Study On Viscosity Behavior Of 45 Steel During Dynamic Deformation

In the process of material deformation,the unique behavior of a certain aspect under certain conditions can generally be described by three ideal properties of elasticity,plasticity and viscosity.In the case of low and medium strain rates,only elasticity and plasticity are required to be considered in the response of materials.However,under the condition of high and ultra-high strain rates,the mechanical behavior of materials is significantly different from that under low and medium strain rates,so the influence of viscosity behavior should be considered.Because of its good cutting performance and mechanical properties,45 steel is widely used in industry,agriculture and national defense.At present,most of the research on 45 steel is limited to quenched 45 steel and medium and low strain rate loading,but there is a lack of research on the viscosity behavior of annealed 45 steel under high and ultra high strain rate conditions.Therefore,this paper analyzed the viscosity behavior of annealed 45 steel under high and ultra-high strain rate loading by combining compression experiment,cutting simulation and theoretical analysis.And from the microscopic point of view to analyze and explain the causes of viscosity behavior.Firstly,the development of dislocation theory is described based on dislocation dynamics.The causes of stress strengthening were analyzed from three aspects of strain rate strengthening,alloy strengthening and grain boundary strengthening.Based on the Frank-Read source proliferation model,the evolution mechanism of micro-dislocation in plastic deformation was analyzed,and the origin of plastic deformation and stress strengthening during deformation was explained from the microscopic point of view.In addition,the relationship between cutting speed and strain rate is deduced and the theoretical cutting force and cutting temperature are calculated.Secondly,the effects of strain,strain rate,temperature and loading history on the viscosity behavior of annealed 45 steel were studied in the range of strain rate(4×103/s～10×103/s)and temperature(20℃～650℃)by using a separate Hopkinson pressure bar device at room temperature and high temperature.And the parameters of Johnson-Cook constitutive equation were obtained by nonlinear fitting of the experimental data.In addition,the difference of viscosity behavior between annealed and tempered 45 steel was compared.The results show that annealing 45 steel not only has positive strain rate effect,but also has temperature effect.The effect of loading history on the viscosity behavior of annealed 45 steel is significant in the early stage and tends to be stable afterwards.The yield phenomenon,elastic modulus and real stress of 45 steel are affected by heat treatment,but the dominant mechanism is not affected,and all obey the thermal activation mechanism.Then,in order to investigate the viscosity behavior of annealed 45 steel at a higher strain rate(>104 s-1),high-speed cutting experiments were carried out to study the change laws of chip morphology,chip hardness and chip microstructure of annealed 45 steel under different cutting speeds(strain rates).The results show that in the range of 150.72 m/min～1507.2 m/min,with the increase of the cutting speed,the chip morphology will appear micro sawtooth,the critical speed is 753.6 m/min.The color of chips will have a yellow-blue-purple transition,which is due to the formation of ferric oxide on the surface of chips exposed to oxygen at high temperature;There is a positive correlation between chip hardness and cutting speed.Finally,two-dimensional cutting simulation was carried out in Abaqus finite element software to analyze the variation of shear stress and cutting force with the increase of cutting speed(strain rate)during the deformation process of annealed 45 steel.The accuracy of the simulation model was verified by comparing with the theoretical calculation of cutting force.In addition,the effects of tool rake angle and workpiece initial temperature on the cutting process are analyzed to determine the optimal cutting conditions.The results show that the stress is concentrated in the first deformation zone during the simulation process,and the shear stress increases with the increase of strain rate,and there is a positive strain rate effect.Under the condition of constant feed and cutting depth,the cutting force decreases gradually with the increase of cutting speed,which is consistent with the theoretical calculation.When the initial temperature of the workpiece is 200℃,the cutting force and the integrity of the machined surface are ideal,and the cutting is more economical.