§1. Aging temperatures and the Concorde skin-temperature limit [00:01]
...give it strength. If I take it and anneal it, or hold it for a long time at 110 degrees Centigrade, after about three or four hundred days — if I want to do a heat treatment for a year — I can get the maximum strength, the maximum hardness, which is related to strength. 130 is also pretty good, but again we're talking ridiculous times. If we want to get down to reasonable times, one to ten hours, I might use 165 C, and that's why before, 340 F — you've got to convert, because some of these metals just don't speak Centigrade like me.
Now, these temperatures are not that hot. So you have a problem when you're welding. You're certainly going to overage a lot of these things in the heat affected zone.
The Concorde, the SST that the French and the British flew, was not limited in speed, it was limited by skin temperature. They had heat treated aluminum alloys, and they had thermocouples — temperature sensors — to measure the frictional heating in the atmosphere. On a really cold day at sixty thousand feet or whatever altitude they were going to fly at, minus 60 degrees Centigrade, they could go across the Atlantic faster. On a warm day up there, minus 30 Centigrade, less cooling. The frictional heat on the skin — I think I knew once — they couldn't exceed something like 200 degrees F on the skin before they were going to get into over-tempering and the life of the aircraft was going to be shortened significantly. You needed a life of the aircraft that's going to go out to ten thousand hours or something, okay.
So when they said they were going to retire them, they made a big deal of it because they had this crash of one of them in Paris — it was taking off and the aircraft ahead of it had dropped a piece of trash on the runway, and the Concorde hit it and caused it to crash. They said, ah, we're going to scrap the fleet. They were about ready to scrap the fleet anyway. It was past its useful life, so this was just an excuse. I actually ended up having to work for the company that dropped the trash on the runway, but anyway.
§2. Filler metal selection for aluminum [02:31]
Any questions on that stuff on aluminum? That's sort of the basic metallurgy of aluminum — easy to weld, the heat treatable alloys. You can put enough alloy content into the ones that are slightly work-hardened in the 5000 series that you can get basically a 100% strength joint in a 5000 series weld. So if I'm making some aluminum ship — and the Navy makes lots of hydrofoils nowadays and lightweight ships, and they're used in 5000 series non-heat-treatable alloys — they don't have strengths of 60 and 70 ksi, they have strengths of 25 and 30 ksi yield strength.
There are some significant limitations. Which filler metal you use makes a big difference in the subsequent properties of what you're going to end up with. As I said before, you want to use a filler metal that either keeps you very lean in alloy or very rich, which means you don't want some filler metal that, when it mixes with the base metal, gives you something in this easily cracked range.
If I'm using 3000 series alloys, which are way over here, I can weld them with 1100 aluminum, nearly pure aluminum. There's not enough alloy, you'll never get over this way. I can use 4043 filler metal, which is a very high silicon aluminum alloy, and my weld metal will end up over here somewhere. I don't want to use some alloy that's going to put me in here. So now that I said that, let's see if I'm right. Here's the test — see if I pass today.
This is a chart of what aluminum alloys can be welded with what filler metals. Let me just show you the whole chart — this chart goes on for two pages in the welding handbook. So it's a two-page chart. If I look on here, I should be able to find — here's my 6061, where's my 1100 aluminum?
So here's my 1100 aluminum, my thousand series — or actually you wanted three thousand series. Three thousand and four, it says I can use 4043 filler metal. I was right. That's the most common filler metal, folks. I wasn't just taking a wild guess there. That's called an educated guess. So with 4043, you can weld something that's very pure with something that's very highly alloyed, and what you get is something alloyed with something that's way over here, and the average is right here. 4043 is just basically an aluminum silicon alloy, very highly alloyed.
You can also use 4145 for 3004. 4043 will be the easiest one to weld for the guy making the weld, whoever it is — man, woman, dog, machine, whatever. This is the easiest operability, and these little footnotes here — there's a bunch of footnotes at the bottom — will tell you something about that. 4145 is probably optimum strength. 5356 might give you the best anodizing, so you can match the color, for example. So there's a whole series of things.
It also depends on what you're welding it to. If I take three thousand and four to three thousand and four, I can go across here and I'll probably find — it says 5356. I guess I was wrong, if it's three thousand four to three thousand four. But three thousand four to eleven hundred, it turns out I was right, 4043 is a good one. It's a complex chart to look at, what filler metal.
