Secondary ageing and formability of an Al-Cu-Mg alloy (2024) in W and under-aged tempers

AA2024, a high-strength aluminum wrought alloy, is widely used in the production of aircraft components, for example, frames and stringers. While the alloy is relatively soft and formable in the solution heat treated and quenched state (W temper), rapid onset of natural ageing increases hardness and decreases ductility within few hours, creating a logistical challenge for the fabrication of frames by roll forming and bending, or other sheet metal forming operations. Here we investigate pre-ageing heat treatments of AA2024 with the aim of creating more stable and formable material conditions. The progression of materials properties after heat treatment is compared to natural ageing by extensive tensile test time series. An inhibition of secondary natural ageing by pre-ageing is demonstrated and a possible microstructural explanation is presented based on transmission electron microscopy and differential scanning calorimetry: S-phase is present in the pre-aged temper, and we suggest that the formation of Mg/Cu-clusters is suppressed. Formability is assessed by forming limit curves and finite element simulation of three-roll-push bending of frame profiles. We found improved processability of the pre-aged temper compared to a one week naturally aged condition: The forming limits are increased, and wrinkling in the bending simulations is reduced.