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%.

Extensive experiments have shown that stress-assisted nanograin rotation is an important grain-boundary-mediated deformation mode in nanocrystalline materials. The nanograin rotation is observed to has close correlation with the enhanced dislocation activity and plasticity in nanocrystalline metals and alloys. However, how nanograin rotation affects the dislocation behaviour remains unclear. In the present study, a theoretical model is proposed to investigate the dislocation emission behaviour in nanocrystalline face-centered cubic crystals as nanograin rotation dominates the deformation. The energy characteristics and the critical shear stress that is required to trigger the dislocation emission from grain boundaries are analysed. The results show that the nanograin rotation process can make the originally energetically unfavourable dislocation emission process favourable. The critical stress can also be extraordinarily reduced as compared with the rotation-free case. As a result, the proposed dislocation emission process can reduce the required external stress to almost zero if the rotation magnitude can reach 8.5 and 2.9 degrees for Al and Pt, respectively. The findings suggest nanograin rotation as an effective dislocation-emission mechanism in nanocrystalline materials and possibly explain the experimentally observed rotation-dislocation correlation in nanocrystalline materials.

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.

Al-Mg-Li alloy is an ideal lightweight structural material for aerospace applications due to its low density, high specific strength, and excellent low-temperature performance. This study examines the mechanical properties and microstructural evolution of Al-Mg-Li alloy subjected to cryogenic and room temperature cold rolling, which induces large plastic deformation. Compared with room temperature rolling, cryogenic rolling significantly reduces surface cavity formation, thereby enhancing the alloy’s rolling surface quality. After cryogenic rolling by 80% and subsequent natural aging, the yield strength of artificially aged Al-Mg-Li alloy reaches 560 MPa, delivering a 60% increase compared to the traditional T6 state with a slight reduction in elongation from 6.5% to 4.6%. The specific strength achieves 2.23 × 10⁵ N·m/kg, outperforming conventional Al-Cu-Li and 7xxx-series Al alloys. The depth of intergranular corrosion decreases from 100 µm to 10 µm, demonstrating excellent corrosion resistance enabled by the new method. Transmission electron microscopy reveals that finely distributed δ′ (Al₃Li) is the primary strengthening phase, with high-density dislocations further enhancing strength. However, coarsening of δ′ (from ~2.9 nm to >6 nm) induced by ensuing artificial aging results in coplanar slip and reduced elongation. Lowering the post-aging temperature inhibits δ′ coarsening, thereby improving both strength and elongation. Our results provide valuable insights into optimizing the properties of Al-Mg-Li alloys for advanced lightweight applications.