| High-strength aluminum alloys are widely used to manufacture key components in aerospace,high-speed railway,and other transportation equipment because of its a series of excellent characteristics,such as low density,high specific strength,high specific stiffness,excellent impact resistance,and high thermal conductivity.Once these components serviced under extreme conditions of high temperature,high pressure,and heavy load fail,a catastrophic accident is likely to occur.The performance of components materials usually determines their service life and performance.The existing strengthening-toughening methods of high-strength aluminum alloys mainly improve the strength of materials by introducing precipitates or dislocations,which are inevitable to reduce the plasticity of the materials.Most processes are faced with “strength-ductility contradiction”.Electromagnetic shock treatment(EST)can improve the strength and toughness of metal materials,and has the advantages of high efficiency and low energy consumption.However,the mechanism of the process is complex due to many physical fields coupling effects including electricity,magnetism,force,and heat during the treatment.At present,only a few phenomenological explanations are reported,which greatly limits its applications in industry.In this work,7075 high-strength aluminum alloy was adopted as the experimental material.The processes to improve the strength,toughness,and fatigue resistance of the alloys by EST were put forward.The evolution laws and mechanisms of the microstructure of the alloys during EST were revealed in this paper.The main contents are as follows:(1)Different EST processes are designed to adjust the mechanical properties of 7075 aluminum alloy.The double-stage electromagnetic shock treatment(D-EST)can improve the plasticity of the alloys at room temperature by 93% in only 10.2 seconds,and the strength and plasticity of T6 aged sample after D-EST are increased by 2.6% and 6.8% respectively compared with T6 aging after traditional heat solid solution treatment(SST).After EST,the plasticity of the pre-deformed 7075 aluminum alloy can be increased by 91.2% without reducing the strength of the material.In addition,the fatigue test and X-ray detection show that EST can also improve the low cycle fatigue life by 32.8% and reduce the residual stress of the pre-deformation alloys by 82.8%.(2)The microstructure of materials determines their properties.The microstructure evolution of 7075 aluminum alloy after EST was revealed by multi-scale microstructure characterization analysis.Compared with SST,it was found that D-EST could not only dissolve the precipitates,but also the rod-shaped dispersed particles and copper-rich intermetallic compounds in 7075 aluminum alloy.Furthermore,the targeted elimination of the 5° ~ 8° grain boundaries were observed after D-EST.By comparing and analyzing the microstructure of samples with and without EST,it can be concluded that EST mainly improves the plasticity of materials by inducing the uniform distribution of dislocations and promoting the formation of edge dislocations.(3)Aiming at the phenomenon of second phase dissolution caused by EST,a thermodynamic model for the dissolution of the second phase of 7075 aluminum alloy during EST was established.By solving this model,it can be concluded that the thermal effect reduces the free energy of the second phases in the alloys via increasing the material temperature.The non-thermal effect can also influence the free energy of the second phase to reduce the dissolution temperature of the second phase,and the reduction of the temperature is positive with the square of the increase of current density.The kinetic models of non-isothermal dissolution of the one-dimensional and three-dimensional second phase,based on Fick diffusion theory and electromigration effect,were established,and the model was solved by numerical simulation.The main driving force sources of different second phase dissolution during D-EST were analyzed,and the dissolution mechanism of D-EST was revealed.The results are consistent with the microstructure characterization.(4)Through the analysis of the combination of theoretical calculation and experimental verification,it is concluded that the existing mainstream views on the electroplasticity mechanism including electronic wind force,magnetoplastic effect,and thermal effect are failed to explain the phenomenon that the electronic current induces the dislocation evolution observed in this study.Based on quantum mechanics and dislocation dynamics,a potential mechanism,the cross slip of dislocations at the stacking site under the combined effects of electron-dislocations non-equilibrium scattering and internal stress,was proposed,which could reveal the mechanism of dislocation evolution of pre-deformed 7075 aluminum alloy induced by EST and the targeted elimination of the small angle grain boundary induced by D-EST.(5)The relationship between microstructure evolution and the strength and toughness of the materials was analyzed via the theories of strengthening and fracture,and the mechanism of EST to improve the strength and toughness of 7075 aluminum alloy was revealed.A microstructure design concept to improve the strength and ductility of metals via “flexible obstacles” was put forward.According to this concept,a combined treatment process of D-EST and EST was designed,which was able to quickly improve the strength and ductility of 7075 aluminum alloy.Compared with the T6 aging process,this process,only taking 2.5 hours of artificial aging,could improve the yield strength of the alloys by 9.8% without reducing the plasticity. |
PDF Full Text Request