Abstract
An atypical grain coarsening phenomenon was reported in welding nugget zone of friction stir welding for the first time. The average grain size and the aspect ratio were determined to be 266.3 µm and 2.9, which were respectively 74.0 times and 1.9 times as those of conventional friction stir welding. A numerical model combining a macro finite model with a micro modified Wagner-Kampmann model was proposed to analyze the evolution mechanism of the atypical microstructure. The heterogeneous precipitation and the dynamic recrystallization were calculated. The unequiaxed coarse grains doped with a small amount of small grains were achieved under the extremely low heat input and the high plastic strain rate. The atypical coarsening phenomenon only occurred at certain conditions: (a) extremely high plastic strain and strain rate by friction stir welding; (b) an elevated temperature slightly above 0.5 melting point to inhibit the dynamic recrystallization refinement under higher temperature and shear induced refinement under lower temperature.
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