| The deformation behaviors of Al-Zn-Mg-Cu-Cr alloy which is used as aerial structural materials were systematically studied in this paper, aiming to provide conferences for the hot machining process and processing parameter's optimization. Hot deformation tests of Al-Zn-Mg-Cu-Cr alloy were performed on Gleeble1500 thermal simulator to analyze the influences of deformation parameters on the flow stress of the test alloys. Optical microscope and transmission electron microscope were employed to study microstructure evolution of deformation alloy during hot compressing and annealing. The effect of cold-rolling on the mechanical properties and microstructures of Al-Zn-Mg-Cu-Cr alloy were investigated by the room temperature tensile test and electric conductivity measurement. Furthermore, the exfoliation corrosion behaviors and mechanism was researched with the aid of electrochemical technique and various microscopy analysis methods. The main results can be summarized as follows:1. The hot compressing results showed that the flow stress decrease with the increasing of deformation temperature and decreasing of the strain rate. The hot deformation mechanisms were mainly influenced by temperature. When the temperature is below 390℃, the dominative restoring mechanism is dynamic recovery. However, the working temperature is over 400℃, dynamic recrystallized occurred. To some extend the strain rate may have a profound effect on the substructure. With the increase of the strain rate the dislocation density decreases, while the sub-grain size decreases.2. The hot deformation activation energy of Al-Zn-Mg-Cu-Cr alloy was 192.6 KJ/mol, and the relationship of flow stress, deformation temperature and strain rate was expressed as follows:ε=e28.78[sinh(0.0128σ)]8.32 exp(-192.6/RT)The favorable hot processing temperature range is 380℃～400℃.3. Hardness measurements and microstructure evolution suggested that the recrystallized temperature of cold-rolled plate rolling about 50% reduction range from 390℃to 420℃and dominant mechanism for nucleation is subgrain boundary merging mechanism. The effects of different time at 400℃on annealing behaviors of the rolled plates were also studied. The recrystallization occurred at 20 minutes and completed within 60～80 minutes. With increasing the annealing time, the grain grew but stopped in 150minutes. In order to further understand the behavior of annealing, the recovery and recrystallization kinetics during annealing of the deformation Al-Zn-Mg-Cu alloy have been investigated, then the dynamic equations of them are obtained.4. The tensile and yield strength of rolled sheet of Al-Zn-Mg-Cu-Cr alloy increases with the increase of the degree of the cold deformation. As the rolling reduction was increased from 50 to 80 pct, the yield strength increased from 464MPa to 480MPa, while the electric conductivity value decreased from 39.9%IACS to 39.2%IACS. The TEM results showed that the main precipitations of the Al-Zn-Mg-Cu-Cr alloy wereη' phase andηphase after a two-stage procedure. The high deformation reduction is attributed to the increasing the volume fraction of precipitates and promoted the precipitation ofηphase at the grain boundary. In additaion, when reduction increased, grains in the samples were elongated with simultaneously increasing grain aspect ratios.5. The exfoliation corrosion behavior of Al-Zn-Mg-Cu-Cr alloy after cold rolling of various reductions was investigated by immersion testing and electrochemical technique. The results show that the exfoliation corrosion resistance decreased with increasing the cold rolling reduction. This was mainly attributed to the elongated grain morphology formed during cold rolling. Furthermore, continuous grain boundary precipitates also contributed to the increasing susceptibility to exfoliation corrosion. A phenomenological model for corrosion crack propagation was established. |
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