D G Staines, W M Thomas, S W Kallee and P J Oakley
Paper presented at Conference in Montreal, COM 2006, Sheraton Centre Hotel, October 1-4 2006.
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 15years ago. The basic principle, recent applications and the continuing development of the FSW technology are described. The paper will introduce some of the variants of FSW, such as Dual-rotation, Twin-stir TM , and the Stir-lock TM technique.
With increasing international competition and the need to reduce the weight of transport structures, this paper introduces new variants of the FSW technology that are proving useful for certain applications. Friction Stir Welding(FSW) was invented and patented in 1991  by TWI and has since then been developed to a stage where it is being applied in production. Currently 130 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and they have filed more than 1500 patent applications related to FSW.
The basic principle of conventional rotary friction stir welding (FSW) and the main terms that define the process characteristics are shown in Figure 1.
Fig.1. Basic principle of conventional rotary friction stir welding
FSW is conducted below the melting point by pressing a rotating tool into the joint line. The wear-resistant FSW tool has a profiled probe and a shoulder with a larger diameter than that of the probe. The probe length is similar tothe required weld depth. The tool is traversed along the joint line, while the shoulder is pressed onto the surface of the workpiece, to provide consolidation of the plasticised workpiece material.
Dual-rotation friction stir welding
The systematic development of friction stir welding has led to a number of variants of the technology. The following describes preliminary studies being carried out on dual-rotation friction stir welding, its effect on lowering welding temperature and minimising the thermal softening of the weld region of certain heat-treatable aluminium alloys.
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 2.
Fig.2. Principle of dual-rotation friction stir welding with rotation of the probe and shoulder in the same direction
The apparatus can enable a range of different rotational speeds to be pre-selected or varied automatically by in-process control to suit 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. For example the shoulder rotational speed can be infinitely varied to almost zero rotational speed while rotating in the same direction or from about 25% less than the probe rotational speed down to almost zero rotational speed when the shoulder is rotated in the opposite direction.
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 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. Figure 3 shows the appearance of the weld surface that is formed beneath the tool shoulder after dual-rotation stir welding.
Fig.3. 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. [2 & 3]
One advantage of dual-rotation FSW is that it reduces the peak temperature reached during the weld thermal cycle. Figure 4 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 of 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.4. 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
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 5 & 6), the dual-rotation friction stir weld shows higher hardness values in the stirred zone, thermo mechanical affect zone (TMAZ) and heat affected zone (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.5. 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 speedof 394 rev/min
Fig.6. 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 the conventional friction stir weld ( Figure 5), 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 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 7 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. Both welds were made in 6.35 mm AA7050-T7451, using similar probes and process conditions.
Fig.7. Photomacrograph of the top surface of a) Conventional friction stir weld; and b) Dual rotation friction stir weld
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 NaCl aerated solution at ambient temperature for 7 days. Both welds were made in 6.35 mm AA7050-T7451 using the same probe geometry and a travel speed of 9.2 mm/secs (552 mm/min). 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 FSW.
Twin-stir TM technique
The simultaneous use of two or more friction stir welding tools acting on a common workpiece was first described in 1998.  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.  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. 
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.  More recently a similar arrangement has been reported with two rotating tools one used to pre-heat and one used to weld.  This disclosure,  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.  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.  Figure 8 shows the three versions of Twin-stir TM welding techniques that are being investigated and developed at TWI.
Fig.8. Twin-stir TM 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 TM
The Twin-stir TM parallel contra-rotating variant ( Figure 8a) 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 TM 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 TM
The Twin-stir TM tandem contra-rotating variant ( Figure 8b) 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 TM 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.
Staggered Twin-Stir TM
The staggered arrangement for Twin-stir TM ( Figure 8c) 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 Figure 9).
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.9. Arrangement of Staggered twin-stir TM 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
A Series of preliminary welding trials has been carried out using an experimental Twin-stir TM head at TWI in order to investigate the characteristics of welds made in a variety of configurations. The welding trials were carried out with the prototype Twin-stir TM head as shown in Figure.10.
Fig.10. Twin-stir TM prototype head assembly
The welding trial demonstrated the feasibility of Twin-stir TM and showed that welds of good appearance were produced as shown in Figure 11.
The two exit holes produced in a tandem weld showed that a similar footprint was achieved for both the lead and following tool (see Figure 12).
Fig.11. Surface appearance of a typical Tandem twin-stir TM weld made in 6083-T6 aluminium alloy
Fig.12. Tandem twin-stir TM 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 alloys. In lap welds, an upturn on both sides of the weld region is also shown ( Figure 13). All sections were prepared in the direction looking towards the start of the weld.
