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.
However
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.
Since,
by definition, the bobbin creates a full penetration weld, 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. 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
