawatts of power, up from about 50 megawatts. They go from 40 tons to 120 or 150 tons that they're melting. Those are big arcs — just think of an arc in the middle of this room that's about the size of a small car, melting tons of steel. There's a lot of radiation to the walls, and when you triple the power you triple the radiant energy to the walls. In the old days the small furnaces could have a ceramic wall maybe 2 feet thick, and it would erode away over time, but you could go in and reline it every six months or
§7. Electric arc furnace water-jacket explosion [24:11]
Those are big arcs. Just think of an arc in the middle of this room that's about the size of a small car, melting tons of steel. There's a lot of radiation to the walls, and when you triple the power you triple the radiant energy to the walls. In the old days the small furnaces could have a ceramic wall maybe two feet thick, and it would erode away over time, but you can go in and reline it every six months. To get it up to higher and higher power, they basically put in steel pipes about two or three inches in diameter, with water running through there at about 1,000 gallons a minute, to extract the heat from the walls.
In this particular case you've got pipes with plates welded to them and another pipe — a pipe here and a pipe here, and in between you've got a web of steel that holds everything together. On top of this whole thing they spray on a ceramic coating that's supposed to be three or four inches thick. They didn't maintain the coating very well. You've got this bundle of ten tons of steel scrap you're trying to melt — actually a bunch of them, kind of like kids' blocks, ten or twelve of these ten-ton bundles of solid that you're trying to melt — and the arc comes down. It's supposed to hit the bundle and come out the other electrode in the bottom of the furnace. But what happened is it jumped from the bundle to the wall because there was a gap in the ceramic coating, and that arc, which is supposed to be melting hundreds of pounds of steel a minute, worked on the pipe pretty quickly. You had a hole this big.
Now they have leaks from the walls all the time — little leaks the size of your finger that spray ten gallons a minute or so. Big deal, just get a little steam coming out of the furnace. But this one was a big leak, almost the diameter — well, a long thin one — but it could let almost all the water into the furnace. They had an explosion. The metal erupted, turned to a foam, and six guys died. In liquid metal shops you have water very close to hot steel or hot aluminum, hot copper, whatever it is. The real rule, if you talk to a foundryman: the water always goes on top of the molten metal, because that way the steam can come off. You never put the molten metal on top. That's the foundrymen's view: never put the metal on top of the water, always put the water on top of the metal, because that gives the steam somewhere to go. If you put the metal on top, you just generate steam underneath the molten metal and spray molten metal all over everywhere.
So these types of hazards exist, and they do kill people, a few people every year. It's sort of like a coal mine — we probably kill 50 or 100 miners a year. It doesn't really hit the news because it's one of those things that happens. It's like gas pipelines. We have gas pipelines all over the country, and some of them have 1,000 PSI gas in a 42-inch diameter pipe, and usually when one blows up it's in some farmer's field and it kills a couple of cows. A few years ago we had one in Edison, New Jersey — 600-foot flames shooting up in the air, like a 60-story building in the middle of Edison, New Jersey at midnight, woke a few people up. So failures occur and they can be pretty dramatic, but fortunately they're not very common.
§8. Defect size scales with casting size: the Kwajalein radar dome bogie [28:28]
We talked about dendrites the other day, and we'll talk a little bit more about them. If you're solidifying an alloy, the alloy wants to segregate, and you get these dendrites forming. If you have a great big casting — you're casting a 200-ton ingot because you're going to forge some big vessel — you can have some pretty big dendrites the size of your fist. And you can have some pretty big inhomogeneities the size of your fist. I used to say that the size of the defect in a casting is proportional to the size of the casting. Call it two percent of the characteristic size of the casting — that's the size of the flaw.
I remember one of my first consulting jobs back in the 70s, MIT Lincoln Lab came in and they said, we have this radar dome in Kwajalein in the Pacific. They had built this for some scientific purposes, but it just happened to be located in the perfect position. Whenever the Soviets were sending a satellite up into space from middle Asia, they could see, with this particular frequency of radar, the boost phase, and get a measure of the trajectory. They could figure out before it got into space what the orbit was going to be, because they could measure the arc of the rocket coming up out of the boost phase. It had been built for scientific purposes — it was about the size of a football field, the dish — and it ran around on big steel railroad tracks so it could spin 360°, take different views. It had big steel bogies that hold railroad wheels — I don't remember if it had eight or sixteen wheels on each bogie. If you've been in a shipyard you've seen big wheeled vehicles that carry 200-ton objects, pieces of ships. This was similar, except railroad-rail rather than rubber-tired wheels.
This thing was at about 3,000% of its useful life, because it didn't have this original military application. When they had built it for scientific purposes, they later found out the military application, and this thing was running around the clock 90% of the time. Whenever it went down for repairs it was classified, because that was when the Soviets could have set something up without our knowing as quickly what the orbit was going to be. So they come to me because one of these bogies needs to be replaced, and the lead time to get the casting was one year. That wasn't really acceptable. If this thing was going to be out for a year, the Soviets might have been able to learn that. They determined they could machine it out of 12-inch thick plate, and they could buy 12-inch plate fairly quickly. But they wanted to know how to specify the testing to make sure that, when they did all this machining on this thing to turn it into the shape they wanted, they wouldn't end up running into some defect.
