The friction stir revolution - Part two

TWI Bulletin, September - October 2006

Part 1

New process variants, new joint configurations, new materials...

Wayne Thomas is a Principal Research Engineer in the Innovation Unit. He joined TWI in 1983. He gained his MPhil from Brunel University (Materials Technology) and is also qualified as Eur Ing, CEng and FWeldI. He is the author of many technical papers and has been responsible for the conception and/or development of a number of emergent technologies.

Peter Oakley originally joined TWI in 1972 with a BSc in Chemistry from Leicester University. Having worked in the Materials and Laser Departments, he left TWI in 1991 and obtained an MBA in 1992. He rejoined the organisation in 2001 as a Business Developer in the Group and European Programme Department, and became Manager of the Department in 2003. His current responsibilities cover TWI's portfolio of Group Sponsored Projects, EU and DTI collaborative projects, and working with the UK regional Development Agencies on the development of TWI's Regional Technology Centres.

David Staines joined TWI in December 1974 after gaining City and Guilds qualifications in machine shop technology and tool making. His work in the machine shop largely involved manufacture of special welding equipment. He spent thirteen years on laser processing and since 1995 has worked in the friction welding department mainly on friction stir. He is a Technician Member of the Welding Institute, registered with the Engineering Council UK, and is currently the laboratory supervisor in the Friction and Forge Processes technology group. He also works with the Innovation Unit.

Edward Watts joined TWI in 1980 as a technician in the Advanced Heavy Section processes department working with friction welding processes. He studied part time and gained his HND in mechanical engineering. Edward worked mainly on non-rotary friction welding processes developing linear friction welding on a dual rotation orbital machine before the construction of dedicated mechanical and hydraulic machines. His main role in the friction group involved developing the equipment and control systems for all the processes including novel types of FSW heads. After 19 years he became the laboratory manager, responsible for all the labs at TWI.

Part one of 'Variations on a theme - the twists and turns of friction stir welding' examined FSW variants Twin-stir and Skew-stir. The sequel, also authored by Wayne Thomas, David Staines, Edward Watts and Peter Oakley looks at the Reversal stir welding process, known as Re-stir, the Dual Rotation process and Pro-stir.

Reversal stir welding - Re-stir ^TM

Preliminary studies are 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 motion, 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 ^TM , 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 4mm/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, the distance and time between each interval depends on the combination of rotational speed and the travelspeed used.

Fig.13. Close up of Re-stir weld surface formed beneath the tool shoulder showing surface rippling and reversal interval. Produced at 4mm/sec (240mm/min) welding travel speed, using 10 revolutions per interval

Macrosections of a lap weld made by Re-stir are shown in Fig.14a and b. This weld was made in 5083-H111 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) using 10 revolutions per interval. The plan view in Fig.4c (see Part I) reveals a patterned weld region surrounded by an 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.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 ^TM probe also suggests that it may be possible to achieve a slight downturn in the overlapping plate/weld interface at the outer regions of the weld which may be beneficial in particular structures and loadingsituations. Figure 15 a, b and c, illustrate this effect in an overlap weld in 5083-H111 condition aluminium alloy.

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 1.6mm/sec (96 mm/min), using eight 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.

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

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 18a and b. 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.

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

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 velocitygradient 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 proberotational 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 alongthe '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 higherrotational speed for the probe.

A double sided butt weld using non-optimised conditions was made to demonstrate that dual-rotation stir welding is practical for certain applications. Figure 19 shows the macrostructural features produced by dual-rotation stir welding using a Flared-Triflute ^TM type.

Fig.19. A dual-rotation stir double-sided butt weld in 16mm thick 5083-H111 aluminium alloy, produced at a welding speed of 3mm/sec (180mm/min), using 584 rev/min for the probe and 219 rev/min for the shoulder. The twoweld 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 16mm 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.

Fig.21. Surface appearance of a dual-rotation stir weld made in 16mm thick 5083-H111 aluminium alloy at a welding speed of 3mm/sec (180mm/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 (eg 2xxx and 7xxx series aluminium alloys) producing a reduction in both the mechanical and corrosion propertiesacross the weld.

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(315mm/min), a difference of approximately 66°C in the maximum temperature of the HAZ region close to the probe (5mm 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 6.35mm AA7050-T7451, using the same probe geometry and a travel speed of 5.25mm/secs (315mm/min). The probe rotation speed was 394 rev/min and 388 rev/min for conventional rotary and dual-rotation stir welding techniques respectively

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, see Fig.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 thedual-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 6.35mm AA7050-T7451, using a travel speed of 5.25mm/sec (315mm/min) and a probe rotation speed of394 rev/min

Fig.24. Hardness traverses as a function of depth through the cross section of a dual-rotary friction stir weld made in 6.35mm AA7050-T7451, using the same probe geometry used in the conventional friction stir weld (seeFig.23), a travel speed of 5.25mm/sec (315mm/min), and a probe rotation speed of 388 rev/min and a should 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. 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 0.25 micron finish before being immersed in a 0.1M NaCl aerated solution at ambient temperature for seven days. Both welds were made in 6.35mmAA7050-T7451 using the same probe geometry and a travel speed of 9.2mm/secs (522 mm/min).

Fig.25. Photomacrograph of the top surface of

a) Conventional friction stir weld; and
b) Dual rotation friction stir weld

a)

b)

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-rotationfriction 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 ^TM

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 fromsheet, 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 fabricationmethods such as milling or turning:

• 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 principle of using the Pro-stir method with the Twin-stir ^TM 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 6x6mm thick sheets welded on top of each other

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 a 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 this two-part paperand 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 weldintegrity and performance. Moreover, the staggered Twin-stir ^TM 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 are expected to prove advantageous for material processing, lap welding, spotwelding 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 tendencytowards 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 theshoulder 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 thedual-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.

Development work will continue at TWI to ensure these techniques can be used commercially.

Acknowledgements

The Authors wish to thank Christoph Wiesner, Paul Evans, Mike Russell, Adrian Duncan, David Saul and Nathan Horrex for their support and contributions.

References

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