In the friction stir welding (FSW) process, onion rings are the result of the periodical deposition of layers of material which have a determining effect on the mechanical properties of FSW joints. The extent and characteristics of the onion rings are quantified in the present study and related to FSW processing parameters. A comprehensive analysis of the steady state model proposed by Jacquin et al. is carried out and an approach is proposed to extend the model for the prediction of onion rings during welding of 6082-T6 aluminum alloy. The model's setting and computation method, based on the SU/PG method, are described and compared to the results of experimental microstructural investigations. The outputs are flow patterns, strain rate and temperature contours, depending on the tool kinematics, the constitutive equation of the material and the friction coefficient at the interface tool/material. The microstructure evolution in the weld is related to the model results, and especially the evolution of banded structure and void defects.