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Friction stir welding, using zero down force, has been successfully demonstrated at TWI on aluminium plate up to 25mm. The technique uses a 'floating' bobbin shaped tool and penetrates the joint line either directly, or from a milled recess, in the material's edge.
Successful demonstrations have shown that thick section friction stir welds up to 25mm can be performed using TWI's large purpose-built FSW machine.
The technique is ideal for joining low temperature softening materials such as 6082 aluminium. TWI has also been successful with joining several two and seven thousand high strength aluminium alloys, as well as grade 5083. The process lends itself to joining closed sections where conventional use of a well supported backing bar is difficult or impossible.
The bobbin creates a full penetration weld so the potential for creating kissing bonds, usually associated with lack of tool penetration, is eliminated. The process uses a one piece tool, unlike some FSW variants which require complex multi piece tooling and it is free to move in the vertical or Z plane about a centimetre up and down.
As the tool is traversed the material passes between the shoulders of the bobbin. So the process involves zero vertical force, unlike earlier adaptations of the FSW process. No backing bar is required because the bottom shoulder supports the underside of the weld.
Laboratory trials have shown that the bobbin variant of the FSW process creates very little distortion because the rotating tool creates a symmetrical heat input throughout the weld section. Since the bobbin tool is free to move up and down, throughout the welding operation, it naturally takes up a vertical position where the top and bottom shoulder forces are equal. This feature also compensates for any misalignment between the tool and the material.
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