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FRICTION stir welding (FSW) is a solid-state joining process developed by TWI Ltd., Cambridge, UK, first patented in 1991.[1] Stationary shoulder friction stir welding (SSFSW) is a variant of FSW developed by TWI Ltd. to improve welding performance in titanium alloys.[2,3] In SSFSW, the welding mechanism (Figure 1) consists of a rotating probe located in a separate shoulder component, which slides over the surface of the material during welding but does not rotate and, thus, does not directly contribute to the heat generated during welding. This results in a more focused heat input around the probe, which produces a more uniform heat distribution through the weld cross section and improves process stability by reducing problems with overheating at the surface and defects at the root due to insufficient heating. FSW has been investigated as a joining method for relatively few Ti alloys. Alloys reported to have been welded include commercial purity,[4,5] the near-a alloy Ti-5111,[6] the a + b alloy Ti-6Al-4V,[2,7–14] and metastable b alloys Ti-15V-3Cr-3Al-3Sn[8] and Timetal 21-S.[15] From the reported microstructures, it is clear that FSW is like other thermomechanical processes in P.S. DAVIES, Development Engineer, B.P. WYNNE, Senior Lecturer, and W.M. RAINFORTH, Professor, are with the Institute for Microstructural and Mechanical Processing Engineering, The University of Sheffield (IMMPETUS), Sheffield S1 3JD, United Kingdom. Contact e-mail: b.wynne@sheffield.ac.uk M.J. THOMAS, Research and Development Engineer, is with Timet UK Ltd., Witton, Birmingham, B6 7UR, United Kingdom. P.L. THREADGILL, Retired, is with TWI Ltd., Great Abington, Cambridge CB21 6AL, United Kingdom. Manuscript submitted February 25, 2010. Article published online January 25, 2011 2278—VOLUME 42A, AUGUST 2011

titanium alloys in that the deformation can take place above or below the b transus temperature. For the majority of welds reported, however, microstructures suggest the deformation was predominantly supertransus.[2,3,5,7,8,12,13] This is most likely due to a wider ‘‘window’’ of processing parameters for supertransus welding, particularly lower welding forces and greater traverse speeds due to the lower flow stress. There were a limited number of studies investigating crystallographic texture during FSW of titanium.[6,11,12,15] Reynolds et al.[15] studied texture in Timetal 21S, in which there was no phase transformation on cooling. Rotations of 20 to 35 deg about the normal direction (ND) were used to bring their observed textures from the center of the welds into alignment with a shear texture reported for bcc tantalum.[16] Mironov et al.[12] in an electron backscattering diffraction (EBSD) analysis of a supertransus weld of Ti-6Al-4V used a technique based on the specific misorientation angles between a variants to distinguish the hightemperature parent b grain structure, but this technique did not extend to a full reconstruction of crystallographic orientations in the parent b grains. b textures were, therefore, based on measurements from the