The high mechanical strength of structurally hardened aluminium alloys is achieved through the fine dispersion of nano-precipitates, which prevent dislocation movement. The combination of high mechanical properties and low density makes these alloys attractive as a replacement for steel in order to reduce the weight of structures in the automotive industry. However, they have poor formability at room temperature, limiting their applications for complex structural parts. In addition, the heat treatments required to achieve their optimal mechanical strength can take several days, making their use uneconomical in the automotive industry. These constraints can be overcome by using warm forming processes. During these warm processes, precipitation occurs at the same time as deformation, which is referred to as dynamic precipitation. The interactions between plastic deformation and precipitates are complex, and the kinetics of precipitation are modified. The result of dynamic precipitation depends on many parameters, such as strain rate, temperature, and precipitation state. Since all these parameters vary during a forming process, it is important to understand quantitatively their influences on precipitation kinetics if one wishes to control the microstructure at the end of the process.
The objective of this thesis is to understand the influence of each parameter on dynamic precipitation, covering a wide range of parameters and implementing a quantitative characterisation of the precipitation state. These quantitative data will be useful for developing a predictive model. Small-angle X-ray scattering (SAXS) experiments were performed in situ during tensile tests to quantify the evolution of precipitate distributions during deformation for different temperatures, strain rates, and initial precipitation states. These results were supplemented by transmission electron microscopy (TEM) observations, atomic probe tomography (APT) experiments, and differential scanning calorimetry (DSC) experiments. The effect of dynamic precipitation was studied for two industrial alloys: AA7449 (Al-Zn-Mg-Cu) and AA2219 (Al-Cu). This allowed the influence of deformation to be compared for two different precipitation systems.
This study revealed that deformation of an unstable precipitation state at the deformation temperature considered significantly accelerates the precipitation kinetics for both alloys studied. This effect is explained by the excess vacancies produced during the non-conservative displacement of jogs during plastic deformation. This acceleration is greater when deformation is applied during precipitation than when it is applied before ageing. If more stable precipitation states are deformed, dynamic dissolution is initially observed, which is due to the shearing of precipitates by dislocations. Competition between dynamic dissolution and dynamic precipitation is highlighted. This competition depends on the temperature and strain rate. The effects of dynamic precipitation and dynamic dissolution are incorporated into a precipitation model, allowing the competition between these two phenomena to be reproduced.