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The SME Summit 2005 Oconomowoc, Milwaukee, USA

August 3-4th 2005

Friction Stir Welding – Process Developments and Variant Techniques

By W M Thomas, I M Norris, D G Staines, and E R Watts wayne.thomas@twi.co.uk

ian.norris@twi.co.uk Staines@twi.co.uk

Ed.watts@twi.co.uk

TWI Ltd Granta Park,

Great Abington, Cambridge,

CB1 6AL United Kingdom

Abstract

Friction stir welding (FSW) is now extensively used in aluminium industries for joining and material processing applications. The (FSW) technology has gained increasing interest and importance since its invention at TWI almost 14 years ago. The basic principle and the continuing development of the FSW technology are described and recent applications are reviewed. The paper will introduce some of the variants of FSW, such as Twin-stir™ Skew-stir™, Re-stir™, Dual-rotation stir and the Pro-stir™ near-net shape processing technique. Particular attention will also be paid to tool probe/shoulder features, in relation to the joint geometry being welded. In addition, this paper makes special reference to the mechanical and structural integrity that can be expected from FSW technology. Keywords: Friction Stir Welding and Processing, Fatigue and variant techniques. Introduction The basic principle of conventional rotary friction stir welding (FSW) is shown in Fig.1.

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Fig.1 Basic principle of conventional rotary friction stir welding The systematic development of Friction stir welding (FSW) has led to a number of variants of the technology. The following describes preliminary studies being carried out on Twin-stir™, Skew-stir™, Re-stir™, Dual-rotation stir, and Pro-stir™ a three-dimensional material processing technique (1-4). Currently, FSW is used particularly for joining aluminium alloys in shipbuilding and marine industries, aerospace, automotive and the rail industry. Furthermore, the technology provides significant advantage to the aluminium extrusion industry. Automotive suppliers are already using the technique for wheel rims and suspension arms. Fuel tanks joined by FSW have already been launched in spacecraft, and many other space advances are under development; commercial jets welded by FSW have successfully completed flying trials, with high volume commercial production forthcoming. Aluminium panels for high speed ferries and panels for rail vehicles are also produced. Moreover, the friction stir welding of 50 mm thick copper material has provided a potential solution for nuclear encapsulation of radioactive waste. Friction stir welding is making an impact as a material processing technique and the prognosis for the successful welding of steel products by FSW looks promising. Twin-stir™ technique The simultaneous use of two or more friction stir welding tools acting on a common workpiece was first described in 1991 (5). The concept involved a pair of tools applied on opposite sides of the workpiece slightly displaced in the direction of travel. The contra-rotating simultaneous double-sided operation with combined weld passes has certain advantages such as a reduction in reactive torque and a more symmetrical weld and heat input through the thickness (6). In addition, for certain applications, the use of purpose designed multi-headed friction stir welding machines can increase productivity, reduce side force asymmetry and reduce or minimise reactive torque (7). The use of a preceding friction pre-heating tool followed in line by a friction stir welding tool for welding steel is reported in the literature 1999 (8). More recently a similar arrangement has been reported with two rotating tools one used to pre-heat and one used to weld (9). This disclosure (9), however, shows a ‘tandem’ technique with the tools rotating in the same direction. A further reference is made to tandem arrangements with tools rotating in the same direction (10). The use of ‘tandem’ contra-rotating tools in-line with the welding direction and ‘parallel’ (Side-by-side across the welding direction) is also disclosed (11). Figure 2 shows the three versions of Twin-stir™ welding techniques that are being investigated and developed at TWI.

