Blade Castings Research Articles

Исследование влияния высокотемпературной газостатической обработки на структуру и свойства интерметаллидного сплава ВКНА-4

The effect of hot isostatic pressing (HIP) on microstructure, microporosity and fatigue strength of Ni₃Al-based alloy VKNA-4 block of blade castings with polycrystalline structure was investigated. Hot isostatic processing significantly increases the fatigue strength due to reduction of casting pores.

Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

Zirconium is always present in Ni base superalloys as it enhances their creep properties. In the present study, the influence of very small Zr additions, 100–400ppm, i.e. 0.01–0.04wt.%, on hot tearing of IN738LC superalloy is experimentally investigated using dedicated turbine blade castings. Although the Zr content remains very small, it has a strong effect on hot tearing tendency. Microstructure of hot tear in as-cast samples reveal that grain size and secondary dendrite arm spacing have no significant effect on hot tearing. On the other hand eutectic phase volume fraction and its dispersion or spreading along grain boundaries drastically affect the hot tearing propensity and strongly increase with increasing amounts of Zr. Hence grain coalescence becomes impossible at grain boundaries covered with eutectic phase films. With increasing Zr content, gain coalescence between two distinct grains with no interdendritic phase requires more undercooling. Coalescence is retarded and occurs deeper in the mush zone, i.e. at lower temperatures resulting in a higher sensitivity to hot tearing. Finally, it is shown that a reduction of Zr content to 0.02wt.% is required to fully suppress hot tearing in polycrystalline IN738LC blades.

Multiscale Modeling and Simulation of Directional Solidification Process of Turbine Blade Casting with MCA Method

Nickel-based superalloy turbine blade castings are widely used as a key part in aero engines. However, due to the complex manufacturing processes, the complicated internal structure, and the interaction between different parts of the turbine blade, casting defects, such as stray grains, often happen during the directional solidification of turbine blade castings, which causes low production yield and high production cost. To improve the quality of the directionally solidified turbine blade castings, modeling and simulation technique has been employed to study the microstructure evolution as well as to optimize the casting process. In this article, a modified cellular automaton (MCA) method was used to simulate the directional solidification of turbine blade casting. The MCA method was coupled with macro heat transfer and micro grain growth kinetics to simulate the microstructure evolution during the directional solidification. In addition, a ray tracing method was proposed to calculate the heat transfer, especially the heat radiation of multiple blade castings in a Bridgman furnace. A competitive mechanism was incorporated into the grain growth model to describe the grain selection behavior phenomena of multiple columnar grains in the grain selector. With the proposed models, the microstructure evolution and related defects could be simulated, while the processing parameters optimized and the blade casting quality guaranteed as well. Several experiments were carried out to validate the proposed models, and good agreement between the simulated and experimental results was achieved.

Surface temperature difference for analysis of distortion of heavy weak-rigidity castings during heat treatment

Temperature field has been widely used to predict the deformation of the blade castings during a heat treatment process. However, it is difficult to predict distortion of the castings. In this article, a new parameter, surface temperature difference of the castings along the thickness direction, has been proposed to analyze distortion of the thin-walled heavy castings with weak rigidity. The parameter was applied to the analysis of distortion of a heavy blade casting under various air speeds of the fans during heat treatment. The calculated minimum temperature in the blade casting was in good agreement with the actual measured value. The surface temperature differences result in the distortion process of the blade during the heat treatment process.

Ensuring the Quality of the Surface of Turbine Blades by the Application of Protective Diffusion Coatings

Scanning electron microscopy and x-ray microanalysis were used to study defects in protective diffusion coatings on the blades of gas-turbine engines. It was established that the defects in the coatings on the flowthrough surface of the middle of the blades are caused by the entry of corundum particles into the surface of the heat-resistant alloy during the pneumoabrasive treatment administered to the blade surface. Defects are formed in the coatings on the inside surfaces of the cooled channels due to the use of an oxidizing atmosphere in the gas-circulation chromizing-aluminizing operation and leaching of carbides from the alloy when ceramic cores are removed from the blade castings. It is shown that the technology of hydrothermal autoclave leaching in a protective argon atmosphere makes it possible to remove corundum particles embedded in the flow-through surface of the middle of the blades and properly prepare the surface of the internal cooled channels for the application of quality protective diffusion coatings.

