Precipitate-assisted Effective Grain Refinement Of Al-Zn-Mg-Cu Alloys And Their Formability

High-strength7000(Al-Zn-Mg-Cu) series Al alloys, as the ideal candidate to replace heavier steel materials for automotive industry, cannot be widely used in structural materials due to their poor formability or corrosion resistance. Currently, much more attention has been paid to modify forming methods to optimize the formability of Al-Zn-Mg-Cu alloys, rather than their intrinsic microstructure and property optimization. The formability of5000(Al-Mg),6000(Al-Mg-Si) series Al alloys as well as BH (bake hardening) steels are enhanced by grain refinement, which may also contribute to improving the formability of Al-Zn-Mg-Cu alloys. To be specific, the grain refinement can be achieved through intermediate thermo-mechanical treatment (ITMT), during which larger MgZn2particles act as preferential nucleation sites (Particles-stimulated Nucleation, PSN) of new recrystallized grains. In present work, fine-grained Al-Zn-Mg-Cu alloys with superior ductility and formability were manufactured by two proposed short-cycled ITMTs (W-TMT and C-TMT) based on deformation-induced precipitation (DIP) and PSN. The optimizing principle for microstructures and processing techniques of Al-Zn-Mg-Cu alloys for automotive applications was also provided.The warm/cold deformation accelerated the precipitation and coarsening of MgZn2in solutionized AA7075and AA7050alloys, and more uniformly distributed MgZn2particles with diameter of0.5-0.6μm throughout the Al matrix were obtained compared to the one with only overaging. The precipitation of MgZn2was controlled by both thermodynamics and kinetics. Final rolled sheets were gained by W-TMT/C-TMT processing within1h, which makes grain refinement of age-hardenable Al alloys more time-saving than classical RI-ITMT that exceeds8h. Furthermore, the final recrystallized fine-grained structures of W-TMT and C-TMT samples were similar to those of RI-ITMT sample under similar total rolling reduction.Effects of several parameters on final recrystallized grain sizes and corresponding recrystallization mechanism were studied. Recrystallized grain sizes decreased with increasing MgZn2size, deformation energy, heating rate during recrystallization and decreasing initial grain size, respectively. However, the solution times hardly affected the final grain sizes. Full recrystallization of W-TMT processed sheet occurred with400℃/6s and480℃/2s treatment, but there was not enough time for the re-dissolution of large MgZn2particles, which can ensure the function of MgZn2as nucleation sites for new recrystallized grains. The PSN mechanism was as follows:large particles decrease recrystallization temperatures and shorten recrystallization times by intensely increasing the densities of nucleation sites for new grains. Furthermore, full recrystallized and fine-grained AA7050alloy was obtained easily by making use of large MgZn2particles, avoiding the conventional processes, such as unlimitedly prolonging the recrystallization times or increasing the stored deformation energy.The ultimate tensile strength or yield strength of W-TMT, RI-ITMT and HR processed7075samples in T6state were similar to each other, however, the total elongation of W-TMT samples was much higher than that of HR samples, indicating that grain refinement do play key role in improving ductility and eliminating strength/plasticity anisotropy while maintaining high strength. Similarly, fine-grained7050samples prepared by W-TMT and C-TMT possessed high strength and much higher elongation (>20%). Obviously, grain refinement contributed little to7000series Al alloys’strengths in T6state, but grain size affected their ductility markedly, and the highest elongation as well as work hardening ability were obtained in both7075and7050Al alloys with grain size of～9μm.For7075sheets, the formability of fine-grained W-TMT samples was similar to that of RI-ITMT samples. At200℃, the Erichsen value of W-TMT samples was10.7mm, which was much higher than that of HR samples (8.5mm). Grain size exerted similar influence on formability and elongation of Al-Zn-Mg-Cu alloys, and the grain size of～9um would be the excellent choice for these two key properties. Moreover, microstructures and properties of W-TMT samples after warm forming at200℃and paint baking were similar to those of retrogression and re-aging treated samples. As a result, high-strength and high corrosion-resistant drawn components were obtained after warm forming and paint baking of fine-grained7000series Al alloy, which makes it feasible to be used as structural component for automotive application.