An Anisotropic Behaviour Analysis of AA2024 Aluminium Alloy Undergoing Large Plastic Deformations

International audienceThe mechanical behaviour of metals during real forming processes must be related to their anisotropic properties. Concerning the analysis of the anisotropic behaviour of aluminium alloys this one has been the subject of various studies, generally in the field of sheet forming processes ((Malo et al., 1998), (Lademo et al., 1999)). In the last decades the majority of the fundamentals works search to define the mathematical description of the anisotropy starting from modified quadratic Hill criteria, non-quadratic ones such as Cazacu-Barlat (Cazacu & Barlat, 2001), Banabic (Banabic, 2002) or other sophisticated models synthesized in (Khalfallah, 2004). On the experimental point of view the many researches describe the rolled sheet properties of aluminium alloys (Choi & Barlat, 1999), (Li et al., 2004), (Park, 1999)) and report the anisotropic response during the mechanical deformation from uniaxial tensile tests, wire drawing or simple shear ones ((Fjeldl & Roven , 1996), (Hu et al., 1998), (Lloyd & Kenny, 1980), (Yonn, 2005)). Until now relatively few studies concerns the use of the channel die compression test, where deformation history is close of the principal sheet forming process such as the cold rolling one (Francillette et al., 1998). In this study an aluminium alloy (AA2024) is analyzed in order to characterize its anisotropy and its mechanical behaviour with this latter mechanical test. In a first part, the microstructure of the material is defined through optical and SEM microscopy, EBSD and X ray measurements. Micro-macro approaches will be used to valid the experimental measurements. Next, mechanical tests mainly the channel die compression one (see Francillette et al, 2003) and the tensile one are used in order to determine the macroscopic anisotropic behaviour of the material. A rigorous analytical model, able to describe the large plastic deformation of the material specimen which occurs during these experimental tests, will be developed. The main idea consist to define analytical equations which permits to compute the stress, the plastic strain rate and the cumulated plastic strain corresponding to a parallelepiped material undergoing a channel die upsetting loading. Final formula will be established to compute all the coefficients corresponding to a quadratic Hill criterion. A comparison with the well known computation model corresponding to the tensile test will be made. Next, a more general Hill criterion, taking into account variation of its coefficients with the plastic strain, will be analyzed. Starting from the previous mathematical description, a general methodology, able to identify rigorously all the parameters defining Aluminium Alloys, Theory and Applications. The laws of variation of the computed quantities with the plastic strain, will be presented. Finally an application for a plan and normal anisotropic formulation, corresponding to AA2024 aluminium alloy rolled plate, will be detailed. A new approach will be then proposed in order to predict the Lankford coefficient values and a validation will be made by a comparison of these ones with those obtained from classical tensile tests