Friction stir welding was invented at TWI in 1992, and has led to a
flurry of research activity to develop applications - initially in
aluminium alloys and more recently in many other metals. The process
uses a rotating tool to generate a plastic zone which is worked along
the joint line. The tool simultaneously provides frictional heating (to
lower the flow stress) and forces the metal to flow round the tool from
the leading edge of the plastic zone to the trailing edge, sealing the
joint.
Developments of the process have so far been entirely empirical - first
finding tool shapes which give sound welds in simple welds in aluminium,
then experimenting with a wider range of joint geometries and
materials. Work has then moved on to measuring joint performance and
characterising microstructure. As the number of variants increases,
experimental costs escalate. Furthermore, the physical limits of the
process (such as maximum viable welding speed in a given thickness of a
given material) are only poorly understood. Modelling is the obvious
remedy, and a number of groups are now doing just that.
This talk will describe progress in using analytical methods to model
friction stir welding, which offers a particularly good challenge,
requiring inter-disciplinary thinking: some plasticity, some heat flow
and some materials science. The results so far give some interesting
insights on the process, and provide a foundation for anyone wishing to
let FE methods loose on the problem. The work is being conducted via
the post-graduate training partnership (PTP) scheme between TWI and the
University, which offers particular opportunities for theoretical
modelling while keeping the real world in view.
