Influence of the heat treatment path on the precipitation sequence in an AlSi7Cu0.5Mg-alloy

After casting and solidification, Al-cast cylinder heads undergo a sequence of heat treatments to achieve the required strength. The phases responsible for the strengthening appear after solution heat treatment, quenching and artificial ageing. Although the state-of-the-art describes the occurrence of phases in this alloy for some treatment conditions, we missed modelled solutions to predict phases and their characteristics after any heat treatment. This work focuses on developing and validating a complete modelling routine to obtain the precipitates along any heat treatment history of an AlSi7Cu0.5Mg alloy. Firstly, we simulated the solidification process of the casting using the Scheil-approach. Secondly, we calculated the phases formed during cooling, solution heat treatment and ageing using the classical nucleation theory with the MatCalc© software based on the CALPHAD method. Finally, we validated the model with heat flow obtained by calorimetry and microstructures obtained by electron microscopy. After validation, the model can simulate the strengthening phases and explain the hardness ageing curves. On the other hand, the model to date could not reproduce the occurrence of the Ti17Al5Si14 phase formed at low temperatures. This Ti-rich phase serves as nucleation sites for a fraction of the Q' phases and decreases the β phases' precipitation kinetics.