Effect of precipitation state on plastic anisotropy in sheets of the age-hardenable aluminium alloys AA 6016 and AA 7021

Abstract

We analyse flow stress, work hardening and plastic anisotropy of two heat-treatable Al alloys of the AA 6xxx and AA 7xxx series in dependence on state and morphology of the age-hardening precipitates. Sheets in the solution heat-treated condition were subjected to various ageing treatments in order to vary the precipitation state while maintaining crystallographic texture and grain shape. Accordingly, differences in the anisotropy obtained for various temper conditions could directly be attributed to the precipitates. Plastic anisotropy was assessed from the in-plane variations in yield strength and plastic strain ratios (r values). The texture-induced anisotropy was further modelled with polycrystal-plasticity simulations; therewith, the anisotropy obtained in the solution heat-treated, i.e. particle-free, sheets could be explained. In alloy AA 6016 with shearable needle-shaped β′′-particles, the effect of precipitates on plastic anisotropy was negligible compared to the pervasive effect of texture. Alloy AA 7021 containing plate-like η′-particles with low aspect ratio displayed a noticeable weakening of the texture-induced anisotropy with progressing heat treatment and the resulting changes in precipitation state. This weakening of anisotropy was attributed to the development of non-directional back-stresses due to the storage of Orowan loops around the non-shearable η′-precipitates during the early stages of straining.