WM_Su2014_27

Remember that, I heard from some Texans, probably from University of Texas. Was a guy comes up to the counter, he says "I'd like a hamburger," and the guy says "you must be an Aggie." And he says "well you're right, how'd you know? It's because of my demeanor? Is it my good looks?" He says "no, this is a hardware store." So okay.

So nickel based alloys have complex structures, which we do lots of things to try to control. This is actually a — in this great — this is from a fairly recent book, this is an artist's depiction rather than a microstructure, so you can see everything in one picture. They have carbides, they have M23C6 carbides, they have gamma prime phases, which is the nickel-3-titanium or nickel-3-aluminum. They have MC carbides, so M1C1. They have nodules of these things, they have these blocky phase of gamma prime. The gamma prime is the stuff that melts in 2000°C and so it has excellent high temperature properties, and has structures down here, and then you have grain boundaries in nickel based super alloys.

Like turbine blades, we may grow single crystals, and the whole structure will look like this blocky phase when it's brand new. It has excellent high temperature strength and that's the basis of being able to run these things at 2000 degrees centigrade or even 2100 degrees centigrade when their melting temperature is barely 2400 degrees centigrade. You can operate these things in within a few hundred degrees centigrade at their melting point.

The problem is, there's something called coarsening in metallurgy, where if you have fine structures that might be these nice rectangular type blocks because of the crystal structure, over time they will get larger. You actually know coarsening — if I go back to my Sprite in there, if you actually pour the Sprite in there you see a bunch of fine bubbles, you come back ten minutes later and you'll see a bunch of coarse bubbles, okay. The bigger bubbles will steal molecules from the smaller bubbles, and so the little bubbles pump up the big breath [big] bubbles.

In fact, when I was interviewing for my Hertz Fellowship, that was one of the questions he asked me. If I had two equal balloons connected with a valve in between, a pipe, a valve in between, uh, uh — actually, one — if I had two balloons and one was smaller than the other, and I had a valve in between, I opened the valve, what will happen? And a lot of people want to say "oh they will equalize the pressure, and you'll end up with two same size balloons, they're identical." That's not true. The little one pumps up the big one because the little one has a higher pressure inside it. So it actually pumps up the big one and it deflates, okay. I knew that answer, so he was happy. He didn't like my other answers, um, so I didn't get the fellowship party.

Oh yeah, we have some skeptics back here, some skeptics about that. Yeah, we'll try it. Okay. Oh okay, well, I'll see if I can do the demonstration, I gotta think of a simple way to do it with equal balloons, um, but it is true okay, it's actually one of these. Anyway, um, I can prove it mathematically. That doesn't work for you? It doesn't, no, okay.

So these are the structures. I'll tell you a little story about these types of structures. So General Electric makes jet engines and, not only that, big land-based turbines for generating electricity. And they came up with a modified alloy, um, back in the early 1980s, that they called GT-100. And GT-100 was not such a big change in composition from other alloys and so they decided they couldn't patent it, they didn't. Well it turns out it was a pretty good alloy, and other people building turbines, like their competitors Siemens or Pratt Whitney [Pratt & Whitney] or others, decided well this is a public domain alloy and we can design it into our systems, and they did.

Until 1997 or 1998, it turns out some patent attorney at General Electric and some engineers got together and said well, the heat treatment — we have a particular heat treatment — and we want to try to patent the heat treatment. Actually I take that back, they applied for the patent back in the 80s on this heat treatment, and it never got allowed until 1997 or 1998, and all of a sudden the patent office after nearly twenty years allows this patent. And all other companies that were competing with General Electric had already designed this alloy into their systems, and all of a sudden General Electric says oh well if you're using my alloy you owe me money, okay, royalties.

Everybody was very unhappy. They were particularly unhappy with the patent office for sitting on this patent application for fifteen years. And General Electric had been prosecuting — the patent office would decline it, GE would come back with their arguments, and anyway they finally got the patent on it. So it turns out, um, these other companies had to scramble and put another alloy and substituted, either that or paid their competitor royalties, which they didn't want to do. But it turns out if you're going to rebuild an engine, if you've already purchased a jet engine component or a land-based engine component that has this alloy in it, when you purchase that component you purchase the license to use that. I mean if it's patented, okay, obviously you don't think about it but that's essentially what you do — when you purchase it from the patent holder, you can use it.

