Modeling of the damage mechanisms in AlMgSi alloys

With the growth in importance of the aluminium industry, has come increased
demand to invest into the quality improvement of the different aluminium based
hot extruded products. One of the main mechanisms, which can influence
deformation at high temperature within the 6xxx aluminium, is linked to the
presence of the AlFeSi intermetallic phases. These phases severely restrict
hot workability when present as hard and brittle plate-like precipitates
b-AlFeSi. Damage initiation occurs in these alloys by decohesion or fracture
of these intermetallic inclusions. The understanding and modeling of the
deformation and fracture behavior of aluminium alloys at room and at hot
working temperature is very important for optimizing manufacturing processes
such as extrusion. The ductility of 6xxx aluminium alloys can be directly
related to chemical composition and to the microstructural evolution occurring
during the heat treatment procedures preceding extrusion if proper physics
based deformation and fracture models are used. In this thesis, room
temperature and hot tensile tests are adopted to address the problem
xperimentally. The damage evolution mechanisms is defined at various
temperatures and a micromechanics based model of the Gurson type considering
several populations of cavities nucleated by different second phase particles
groups is developed on the basis of the experimental observations. This model
allows relating quantitatively microstructure and ductility at various
temperatures strain rates and stress triaxialities. Finite element simulations
based on an enhanced micromechanics-based model are used to validate the
model. Finally, the effect of some key factors that determine the
extrudability of aluminium is also discussed and a correlation between the
ductility calculations in uniaxial tension and the maximum extrusion speed is
developed for one defined profile.