| Creep aging forming technology can be used to accurately form high strength Al-Zn-Mg series aluminum alloy large components.It is an advanced manufacturing technology for lightweight panel components of aerospace equipment that needs to be researched and developed urgently.Creep aging forming was carried out under the coupling effect of heat treatment and creep stress,both of which have a significant impact on the evolution of creep behavior,microstructure,and mechanical properties.However,there are few studies on direct quasi-in situ observation of the microstructure evolution of Al-Zn-Mg alloys during creep aging and quantitative analysis of the strength evolution during creep aging.Therefore,this paper applies the creep aging test,microstructure observation,mechanical property test,and other test methods to study the effects of the initial heat treatment state to load stress state on the microstructure and mechanical properties of Al-Zn-Mg alloys during creep aging,the main research contents and results are as follows:Different initial heat treatment states have direct effects on the creep behavior and microstructure evolution of Al-Zn-Mg alloys.The initial creep strain of the sample in the T4 state reaches 0.49%,showing the best creep formability,followed by the T6 state reaching 0.18%,and the solid solution state is the lowest at 0.08%.Quasi-in-situ EBSD characterization of microstructures found that creep would increase the average misorientation within the grains,creep will increase the average misorientation within the grain,and different orientations increase in different proportions.However,the grains with the(001)orientation always show the highest increase ratio in different initial states;after creep aging,the proportion of low-angle grain boundaries increased by 15.5%for the solid solution sample and 8.8% for the T4 state,while T6 status dropped slightly,but remained largely stable;Short-time creep aging can promote dislocation movement,and the change law of dislocation density during creep aging is as follows: with the increase of the degree of precipitation in the initial state of the material,the dislocation density increases from 41.0% in the solid solution state to 34.1% in the T4 state,and the lowest is 11.2% in the T6 state.The magnitude of creep stress has a significant effect on the shape evolution of Al-Zn-Mg alloys during creep aging.The larger the creep stress,the higher the creep strain and creep rate.The mechanical properties of the alloy were sensitive to creep stress,and the creep-aged specimen at260 MPa always shows the best yield strength.TEM results show that theη’ phase is mainly in the crystal,and with the extension of creep aging time,the size of the intragranular precipitates increases continuously,and the volume fraction first increases and then decreases.EBSD results show that the distribution of geometrically necessary dislocations(GND)first decreases and then increases with the prolongation of creep aging time.Through the calculation of the yield strength contribution model,it is found that the contribution of solid solution strengthening and grain boundary strengthening contributions are maintained at around 82 MPa and 17 MPa,and the coupling effect of dislocation strengthening and precipitation strengthening is the main reason for the increase in strength.After creep aging,the widening of PFZ and the coarsening of GBPs are effectively suppressed,and the number of grains with a high average Schmid factor is higher than that of the artificially aged samples,which allows the material to maintain high strength while increasing elongation by 15%. |
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