I went and looked at the ASM specs. The tightest specification for any plate above six inches that you could have for ultrasonic testing meant that the defect had to fit within a 12-inch diameter circle. So if you're running ultrasonics across the top of the plate, if you find a defect, as long as the projection of that defect would fit within a 12-inch circle, it was an acceptable plate. So when I talk about fist-sized defects, when you get to really heavy sections, that's acceptable. Now, it may not be — frankly, you saw the piping porosity from the solidification of the ingot, and I showed you the football-sized defect in that big shaft. When you make big things you get big imperfections. The converse is that when you make small things you have smaller imperfections, and they're still a fraction of the diameter of whatever you're trying to make.
§9. Rapid solidification and amorphous metals [33:24]
So it turns out, in the 1960s, the guy at Caltech took some iron-boron alloys — that are actually basically just brazing alloys — which melt at a fairly low temperature, sort of like cast iron. He took a spinning copper wheel going at thousands of surface feet per minute, and they would pour the molten metal on it as a liquid spray, and it would come spinning off as a thin sheet. You could roll off sheets two to six, twelve inches wide, about as thick as a piece of paper — rapidly solidified metal, and you lose all crystalline structure. You have no dendrites. Ordinarily these things would be full of dendrites, but it's so thin it cools so quickly. They called it rapid solidification. In the best cases they got solidification rates of a million degrees per second. So this thing's solidifying so fast that you end up getting a very homogeneous structure. It's not all that different than the Hazelett process except you don't even need a top band, because it's just a thin layer, and it's solidifying so fast that this thing only has to be about two inches long.
The only problem is you can only make thin sheets.
Student: It depends on which volume you're pouring the metal on the wheel, right? If you pour too much, you won't have the thin sheets. What's the maximum volume that they pour on?
For the original alloys, you had to make sure that you poured it out so it came out as a big, wide, very thin stream that would then be thinned out by the spinning wheel. You're sort of stretching the liquid out, if you will. If you made it ten-thousandths of an inch thick — a quarter of a millimeter — that's too thick to get rapid solidification, and you wouldn't get an amorphous metal, you would get a semi-crystalline metal. Professor Schuh here now works on these kinds of semi-crystalline metals.
The Defense Department and the National Science Foundation were spending hundreds of millions of dollars a year in the 1970s and early 1980s, because these things were uniform, they had no grain boundaries, they had tremendous corrosion resistance. It was like getting stainless steels without the chromium. They were strong, they had 500 ksi strength. The trouble was, they were very thin, and you didn't have bulk material. They thought, okay, we'll figure out some way to consolidate it later, let's just study the properties. So they were spending hundreds of millions of dollars on this.
§10. Bulk metallic glasses and the golf-club market [36:55]
It turns out another guy, a student of Pol Duwez — who was the guy who discovered this at Caltech in the early or mid 60s — his student Bill Johnson, who's a faculty member at Caltech now, about my age, in the late 80s came up with bulk metallic glasses. He started throwing all kinds of other metalloids from that part of the periodic table in with the iron. Actually it wasn't just iron, it was iron-beryllium alloys originally. Beryllium's toxic, but he was able to make bulk metallic glasses. You could make things about the size of a golf ball, and still get the amorphous structure. There's actually a company out there now — they haven't been all that successful, they've been around for fifteen years trying to sell bulk metallic glasses. All the way through the 1980s and 1990s no one figured out how to consolidate these thin sheets into a solid bulk material. They tried explosive bonding, where you just slam them together with an explosive charge, they tried everything and nothing worked. Still today nothing has worked, except Bill Johnson has gotten some alloys where, if you cool them at 100° centigrade per second, you can actually make them bulk — so you can get bigger things if you cast them the right way.
The only commercial product they've come up with is golf clubs. The metal woods — the same type of thing I passed around for investment casting out of stainless steel. They've been around for about fifteen years. I always say you can sell anything to a golfer. Golfers are a little older, they have a little more disposable income, and they will pay anything to be better than their executive friend down the road, to beat him in a golf game. If they can get some advantage they'll pay thousands of dollars for it. When I was department head fifteen years ago, we were interviewing one of Johnson's postdocs for a potential faculty position. I said, what are the markets for this? He says, well, we've made golf clubs. I said, anything else? He said no. And I just learned about six months ago that they are selling the golf clubs, but apparently their fatigue life is such that they get about 50 shots before it breaks. But you can sell anything to a golfer — 50 shots, that's a whole round. If he beats his buddy, it's worth $10,000. They may have a $50,000 bet on the game, I don't know.
§11. Atomization and powder metallurgy [39:44]
That's rapid solidification by sheet. Another way to do rapid solidification is atomization, making powders. We make probably 5 million tons of powder metallurgy steel, aluminum, and other things each year. What you do is you basically pour a molten stream and you hit it with a supersonic gas jet. Getting the right type of jet is very important. Professor Grant, who was a powder metallurgist here — passed away now, but he was here in the 40s and 50s all the way up until the 90s, well over 50 years as a faculty member — Professor Grant was a specialist in this, and he had a patent on a whistle where you basically had a little cavity and you blow the air in, and it would come shooting out, and you would get shock waves that would help break up these jets into very small powders. You're trying to solidify these powders. Lots of surface-to-volume ratio: the smaller the diameter, the more surface-to-volume ratio. It goes as 1 over r. The surface area is 4π r² of a sphere, and the volume is 4/3 π r³, so the ratio of surface to volume is 1/r with 3 over π in front, or whatever it is.