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Fig. 2 Twin-stir™ variants a) Parallel side-by-side transverse to the welding direction b) Tandem in-line with the welding direction c) Staggered to ensure the edges of the weld regions partially overlap. Parallel twin-stir™ The Twin-stir™ parallel contra-rotating variant (Fig. 2a) enables defects associated with lap welding to be positioned on the ‘inside’ between the two welds. For low dynamic volume to static volume ratio probes using conventional rotary motion, the most significant defect will be ‘plate thinning’ on the retreating side. With tool designs and motions designed to minimise plate thinning, hooks may be the most significant defect type. The Twin-stir™ method may allow a reduction in welding time for parallel overlap welding. Owing to the additional heat available, increased travel speed or lower rotation process parameters will be possible. Tandem twin-stir™ The Twin-stir™ tandem contra-rotating variant (Fig. 2b) can be applied to all conventional FSW joints and will reduce reactive torque. More importantly, the tandem technique will help improve the weld integrity by disruption and fragmentation of any residual oxide layer remaining within the first weld region by the following tool. Welds have already been produced by conventional rotary FSW, whereby a second weld is made over a previous weld in the reverse direction with no mechanical property loss. The preliminary evidence suggests that further break-up and dispersal of oxides is achieved within the weld region. The Twin-stir™ tandem variant will provide a similar effect during the welding operation. Furthermore, because the tool orientation means that one tool follows the other, the second tool travels through already softened material. This means that the second tool need not be as robust.

a)

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Staggered twin-stir™ The staggered arrangement for Twin-stir™ (Fig 2c) means that an exceptionally wide ‘common weld region’ can be created. Essentially, the tools are positioned with one in front and slightly to the side of the other so that the second probe partially overlaps the previous weld region. This arrangement will be especially useful for lap welds, as the wide weld region produced will provide greater strength than a single pass weld, given that the geometry details at the extremes of the weld region are similar. Residual oxides within the overlapping region of the two welds will be further fragmented, broken up and dispersed. One particularly important advantage 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 in the first weld. This will be achieved by locating the retreating side of both welds on the ‘inside’ (see Fig. 3).

For material processing, the increased amount of material processed will also prove advantageous. In addition, for welding it would enable much wider gaps and poor fit up to be tolerated.

Fig. 3 Arrangement of Staggered twin-stir™ contra-rotating tools with respect to rotation and direction a) Advancing sides of the ‘common weld region’ are positioned outwards with left-hand

tool leading. b) Retreating sides of the ‘common weld region’ are positioned outwards with left-hand

tool leading c) Retreating sides of the ‘common weld region’ are positioned outwards with right-hand

tool leading d) Advancing sides of the ‘common weld region’ are positioned outwards with right-hand

tool leading Welding trials A Series of preliminary welding trials has been carried out using an experimental Twin-stir™ head at TWI in order to investigate the characteristics of welds made in a variety of

a)

b)

c)

d)

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configurations. The welding trials were carried out with the prototype Twin-stir™ head as shown in Fig. 4.

Fig. 4 Twin-stir™ prototype head assembly The welding trial demonstrated the feasibility of Twin-stir™ and showed that welds of good appearance were produced as shown in Fig. 5.

Fig. 5 Surface appearance of a typical Tandem twin-stir™ weld made in 6083-T6 aluminium alloy The two exit holes produced in a tandem weld showed that a similar footprint was achieved for both the lead and following tool (see Fig. 6).

Fig. 6 Tandem twin-stir™ lead and follow exit holes Metallographic observations revealed a marked refinement of grain size in the weld region and comminution of oxide remnants and particles. This is consistent with the microstructural features previously observed in conventional rotary stir welds in aluminium

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alloys. In lap welds, an upturn on both sides of the weld region is also shown (Fig. 7). All sections were prepared in the direction looking towards the start of the weld.

Advancing side follower tool Retreating side follower tool

Fig. 7 Macrosection of Tandem twin-stir™ lap weld in 6 mm thick 6082-T6 aluminium alloy Metallographic examination of Staggered twin-stir™ lap welds revealed a ‘common weld region’ that measures 430% of the sheet thickness as shown in Fig 8.

