Based upon a thermomechanical simulation of the FSW process, this work highlights the development of the characteristic microstructures generated during the assembly in the affected zones. The analytical model developed in [2] has been improved by the use of a FE calculated velocity field enabling a description of the vertical flow of the bulk material subjected to the weld. Investigations using TEM and FEG-SEM have been carried out in the various weld zones to establish a global thermomechanical scenario of the latter. This history of the weld determines the microstructure characteristics previously described in the literature, and is also enriched by a study of the various precipitation states.