You can get amorphous metal powders. People tried explosive compaction, they tried all kinds of things. The problem is these amorphous materials — even if it's an iron-based alloy, you heat them up to 400° centigrade and they crystallize, they transform. It's a non-stable structure, it's a metallic glass. People have been trying this for years to make ultra-fine structures.
§12. The Conform process and the Army's "wonder armor" [41:15]
Last week when I was down at the Army, they were trying to develop new types of armor, not by rapid solidification, but they have a process the British developed and tried to commercialize 30 years ago, where they really sort of knead the metal. You put 30 times the energy into it, and you squeeze it out an extrusion hole — actually you squeeze it this way and it comes out at 90° — and it's called the Conform process. You can get some incredible properties. I mentioned the trolley wire — the trolley wire they ended up buying from Phelps Dodge for the New Hampshire-Boston connection was made by the Conform process. Fantastic strength and ductility. Remember we talked about that last time.
The Army's been doing this on a lightweight alloy — I won't tell you what type — and they get tremendous strength and ductility. It's sort of known you can do this. So this is one of the promising areas for the Ground Combat Vehicle, which is the Army's new program. The problem is, you can make it into rods because you're extruding it out, but how do you consolidate those rods into a plate? Usually you like your armor to be plate-shaped rather than rod-shaped. Don't stop a lot of bullets unless they're shooting for your armor. They might try to shoot around your armor if it's just a rod shape. So you have to have a very cooperative enemy, shooting for your armor.
I raised my hand and said, how are you going to consolidate this? Oh, we'll worry about that later. I said, that's what they said about rapidly solidified powders 30 years ago, and we spent hundreds of millions of dollars and got nowhere. They said, oh no, we'll solve the problem. I said, good — they couldn't solve that problem before. And I quoted George Santayana — does anyone know who Santayana was? He was a philosopher at Harvard. One of his sayings was, "those who cannot remember the past are condemned to repeat it."
You can get great properties in a little piece of material. It costs a small fortune but you can sell the next research project on that. You're not looking far enough ahead to say, how can I actually make a product out of it? You have made a research product out of it. You get more research money to study it, until finally you get a manager who's going to kill it. It's true, I've been watching it for 35 years.
§13. The rotating electrode process and depleted uranium at Nuclear Metals [44:29]
There's another process to make metal powders. Making metal powders is a good thing because they solidify rapidly — might be 100 or 1,000 degrees per second. They may have dendrites, but they're very small. The micro-segregation you get because of the enriched liquid between the dendrites versus the lightly alloyed dendrite — you actually now don't have to wait tens of years to homogenize it in a furnace, you can homogenize it in minutes. You could be a thousand times thinner, and it goes as the square, so you'd be a million times faster on your heat treatment times. You actually can get good homogeneous material if you rapidly solidify it. You don't have to get amorphous metal glass, but you can get good material.
One of the things they do is take a plasma torch — you've got an electrode, you've got copper, you have argon, and you're going to heat this material. A company up in Concord, Massachusetts called Nuclear Metals, which is a spin-off of the MIT Mechanical Engineering Department — they were the people making depleted uranium back when we used depleted uranium for rounds. Actually the Navy may still use it. It's very good for things like Phalanx and stuff. But the Army had to stop because of the first Gulf War — they contaminated most of Kuwait and southern Iraq with depleted uranium, and they decided it was environmentally unsound.
All that depleted uranium was made by extrusion up at Nuclear Metals. One of the things Nuclear Metals developed about 20 years ago was the rotating electrode process. You have a plasma torch and an ingot of, say, a superalloy for jet engines, very homogeneous, no segregation. You machine the ingot so you can spin it at thousands of surface feet per minute. Then you come in just like a welding torch, you melt the top of it, and as it's spinning you shoot off the metal as powder spatter, and you catch it in your chamber. You can get very good metal powders — not very cheap, but very good. You can then take those powders and forge them, using various techniques we'll talk about in a little bit, into something like a rotor for a jet engine. Excellent homogeneity, excellent properties.
When we talk about some of these things, the applications are not going to be for Ford Tauruses or for ships or railroads where we're talking about a couple of bucks a pound. They're going to be for jet engines where you're willing to pay $200 a pound, or for aerospace applications. If it's space, the value could be $20,000 a pound. We went through all that before. So when we talk about these sophisticated processes where you're melting with an arc and spinning this electrode, we're not making automotive parts, we're making aerospace parts.
But you as managers are going to have people come and say, oh, we got this wonder material — just like last week the Army, they got this wonder armor. They can shoot little bullets at it and they show that armor is fantastic. The question is, they can never make it into a plate. They can make little things that you can shoot little bullets at, and it stops little bullets just great, but you'll never stop the big stuff because you can't make it big. How do you scale it up?
§14. Reverberatory furnaces [48:19]
I'll just mention another type of furnace for melting, which goes way back in time — the reverberatory furnace. This is nothing more than the open hearth furnace. You have a burner with —
§15. The reverberatory furnace [48:48]
Let me mention another type of furnace for melting, which goes way back in time — a reverberatory furnace. This is nothing more than the old open hearth furnace. You have a burner with a flame, a furnace exhaust, and your molten material down inside a refractory crucible. This is basically the old open hearth where they made steel, and it is still the way they make float glass today. We talked about float glass on a bed of tin. You just have a flame shooting in, and the radiant heat from the flame melts something inside. The float glass is being pulled out of that furnace very slowly — maybe a foot or two per minute, you're pulling this little layer of glass out. You want everything nice and smooth; you don't want high turbulence that's going to mess up the surface of your liquid.
