In this paper, the microstructure in the nugget zone of friction stir welding (FSW) 7075 aluminum alloy thick plate weld is characterized by Optic Microscope (OM), Scanning Electron Microscope (SEM) and Electron Back Scattered Diffraction (EBSD) technology, and the microstructure evolution rule along the thickness direction of thick FSW weld is investigated. The results show that, the dynamic recrystallization occurs in the nugget zone, making the original course lath-shaped structure change to the fine equiaxed grain. With the increasing distance to the weld top surface, the grain size of the equiaxed grain shows an increasing trend. The grain size at 4 mm above the weld top surface is the smallest, being only 7.8 μm. However, the size is the biggest at 19 mm, reaching 18.6 μm. Moreover, with the increase of the distance, the fraction of the dynamic recrystallization reduces from a maximum value of 90.4% at 1 mm to a minimum value of 57.5% at 19 mm. Meanwhile, a large number of High Angle Grain Boundaries (HAGBs) are mainly distributed in the nugget zone due to the effect of the dynamic recrystallization, and the HAGBs show a decreasing trend with the increase of the distance. But, the HAGBs in each one typical position increase firstly and then decrease, with the largest value of the HAGBs at 45°. Also, no strongly oriented structure is found in the nugget zone. At the same time, many fine and disperse reprecipitated phases are distributed in the nugget zone, and they are mainly η phase. The particle size of the reprecipitated phases increases with the increase of the distance, while their quantity shows an inverse trend.
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