| The 7-series aluminum alloy prepared by spray deposition has the characteristics of short process,flexible composition adjustment,uniform equiaxed crystal structure and good comprehensive properties.It is of great significance for the application and popularization of this material to study the evolution of its microstructure during hot working and determine the best hot working process range.In this study,Gleeble-3500 hot compression tester was used to study the rheological behavior of Al-5.8Zn-1.6Mg-2.2Cu aluminum alloy prepared by spray deposition under different deformation conditions.The influence of strain rate and temperature on rheological behavior is studied,and the rheological stress constitutive equation and hot working diagram of aluminum alloy are constructed by linear fitting.SEM,TEM,EBSD and other characterization methods were used to analyze the evolution of microstructure and the change of properties of the material,which provided theoretical basis for the optimization of hot working process and the improvement of properties of the material.The main conclusions are as follows:The rheological behavior of Al-5.8Zn-1.6Mg-2.2Cu aluminum alloy prepared by spray deposition process under different deformation conditions was studied by Gleeble-3500 hot compression tester.The results show that the stress increases with the increase of strain rate,and the rheological stress decreases with the increase of deformation temperature.At the same time,the constitutive equation of Fiels-Backenfen flow stress of spray-deposited aluminum alloy at different temperatures is:σ=Cεnεn.The calculated stress peak value is compared with the actual peak value,R2=0.953,and the correlation coefficient is large,which is in good agreement and verifies the accuracy of the model.The hot working diagram of Al-5.8Zn-1.6Mg-2.2Cu aluminum alloy prepared by spray deposition process is drawn.The optimum hot working temperature is 613K-643K and the strain rate is 1 s-1-10 s-1.The strain rate is 713K-723K at higher hot working temperature,and the optimal strain rate is 0.1s-1.Based on the established material constitutive equation,the thermal compression process is numerically simulated by finite element method.From the microstructure diagram,it can be seen that the center position of the sample has obvious changes in the grain length,and the grain structure in the corresponding area is relatively fine.The periodic fluctuation in the stress-strain curve is caused by the joint action of work hardening and dynamic softening.Dynamic softening is the process of discontinuous dynamic recrystallization(DDRX).Because of the uneven deformation and the formation of orientation gradient near the original grain boundary,the grain boundary appears obvious bowing phenomenon,which leads to the grain boundary moving to the side with high dislocation density,resulting in the formation of recrystallization nuclei,which reduces the dislocation density in the grain and releases the stored energy,which increases the softening effect and makes the flow stress curve decrease.Work hardening is dominated by the mechanism of continuous dynamic recrystallization(CDRX),that is,with the continuous deformation,the newly generated grains will undergo work hardening again.In this way,there will be periodic fluctuation pattern and sawtooth fluctuation on the stress-strain curve.The compressive samples were characterized by backscattered electron diffraction.When the temperature was kept at 723K and the strain rate increased from 0.1s-1 to 10s-1,the recrystallization volume fraction in the alloy increased from 14.69%to 27.3%.KAM diagram analysis shows that the increase of dislocation density increases the volume fraction of dynamic recrystallization.The Schmidt factor is large,which shows that the grain migration in the alloy is always going on during compression,and the orientation of most grains is beneficial to slip.The main factors leading to dynamic recrystallization are the increase of HAGBs and the slip of dislocations.According to the results of XRD,SEM and TEM analysis,it is known that the second phase particles dispersed in the matrix are MgZn2 phase,and the transition phase between the second phase particles and the matrix is Al15Mg11Zn2 phase.When the strain rate is constant at 1s-1,the necklace-like structure in the microstructure increases significantly and the recrystallization is more complete.The orientation of the newly generated grains is quite different from that of the original grains,from<001>and<101>parallel to the compression direction to<111>parallel to the compression direction.Continuous dynamic recrystallization occurs strongly at low temperature,which leads to the merger and growth of subgrain.Discontinuous dynamic recrystallization occurs very strongly at high temperature,which leads to obvious bowing of grain boundaries,that is,the grain boundaries have obvious protrusions.Deformation temperature and strain rate have significant effects on the microstructure of Al-5.8Zn-1.6Mg-2.2Cu aluminum alloy prepared by spray deposition process.The dislocation density,recrystallization behavior,grain orientation,large and small angle grain boundaries and second phase particles in the microstructure determine the properties of the material.This study provides a micro-analysis and theoretical basis for other hot working treatments using spray-deposited high-strength aluminum alloy. |
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