They used it in the old open hearths, but that was bad kinetics — took a day to make the steel. The basic oxygen furnace was a lot faster. But they still use the reverberatory furnace for glass melting furnaces and a lot of other things.
One of my favorite gradings of doctoral exams was back in my second year on the faculty. Professor Bowen, who's no longer here — he's retired from Harvard Business School now, but he was a professor of ceramics before he went up there — asked a question on the general exam asking a doctoral candidate to design furnaces for three different applications. One student had a reverberatory furnace, but for whatever the application was, he wanted to do it in a vacuum. So he had this flame shooting right into the vacuum, and he had a picture of it. I was grading this question, and Professor Clark, who was in charge, had his office right next to mine. I went over and said, "Did you make this up? This couldn't be a student's answer. Is this real?" And we looked at it — no, he hadn't, he wasn't trying to pull an April Fool's joke. It was real. We sent it back to Professor Bowen and graded it with a very low score. Actually it was the student's second time around, and he never did get a doctorate. So MIT does have some standards. Our standards may be low, but we have them.
§16. Spray casting and the limits of new processes [52:02]
Another technique is spray casting. You want something that can cool fairly rapidly, so you take the atomization process and come up with a spray gun that uses air or a flame, and you shoot out metal powders just like an oil burner shoots out a mist of oil. You turn it into a metal spray gun, spray that on some mold, and you can make different shapes.
The reason you should know about this is the Navy thinks this is one of the greatest repair techniques going. They've got worn-out shafts, worn-out bearings, worn-out all kinds of things, and so you put metal spray all over the surface — flame spray, plasma spray, different heat sources. You're basically casting by shooting little droplets, and the microstructure will have some porosity and some inclusions. Plasma spray can do steel; it's very good for copper alloy bearings, and for aluminum alloys you spray on a thick, somewhat porous coating that's great for corrosion resistance in some applications.
Student: Does it hold the shape that well?
You can machine it afterwards. Sometimes they build it up and machine it back. Over the last twenty years I've started to try to categorize these things by energy source — if you've got a certain heat source, or a certain size you want to make (fine powders, great big castings, ingots), and you start thinking about it, we've sort of eliminated most of the possibilities. There's electrical energy, chemical energy, thermal energy. If you want to heat something up, there are only certain types of energy you can use. I did this because in welding we've tried virtually every combination of energy source, material, and shape forming, and there's not a whole lot of them left.
There's mechanical energy. In fact, the only new welding process developed in the last twenty-five years is friction stir welding. If you look at the two hundred welding and cutting processes listed in the back of the welding handbook, you'll find that 150 of them were developed between 1875 and 1925. Another fifty were developed between 1925 and 1975. From 1975 to 2000, there was one new welding process developed. They call it friction stir welding. They take a metal electrode basically like a milling machine, ram it into the surface of the plate between two plates, and forge them together at room temperature. It generates a little friction so it gets warm, but it's a solid-state welding process — there's no melting and almost no distortion. Boeing put in a $10 million facility to weld aluminum plates.
For the last twenty years people have been trying to do steel. The problem with steel is there's no electrode material that really works well — you've got to have something with good mechanical strength at welding temperatures that doesn't wear out. You can do it with aluminum, with magnesium alloys, with a number of things, but the high-melting alloys get harder and harder. And since aluminum is only two percent of all metals, it's a limited application.
Student: You said the Navy interest is laser cladding — is that really just like flame spray?
It's not flame spraying, it's laser cladding. You're melting the surface. Rather than spraying a powder, you take a laser or a plasma torch or even a flame and just melt the top surface. That's another way of building it up, and those are often machined afterwards. Almost any variation you can think of, someone has said, "Why don't we just put these things together?" Sometimes it was a great scientist who thought of it, but more often than not it's like the artist who came in with his little lost-foam Styrofoam model that he was pouring sand around. A lot of the people who do the real invention don't know what they're doing.
The guy who invented the jet engine — I have a quote somewhere in my office. Lord Kelvin, or some great scientist from a number of years before, had said it was impossible to build something like a gas turbine. They didn't call it a gas turbine then. But Whittle, who designed the first gas turbine in England in the late thirties, said it's a good thing he didn't know that this scientific fact was true, or he never would have tried. Often it's someone who doesn't understand things very well who goes ahead and tries something, and it ends up working. For every one of those, there are 999 who tried something that didn't work, but every now and then there's someone who comes along — and dumb luck is great. The flame in the vacuum chamber: you won't get any damage to your part; after a while the oil will just fill up the vacuum chamber and your part will be completely preserved in oil. I guess that's creativity.
§17. Gases in metals [59:10]
Let me back up. I talked about extracting the heat of fusion — you have macrosegregation, you get these big dendrites, and the fluid flow, and the highly alloyed liquid and the lightly alloyed solid that first solidifies. Then you end up worrying about microsegregation. The way you get rid of that is to solidify rapidly, but then you end up with small powders or thin sheets or bulk metallic glasses. People have tried a lot of different things.
