Formation And Control Of Freckles On Directional Solidified Nickel-based Superalloy Casting With Abruptly Varying Cross-section

Because of the advantages of gas turbines in electricity and marine power,gas turbines have attracted much attention around the world.However,the wide application of gas turbines is limited by the preparation technology of gas turbine blades,especially for our country.Due to the increase of gas turbine blade size,the addition of high temperature refractory elements in the blade materials,and the design of more complex cooling channels in the blade,the formation tendency of freckles and other defects in the gas turbine blade prepared by traditional preparation technology of aeroengine blade is increased.The freckle defects reduce the high temperature mechanical properties of castings,and cannot be eliminated by later heat treatment and machining.At present,most of the researches focus on the numerical simulation and visualization of freckle formation and the improvement of the criteria for predicting freckle formation.The specimens used are simple in cross section and small in size,or low melting point alloys are employed as research objects.This is quite different from the characteristics of real gas turbine blades.The existing prediction methods are complex and are not suitable for predicting the formation of freckles in casting with a large size and complicated shape.Meanwhile,the existing methods of reducing freckles tend to have some shortcomings in practical applications.In addition,the travelling magnetic field has not been applied to the directional solidification of superalloys to control freckle formation,and whether the travelling magnetic field can be used to reduce freckling tendency is still unknown.Therefore,it is necessary to study the above problems.In this study,the specimens with abruptly varying cross-sections which are consistent with the variation characteristics of real blades are employed,the CMSX-4 superalloy for gas turbine blade was used as the research alloy and directionally solidified in the Bridgman furnace.The freckle formation in the specimens was studied by means of macroscopic and microscopic corrosion techniques,electron microscopy and numerical simulation under different experimental conditions.The main research results are as follows.(1)Freckles were usually observed on the specimen surface within a range of 2-15 mm above the platform,and the freckles on both sides were slanted to respective edge.The abruptly variation in cross-section can promote freckles formation.Because the abruptly variation in cross-section makes the growth rate of dendrite above the platform change from fast to slow,then increases the tendency of the fact that melt flow rate in the mushy zone is greater than the dendrite growth rate,and thus increasing the freckling tendency.Due to the concave solidification interface,the segregated liquid in the mushy zone suffer the pressure difference in horizontal direction,flow toward the surfaces and edges of the specimens,and then the freckles appear on the surface of the specimens and slant to the each edge finally.The top state of the specimen influences the freckle formation by influencing the solidification conditions,and the effect of alloy composition and specimen size on the freckle formation is consistent with that in the specimen with uniform cross-section.The freckling number was reduced with increasing withdrawal rate for the refined dendrite microstructure.(2)A method for predicting the freckle formation in directionally solidified castings with large size and complex shape is presented.The method is combined with the simplified theoretical model and simulation.It is verified with the directional solidification experiments of the specimens with abruptly varying cross-section under various parameters,and applied to the prediction of freckles in dummy blade.(3)The freckling tendency can be reduced by embedding the graphite block into the bottom of platform.The experimental results showed that the freckles on the specimens were completely eliminated after the application of graphite block.Embedding the graphite block into the bottom of the platform reduces the thermal resistance,increases the effective heat transfer coefficient and improves the local cooling conditions.Firstly,the fined microstructure with a reduced primary dendrite arm spacing was obtained for the increasing temperature gradient and local cooling rate.Secondly,due to the increasing G/R value,the dendrite transforms to cellular dendrite was promoted.Finally,the solidification interface in the platform region was flattened.The adding of radiant baffles on the top of the mold shell can significantly increase the solidification rate,increase the tendency of the fact that the dendrite growth rate is greater than the liquid flow rate in the mushy zone,reduce the possibility of dendrites being remelted and fractured,and thus reduce the freckle formation tendency.(4)The proper travelling magnetic field can employed to reduce freckling tendency,and the travelling magnetic field with the upward direction can significantly reduce the freckle formation tendency in the inner of the specimen.The travelling magnetic field affects the freckle formation through the following aspects.The Lorentz force induced acts on the liquid phase,changes the liquid flow rate,causes the change of solute segregation in the mushy zone,and then changes the remelting to the dendrite arms.Meanwhile,this change of liquid flow rate also changes the shearing force on the dendrite arms;In addition,the Lorentz force acts on the dendrite arms directly,affects the formation of dendrite fragments.Under the effect of travelling magnetic field,the traditional method of predicting freckling tendency by Rayleigh number is no longer applicable.