Modeling the Deformation of a Strongly Anisotropic Aluminum Alloy

The 6000-series aluminum alloys are generally used as structural materials in building structures and automotive parts where the engineering design requires the use of a light weight material. The credibility of any finite element analysis conducted for these materials depends critically on the constitutive models used to describe the plastic deformation and fracture of these alloys.

Extruded aluminum alloys typically have either recrystallized, equi-axed or non-recrystallized, fibrous grain structure. The accompanying crystallographic texture is typically a strong cube texture with a minor Goss component, or a deformation texture with a cube component in addition to components along the β-fiber (Frodal et al. (2017)). As a result of this strong texture, the major challenge in modeling the deformation response of extruded aluminum alloys is their strong anisotropy, where the flow stress, plastic strain and ductility depend markedly on the loading direction with respect to the principal axes of anisotropy (Fourmeau et al. (2011, 2013)).

In this paper, the plastic deformation up to failure of a strongly textured AA6060 alloy is modeled using an elastic-plastic model with yielding described by the orthotropic yield criterion of Cazacu (2018). This yield criterion is used in conjunction with a Swift-type isotropic hardening law. In order to ascertain the coefficients of anisotropy involved in the Cazacu (2018) criterion, we use a combination of the available mechanical tensile test data and results of virtual tests. More precisely, to obtain flow stresses for loadings where experimental data were not available, we use the recent polycrystalline model of Chandola et al. (2017) which in turn is based on the single crystal criterion of Cazacu et al. (2018). The capability of the macroscopic elastic-plastic model with yielding described by the orthotropic Cazacu (2018) yield criterion to account for the pronounced anisotropy of the AA6060 alloy is demonstrated through comparisons between the finite element simulations and the experimental data on axisymmetric tension test specimens for both smooth and notched geometries reported in Khadyko et al. (2015). It is shown that for the smooth specimen, the model correctly predicts that the minimum cross-section evolves from a circle to an ellipse while for a notched specimen of 2 mm notch radius, the predicted final cross-section is  rectangular in shape while for a more acute notched specimen (0.8 mm notch radius) the minimum cross-section evolves from a circular shape to an approximately rhomboidal shape, respectively, as was observed in the experiments. Thus, the Cazacu (2018) model predicts correctly the influence of the notched geometry on the deformation. Moreover, this elastic-plastic model can be easily implemented in finite element codes, requires less CPU time compared to crystal plasticity finite element method (CP-FEM) based analysis, and can be used routinely for detailed analyses of complex deformation processes.