But in addition to extracting the heat of fusion, you've got to worry about the fact that you can dissolve gases, you get piping porosity from the ingots that leaves big voids, or you can get inclusions in the material. The inclusions often form from the gases and the alloying elements. For example, if you make pure copper and you're melting it in a reverberatory furnace in the air, it'll pick up lots of oxygen. That dissolves right in the copper. The problem is when you solidify it, that oxygen forms copper oxide, and you end up not with pure copper but copper peppered with little copper oxide inclusions. That weakens the copper and destroys part of its electrical conductivity, which is one of the things you want out of copper.
They have all kinds of ways of getting rid of that oxygen. One of the simplest is you throw a little phosphorus in, and the phosphorus forms a phosphorus oxide. Now I have phosphorus oxide inclusions, but many of those will float off before you cast it. You'll have fewer inclusions with phosphorus-deoxidized copper. You can also go to oxygen-free high-conductivity copper, which is melted in a vacuum furnace. You pull the oxygen out while it's molten, because there's no oxygen to dissolve in it. It's kind of like letting your Sprite go flat — the CO₂ comes out when you put it in a vacuum.
§18. Hydrogen, humidity, and unenforceable standards [61:34]
One good example of dissolved gases is aluminum alloys and hydrogen. Hydrogen is everywhere. We call it water. There's humidity in the air. If you go down to Electric Boat, you want to keep hydrogen out of your steel welds. NAVSEA has a spec that you cannot weld on days that have more than a certain percent humidity. I don't remember the numbers — they're probably classified anyway. A certain level of humidity and a certain temperature in the air tells you how much moisture is in the air. If you're doing stick-electrode welding, you're going to pick up some of that air, get hydrogen into the steel, and you can get cracking.
Well, that NAVSEA spec is violated eighty percent of the days that they're welding. You could only weld twenty percent of the time at Electric Boat or any other shipyard, because shipyards tend to be located next to a lot of water, and water creates humidity. So we have this spec that is just ignored.
It's sort of like the Air Force — or maybe Army; the government tends to share these specs — has a spec that you have to have a shell around the engine compartment for the turbine on a helicopter that will contain the rotor if there's a rotor failure, so it won't go through the pilot's compartment and cut him in two. He doesn't fly as well when he's cut in two. They've done studies on this — probably very expensive studies — and there is no material that can stop a rotor from going through. Well, there are materials, but you're talking about armor. If you want to put two-inch armor plate around your engine, that's fine; you just can't get your helicopter off the ground. The FAA has adopted the standard, but no one can meet the standard, so everyone ignores the standard. It's like the Navy's humidity requirement for welding in shipyards. You just ignore the standard, because otherwise you couldn't do anything. You get certain groups of people who develop standards or ideas, and they don't look at the practical reality.
§19. Hydrogen in aluminum [64:32]
So far as aluminum goes, if you just melt it in the air, there's enough humidity in Cambridge that you'll pick up a fair amount of hydrogen. This is solubility of hydrogen versus temperature — this is the melting point, and this is on a log scale, in cubic centimeters per 100 grams. For steel, one cubic centimeter per 100 grams of hydrogen is about one part per million, like 1.1. In aluminum, for 100 grams it may be three, because aluminum has about a third of the density. We're talking about a number of parts per million of hydrogen in the aluminum. Hydrogen is pretty light compared to aluminum, so it's a lot of parts per atom — atomic percent, it's like 0.03. It dissolves a lot, and then the solubility drops by about a factor of fifteen when it solidifies. So you're going to give off the gas.
If you tried to take seltzer water with CO₂ in it and turn it into ice, you'd find that you give off a lot of gas as it solidifies, because you can't dissolve as much CO₂ in the ice as in the liquid water. So you'll always end up, unless you degas it, with some porosity when you solidify your aluminum alloys. There are ASM specs — if I'd brought reference radiographs — where you have a little bit of porosity or a lot of porosity in an aluminum weld or aluminum casting. The best quality is something where you can find some little ten-mil inclusions; in a one-inch square you might have ten of these little inclusions on a quarter-inch-thick piece of aluminum. The worst is quarter-inch holes in your aluminum.
How do you get rid of it? Back in the old days when I was a student working in the foundry at MIT, we just took some chlorine gas, had a little steel lance, stuck it in there, and bubbled chlorine through. The chlorine would react with the hydrogen and aluminum, form hydrogen chloride gas, which we just blew off into the air in Cambridge. They don't let us do that anymore. How do you degas your aluminum now?
Student: [inaudible — suggests an alternative]
Same idea, or bubble nitrogen. They don't care if they get porosity, because they're just making little medallions for MIT students, and no one's going to cut them open and see they look like Swiss cheese on the inside. But if you're trying to make a high-quality casting, you've got to degas it. Hexachloroethane — they have these as solid pellets rather than gases, because it's messy to fool around with chlorine gas in the foundry. Someone pours something or cuts the rubber cable, and all of a sudden you have a chlorine leak in the room. Lose more undergraduates that way.
You've got to remember, back when I was a student I was sawcutting transite on the bandsaw with no mask or anything, because transite is asbestos plate. I've always figured if I ever get asbestosis I can sue MIT. I can't as a faculty member, because I'm an employee, but as a student I wasn't an employee — so there's a deep pocket if I ever get asbestosis. We had asbestos gloves and they were always falling apart, holes in the fingers. You'd go around the foundry for twenty minutes looking for a pair of gloves that didn't have holes in the fingers.