§3. Foreshadow: the Canadian aluminum casting repair case [07:08]
What I'm going to do on Friday is take you through some free consulting I did for a shop — I mentioned this a little while ago — that repairs aluminum castings for aircraft engines. They are one of the only shops in the country that's authorized by the Federal Aviation Administration to do weld repair on these castings. They had developed a process — they think they were using 4043 on this particular casting alloy, I had to look it up — and we'll go through on Friday how I use this chart.
They said, well, is this the best alloy? Because someone else — there was a crash in Canada, and the Canadian authority — ours is the Federal Aviation Administration, in Canada it's the Canadian Aviation Administration, but you've got to say it in French and English in Canada, right? I can't remember what the initials are. I think it's CAA.
Student: [unintelligible]
What Canadian are you? What good are you as a Canadian, anyway? Anyway.
So they were being criticized in a report by the Canadian Aviation authorities for the filler metal they were using. They contacted me and said, Tom, we've been doing this since 1976 — should we re-look at what we've been doing for the last forty some years? And so we'll talk about that on Friday. It's a little case study on how do you pick an aluminum filler metal. And it depends — what are you trying to get? Best strength, over-matching, under-matching — we talked about matching of strength yesterday. You want better corrosion resistance? In some cases you want to have a color match when you anodize the structure afterwards. You don't want the weld to look like a weld after you've anodized it blue or green or whatever color. So there's lots of different filler metals that help match these things.
I didn't tell you this, but in steels we have what we call weathering steels. You have 8018 electrodes, and they have a designation. If you're buying the steel from this steel company that uses this composition for their high strength steel, you've got to use this electrode, which is different than if you were buying the steel from another one, if you want to match the rust color on the weathering steel. There are lots of little details out there that it's not worth spending a lot of time on. Any questions in the next two minutes?
§4. Residual stress in heavy aluminum plate: the Davenport stretcher [09:48]
When we're talking residual stresses, when you heat treat aluminum you get residual stresses through the thickness. There's actually a student I'm working with who's doing his bachelor's thesis on something called up-quenching of aluminum.
This is aluminum alloy 7050 plate and sheet manufactured by Alcoa. It's an alloy they developed, and of course they're supplying the world's best — I don't know if they mean best products or best companies that they're selling to, they're trying to pander to their customers anyway. They have a bunch of different heat treatments here. You can get 7050 up to six inches thick. That's what I want you to see here. Here's another one up to six-inch-thick plate.
It's all made in Davenport, Iowa, on the world's largest rolling mill — larger than any steel rolling mill, because you have enough separating force on a big aluminum rolling mill that you can roll very thick plates, very wide plates. And what do we do with them? We turn them into aircraft wings. The Davenport Works makes most of the aircraft wing material for the big Boeing jets and the big Airbus jets. Airbus may buy it from somebody else, but.
When they heat treat this six inches thick to get the strength, they get tremendous residual stresses. And when you start machining this — all the little cavities and everything — they may throw away 80, 90 percent of the weight of the aluminum after they take this solid chunk and turn it into a thinner section with all these bosses and stuff. You don't want to have to weld it; they machine the whole thing. They've got tons and tons of aluminum, they may throw away 80, 90 percent of the weight as machining chips. But it's got tremendous residual stresses, and if you don't get rid of them you're going to have fatigue problems.
You can't heat treat it to relieve the residual stresses for various metallurgical reasons. So what they do — this 51 designation is a one percent strain — they have this huge hydraulic jack that can take a six-inch plate by about 10 or 12 feet wide and just stretch it one percent. Now remember, this is 70 ksi material, so go home and do that homework. Six inches by about 12 feet by 70,000 PSI, and you're going to do a one percent strain. How many million tons is that? I've been to that room. They weren't stretching something at the time so I didn't see it being stretched, but it's a big hydraulic machine. Just got big jaws on the ends, and they just grab it just like a big tensile machine and pull it. One to three percent strain.
Student: Does that activate the aging?
Yes, that can activate the aging. I'm not going to get into why these are 7451, 7351 — but yes, you're right, it does.
Student: Is it done before or after aging?
Depending on the alloy — you quench it and you age it, but you may do a double age. You may age it to get strength, and then you stretch it, and then you might age it again to get even more strength, or overage it to get corrosion resistance. I'm not enough of an aluminum metallurgist to be able to tell you all that.