Well the problem was, this alloy was a gamma prime strengthened alloy, and after 10,000 hours of use in a jet engine or whatever, these blocky carbides — had a blocky gamma phase — had become rounded and much larger, and didn't have the same properties. But if you could somehow do some sort of heat treatment that could bring them back to this without completely melting it again, you already had a right to use that part, you had bought that with the original thing.

So it turns out it was a Greek Navy student who worked with me, with a company called Chromalloy, and we basically — couldn't melt it, but we took these old turbine blades and we gave them a hot isostatic press heat treatment. I talked about this hot isostatic press, where you go up to 20,000 psi in an argon atmosphere and heat it up, and you re-heat-treated it. And by doing hot isostatic press on these parts, heat treatment, we could bring it back to its original structure without melting it and re-solidifying it and doing all the other heat treatments. We could get back the very fine phase, uh, getting it very close to the melting temperature but not melting it, uh, because it came back out of the HIP unit with the same shape that it went in at, so we obviously didn't melt it. And so they could reuse the ones they had bought, okay. But just one of these — doesn't happen very often, but someone gets a patent late and it surprised everybody and they were not happy.

So one of the things about these high temperature alloys, of course, you'd like to weld them. And Inconel came out — our Inco, International Nickel came out with 718 alloy I think around late 1970s or so. So 718, Inconel 718, had excellent weldability properties. Here it is down here. And it has fairly low aluminum and titanium. If you look at the aluminum and titanium in these precipitation heat treated alloys, strengthened alloys, you find that the more aluminum and titanium you have, which goes to higher and higher temperatures, like Inconel 100, very high aluminum and titanium, very high strength — Martin Marietta 200, Udimet 700, Astroloy, these are all proprietary kind of names — but Inconel 718 has sort of become the workhorse nickel based super alloy for making jet engine parts. Maybe not the hottest parts, which might have to be castings and whatnot, can't be welded. But if you've got to make cases for your turbine and stuff and it has to take high temperatures, Inconel 718 is the choice in general because it has very high weldability.

There are some variations on it that use niobium rather than aluminum and titanium, and niobium is easier to weld in many cases. It gives you the same nickel — in this case nickel-3-niobium — high temperature phase. But you can see that in general, titanium substituting for aluminum gives you much better weldability, so you can only tolerate about one half as much titanium [aluminum], or aluminum as you can titanium. So over the years people have been making modified Inconel 718 that sort of shifts over in this direction. But one of the first things to look at when someone's trying to weld a nickel base alloy is to come to a plot like this, and kind of see where you are in your aluminum-titanium ratio. Some of them just aren't weldable okay, no matter what you do. You can only use them as castings.

Turns out one simple little test — you often don't have a lot of material, so they do a little circular patch test. This might be a one-inch diameter TIG weld, and these things will crack very fairly readily so far as that goes. You also — they will tend to give you grain boundary cracking, and you get this rock candy intergranular fracture. That's the same thing you get with stress corrosion cracking — that's the same appearance you get with stress corrosion cracking in the stainless steels, as far as that goes. So that's all I'm going to do on nickel based alloys. They are very susceptible to cracking.

The Monels and the cupronickels, which the US Navy uses more than anyone else that I know of — they, sea water corrosion resistance. But in fact, when we want to do solidification cracking experiments in the laboratory, those are the alloys we use. They're the easiest ones in the world to do solidification cracking on. And the Navy happens to use them, okay. So you got to follow the procedures, you can develop procedures, right heat inputs and things, but you got to have the right joint preparation and other things. Yes?

Student: So we had a problem with some cracks in some of the Inconel that we had, and one of the solutions was going to be — to propose, I guess — not one that we use — was to basically crush the pipe in order to close the cracks. How viable is that as a long-term solution?

When you crush it, you mean take a cylinder and just — it's no longer working as a pipe? Student: Well, is it not, you know, not completely flat, but you know like lower the diameter maybe slightly. Oh so you're kind of rolling the outside to just shrink the diameter, okay. Well all you're doing — you really had cracks, I mean you have, we peach [breach] through it? Student: Uh no, I think there was like maybe sixty percent. Oh okay. All you're doing is trying to introduce compressive residual stresses. You may have had from welding tensile residual stresses on the outside. These are cracks from the outside or inside? Student: I assume — yeah, they should have been on the outside. Because what you — from what you're telling me I would assume they're on the outside.

So I got a weld here, and I got some little cracks in the heat affected zone, and they might go fifty percent of the way through. So it's going to be a pain in the neck to have to cut the whole thing out and re-weld it and hope that you don't get cracks again. And if you were at sea and there's a temporary repair, is this a permanent — Student: No, this new construction. No okay. Older boat, one that's only going to go for another five years, right?

