A model for predicting the yield stress of AA6111 after multistep heat treatments

Abstract

A model has been developed to predict the yield stress (YS) of the aluminum alloy AA6111 after multistep heat treatments that involve combinations of ambient-temperature aging and high-temperature artificial aging. The model framework follows the internal state variable framework where the two principal state variables are the volume fraction of clusters that form at ambient temperature and the volume fraction of metastable phases that form during high-temperature aging. The evolution of these state variables was modeled using a set of coupled differential equations. The mechanical response (the YS) was then formulated in terms of the state variables through an appropriate flow stress addition law. To test the model predictions, a series of experiments were conducted that examined two scenarios for multistep heat treatments. In general, good agreement was observed between the model predictions and the experimental results. However, for the case where a short thermal excursion at 250 °C was applied immediately after the solution treatment, the results were not satisfactory. This can be understood in terms of the importance of the temperature dependence for the nucleation density of metastable precipitates.