Strong in-plane anisotropy of creep ageing behavior in largely pre-deformed Al-Cu alloy: Experiments and constitutive modeling

Abstract

Deformation anisotropy is one of the key factors affecting the dimensional accuracy and performance consistency of components manufactured from aluminum alloys. Understanding and modeling their anisotropic creep ageing behavior is thus essential for achieving precise creep age forming of Al alloy components. Creep ageing responses along varying loading directions have been investigated in an Al-Cu alloy heavily cold-rolled by 82.6% at room temperature. Although the large pre-deformation imparts markedly enhanced creep formability and strength-ductility balance, significant in-plane creep anisotropy (with an index of 49%) and slight anisotropy of the obtained yield strength (with an index of 5.6%) are observed for the creep ageing at 140 ℃ under 150 MPa. The creep deformation decreases evidently with the increasing angle between the loading direction and the rolling direction. The microstructure (grain morphology, texture, dislocation and precipitation, etc.) of the largely pre-deformed Al-Cu alloy before and after creep ageing are characterized in detail by X-ray diffraction, Electron backscattered diffraction and Scanning transmission electron microscopy. The multi-scale examinations unveil a hierarchical structure consisting of flattened grains, dislocation sub-structures considerably elongated along the rolling direction and plate-like precipitates in the investigated alloy. This comparative analysis suggests the strong creep anisotropy is mainly determined by the preferential alignment of dislocation cells with a width of ∼500 nm rather than the texture and precipitation. By introducing an orientation-dependent creep resistance caused by back-stress of the elongated dislocation nanostructure, a mechanism-based constitutive model is established to accurately describe the in-plane creep anisotropy. This work also provides guidance for tuning the creep anisotropy in largely pre-deformed Al-Cu alloy.