A Coupled Thermal/Material Flow Model of Friction Stir Welding Applied to Sc-Modified Aluminum Alloys

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OVER the last 20 years, numerous investigations have sought to characterize the principles of friction stir welding (FSW) and to model the material flow behavior, the temperature distribution, and the microstructural evolution within the weld. The review papers of Nandan et al.[1] and Threadgill et al.[2] provide an excellent account of past and current FSW research. Early numerical simulations, such as those of Colegrove and Shercliff[3] or Khandkar et al.,[4] focused on the temperature distribution during welding and studied its potential influence on the weld microstructure and precipitation kinetics. More recently, researchers have been able to model both the material flow behavior and temperature characteristics during FSW despite the complex material flow associated with the process. Robson and Campbell[5] developed a grain growth and recrystallization model of FSW that successfully predicted the weld nugget size during the joining of 2524 aluminum alloy plates. Colegrove et al.[6] created a numerical model that combined material hot deformation and thermal properties to predict temperature, flow stress, and strain rate in age hardenable aluminum

CARTER HAMILTON, Associate Professor, is with the Department of Mechanical and Manufacturing Engineering, Miami University, Oxford, OH. Contact e-mail: [email protected] MATEUSZ KOPYS´CIAN´SKI, Ph.D. Candidate, and STANISLAW DYMEK, Professor and Chair, are with the Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Krako´w, Poland. OLEG SENKOV, Senior Scientist, is with UES, Inc., 4401 Dayton-Xenia Rd, Dayton, OH, 45432-1894 Manuscript submitted April 10, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A

alloys 2024, 7449, and 6013. The current investigation presents a coupled thermal/flow model of friction stir welding applied to Sc-modified Al-Zn-Mg-Cu extrusions (Al alloy 7042-T6). Additions of scandium (Sc) and zirconium (Zr) to 7000 series alloys stabilize the microstructure at temperatures greater than 423 K (150 C) through the formation of fine, secondary strengthening phases such as Al3(Sc,Zr).[7,8] The nanometer-sized Al3(Sc,Zr) particles also stabilize the microstructure formed during hot working operations and inhibit recrystallization during heat treatment, thus potentially enhancing the residual properties after joining operations such as FSW.[9] These additions also affect the kinetics of precipitation and growth of the primary strengthening precipitates (GP zones, g¢), thus modifying heat treatment conditions for enhancing the mechanical properties of these alloys.[10] The numerical simulation proposed here gives insight into the material flow and temperature distribution of the weld zone during the joining of 7042-T6 extrusions. Combined with thermal analysis data from differential scanning calorimetry (DSC), the precipitation behavior within the weld is discussed in terms of the volume fraction of the metastable (GP zones and g¢) and equilibrium [g (MgZn2) and/or T (Al2Mg3Zn3)] strengthening particl