Oh well okay. In any case, if you now compress this whole thing, um, in here, instead of having tensile stresses on the surface you can introduce compressive stresses. You're going to bow it in, like, you know, the thing is going to — the weld is going to — and now I've got compressive stresses at that crack and hopefully it won't grow, okay. Cracks only grow under tension. So they've now tried to introduce a compressive residual stress by compressing the — this is, there's a center line through both of these right, this is just one half of the pipe, right, twelve o'clock position or whatever. So that's what they were trying to do. First time I've ever heard of that trick.

Oh, okay well, I was going to say I've never heard of it before and I'm not sure I really think a whole lot of it, and obviously someone else didn't think a whole lot of it either, right. Okay, I'd like to know the application, and if it's a critical application I want to make sure I'm not near that, okay. And if it's a non-critical application, okay fine, okay. I mean you don't want to repair it and no one's going to get hurt if it fails, but —

I don't, except for this one code from Westinghouse Bettis, I don't know of any welding code that says if you find a crack you can leave it, okay. All codes say if you find a crack you got to grind it out. Whether you just — if you can grind it out to a certain depth and you don't even have to repair it, you can just, you know, grind out the crack, get rid of the thing. Now a lot of times on a plate or a piece of sheet metal you stop drill the hole, you just drill a little round hole at the tip of the crack to get rid of the stress concentration of the head of the crack. That's a temporary fix that's used all the time, has been for a hundred years. Doesn't always work but it can stop fatigue cracks.

Yeah, did you have a comment? No, okay. Um, but stop drilling — stop drilling holes, stop drilling cracks, okay. Just, but one of the problems is if you're not careful, okay, and you think the crack goes to here and you put the hole here but in fact the crack went through here, uh, deeper and you can't always see it, then you didn't do anything, okay, to stop the stress concentration. So, and it works in sheet metal. It doesn't work so well plate, because if you start thinking of the other dimension, how are you going to stop drill it at, you know, the crack is not necessarily just perpendicular to the surface, okay.

So, um, now most things don't allow you to leave cracks. Uh, I'm sure the Westinghouse Bettis thing, someone did fracture mechanics and did all kinds of studies and determined it was okay and you can leave small cracks. I told you the story about the Air Force wanted to do a test on acoustic emission, and they wanted to fly an airplane with a crack in it and follow the — with a crack in it and follow the crack — and the Air Force general says you can't fly a plane with a crack, you can't fly a plane with a crack in it. Well every plane in the world has got cracks in it. Don't tell Mr. General that, you know, every one of his planes has got cracks, he just doesn't know where they are, okay. But it's because we know how fast cracks grow, and we can — people have done the fracture mechanics. All kinds of structures have cracks in them.

And if you have a tough enough material, your fracture toughness tells you small cracks won't grow. When you do inspections on things, you're only looking for indications that are larger than an eighth of an inch generally, okay. Which means oh I could have a crack that's a sixteenth of an inch, or three thirty-seconds, okay, and it would pass. The code says no cracks, but you could have pores or other slag inclusions or whatever that are smaller than that. Other codes, and porosity — the structural welding code will tell you you can have no more than one inch of undercut in twelve inches of weld, okay. So it doesn't mean you can't have undercut, you can. But that undercut typically is not more than one or two millimeters, and someone already knows the critical flaw size even in the structural steels is not ten or twenty inches like a submarine steel, it's more like an inch or two inches. So who cares about a two millimeter flaw — unless it's a bridge.

And they have — the codes will have stronger requirements for bridges than they do for static structures. So they have inspection for dynamically loaded structures where you can have fatigue, or statically loaded structures where you don't expect fatigue, but you've got to have someone do a fracture mechanics analysis to determine whether you can leave cracks there. In general the codes say no cracks. Other people may decide it's okay to leave a crack.

I've told people leave cracks. I told people thirty years ago here at MIT, they had a steam pipe — they got steam pipes right all through the campus, and they generate steam over the generating plant. And they had a weeping leak in the steam plant over here near 100 Memorial Drive. And they came to me and said we've tried to weld it four or five times and it keeps on cracking, because they're trying to hot weld it while it's still got steam in it and you got moisture coming out of the crack and guess what you get? Hydrogen, right. And they said — and they were going to have to shut down the steam plant, and most of them might use heat for that in the campus, um, to do this, and it was the middle of the winter and not everybody wanted to, you know, they probably have freeze-ups on other places and things. And they wanted to know how they could weld it. And I said well, why don't you just put a patch over it, and then fix it in the summer, okay.