Advancing side of follower tool Advancing side of leading tool

Fig. 8 Macrosection taken from the ‘common weld region’ of a Staggered twin-stir™ lap weld in 3mm thick 5083 –H111 aluminium sheets. The tool arrangement used to produce this Staggered twin-stir™ weld is that illustrated in Fig. 3a; whereby the advancing sides of the ‘common weld region’ are positioned outwards. Consequently, both retreating sides face inwards with the lead weld retreating side receiving further friction stirring treatment from the retreating side of the follower tool.

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Skew-Stir™ The skew-stir™ variant of FSW differs from the conventional method in that the axis of the tool is given a slight inclination (skew) to that of the machine spindle (3), as shown in Fig. 9a, b and c. The skew-stir™ technique enables the ratio between the ‘dynamic’ (swept) volume and the static volume to be increased by the skew motion of the tool. This can be additional to that provided by the use of re-entrant features machined into the probe. It is this ratio that is a significant factor in enabling a reduction or elimination of void formation and improving process efficiency. The arrangement shown in Fig.9a, results in the shoulder face being oblique to the axis of the skew tool and square to the axis of the machine spindle. This shoulder face remains in a fixed relationship with respect to the plate top surface. Tilting the plate or the machine spindle will produce a plate to tool tilt that can be varied to suit conditions. The focal point of a skewed tool affects the amplitude of the orbit of the tool shoulder and probe. With the focal point at the shoulder position, i.e. at the top of the workpiece, the shoulder essentially has a rotary motion with no off-axis orbit. When the focal point is positioned slightly above the top surface of the work piece, or at any position through the thickness of the workpiece, the shoulder contact face has an off-axis orbital movement. In addition, the off-axis orbital motion of the shoulder is dependent on the angle of skew and the distance that the intersection (focal point) is away from the top of the plate. The greater the skew angle and the greater the distance that the focal point is away from the workpiece surface, the greater is the amplitude of the shoulder off-axis movement. The skew action results in only the outer surface of the probe making contact with the extremities of the weld region. The FSW tool does not rotate on its own axis, and therefore only a specific part of the face of the probe surface is directly involved in working the substrate material. Consequently, the inner part of the tool can be cut away to improve the flow path of material during welding, (see Fig.9a). This probe type is termed A-Skew™.

Fig. 9 Details of Prototype A-Skew™ Probe. a) Side view b) Front view, showing tip profile c) Swept region encompassed by skew action The skew-stir™ technique provides an easier material flow path than conventional FSW and a weld nugget region of width greater than the diameter of the probe. In addition the skew

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action provides an orbital forging action at the root of the weld, which improves weld quality in this region. Work has been undertaken to establish the fatigue performance of welds made using the skew-stir technique and a fatigue-tested sample is shown in Fig. 10.

Retreating side Advancing side

a) b)

c) d) Fig. 10 Lap weld made with the retreating side near the top sheet edge (RNE configuration) using Skew-stir™ with an A-skew™ probe (8.25mm in length) in 6mm thick 5083-H111 aluminium alloy at a welding speed of 3mm/sec (180mm/min). a) Macrosection b) Detail of fracture, bottom sheet retreating side c) Detail of fracture top sheet advancing side d) Detail of the form of the notch at the edge of the weld – advancing side Typically these Skew-stir™ lap welds gave good fatigue performance when compared with an artificial weld made from parent material of similar geometry as shown in Fig. 11.

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Fig. 11 Fatigue results of welds carried out with different lap configurations with a 8.25mm long probe Reversal Stir Welding - Re-stir™ The following describes preliminary studies being carried out on Re-stir™ welding at TWI. The salient features of the Re-stir™ welding technique are illustrated in Fig.12. This illustration applies to both angular reciprocating, where reversal is imposed within one revolution, and rotary reversal, where reversal is imposed after one or more revolutions.

Fig. 12 The basic principle of Re-stir™, showing the reversal technique. The use of the Re-stir™ welding technique provides a cyclic and essentially symmetrical welding and processing treatment. Most problems associated with the inherent asymmetry of conventional rotary FSW are avoided. Figure 13 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

Lap weld configuration (A-Skew TM , 584revs, 3mm/sec, 8.25mm probe)

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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™, the distance and time between each interval depends on the combination of rotational speed and the travel speed used.

