A technique developed in 1991 to supplement the existing forms of welding. The two items (typically plate or sheet metal) to be welded are placed edge to edge, and a cylindrical tool is spun up to a working speed, typically in the thousands of rpms. It is then pressed down with a large amount of force onto the seam and dragged along the seam slowly. The friction stirs the metal without the temperature exceeding the melting point of the metal.
The tool consists of two small cylinders joined to each other, one with about the diameter of a pencil eraser and the other on the order of one inch in diameter. It's made of highly wear resistant materials which can also resist the extreme temperatures of the process. Relative to the tool path, it appears to lean backwards, so that the leading edge of the tool doesn't touch the metals to be welded, but the trailing edge does. While spinning, the business end of the tool--the end with the small cylinder--is plunged into the seam, burying the small cylinder and pressing the face of the larger cylinder against the seam.
Because the metal never exceeds its melting temperature, you don't find the grain boundary flaws you get in ordinary fusion and resistance welds. Because no second material is used, you don't get any cathodic weld decay. The normally unpredictable and non-uniform strebgth profile of fusion welds is replaced by a weak area whose location is eminently predictable. In most welds, there is also an expected percentage of flaws per length of weld. A representative of the Boeing Co. told me that, for one application, the norm is a flaw in every two feet of weld. For friction stir welding, there have been instances where the machine was able to make perfect welds measurable in miles; even then, the limit to the length of a perfect weld is limited only by the quality (and therefore, cost) of the tool used.
The bottom line
This process has applications in the aerospace sector, where long, high-quality welds on thin pieces of difficult-to-weld metals are often desired. Examples include aluminum aircraft and rocket fuselages, or titanium turbine blades in a jet engine, where a weld eliminates the need to use rivets. In many other applications, welding aluminum or titanium is prohibitively expensive and flawed. Friction stir welding is changing all of that.