The Mechanism of Grain Coarsening in Friction-Stir-Welded AA5083 after Heat Treatment
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FRICTION stir welding (FSW) is a relatively new welding technique, invented in 1991.[1] It has broad promise for joining similar and dissimilar materials, which are difficult or impossible to weld conventionally. FSW can produce a fine microstructure with fewer defects, lower residual stresses, less distortion, better retained mechanical properties, and better dimensional stability as compared with conventional welding.[2–5] FSW is also a candidate process for joining sheets of dissimilar thickness or composition to create tailorwelded blanks[6–8] that retain the capacity for enhanced ductility and strength via fine grain structure. Recent research focuses on FSW in joining materials such as magnesium alloys and steel.[9–12] Commercialization of FSW, mainly for aluminum alloys, has occurred in the transportation industry, for applications such as automobiles, railway vehicles, ships, and rockets.[13–17] KE CHEN, formerly Graduate Research Associate, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, is Assistant Professor, College of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China. WEI GAN, Senior Engineer, is with Medtronic Inc., Mounds View, MN 55112. K. OKAMOTO, Senior Researcher, is with the R&D Division, Hitachi America, Ltd., Farmington Hills, MI 48335. KWANSOO CHUNG, Professor, is with the School of Materials Science and Engineering, Engineering Research Institute, Seoul National University, Kwanak-gu, Seoul, 151-742, Korea. R.H. WAGONER, Professor, George R. Smith Chair, is with the Department of Materials Science and Engineering, The Ohio State University. Contact e-mail: [email protected] Manuscript submitted September 4, 2009. Article published online November 2, 2010 488—VOLUME 42A, FEBRUARY 2011
A. FSW Background FSW is achieved by severe plastic deformation and heating induced by friction and plastic work created by the rotation of a nonconsumable tool embedded in the workpiece (Figure 1).[4] Severe microstructural changes can occur in the welded region.[4,18–25] Peak temperatures, reportedly as high as 0.6 to 0.8 times the melting temperature,[26–28] have been correlated to combinations of welding parameters m (weld feed rates) and x (tool rotation speeds) through a ‘‘heat input parameter’’ or a ‘‘heat index,’’ x2/m[26,29] or x/m,[30] respectively. Dynamic recrystallization and texture development occur in the weld nugget zone (WNZ) (delineated approximately by the dashed line in Figure 1(b)) during FSW,[5,28,31–35] while precipitate dissolution and coarsening occur not only in the nugget zone but also in the thermomechanically affected zone and heat affected zone.[31,35,36] A fine and equiaxed grain structure is obtained after FSW, with grain sizes ranging from 1 to 10 lm.[18,31,33,35–37] Ultrafine-grained microstructures with average grain sizes smaller than 1 lm have also been obtained using special cooling methods.[18,38] B. Recrystallization after FSW Post-FSW heat treatment may be carried out for annealing or
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