Numerical Simulation Study Of Microstructural Evolution And Multiphase Flow Mechanism Of Semi-solid Slurry

The lightweight of automobiles is an important way to protect the environment and the green mountains.Aluminum alloy auto parts play an important role in the lightweight journey.How to prepare high-quality aluminum alloy parts is a new challenge for the current manufacturing industry.The semi-solid forming technology proposed in the 1970s is a new forming process between liquid and solid forming.Its products have the advantages of low cost and high quality.It is the preferred technology for lightweight vehicles.This research focuses on the basic theoretical exploration of the two bottlenecks of the semi-solid die-casting process about the nonuniform microstructure of slurry and instability of filling flow.This paper focuses on numerical simulation modeling and application based on experiments to investigate the evolution mechanism of grain morphology during the preparation of 357.0 aluminum alloy semi-solid slurry and the multiphase flow mechanism of slurry during the die-casting process.Firstly,taking the process characteristics of semi-solid slurry preparation for reference,a Phase-Field-Lattice-Boltzmann model coupled with solute,temperature,and velocity fields was established.Based on the established model,the parameters of the specific range in semi-solid slurry preparation were studied.The mechanism of grain morphology evolution reveals the key factors for the formation of spherical grains.Secondly,a multi-phase flow model was established for the simulation of solid-liquid segregation in the filling process of semi-solid slurry.The effects of process parameters,cavity shape,etc.on phase segregation were studied based on the multiphase model.The interaction mechanism between macroscopic flow and microcosmic movement of particles in the multi-phase flow of semi-solid slurry is revealed,the flow tendency of particles in different regions is illustrated,and the separation of solid-liquid with different characteristics is explained.The whole paper has obtained the following research results:(1)A Phase-Field-Lattice-Boltzmann model suitable for the simulation of microstructure evolution in the semi-solid slurry is established and used to accurately simulate the influence of different parameters on the grain morphology.The distributions of velocity and solute in the study area were also given to provide a basis of quantitative analysis for revealing the inherent control mechanism of grain morphology evolution.(2)Through the microstructure simulation study,the following conclusions are given for the process optimization:the smaller the growth space,the better the formation of spheroidal crystals.The smaller the local undercooling for stable growth of grain(2.3?16.3?),the greater the probability of spheroidal crystal formation.The sphericity of the grain increases first and then decreases with the increase of the steady-state cooling rate(0.0162?1.62?/s).The natural convection caused by solute expansion(?c=-4.0?7.3)can make the secondary dendrite arm thinner with the increase of natural convection intensity,but not significant.Forced convection(0.0001?0.2 dimensionless unit)has little influence on the morphology of grains grown under low undercooling degree,and a significant effect on grain morphology under high undercooling degree.Grain growth at the downstream side is suppressed,and the upstream side is promoted.(3)According to the unique properties of semi-solid slurry,a model suitable for multiphase flow simulation of its flow process is established.The combination among the viscosity model-particle-particle interaction model-particle-liquid interaction model is(k-? realizable)-(Syamlal-O'brien)-(Gidaspow).(4)Based on the established multiphase flow model,the influence mechanism of different parameters on the solid-liquid segregation in the slurry flow process was studied.In general,the higher the filling speed,the more uniform the distribution of solid particle phase.As the particle size(50 ?m,100 ?m,and 150 ?m)increases,the degree of solid-liquid segregation at the flow front decreases first and then increases.The degree of solid-liquid segregation is exacerbated as the channel curvature increases.(5)The flow behavior of particles in different regions(boundary particle zone and central particle zone)was explained,the mechanism of solid-liquid segregation at the wall and flow front was revealed.According to the theory of fluid mechanics and the influence of various parameters on the solid-liquid segregation during the semi-solid slurry flow,the internal mechanism of phase segregation was investigated.It was found that when the slurry with a solid fraction of 0.5 and particle size of 100?m,the critical value of filling velocity is 20.799 m/s to ensure a thin solid-liquid separation layer and a suitable flow state.(6)This study fills in two blanks for the development of full process semi-solid die-casting multi-scale numerical simulation technology:microstructure simulation and multi-phase flow simulation.And initially establishes the interface between the microstructure simulation and multi-phase flow simulation,connects the front-end material calculation and subsequent defects prediction.