Friction Stir Welding - Recent Developments in Tool and ... - Research

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Friction Stir Welding -Recent Developments in Tool and Process Technologies** By Wayne M. Thomas,* Keith I. Johnson, and Christoph S. Wiesner Friction stir welding (FSW) was invented by TWI in 1991[1] and substantial development has been conducted subsequently. It allows metals, including aluminium,[2±12] lead,[13] magnesium,[14] steel,[15] titanium,[16] zinc, copper,[17] and metal matrix composites[18,19] 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 Figure 1. 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 to the abutting faces of the workpieces and rotated, thereby generating frictional heat, which creates a softened plasticized region (a third-body) around the immersed probe and at the interface between the shoulder of the tool and the workpiece. The shoulder provides additional frictional treatment to the workpiece, as well as preventing plasticized material from being expelled from the weld. The strength of the metal at the interface between the rotating tool and the workpiece falls to below the applied shear stress as the temperature rises, so that plasticized material is extruded from the leading side to the trailing side of the tool. The tool is then steadily moved along the joint line giving 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 fusion welding, 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 75 mm. The present paper describes recent developments in FSW tool design, as this is the key to the successful application of the process.

Fig. 1. Principle of friction stir welding.

Tools and Techniques: 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 welding motion. The importance of the tool is illustrated in the following recent example involving the lap welding of 6 mm 5083-O, aluminium alloy wrought sheet. In preliminary trials a conventional cylindrical threaded pin probe tool was used which gave a good as-welded appearance. A typical pin type probe is shown in Figure 2. 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). 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 gave satisfactory 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 when butt-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).

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[*] W. M. Thomas, K. I. Johnson, C. S. Wiesner TWI Ltd, Great Abington Cambridge, CB1 6AL (UK) E-mail: [email protected]

[**] The authors thank I. M. Norris D. G. Staines, E. D. Nicholas, E. R. Watts, S. M. Norris, M. V. Dobinson, and P. Evans.

ADVANCED ENGINEERING MATERIALS 2003, 5, No. 7

Fig. 2. Cylindrical threaded pin type probe.

DOI: 10.1002/adem.200300355

Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Thomas et al./Friction Stir Welding -Recent Developments in Tool and Process Technologies

Fig. 6. Basic principle of skew-stir . TM

tion of the Re-stir technique is described later, and is illustrated in Figure 9. Fig. 3. Hand bend tested lap weld in 6 mm thick, 5083-O aluminium alloy sheet made Triflute type probes can be designed with any combination at a welding speed of 2 mm/s (120 mm min±1). Severe plate thinning on the retreating of neutral, left or right-handed flute, or ridge groves to suit side of the top plate (see Fig. 1) is evident. the material and joint geometry being welded. Moreover, Figure 5d 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 plasticized material and move the fragmented oxides upward or downward as required with every 120 degree part rotation of the probe. Skew-stir Lap Welds: 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, as shown in Figure 6. A lap joint made with a Flared-Triflute probe is shown in Figure 7a. 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 Figures 7b,c. It should be noted that the notch at the retreating side Fig. 4. Macrosection of a lap weld in 6 mm thick 5083-O condition aluminium alloy (Fig. 7b) does not lie in a direction perpendicular to the sheet produced with a pin type probe, at a weld travel speed of 2 mm/s (120 mm min±1). interface as it does in a weld made with a conventional pin This example illustrates that good welding can only be probe. The notch at the advancing side (Fig. 7c), however, achieved by the use of a tool appropriate to the application.[20±22] turns in a direction perpendicular to the sheet interface, but this With regard to modification of the notch at the edge of the is much less pronounced than when a conventional pin is used. weld, special tools and techniques are under development, Promising results have also been achieved with the A-Skew which will accomplish this, specifically the Flared-Triflute tool.[23] Figure 8 shows a weld made at the same conditions as probe Skew-stir , and Re-stir . The forms of the first two Figure 7, but using this tool. Figures 8b,c show an improved tools are shown in Figures 5 and 6 whilst detailed explanaorientation of the edge notch, even on the advancing side. Reversal Stir Welding ± Re-Stir : 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 welding at TWI. The salient features of the Re-stir welding technique are illustrated in Figure 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. The use of the Re-stir welding technique provides a cyclic and essentially symmetrical welding Fig. 5. Basic variants for the Flared-Triflute 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. and processing treatment[1,24,25] Most problems assoTM

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Fig. 7. Lap weld made using a Flared-Triflute probe in 6 mm thick 5083-0 condition aluminium alloy, at a welding speed of 4 mm/s (240 mm min±1). a) Macrosection. b) Detail of notch at the retreating side. c) Detail of notch at the advancing side.