So you do things like that and you left a crack there, but it was buried in the ground, I mean it wasn't going to explode. Well it could have exploded if they left the crack and didn't do something to support it, but putting a patch over it works. It's not a permanent solution but it's a temporary fix. And you guys don't know all about temporary fixes — except for the Coast Guard, your temporary fixes are permanent, right? Anyway. Uh, sorry, didn't mean to give you uh, palpitations.

Okay so that's all I was going to talk about nickel alloys. Let's talk about alloys in general. And here's a nice fictitious slide. I'll tell you what's wrong with this slide. Um, it looks sort of interesting in the beginning.

[Tom hands out materials.] In the meantime you can pass this out for a little bit later. So, um, here is tensile strength in KSI or megapascals for four different types of alloys: steels, titanium, aluminum, magnesium alloys. It tells you the density, decreasing order. Steels are heavy, titanium's intermediate, aluminum's light, and magnesium is even lighter. And the Department of Energy has got a huge program to put more magnesium in automobiles. They've had this program for forty or fifty years. So magnesium is the material of the future for automobiles. It always has and always will be.

And I understand the Navy's now, since they're looking at aluminum craft, they're also looking at some magnesium because hey it's really light. [Tom indicates a sample.] That you have a bar of, in order to bring it in sometime. And you can see how much lighter it is.

Well here are different strengths from just a plain old carbon 1015 steel, kind of like old automotive sheet — easily, it's mild steel, whatever you want to call it — all the way up to A640, and I think it goes even up to 300M, yeah, which is basically aircraft landing gear type steels, 300 KSI strength okay. And so it looks like steels can go to very high strength, but if you really talk about readily weldable steels, we're all down in here. Titanium, the workhorse alloy, six aluminum four vanadium [Ti-6Al-4V], developed over here at Watertown Arsenal which is now Home Depot — um, it wasn't Home Depot at the time that developed 6-4. But it was Watertown Arsenal.

And then here's the aluminum alloy 7075, used to be the workhorse alloy for the skins of Boeing jets, or Boeing aircraft and whatnot. Uh, 2024, which isn't on here but it would be down — be like 2014, um, 3003 is beer can stock okay. 2024, 2014 is similar in strength in the heat treated or otherwise conditioned to the aluminum copper alloys that we've been using for a hundred years, okay, in aluminum. The Wright Brothers, uh, engine — the case of Wright where this engine was a cast 2000 series aluminum like this, um, and it's in the Smithsonian.

The magnesium alloys have this strength, and you say "gee everything looks like it's got" — particularly if we do a specific tensile strength, they all come scrunching down. Well that's not really true. The steels that are weldable are down here. The titanium is three times stronger. The typical 6061 aluminum are significantly stronger than the steel in a specific strength, and magnesium is right up there with the best of things. So this sort of says gee everything's sort of the same. Not really, because these steels over here are very difficult to weld okay, or use in many applications, very susceptible to hydrogen. So it turns out these lighter alloys really are better, okay. And there's places where you have to use them and that's why you use them.

And that's why, you know, back in the 70s the Navy was interested in aluminum for superstructures on surface ships. And then after the Falklands War and the Sheffield caught fire and the Belknap caught fire, um, with the JFK — the first JFK carrier — the Navy then got out of aluminum. And then with the littoral battlefield they got back into aluminum as the only thing that's going to work. That's why we built aircraft out of aluminum, because it's the only thing that's going to work. That's why we would like to — and you already have this, it's, you're not going to be able to read it if I get it all the thing on here — uh, we'd like to build automobiles out of aluminum, and we are heading towards aluminum and magnesium because lightweight materials save energy, okay, they also allow you to go faster, um, it's sort of obvious.

And steel's — this is the Achilles heel of steel — is, it's heavy, okay. And hydrogen, I guess if I had to pick two. And hopefully you've gotten that. But we know how to deal with the hydrogen if we're careful, and we don't have to worry about too much if we don't need to move things very fast. We can use steel and take the advantage of the low cost.