THREE DIMENSIONAL DYNAMICAL MEASUREMENT OF DISTORTION OF HEAVY STEEL CASTINGS DURING HEAT TREATMENT PROCESS

At the forced air cooling stage in normalizing process of heavy steel castings, the nonuniform cooling might result in thermal and phase transformation stresses, which might cause the distortion, especially for the heavy yet thin blade castings. Dynamical measurement of the distortion of castings during heat treatment process will be significant for understanding the mechanism of dis- tortion and then taking the control measures in productions. In this paper, a novel measuring method, fixed direction method, was proposed, which could three-dimensionally and dynamically measure the distortion of castings without index points at high temperature, and the corresponding analysis method was established. The method was employed to measure the distortion of a hydro blade casting at the forced cooling stage in normalizing process, and the dynamical distortion behavior in three dimensions of the blade casting was obtained. And the measurement error of blade casting was analyzed. The results show that under the conditions of no index points and high temperature, the three- dimensional distortion of castings could be dynamically measured through the fixed direction method.

Modelling of Grain Microstructure of IN-713C Castings

This paper provides an analysis of experimental research results and numerical simulation of grain microstructure of turbine blade castings made of the IN-713C nickel superalloy. The numerical simulation was carried out by applying the ProCAST program. The geometric description of model assemblies and three-dimensional ceramic mould enclosure was developed. The boundary conditions as well as the thermal and physical coefficients for alloy and ceramic shell mould were selected for simulation purposes. The parameters of nucleation law based on normal (Gaussian) distribution as well as the values of equation coefficients were established in order to determine the growth rate of dendrite tips for the IN-713C alloy, depending on the undercooling. The experimental verification of boundary conditions for numerical simulation was carried out by comparison with the results of temperature distribution measurements performed in the castings. The analysis of grain microstructure was conducted on the surface and cross-sections of castings. The forecasted grain microstructure was determined using the CAFE module (ProCAST software). The cast microstructure as well as the value of grain growth and nucleation coefficients, which were used for numerical simulation of solidification process, were experimentally verified.

Fabrication of Integral Core/Shell Ceramic Casting Mould for Hollow Turbine Blade

The paper presents an integral core/shell fabrication of ceramic casting mould for hollow turbine blades by combining SL (Stereolithography) with gelcasting. This method could guarantee the positional accuracy between ceramic cores and shell, thus achieving the rapid fabrication of complex turbine blade castings. The paper focuses on the design of resin mould for gelcasting, the preparation of ceramic slurry, the pyrolysis of resin prototype and the sintering of ceramic green body. The feasibility and effectiveness of above mentioned method were verified through successful manufacturing of hollow turbine blades with double-walled structure.

Stress analysis and deformation prediction of a heavy hydraulic turbine blade casting during casting and heat treatment

Owing to their large and curved shape, blade castings, a key component for heavy hydro turbines, are susceptible to deformation during casting and heat treatment. In the present paper, the stress analysis of a blade casting during both casting and heat treatment is performed. The coupled thermo-stress and thermo-phase transformation stress models are used for casting and heat treatment respectively. Machining allowance distribution is used as the deformation criterion and an algorithm of inverse deformation determination is presented. The mechanical properties of the martensitic stainless steel ZG0Cr13Ni4Mo (13Cr–5Ni–1Mo) at different temperatures are measured under as cast and heat treated status. Finally, the inverse deformation of the blade during both casting and heat treatment processes is obtained, and a series of sections of the blade casting with inverse deformation design are given for pattern making. The calculated deformation results are compared to the measured one, and they are basically in agreement.

Simulation and Control of Distortion of Hydro Turbine Blade Steel Casting in Heat Treatment Process

Blades are key part of hydro turbines, which often distorts during heat treatment process for their special structures. In this paper, thermal fluid finite element simulation of the forced air cooling process of a blade casting was carried out under a variety of distances between fans and blades, air speeds, groups of fans and circumstance temperatures. The temperature fields of blade castings were obtained. A novel parameter, temperature difference between surfaces of castings along thickness direction, was proposed to analyze the distortion of blade castings. The distortion behavior of blade castings with martensitic stainless steel were discussed, which is in good agreement with distortion regularity of the experimental ones. The temperature differences between blade casting surfaces are always greater than zero, resulting in distortion which could be divided into three stages. Finally, we focused on discussing the control methods of distortion behavior of blade castings which could be operated in actual production.

