Precipitation microstructures in an AA6056 aluminium alloy after friction stir welding : Characterisation and modelling

The effect of friction stir welding on the microstructure and local properties of the AA6056 alloy has been investigated for both T4 and T78 initial states. A particular attention was devoted to the relationship between the fine hardening precipitation within the affected zones and local hardness. In the heat-affected zones of the T4 weld an extensive heterogeneous precipitation is occurring on dislocations and dispersoids whereas the heat-affected zones of the T78 weld are characterised by the coarsening and dissolution of initial hardening precipitates and by heterogeneous precipitation on dispersoids. Modelling tools dedicated to the weld behaviour understanding have been developed: a physically based model for precipitation and hardening has been coupled to a model for thermal cycles. This model describes correctly both the microstructural evolution through the FSW joint and its consequence on hardness variations.