Fig. 13 Close up of Re-stir™ weld surface formed beneath the tool shoulder showing surface rippling and reversal interval. Produced at 4 mm/sec (240 mm/min) welding travel speed, using 10 revolutions per interval. Macrosections of a lap weld made by Re-stir™ are shown in Fig. 14 a, b & c. This weld was made in 5083-H111 condition aluminium alloy, using a Flared-Triflute™ type probe designed for rotary stir, at a travel speed of 3.3 mm/sec (198 mm/min) using 10 revolutions per interval. 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.

a) b) Fig. 14 Metallurgical sections showing the effect of the Re-stir™ technique on the weld shape, produced at a welding speed of 3.3 mm/sec (198 mm/min), using 10 revolutions per interval. a) Longitudinal macrosection showing regular patterns caused by rotation reversal. b) Plan macrosection taken mid thickness showing the effect of reversal motion. The Re-stir™ process requires further optimisation to achieve welds of reproducibly high quality and freedom from defects but early trials suggest benefits in terms of weld symmetry. Initial work using an A-skew™ probe also suggests that it may be possible to

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achieve a slight down turn in the overlapping plate/weld interface at the outer regions of the weld which may be beneficial in particular structures and loading situations. Figure 15 a, b & c, illustrates this effect in an overlap weld in 5083-H111 condition aluminium alloy.

a) b)

c)

Fig. 15 Detail of the outer regions of a Re-stir weld made with an A-skew™ probe in combination with a skew motion, at a travel speed of 1.6 mm/sec (96 mm/min), using 8 revolutions per reversal interval. a) Macrosection b) Detail of notch (that would formerly have been at the retreating side with conventional

rotary FSW) c) Detail of notch (that would formerly have been at the advancing side with conventional

rotary FSW) Typically these Re-stir™ lap welds gave very good fatigue performance when compared with an artificial weld made from parent material of similar geometry as shown in Fig. 16.

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Re-StirTM with skew action (900 rev, 8 revs per reversal interval, 1.7mm/sec, 7mm long probe)

1.0

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ange

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R69 Re-Stir

R70 Re-Stir

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Artificial lap weld mean curve

R=0.5

Fig. 16 Fatigue results of welds carried out with reversal motion Skew-stir™

Dual-rotation friction stir welding A dual-rotation FSW variant is being investigated at TWI, whereby, the probe and shoulder rotate separately. The dual-rotation FSW variant provides for a differential in speed and/or direction between the independently rotating probe and the rotating surrounding shoulder as shown in figure 17.

Fig. 17 Principle of dual-rotation friction stir welding with rotation of the probe and shoulder in the same direction. The apparatus used for this investigation is shown sequentially in Figures 18 a & b.

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a) b) Fig. 18 Dual-rotation apparatus complete with white marks on the shoulder and probe to indicate relative rotational movement. a) White marks on shoulder and probe almost in line. b) White marks on shoulder and probe moving apart The apparatus can enable a range of different rotational speeds to be pre-selected or varied automatically by in-process control to achieve the desired welding conditions. In conventional rotary FSW, the relative velocity of the tool increases from zero at the probe centre to maximum velocity at the outer diameter of the shoulder. The dual-rotation technique can significantly modify the velocity gradient between the probe centre and the shoulder diameter. This technique provides a differential in rotation speed and the option for rotation in opposite directions. This dual-rotation technique effectively allows for a high probe rotational speed without a corresponding increase in shoulder peripheral velocity. This technique can provide for a more optimised rotational speed for both probe and shoulder. Dependent on the material and process conditions used, over-heating or incipient melting along the 'near shoulder side' of the weld surface of certain friction stir welds can occur. Melting can lead to fusion related defects along the 'near shoulder side' weld surface. The dual-rotation technique can be used to reduce the shoulder rotational speed as appropriate and, therefore, help reduce any tendency towards over-heating or melting, while maintaining a higher rotational speed for the probe. A double sided butt weld using non-optimised conditions was made to demonstrate that dual-rotation stir welding is practicable for certain applications. Figure 19 shows the macrostructural features produced by dual-rotation stir welding using a Flared-Triflute™ type probe.

