Crocodile Dundee once said: "That's not an Ares I rocket welder. This is an Ares I rocket welder." He was obviously referring to this toy in NASA's garage, which fuses aluminum-lithium 2195 alloy via friction.
Wired's Bob Parks, toolman extraordinaire, describes the process far better than I ever could:
Stir welding plunges a rapidly rotating pin about the size of a pencil into the joint between two panels with more than 5,000 pounds of force per square inch; the friction makes the alloy pliable, and the rotation forces grains of metal to mingle behind the pin as it crawls up the joint. The resulting welds are strong, defect-free, and actually shave material (and weight) from the craft rather than leaving a bulky seam.
The process is necessary because the ultralight aluminum-lithium 2195 alloy becomes too brittle when melted by a traditional blowtorch. [Wired, photo by Brent Humphreys]