To create the new iMac, Apple's thinnest desktop yet, the designers used friction-stir welding to join the aluminum body. Unlike arc welding, the more standard way to fuse metal plates, a friction stir just needs a good rubbing—and a few thousand pounds of pressure—to stick together.
When you use an arc welder, you have to heat two metal plates on either side of a seam to the point of melting in order to join them. The process works fine for thin-gauge sheets, but it's slow on thicker plates—it requires as many as a dozen passes to fuse two pieces of inch-thick aluminum. Plus, changes the molecular properties of the metal itself. Friction-stir welding, on the other hand, was developed specifically to maintain thick metal's atomic integrity as much as possible—by heating it as little as possible.
Friction-stir welding machines utilize a head unit, outfitted with a small nib and spinning at 275 rpm to generate heat along the butt joint of two pieces of clamped metal. This softens aluminum to the texture of modeling clay. As the head passes, the two plates are squeezed together by the machine with upwards of 15,000 pounds of force, smooshing and fusing them together in what's known as dynamic recrystallization.
With so much lateral pressure on the plates, friction-stir welding also has to create huge amounts of downward pressure, known as Z-force, to help hold the plates in place. This used to require heavy, expensive add-on equipment that limited the length of weld. This made friction-seam welding cost-ineffective for anything smaller than the biggest budget government projects, like the Ares I-X test rocket. However, the recent addition to the rotating head—a more mobile tool called a bobbin—has effectively eliminated the shortcomings of all that extra Z-force equipment.
Already cleaner, faster, and greener than conventional arc welding—a single tool steel-quality nib nib can join a kilometer of aluminum without toxic fumes, splattery molten metal, or need for further machining—friction-stir welding is easily adaptable to current CNC mills. [Popular Mechanics - Wikipedia 1, 2 - NASA]