Age forming is an advanced manufacture technology for forming large aluminum panels. Temperature, initial stress level and pre-strains have a great effect on the formability and performance. The stress relaxation aging behavior of AA7150-T7751 under different temperatures, initial stress levels and pre-strains was studied through stress relaxation tests, tensile tests and TEM observations. The results show that the formability can be improved with the increase of temperature, initial stress levels and pre-strains. Deformation mechanisms during stress relaxation of the material were analyzed on the basis of creep stress exponent and apparent activation energy. The aging precipitates of the studied alloy were not sensitive to the age forming conditions, but drastically coarsened at over aging temperature, which decreased the mechanical properties. In addition, the relationship between stress relaxation behavior and aging strengthening is discussed. Based on the dislocation theory and the modified Arrhenius equation, a stress relaxation constitutive equation considering the initial mobile dislocation density and temperature dependent activation energy was established. This model can predict very well the stress relaxation behavior under various temperature, stress level and pre-strain conditions, with an average error of 2%.

A set of creep aging tests at different aging temperatures and stress levels were carried out for Al-Cu-Mg alloy, and the effects of creep aging on strength and fatigue fracture behavior were studied through tensile tests and fatigue crack propagation tests. The microstructures were further analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that temperature and stress can obviously affect the creep behavior, mechanical properties, and fatigue life of Al-Cu-Mg alloy. As the aging temperature increases, the fatigue life of alloy first increases, and then decreases. The microstructure also displays a transition from the Guinier-Preston-Bagaryatsky (GPB) zones to the precipitation of S phase in the grain interior. However, the precipitation phases grow up and become coarse at excessive temperatures. Increasing stress can narrow the precipitation-free zone (PFZ) at the grain boundary and improve the fatigue life, but overhigh stress can produce the opposite result. In summary, the fatigue life of Al-Cu-Mg alloy can be improved by fine-dispersive precipitation phases and a narrow PFZ in a suitable creep aging process.

A novel methodology combining the macro- and micro-creep techniques was employed to study the effect of S′/S precipitate growth on the creep mechanism of an Al–Cu–Mg alloy. An AA2524 alloy was pre-aged at 180 °C to obtain S′/S precipitates with various sizes. The results showed that the precipitate size increased approximately linearly to ≈32 nm, ≈60 nm, and ≈105 nm after 3 h, 6 h, and 12 h of pre-aging, respectively. The growth of precipitate could significantly shorten the primary creep stage, despite the fact that the steady-state creep behavior was similar to that of the as-received alloy, as revealed by the macro tensile creep tests at 180 °C and 180 MPa. This led to a stress exponent (2.4–2.5) of the Al alloy with various precipitate sizes that was quite close to that of the as-received Al alloy, implying a steady-state creep mechanism dominated by grain boundary sliding and dislocation interactions. Finally, the micro-creep tests showed a minor role of the precipitate size on the steady-state creep mechanism, as evidenced by the similar strain rate sensitivity (0.0169–0.0186), activation volume (≈27 b³), and the results of a detailed transmission electron microscopy analysis of all tested alloys.

The creep aging behaviors of retrogressed Al-Zn-Mg-Cu alloy were studied by uniaxial tensile creep tests at 140 °C. The effects of creep aging time and applied stress on microstructures and properties of the studied alloy were investigated by using transmission electron microscope (TEM), hardness, and corrosion resistance tests. Results show that the effects of the creep aging process on microstructures and properties are significant. The size of matrix precipitate (MPt), distance between MPts, width of precipitate-free zone (PFZ), and distance between grain boundary precipitates (GBPs) increase with the increase of creep aging time and applied stress. With the increase of creep aging time and applied stress, the corrosion resistance of the studied alloy improved. After creep aging for 20 h, the electrical conductivity varied with different applied stress from 35.99% to 37.24% International Annealed Copper Standard (IACS), and the exfoliation corrosion (EXCO) resistance increased to the corrosion rating of “EB”, which express slight surface corrosion. Compared with the traditional retrogression and re-aging process (RRA), the retrogression and creep aging process (RCA) can increase the MPt size, widen the precipitates distribution, narrow the PFZ width, and enhance the corrosion resistance while offering the hardness comparable to that of the RRA process.

Creep aging behavior of a pre-strain and under-age treated 7B04 aluminum alloy (7B04-P for short) was systematically investigated under different temperatures and applied stresses. A lot of dislocation tangles and η’/GPzs were formed in the Al matrix of the 7B04-P al alloy. With the increase in temperature and applied stress, the total creep strain and steady-state creep rate increased significantly. However, the mechanical properties of creep-aged 7B04-P al alloy are sensitive to temperature rather than applied stress. The age-hardening precipitates and grain boundaries change obviously when the temperature rises to 160 °C. Compare to the low temperature (less than 160 °C) creep-aged samples, the size of precipitates is much larger, the width of PFZ is broader, and the grain boundary precipitates are more discontinuous and coarsen. As expected, the electrical conductivity is improved after the high-temperature creep aging process at 160 °C. Last but not least, the creep deformation of 7B04-P al alloy almost retains that of AA7B04-T7451. Meanwhile, the mechanical properties after the creep aging process of 7B04-P al alloy are better than that of AA7B04-T7451. It can be suggested that the novel high-temperature creep age forming of the thermo-mechanical treated 7B04 aluminum alloy can enhance the forming efficiency and comprehensive properties for aerospace industries.

Creep age forming (CAF) is a new technology developed for manufacturing large aluminum components in the aerospace industry. Aluminum–lithium alloys may be used in aerospace components because of their high modulus, specific strength and specific stiffness. Therefore, the creep deformation, mechanical properties and aging precipitation of Al-Li-S4 alloy under CAF conditions were studied. It was found that the creep behavior presents double steady state creep stages during the creep aging process. With the increase of stress level, the first steady creep rate increased, but the second steady creep rate was slightly reduced. Coincidentally, in the first steady state creep stage, the yield strength of the studied alloy also showed a slow increase stage. TEM observation showed that Al-Li-S4 alloy mainly contains two precipitation phases, T₁ phase and θ’ phase. A few precipitates form during the first steady creep stage. Then, a lot of nucleation and growth of T₁ phase resulted in rapid increase of yield strength. At the same time, the increase of stress level effectively inhibited the growth of T₁ phase, which resulted in these strengthening phases being more uniform, and thus improved the mechanical properties of materials. On this basis, the relationship between the multi-step behaviors of creep, mechanical properties and aging precipitates are discussed. It is considered that the main reasons for the multi-step phenomenon of creep and mechanical properties are strongly related to the nucleation, growth and distribution of T₁ phase.