Friction Stir Welding - Recent Developments in Tool and Process Technologies
W M Thomas, K I Johnson, and C S Wiesner, TWI Ltd
Paper published in Advanced Engineering Materials, Volume 5, Issue 7, Date: July, 2003, Pages: 485-490.
Friction stir welding (FSW) was invented by TWI in 1991  ) and substantial development has been conducted subsequently. It allows metals, including aluminium, [2-12] lead,  magnesium,  steel,  titanium,  zinc, copper  and metal matrix composites  to be welded continuously. Many alloys, which are regarded as difficult to weld by fusion processes, may be welded by FSW. The basic principle of the FSW process is shown in Fig.1.
Fig.1. Principle of friction stir welding
A non-consumable rotating tool is employed of various designs, which is manufactured from materials with superior high temperature properties to those of the materials to be joined. Essentially, the probe of the tool is applied tothe abutting faces of the workpieces and rotated, thereby generating frictional heat, which creates a softened plasticised region (a third-body) around the immersed probe and at the interface between the shoulder of the tool and theworkpiece. The shoulder provides additional frictional treatment to the workpiece, as well as preventing plasticised material from being expelled from the weld. The strength of the metal at the interface between the rotating tool andthe workpiece falls to below the applied shear stress as the temperature rises, so that plasticised material is extruded from the leading side to the trailing side of the tool. The tool is then steadily moved along the joint linegiving a continuous weld.
Although incipient melting during welding has been reported for some materials, FSW can be regarded as a solid state, autogenous keyhole joining technique. The weld metal is thus free from defects typically found when fusionwelding, e.g. porosity. Furthermore, and unlike fusion welding, no consumable filler material or profiled edge preparation is normally necessary.
The process has already made a significant impact on the aluminium-producing and user industries worldwide and FSW is now a practical technique for welding aluminium rolled and extruded products, of thickness ranging from 0.5 to 75mm. The present paper describes recent developments in FSW tool design, as this is the key to the successful application of the process.
2. Tools and techniques
2.1 Conventional Rotary Welds
Although FSW consistently gives high quality welds, proper use of the process and control of a number of parameters is needed to achieve this. A key factor in ensuring weld quality is the use of an appropriate tool and weldingmotion.
The importance of the tool is illustrated in the following recent example involving the lap welding of 6mm 5083-O, aluminium alloy wrought sheet. In preliminary trials a conventional cylindrical threaded pin probe tool was usedwhich gave a good as-welded appearance. A typical pin type probe is shown in Fig.2.
Fig.2. Cylindrical threaded pin type probe
However, bend testing showed the weld to be weak due to excessive thinning of the top sheet and thickening of the bottom sheet caused by a pressure differential during welding, see Fig.3.
Fig.3. Hand bend tested lap weld in 6mm thick, 5083-O aluminium alloy sheet made at a welding speed of 2mm/sec (120mm/min). Severe plate thinning on the retreating side of the top plate (see Fig.1) is evident
The failure followed the original interfacial surface oxide layers, which in 5083-O condition aluminium alloy, are known to be particularly tenacious. The above problems were caused because, although the tool employed gavesatisfactory welds when butt-welding plate components, its use when lap welding was inappropriate. Lap welding requires a modified tool to ensure full disruption of the interfacial oxide layers and a wider weld than is required whenbutt-welding.
A transverse macrosection taken from a weld produced with a pin type probe shows extreme plate thinning on the retreating side and a serious hook feature on the advancing side of the weld, whilst porosity is also clearly visible,see Fig.4.
Fig.4. Macrosection of a lap weld in 6 mm thick 5083-O condition aluminium alloy produced with a pin type probe, at a weld travel speed of 2 mm/sec (120 mm/min)
This example illustrates that good welding can only be achieved by the use of a tool appropriate to the application. [20,22]
With regard to modification of the notch at the edge of the weld, special tools and techniques are under development, which will accomplish this, specifically the Flared-Triflute TM probe Skew-stir TM , and Re-stir TM . The forms of the first two tools are shown in Figs.5 and 6 whilst detailed explanation of the Re-stir TM technique is described later, and is illustrated in Fig.9.
Fig.5. Basic variants for the Flared-Triflute TM type probes
a) neutral flutes
b) left hand flutes
c) right hand flutes
d) ridge detail showing that ridge groves can be neutral, left, or right handed
Triflute type probes can be designed with any combination of neutral, left or right-handed flute or ridge groves to suit the material and joint geometry being welded. Moreover, Fig.5(d) shows that the individual ridges on the probe can be regarded as independent features. This effectively enables neutral, left or right hand inclined ridge grooves to deflect plasticised material and move thefragmented oxides upward or downward as required with every 120 degree part rotation of the probe.
2.2 Skew-stir TM lap welds
The Skew-stir TM 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, as shown in Fig.6.
Fig.6. Basic principle of Skew-stir TM
A lap joint made with a Flared-Triflute TM probe is shown in Fig.7(a). In this example the width of the weld region is 190% of the plate thickness and little upper plate thinning is apparent. (The corresponding weld width achieved when using a conventional threaded pin probe is 110%).The notch at the edge of the weld achieved using this tool is shown in Figs.7(b) and (c). It should be noted that the notch at the retreating side ( Fig.7(b)) does not lie in a direction perpendicular to the sheet interface as it does in a weld made with a conventional pin probe. The notch at the advancing side ( Fig.7(c)), however, turns in a direction perpendicular to the sheet interface, but this is much less pronounced than when a conventional pin is used.