Fig. 8. Lap weld made using a A-SkewTM probe in 6 mm thick 5083-0 condition aluminium alloy at a welding speed of 4 mm/s (240 mm min±1). a) Macrosection. b) Detail of notch at the retreating side. c) Detail of notch at the advancing side

ciated with the inherent asymmetry of conventional rotary FSW are avoided. The results from preliminary Re-stir welding trials with 6 mm thick 5083-O condition aluminium alloy show consid-

erable promise. A transverse macrosection of a Re-stir butt weld made in this material using a conventional MX-Triflute probe is shown in Figure 10. The weld region is essentially symmetrical in shape tending to become narrower towards

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Thomas et al./Friction Stir Welding -Recent Developments in Tool and Process Technologies Surface Appearance of Re-stir 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/s (96 mm min±1), using five revolutions per interval. Figure 12 shows the detail of the surface of a weld made at 4 mm/s (240 mm min±1) travel speed, using ten 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 travel speed used. Macrosections of a lap weld made by Re-stir are shown in Figures 13a±c. This weld was made in 5083-O condition aluminium alloy, using a Flared-Triflute type probe. Figure 13a shows a weld with detrimental plate thinning/hooking owing to the non-optimization of welding parameters, but does serve to illustrate the symmetrical nature of the weld produced by the Re-stir technique. The longitudinal section shown in Figure 13b 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 Figure 13c reveals a patterned weld region surrounded by a HAZ. There is some evidence that during the reversal stage some of the ªThird-bodyº plasticized material close to the probe is ªre-stirredº back in the opposite direction. The Re-stir process requires further optimization to achieve welds of reproducibly high quality and freedom from defects, but early trials suggest benefits in terms of weld symmetry. Initial Re-stir work using an A-skew 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). Figures 14a±c illustrates this effect in a lap weld in 5083-O condition aluminium alloy. This paper describes recent tool and process developments for FSW butt and lap welding, particularly when using a FlaredTriflute probe, and the Skew-stir and Re-stir techniques. The TM

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Fig. 9. The basic principle of Re-stirTM, showing the reversal technique.

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Fig. 10. Microsection of a Re-stir butt weld, produced at a welding travel speed of 4.2 mm/s (250 mm min±1), using 8 revolutions per interval, which shows an essentially symmetrical, shaped weld region. TM

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Fig. 11. Surface appearance of Re-stir welds made in 6 mm thick 5083-O condition aluminium alloy at a welding travel speed of 1.6 mm/s (96 mm min±1) using 5 revolutions per reversal interval. TM

the top of the plate. This is in marked contrast to conventional rotary friction stir welding in which an asymmetrically shaped weld is obtained.[5]

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Fig. 12. Close up of Re-stir weld surface formed beneath the tool shoulder showing surface rippling and reversal interval. Produced at 4 mm/s (240 mm min±1) welding travel speed, using 10 revolutions per interval.


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Fig. 13. Metallurgical sections of a Re-stir lap weld produced at a welding speed of 3.3 mm/s (198 mm min±1), using ten 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.

Fig. 14. Metallurgical sections of a Re-stirTM lap weld made with an A-skewTM probe in combination with a skew motion, at a travel speed of 1.6 mm/s (96 mm min±1), 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).

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.

Although significant improvements have been achieved, additional tool development work is underway to further optimize integrity and appearance of FSW lap welds. Butt welds pro-

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duced with Whorl and MX-Triflute frustum-shaped probes that gave acceptable weld quality are also described. In addition, initial investigation of the Re-stir 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 inherent in conventional rotary friction stir welds. Moreover, ongoing investigations are expected to establish the advantage of using Re-stir 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 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. TM

