Friction stir welding mixes the metals of different components to form a joint between the components. The metals do not melt in this process. Rather a friction stir welding tool plasticizes the metals and then stirs them together by its motion while in contact with the materials. This is known as a solid-state welding process because the metals involved never melt but instead retain a solid form through the whole process.
The motion of a friction stir welding pin tool is both linear and rotational. As the tool moves linearly through the materials to be joined, it generates heat due to the friction between the tool and the components. This causes the region very close to the tool to take on the physical properties of plastic. Material then moves from one side of the tool to the other as the tool rotates. As material crosses the seam between the two components, it is mixed together and a joint is formed.
A variety of metals may be joined by friction stir welding, including steel, aluminum, copper, and others. Components of the same metal may be joined with this technique, as can components made of different metals. While friction stir welding is used to form a variety of joints including butt welds, lap fillets, and others, it cannot produce the common tee fillet joint.
Welds constructed using this process have excellent strength and fatigue properties. This solid-state joining process yields a final product with less distortion and lower levels of residual stress than many other joining techniques. The consistently solid physical state of the metals in this process achieves these features in the final product.
It is often easier to use this technique in a factory setting than in other locations. This type of welding is less sensitive to environmental variation than other methods, making process control simpler. The environmental impact of this process also compares favorably to other methods, as no contaminating fumes or spatter are produced. Neither does a friction stir welding machine involve the concerns associated with arc glare and reflection of laser beams. Process speed of this method is, however, somewhat slower than other welding methods.
Friction stir welding is used across a number of industries and in a wide variety of applications. Where weight is a significant concern and aluminum and other lightweight alloys are favored construction materials, friction stir welding is an attractive option. It has been used in shipbuilding, automotive production, manufacture of rail cars, and to build aircraft and spacecraft. A notable example of its use is in building the external fuel tank of a space shuttle, which is its largest component.