Prediction And Control Of Stray Grain And Freckle In The Directional Solidification Of Nickel-based Single-crystal Superalloys

Aero-engine is one of the national major strategic demands,and the properties of turbine blades directly determine the performance and life of the engine.Since the single crystal structure has better creep resistance,the turbine blade is required to be single-crystal in the manufacture.However,it is very difficult to ensure the single-crystal structure in the blade with such a complex geometry.Stray grains and freckles can often be found in the castings,which break the single-crystal structure and make a negative influence on the mechanical properties of the blades,reducing yield and increasing cost.It is rather necessary to gain insights to the mechanism so as to find effective ways to control these casting defects.On the characteristic distribution of freckles,previous theoretical analysis and simulation results contradict experimental observations,and the mechanism has not been well explained yet.Combined with the characteristics of the temperature field in the Bridgman furnace,the freckle distributions shown by the experiments are reproduced by numerical simulations,and the mechanisms of geometric sensitivities are illustrated.Although chimneys are formed during the solidification process,channels are not formed under all of them.The perturbations caused by asymmetric melt flow in the mushy zone inhibit accumulation of solute enrichment and the continued development of segregation channel.It is difficult for channels to develop upward continuously in the center where the perturbation induced by melt convection is rather significant.In contrast,the flow field in the near-wall region is less affected by the melt convection in the main bulk region,so channel segregation is more likely to form and develop here.Radial heat loss is an important contributor to the formation of freckles in near-wall regions,sharp and thin-walled features.Increasing the cross-section area dilutes the solute in the melt and inhibits freckles,while decreasing the cross-section area promotes solute enrichment and freckle formation.Based on Eulerian multiphase framework,remelting mechanism is included in the cellular automaton model as a complement for the current microstructure models.Solute-enriched induced dendritic fragmentation during the directional solidification of Ni-based single-crystal superalloys is successfully predicted,i.e.,the enriched solute melts the dendritic trunk and turns the dendritic tip into an isolated dendritic fragment.Although the bottom of the dendritic trunk melts,the dendritic tip can still grow in the undercooled melt.Even though the segregation channel is completely occupied by side branches,the undercooling degree in the channel does not reach the critical nucleation undercooling degree,indicating the origin of freckle grains is dendritic fragments rather than the newly nucleated grains.A mathematical model of radiant heat transfer based on the discrete ordinate method is developed for a more in-depth understanding of the heat transfer characteristics of the directional solidification process,and multiple adjustments of the withdrawal rate are implemented in the simulation.Height range of the liquidus isotherm,defined as the height difference between the top and bottom,is used to describe the shape of the solid/liquid interface and to estimate the volume and undercooling degree of the undercooling region in the platform.A variable withdrawal rate approach based on numerical simulation is proposed,where solid/liquid interface advancing velocity and height range are employed as criterions to adjust the withdrawal rate.Compared with the conventional one,this new method successfully eliminates the stray grains formed in the platform.Freckle formation is simulated on multiple scales to illustrate its mechanism and gain insights into the characteristics of freckle distribution,and a new approach of real-time adjusting withdrawal rate and design principles are proposed.These research efforts provide theoretical bases for controlling the freckles and stray grains by adjusting operating parameters and improving the single-crystal blade mold design.