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What is friction stir welding of aluminum?

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Q - I have heard of a welding process called Friction Stir Welding. Apparently, it is quite new and it is a particularly good process for welding aluminum.  Can you give me some information on this process and the types of applications for which it is used?

A -  Invented in 1991, the friction stir welding (FSW) process was developed at, and is patented by, The Welding Institute (TWI) in Cambridge, United Kingdom.  The first purpose built and commercially available friction stir welding machines were produced by ESAB Welding and Cutting Products at their equipment manufacturing plant in Laxa, Sweden.  The development of this process was a significant change from  the conventional rotary motion and linear reciprocating friction welding processes. It provided a great deal of flexibility within the friction welding process group. 

The conventional rotary friction welding process requires at least one of the parts being joined to be rotated and has the practical limitation of joining regular shaped components, preferably circular in cross-section and limited in their length.  Short tubes or round bars of the same diameter are a good example. 

The linear reciprocating process also requires movement of the parts being joined. This process uses a straight-line back and forth motion between the two parts to generate the friction.  Regularity of the parts being joined is not as necessary with this process; however, movement of the part during welding is essential. 

The obvious limitation of both these processes is the joint design and component geometry restriction. At least one of the parts being joined must have an axis of symmetry and be capable of being rotated or moved about that axis.

Friction stir welding (FSW) is capable of fabricating either butt or lap joints, in a wide range of materials thickness and lengths.  During FSW, heat is generated by rubbing a non-consumable tool on the substrate intended for joining and by the deformation produced by passing a tool through the material being joined.  The rotating tool creates volumetric heating, so as the tool is progressed, a continuous joint is created.  FSW, like other types of friction welds, is largely solid state in nature.  As a result, friction stir welds are not susceptible to solidification related defects that may hinder other fusion welding processes.  The FSW process is diagrammed in fig 1.  The parts intended for joining are usually arranged in a butt configuration.  The rotating tool is then brought into contact with the work pieces.  The tool has two basic components: the probe, which protrudes from the lower surface of the tool, and the shoulder, which is relatively large diameter.  The length of the probe is typically designed to match closely the thickness of the work pieces.  Welding is initiated by first plunging the rotating probe into the work pieces until the shoulder is in close contact with the component top surface.  Friction heat is generated as the rotating shoulder rubs on the top surface under an applied force.  Once sufficient heat is generated and conducted into the work piece, the rotating tool is propelled forward.  Material is softened by the heating action of the shoulder, and transported by the probe across the bondline, facilitating the joint. 

One limitation of the FSW process is mechanical stability of the tool at operating temperature.  During FSW, the tool is responsible for not only heating the substrate material to forging temperatures, but also providing the mechanical action of forging.  Therefore, tool material must be capable of sustaining high forging loads and temperatures in contact with the deforming substrate material without either excessive wear or deformation.  As a result, the bulk of the FSW applications have involved low forging temperature materials.  Of these, the most important class of materials has been aluminum.  A range of virtually all classes of aluminum alloys have been successfully friction stir welded.  These include the 1xxx, 2xx, 3xxx, 4xxx, 5xxx, 6xxx and 7xxx alloys, as well as the newer Al-Li alloys.  Each alloy system is metallurgically distinct.  Furthermore, different alloys within the given class may have different forging characteristics.  As a result, processing for each alloy may vary.  However, high-integrity joints can be obtained in all classes.

Some Applications:

Because of the potential of advantages over arc welding in some applications associated with this processes, FSW has received interest from many areas of industry working with aluminum. The advantages include the ability to produce long lengths of welds in aluminum without any melting of the base material.  This provides important metallurgical advantages when compared to conventional arc welding. Melting of the base material does not occur with FSW and this eliminates the possibility of solidification cracking which is often a problem when arc welding some aluminum alloys.  Other advantages may include: low distortion associated with lower heating during the welding process; elimination of porosity problems that are challenging when arc welding aluminum; minimum edge preparation, as butt joints are typically performed with a square-butt preparation; and, the absence of welding consumables such as shielding gas or filler material.

The friction stir welding process is being used and/or evaluated for use within the aerospace, military vehicle, aircraft, automotive, shipbuilding, railway rolling stock industries and most likely others.

Friction Stir Welding Chart