Study On The Microstructure Of 7150 Aluminum Alloy Billets Regulated By Electropulsing And Their Creep And Corrosion Behavior

Creep aging forming technology is an advanced technology that utilizes the creep and aging strengthening properties of metals to synchronize the formability of materials,and is developed for the preparation of large thin-walled components.It has a broad application prospect in the development of large aerospace vehicles in China.During the creep aging deformation evolution process of high-strength aluminum alloys,represented by 7000 series aluminum alloys,the microstructure of the billets not only affects its creep deformation behavior,but also has an important impact on the comprehensive service performance of its formed parts,such as mechanical properties and corrosion properties.Therefore,a suitable microstructure of the billets is a prerequisite for shape and performance collaborative manufacturing of high-strength aluminum alloy large components.Most previous studies have focused on controlling the single microstructure state such as solid solution heat treatment to achieve second phase dissolution or pre-aging heat treatment to achieve fine second phase dispersion and precipitation,which cannot meet the requirements of comprehensive control of material multi-scale microstructure（grain structure,dislocation configuration,precipitation phase morphology,etc.）.Moreover,traditional heat treatment methods based on convective heat transfer have shortcomings such as low efficiency,high energy consumption,and complex processes.In this study,it is found that electropulsing treatment,as an efficient and energy-saving heat treatment method,can rapidly and simultaneously change the grain structure,dislocation configuration,and precipitate morphology of metal materials.Based on this,a new method for controlling the microstructure state of high strength aluminum alloy creep aging billets by electropulsing treatment is proposed.Taking 7150 aluminum alloy as the research object,a study is conducted to control the microstructure state of the billets by conducting different electropulsing treatment process（electropulsing cycle pre-treatment process,electropulsing pre-aging process,and electropulsing solid solution pre-treatment process）,with a view to identifying the electropulsing treatment process and the microstructure of the billets that achieve the synergistic evolution of creep aging deformation,which provide new methods and technical support for efficient and low consumption creep aging collaborative manufacturing of high-strength aluminum alloy large components.The main research content and innovative achievements of the thesis are as follows:（1）The effects of electropulsing cycle pre-treatment process on the microstructure and properties of 7150 aluminum alloy billets are studied.It is found that electropulsing cycle pre-treatment significantly promotes the formation of coherent GP zone and earlyη′phases.With the increase of electropulsing cycle number,the dislocation density and high angle grain boundary volume fraction of the sample increase,indicating that electropulsing cycle heat treatment promotes the generation of dislocations and the occurrence of recrystallization.When the cycle number increases to 10（ECHT2 sample）,the tensile strength of the sample is 592 MPa,the yield strength is 520 MPa,and the elongation is 12.7%,its tensile strength is similar to that of the As-received sample,and the elongation is higher than that of the As-received sample（9.6%）,that is,the electropulsing cycle pre-treatment process can achieve a synchronous increase in the strength and elongation of the sample.Through quantitative calculation of the strengthening mechanism,the grain boundary strengthening,dislocation strengthening,and precipitation strengthening values of the ECHT2 sample are 27 MPa,91 MPa,and 288 MPa,respectively.It can be seen that the strengthening mechanism of the electropulsing cycle heat pre-treatment sample is mainly dislocation strengthening and precipitation strengthening.（2）The effects of different electropulsing pre-aging process on the microstructure and properties of 7150 aluminum alloy billets are studied.The results indicate that when the aging temperatue increases to 115℃,135℃ and 155℃,the strength of electropulsing pre-aging samples is greater than that of furnace heat aging samples under the same aging temperature conditions.When the aging temperature increases to 175℃ and 195℃,the strength of the electropulsing pre-aging samples is lower than the strength of the furnace heat aging sample under the same aging temperature conditions.Compared with furnace heat aging at the same aging temperature,the average size and volume fraction of precipitates in the electropulsing pre-aging samples are larger than those in the furnace heat aging samples,indicating that the electropulsing pre-aging promotes the precipitation and growth of the precipitate phases,and the average grain size and texture strength of the electropulsing pre-aging samples are smaller than those in the furnace heat aging samples.At the same aging temperature,the activation energy of the electropulsing pre-aging samples is smaller than that of the furnace heat aging samples,and the electropulsing pre-aging treatment improves the atomic diffusion rate,thereby promotingη′/ηphases formation and growth.Additionally,the coupling effect of pre-deformation and electropulsing pre-aging promotesη′phases growth and dislocation annihilation.