Space Shuttle External Tank (2219 aluminum)

NASA prefers variable-polarity plasma arc (VPPA) over GTA for welding the Shuttle main tanks because the higher heat intensity is better matched to aluminum's high thermal conductivity, and the AC component helps disrupt the stable aluminum oxide.

It turns out NASA likes to weld the space shuttle main tanks with what they call variable polarity plasma arc. We might talk about why they would use some sort of AC rather than straight DC for welding of aluminum, but on a high-conductivity material like aluminum, they like to use the plasma arc rather than gas tungsten arc, because the higher heat intensity with the higher thermal conductivity metal gave better results. So that's why we use plasma arc.

2219 chosen as the rare aluminum-copper alloy with good weldability. Tank had to be welded; leaks were unacceptable.

One alloy in the aluminum-copper series, 2219, has very good weldability. That's what the Space Shuttle main tank was made out of for many years, because you had to weld it. You don't like leaks — they found out with Space Shuttle Challenger what happened with leaks of the O-ring. So they used 2219 aluminum for the tank when we had a Space Shuttle. If you get down to the 6000 series, you get a lot of A and B weldability. 6061 and 6063 — the difference is, this is typically plate or sheet and this is typically extrusions — they have A-grade weldability. The largest tonnage of aluminum alloys is 6061: architectural aluminum, door frames, window frames, you name it. You can get good strength in the base plate, but we're going to see you only get about half the strength in the weld as heat-treated. The 7000 series — some of them have very good weldability. Some of them, like 7075, which was the workhorse aviation alloy — 2024 was the workhorse alloy from 1920 to 1950, and from somewhere around 1960 to 1970, 7075 was the workhorse aircraft alloy for Boeing and McDonnell Douglas. Since then, Alcoa has come up with other alloys that have better combinations of properties.

Example of why 2219 was chosen for the Space Shuttle main tank — more weldable than other 2000-series Al-Cu alloys. ~4.5% copper.

This is percent copper, increasing. This is a hundred percent aluminum, and this is five percent. 660 centigrade is the melting point here, 540 or so here — about a 120 degree centigrade temperature differential. Since aluminum melts at 660, it doesn't have really good high-temperature properties. In fact some of these alloys are not very good at all at temperatures above even 200 degrees Fahrenheit. Boil water and you wipe them out. So these are heat-treatable alloys, and 2219 is a modern alloy. That's what they used to build the Space Shuttle main tank. These are lightweight. 2219 was a four-and-a-half percent copper alloy that was more weldable than some of the others. If you tried to weld the Wright brothers' crankcase or the cylinder housing for their engine with gas tungsten arc, you'd crack it. You'd melt it and you'd have nice cracks — pull that rope, melted metal right out. Same type of problem as you have on Monel — you get cracking, equation melting in the heat-affected zone in the copper interiors.

Tom uses 2219 — the external expendable tank alloy — to illustrate aluminum-copper liquation cracking at grain boundaries.

The problem is liquation cracking. When you get a liquid up there — this is actually 2219, which is what they used to make the space shuttle great big external expendable tank out of. The problem is the copper alloy — we talked about liquation cracking in nickel-based alloys, but we have the same thing in aluminum alloys. It's an aluminum-copper alloy, and the copper will segregate at the grain boundaries and it will melt while you're heating up the weld. You can see the cracks just running right through the grain boundaries, which basically turned into a slurpee — a liquid-solid solution.