| Finite element numerical simulation is one of the most effective methods to verify the mechanical properties of materials under complex working conditions.The accuracy of the simulation is determined by the constitutive model.During dynamic loading,the mechanical properties of 7075 aluminum alloy are affected by the viscous effect,which causes the flow stress to vary with the stress jump.The existing constitutive models cannot accurately describe the mechanical properties of aluminum alloy under different strain rates.Therefore,it is necessary to develop a suitable constitutive model to study the mechanical behavior of materials under high strain rates.This article is based on the Frank-Read source dislocation theory,which reveals the mechanical behavior of materials at different strain rates from a microscopic perspective and constructs a constitutive model considering mechanism transformation and viscous behavior.The viscous behavior of 7075 aluminum alloy at intermediate and high strain rates was studied through compression experiments,and Abaqus was developed for secondary development to simulate cutting and cutting experiments to study the viscous behavior of materials at ultra-high strain rates.This paper presents the following research:1.Based on dislocation dynamics,the microscopic deformation mechanism of materials in plastic deformation was analyzed through theoretical derivation.According to the Frank-Read source dislocation theory,the control mechanism of different strain rates in plastic deformation was revealed,and a material viscosity behavior constitutive equation considering the plastic deformation mechanism transition was established.In addition,the relationship between strain rate and cutting speed was derived,which provided a theoretical basis for obtaining ultra-high strain rates in subsequent cutting experiments.2.The mechanical behavior of 7075 aluminum alloy was studied using the Hopkinson bar experiment at strain rates ranging from 700 s-1 to 15000 s-1 and temperatures ranging from 20℃to 500℃.The plastic deformation mechanisms and the transition of the mechanisms were investigated based on the stress-strain curves,and the experimental data were fitted to a constitutive equation for the viscous behavior.The study revealed that the dynamic deformation of the 7075 aluminum alloy exhibited strain rate effects,and the transition of plastic deformation mechanisms occurred between strain rates of 10000 s-1 to 13000 s-1,accompanied by a significant increase in yield stress.In addition,the 7075 aluminum alloy showed a clear temperature softening effect,with the flow stress initially decreasing slowly and then sharply as the temperature increased before stabilizing.The fitting results were also verified,demonstrating that the constitutive equation for the viscous behavior provided a good fit.3.Based on the secondary development of the Vumat subroutine in Abaqus,the relationship between the yield stress and the equivalent plastic strain was derived according to the theory of elastoplasticity.Microstructure models were constructed based on dislocation theory.The viscous behavior constitutive model and microstructure model were embedded into Abaqus,and the viscous behavior constitutive model developed by Vumat was compared and verified with the Johnson-Cook constitutive model provided by Abaqus.The results indicate that under the same conditions,the Mises stress and cutting force of the viscous behavior constitutive model and the Johnson-Cook constitutive model vary similarly over time.Furthermore,the maximum error in the simulation values of the cutting force for the two models at different cutting speeds is only 8.2%,which demonstrates the reliability of the Vumat subroutine.4.The parameterized modeling of the cutting simulation model was completed based on the Python language,and the temperature during the milling process was measured using the thermocouple method.Different cutting speeds(strain rates)were used to study the chip hardness,cutting temperature,stress,dislocation density,and grain size of7075 aluminum alloy using both simulation and cutting experiment methods,and mutual verification was carried out.The results show that the microhardness of the measured chips in the experiment and the dislocation density of the simulated chips have consistency in their distribution laws and changing trends with cutting speed.As the cutting speed increases,the dislocation densities and grain sizes in the first,second,and third deformation zones change differently,and the flow stress increases with the strain rate,while the cutting temperature increases first and then gradually stabilizes.In addition,the average temperature obtained from the simulation was compared with the temperature measured in the experiment,and the average error was 2.52%,which demonstrates the reliability of the simulation results. |
