The standard riveting process used for joining stiffeners (AA 7075) to panels (AA 2024) in aeronautic industry has a detrimental effect on the weight of planes. Friction stir welding (FSW) process is seen as an interesting option and a promising alternative process to lighten aircraft structure. However the mechanical properties in these grades are linked to structural hardening due to the solid state development of fine and densely distributed precipitates. FSW processes are known to affect these precipitates and the final properties of the welds. Thus the estimation of precipitate evolutions is of prime importance in order to deliver reliable welds to the aircraft industry. A time-efficient solid-state precipitation modelling based on a reliable thermodynamic description has been developed in order to predict the precipitate distribution and the related mechanical properties. At a micro scale, a particle size distribution approach (PSD) [1] is proposed for multi-components alloys to predict the nucleation, growth, dissolution and coarsening of both stable and metastable phases. The model is coupled to thermodynamic computations [2] in order to determine precipitate/matrix equilibrium compositions and associated growth velocity. This microstructural model is applied on a AA 2024 aluminium alloy in order to model both stable (S-phase) and metastable (GPB zones) phases. A DSC (Differential Scanning Calorimetry) calibration procedure has been developed to determine the nucleation parameters (Fig. 1 a). At a larger scale, a macro-structural model is used to define precisely the thermo-mechanical evolutions of the metal parts and thermal evolution inside welds [3]. Then, it is possible to couple the micro/macro models in order to define precisely the precipitate evolutions in nuggets, TMAZ and HAZ domains leading to heterogeneous mechanical properties (Fig. 1 b). The final mechanical properties are presented and discussed. [1] V. Legrand, Modélisation des processus de précipitation et prédiction des propriétés mécaniques résultantes dans les alliages d’aluminium à durcissement structural – Application au soudage par Friction Malaxage (FSW) de tôles AA2024, Doct. MinesParisTech, 2015 [2]TCAL3, TCS Al-based alloys thermodynamic database, TC software AB (Stockholm, Sweden), 2014. [3]S. Gastebois, Simulation numérique 3D du FSW à l’aide d'une formulation ALE, Doct. Mines Paristech, 2015