Study On Superplastic Behavior And Cavity Of 7B04 Aluminum Alloy

7xxx aluminum alloys possess excellent and comprehensive mechanical properties.They are widely used in the aerospace industry and parts manufacturing.The parts used in aircraft have complicated structures and high requirements are brought on the strength and the formability of the materials.Research on the superplastic deformation mechanism and the laws of microstructure evolution can provide bases and references for hot working processes and controlling deficiencies.The thesis takes a 7B04 aluminum alloy as the research object.Sheets with fine and coarse grains(the average grain sizes are 8?m and 16?m)were chosen to conduct the high temperature tensile tests in different deformation conditions and different amount of deformation.The hot tensile deformation behavior was analyzed and calculated by the true stress-true strain curves.Superplastic deformation mechanism was obtained and analyzed by considering the microstructure evolution and the fracture appearances.Cavity behavior of the aluminum alloy was presented and analyzed.The main results are as follows:(1)The maximum elongation of 1663%and the optimal superplastic deformation condition 530?,3×10-4s-1 were obtained through the high temperature tensile tests of the fine grained sheet materials in different conditions.At certain temperatures,the elongation was gradually increasing with initial strain rate decreasing.The elongations showed little change with temperature changing at high initial strain rates.And the elongation was gradually increasing with temperature increasing at low initial strain rates.At certain temperatures,the peak stress was gradually increasing with initial strain rate increasing.At certain initial strain rates,the peak stress was gradually decreasing with temperature increasing.(2)The strain rate sensitivity index m was gradually increasing with temperature increasing.The maximum m was 0.63 at 530?.The values of deformation activation energy Q were 158.44 kJ/mol,188.13 kJ/mol,177.78 kJ/mol,250.54 kJ/mol at the initial strain rates chosen in the tests.A constitutive equation was established to describe the hot deformation process.R-W-S deformation mechanism maps at different temperatures of the alloy were established.The superplastic deformation mechanism in the experimental conditions of the 7B04 aluminum alloy was dislocation slip accommodated grain boundary sliding controlled by lattice diffusion.(3)It was found that the deformation induced grain growth existed in deformed section by analyzing the microstructures of several samples from fine grained sheet material with different amount of deformation.Equiaxial grains and arc-shaped grain boundaries could provide good conditions for the superplastic deformation.Substances like whisker existed on the fracture surface of high temperature tensile tests.Their content was increasing with temperature increasing and it was decreasing with initial strain rate increasing.It was indicated that liquid phase coordinated mechanism existed in the deformation process.(4)Cavity behavior of the fine grained and coarse grained sheet materials in the high temperature tensile tests was presented and analyzed.It was found that the average diameter of cavities was increasing linearly and the volume fraction was increasing exponentially.The development of cavities in fine grained sheet material was slower.The tolerability for cavities of the 7B04 aluminum alloy was about 16%.Cavities growth mechanism map was obtained.The converting radius between diffusion controlled and plasticity controlled cavity growth mechanisms is?1.64?m.