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Advancing side Retreating side

Advancing side Retreating side Fig. 19 Macrosection of a dual-rotation stir double-sided butt weld in 16 mm thick 5083-H111 aluminium alloy, produced at a welding speed of 3 mm/sec (180mm/min), using 584 rev/min for the probe and 219 rev/min for the shoulder. The two weld passes were made in opposite directions, with the first pass shown on top. Guided bend testing demonstrated freedom from gross defects as shown in Fig. 20.

Fig. 20 Guided side bend test, carried out on a double-sided butt weld, in 16 mm thick 5083-H111 aluminium alloy, achieved 180°. Figure 21, shows the appearance of the weld surface that is formed beneath the tool shoulder after dual-rotation stir welding.

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Fig. 21 Surface appearance of dual-rotation stir weld made in 16 mm thick 5083-H111 aluminium alloy at a welding speed of 3 mm/sec (180 mm/min), using 584 rev/min for the probe and 219 rev/min for the shoulder. Owing to the relatively low temperature reached, with solid-phase welding techniques such as FSW, the problems of solidification and liquation cracking when fusion welding certain materials, can be significantly reduced. However, the thermal cycle produced in FSW is sufficient to modify the original alloy temper in certain heat-treatable materials (e.g. 2xxx and 7xxx series aluminium alloys) producing a reduction in both the mechanical and corrosion properties across the weld (12&13). One advantage of dual-rotation FSW is that it reduces the peak temperature reached during the weld thermal cycle. Figure 22, shows a comparison of thermal profiles produced by conventional rotary and dual-rotation friction stir welds made in AA7050-T7451 using similar probes and process conditions. For a given travel speed, 5.25 mm/sec (315 mm/min), a difference of approximately 66 °C in the maximum temperature of the HAZ region close to the probe (5 mm from the weld centre line) is shown.

Fig. 22 Thermal profiles of conventional rotary friction stir welds and dual-rotation friction stir welds made in 6.35 mm AA7050-T7451, using the same probe geometry and a travel speed of 5.25 mm/secs (315 mm/min). The probe rotation speed was 394 rev/min and 388 rev/min for conventional rotary and dual-rotation stir welding techniques respectively.

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The lower temperatures reached in the dual rotary weld reduce the change in mechanical properties produced during friction stir welding. After two months natural ageing (figures 23 & 24), the dual-rotation friction stir weld shows higher hardness values in the stirred zone, TMAZ and HAZ compared to the conventional friction stir weld. This indicates that the lower temperatures produced by the dual-rotation technique reduced thermal softening resulting in an increase in weld hardness.

Fig. 23 Hardness traverses as a function of depth through the cross section of a conventional friction stir weld made in 6.35 mm AA7050-T7451, using a travel speed of 5.25 mm/sec (315 mm/min) and a probe rotation speed of 394 rev/min

Fig. 24 Hardness traverses as a function of depth through the cross section of a dual-rotary friction stir weld made in 6.35 mm AA7050-T7451, using the same probe geometry used in

