Warm Forming Behaviour, Ageing Response, and Microstructure-Formability Relationship of AA7075-T6 Aluminium Alloy

High-strength aluminium alloys of the 7xxx series, particularly AA7075-T6, present significant challenges during conventional cold forming due to their limited ductility, high flow stress, and pronounced springback, which restrict the manufacturability of complex structural components. In this study, a comprehensive experimental and analytical investigation is conducted to evaluate the warm forming behaviour and forming limits of AA7075-T6 by integrating tensile testing, hardness evolution, transmission electron microscopy (TEM), and forming limit diagram (FLD) analysis. Solution heat treatment (SHT) optimisation demonstrated that a holding time of 15 min at 477 °C is sufficient to achieve complete precipitate dissolution, yielding mechanical properties and hardness recovery comparable to longer treatments while substantially reducing processing time. Warm tensile tests performed over a temperature range of 100–180 °C and strain rates of 0.1–1.0 s−1 revealed a pronounced reduction in flow stress and enhancement in ductility relative to room-temperature forming. An optimal forming window was identified at 160 °C and 0.5 s−1, achieving up to a 28% reduction in ultimate tensile strength while maintaining high elongation. Strain-assisted ageing led to significant post-forming hardness enhancement due to dislocation-assisted precipitation, with peak hardness exceeding the original T6 condition under appropriate ageing temperatures. FLD results showed a clear expansion of the safe deformation domain with increasing forming temperature, indicating delayed strain localisation and improved biaxial formability. The experimentally determined forming limit curves were accurately represented using a shifted parabolic model implemented in MATLAB, enabling consistent quantitative comparison across different forming conditions. TEM analysis revealed deformation-induced microstructural evolution, including dislocation structures and precipitate–dislocation interactions, which directly govern strain redistribution and forming limits. The combined experimental–analytical framework establishes a clear structure–formability relationship and confirms that warm forming is an effective and industrially viable strategy for improving the formability of AA7075-T6 while preserving or enhancing mechanical performance.