Modelling of friction stir welding of 7xxx aluminium alloys

A combined modelling approach was applied to predict the behaviour of high strength 7000 aluminium alloys. Thermal modelling, microstructure modelling and strength modelling were performed in succession to give some insight into the complex precipitation mechanism occurring during friction stir welding (FSW). A quantitative assessment of a recent numerical model to predict the evolution of the precipitate distribution is performed for a high strength 7449 aluminium alloy subjected to a FSW process. An optimised model calibration procedure is also presented for the 7449 alloy. The robustness of this calibration is subsequently tested by applying the model to a different 7000 series alloy, 7150 in peak-aged condition, after FSW. Microstructure predictions are found to be highly dependent on the peak temperature reached during the weld thermal cycle as well as heating and cooling rates. A range of precipitation sequences involving metastable to equilibrium phase transformation and dissolution/coarsening of precipitates are predicted. The predicted microstructures are found to be in good quantitative agreement with the characterised experimental microstructures. Predicted precipitate distributions are used to estimate the strength of the material. These predictions generally agree well with measured hardness values.