A physical-based unified constitutive model of AA7075 for a novel hot forming condition with pre-cooling

Hot forming high strength aluminium alloys with a pre-cooling stage can overcome detrimental effects of the low-melting phases defect, and enhance hardening. The introduced pre-cooling brings significant changes in microstructures of the alloys at elevated temperatures, which directly affect the hot deformation and strength behaviour under the novel hot forming condition. In this study, systematic experimentation was performed to obtain the detailed effects of pre-cooling strategies on hot forming, and a physical-based unified material model considering the mutual effects of underlying microstructures evolution (precipitates, solute concentrations and dislocations) and mechanical properties (yield strength, work-hardening and ductility) was established. The developed model enables the precise prediction of hot flow behaviour, microstructure evolution (solute concentration, fraction of precipitates and dislocation density) and post strength (yield strength/hardness) of AA7075 conditioned by different pre-cooling strategies. The effectiveness of the developed model has been successfully supported and validated by corresponding experiments, including Gleeble thermal mechanical testing and transmission electron microscopy (TEM) observations for both precipitates and macro-properties. In addition, effects of the key parameters of pre-cooling, such as cooling rate and cooled temperature, on hot flow and post treated hardness were discussed based on both experimental and numerical results. It is found that the detrimental effect using a lower cooling rate, i.e. 5 K/s, is the induced coarse precipitates, which would alter the subsequent hot flow behaviour and deteriorate post strength. In addition, the lowest hardness in the formed alloy occurs within a holding temperature range of 598–623 K for AA7075. The developed model can provide direct guidance for process design and parameter optimisations for the hot forming processes of high strength AA7075 with pre-cooling.