|Kennedy Space Center, Florida, Feb. 2, 2015. Charles Bolden delivers a “state of the agency” address. Left to right, the three spacecraft on display are the Boeing CST-100, NASA's Orion and the SpaceX Dragon. Photo: NASA/Amber Watson|
|This diagram shows the seven pieces of Orion’s primary structure and the order in which they are welded together. Credits: NASA|
I came across this photo while I was reading about the welding operation NASA was using to build the Orion. The are going to a great deal of trouble, building one complete shell structure just to check their tools and procedures. A spacecraft is a pressure vessel, and while the pressure is not that high, the spacecraft itself is rather large. I have an air compressor in my garage that holds 100 PSI, but it's only about a foot in diameter. These craft are considerable larger, and while the pressure is lower, when multiplied by the area it gets big, as in tons of force. The shell has to be able to withstand the strain, and it probably doesn't weigh any more than my air compressor.
Robotic Friction Stir Welding Automation - Courtesy of CRIQ
To my surprise, I found they are using friction stir welding, which is kind of a bizarre technique. Watching the way aluminum will gum up a file or a grindstone might give you some idea of why it works.
Update: Jack provided this snippet from a different Wikipedia article:
Friction welding (FRW) is a solid-state welding process that generates heat through mechanical friction between work pieces in relative motion to one another, with the addition of a lateral force called "upset" to plastically displace and fuse the materials. Technically, because no melt occurs, friction welding is not actually a welding process in the traditional sense, but a forging technique