Study On Experiment-based Constitutive Model Of Magnesium Alloy And Its Application In Numerical Simulation Of Rolling Process

This research is supported by a sub-project under the National Key Technology R&DProgram (No.2012BAF09B01): Research on key technology of magnesium alloy furtherprocessing and development of large equipments. Different constitutive models, such asArrhenius hyperbolic sinusoidal flow stress constitutive equation, flow stress constitutiveequation with a constant strain-softening factor, anisotropic constitutive model undernon-associated flow rule and anisotropic-asymmetric constitutive model were built based onthe tension and compression stress-strain data of the AZ31magnesium alloy at varioustemperatures, different strain rates and along different directions. The deformation rule ofspread and head warping of the hot-rolled magnesium alloy sheet were analyzed bynumerical simulation and experiments to instruct the design of magnesium alloy rollingprocess and control the quality of the sheet forming.The constitutive model of the magnesium alloy is the basis of the numerical simulationof rolling process. An overview of the relevant research achievements on the numericalsimulation technology of the magnesium alloy hot-rolling process was provided, applicationof the anisotropic constitutive theory in metal forming was summarized, features of thecommonly used constitutive model were discussed. Under the condition that the range oftemperature is25℃~375℃and strain rate changes from0.001s-1s to0.1s-1s, uniaxialtension and compressive tests were carried to study the tensile and compression mechanicalproperties of AZ31magnesium alloy at various temperatures. Results indicates thattemperature and strain rate has a great effect on the flow stress of AZ31magnesium alloy,and the difference of the initial yield stress between tension and compression stage undertemperature of25℃is large. The difference decreases as the temperature rises, but thehardening relationship between two states still has a big difference. Besides, uniaxial tensionand compression tests along each direction of the rolling field and vertical direction werecarried out to obtain anisotropy in the tensile and compressive yield stress and the plasticstrain ratio of AZ31magnesium alloy. The experimental result shows that the rolling AZ31magnesium alloy has strong stress-strain anisotropy effect.Arrhenius hyperbolic sinusoidal flow stress constitutive equation was built to generallyconsider the effects of deformation temperature and strain rate on peak stress during theprocess of the magnesium alloy deformation at high temperatures, while the flow stressconstitutive equation with a constant strain-softening factor reflects the effects on the wholestress-strain behavior, and the simulation results of both the equations correspond well withthe experimental data. The coupled thermo-mechanical finite element method was used to analyze the temperature field of the rolled piece, the roller and the conveyor roller, thecomparison between the simulation results with the experimental warping shows thefeasibility of the coupled thermo-mechanical finite element method in predicting thewarping.Because the anisotropic property in stress and strain of AZ31magnesium alloy was nottaken into consideration in the the flow stress constitutive equation with a constantstrain-softening factor, so the error of the simulation is unavoidable, this paper introducedthe anisotropic constitutive model in the simulation of the rolling process of magnesiumalloy. According to the compressive yield stress and the plastic strain ratio of AZ31magnesium alloy along different directions in the plane of rolled piece and the verticaldirection, the three-dimensional Hill48and Hu2005constitutive models undernon-associated flow rule were established. Compared with the experimental data and theYld2004-18p constitutive model which has18anisotropic parameters, the result shows thatthe constitutive models under non-associated flow rule could accurately predict thecompressive yield stress and plastic strain ratio of AZ31magnesium alloy at each direction.The non-associated Hill48and Hu2005constitutive models were implemented into auser-defined material subroutine (VUMAT) to simulate the rolling process of AZ31magnesium alloy at constant temperature in Abaqus/Explicit. The simulation predicted thespread of the magnesium alloy bar. The simulation result is very close to the experimentresult. Thus the accuracy of the constitutive models under non-associated flow rule isverified.According to the tensile and compressive yield stress and plastic strain ratio alongdifferent directions in the plane of the rolled AZ31piece and the vertical direction, atension-compression asymmetric&anisotropic constitutive model of CPB06ex2under thethree-dimensional stress state was built. The result shows that the predicted head warpingwith the tension-compression asymmetric&anisotropic constitutive model of CPB06ex2matches well with the measured data. Then, the effects of speed ratio of the upper roller tothe lower roller, the rolling speed and the friction condition between the rolled sheet and theroller surface on head warping of the rolled AZ31sheet were analyzed. The achievements ofthis thesis have significant meaning in improving the precision of rolling simulation and thequality of magnesium alloy sheet metal.