Well here are the aluminum alloys, and they have these fancy designations, just like the steels. AISI has a four digit number for the steels, like 1015 steel, 1020 steel, 4340 steel. The aluminum alloys have a one thousand, two thousand up through nine thousand classification, although nine thousand is not yet used. Uh, one thousand is basically nearly pure aluminum alloys. The example is 1100 aluminum, 99 percent aluminum minimum, okay, doesn't have any real properties other than, has very good electrical and thermal conductivity. So we use it for wiring, electrical wiring. We use it when we need heat sinks. We use it on aluminum, um, the plates of my favorite, uh, pot that I brought in and showed you, the stainless steel, they actually forged on there a piece of aluminum on the bottom to distribute the heat, okay.

Nowadays they actually use composites of up to five layers of metal, and sometimes it's copper in really expensive pots. But the cheaper pots, uh, have, you know, be stainless steel with aluminum with stainless steel, so a three layer peanut butter sandwich if you will. So 1100 aluminum, very good corrosion resistance because it has no alloy content in there to create pits and other things.

The heat treatable alloys — the oldest alloy is like the 2024, it's an aluminum copper alloy. Like I said the Wright Brothers aircraft engine is made out of aluminum copper, precipitation hardened, for strength. Doesn't have particularly good corrosion resistance, most of it doesn't have very good weldability. 2219 is a modern version which NASA was using for the early space shuttle main tanks and stuff because it was weldable. They had to weld the great big, uh, tank. They've now gone to aluminum lithium alloys which are even lighter, which are part of the 8000 series that you're not — on this thing.

The ones — they throw about one percent manganese in — manganese, manganese, not magnesium but manganese — is for formability. The 3000 series, beer can stock okay. Um, some of your aluminum foils are 3 — thin, you know, you've got to roll them down very thin, they're 3000 series. You don't want something as soft as this. These things really pure aluminum, like a 1040 aluminum is 99.6 percent aluminum, may have a strength of two or three KSI, I mean it's dead soft, okay. Softer than most plastics in terms of strength. You put something over here, you get 10 KSI with just one percent manganese, and high formability.

The 4000 series basically are filler wires or certain cast alloys. The workhorse — two workhorse aluminum filler alloys are 1144 [4047] and 4043. Weld a lot of aluminum alloys, not all of them though. The non-heat-treatable — most of what the US Navy uses is just an aluminum magnesium alloy. It's like 20 or 25 KSI strength, it's nothing compared to a steel, but it's weldable and you can get 100 percent joint efficiency in the weld. So 5054, 5456 are two popular Navy alloys. Huh? Student: 5083 is actually popular with commercial stuff too. Yeah, uh, 5083 is more economical than 5054 or 5456. 5083 is probably the most common of the 5000 series, widely used in commercial applications. And the Navy is probably going that way, just to — that have quite the properties as the alloys that the Navy likes to have. But the Navy doesn't have the purchasing power they used to have, okay.

Student: [inaudible question about 6061] Yeah, because 6061 is a heat-treatable, precipitation hardening, ball [aluminum] alloy. It is probably the largest — I don't know if it's the largest volume is just pure aluminum, when you need electrical conductivity. But the number two alloy in aluminum is 6061. I think it's better — well yes, well 6061 has a wonderful combination of good precipitation hardenable strength, very good corrosion resistance, easily weldable, okay. Just sort of like, I said, steels, you know, steels were low cost, high strength, high toughness, you know, I could rattle off seven, or you know, availability. 6061 is an available alloy. You can find it. You go order it as an extrusion, extruded tube, as an angle, as a plate, as a sheet, it's there in the local supply house, you know, in huge quantity.

You're going to start picking some other aluminum alloys — we don't make one and a half billion tons of aluminum every year. We make, maybe we make, you know, we probably do make one half billion pounds of aluminum. If I make three or four billion pounds of aluminum, but we're different orders of magnitude. We make 45,000 — 45 million tons of aluminum each year, and forty percent of that goes into beer cans. So maybe I'll take it back. The 3000 series alloys are probably the highest volume because of can stock. But after that it's probably 1100 for electrical, and 6061 in structural, unless you're in the aerospace industry where you go to very high strength. You lose a lot of corrosion resistance, but those commercial jets are going to be maintained, okay, they're too valuable and the cost of the failure are too great.

And this doesn't give you the 8000 series alloys. 8000 series is sort of a catch-all, that's where your aluminum lithium alloys are. And I didn't bring — I'll have to bring tomorrow — one of the highest strength aluminum alloys, actually the highest strength I know of. Does anyone know what the application is? It's some of the — they would be 8000 series. Baseball bats. Okay, aluminum bats.

