Microstructural evolution during friction stir welding on AA2024 aluminium alloys - Application to the prediction of the mechanical properties

Abstract : Friction stir welding on aluminium alloys is seen as an interesting option to lighten aircraft structure. In particular, this process is considered as an alternative to the standard riveting process used for joining stiffeners to panels. However in aluminium alloys, mechanical properties are linked to structural hardening due to solid state precipitation. This phenomenon is known to affect the final properties of the welds after FSW. 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 for aluminium alloys based on a reliable thermodynamic description has been developed. A particle size distribution approach (PSD) is proposed for multi-components alloys to predict the nucleation, growth, dissolution and coarsening of both stable and metastable phases. At a larger scale, a macro-structural model is used to define precisely the thermo-mechanical evolutions of the metal parts. It allows to know the thermal evolution during the whole FSW process. Then, it is possible to couple the models in order to define precisely the precipitate evolutions in HAZ, TMAZ and nuggets domains leading to heterogeneous mechanical properties. Thereafter an accurate yield strength model is developed knowing the final precipitate radius distribution and precipitates properties. The calibration of this model is performed on differential scanning calorimetry studies (DSC), hardening test and tensile tests for both isothermal / non-isothermal treatments. This microstructural model is applied on a 2024-T3 aluminium alloy. The final mechanical properties are presented and discussed.