| Superplastic forming can effectively solve the difficult problem of local large plastic deformation of alloy components.It is more and more widely used in aviation,aerospace,architecture,transportation,electronics and so on.Especially in the aerospace field,superplastic forming has become an indispensable processing means and has attracted universal attention all over the world.However,superplastic deformation mechanism has been a controversial topic for a long time.In order to further clarify the superplastic deformation mechanism and expand the application field of superplastic deformation,it is of great scientific significance and engineering value to study the superplastic deformation mechanism and characterization methods of alloys.Combined with the grain boundary slip mechanism,a theoretical model of stress-strain relationship during superplastic deformation at constant tensile speed is established in this paper.Based on the characteristics of superplastic tensile stress-strain curve,three new methods for predicting superplasticity are proposed:slope method,stress difference method and stress change factor.On this basis,taking 2A97 Al-Li alloy as an example,the superplastic deformation behavior,forming limit and cavity evolution of 2A97 Al-Li alloy at low temperature and high speed are studied,and the superplastic constitutive equation of 2A97 Al-Li alloy is established.Based on the grain boundary slip deformation mechanism,a theoretical model of superplastic deformation stress-strain relationship under constant tensile speed is proposed from the perspective of energy.The theoretical model contains two parameters that have an important impact on the Superplasticity of the material,namely,the grain boundary slip index,M,and the grain boundary slip resistance index,N.Combined with the theoretical model,the mechanical behaviors of superplastic deformation of DSS2507,SAF2205,TC4 and Mg-Li alloy are analyzed.The theoretical values are in good agreement with the experimental data.The results show that whether the material is in superplastic deformation state can be judged according to M and N-M.When M≥0.5 and M<N<Al0/4Ar0,it is in superplastic deformation state.M=0.5 is the threshold value for achieving superplastic deformation.The influence of M-value on the curve change trend runs through the whole superplastic deformation process.When N-M is a certain value,the stress-strain curves under different M-values have an intersection.Before and after the intersection coordinates,the influence of M and N-values on flow stress is opposite.When η+ε<1,the greater the M-value,the higher the flow stress during superplastic stretching,and the greater the N-value,the lower the flow stress;Whenη+ε>1,the greater the M-value,the lower the flow stress during superplastic stretching,and the greater the N-value,the higher the flow stress.According to the characteristics of superplastic tensile stress-strain curve,three new methods for predicting superplasticity are proposed:slope method,stress difference method and stress change factor.According to whether there is an inflection point after the peak strain on the engineering stress-strain curve of unidirectional tensile test,whether the material is superplastic can be judged,that is,superplastic material with inflection point and conventional superplastic material without inflection point.For duplex stainless steel,when superplastic uniaxial tension is carried out at a constant tensile speed,as long as the specimen is stretched to 500%,the elongation can be predicted according to the functional relationship between D(300,350)e and δ.Thus,the experimental cycle of high elongation materials can be significantly shortened.From an economic point of view,high temperature and low strain rate are the main limiting factors for superplastic application.Therefore,it is of great commercial value to study the deformation behavior and mechanism of low temperature and high speed superplastic deformation.Based on the demand for aluminum lithium alloy thin-walled complex parts in the aerospace field,taking the third generation high-strength 2A97 aluminum lithium alloy as the research object,this paper studies the microstructure evolution and deformation mechanism 2A97 under the superplastic deformation conditions of high strain rate(1×10-2s-1/450℃)and low temperature(390℃/3×10-3s-1).The results show that the flow strain stress curves under the above two deformation conditions are divided into three stages,and the proportion of each stage in the deformation process is different.The elongation of 2A97 Al-Li alloy can reach 760%under the high-speed superplastic deformation condition of 1×102s-1/450℃.Through the study of microstructure and texture evolution during deformation,it is found that dislocation creep is dominant in deformation stage I,resulting in the decrease of the proportion of small angle grain boundaries and the increase of texture at the same time.In deformation stage Ⅱ,due to high-speed deformation,there is not enough time to complete dynamic recrystallization,and the fairly strong texture and banded structure remain until the true strain is 1.39.Dislocation creep and coordinated grain boundary slip are the main deformation mechanism.In the third stage,there are enough large angle grain boundaries to support grain boundary slip,and the deformation mechanism is mainly grain boundary slip.The elongation of 300%can be obtained under the high-speed superplastic deformation condition of 390℃/3×10-3s-1.It is found that in deformation stage Ⅰ,the texture and grain boundary orientation difference increase at the same time,and dislocation creep and subcrystalline rotation are dominant.In deformation stage Ⅱ,dislocation creep coordinated grain boundary slip is the main deformation mechanism,which is manifested in the increase of m-value,the decrease of texture and the growth of sub grain.In the third stage,the deformation mechanism is mainly grain boundary slip.Therefore,dislocation creep and grain boundary slip can occur simultaneously under the experimental conditions studied.The proportion of deformation stage I in the whole deformation process is much larger than that in previous studies.Therefore,dislocation creep contributes greatly to superplastic deformation.Through the research work of this paper,a new stress-strain model for superplastic uniaxial tension at constant tensile speed is proposed.By studying and analyzing the characteristics of stress-strain curves of various materials,a method which can better characterize the superplastic properties of materials is proposed.The deformation behavior and mechanism of 2A97 Al Li alloy were analyzed by constant speed superplastic tensile test,and the above theory was verified. |
