Modelling Heterogeneous Precipitation in 7xxx Aluminium Alloys during Complex Processing

A numerical model based on the Kampmann and Wagner method was developed to predict the evolution of precipitate distribution in 7xxx aluminium alloy during non-isothermal heat treatments. The model considers the nucleation, growth and coarsening/dissolution of the metastable and equilibrium precipitate phases, η' and η with their stochiometric composition, MgZn2. Constitutive model equations for nucleation were based on the classical theory of nucleation whilst growth and coarsening were treated using classical phase transformation theory. The transition between η' and η, where η' acts as a precursor for η was also accounted for in the model. Differential scanning calorimetry was used to calibrate the homogeneous precipitation kinetics. The model also predicts the evolution of grain boundary precipitates and their effect on precipitate free zone size. Jominy end quench tests were performed to calibrate grain boundary precipitation kinetics. Precipitation on dislocations and dispersoids was considered. The dislocation and dispersoid densities were varied to represent different regions of a grain and therefore account for the spatial distribution of preferential heterogeneous precipitation sites. Comparison between the model prediction and experimental characterisation of the microstructure evolution of a friction stir welded 7449 aluminium alloy was found to be reasonably consistent.