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'Floating' bobbin tool eliminates friction stir-welding shoulder forces

Case Study

A variant of friction stir welding has been successfully tested at TWI which could enable FSW to be performed using traditional unmodified machine tools.

It uses a bobbin shaped tool and penetrates the material to be joined from the edge.

The technique is ideal for joining low temperature softening materials such as aluminium. Already successful welds have been achieved on sheet between 2.5mm and 25mm thickness.

The process lends itself to joining closed sections where conventional use of a well supported backing bar is difficult or impossible.

Since, by definition the bobbin creates a full penetration weld, the potential for introducing 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.

As the machine bed is traversed the material passes between the shoulders of the bobbin. So the process involves zero vertical force, unlike traditional adaptations of the FSW process.  This works shows that it could be operated on a conventional toolroom milling machine.

No backing bar is required because the bottom shoulder supports the underside of the weld. Laboratory trails have shown that the bobbin variant of the FSW process creates very little distortion. It is presumed that this is 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.

To learn more about bobbin welding please contact us.