Dynamic Compression Deformation Mechanism And Numerical Simulation Of AZ31 Magnesium Alloy

Due to the diversity and complication application forms of magnesium alloys in aerospace,automotive and 3C products,the performance requirements of magnesium alloys are changing from static or quasi-static to dynamic.The deformation mechanism of magnesium alloys under static or quasi-static conditions is quite different from that of dynamic conditions.At present,the main application forms of AZ31 magnesium alloy are as-cast and wrought state.However,the investigation of the dynamic deformation mechanism of these two states is not systematic enough,and no modified Johnson-Cook material constitutive relation can accurately reflect the dynamic deformation behavior of AZ31 magnesium alloy,including casting state and processing state,especially the adiabatic shear behavior.The as-cast state and rolling state with strong{0002}texture of AZ31 magnesium alloy are selected as a research material.The dynamic mechanical properties and the deformation mechanism of AZ31 magnesium alloy are systematic investigated in the present paper.A modified temperature term on original Johnson-Cook constitutive model is proposed to accurately reflect the material deformation behavior,including the as-cast state and wrought state.The modified Johnson-Cook?J-C?constitutive model and cumulative damage model are embedded into the Lsdyna software by the way of Key type of file.And the dynamic deformation process of AZ31 magnesium alloy is simulated numerically based on these two models,especially the adiabatic shear process.In order to verify the correction of the two models,a new type of stop ring is designed to capture the microstructure evolution of AZ31 magnesium alloy during the adiabatic shear conditions,which is benefit for analyzing and discussing the adiabatic shear deformation mechanism of AZ31 magnesium alloy.In this study,the samples are cutting from ND?normal direction?,TD?transverse direction?,RD?rolling direction?of hot rolling sheet and as-cast sample of AZ31magnesium alloy.The temperature range is 20-350?,and the strain rate range is0.001-2200s^-1.The microstructure is observed by optical microscope?OM?and electron backscattering diffraction?EBSD?.The deformation behavior,microstructure evolution and dynamic deformation and failure mechanism of AZ31 magnesium alloy under dynamic loading are revealed.It is found that the true stress-strain curves of the cylindrical samples of as-cast and as-rolled AZ31 magnesium alloy at room temperature can be divided into four stages,the elastic stage,strain hardening stage,thermal softening enhancement stage,unloading recovery or failure fracture stage.At the same strain rate the peak stress is decreased and the maximum strain is increased with the increasing of temperature.The anisotropy of AZ31 magnesium alloy rolling sheet is decreased with the increasing of temperature,while it is enhanced by the strain rate.With the increase of strain rate,the twins of as-rolled AZ31 magnesium alloy are gradually converged into the grain boundary area and a great amount of dislocations are piled-up at the grain boundaries.It is also found that the dynamic recrystallization mechanism of cylindrical samples of as-rolled AZ31 magnesium alloy is determined by deformaiton temperature,initial crystal orientation and loading direction.The Adiabatic Shear Band?ASB?of AZ31-ND is consist of a large number of dynamic recrystallization grains,a small amount of deformed grains and sub-structures.On both sides of the ASB,a large number of long needle-shaped{10-12}tension twins and a small amount of long needle-shaped{10-11}compression twins are located.Based on the Schmid Factor?SF?analysis,the activation of the deformation mode of AZ31-ND can be predicted from hard to easy as:{10-12}tensile twinning,{10-11}compression twinning,prismatic slip,pyramidal<a>slip,basal slip and pyramidal<c+a>slip.The adiabatic shear band of AZ31-TD are consist of a small amount of equal axisl grains,a large number of deformed grains and sub-structure.On both sides of the adiabatic shear band,the shape of the tension twinnings{10-12}and compression twinnings{10-11}are all in short needle-like shape.Based on the Schmid Factor?SF?analysis,the activation of the deformation mode of AZ31-TD can be predicted from hard to easy as:{10-11}compression twinning,prismatic slip,basal slip,pyramidal<c+a>slip,pyramidal<a>slip and{10-12}tensile twinning.Based on the deformation heat and environment temperature of magnesium alloy,the temperature softening term in J-C constitutive model is modified.In order to correcting the softening coefficient of J-C constitutive model,the environmental temperature is set as a variable factor.The constants in the modified J-C constitutive model and the cumulative damage model are solved by fitting the AZ31 magnesium alloy compression true stress-strain curves,including as-cast and as-rolled state.The accuracy of modified J-C constitutive model is verified by comparing the experimental data and the calculated data under same condition.By comparing the theoretical calculation with the measured results,it is found that the modified Johnson-Cook model can accurately reflect the alloy deformation behavior.When{10-12}tensile twinning is initiated,a slight error between the theoretical calculation and the measured results is still existed.Based on the modified J-C constitutive model and damage model,Ls-dyna software is used to simulate the dynamic compression stress state of the hat shaped samples of AZ31 magnesium alloy rolling sheet.The critical values of recrystallization and failure under various conditions are determined.It is revealed that at the initial stage of external loading,an"ear"plastic enhancement zone is formed at the corner of hat shaped sample.And with the increasing of the external loading,the upper and lower cornor of the hat-shaped sample are suppressed,the expansion process of the plastic region from both the up and low cornor to the middle,which connecting the two cornorsunder dynamic loading.The plastic work inside the material is converted into heat and then give rise to the temperature increasing,as a result,the adiabatic shear bandis formed.The numerical simulation results show that cracks occurred in the two corners of the hat shaped sample,the occurrence of cracks relieved the stress to a certain extent.As the load continues to increase the crack gradually from the corner to the middle of the expansion,leading to failure.