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the conventional friction stir weld (see fig. 23), a travel speed of 5.25 mm/sec (315 mm/min), and a probe rotation speed of 388 rev/min and a shoulder rotational speed of 145 rev/min. The heat affected zone (HAZ) of conventional friction stir welds in both 2xxx and 7xxx series aluminium alloys has been shown to be the region most susceptible to localised corrosive attack (14). Figure 25, shows a comparison of the extent of corrosion in specimens from conventional and dual-rotation friction stir welds that were exposed to the same test. These tests were carried out after two months natural ageing the ‘near shoulder side’ of the weld surface was removed and the surface prepared to a ¼ micron finish before being immersed in a 0.1M NaCi aerated solution at ambient temperature for 7 days. Both welds were made in 6.35mm AA7050-T7451 using the same probe geometry and a travel speed of 9.2mm/secs (522 mm/min). Advancing side Retreating side Fig.25 Photomacrograph of the top surface of (a) conventional friction stir weld and (b) Dual rotation friction stir weld. The probe rotation speed was 394 rev/min and 388 rev/min for conventional rotary and Dual-rotation stir welding techniques respectively. A shoulder rotational speed of 145 rev/min was used for dual-rotation. In the conventional friction stir weld the high temperature HAZ is shiny due to severe localised attack that has occurred in this region, therefore cathodically protecting the surrounding areas in the HAZ. In the dual-rotation friction stir weld there is no shiny region evident in the HAZ suggesting the degree of localised attack occurring in this region to be lower than in conventional friction stir welding. Additive FSW technology - Pro-stir™ A novel near-net shape prototyping technique is under development at TWI. Rapid prototyping is the most widespread name given to a host of related additive technologies that are used to fabricate physical objects directly from sheet, or powder material. These methods are unique in that they add and bond materials in layers to form objects. Near net shape additive technologies offer advantages in many applications compared to classical subtractive fabrication methods such as milling or turning:

HAZ

(a)

Weld TMAZ HAZ HAZ

(b)

Weld TMAZHAZ HAZ

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• Near-net shape manufacturing systems reduce the construction of complex objects to

a manageable, clear-cut, and relatively fast process. • Objects can be fashioned that have geometric complexity or sophistication without

the need for elaborate machine set up. • Environmental benefits include reduced machined waste, energy, and waste

disposal. Many welding techniques have been adapted and developed for rapid prototyping and near-net shape manufacture. Figure 26, shows the notion of using the Pro-stir™ method with the Twin-stir technique to manufacture near-net shape components.

Fig. 26 Principle of Near-net shape manufacture by Pro-stir™ Figure 27, shows a small test trial using 6 mm thick 5083 aluminium alloy. The same principle would apply for much thicker plate material.

Fig. 27 Near-net shape test sample, 6x6 mm thick sheets welded on top of each other

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The advantages of FSW near net shape, prototype-processing techniques can be summarised as follows: • High deposition rate is possible when used with comparatively thick plate. • Able to use comparatively thin sheet as well as thick plate. • Low distortion • Three-dimensional processing technique • Strategic regions of the component can be tailored with material to provide different

properties. • The product comprises processed, hot forged material. • It is solid-phase technique not subject to gravity (most rapid prototyping systems are

gravity restricted) This means that it is potentially possible to ‘grow’ additional parts in situ on large, complex structures if required

Discussion and Concluding Remarks The basic principles and the continuing development of the FSW technology such as Twin-stir, Skew-stir, Re-stir, Dual-rotation stir and the Pro-stir near-net shape processing techniques, have been described in the paper and the following concluding remarks are made: It is to be expected that the tandem and staggered Twin-stir™ variants will further fragment and disperse tenacious residual oxides within the weld region or part of the weld region respectively. This will lead to improved weld integrity and performance. Moreover, the staggered Twin-stir™ method is likely to provide advantage and in some instances be preferred for safety critical applications for both butt and lap joints. All contra-rotating systems help to reduce the reactive torque necessary to secure plates to the machine during welding. The use of twin-stir™ techniques is expected to prove advantageous for material processing, lap welding, spot welding and it would enable much wider gaps on butt welds to be tolerated. Rotary motion Skew-stir™ lap welds and reversal motion Skew-stir™ lap welds provided good fatigue performance when compared with artificial lap welds made from parent material. The initial investigation of dual rotation stir welding has demonstrated the feasibility of the technique for butt welding 5083-H111 and 7050-T7451 aluminium alloys. The dual-rotation technique is capable of minimising any tendency towards over-heating or incipient melting associated with the 'shoulder near side' weld surface. The results confirm that the dual-rotation technique can significantly modify the velocity gradient between the probe centre and the shoulder diameter. These trials confirm that use of slower shoulder rotational speed lowers the HAZ temperature during the welding operation. This effectively reduces thermal softening in the HAZ region. The results shows that dual-rotation technique reduces the susceptibility to corrosion in 7xxx series aluminium alloys HAZs. Work will continue at TWI to investigate the use of dual-rotation on spot, butt, and lap welds. Preliminary trials using a FSW method of near-net shape manufacture and three-dimensional material processing show promise, but much more work will be required to develop and perfect the technique.

