Phase Field Modeling Of Microstructure Evolution During Hot Deformation For AZ80 Magnesium Alloy And Its Application

The components of magnesium alloy can significantly reduce the weight of the equipment,which has a good application prospect in the field of aerospace lightweight.AZ80 magnesium alloy has been widely concerned due to its excellent comprehensive performance.Owing to the hexagonal close packed(HCP)crystal structure with limited slip systems,magnesium alloys exhibit relatively poor mechanical properties at ambient temperature.Fortunately,grain refinement in thermoplastic deformation,such as isothermal forging,is an effective way to improve the microstructure of magnesium alloy and its mechanical properties.Considering the complex interaction mechanisms of grain growth,dynamic recrystallization(DRX)and second phase pinning during the isothermal forging of AZ80 magnesium alloy,it is urgent to grasp the mechanism of microstructure evolution.In this dissertation,the phase field(PF)model of microstructure evolution of AZ80 magnesium alloy during hot deformation was established by the means of experiment,theory and numerical simulation,which was applied to the optimization of isothermal forging process.The main research of this dissertation are as follows:Basing on isothermal compression experiments,the work hardening effect,processing parameter sensitivities and DRX behavior of AZ80 magnesium alloy in hot deformation were revealed.The results show that both work hardening and DRX softening show significant strain rate sensitivity and temperature sensitivity.Strain rate sensitivity coefficient becomes stronger due to intensive DRX and even-distributed particles.Deformation heat and second phase content effects on temperature sensitivity coefficient are critical.Based on the diffusion interface theory,the PF model of grain growth considering the pinning effect of the second phase was proposed.The results show that the number of order parameters(more than 36)and the value of second phase characterization factors have little influence on the simulation results.It can reflect the real symmetrical diffusion interface when the coefficient of coupling term is 1.5.Moreover,the small rod-like second phase with high content and uniform distribution has more remarkable grain boundary pinning effect.Compared with the experimental results,the reliability of the PF model in predicting the grain growth dynamics and its topological transformation,the second phase pinning and its Zener relationship was verified.The deformed grains,recrystallized grains and second phase were described by using different order parameters.The storage deformation energy related to dislocation activity and the pinning effect of the second phase were introduced.The PF model coupled grain growth,DRX and the pinning effect of second phase during hot deformation was established.The contribution of DRX softening and the Orowan mechanism was considered in the PF model.Combined with the experimental hardening rate curve and graphic tangent method,the judgment rules of DRX starting point and dynamic recovery steady stress value were determined.Based on the variation of flow stress and dislocation evolution criterion in the initial DRX stage,an experimental calibration method for the key parameters of nucleation rate model was proposed.The determination of the above model parameters correlates well with the stress response obtained from the thermal deformation experiment.Based on the topological generation method,the PF model of microstructure evolution of AZ80 magnesium alloy under hot deformation was established by embedding the initial topological structure of the characterization results.The PF model can well reflect the necklace structure characteristics of discontinuous DRX,the pinning effect of the second phase and its macroscopic mechanical response.Moreover,the macroscopic stress response is closely related to the content of second-phase particle.Compared with the second phase with high content,the low content of the second phase can promote DRX nucleation.However,the corresponding Orowan stress strengthening is lower than the DRX softening due to its weak ability of pinning grain boundaries.Based on the results of microstructural evolution predicted by PF model and the sensitivity analysis of process parameters,an optimized processing window with fine DRX grain was obtained for AZ80 magnesium alloy.The window can be applied to guide the multi-step variable speed process during isothermal forging process considering microstructure control.The fine-grained AZ80 magnesium alloy with high rib and thin web has been successfully obtained.The results show that the maximum yield strength and tensile strength of the component were 268 MPa and 342 MPa respectively,the average grain size was 9 ym,and the grain refinement degree was 77.5%.