| The demand for high thrust and light weight of advanced commercial aerospace engines has made complex thin-walled high-temperature alloy castings widely used.For complex structural castings,the flow and solidification process in the filling process is very complicated,and the solidification structure is not only affected by the solidification sequence,solute and heat distribution,but also by the convection of the melt.Therefore,studying the mechanism of solidification microstructure evolution of superalloy under forced convection in complex space,and providing a theoretical basis for casting of aero-engines is not only a basic problem that requires in-depth system research,but also has practical engineering value.Based on the lattice Boltzmann method,this paper establishes a three-field LBM transmission model that can quantitatively simulate the flow field,solute field and temperature field around the dendrite growth under convection,and it is combined with the cellular automata method based on Zhu-The Stefanescu model was coupled to establish a convective dendritic growth model of Ni-Nb binary alloy with K4169 nickel-base superalloy as model alloy.Using this model,the basic flow and mass transfer heat transfer problems during solidification and the dendritic growth of nickel-base superalloy under forced convection are simulated to simulate the plate flow and pipe flow when the alloy melt is filled in the casting process.And the top cover drive flow simulating the evolution of eddy currents,as well as the solute convection diffusion problem in the melt and the natural convection of the square cavity.According to the above simulation,the established model has certain applicability and good accuracy.Based on the established convective dendritic growth model,the influence mechanism of the formation and evolution of nickel-based superalloy dendrites was studied by adjusting the initial subcooling degree and initial concentration.The simulation results show that increasing the initial degree of subcooling can accelerate the growth rate of dendrites and reach a higher solid fraction in the same time,which is more likely to promote secondary dendrite growth.Increasing the initial concentration will slow the dendrite growth rate,decrease the dendritic growth driving force,and suppress the secondary dendrite growth.Based on the established convective dendritic growth model,the influence mechanism of melt transport physical properties such as melt flow velocity,kinematic viscosity and diffusion coefficient was studied,and its influence mechanism on the formation and evolution of dendrites in forced convection was studied.The results show that the increase of the melt momentum caused by the increase of the convection velocity will significantly promote the dendrite growth,and the secondary dendrite growth upstream of the dendrites is also significantly enhanced.The downstream dendrite growth is inhibited.Increasing the viscosity of the melt has no significant effect on the main growth rate and dendritic morphology of the dendrites,but it will reduce the melt fluidity,increase the spacing of the secondary dendrites,and decrease the number of secondary dendrites.The increase of the diffusion coefficient will increase the dendrite growth rate,obtain a higher solid fraction in the same time,and aggravate the secondary dendrite maturation,and the number of secondary dendrites is less under the same solid fraction.The convective dendritic growth model of Ni-Nb binary alloy was established in this paper to realize the quantitative simulation of dendritic formation and evolution of convective nickel-base superalloy.The effects of solidification process parameters on dendrite growth and the effect of transporting physical properties of the melt on the dendrite growth were investigated.The simulation results have great significance for understanding the relevant solidification theory,and have certain reference significance for the actual casting process. |
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