So you've got this problem of gases and metals. It can be oxygen in copper, hydrogen in aluminum, hydrogen in steel, oxygen in steel. Different alloy systems have different gases that are a concern. You can bubble nitrogen through, or argon, and those bubbles will help pull out some of those things. It's like the argon-oxygen decarburization — you bubble argon through to get the carbon and oxygen coming out as carbon monoxide. You can degas by bubbling something through, or with hexachloroethane as a solid pellet. I've seen people put vials of liquid in too — you submerge it underneath and you get a nice little boil going. You start boiling the metal, generating bubbles of gas, but those bubbles are carrying away the harmful gas.
This is hydrogen solubility in different aluminum alloys as a function of temperature, and if you have lots of magnesium it's a lot worse. Magnesium will dissolve more hydrogen than aluminum. So there are lots of bubbling studies on how to get bubbles in the metal to help pull things out.
§20. Going the other way: foamed aluminum and atomized powders [70:25]
Ordinarily you're trying to get rid of the gas. Here's an example where you go the exact opposite direction. A lot of times you come up with some pretty clever things if you do the opposite of whatever the conventional wisdom is. Here's foamed aluminum, made by a similar process to styrofoam. With styrofoam you take molten plastic and inject gas — they used to use Freon, probably some hydrocarbon now. You don't inject oxygen or air into molten plastic, it tends to burn, so you inject something that won't cause burning. In this case I think they foamed it with nitrogen. Very light. It has all the strength of a piece of styrofoam too — so don't push on it too hard.
No one's really found a good application for this, but it's easy to make. Professor Gibson had a big project on this. They had sheets four or five inches thick, four or five feet wide. They never figured out an application for it, but they made it, and I'm sure they got a few million dollars to make it. That's going the opposite direction — where you put gases in.
Another fellow around here had a bright idea once. He wanted to melt various metals under a higher pressure of a gas, drive more of the gas into solution in the molten metal, and then atomize it. He'd atomize it in a vacuum chamber, so all that extra solubility of gas was going to cause the drops to explode. You'd get even finer metal powders. It's hard to get metal powders smaller than about 30 microns — 25 microns is 1/1000 of an inch. He started a company and was actually able to do it, but I'm not sure the economics ever worked out, or that he really found an application for powders that fine.
Does anybody know why, when you get down to five- and ten-micron powders, they're a problem?
Student: Breathing them in.
Yeah, but before you even breathe them in, they tend to be pyrophoric. There's enough surface area that they burn really well. You've seen a metal powder fire — you've been to fireworks demonstrations, you've seen incendiary flares. That's nothing more than a metal powder fire. Or you've seen rockets take off — a solid rocket motor, that's a metal powder fire. You get small fine metal powders, lots of surface-to-volume ratio, heat them up a little bit, and you get some really good fires.
We don't have any Coast Guard folks here, but Professor Szekely had a big project once where someone was transporting a bunch of old machine turnings in a big barge, taking them to the steel mill —
§21. Metal fires and the titanium heat exchanger fire in New York [73:28]
A solid rocket motor is nothing more than a metal powder fire. You get small fine metal powders, lots of surface-to-volume ratio, heat them up a little bit, and you get some really good fires. Professor Szekely had a big project once where someone was transporting a bunch of old machine turnings in a barge to the steel mill for scrap, and the barge caught on fire. It's not easy to put out a metal fire. You can't put water on it, because the oxygen in the water reacts with the metal and gives off hydrogen, and in the fire that hydrogen will sometimes go boom as you give it off.
I've had a couple of cases of titanium fires. One was in New York City not long after 9/11, just a few blocks down. These guys were cutting up a titanium heat exchanger in the basement of a building. They had all these people, none of whom could speak English because they could be hired for a nice low wage, and they gave them oxyacetylene torches and sent them in to flame-cut this huge titanium heat exchanger. When you do that you create fine powders, some completely combusted but some not. When things would get too hot they had a garden hose, and they'd spray it on there — and the workers noticed that sometimes when they sprayed the garden hose the flames got bigger rather than smaller. That's because they were generating hydrogen — oxidizing the titanium and generating hydrogen.
They did this for a couple of days. On the third day, Saturday morning, all the heat was building up in the bottom of the vessel, and the fire was getting worse. They kept spraying water on it; it kept getting bigger. Finally they decided to call the fire department. The chief came down to the third level of the basement, saw what they were doing, and I think the chief probably knew about metal fires. They ordered the truck — the city of New York owns one truck with Met-L-X [?], which is basically a salt, because sodium chloride won't burn and you can smother a fire with it. They keep that truck at LaGuardia Airport because that's where you might have a metal fire from an aircraft crash.
You can't smother a titanium fire with sand, because it will take the oxygen from the silicon — titanium and silicon form a very stable alloy and give off heat. So while they were waiting for the Met-L-X truck, they had a small hydrogen explosion in the basement. It didn't do terrible damage but did a few million dollars worth. And now these people are suing the fire department in New York because the fire department didn't put the fire out. These guys had been generating the fire for three days without any help, called the fire department only when it got out of control, and then blamed the fire department for not being able to control it. It's an interesting theory.
