| As aircraft engine core component, Turbine blades has long been a worst working conditions of aircraft parts. The compound cooling design and preparation of single crystal hollow turbine blade has become the major technical hurdles of the development of aircraft engine with the constant improvement of the aeroengine in thrust-weight ratio. The importance of the core technology development has been realized at home and abroad. Based on this, The solidification laws of single-crystal hollow blade during directional solidification were studied through analyzing the molding characteristics of hollow blade. The ceramic core and blade shape structure suitable for the preparation of single-crystal hollow blade were designed in this paper.This paper studied the preparation process of nickel-based single-crystal hollow turbine blade. The formation laws and mechanism of casting defects, such as insufficient filling, shrinkage, contraction cavity, mixed crystal and low-angle boundary, were studied. According to high-temperature tension and creep test,mechanical properties of single-crystal hollow blade were studied. These results will provide theoretical and experimental basis for the research and development of high performance single crystal hollow turbine blade.The second generation of nickel-based single crystal alloy DD5 is used for the preparation of single crystal hollow turbine blade by directional solidification technology. The solidification laws of single-crystal hollow blade during directional solidification were studied. The results showed that the primary dendrite spacing was gradually increased, the grain deflection angle was gradually decreased and the orientation was optimized with the continuous progress of seeding as the solid liquid interface. The helical structure of seeding section had no optimizing effect on the grain orientation. Grain growth and elimination were not related to crystallography,but only related to the position of grains in the spiral channel. The solidification characteristics of blade was that the dendrite spacing was gradually increased with the increase of blade wall thickness in the same axis, and the size of γ’ strengthened phase in dendrite arm was increased with the increase of blade wall thickness in the same cross section. The solidification characteristic of variable cross-section is that the order of solidification is opposite to its cooling order. The crystal growth starts from the lowest edge of temperature to the blade, and finally merges with the single-crystal structures of the blade.According to the molding characteristics of nickel-based single-crystal turbine blade used in the aircraft engine and the typical cooling method of hollow blade, the core and blade structure suitable for the preparation of single-crystal hollow blade were designed in this paper. This paper studied the preparation process of DD5 single-crystal hollow turbine blade, including the preparation of silicon oxide-based ceramic core, the connection of shell and core, the preparation of corundum ceramic shell, the selection of directional solidification experiment parameters and the removal of ceramic core via chemical method. Through the thermogravimetry -differential thermal analysis (TG-DTA) test, the phase-transition temperature and melting point of DD5 single-crystal alloy were investigated, providing scientific basis for directional solidification experiment and selection of heat treatment process parameters. Through studying the directional solidification process, the control parameters of blade forming were analyzed.The formation laws and mechanism of casting defects, such as insufficient filling,shrinkage, contraction cavity, mixed crystal and low-angle boundary, were studied.The results showed that the insufficient filling usually occurs in the thin walls of single-crystal hollow blade, especially the exhaust edge with the wall thickness of only 0.5mm. The filling capacity of molten metal is related to the alloy characteristics,casting temperature and wall thickness. The macroscopic shrinkage usually occurs in the dendritic structure along the direction of gravity, and sometimes the "tunnel-like shrinkage" can form on the surface of single-crystal hollow blade, or inside the blade.And the microscopic shrinkage occurs near the interdendritic γ/γ’ eutectic structure,mostly showing a circular and irregular long-strip or triangle; the mixed crystal usually forms at the corner of blade variable cross-section, and mostly the variable cross-section away from the muff heater. The grain boundary migration formed by original grain and mixed crystal is non-linear, and dendritic crystals in some places penetrate and intersect with each other, but the whole trend of crystal boundary remains unchanged. And the mixed crystal boundary is composed of the chain y’phase and large y/y’ crystal eutecticum; the low-angle boundary is located in the primary dendrite arm, which is composed of three-dimensional surfaces. And the secondary dendrites formed in the transition zone asymmetrically grow. In addition, a certain amount of carbide and boride will be precipitated in the grain boundary, and the y’ phase cube precipitated is not complete with irregular arrangement.The research on the effects of heat treatment of single-crystal hollow turbine blade on microstructure showed that the dendrite arm and interdendritic boundary are vague after solution treatment, but there is still a clear boundary. The elemental segregation between dendrite arm and interdendritic is decreased and the large y’phase and interdendritic γ/γ’ eutectic structure are almost eliminated, and the small and irregular γ’ phase with relatively free distribution is precipitated. With the increase of blade wall thickness, the residual eutectic crystals are gradually increased, and γ’phase is gradually larger. After the aging treatment, the size distribution of y’ phase is concentrated, and the difference of size is reduced with regular shape and increased cubicity; and the unevenly-distributed, irregular and fine cubic y’ phase is precipitated in the γ matrix channel.The research on recrystallization of blade tenon showed that after the DD5 single-crystal samples received the ac solution treatment at 1230℃/4h under different loading, no new crystals are found. And the depth of region affected by the loading is increased with the increase of loading. The interdendritic spacing is increased, among which a number of γ’ phase particles coherent with y’ phase exists around the undissolved eutectic structure, and γ’ phase particles are re-precipitated in a cellular form, and the size and depth are increased with the increase of loading. The recrystallization occurs after the ac solution treatment at 1315℃/4h, followed by nucleation at the maximum distortion caused by plastic deformation. With the increase of loading, the recrystallization area is gradually enlarged, the dendrite arm becomes blunt in the affected area, and the dendrite arm and interdendritic boundary disappears, and y/y’ eutectic structure exists between recrystallized grains.The high-temperature tension test for DD5 single-crystal alloy and thin-walled castings showed that the wall thickness has a certain influence on the tensile strength at 800℃, but its law is not obvious. With the decrease of wall thickness, the elongation is decreased gradually and the appearance of fracture of thin-walled castings is relatively flat. The tensile strength difference between the standard specimen and the plate specimen with different thickness at 1000℃ is lower than that at 800℃, and the fracture of standard specimens has obvious constriction with dark gray color and a lot of equiaxial dimple under the scanning electron microscope. The fracture surface of thin-walled specimens is similar,and a lot of dimples also exist,but a certain cleavage cracks exist on the edge of fracture with the wall thickness of 1.3mm and 1.6mm.The high-temperature creep test showed that the deformation and fracture life of the thin-walled castings are decreased by 10.676% and 20.84h, respectively,compared with those of standard castings. But the deformation rate is larger and the dimple size in the creep fracture of thin-walled specimens is relatively small with larger density and more micropores. The γ’ phase directional coarsening of DD5 single-crystal alloy is the N (normal)-rafting. The degree of γ’ phase directional coarsening is gradually decreased with the distance away from the fracture. The original γ’ phase size after the heat treatment of thin-walled castings is small, and the thickness of γ’ phase raft thickness during the creep process is increased significantly,but that of the standard specimens is also increased but not significantly. |
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