（3）The effects of electropulsing solid solution pre-treatment process on the microstructure and properties of 7150 aluminum alloy billets are studied.Through comparative analysis of microstructures under different electropulsing solid solution pre-treatment time,it is found that a suitable electropulsing solid solution pre-treatment time can enable the precipitation phase to undergo rapid dissolution at lower temperature.The electropulsing solid solution pre-treatment causes rapid recrystallization of 7150 aluminum alloy at lower temperature.The electropulsing solid solution pre-treatment reduces dislocation density while promoting the formation of regularly arranged and stable dislocation.Through qualitative analysis,it can be seen that compared with traditional solid solution treatment,electropulsing solid solution pre-treatment reduces the dissolution temperature of the precipitation phases,which leads to the dissolution of the precipitation phases at a lower temperature;In addition,electropulsing solid solution pre-treatment reduces the effective nucleation barrier of recrystallization,improves the nucleation rate of recrystallization,and promotes recrystallization.（4）The effects of electropulsing cycle pre-treatment,electropulsing pre-aging,and electropulsing solid solution pre-treatment on the creep aging behavior of 7150 aluminum alloy are investigated.The results show that the creep deformation and steady-state creep rate in the electropulsing cycle pre-treatment samples(after 10 cycles,the precipitation phases are GP zone andη′phases,with an average diameter of 5.6 nm and an average grain size of 18.2μm,and the dislocation density is 9.15×10¹⁴ m^-2)increase with the increase of applied stress,while the tensile strength of the electropulsing cycle pre-treatment samples decreases to 539.5 MPa,and its yield strength decreases to 494 MPa under high stress（350 MPa）conditions.In summary,it is difficult to achieve synergistic improvement of creep and performance through electropulsing cycle pre-treatment.The creep deformation and steady-state creep rate of the electropulsing pre-aging samples（after electropulsing pre-aging at 155℃,the precipitation phases are GP zone andη′phases,with an average diameter of 4.53 nm and an average grain size of 14.68μm）increase with the increase of applied stress,while the tensile strength decreases to 576MPa,and its yield strength decreases to 529 MPa under high stress conditions.Therefore,it is difficult for electropulsing pre-aging treatment to achieve synergistic improvement of creep and performance.The creep deformation,steady-state creep rate and strength of the electropulsing solid solution pre-treatment samples(after 20 s of electropulsing solid solution pre-treatment,the precipitatation phases are GP zone andη′phases,with an average diameter of 4.25 nm and an average grain size of 8.7μm,the dislocation density is 8.43×10¹⁴ m^-2)increase with the increase of applied stress,and the tensile strength increases to 604.4 MPa,the yield strength increases to 568 MPa,the elongation is 10.24%under high stress conditions.Its mechanical properties are significantly better than those of the As-received sample after creep aging,which is mainly attributed to the dislocation assisted nucleation and grain boundary assisted nucleation promoting the precipitation of fine phases and inhibiting their growth.In summary,the optimal microstructure of the billets（relatively small grains,regular arrangement and high stability of multiple dislocations,relatively small precipitates）obtained by electropulisng solid solution pre-treatment can achieve synergistic improvement of creep and performance.Further exploration was conducted on the corrosion performance of the electropulsing solid solution pre-treatment samples after creep aging.It is found that after 20 s of electropulsing solid solution pre-treatment and creep aging of 7150 aluminum alloy,the intergranular corrosion resistance and spalling corrosion resistance of the sample improve,mainly due to a smaller width of precipitation free zone,while an increase in electrochemical corrosion sensitivity,mainly due to a larger degree of lattice distortion.Increasing stress helps to improve the intergranular corrosion resistance and exfoliation corrosion resistance of the sample,and reduce its sensitivity to electrochemical corrosion.