Multi-physics Fully-Coupled Modelling And Analysis Of Solidification Defects Formation For Directionally Solidified Hollow Turbine Blades In Large-Size

Nickel-based superalloys are widely used as components in hot end of aerospace engines because of its excellent high temperature strength,fatigue resistance,high temperature corrosion resistance and microstructure stability,such as large-size gas turbine blades and high pressure turbine blades,etc.At present,the nickel-based superalloy turbine blades are usually prepared by directional solidification(DS).However,the application prospect of nickel-based superalloy in the preparation of large gas turbine blades is greatly limited during DS because of the existence of freckle,stray grain,shrinkage and porosity.The formation of these solidification defects has a high correlation with distribution characteristics and evolution rules of temperature field,flow field and solute field.At present,it is not clear aiming at interaction relationship among multi-physics fields in the process of manufacturing large-size nickel-based columnar crystal superalloys during solidification,effective criterion or theoretical model has not been established to forecast perfectly the formation position of solidification defects in large-size turbine blades.In the present work,third generation superalloys CMSX-4 was selected as research object.The distribution and evolution of fully-coupled multi-physical fields such as temperature field,flow field,solute field and electromagnetic field of large-size hollow turbine blades during directional solidification are studied in detail.Besides,a new freckle formation criterion based on dynamic pressure is proposed to predict the position of freckles in the blades.In addition,a series of corresponding optimization process methods are proposed and provide an important material science foundation and engineering theoretical support for realizing the high quality nickel-based superalloy turbine blades.The main conclusions of the present study can be drawn as follows.The characteristics and evolution of temperature gradient distribution at the front of solid-liquid(S/L)interface during directional solidification of hollow turbine blades have been revealed.The simulation results show that,in an upturned blade,the temperature gradient can dramatically drop to 11K/cm from 40K/cm when the S/L interface enters into the platform.Besides,there is a great difference for temperature gradient at some points on the front of S/L interface,especially at several top ends on blade platform,the temperature gradient drops dramatically.Based on the results of the experiment,the sudden drop of temperature gradient is the primary cause of solidification defects,such as freckle,stray grain and shrinkage porosity.A criterion for predicting freckles in hollow turbine blades is established.Based on Darcy's law,the criterion of freckle formation including dynamic pressure is deduced,and the critical value is about 1.In this work,the results show that the critical value of criterion in CMSX-4 alloy is about 0.85,the criterion value of hollow turbine blade on surface of platform and near internal ceramic core are 2.8 and 1.2 respectively,that means the freckles are more easy to form in such two positions.A method for reducing freckles in directionally solidified superalloy blades by traveling magnetic field(TMF)is proposed.Meanwhile the ranges of electromagnetic field parameter to reduce(or eliminate)freckle are given based on our new freckle criterion.The research results show that,as the downward TMF intensity increased,the flow velocity of solid liquid interface front presents a tendency of decreasing first and then increasing.The critical range(in this work 0~150mT)exists in downward TMF intensity can reduce even eliminate the freckle defects in turbine blade;otherwise,the upward TMF can only intensify the flow in front of S/L interface and then result in more freckles.For large-size gas turbine blade,the effects of withdrawal rate and hot zone temperature on solute field are investigated.The results indicate that the increase of withdrawal rate and hot zone temperature have obvious influence on the solute distribution in the blade.Raising the withdrawal rate or hot zone temperature can reduce the external segregation of the blade.Conversely,the segregation near the inner ceramic core is intensified.The geometric shape of casting has significant influence on the flow pattern at the front of S/L interface and freckle distribution.According to the geometric characteristics of the blade platform,the casting with abruptly varying cross-sections is designed to study the influence of geometry on flow in front of S/L interface and freckle formation.When the S/L interface enters the platform,the axial flow intensity of the front edge of the S/L interface suddenly increases,at the same time the temperature gradient is greatly reduced,and a combination of the two factors can lead to the formation of the freckles.At the end of platform,the flow pattern at the front of the S/L interface can be switched from the axial flow to the radial flow,so the freckle defect correspondingly disappears.Two types of non-uniform thickness mould are designed to obtain higher temperature gradient ahead of S/L interface and flat S/L interface.The simulation results show that,when ratio comes to 2,the axial temperature gradient ahead of S/L interface on the platform increases from 7 K/cm to 17 K/cm.Moreover,when ratio is 0.5,the S/Linterface shape remains flat in whole process,especially in the platform of blade.Further,a compound mould is designed to solve the stubborn problem of blade removal after solidification.In addition,a new baffle with movable part is also designed.Its movable part can be separated by itself when the geometric shape of the casting changes suddenly.The results indicate that the axial temperature gradient at the platform of the blade is greatly enhanced,which provides a new technical scheme for controlling the freckles and stray grains formed in the large-size gas turbine turbine blade.