Textures in Single-Crystal Aluminum Friction Stir Spot Welds
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TRODUCTION
THE microstructural features of polycrystalline materials are generally characterized by the following factors: the phases present, the size and shape of grains, the crystallographic orientation of the grains, the grain boundary misorientation distribution, and the dislocation structure. The mechanical properties of different materials are determined by these factors, e.g., the yield strength increase resulting from cold working, precipitation hardening, grain refinement via the Hall–Petch relationship,[1,2] and tailoring of the crystallographic texture through rolling.[3] The microstructures and orientation distributions or textures produced when different manufacturing procedures such as rolling, extrusion, drawing, joining, and heat treatment are applied reflect the deformation mode and thermal history that the material experiences. For example, {110} planes are perpendicular to the loading axis when fcc metals are compressively loaded.[4] In contrast, when they are deformed in tension the {111} and {112} planes are parallel to tensile axis.[5,6] The deformation mode is therefore a key issue determining texture development.
TOSHIYA SHIBAYANAGI, Associate Professor, is with the Joining and Welding Research Institute (JWRI), Osaka University, 11-1 Mihogaoka, Ibaraki-shi, Osaka, Japan. ADRIAN P. GERLICH, Assistant Professor, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2V4. Contact e-mail: [email protected] KEIZO KASHIHARA, Associate Professor, is with the Department of Mechanical Engineering, Wakayama National College of Technology, 77 Noshima, Nadacho, Gobo-shi, Wakayama 644-0023, Japan. THOMAS H. NORTH, Professor, is with the Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, Canada M5S 3E4. Manuscript submitted May 20, 2008. Article published online January 30, 2009 920—VOLUME 40A, APRIL 2009
The use of electron-channeling patterns or electron backscatter diffraction (EBSD) patterns when examining texture has revealed that the three-dimensional arrangement of grains in a given microstructure is as important as statistical descriptions such as average grain sizes, misorientations, and pole figures.[7] The unique orientation of grains influences how the microstructure develops during deformation.[8] Grain growth in aluminum, for example, is determined by the local grain boundary character or microtexture, which is a function of the local orientation distribution.[9] Microtexture is therefore a key aspect when the orientation distribution in materials experiencing different deformation modes is characterized. Although friction stir welding was originally developed for joining aluminum alloys, it is now being applied during the fabrication of a wide range of materials such as magnesium, copper, high-strength low-alloy, and stainless steels, Ni-based superalloys, and titanium alloys.[10] During friction stir welding a rotating tool comprising a shoulder and pin is plunged into the contacting sheets a
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