Connect, no.142, May/June 2006, p.7

Better joints using two contra-rotating FSW tools ... and all with less clamping?

Twin-stir prototype head assembly

A new variant of friction stir welding has been developed which could make clamping difficulties a thing of the past.

Securing parts to be welded, say long sheets of aluminium, can be both costly and time consuming. Invariably it involves bolting or hydraulic clamping and, since the lateral forces inherent in the process can often be high the jigging has to be of a robust nature.

Twin-stir ^TM friction stir welding, as it is known, uses two contra-rotating tools. Their respective reactive process torques practically counter each other, so the parts to be welded require relatively low securing forces.

To date there are three variants of Twin-stir.

As the name suggests the tandem version involves two tools, sharing the same traversing line and rotating in opposite directions. It can be applied to all FSW joints. The tandem technique improves weld integrity by disrupting and fragmenting any residual oxide layer remaining within the first weld region by the following tool.

Already welds have been produced by conventional rotary FSW which demonstrate that a second weld made over the first run, but in the reverse direction, creates no loss of mechanical properties but produces further break-up and dispersal of oxides.

Tandem Twin-stir can do this in one welding operation and, since the second tool travels through already softened material, it does not have to be as robust as the leading tool.

Parallel Twin-stir, the second variant, is ideal for lap joints and uses two contra rotating tools running in parallel along the weld line. This approach enables defects associated with lap welds to be positioned on the 'inside' between the two welds.

The third version, staggered Twin-stir, allows an exceptionally wide common weld region to be created. The tools are positioned one in front, and slightly to the side of the other. The second probe partially overlaps the previous weld region.

The wide weld region created provides greater strength than a single pass weld. Residual oxides within the overlapping region of the two welds will be further fragmented and dispersed. A particularly important aspect of the staggered variant is that the second tool can be set to overlap the previous weld region and eliminate any plate thinning that may have occurred during the first weld.

Metallographic observations have revealed a marked refinement of grain size in the weld region and break up and dispersion of the oxide remnants and particles.

It's early days for the Twin-stir process. So far TWI is able to conclude that the simultaneous use of two or more friction stir welding tools rotating in opposite directions adds a new degree of freedom to the process which may open up new application areas. There is for instance considerably more thermal energy available which may prove to be an advantage for FSW of steel.

However the advantages of tandem, parallel and staggered Twin-stir techniques look likely to far outweigh the disadvantages. All contra-rotating systems help to reduce the reactive torque necessary to secure the substrate material. Use of the staggered Twin-stir technique is expected to prove advantageous for materials processing and lap welding and potentially allows much greater gaps to be tolerated in butt welds.

More details from Wayne Thomas or David Staines.
E-mail: wayne.Thomas@twi.co.uk