Fig.7. Lap weld made using a Flared-Triflute TM probe in 6mm thick 5083-O condition aluminium alloy, at a welding speed of 4mm/sec (240mm/min)
b) Detail of notch at the retreating side
c) Detail of notch at the advancing side
Promising results have also been achieved with the A-Skew TM tool.  Fig.8 shows a weld made at the same conditions as Fig.7, but using this tool. Figs.8(b) and (c) show an improved orientation of the edge notch, even on the advancing side.
Fig.8. Lap weld made using a Skew-stir TM probe in 6mm thick 5083-O condition aluminium alloy at a welding speed of 4mm/sec (240mm/min)
b) Detail of notch at the retreating side
c) Detail of notch at the advancing side
3. Reversal stir welding - Re-stir TM
3.1 Introductory remarks
The continuing development of friction stir welding (FSW) has led to a number of variants of the process. The following describes preliminary studies being carried out on Re-stir TM welding at TWI. The salient features of the Re-stir TM welding technique are illustrated in Fig.9. 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.9. The basic principle of Re-stir TM , showing the reversal technique
The use of the Re-stir TM welding technique provides a cyclic and essentially symmetrical welding and processing treatment. [1,24 & 25] Most problems associated with the inherent asymmetry of conventional rotary FSW are avoided.
The results from preliminary Re-stir TM welding trials with 6 mm thick 5083-O condition aluminium alloy show considerable promise. A transverse macrosection of a Re-stir TM butt weld made in this material using a conventional MX-Triflute TM probe is shown in Fig.10. The weld region is essentially symmetrical in shape tending to become narrower towards the top of the plate. This is in marked contrast to conventional rotary friction stir welding inwhich an asymmetrically shaped weld is obtained. 
Fig.10. Microsection of a Re-stir TM butt weld, produced at a welding travel speed of 4.2 mm/sec (250 mm/min), using 8 revolutions per interval, which shows an essentially symmetrical, shaped weld region
3.2 Surface Appearance of Re-stir TM Butt and Lap Welds
Figure 11 shows the appearance of the weld surface that is formed beneath the tool shoulder produced at a welding travel speed of 1.6 mm/sec (96 mm/min), using 5 revolutions per interval.
Fig.11. Surface appearance of Re-stir TM welds made in 6 mm thick 5083-O condition aluminium alloy at a welding travel speed of 1.6 mm/sec (96 mm/min) using 5 revolutions per reversal interval
Fig.12 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 theless 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.
Fig.12. Close up of Re-stir TM 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 TM are shown in Fig.13(a) (b) and (c). This weld was made in 5083-O condition aluminium alloy, using a Flared-Triflute TM type probe. Figure 13(a) shows a weld with 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.13(b) 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 of Fig.13(c) 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 oppositedirection.
Fig.13. Metallurgical sections of a Re-stir TM lap weld produced at a welding speed of 3.3 mm/sec (198 mm/min), using 10 revolutions per interval
a) Macrosection showing an essentially symmetrical dovetail shaped weld with a similar amount of upturn of the plate interface each side of the weld
b) Longitudinal macrosection showing regular patterns caused by rotation reversal
c) Plan macrosection taken mid-thickness showing the effect of reversal motion
The Re-stir TM 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 Re-stir TM work using an A-skewTM probe shows that only a slight down turn in the overlapping plate/weld interface occurs at the outer regions of the weld that should be beneficial in structural applications (cf. Fig.4). Figure 14(a) (b) and (c) illustrates this effect in a lap weld in 5083-O condition aluminium alloy.
Fig.14. Metallurgical sections of a Re-stir TM lap weld made with an A-skew TM 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
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)
4. Concluding remarks
This paper describes recent tool and process developments for FSW butt and lap welding, particularly when using a Flared-Triflute TM probe, and the Skew-stir TM and Re-stir TM techniques. The latter lap welding techniques gave an improvement in weld integrity; a reduction in upper plate thinning and an increased welding speed compared with the conventional pin type probe. Althoughsignificant improvements have been achieved, additional tool development work is underway to further optimise integrity and appearance of FSW lap welds. Butt welds produced with WhorlTM and MX-Triflute TM frustum-shaped probes that gave acceptable weld quality are also described.
In addition, initial investigation of the Re-stir TM technique has demonstrated that it may offer significant benefit in generating essentially symmetrical welds and hence has the potential to overcome some of the problems associated with the asymmetry inherentin conventional rotary friction stir welds. Moreover, ongoing investigations are expected to establish the advantage of using Re-stir TM for the welding of dissimilar materials; by using more revolutions in one direction interval and less in the opposite direction interval to compensate for material with widely differing flow properties.
Although it is early days and much more development work is required before the technique can be used commercially, it seems possible that the Re-stir TM may well become the preferred option for certain butt and lap weld configurations, tailor welded blanks, compound lap, spot and welding, dissimilar materials and other material processing applications.
The authors wish to thank I M Norris D G Staines, E D Nicholas, E R Watts, S M Norris, M V Dobinson and P Evans.
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