The continuing development of Friction Stir Welding (FSW) has led to a number of process variants. This article describes preliminary work carried out on Re-stir TM welding at TWI. The main features of the process are shown in Fig.1. The process may be applied as both angular reciprocating, where reversal is imposed within one revolution, and rotary reversal, where reversal is imposed after one or more revolutions.
The Re-stir TM welding technique provides a cyclic and essentially symmetrical process. Problems associated with the inherent asymmetry of conventional rotary FSW are avoided.
Figure 2 shows the appearance of the weld surface that is formed beneath the tool shoulder produced at a welding speed of 1.6 mm/sec (96 mm/min), using 5 revolutions per interval.
Figure 3 shows the detail of the surface of a weld made at 4 mm/sec (240 mm/min) travel speed, using 10 revolutions per interval. The fine surface ripples reveal the number of rotations and the extent of the interval, while the less frequent, coarser and wider surface ripples reveal the position of the change in rotation direction. For Re-stir TM, the distance and time between each interval depends on the combination of rotational speed and the travel speed used.
Macrosections of a lap weld made by Re-stir TM are shown in Fig.4a, b & c. This weld was made in 5083-O condition aluminium alloy, using a Flared-Triflute TM type probe designed for rotary stir, at a travel speed of 3.3 mm/sec (198 mm/min) and at 10 revolutions per interval. Figure 4a shows an essentially symmetrical weld. It should be noted that this weld shows detrimental plate thinning/hooking owing to the non-optimisation of welding parameters but does serve to illustrate the symmetrical nature of the weld produced by the Re-stir TM technique.
The longitudinal section shown in Fig.4b is taken at a position at the edge of the weld region and shows the effect of the change in the direction of rotation. The plan view in Fig 4c reveals a patterned weld region surrounded by a HAZ. There is some evidence that during the reversal stage some of the 'Third-body' plasticised material close to the probe is 're-stirred' back in the opposite direction.
Fig.4. Metallurgical sections showing the effect of the Re-stir TM technique on the weld shape
Although it is early days and much more development work is required before the technique can be used commercially, the prognosis for the success of Re-stir TM welding is good. The process has been found to be robust in trials to date although the probe geometry, the rotational speed, and the reversal frequency have yet to be optimised. Work is continuing at TWI with purpose-designed tools with straight flutes and neutral or balanced ridges. It seems likely that an essentially symmetrical technique like Re-stir TM may well become the preferred option for certain butt, lap, compound lap and spot welding and material processing applications.
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