Effect of natural ageing time and storage temperature on mechanical characteristics and hemmability of a 6014 Al alloy

In this work, the effect of natural ageing on the hemmability of AA6014-T4 aluminium alloy was investigated near and beyond the nominal expiry date using a combined experimental-numerical approach. Tensile tests and hemming trials were performed on material stored under different conditions, including room temperature ageing and −20 °C storage, to quantify changes in strength and work-hardening response. True stress-true plastic strain curves were identified up to the onset of necking and used to calibrate isotropic hardening laws (Ludwik, Swift, Voce and Swift–Voce), with performance assessed over low- and moderate-strain regimes relevant to bending. A 2D plane-strain finite element model of the hemming sequence was then employed to analyse the strain distribution on the outer fibre of the bend for multiple hemming radii. The Swift–Voce formulation provided the most robust representation of the measured flow curves across ageing conditions. Numerical results show that constitutive changes driven by natural ageing can alter strain distribution and thus hemming robustness, highlighting the need for condition-specific material calibration in mechanical analyses of hemming. Room-temperature ageing increased the yield strength by up to approximately 10–15% relative to the expiry-date condition and promoted crack-related hemming ratings at small radii, whereas −20 °C storage kept the tensile response close to the expiry reference and preserved a larger crack-free hemming window.