Fig.13. Macrosection of Tandem twin-stir TM lap weld in 6 mm thick 6082-T6 aluminium alloy
Metallographic examination of Staggered twin-stir TM lap welds revealed that the r width of the a 'common weld region' measured 430% of the sheet thickness as shown in Figure 14.
Fig.14. Macrosection taken from the 'common weld region' of a Staggered twin-stir TM lap weld in 3mm thick 5083 -H111 aluminium sheets
The tool arrangement used to produce this Staggered twin-stir TM weld is that illustrated in Figure 9; 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.
Stir-lock TM is an 'in-process' forge/forming seam joining technique. One side of the Stir-lock TM joint can be compared with riveting, whereby a rivet head is formed into a countersunk hole, for example, to provide a mechanical interlock between two or more plates. The countersunk holes are made in the comparatively harder sheet or plate material. However, the material that forms the interlock or 'rivet head' remains integrally part of the comparatively softer, more easily formable sheet or plate material. The Stir-lock TM technique can also be applied to any perforated material. Figure 15 shows a possible application for steel-to-aluminium joining in a T-joint configuration.
Fig.15. Stir-lock TM technique for joining dissimilar metals
Demonstration examples of steel to aluminium transition joints are shown in Figures 16 and 17.
Fig.16. Double sided transition joint showing hole cross-section
a) Friction treated near-side, continuous weld track
b) Far-side showing aluminium extruded into re-entrant holes
Fig.17. Single sided, Stir-lock TM aluminium-to-steel transition joint a) Friction treated near-side, continuous weld track b) Far-side showing aluminium extruded into re-entrant holes
A simple tensile test on initial samples showed promising results and failed in the steel along the line of holes. In this respect, the joint can be designed to fail in the steel or in the aluminium material, depending on the hole pattern.
Composite Transition Joints Using Stir-Lock TM
Transition joints between metals and composite materials are also becoming increasingly important in the aerospace, marine and automotive industries. Using the Stir-lock TM technique, reinforcement transition joints can also be produced for composite/metal applications. Figure 18 shows a stainless steel mesh joined to aluminium sheets by friction. The mesh provides a skeleton reinforcement for the application of resin based, polymer or rubber materials. This technique differs from other transition jointing techniques in that the reinforcement itself can provide a degree of flexibility, which can be important for certain applications eg for polyurethane or rubber-to-metal composite applications, where appropriate compliance and flexibility is required.
Fig.18. Stainless steel mesh reinforcement joined to aluminium sheets by the Stir-lock TM technique
Peel tests were carried out on initial welded samples, which showed that the mesh was substantially joined to the aluminium sheet material. Figures 19a and b show the mode of failure of the peel tested sample in which both the aluminium sheet material and stainless steel mesh have undergone significant deformation prior to joint failure. The results of test show that the weld region remained attached to one side of the sheet, and pulled material out of the other sheet.
a) Weld region pull-out with embedded and part ruptured mesh
b) Weld region attached with some embedded mesh
Fig.19. Transition joint between stainless steel mesh and aluminium sheets a) Weld region pull-out with embedded and part ruptured mesh b) Weld region attached with some embedded mesh
Different forms of material, such as perforated metal or other non-solid forms, could be welded as an alternative to mesh. Furthermore, different steels, uncoated and coated could be welded, depending on the application and other weldable materials could also be considered.
Discussion and concluding remarks
This paper provides examples of the growing use of friction stir technology. Further developments of the technology are likely to increase the types of applications that can be joined by FSW.
Results are shown for the dual-rotation technique that 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.
In-process forging, forming, embossing and mechanical joining of seam joints by friction techniques are well known. [5, 11-14] More recently a friction stir spot welding method has been used to fill individual holes by a series of one-by-one separate FSW spot welds in order to provide a series of mechanical locks.  The Stir-lock TM technique differs from the latter because it can fill individual holes along a common seam in a continuous and uninterrupted manner.
Transition joints between dissimilar materials and composite materials are frequently required in a range of demanding engineering structures and are of growing importance for many applications. The use of structural composites provide the opportunity for reduction in the weight of structures provided that the transition joints are able to transfer stresses homogeneously and in such a manner as to achieve the required design life.
Initial investigation of transition joints has demonstrated the potential of using friction techniques for producing mechanical joints between dissimilar metals and skeletal reinforcement for composite materials that would allow the joining of polymer, rubber or composites to metals.
Acknowledgements are made for the support and contributions provided by C S Wiesner, I M Norris, P Woollin, C Goodfellow, and E R Watts.
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