§22. Inclusions, porosity, and vacuum arc remelting [77:53]
We have to worry about the gases. You end up with porosity, and there are various specs for the amount of allowable porosity — but as I said with the football-size defect in the 33-inch diameter shaft, the smaller the casting, the smaller the allowable defect. You also get inclusions. The difference between a pore and an inclusion is that an inclusion has solid material in it, typically a ceramic. In steel, to get rid of oxygen you throw in some silicon, which forms SiO₂ — basically molten quartz. Or you throw in some manganese and silicon and get a manganese silicate, which is liquid in the liquid steel. Those little inclusions can degrade the fatigue strength, so we have to do things to get rid of them.
One thing we do is pour the metal through a shallow layer with a slag on top — a molten glass — and hopefully the inclusions float up and are absorbed into the slag. That doesn't work perfectly. So you can cast an ingot and then vacuum arc remelt it. In the vacuum you'll get rid of some of that stuff. This was a big Air Force project — they were trying to build a $10 million prototype facility to make titanium powders for aircraft parts that were very clean. It's basically electron-beam or laser surface melting. You have a water-cooled copper hearth, you bring in a horizontal bar — Air Force titanium in this case, but it could be a nickel-based superalloy for an aircraft part. They like electron beams because you can get a lot of heat intensity. You overheat the surface and start to vaporize material — and you'll preferentially vaporize some of those ceramic inclusions.
§23. Figuring out the Air Force's classified laser power [80:27]
A guy at Naval Research Laboratory had a big 25-kilowatt laser back in the '70s, which was the biggest laser you could buy commercially. The Air Force had bigger ones, but you couldn't talk about them commercially. The Air Force's laser weapons back then were about 50 megawatts. That was classified — but you could figure it out by three different techniques in the open literature.
One: they were trying to build a superconducting generator with a 50-megawatt output. That was going to fly in a plane and power a laser. Another: Professor Bowen, about the time I came back as a faculty member, had a big project to make halide glass windows two or three feet in diameter. If you knew the damage tolerance for the power density of laser light going through them — these were going to be windows for high-power lasers — you could calculate, and it came out to about 50 megawatts. There was a third way; I don't remember what it was.
In the early '80s I had a friend who worked for Rockwell International, a laser expert. I said, "Charlie, I've kind of figured through these other things — the Air Force laser weapons must be on the order of 50 megawatts." He said, "Tom, I can't really tell you because that's classified, and Rockwell is the prime contractor for the Air Force on this. But I will tell you that the size of the weapon fits in the space shuttle cargo bay." And that was how they sized the space shuttle. Or at least one of the ways — in the late '60s they knew approximately what size space shuttle they wanted to build. Just happened to be one that could carry a laser weapon into space. So you can learn all kinds of things if you just know the open literature.
§24. Laser remelting and clean weld metal [82:47]
So they heat this thing and it superheats on the surface. What Ed Metzbower did at Naval Research Laboratory — he was welding HY-80 steel with a laser, and he found that the weld metal was cleaner than the base metal he was welding. He figured out that the superheating from the laser beam in the weld metal was vaporizing all the inclusions, and he got better properties in the weld metal than in the base plate because he was getting rid of those inclusion particles. So one of the things the Air Force does to make very expensive material is remelt the material and let it resolidify, but in vacuum. They can have another beam rotating on top of the pool to keep it liquid. It's a fairly expensive facility. They were building one of these for titanium about 20 years ago right over here in Worcester, Massachusetts at the Wyman-Gordon facility.
§25. Filters in castings and the catalytic converter [83:51]
We do all kinds of things in castings to put filters in. Typically for aluminum alloys or copper alloys, to get rid of the inclusions. You pour the metal into the runner, the casting is down at the bottom, and you put a filter in between. The filters are various types of ceramic. [Tom holds up a ceramic catalytic converter.] I don't have a ceramic metal filter, but I have a ceramic catalytic converter, which shows you can make fairly complex shapes out of ceramic. It's a bunch of square holes in a pattern — the support for the platinum catalyst in a car. They actually make these now and plug up half the holes for diesel engine filters, to trap the ash particulates.
You can make various ceramic filters that hopefully filter out the inclusions. However, if you get great big castings, sometimes your filters or your mold wall will fall in, and in a casting a couple of feet in diameter you could have inclusions the size of your fist. I always say you might find a brick in the middle of your casting, because sometimes those things fall in during the casting process. It's not always people that fall in — sometimes it's material. And if it gets frozen in, that's why the best-quality casting you can measure with ultrasonics has to fit within a 6-inch diameter circle. That's just the way it is, folks.
§26. Double vacuum arc remelting for aircraft critical parts [85:55]
Once inclusions get in there, you've got to remelt to get them out. That's electroslag remelting and vacuum arc remelting. For the best — actually, even for aircraft landing gear, for the shafts and the rotors for commercial materials — it's typically two vacuum arc remeltings. You cast it once by vacuum induction melting to get the impurities down as low as possible. You then do vacuum arc remelting to vaporize off the inclusions, and then you do it again. I've heard of people trying a third time, but you get to the law of diminishing returns. Typically aircraft landing gear for a 747 or any commercial aircraft is double vacuum arc remelted. One reason is you want to get your imperfections down to less than about 10 microns in size. When you get to strength levels of 250 ksi, your fracture mechanics tells you that flaws 100 microns in size — the thickness of a sheet of paper — can be harmful for fatigue life. So the rotors for the engines, the shafts, the landing gear — you're talking about material that's going to be hundreds of dollars a pound because of all this extra processing.
