Microstructure heterogeneity effects on damage and fracture of 6xxx aluminium alloys

Hannard, Florent [UCL]

The ductility of Al alloys is dictated by the nucleation, growth and coalescence of small internal voids originating from intermetallic particle fracture and from the presence of pre-existing porosity. A low ductility adversely impacts both forming operations and the integrity of structural components. Ductile failure of the three aluminium alloys Al 6056, Al 6061 and Al 6005A has been characterized and modelled for a wide variety of heat treatment conditions. These alloys involve relatively similar composition and volume fraction of second phase particles but show major differences in terms of fracture strain for the same yield strength. The origin of these differences is unraveled by detailed characterization of the void nucleation, growth and coalescence process involving in situ 3D microtomography. The key element setting the fracture strain is the effect of particle size distribution and spatial distribution on the void nucleation and coalescence processes. Furthermore, the onset of yielding and strain hardening behaviour do not significantly depend on the loading direction for the three alloys. However, while the fracture strain is close to isotropic in the alloys Al 6061 and Al 6005A, the alloy Al 6056 exhibits clear fracture anisotropy. The propensity to fracture anisotropy is quantitatively linked to the degree of connectivity or percolation between clusters of second phase particles. Moreover, the fracture toughness of thin Al sheets is characterized by the Essential Work of Fracture (EWF) method. In situ synchrotron laminography combined with a correlation technique is also used to study the ductile damage micromechanisms and to measure the strain field in the immediate vicinity of the notch of CT-like specimens. This technique allows the study of the evolution and the interaction between microstructure, damage and plasticity. Finally, friction stir processing (FSP) is shown to be an efficient way to improve the fracture strain of the Al 6056 while making it more isotropic. From an applicability viewpoint, this method has the potential to locally improve ductility of sheets at locations where forming involves large strains. Sheet bending is used as a proof of concept and FSP is shown to improve the bendability and to reduce the bend limits associated with the anisotropy of the sheet.