Abstract
Pre-deformation prior to age hardening is an effective approach to increase strength in extruded 6xxx-series aluminum profiles. We systematically investigated how 0%–35% pre-deformation combined with artificial ageing affects mechanical properties, precipitate characteristics, and phase evolution in aluminum alloy 6061, with and without 0.1 wt% Zn. Adding Zn increased strength without reducing elongation in both the undeformed and the pre-deformed condition. For a given artificial ageing time and temperature, strength increased with deformation up to 15% strain before reaching a plateau. At higher strains, the additional work-hardening contribution could be counterbalanced by faster coarsening of the precipitates, resulting in an overaged microstructure. Transmission electron microscopy showed that Zn addition increased precipitate density without affecting size, consequently raising the volume fraction. Pre-deformation also increased precipitate density, but a decrease in size resulted in a similar volume fraction as the undeformed condition. High-angle annular dark field scanning transmission electron microscopy revealed a transition from needle-like ordered phases to a mix of structurally disordered needle-like precipitates and lath-like phases. Strength modeling without phase specificity reproduced the behavior of the Zn-free alloy, including a 60 MPa contribution from work-hardening after recovery. While pre-deformation was shown to slightly reduce precipitate strengthening, work-hardening fully explained the net increase in strength.