iFixit

Though the rear case is the last piece we reach, it's where the design of the new iMac began.

In order to carefully join together the pieces of the case, Apple turned to friction stir welding, a process previously reserved for applications such as ship building and aerospace.

Friction stir welding is more like joining clay than welding—it doesn't melt the workpieces, but rather softens the area between them and forces material together, creating a strong weld with no weakened heat affected zone.