The stress (from elastic to plastic) and temperature dependence of stress relaxation ageing (SRA) behaviour of an Al-Zn-Mg alloy, AA7B04-P, has been experimentally investigated in this study. A series of SRA tests have been carried out under various initial stress levels in both elastic and plastic regions and at different temperatures. Corresponding microstructural evolution during SRA has been characterised using transmission electron microscopy (TEM).It is found that increasing the initial stress and/or temperature enhance the stress relaxation in both elastic and plastic regions. The dislocation creep mechanism plays the dominant role at the investigated temperatures during SRA, with the stress exponent n ranging from 3 to 8, decreasing with increasing temperature. External stresses accelerate the coarsening of GP zones and η' precipitates and, when loaded to the plastic region, promote the formation of large rod-shaped η precipitates within 2 h of SRA tests, due to the high energy sites provided by dislocations from plastic loading. Yield strength shows a much higher sensitivity to the temperature than creep strain has during SRA tests. A temperature below 165 °C is suggested for SRA of AA7B04-P, so that a high stress relaxation level with less than 15% strength loss can be obtained after 16 h forming.

Creep age forming (CAF) of aluminium alloy 7B04 (AA7B04) stiffened plates has been experimentally and numerically investigated in this study. Creep-ageing experiments of AA7B04-T651 were conducted under different tensile stress levels at 140 °C for up to 20 h, and a set of unified constitutive equations was calibrated based on the experimental results of the evolutions of creep strain, yield strength and precipitate size, which was implemented into ABAQUS for CAF process modelling. CAF experiments and corresponding simulations of AA7B04 stiffened plates were then carried out and the effect of stiffener height and die radius on springback and yield strength was studied. It was found that the springback percentage decreases with increasing stiffener height and decreasing forming die radius, and the yield strength is slightly lower in the stiffener than in the skin of the CAFed stiffened plates due to stress effect on ageing progression. A good agreement has been achieved between experimental and corresponding FE results, with maximum deviations of 6.7% and 3.3% respectively for springback and yield strength.

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Unified creep-ageing constitutive equations for aluminium alloy 7050 (AA7050) determined from experimental data were implemented into the finite element (FE) code, ABAQUS, through the user-defined subroutine, CREEP, for creep age forming (CAF) process modelling. The outer contour and yield strength of a saddle-shaped component were predicted, and the accuracy of FE prediction was analysed with experimental results. The research results show that, stress relaxation and creep deformation occur mainly in the early stage of the CAF process. A close agreement has been achieved between the simulation results and measurement data for the formed shape. More than 90% of the area has a surface gap less than 0.5 mm, and the relative error of the yield strength is between -1.18% and +7.22%, which prove the validity of the numerical computation