Directional Solidification Process Control And Microstructure Optimization Of Nickel-base Single Crystal Hollow Turbine Blade

Aeroengine is the power plant of the aircraft and is known as the"heart"of the aircraft.As one of the most critical core components in aeroengine,turbine blades have high working temperatures,complex stress,and harsh environment.With the development of high thrust ratio engine,the operating temperature of turbine blades is getting higher and higher,the material of the blades is developed from nickel-base columnar crystals to single crystals,and the structure of blades is developed from solid to hollow.The nickel-based single crystal hollow turbine blades are the trend of international development.However,domestic research in this area has just started and there is a big gap with foreign countries.According to the national strategic requirements,this paper used a new type nickel-based superalloy as the research object,uses the crystal selection method and the vacuum directional solidification method to cast the single crystal hollow turbine blade,studied and analyzed its microstructure.The influence of pulling rate on the microstructure of single-crystal hollow turbine blade was studied.By studying the law of variable cross-section growth of single crystal hollow turbine blade,a new type of thermal conduction technology was designed,by moving the thermal barrier downward,uniform the heat of blade edge plate.The method of temperature is used to eliminate or reduce the generation of leaf stray crystals,verified by directional solidification experiments;The anti-oxidation performance of single crystal turbine blade at the use temperature was explored,providing theoretical and experimental basis for the design and application of oxide coating.During these researches,we got the following research results:（1）The effect of uniform speed pulling on the microstructure of single crystal hollow turbine blades showed that the solidification structure of hollow blades grows in the dendrite form at different drawing speeds,but the solidification structure of the blades with the pulling speed increases change from coarse dendritic to twig.The size of theγ′phase between the dendrite and the dendrite decreases,and the size of the（γ+γ′）eutectic decreases gradually.The size of the cross-shaped“petal”at the dendrite of the hollow leaf blade becomes gradually uniform.When the blade is pulled at a constant speed,the solidification structure of the leaf blade and the blade body can be obtained at a higher drawing speed,and the variable cross-section position is more prone to miscellaneous crystals when the pulling speed is faster.Grains selected by the grain-selective structure will exhibit large deviations from the[001]orientation.（2）The research on the effect of variable pulling speed on the microstructure of single crystal hollow turbine blades showed that:Stepwise speed change will still leave an unsteady zone on the blade,which will affect the performance of the blade.Due to sudden overspeed,the temperature gradient will suddenly become larger and probably produced a miscellaneous crystal.The uniform variable speed scheme succeeded in obtaining a better organization in different parts of the blade.The non-steady-state zone organization of the uniform speed shift is relatively orderly compared to the step speed change,but the dendrite pitch is larger than the steady state zone.（3）The study of stray crystal control of single-crystal hollow turbine blades’edge plate showed that:The occurrence of miscellaneous crystals is mainly due to the fact that the heat dissipation conditions of the near-wall and far-wall side of the shell are too different,and that the thermal barriers hinder the growth of the dendrites to variable cross-sections.The addition of a heat-conducting body ring makes the organization at the variable cross-section even smaller and more uniform.The grain structure that spreads from the length of the blade body to the variable cross-section to the edge increases.The microscopic shrinkage at the cross section is significantly reduced and the dendrites grow outward in an orderly manner.A solidification structure that is clearly superior to the ordinary directional solidification process is obtained.（4）The oxidation resistance of single-crystal hollow turbine blades shows that the outer oxide layer of the new alloy is mainly composed of NiO,and the intermediate layer oxidation products are mainly（Ni,Co）（Al,Cr）O₄ spinel phases,and the inner oxidized layer is Al₂O₃.Due to the low content of Cr in the new alloy,a NiO layer forms rapidly on the surface during oxidation.When the new alloy was oxidized at1100°C for 200 h,the oxide film was peeled off and cracked,showing that the inner oxide film was continuously breaking through the continuously generated oxidation behavior.The oxidative weight gain curve deviates from the parabolic law under the action of this oxidation behavior.