Mitigating creep anisotropy of largely pre-deformed Al-Cu alloys by shape tailoring of dislocation sub-structures

Abstract

Introducing high-density dislocations by large pre-deformation could afford a significant increase in both the creep formability and mechanical properties of aluminum alloys. However, the strong creep anisotropy typical of the alloy sheets prepared by this method, e.g. cold-rolling with a large thickness reduction, leads to the difficulty in accurate creep age forming of doubly curved panels. Clarifying the influence of different types of microstructures on the creep deformation is pivotal in tailoring the creep anisotropy of largely pre-deformed Al alloys. This study investigated the creep aging responses of largely pre-deformed Al-Cu alloys prepared using two different processes, i.e. unidirectional rolling and cross-rolling, with a same total thickness reduction of 80%. Cross-rolling was applied by changing the rolling direction by 90° about normal direction in the second step. The in-plane creep anisotropy index of the sample prepared through the 3:1 (ratio of the reductions in two steps) cross-rolling scheme is about 13%, much lower than that (about 49%) of the unidirectional rolling sample. The as-rolled and creep-aged samples for unidirectional rolling, 1:1 and 3:1 cross-rolling exhibited similar tensile properties and had a yield strength anisotropy index less than 6%. Detailed characterizations by electron back-scattering diffraction (EBSD) and scanning transmission electron microscopy (STEM) reveal that cross-rolling mainly changes the dislocation substructures rather than the grain orientations and precipitates in the largely pre-deformed alloys. The dislocation cells with a diameter ranging from 400 nm to 1.8 μm changed from the "elliptical" shape in the unidirectional rolling sample to a "circular" shape in the 3:1 cross rolling sample. The crystal plasticity finite element models considering the effect of dislocation substructures were established to simulate the creep deformation in the largely pre-deformed Al alloys. Multi-scale experimental characterizations and crystal plasticity finite element simulations demonstrate that the texture and grain shape have little influence while the dislocation substructure plays a dominant role in the creep anisotropy in the largely pre-deformed Al-Cu alloys. Our findings inspire alleviating creep anisotropy of largely pre-deformed Al alloys by shape tailoring of dislocation sub-structures.