Intermixing in Dissimilar Friction Stir Spot Welds
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I.
INTRODUCTION
THE friction stir welding process was developed by TWI (Abington, United Kingdom) in 1991 as a novel method for joining aluminum alloys.[1] Since that time, the welding process has been employed in aerospace, rail, automotive, and marine industries for joining aluminum, titanium, magnesium, zinc and copper alloys, steel, and thermoplastics in thicknesses ranging from 1 to 50 mm.[2–14] Friction stir spot welds are produced by plunging and retracting a rotating tool into and out of the sheets being joined. A keyhole region remains at the end of the welding operation. The spot welding operation typically lasts from 2 to 5 seconds and within this brief time envelope heat generation and stir zone formation create a joint between the two contacting sheets. The power density resulting from tool rotation is particularly high during spot welding (around 1010 W/m3), and as a consequence, remarkably high peak temperatures are attained at the contact interface between the rotating pin and adjacent material within the stir zone.[15] For example, peak temperature values approaching the solidus temperature of different aluminum or magnesium alloy sheet materials have been reported during friction stir spot welding.[16–19] Intermixing of dissimilar sheet materials during friction stir spot welding has not been previously investigated. Also, only a limited amount of research has been published concerning dissimilar intermixing in friction stir welds where the rotating tool is traversed across the component.[20–23] Larssen et al.[20] examined the dissimP. SU, Postdoctoral Fellow, A. GERLICH, Postdoctoral Student, T.H. NORTH and G.J. BENDZSAK, Professors, are with the University of Toronto, M5S3E4 Toronto, ON, Canada. Contact e-mail: [email protected] Manuscript submitted July 17, 2006. Article published online March 29, 2007. 584—VOLUME 38A, MARCH 2007
ilar microstructures formed during friction stir welding of Al 5083 and Al 6082 sheets. The dissimilar Al 5083 and Al 6082 lamellae in completed welds were differentiated by their different etching behaviors. There was no evidence of chemical mixing and negligible evidence of interdiffusional mixing at the boundaries between Al 5082 and Al 6082 lamellae. Also, the boundaries between Al 5082 and Al 6082 lamellae were decorated by large numbers of small diameter oxide particles. The formation of intermixed weld microstructures containing intermingled lamellae of the dissimilar materials being welded has been associated with chaotic flow of viscous material at temperatures close to the solidus temperature[21] and with dynamic recrystallization at temperatures below 0.7 Ts, where Ts is the solidus temperature of the base material in degrees Celsius.[22,23] Neither of these proposals associated dissimilar intermixing during friction stir welding with the presence of a thread on the rotating tool. However, the critical role that the thread on the rotating pin plays in terms of facilitating intermixing during dissimilar spot welding of Al 5754 and Al 6111 sheets is shown in F
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