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Overview of Friction Stir Welding

Friction stir welding, or FSW, is a solid state joining technique that is used on metals where the original characteristics of the metal must remain as unchanged as possible.  The technique of friction stir welding is most commonly used on aluminum and larger pieces that cannot be heat treated after welding in order to retain the temper characteristics of the metal.  This joining technique was invented in 1991. 

The process of friction stir welding uses a cylindrical tool with a profiled threaded probe which is rotated at constant speed and is then placed into the joint between the two pieces of metal which are to be joined together.  The metals that are going to be joined must be clamped onto a backing bar so that their joint faces cannot be forced apart by the process. 

Once the process is started, frictional heat is created between the welding tool shoulder, nib and the metals being joined.  The heat created along with the heat created by the mechanical mixing process and the adiabatic heat in the material cause the metals to soften to a point just higher than their melting point.  This allows the tool to move along the weld line in a plasticized tubular shaft of metal. 

The process of friction stir welding has many advantages over other fusion welding techniques.  The problems that typically occur from the cooling of the liquid phase are not present in friction stir welding.  Problems such as, solidification cracking, porosity, solute redistribution are not a problem when friction stir welding is used.  The amount of defects typically reported from the use of friction stir welding are minimal; while those problems are eradicated, there are others that can arise from the use of friction stir welding.  Some problems with friction stir welding are insufficient weld temperatures, long tunnel like defects, poor continuity of the bond between materials from each side of the weld and the formation of a kissing bond which can be very difficult to notice without the use of an x-ray or ultrasonic testing of the weld area.

Other advantages of this type of joining process include the absence of toxic fumes or splatter.  No consumables are required to produce the weld; it is a process that can be easily automated which makes it perfect for industrial use.  It can be performed in all positions and doesn’t create a weld pool.  It is also a better welding process for the environment, because there aren’t any toxins created during the process, it is a much more environmentally friendly way of joining two pieces of material.

Some other disadvantages of the technique include leaving an exit hole in the material when the tool is removed; heavy duty clamps must be used to hold the materials in place while they are being joined, and the process creates a less flexible arc than the arc that is created during manual welding. 

As with any welding process there are always pros and cons, it is important to find out if the cons outweigh the pros for the type of job you will be performing.  The process of friction stir welding is one that is most commonly found in an industrial setting, it is not something that an everyday welder will be performing.  But if the need should arise, it is important that the welder be familiar with the type of equipment that is necessary and aware of all risks that may be associated with the process.  As with any welding process, safety should always be your first concern and the proper guidelines should always be followed to avoid injury.

Ed C.


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