Numerical Simulation On Channel Segregation Defects During Solidification Process Of Nickel-Based Superalloys

As the most widely used superalloy,nickel-based superalloys have the highest strength at high temperature at the moment.Therefore,they are mainly intended to manufacture the key hot-end components of aero-engines and large gas turbines.Due to inherent segregation defects in the practical solidification process,several solute enrichment channels are usually formed inside the casting products.The existence of such channel segregation defects will seriously affect the comprehensive properties of castings and even lead to their direct scrapping.However,there are still many problems in studying the formation of channel segregation during solidification process by experimental methods,so this paper adopts numerical simulation technology to analyze the transport phenomenon in this process.Based on the above,we can explore the formation mechanism of channel segregation defects and grasp the basic relationship between the development trend and main process parameters,which has extremely important theoretical and engineering practical significance for improving the production process and the quality of casting products.Firstly,a three dimensional coupled mathematical model based on the continuum model is established to predict channel segregation defects of multicomponent alloy solidification process under the effect of nature convection.In order to verify the reliability of this prediction model,we apply it to the castings of common binary and nickel-based superalloys,respectively.The formation process of channel segregation under natural convection is simulated completely,while the influence of cooling rate on channel segregation is discussed.The simulation results show that the flow causing freckle formation originates from the liquid region rather than the mushy zone.The solute-rich fluid continuously converges toward the channels of mushy zone and continues to replenish the flow in the channel from the liquid region through the mushy zone.During the solidification process,these channels keep developing and changing all the time.As multiple unstable channels merge or disappear directly,stable segregation channels are finally formed.What's more,channel segregation could become more severe with the lower cooling rate,which is verified by the segregation of practical ingots.Secondly,although vacuum arc remelting technology has been widely used in the practical production of superalloy castings as an effective means to improve the purity of remelted metals,channel segregation defects in the casting products are still inevitable.Therefore,magnetic vector potential method is used to perfect the segregation defect prediction model on the basis of the research under natural convection.A mathematical model describing the coupling of flow,heat transfer and mass transfer is established in vacuum arc remelting process,which is used to simulate the channel segregation defects of nickel-based superalloy castings during vacuum arc remelting process.Finally,the effect of current intensity on the formation of channel segregation defects in vacuum arc remelting process is systematically discussed.The results show that the channel segregation in nickel-based superalloy castings will become more serious with the increase of current intensity.In the process of practical production,we should not only reduce segregation defects,but also take the production costs into account.The numerical simulation results in this paper can provide a scientific reference for selecting appropriate cooling rate and current in the practical production process.