The Mechanism of Grain Coarsening in Friction-Stir-Welded AA5083 after Heat Treatment |
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Authors: | Ke Chen Wei Gan K Okamoto Kwansoo Chung and R H Wagoner |
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Affiliation: | (1) Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA;(2) College of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China;(3) Medtronic Inc., Mounds View, MN 55112, USA;(4) R&D Division, Hitachi America, Ltd., Farmington Hills, MI 48335, USA;(5) School of Materials Science and Engineering, Engineering Research Institute, Seoul National University, Kwanak-gu, Seoul, 151-742, Korea;; |
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Abstract: | Friction stir welding (FSW) takes place in the solid state, thus providing potential advantages of welds of high strength
and ductility because of fine microstructures. However, post-FSW heat treatment can create very coarse grains, potentially
reducing mechanical properties. AA5083-H18 sheets were friction-stir butt welded using three sets of welding parameters representing
a wide range of heat input. They were then heat treated for 5 minutes at 738 K (465 °C), producing grain sizes exceeding 100 μm near the top weld surfaces, with the coarse grains extending toward the bottom surface to various degrees depending on the
welding parameters. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy
(SEM), optical metallography, inductively coupled plasma–mass spectrometry, and Vickers hardness testing were used to characterize
the regions within welds. Particle pinning was determined quantitatively and used with Humphreys’ model of grain growth to
interpret the behavior. The mechanism responsible for forming the large grains was identified as abnormal grain growth (AGG),
with AGG occurring only for regions with pre-heat-treatment grain sizes smaller than 3 μm. Second-phase particle volume fractions and sizes, textures, and solute concentrations were not significantly different
in AGG and non-AGG regions. Ultrafine grain layers with grain diameters of 0.3 mm were characterized and had high densities
of pinning particles of MgSi2, Al2O3, and Mg5Al8. Strategies to eliminate AGG by alloy and weld process design were discussed. |
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