§27. Hot isostatic pressing [87:34]
You can tolerate some inclusions, but porosity can be particularly harmful. You try to degas it, add alloying elements that form inclusions, vacuum it, stir it with gas — and you still end up with a casting that has some porosity. What you'd like to do is get rid of it. One way is to squeeze it out at high temperatures after you've made the casting. You can make it much better if you put it in a big pressure vessel. It's called hot isostatic pressing.
Developed at Battelle Memorial Institute right after World War II. The first unit was at Battelle. The second one in the world — well, the room where you have your refrigerator now — was the second hot isostatic press, around 1950. Professor Grant, who did all this powder metallurgy work, had one. In about 1985 I got a brand new hot isostatic press, and I put it in that room. I saw Professor Grant in the hallway and said, "Hey Nick, we got a HIP unit." He said, "Oh, you did?" I took him in and showed it. He said, "That's the same spot where the second one in the world was. I had it there in the early 1950s." And I had it there in 1985.
So hot isostatic pressing: you take a big pressure vessel, basically a big cylinder, and pump in argon gas at maybe 2,000 psi. You do everything cold, seal it off, then heat it up to maybe 2,000 degrees Fahrenheit, and that 2,000 psi goes to 20,000 psi. Some units will go to 30,000 psi. You can't pump 20,000 psi directly — you generate it by pumping in 2,000 and heating it up, using PV = nRT (although it's a real gas at that point, not an ideal gas).
Remember I told you that going from liquid to vapor at room temperature is about a factor of a thousand. The cube root of a thousand is ten, so the distance between gas atoms at room temperature is about 10 atom distances. If I go to 15,000 psi, that's about 1,000 atmospheres. At 1,000 atmospheres, I've condensed this thing — at room temperature, though I'm not at room temperature anymore — to where it has roughly the same density as the liquid. It's a gas according to thermodynamics, but you've got the atoms as close together as in a liquid, and you really can't get them much closer. 30,000 psi is about the limit you can compress gases, because at that point they have liquid density.
Student: [Question about whether internal volume changes as gas is heated.]
Yeah — the atoms aren't closer together, they're moving faster. The average distance between them is a function of the velocity. Due to the kinetic theory of gases, their velocity is such that they're still a gas, and there is still a separation. I have to think about that a little more carefully — it's a good point. But if you run through the numbers, you basically get to near-liquid densities at those temperatures and pressures.
Anyway, the metal gets soft, just like in a blacksmith shop. But you're squeezing it isostatically from all directions — like being at the bottom of the ocean. You squeeze the porosity out as the metal becomes plastic at high temperature.
§28. Harley cylinders and turbine rotors [93:48]
Take complex shapes. Anybody here ride Harleys? The Harley engine block is aluminum, with all those fins. They cast it. You'd like to know it's not going to blow up when those little explosions are going on between your knees. It turns out each one of those cylinders is hot isostatically pressed. That's why you pay a little more for that engine. They don't bother on an aircraft piston engine, which is also a finned aluminum cylinder head — if you lose one cylinder on an aircraft, well, it's a big deal, but you can glide down. On a motorcycle it's a little more critical.
Many of the rotors that hold turbine blades — some of those alloys can't be forged, they can only be cast. But you can't tolerate pores a millimeter in diameter, and these are big enough castings, weighing 10 tons or so when they start out, that you would have big pores. So they're hot isostatically pressed.
§29. The North Andover hot isostatic press failure [95:33]
About 15 years ago — let me give you the preface. The world's largest hot isostatic press was located up here in North Andover, Massachusetts. The cylinder weighed 200 tons. The top was beveled on the side, coming down to save steel. The top was 17 inches thick, the main wall 10 inches thick. It let go one night. One 16-ton piece from this 200-ton vessel — the caps were about 25 tons each and they were just threaded in — one 16-ton piece was found a quarter of a mile away. You guys in the battleship business, that's no big deal — they can lob shells a lot more than a quarter mile. But this was more than they usually do in North Andover.
We'll talk next time about why it failed. The other interesting thing: at the time it failed, it was the world's largest hot isostatic press. To build another one — to get the forgings and everything — would have been three years. This was the only one in the world at the time that would take the 60-inch diameter rotors from some of the major aircraft engines. Fortunately, the company that owned it had decided about two and a half years before to build another one, and that second unit was scheduled to come online three months after this one blew up. So we almost had a situation where we couldn't make aircraft engine rotors anywhere in the world for three years. But it was only three months.
They dodged a bullet. Can you imagine what that would have been? A problem for a lot of industries — commercial, military, everybody. And old Harleys would have been a lot more valuable, because you wouldn't have been able to buy new ones. I remember going out to the plant. After they cleared up the building — it had lost its top and walls — the head guy from Travelers Insurance and I were the first two guys in the pit. They took a crane and lowered us into the pit to look at what was left. And I remember looking over in the corner, and they had this rack of Harley cylinders. That's when I learned that Harley HIPs their cylinders.
End of lecture (chunk 4 of 4). Tom indicates he'll return to the cause of the North Andover HIP failure in the next session.