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Received: February 28, 2003

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[1] M. Thomas, E. D. Nicholas, J. C. Needham, M. G. Murch, P. Temple-Smith, C. J. Dawes, EU Patent 0 615 480 B1. [2] K. V. Jata, K. K. Sankaran, J. J. Ruschau, Metall. Mater. Trans. A 2000, 31, 2181. [3] M. W. Mahoney, C. G. Rhodes, J. G. Flintoff, R. A. Spurling, W. H. Bingel, Metall. Mater. Trans. A 1998, 29, 1955. [4] K. J. Colligan, J. J. Fisher, E. J. Gover, J. R. Pickens, in Adv. Mater. Processes Technol. 2002, p. 39. [5] W. J. Arbegast, P. J. Hartley, Friction stir weld Technology Development at Lockheed Martin Michoud Space System-An Overview, Proc. of the 5th Inter. Conf. onTrends in Welding research 1, Pine Mountain, Georgia, USA 1998, ASM International, OH 1998. [6] C. J. Dawes, Proc. of the 3rd Int. Forum on Aluminium ships Hauesund, Norway 1998. [7] D. C. Donne, G. Biallas, T. Ghidini, G. Raimbeaux, 2nd Int. Symposium on Friction Stir Welding, Gothenburg Sweden. 2000. [8] A. J. Leonard, Proc. of the 2nd FSW Symposium, Quality Hotel 11, Gothenburg, Sweden 2000, TWI, Abington, UK 2000. [9] S. Tanaka, M. Kumagai, Proc. of the 3rd Int. Friction Stir Welding Symposium 2001, Kobe, Japan, TWI, Abington, UK 2001. [10] J. F. dos Santos, G. Cam, A. von Strombeck, V. Ventzke, M. Kocak, Proc. of the Minerals, Metals, & Materials Society (TMS) 2001, 130th Annual Meeting and Exhibition, New Orleans 2001. [11] L. E. Svensson, L. Karlsson, H. Larsson, B. Karlsson, M. Fazzini, J. Karlsson, Sci. Technol. Joining 2000, 5, 285. [12] S. W. Kallee, E. D. Nicholas, W. M. Thomas, Structures and Technologies±Challenges for Future Launchers, Proc. of the 3rd European Conf., Strasbourg 2001.

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[13] T. Boon, W. M. Thomas, P. Temple-Smith, US Patent 6 325 273 B1, 1996. [14] S. A. Lockyer, M. J. Russell, Proc. of the 3rd Int. Symposium on Friction Stir Welding, Kobe, Japan 2001. [15] W. M. Thomas, P. Woollin, K. I. Johnson, Steel World 1999, 4, 55. [16] M. J. Russell, FSW of Titanium Alloys - A Progress Update, presented at the Ti - 2003, 10th World Conf. on Titanium, Hamburg 2003. [17] C. G. Andersson, R. E. Andrews, B. G. I. Dance, M. J. Russell, E. J. Olden, R. M. Sanderson, 1st FSW Symp., Thousand Oaks, CA 1999, TWI, Abington, UK 1999. [18] W. T. Nelson, H. Zhang, T. Haynes, Proc. of the 2nd Int. Symp. on Friction Stir Welding, Gothenburg, Sweden 2000, TWI, Abington, UK 2000. [19] K. Nakata, Y. G. Kim, M. Hashimoto, T. Ushio, S. Jogan, Trans. Joining Weld. Res. Inst. 2000, 29, No. 2. [20] B. Christner, ICAS Congress 2002. [21] L. Cederquist, A. P. Reynolds, ªFactors affecting the properties of friction stir welded aluminium lap jointsº Welding Journal (Research supplement), December 2001, pp. 281±287. [22] K. Matsumoto, S. Sasabe, Proc. 3rd Int. Friction Stir Welding Symp., Kobe, Japan 2001, TWI, Abington, UK 2001. [23] W. M. Thomas, A. B. M. Braithwaite, R. John, Proc. 3rd Int. Friction Stir Welding Symp., Kobe, Japan 2001, TWI, Abington, UK 2001. [24] E. D. Nicholas, W. M. Thomas, C. J. Needham, WO Patent 9 638 256, 1996. [25] W. M. Thomas, E. D. Nicholas, M. Murch, Connect, March 1993.

High Temperature Deformation Behavior of the Bond±Coat Alloy PWA 1370** By Jochen Schwarzer and Otmar Vöhringer* The application of thermal barrier coatings (TBCs) is generally recognized as being essential to the development of high efficiency gas turbines, both for aeroplane engines and stationary use. A TBC system consists of the superalloy sub-

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[*] Prof. O. Vöhringer, J. Schwarzer Institut für Werkstoffkunde I Universität Karlsruhe (TH) Kaiserstr. 12, D-76131 (Germany) E-mail: [email protected]

[**] The financial support of the Deutsche Forschungsgemeinschaft (German Science Foundation) is gratefully acknowledged. The provision of the material from the Institute of Materials Research at the German Aerospace Center, Cologne, is appreciated.

DOI: 10.1002/adem.200300354
