Investigation of large strain plasticity, strain localization and failure in AA7075-O aluminum sheet through microstructure-based FE modelling

Abstract

AA7075 is a precipitation hardening structural aluminum alloy, which has garnered considerable interest in automotive industry, primarily due its lightweighting capacity compared to many other aluminum alloys from 2xxx and 6xxx series. However, the damage evolution in AA7075 is quite complex due to the presence of different second phase particles in the microstructure and their contribution on damage evolution is largely unknown at large plastic strains. The common second phase particles are η precipitates, θ precipitates and Fe-rich intermetallic particles. The current work presents an extensive multiscale numerical framework, which in conjunction with complementary experiments, is applied to study strain localization, void nucleation, growth, and coalescence in a particle rich matrix. Experimentally, void nucleation is observed to be driven by particle decohesion and particle fracture. Nanoscale molecular dynamics (MD) simulation is carried out to estimate interface properties of the three distinct particle types. The extracted properties are used as input for real particle field 2D and 3D microstructure based finite element (FE) models. The stochastic nature of particle fracture is described using a Weibull distribution, while the effect of grains is incorporated in terms of their Taylor factors. Ductile matrix is described using the well known Gurson Tvergaard Needleman (GTN) void damage model. Complementary experiments included uniaxial tensile tests carried out in-situ in Scanning Electron Microscope (SEM) and X-ray Computed Tomography (XCT), ex-situ high resolution XCT and Electron Back Scattered Diffraction (EBSD) tests. The FE models with three distinct particle stoichiometries and three competing damage mechanisms, show good agreement with experimental observations. Particle fracture marginally dominates particle decohesion. At low plastic strains, void nucleation is initiated by decohesion and fracture of larger Fe-rich particles, which facilitate formation of localized deformation bands. At large plastic strain, elevated stresses within the localized bands facilitate decohesion and fracture of more resistant η and θ precipitates. Due to their inherent larger size and more irregular morphology, θ precipitates contribute to voiding more than η precipitates. Under uniaxial tensile loads, void growth takes place in the middle of the specimen, driven by higher triaxiality stress state in the middle, relative to the surface. Void coalescence occurs along deformation bands driven by higher stresses due accumulated plastic strain within the bands, in a process known as void sheeting.