Research on stir welding

by Wayne M. Thomas, David G. Staines, Ian M. Morris, Edward R. Watts
Edited by Kimberley A. Gilles, associate editor

Fig. 2. Weld surface appearance formed beneath the tool produced at a welding speed of 1.6 mm/sec using 5 revolutions/interval.

Fig. 3. Close up of a Re-stir weld formed beneath the tool shows surface ripping at the reversal interval (10 revolutions per interval, at 4 mm/sec welding speed).

Fig. 4. Metallurgical sections showing weld shape produced by Re-stir at 10 reversals/interval and a welding speed of 3.3 mm/sec; a.) macrosection shows a symmetrical dovetail-shaped weld with a similar amount of upturn of the plate interface at each side of the weld; b.) longitudinal microsection showing regular patterns caused by rotation reversal; and c.) plan macrosection taken mid-thickness showing the results of reversal motion.

Fig. 5. Outer regions of a weld made with an A-skew probe and skew motion at a travel speed of 1.6 mm/sec at 8 revolutions/reversal interval.

Cambridge, England-based TWI's research on friction-stir welding has led to development of Re-stir, a reversal-stir welding process. It 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 process was tested on and is now considered appropriate for butt, lap, and spot-welding applications because it generates symmetrical welds that eliminate problems associated with asymmetry inherent in conventional friction-stir welding.

Surface appearance
Butt Welding For butt welding tests, technicians used a 6 mm-thick 5083-O condition aluminum alloy and a MXTriflute probe. The result was an essentially symmetrically shaped weld region that narrows toward the plate's top. By comparison, conventional rotary-friction stir welding produces an asymmetrically shaped weld.

Butt and Lap Welds The appearance of the weld surface formed under the tool shoulder using 5 reversals/interval is shown in Figure 2, and a weld surface made at 4 mm/sec travel speed is shown in Figure 3. Fine surfaces ripples reveal the number of rotations and the extent of the interval, while the less-frequent, coarse and wide surface ripples present the position of rotational change. For Re-stir, the distance and time between each interval depends on the combination of rotational and travel speeds used.

Lap Welds A lap weld was made using 5083-0 condition aluminum alloy and a Flared-Triflute type probe designed for rotary stir, at a travel speed of 3.3 mm/sec and at 10 revolutions/interval. Figure 4a shows a symmetrical weld, but there is detrimental plate thinning/hooking because of non-optimization of welding parameters.

Figure 4b shows a longitudinal section taken from the edge of the weld region and the effects of changing the rotational direction. Figure 4c shows a patterned weld region surrounded by a heat-affected zone (HAZ) and that some of the third-body plasticized material close to the probe may have been "re-stirred" back in the opposite direction during the reversal stage.

Tests using an A-skew probe indicate a slight downturn in the overlapping plate/weld interface occurs at the outer regions of the weld, which is beneficial in particular structure and loading situations. Figure 5 shows this effect in an overlap weld in 5083-O condition aluminum alloy. While the Re-stir process is in further development work, early trials suggest commercial benefits in terms of symmetry — it could overcome some problems associated with the asymmetry of conventional friction-stir welds and help control the morphology of the notch tips on either side of a lap weld and in welding dissimilar materials with widely differing flow properties. The probe geometry, rotational speed, and reversal frequency need to be optimized.

TWI continues to work using purpose-designed tools with straight flutes and neutral or balanced ridges. Its studies include dual-rotation stir welding, multi-stir, simultaneous double side stir, and several Twin-stir techniques.