Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg₅Si₆, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.