Modelling the Combined Effect of Room Temperature Storage and Cold Deformation on the Age-Hardening Behaviour of Al-Mg-Si Alloys-Part 2

Abstract

The present investigation deals with modelling of the age-hardening behaviour of 6xxx series automotive sheet alloys. The basis for this work is the established precipitation model NaMo developed for coupled nucleation, growth, dissolution and coarsening in Al-Mg-Si extrusion alloys. It has recently been extended for applicability for Al-Mg-Si automotive sheet alloys by incorporating the important effects of room temperature (RT) storage and deformation prior to the final artificial ageing of Al-Mg-Si sheet alloys. The 6xxx automotive sheet alloys change due to natural ageing during the time elapsing between their processing and their paint baking in the customers process. This RT storage time has an impact on the artificial ageing response during the OEMs paint baking cycle. A second effect originates from the deformation introduced in the material during the part forming process prior to the artificial ageing in the paint bake cycle. This deformation leads to the introduction of dislocations which further modify the artificial ageing response by providing heterogeneous nucleation sites for nucleation of additional strengthening phases. Part 1 of this work deals with the theoretical background and experimental validation of the extended version of NaMo, while Part 2 focuses on the new applications of the extended model by simulation of ageing during paint baking according to typical customer requirements. The model validation is based on a comprehensive set of tensile tests. A comparison between model predictions and measurements shows reasonable agreement, and it is concluded that, after some further development, the model can be used to model the yield strength response of 6xxx automotive sheet alloys incorporating the (combined) effects of natural ageing, deformation and the accurate heat treatments in the paint bake cycle.