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Development work will continue at TWI to ensure these techniques can be used commercially. Acknowledgements The Authors wish to thank C S Wiesner, P J Oakley, P Evans, M J Russell, A Duncan, D Saul and N L Horrex for their support and contributions. References 1. Thomas W M, Wiesner C S, Staines D G & Watts E R: ‘Friction stir welding – Process

developments for aluminium applications’. International Conference & Exhibition, ‘Aluminium in Transport’, Moscow, Russia, April 24-29, 2005.

2. Catin G M D, David S A, Thomas W M, Lara-Curzio E, and Babu S S: ‘Friction Skew-stir welding of lap joints in 5083-0 aluminium’ Science and Technology of Welding and Joining, Vol. 10, No. 1, 2005

3. Thomas W M, Braithwaite A B M and R John: ‘Skew-Stir™ technology’. TWI 3rd International Symposium on Friction Stir Welding, Kobe, Japan, 27-28 September 2001.

4. Thomas W M, Norris I M, Smith I J, and Staines D G: ‘Reversal sir welding – Re-Stir - Feasibility Study’. Fourth International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 may 2003.

5. Thomas W M, Nicholas E D, Needham J C, Murch M G, Temple-Smith P and Dawes C J: 'Improvements relating to friction welding'. European Patent Specification 0 615 480 B1.

6. Thomas W M, ‘Friction Stir Welding and Related Friction process Characteristics’, Inalco 98 7th International Conference, joints in Aluminium, Cambridge, UK. April 1998.

7. Thomas W M, Nicholas E D, Watts E R, and Staines D G: ‘Friction Based Welding Technology for Aluminium’, The 8th International Conference on Aluminium Alloys 2nd to 5th July 2002, Cambridge, UK

8. Thomas W M, ‘Friction Stir welding of Ferrous materials; A Feasibility Study’, 1st Symposium on Friction stir Welding, 14-16 June 1999, Rockwell Science Center, Thousand Oaks, California, USA.

9. H Mitsuo; ‘Friction agitation joining method and Frictional Agitation joining device’ Patent Abstracts of Japan, Publication number 2000-094156, Date of publication of application 04.04.2000.

10. K Atsuo, O Yoshinori, and Y Mutsumi: ‘Friction Stir welding Method’ Patents Abstracts of Japan, Publication number 2003-112272, Date of publication of application 15.04.2003.

11. K Atsuo, O Yoshinori, and Y Mutsumi: ‘Friction stir welding method’ Patents abstracts of Japan, Publication number 2003-112271, Date of publication of application 15.04.2003.

12. Mahoney M W, Rhodes C G, Flintoff J G, Spurling R A and Bingel W H: ‘Properties of friction stir welded 7075-T651 aluminium’. Metallurgical and Materials Transactions, Vol. 29A, pp1955-1964, 1998.

13. Biallas G, Braun R, Dalle Donne C, Staniek G and Kaysser W A: ‘Mechanical properties and corrosion behaviour of friction stir welded 2024-T3’. 1st International Friction Stir Welding Symposium, Thousand Oaks, California, USA, 1999.

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14. Hannour F, Davenport A J and Strangwood M: ‘Corrosion of friction stir welds in high strength aluminium alloys’. 2nd International Symposium on Friction Stir Welding, Gothenberg, Sweden, 2000.