Numerical Simulation Of Freckle Defects In Nickel-Based Superalloys

The freckle defects formed in the directional solidification process of nickel-based superalloys are very destructive to the castings.Although the experimental research can explain the formation characteristics of freckles to a certain extent,it consumes a lot of manpower and material resources.With the development and application of computer technology in the field of materials science,the use of computer simulation of the solidification process is more conducive to understanding the influence of process parameters on the formation of freckles.Based on the volume element average technique,this paper establishes a three-dimensional prediction model of macrosegregation with hot solute convection as the main driving force.The semi-implicit algorithm is developed,that is,the energy conservation equation and the solute conservation equation use the explicit finite difference algorithm,and the momentum conservation equation uses the implicit SOLA(Solution Algorithm for Transient Fluid Flow)algorithm.The simulation results were first compared with the results in the literature,and the results were compared with the experimental results.The qualitative agreement between the two can verify the reliability of the model algorithm.The effects of different withdrawal rate on the formation of freckles were studied by using the developed models and algorithms.The calculation results show that the freckles of the castings with a withdrawal rate of 0.10 cm/min are more serious than the castings with a withdrawal rate of 0.60 cm/min.An increase in the withdrawal rate indicates an increase in the cooling rate,so increasing the withdrawal rate can effectively suppress the formation of freckle defects.Simulation studies of the effects of different alloy compositions on freckle formation indicate that castings without Ta element have significant freckle defects on the surface compared to castings containing all elements,so Ta is an element that inhibits freckle formation.Freckles are formed at the corners of the casting without Al element,but the overall macrosegregation is reduced.The addition of Al increases the coefficient of solute expansion and also increases the equilibrium partition coefficient,so its effect on freckle formation is more complicated.The simulation studies on the formation of freckles caused by different geometric shapes show that the freckle defects are more likely to be formed in the shrinkage structure castings compared with the castings without cross-section changes,and the freckle defects and macrosegregation in the expanded structure castings are reduced.The three-dimensional macrosegregation prediction model can reproduce the influence of the placement of the casting in the furnace on the formation of freckles,which cannot be achieved by the two-dimensional model.The numerical simulation research carried out in this subject has certain accuracy and practicability,and provides theoretical guidance for the control of freckle defects in industrial production of nickel-based superalloys.