Simulation Of Dendrite Growth Of Nickel-Based Superalloy Under Convective Conditions By Phase Field-Lattice Boltzmann Method

The microstructure of nickel-based superalloys during solidification is mainly composed of equiaaxial crystals and columnar crystals.The microstructure is determined by the interaction between melt flow,heat and mass transport,dendrite growth and dendrite movement,and affects the final properties of the alloys to a great extent.Therefore,it is of great scientific and engineering application value to study the influence mechanism of melt flow on solidification structure.In this paper,the phase field model which simulates dendrite growth is coupled with the Lattice Boltzmann model which computes fluid flow efficiently,and Newton’s laws of mechanics are used to solve the velocity of solid phase motion.A two-dimensional Phase Field-Lattice Boltzmann transport model which is suitable for simulating the dendrite growth and movement of nickel-based superalloy under convective environment is constructed.The influence of melt convection on microstructure evolution was analyzed by solute concentration field and flow field distribution.The dendrite growth process was simulated under natural convection environment.The results showed that the intensity of natural convection was different with different solute expansion coefficient β,which had different effects on the microstructure evolution process.When β<0,the solute sank and the lateral dendrite arms grew faster.When β<0,the columnar morphology was less affected by convection.When β>0,the solute floats upward,and the lateral dendrite arm grows faster under the equiaxed crystal.When β>0,the change of β has a more significant effect on the dendrite morphology.With the increase of the solute expansion coefficient β,the dendrite tip splits gradually,the dendrite arms increase,and the primary spacing decreases.The solute expansion coefficient β is different when the preferred orientation is different,which is closely related to the longitudinal average flow velocity of the dendrite tip melt.The dendrite growth process was simulated under forced convection environment.The results show that the dendrite arms of equiaxed crystals on the side of the oncoming flow grow faster under forced convection environment,while the dendrite growth in the "downstream"direction is inhibited,forming different dendrite morphology than that under the condition of no convection,and the influence is more significant with the increase of initial melt flow velocity.The existence of forced convection increases the growth rate of oncoming side dendrite arms in the process of multi-dendrite growth,which gives them a greater competitive growth advantage.The forced convection has great influence on the primary spacing and the actual growth direction of slanted dendrites.With the increase of the inflow velocity of forced convection,the primary spacing of dendrites increases,and the actual growth direction of dendrites rotates towards the preferred growth direction.The motion and sedimentation behavior of equiaxed crystals were studied under the condition of melt convection.The results show that the "upstream" main arm grows faster than the "downstream" arm when forced convection drives dendrite movement.The dendrite morphology gradually becomes asymmetrical and the dendrite moves at a speed close to the surrounding molten liquid.The dendrite deposition occurs under the action of gravity,and the higher the solid-liquid density ratio,the higher the dendrite falling rate and the molten flow rate,and the dendrite growth rate is also accelerated.The solid fraction fs and dendrite mass ms increase with the increase of the solid-liquid density ratio at the initial solidification stage,and the oscillations of fs and ms decrease until the dendrites gradually settle outside the calculated region.