Study On Macro-micro Deformation Mechanism Of 6008 Aluminum Alloy With High Strain Rate And Temperature Dependence

As one of the most commonly utilized and significant engineering alloys,aluminum alloys are widely employed in the structural design of automobiles,high-speed trains,and aircraft bodies owing to their excellent properties such as high specific strength and low density ratio.However,at present,the plastic flow response,constitutive equation,and microstructure evolution of aluminum alloys under different strain rates and temperatures and their coupling conditions need to be improved and systematically investigated.Particularly,from the standpoint of revealing the plastic deformability of materials or the safety design of practical engineering structures,it is necessary to characterize the mechanical properties of aluminum alloys over a wide range of temperatures and strain rates.The macroscopic impact compression and tensile mechanical responses of the 6008aluminum alloy are studied using an RPL-100 testing machine,Split Hopkinson pressure bar(SHPB),and Split Hopkinson tension bar(SHTB)at temperatures from 25 to 300°C and a strain rate of 2×10^-4–3.5×10³ s^-1.Furthermore,the microstructure of this alloy after compression and tensile loading over a wide range of temperatures and strain rates is observed and analyzed via optical microscopy(OM),transmission electron microscopy(TEM),and scanning electron microscopy(SEM),and the plastic flow behavior due to the change in microstructure is further characterized.The experimental results show that under the loading conditions of quasi-static compression and tensile strength at room temperature,the 6008 aluminum alloy can be considered to have no strain rate effect.However,under the loading conditions of higher strain rates and temperatures,the material exhibits obvious strain rate strengthening and temperature softening behaviors,accompanied by a clear adiabatic temperature rise.Under the loading condition of impact compression at room temperature,an increase in strain rate would lead to a surge in dislocation density,refinement and fragmentation of grains,and precipitation of second-phase particles.The coupling actions of these deformation mechanisms at high strain rates strengthen the material.With the increase in temperature,the escape rate of the atomic and softening rate of the second particle increase.When the temperature is increased to approximately 300°C,dynamic recovery occurs.Simultaneously,the slip systems increase owing to the weakening of the binding force between atoms,and grain boundary sliding is easy to carry out.The coupling actions of these deformation mechanisms at high temperatures soften the material.When the loading condition of the impact tensile strain rate exceeds 1450 s^-1,a large number of circular and oval dimples are observed in the center of the fracture,and a small proportion of shear planes are distributed concurrently.This shows that the impact tensile fracture behavior of the aluminum alloy is dominated by the dimple fracture mode and supplemented by the transgranular shear fracture mode.Based on the systematic macro and micro experimental research,three impact constitutive models of aluminum alloy are proposed from different perspectives of model construction to meet the development requirements of the impact constitutive model with high precision,few parameters,and rich physical meaning via the process of engineering numerical simulation.Based on the theory of dislocation dynamics and by fully considering the adiabatic temperature rise effect of materials during impact,a dislocation-related impact constitutive model of aluminum alloy is constructed.The model can accurately describe and predict the impact compression mechanical behavior of the 6008 aluminum alloy for a wide range of strain rate at room temperature,and it exhibits high computational efficiency,whereby theoretical support is provided for the development of an impact dynamic numerical simulation of this material.Based on the microstructure evolution,macroscopic strain rate strengthening,and temperature softening effect of the aluminum alloy during impact loading,the impact constitutive model of the dislocation density evolution of the aluminum alloy is further established.Considering the coupling effect of the strain rate and temperature on the material in the thermal activation stress,a new dislocation density evolution equation is proposed.The grain size effect of the material is considered in the thermal independent stress,which is in turn considered to be independent of strain and temperature.The results show that the model can accurately characterize the impact compression mechanical behavior of the 6008aluminum alloy over a wide range of strain rates and temperatures.Aluminum alloy,as a face-centered cubic crystal(FCC),can be regarded as a polycrystal material formed by multiple single crystals through a series of specific arrangements and combinations.As an important link between macroscopic deformation and microcosmic texture evolution of materials,the crystal plasticity theory reveals the mechanical behavior of crystal materials from the point of view of material grains.Based on the above assumptions,the plastic impact constitutive model of aluminum alloy crystal is established by considering the influence of the loading temperature and adiabatic temperature rise on the deformation mechanism of the material,and introducing the strain rate sensitivity function,.The numerical algorithm of the model and the method for obtaining the parameters are given.The calculation results show that the impact constitutive model is suitable for describing the mechanical behavior of impact compression and tension of 6008aluminum alloy over a wide range of strain rates and temperatures.It is particularly effective to describe the temperature softening phenomenon and strain rate sensitivity of aluminum alloys.