Effects of texture gradients on yield loci and forming limit diagrams in various aluminum-lithium sheet alloys

Abstract

Marked through-thickness variations of preferred crystallographic orientations in aluminum-lithium (Al-Li) sheet alloys have been observed and documented. These metallurgical features could have an effect on the way in which these materials distribute strain during plastic deformation. From a theoretical or a practical point of view, it is important to investigate these texture effects on plastic-deformation properties and particularly on forming limit strains. In this work, quantitative texture data, which were determined by X-ray and neutron diffraction techniques, were used with a polycrystal model to predict the yield locus of recrystallized and unrecrystallized AA8090 and AA2090 Al-Li sheets. The conventional AA2024 alloy in the annealed condition was also investigated as a reference material. Subsequently, these yield loci were used to calculate forming limit diagrams (FLDs) in the stretching range, using the Marciniak-Kuczynski (M-K) approach with strain rate potentials to describe the constitutive properties of the sheets. A simple critical-thickness-strain criterion was used to predict the FLD in the drawing range. The predicted FLDs were found to be in fair agreement with experimental curves obtained under punch-stretching conditions. In general, experimental trends were accounted for by the results predicted using the average texture data. However, the texture gradients do not completely explain the large scatter observed in the experimental forming limits and the high average limit strain of the recrystallized AA8090.