Abstract
In order to obtain fine grained structure efficiently, a new multi-step rolling process (MSR: pre-deformation + intermediate annealing + hot deformation) was applied in Al-Zn-Mg-Cu plates. Conventional hot rolling (CHR) was also carried out as a contrast experiment. The evolution of microstructures and improvement of mechanical properties were analyzed by optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffractometer, and tensile tests. The results show that the MSR process can obtain finer longitudinal grain size and better mechanical properties than CHR, which can be explained as follows: spheroidization of precipitates wrapped by high density dislocations could be promoted by increased pre-deformation; numerous ordered substructures were formed during short-period intermediate annealing at high temperature; in the subsequent hot rolling process, the retained spherical precipitates pinned dislocations and boundaries. With the increase of accumulated strain, low angle grain boundaries gradually transformed into high angle grain boundaries, leading to grain refinement. With the increased pre-deformation (MSR1 20 + 60%, MSR2 40 + 40%, MSR3 60 + 20%), the effect of grain refinement and plasticity improvement gradually weakened. The optimum thermomechanical process (MSR1 solid solution + pre-deformation (300 °C/20%) + intermediate annealing (430 °C/5 min) + hot deformation (400 °C/60%)) was obtained, which can increase elongation by ~25% compared with the CHR process, while maintaining similar high strength for reduced longitudinal grain size.
Highlights
Aluminum has been widely used in aerospace, construction, machinery manufacturing, chemical industry, electrical appliances, and other industries [1]
Al-Zn-Mg-Cu alloys with excellent comprehensive properties, such as high strength-to-weight ratio, good toughness, and stress corrosion cracking resistance, has become one of the most important structural materials applied in aerospace, transportation, and other fields [3]
In order to make research meaningful, the initial alloy should return to the homogenization state before hot deformation, that is, the solid solution alloy with uniform microstructure and no segregation
Summary
Aluminum has been widely used in aerospace, construction, machinery manufacturing, chemical industry, electrical appliances, and other industries [1]. With the development of the industry, the requirements for strength, toughness, and light weight in aerospace are advancing to ever higher levels [2]. Al-Zn-Mg-Cu alloys with excellent comprehensive properties, such as high strength-to-weight ratio, good toughness, and stress corrosion cracking resistance, has become one of the most important structural materials applied in aerospace, transportation, and other fields [3]. The main alloying elements of 7xxx series high strength aluminum alloy are Zn, Mg, Cu, Zr, and impurity elements Fe and Si (Zn 7–12%, Mg wt% 2–3%). Increasing the content of Zn and Mg can improve the strength, but decreases the toughness and SCC
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