Control Of Deformation And Property Of 7050 Aluminum Alloy During Creep Aging Process Assisted By Electric Pulse

Electric pulse assisted creep age forming(ECAF)is an advanced metal sheet forming technology developed from the creep deformation and aging strengthening behaviors of metals.Compared with the traditional creep age forming(CAF)process,it can further improve the creep deformation and shorten the aging strengthening time,which is expected to achieve the goal of integrated manufacturing of aluminum alloy integral panel by rapid and efficient forming and strengthening in aerospace field.However,the electric field,temperature field and stress field are coupled with each other in the process of ECAF,and there is an interplay of dislocation movement inside the material and the nucleation and growth of precipitates.Many interactions jointly determine the creep deformation and aging strengthening of materials,and it is difficult to obtain the desired shape size and excellent mechanical properties at the same time,that is,the coordination of shape forming and property tailoring is very difficult.In this work,to achieve the aim of rapid and efficient cooperative control of shape and property of 7050 aluminum alloy in ECAF,the experimental,theoretical analysis and numerical simulation research methods were used to study the evolution law of shape and property,and determine the process parameters The main research contents and results are as follows.The creep deformation and microstructure evolution of 7050 aluminum alloy during the whole process of creep aging assisted by electric pulse were obtained.Through the creep and stress relaxation experiments with or without electric pulse,it is found that electric pulse treatment promotes dislocation motion,reduces dislocation entanglement,accelerates precipitate phase precipitation and growth,improves dispersion degree,and exhibits more creep deformation and higher mechanical properties.Through the creep and stress relaxation experiments with different pulse parameters and application modes,it is found that under the same Joule heat condition,the high current density and the initial creep aging stage are more powerful.Based on the internal variable method,viscoplastic theory and metal strengthening theory,the relationship between macroscopic creep deformation,mechanical properties and microstructure was established by using three characteristic state variables of microstructure,namely,precipitate phase radius,precipitate phase volume fraction and dislocation density.A unified constitutive model for the whole process of electropulse-assisted creep aging of 7050 aluminum alloy was established.The unknown parameters in the constitutive equation were determined by using MATLAB genetic optimization algorithm(GA)toolbox and Romberg product formula fitting test data fitting.Through the secondary development of user subroutine CREEP and USDFLD in ABAQUS software,the strain-time curves of electric pulse assisted creep aging test and simulation of 7050 aluminum alloy are compared.The results show that the maximum deviation between the simulated values and the acquired values is 4.7%.The results show that the constitutive model has good prediction accuracy for creep deformation and microstructure evolution.The finite element model of electropulse-assisted creep aging forming of 7050 aluminum alloy sheet was established to simulate the shape evolution of 7050 aluminum alloy sheet during creep aging forming under different equivalent current density,charging stage,charging time and charging frequency of electric pulse,and to compare the residual stress,yield strength and resilience of sheet forming parts.The forming parts with stable shape,good mechanical properties and low rebound can be obtained by applying pulse current with equivalent current density of 1 A/mm2 and 0.5 A/mm2 for 1 h and 0.5 h respectively in the early and late loading stages.