Abstract: The 5083-H321 aluminum magnesium alloy plates were welded on a minitype friction stir welding machine. The microstructure of friction stir welding (FSW) in each nugget zone was studied via the grain morphology and misorientation distribution of nugget advancing side, nugget median zone and nugget retreating side. The results show that there are small equiaxial grains into which lathshaped grains turn in nugget zones after friction stir welding. However, there are differences in the grain size between the nugget advancing side, nugget median zone and nugget retreating side, and the average grain sides are 17, 15.8 and 13.5 μm, respectively. Compared with the base metal, the high angle grain boundaries(HAGB) fraction in nugget zones increase. Meanwhile the content is also different in the three zones, and they are 76.4%, 68.8% and 73.2%, respectively. 

Key words: friction stir welding; nugget advancing side; nugget median zone; nugget retreating side; microstructure