What Is Friction Stir Processing?

Friction stir processing is a metallurgy technique that joins metals through increased friction without melting them down. The friction stir processing method starts by placing a tool through a piece of metal. After this, the tool moves the metal around rapidly, increasing heat and friction until the separate pieces of metal in the machine join together. This normally does not cause any phase change, which commonly is needed to combine metals. Along with using less energy, this also improves the metal’s microhardness and its tensile and fatigue strength.

To start friction stir processing, several pieces of metal are placed in a stir processor. The main piece of metal, the one the other metals are joining, has a rod piercing it. This rod is metal, but it is not absorbed during the processing, because it is only meant to increase friction and assist in processing.

The rod then begins to work by moving the main piece of metal. Movements become more intense as time progresses, causing friction to build up between all the different pieces of metal. When enough friction is produced through friction stir processing, all the metals conjoin into one.

While there are many ways of conjoining pieces of metal, friction stir processing is different from most because there is no phase change during the conjoining process. Metal normally has to be melted down, or turned from a solid to a liquid. With the friction process, the intense friction has enough force to cause the metals to conjoin, though they all remain solid during the process.

There are many benefits to using friction stir processing. One such benefit is the conservation of energy. When metal has to be melted down, this requires a vast amount of heat, which requires a lot of energy and many specialized tools made to hold incredibly hot molten metal. This is expensive and can be very hazardous if any of the molten metal escapes and workers are exposed to it.

Another benefit to this process is that the metal itself often is improved much more than it would be through other metallurgy operations. For example, the microhardness, fatigue strength and tensile strength usually double or triple, depending on the metals being used and conjoined. The use of high heat also may soften the metal after processing, which can be a problem if extremely hard metal is needed for construction, lab tests or any other purpose.