| As the main structural material used in aerospace,aeronautical and ships field,titanium alloys have high specific strength,excellent corrosion and heat resistance.Because of the high deformation resistance and poor thermal conductivity,it is difficult to proform large and complex structural components with uniform microstructure and excellent performance by the conventional forming method.Therefore,superplastic forming is an effective approach to solve this problem.The main work and research results of this paper are as follows:Through isothermal constant strain rate compression tests at the temperature of800?950?and strain rate of 0.001 s-110 s-1,the influence of the hot deformation parameters on the flow stress of the hot rolled TC4 alloy was studied.The average hot deformation activation energy of the alloy was 403 kJ/mol under the experimental condition.The developed Arrhenius constitutive equation of the alloy considering the strain was established.The microstructure evolution law of the alloy during the deformation process was analyzed,and the critical strain models of dynamic recrystallization were established.By the processing map technology based on Dynamic Material Model,the hot processing performance of hot rolled TC4 alloy in the alpha and beta phase region was predicted,and the hot processing parameters of the alloy were optimized.The preferable processing region of hot rolled TC4 alloy is at the temperature of 830?880?and the strain rate 0.001 s-10.01 s-1,and the microstructures shows a large proportion of equiaxed alpha phase.The suitable processing regions are at the temperature of 800?830?and the strain rate 0.001 s-10.18 s-1,the temperature of 830?880?and the strain rate 0.01 s-10.18 s-1,the temperature 880?950?and the strain rate 0.001 s-110 s-1.The instable region of plastic flow is mainly at the temperature of 800?880?and the strain rate 0.18 s-110 s-1.Flow localization and macro shear crack are the instability defects of plastic flow.Submicron-grained TC4 alloy sheet was prepared by multi-pass warm rolling.The alpha phase was refined to nanoscale,and the beta phase was broken and exhibited uniformly distribution.The preferable temperature of multi-pass rolling is700?750?,and as the temperature increasing the alpha grain grows and the beta phase aggregates.At low temperature,the refinement mechanism of multi-pass rolled TC4 alloy is mainly dislocation refinement mechanism.At high temperature,dislocation and dynamic recrystallization lead to the grain refinement.The fabricated submicron alloys show good thermal stability and room temperature mechanical properties.The superplasticity of multi-pass rolled TC4 alloy was investigated by constant strain rate tensile test.Temperature of rolling deformation has an important influence on superplastic elongation.The optimum temperature was 700?,and the maximum elongation was 1550%at the superplastic tensile temperature of 800?and the strain rate of 0.001 s-1.At the low temperature of 700?,the elongation was 576%and 356%at the strain rate of 0.001 s-1 and 0.01 s-1,respectively.The enhanced superplasticity is related to the fine equiaxed alpha phase,broken and dispersed beta phase,high grain boundary dislocation density which formed by multi-pass rolling and the dynamic recrystallization during superplastic deformation.The activation energy of the superplastic deformation at temperature of 800?870?is about 223.5 k J/mol,which is closed to the self-diffusion activation energy of the grain boundary of TC4 alloy.The dominant superplastic deformation mechanism is grain boundary sliding accommodated by the diffusion of grain boundary and grain growth which is induced by deformation.At the temperature of700?,the activation energy increases to 377.5kJ/mol,which is far greater than the self-diffusion activation energy of the grain boundary,and the superplastic deformation mechanism is mainly grain boundary sliding controlled by dislocation motion.The mode of grain boundary sliding is synergistic sliding of grain cluster.During the superplastic deformation process,the grain boundary sliding leads to the formation and connection of the cavity,especially near the fracture surface of the specimen.Under different conditions,the fracture of the tensile specimen has an obvious macro necking,and the superplastic fracture is caused by the external geometric necking and the inner cavity. |
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