| In recent years,driven by the demand for energy conservation and emission reduction and the trend of lightweight,magnesium alloy parts prepared by semisolid forming technology are widely used in the fields of automobile,aerospace,information electronics,military industry and national defense because of their advantages of low density,high specific strength and high specific stiffness,low elastic modulus,good vibration damping,high thermal conductivity and excellent mechanical properties,At the same time,semi-solid forming technology has the advantages of dense internal structure,good mechanical properties,long mold life and energy saving.However,due to the controversy on the formation mechanism of non dendritic structure in the process of preparing semi-solid slurry by mechanical stirring,there is a gap in the numerical simulation of grain growth in the process of preparing semi-solid slurry by mechanical stirring.Therefore,the establishment of grain growth model suitable for the preparation of semi-solid slurry by mechanical stirring is of great significance to study the semi-solid forming theory,predict the semi-solid microstructure and optimize the semi-solid preparation process.Taking the semi-solid microstructure of AZ91 D magnesium alloy prepared by mechanical stirring as the research carrier,a single grain growth model suitable for the simulation of semi-solid grain microstructure evolution of magnesium alloy is established by coupling phase field method,lattice Boltzmann method and rigid body rotation equation.Then the orientation field is introduced to establish the multiple grains growth model of semi-solid microstructure of magnesium alloy.The growth law and evolution mechanism of semi-solid microstructure were deeply studied.In addition,combined with the semi-solid microstructure experiment prepared by mechanical stirring method,the grain growth behavior and morphology evolution are further studied from the perspective of experiment,and the experimental results are compared with the simulation results to further verify the reliability of the simulation results.Finally,based on neural network algorithm and multiple regression analysis,a grain morphology prediction model is proposed.The single grain growth process of AZ91 D magnesium alloy under different flow fields was simulated by using the semi-solid single grain growth model.The results show that the grain shows six fold symmetric anisotropy under pure diffusion,and the grain is typical ‘snow-shape crystal’.Under the condition of uniform intensity convection,the upstream side has a faster growth rate and the secondary dendrite arm is thicker,while the downstream side grows slowly.Under the action of shear flow,when the stirring rate is low,the overall grain morphology presents a ‘rose-shape’,while at high stirring rate,the grain morphology is spherical or nearly spherical.The effects of different parameters on the semi-solid grain morphology of magnesium alloy under shear flow conditions were studied.The grain size was large and the growth rate was fast at low solidification temperature,while the branching and splitting phenomenon decreased and the grain size was small at high solidification temperature.Increasing the shear flow intensity will inhibit the growth of dendrite arm and reduce the tip splitting of main dendrite arm.Increasing the shear rate will accelerate the grain rotation,and then reduce the asymmetry of dendrite arm growth caused by forced convection.Under the influence of high shear flow intensity and high shear rate,the grains show a nearly spherical morphology.Analysed of the flow field distribution around the grain under shear flow,the forming mechanism of the grain in the semi-solid structure prepared by mechanical stirring is studied.The results show that the forced shear flow caused by mechanical stirring makes the flow field distribution around the grain uneven,the resulting velocity difference makes the grain rotate,and the rotating grain makes the solute distribution precipitated during grain solidification uniform,The evenly distributed solute inhibits the growth of dendrite arms and promotes the grain morphology to appear ‘rose-shape’ or ‘nearly spherical’.The multiple grains model of semi-solid microstructure growth of magnesium alloy was adopted to study the growth morphology of semi-solid two grains and multiple grains.It is found that the grains close to each other will affect and interact in the process of multiple grains growth,and the precipitated solute is enriched among the grains,which inhibits the growth of grains.At the same time,the grains coalesce due to contact and collision during the growth process.The simulation results are compared with JMAK theory and Starink phenomenological theory.The results show that JMAK theory can well characterize the volume conversion fraction in the early stage of growth,but there will be a large deviation with the progress of solidification,because there is no consideration of the interaction between grains and the influence of soft collision.In the case of multiple grains solidification with the combination of pre nucleation and continuous nucleation,the deviation between JMAK theory and simulation results will always increase gradually.In both cases,the simulation results are more consistent with the calculation results of Starink phenomenological theory.Through the semi-solid microstructure experiment of AZ91 D magnesium alloy prepared by mechanical stirring,the effects of different process parameters on the microstructure are studied.Reducing the stirring temperature can promote the grain growth and increase the grain size,but it can also promote the nucleation rate and nucleation density and improve the solid rate.Too low stirring temperature limits the grain growth space,and the growth of grains is inhibited.Increasing the stirring rate will increase the forced convection intensity and shear rate of molten metal,and then refine the grain and improve the degree of spheroidization.With the increase of stirring time,the grain size increases,and the fine grains grow gradually.It can be observed that the growth modes such as coalescence and merging occur with the contact between grains.The experimental results are compared with the simulation results of single grain and multiple grains,and the reliability of the previous simulation results is verified.The prediction model of semi-solid grain morphology of magnesium alloy is established by using neural network and multiple regression analysis,and the prediction formulas of average grain size and shape factor are obtained.Based on the prediction model,an optimization method for preparing semi-solid microstructure of magnesium alloy is proposed,with solidification temperature of 850k-875 k and stirring rate of 200rpm-2000 rpm,Taking the stirring time of30s-600 s as an example,the optimal process parameters in the sample set range are as follows: the solidification temperature is 870 k,the stirring rate is 2000 rpm and the stirring time is 60 s. |
