Mechanical Behavior And Microstructure Evolution Of 7003 Aluminum Alloy Under Dynamic Load

Aluminum alloy is an ideal lightweight material for automobile due to its excellent energy absorption performance and high specific strength.In recent years,more and more high performance extruded 7000 series aluminum alloy profiles are applied in the safety components of car body.Under the requirement of car body strength and safety performance,the performance of such materials and components under impact condition is extremely critical.Although domestic and foreign scholars have studied the dynamic mechanical behavior and microstructure evolution of 7000 series aluminum alloy,but most of the existing research are mainly concentrated in the aerospace aluminum alloy of high strength and high alloy content,such as 7075,7050,7055 aluminum alloy,studies on the medium strength and low alloy content of7000 series aluminum alloys used in automobile bodies are still scarce and unsystematic.Therefore,it is an important demand of automobile engineering to study the mechanical behavior of 7000 series aluminum alloy profiles used in automobile body under dynamic load.The extruded 7003 aluminum alloys are studied in this paper,dynamic compression experiments are carried out by split hopkinson bar,the mechanical behavior of the alloys under different loading directions,loading conditions and heat treatment conditions are studied systematically,combination of OM,TEM and XRD techniques,the mechanical behavior and microstructure evolution mechanism of the alloys during the deformation are discussed,the main conclusions are obtained:As extruded 7003-t6 aluminum alloy profile has no obvious strain strengthening effect in different loading directions.When the strain rate increases in magnitude,the alloy has obvious positive strain rate effect and obvious mechanical behavior anisotropy.In higher strain rate condition,the yield strength and flow stress parallel to the extrusion direction are significantly higher than that of perpendicular to the extrusion direction.When the dynamic loading direction is parallel to the extrusion direction,the stress,strain and higher temperature rise concentration tends to occur in the area 45°from the axis of the specimen.When the dynamic loading direction is perpendicular to the extrusion direction,the distribution of stress,strain and temperature in the specimen are relatively uniform.The reason for the anisotropy of mechanical behavior is that the yield strength,strain hardening coefficient,strain rate sensitivity coefficient and the thermal softening index along the extrusion direction are greater than the loading direction perpendicular to the extrusion direction.The forming process of adiabatic shear band in extruded 7003-t6 aluminum alloy profile is the transformation process of deformation localization,deformation band and phase transformed band.The whole process is accompanied by the evolution of dislocation,grains and precipitated phases.In the process of grain evolution,the deformation of original grain,the separation of grain by the newly formed dislocation wall and the formation of dynamic recrystallization occurred in sequence.The evolution process of precipitated phase is that precipitation,transformation and coarsening take place successively in the matrix,the precipitated phase in the center of the adiabatic shear band dissolve.The temperature rise during the deformation prompts the formation of recrystallized grains in the adiabatic shear band,the transformation of deformation band into the phase transformed band and the dissolution of precipitated phase in the center of the adiabatic shear band.The texture evolution during deformation is also closely related to the formation of adiabatic shear band: with the formation and development of adiabatic shear band,shear texture and recrystallization texture are generated in the material.The adiabatic shear behavior of extruded 7003-T6 aluminum alloy profile is anisotropic,it’s caused by the difference of grain morphology and the anisotropy of mechanical behavior: the grain along the extrusion direction is fibrous,the grain size is not uniform in different directions,and the deformation of the grain is prone to uncoordination.Therefore,it is easy to form an adiabatic shear band.The grain perpendicular to the extrusion direction is equiaxed,the deformation coordination is easier.Therefore,it’s not easy to form adiabatic shear band.Compared with perpendicular to the extrusion direction,the strength of parallel to the extrusion direction is higher and the energy barrier to be overcome is lower,which also leads to the formation of adiabatic shear band more easily when the loading direction is parallel to the extrusion direction.The heat treatment condition affects the mechanical behavior and adiabatic shear behavior of extruded 7003 aluminum alloy.The stress,strain and temperature of adiabatic shear band in 7003-T6 aluminum alloy is higher than that of T4 temper.This results in the formation of recrystallized grains in adiabatic shear bands under the combination of heat and force in 7003-T6 aluminum alloy.Fine and uniform banded grains are formed under the action of strain concentration in the adiabatic shear band of 7003-T4 aluminum alloy.Compared with T6 temper,7003-T4 aluminum alloy has obvious strain-hardening effect.Under high strain rate loading,the adiabatic shear bands are formed both in T4 temper and T6 temper 7003 aluminum alloy,the dynamic recrystallization has occurred in the adiabatic shear band of 7003-T6 aluminum alloys,and the grains in the adiabatic shear zone of 7003-T4 aluminum alloys are uniformly striped.No matter in T4 temper or T6 temper,the adiabatic shear band is formed when the loading direction is parallel to the extrusion direction,the strain and strain rate of the 7003 aluminum alloy reach the critical condition.The critical strain rate and critical strain rate of the 7003 aluminum alloy in T4 temper is greater than that of T6 temper.That’s because the strain rate effect of 7003 aluminum alloy are not obvious,7003-T4 aluminum alloy has obvious strain hardening effect,the strain hardening effect of 7003-T6 aluminum alloy is not obvious.Therefore,under the same loading condition,compared with T6 temper,7003-T4 aluminum alloy is difficult to produce plastic instability,which leads to the need for larger strain rate and strain,and more thermal softening effect,so as to overcome strain hardening and strain rate effect and produce adiabatic shear band.