Research On Superplasticity And Cavity Behavior Of Al-Zn-Mg-Cu Alloy

7xxx group A1 alloy is a kind of super high strength A1 alloy.It has wide applications in aircraft and aerospace industry because of its high strength and excellent comprehensive performance.Based on 7050 alloy,we studied the effect of friction stir processing on the microstructure and mechanical properties of the materials.In addition,based on 7B04 alloy,we studied the cavity behavior during superplastic deformation.The conclusions are as follows:The grain size and mechanical properties of the original 7050-T7541 plate are obviously different along the thickness direction.The grain of stir zone is obviously refined after friction stir processing(original 7050-T7541 plate 25-300?m,after FSP 3?9?m),while the grain size decreases from the shoulder flow zone to the swirl zone along the thickness direction and the elongation increased significantly.The results of tensile test at high temperature showed that the superplastic properties of 7050 alloy were obviously improved after FSP.Based on uniaxial superplastic tension of 7B04 alloy,we studied the effect of initial strain rate and variable strain on cavity volume fraction respectively in 470??485??500?.We found that as temperature increased from 470? to 500?,the cavity volume also increased.As initial strain rate increased,the number and size of the cavity increased and the volume fraction reached the highest at 10-3s-1.The cavity volume fraction increased with strain almost exponentially.It is deduced that the cavity growth in 7B04 alloy is a cooperative control of diffusion and plastic deformation.Based on the effect of temperature,strain rate,strain as single variable on cavity volume fraction,we used mathematical method to summarize a cavity growth model.Considering the neglected factors in the design of the experiment,we introduced a correction function which makes the model more accurate and practical.According to the theoretical values of the model,the results are in a good agreement with the experimental values.Through microstructure analysis,it is found that not all particles and grain boundary junctions have voids.This indicates that in addition to the pre-existing voids,the void of new nucleation in the alloy is also present.Considering the grain boundary tension stress and the view of energy,the critical radius of void nucleation,the change of Helmholtz free energy in void nucleation site and the system.