Fracture of 7xxx aluminium alloys with tailored friction stir processed microstructures

The present thesis investigates the influence of 7XXX aluminium alloys microstructure on damage mechanisms. 7XXX alloys are widely used as structural materials due to their high strength and low density. However, strength was obtained at the expense of ductility, which is a limiting factor for forming applications. Ductility can be related to the uniform elongation εu prior to material strain localization, or related to the fracture strain εf that is related to the ultimate strain level that the material reaches at failure. It was revealed in the literature that the microstructure of 7XXX alloys is affecting damage initiation, propagation and coalescence. This correlation between microstructural features and damage mechanisms opens the way for mechanical performances optimization by microstructural changes. Indeed, the 7XXX alloys microstructure is complex and its modification by thermomechanical and heat treatments is not obvious. This work focusses on manufacturing a variety of 7XXX microstructures prior to testing their mechanical properties. Friction Stir Processing (FSP) was selected for refining the 7XXX alloys microstructure. Obtaining FSPed material in full strength condition was a first achievement presented in this work. This specific FSPed material has a significantly higher ductility compared to the standard base alloy at equivalent tensile strength, which is a very interesting result of the present thesis. In situ X-ray tomography is used for revealing the changes in fracture mechanisms after FSP on 7XXX alloys. However, despite higher ductility, the FSPed 7XXX alloys do not show higher crack propagation resistance, which is an unexpected decoupling of the ductility versus toughness. Investigations were pursued for understanding better the damage mechanisms and the crack propagation behaviour of the manufactured 7XXX alloys microstructures. The understanding of damage sequence in 7XXX aluminium alloys is then transposed to a numerical model that simulates intrinsic components of the real microstructure in order to predict the material failure. This model offers an outlook for designing new 7XXX microstructures, reaching high strength, ductility and toughness simultaneously.(FSA - Sciences de l'ingénieur) -- UCL, 202