There's some interesting stories. They actually use scandium — when they only use this for, scandium in metallurgy, to get very fine grain size, and they extrude these tubes very thin wall and they quench them right off the extrusion machine, actually some of them are self-quenching. But they get very fine grain size, they can get 100 KSI yield in baseball bats. But they're very thin wall, I mean. But there's a big business there, I mean, it's three or four hundred dollars a bat. Actually the one I bought cost 99 on Amazon, or 120 off Amazon. But the good ones, um, are, uh, can be three or four hundred dollars, and they break readily so you have to buy new ones okay. And we could talk about that. But, um, but the 7000 and 8000 series are now [not] the strongest aluminum alloys, because the scandium batch can only be made in like two millimeter thick.

Student: [I served on a] ship with 5456 aluminum as the superstructure, and it was subject to severe corrosion cracking as well as some of the fatigue. Is that just because, you know, the 5000 series, you don't get that heat treatment on it to help prevent some of those?

In your case it was probably half inch or greater thickness, okay. And the reason is residual stresses. You're getting fatigue cracks, it's not that it's inherently so bad, but when you start making relatively heavy structures — I mean by heavy, by thick wall — the residual stresses, remember aluminum shrinks six percent, and so you get very severe distortion or residual stresses if you're more than half an inch thick. You're getting more residual stress and less distortion. Remember my kind of little thing, general rule, three-eighths is kind of the worst of both — distortion but not, you know, it's the worst distortion. Very thin is not a lot of stress, residual stress or distortion. Three-eighths is the worst distortion. You get thicker than that you've got lots of residual stress.

So yes, 5456 is more highly alloyed than 5083, which does cause pitting, and the pitting leads to fatigue cracks, okay, that's the initiation sites for fatigue cracks. You're probably talking about a, you know, well you were on a cruiser, so you're talking a thirty year old ship, right? And that's what they were using back in the 1980s, was 5456. I mean I think it was probably David Taylor with Alcoa that developed those alloys. And what people are telling me now is for the LCS's, the Navy said okay we don't want that extra 10 or 20 percent of strength, we'll go with the 5083 which is readily available, and we'll design with that too. And we give up a little bit of the strength but we don't have the pitting problems that lead to fatigue cracks and all the other problems.

The Navy has had a lot of problems with the 5000 series alloys when they started using them in the 70s and 80s on the superstructures. It took a few years for all the stuff to develop, so now you're living with what didn't show up for thirty years right. But it does show up now, but it is because of the thickness — of the fatigue cracking, more than anything else. And the pitting, the pitting is also a problem, and the pitting is a function of alloy content.

I don't know if I got anything here, uh, well let's see what it says about — I do have something that least 5054. Some of these — so this is just listing some typical aluminum alloys. And so 5083, marine components, tanks, unfired pressure vessels, railroad cars, drilling rigs, I mean obviously used for commercial as well as, uh, now military applications. Uh, 5454, structural applications and tanks for sustained high temperature service. 5456, structures, tanks, unfired pressure vessels, marine components. But I have seen 5456 used in non-naval applications, but 90 to 95 percent of it was naval applications. And what I'm sort of hearing here is the Navy's getting away from it because of these problems in the heavier structures, okay. Heavier being thicker, not heavier in weight but heavier in thickness, okay.

Um, now the aluminum alloys come in a number of different what we call tempers. My children used to come in different tempers — now it's my grandchildren come in different tempers, um, but a different type of temper. The zero temper is annealed. The F is as fabricated. The H is hot worked, so they've rolled the steel — enough steel — the aluminum, and it's strengthened because of the cold work that's in it. H2 is basically partially annealed, I mean they have lots of different numbers, strain hardened and then stabilized, is a heat treatment that keeps it from developing precipitates later.

There are whole books written about aluminum tempers. I mean you can buy a book and the whole book just tells you all the variations on aluminum tempers. Here's another thing — there's something I hadn't seen before, about a year ago, was W temper. That's where it's been solution heat treated but you haven't done anything to precipitation harden it yet. And I had a — I can't remember the case right now, the situation — but someone was selling something in a W temper and they were supposed to heat treat it later, but they didn't, or something, but anyway.

We've already talked about the stress relieving by stretching, or the stress relieving by compression. You get, because of the greater shrinkage with aluminum, you can get more severe residual stresses. Here's the Navy component painted gray, you can tell it's a Navy component right? [Tom holds up a part.] Here's a — this is actually a piece of steel, but this goes on the back of a Seahawk. This goes —