On the coupling between precipitation and plastic deformation in relation with friction stir welding of AA2024 T3 aluminium alloy

We have investigated the influence of plastic deformation on the precipitation kinetics of the S′(S) phase in the AA2024 aluminium alloy, with the objective of understanding the microstructures found in friction stir welds of the same alloy. Different types of thermo-mechanical treatments have been investigated, where plastic deformation has been carried out either before heating in a salt bath, or concurrently with the thermal treatment in a Gleeble compression apparatus. The volume fraction of precipitates has been measured by differential scanning calorimetry and the precipitate morphology has been observed by transmission electron microscopy.It is observed that plastic deformation increases the precipitation kinetics in the temperature range below 300 °C. This can be attributed both to the increase in the number of nucleation sites and to the increase in the effective diffusion coefficient. At higher temperatures the influence of plasticity on the volume fraction is negligible, however, in the presence of deformation during the heat treatment, the precipitate morphology is profoundly modified. The effect of plastic deformation is observed to be similar, whether plastic deformation is carried out before or during the heat treatment.These results are shown to be valuable tools in understanding the microstructures of friction stir welds of the same material. It is shown that taking into account the effect of plastic deformation is not absolutely necessary in the thermo-mechanically affected zone. However, we show that the precipitate microstructure in some parts of the heat-affected zone is strongly influenced by a small amount of plasticity.