The Deformation of Wax patterns and Castings in Investment Casting Technology

The dimensional accuracy of the final casting of Inconel alloy 738 LC is affected by many aspects. One of them is the choice of method and time of cooling wax model for precision investment casting. The main objective was to study the initial deformation of the complex shape of the casting of the rotor blades. Various approaches have been tested for cooling wax pattern. When wax models are cooling on the air, without clamping in jig for cooling, deviations from the ideal shape of the casting are very noticeable (up to 8 mm) and most are in extreme positions of the model. When blade is cooled in fixing jig in water environment, the resulting deviations compared with cooling in air are significantly larger, sometimes up to 10 mm. This itself does not mean that the final shape of the casting is dimensionally more accurate with usage of wax models, which have deviations from the ideal position smaller. Another deformation occurs when shell mould is produced around wax pattern and furthermore deformations emerge while casting of blade is cooling. This paper demonstrates first steps in describing complex process of deformations of Inconel alloy blades produced with investment casting technology by comparing results from thermal imagery, simulations in foundry simulation software ProCAST 2010 and measurements from CNC scanning system Carl Zeiss MC 850. Conclusions are so far not groundbreaking, but it seems deformations of wax pattern and deformations of castings do in some cases cancel each other by having opposite directions. Describing entirely whole process of deformations will help increase precision of blade castings so that models at the beginning and blades in the end are the same.

Integration of reverse engineering and rapid technologies for rapid investment casting of gas turbine blades

The aim of this research work is to offer a unique procedure of integrated Reverse Engineering (RE) and Additive Manufacturing (AM) technologies through implementation of a comprehensive experimental study to present an extensive comparison between applicable rapid technologies for blade rapid investment casting. Different direct and indirect AM techniques were used to produce sacrificial investment casting patterns. In addition, an aluminium mould of a blade for wax injection was fabricated using Computer Numerical Control (CNC) machining to compare conventional investment casting with the proposed rapid investment casting process. Dimensional inspection of cast blades showed that the MultiJet Modelling (MJM) method has the most economic justification and compatibility with blade rapid investment casting and it can be considered as an alternative to conventional wax precision CNC machining. On the other hand, sacrificial patterns produced by the Perfactory system showed lower applicability for investment casting due to pattern flexibility and difficulties in burn-out step. As for examined rapid tooling technologies, silicone rubber moulding was not a stable technique for small batch fabrication of blade wax patterns. In contrast, the results proved that epoxy resin tooling could make possible new cost-effective approaches for low volume production of gas turbine blades.

Hot Isostatic Pressing Improves the Quality of the Blades from Nickel Base Superalloys for Turbine Engines

Typical defects of the cast engine blades from Ni base superalloys in the as-cast state and after long operation were investigated: the cast porosity, internal and external microcracs. The initial cast porosity and voids, which have appeared during long operation, reduce the fatigue and other service characteristics of the blades from Ni base superalloys. Hot isostatic pressing (HIP) reduces the size and number of micropores in the cast blades from Ni base superalloys with a polycrystalline (alloy GS6U) and single crystal (alloy GS32) structures. As a result, HIP improves the mechanical properties of the blades. Application of the HIP, together with the standard heat treatment in the renovation of the blades after long operation can provide a virtually complete restoration of their structure and efficiency.

Advances in multi-scale modeling of solidification and casting processes

The development of the aviation, energy and automobile industries requires an advanced integrated product/process R&D systems which could optimize the product and the process design as well. Integrated computational materials engineering (ICME) is a promising approach to fulfill this requirement and make the product and process development efficient, economic, and environmentally friendly. Advances in multi-scale modeling of solidification and casting processes, including mathematical models as well as engineering applications are presented in the paper. Dendrite morphology of magnesium and aluminum alloy of solidification process by using phase field and cellular automaton methods, mathematical models of segregation of large steel ingot, and microstructure models of unidirectionally solidified turbine blade casting are studied and discussed. In addition, some engineering case studies, including microstructure simulation of aluminum casting for automobile industry, segregation of large steel ingot for energy industry, and microstructure simulation of unidirectionally solidified turbine blade castings for aviation industry are discussed.

Gas turbine blade manufacturing by use of epoxy resin tooling and silicone rubber molding techniques

PurposeConventional investment casting of turbine blades is a time consuming and expensive process due to the complications in wax injection steps and the complex shape of airfoil surfaces. By using rapid investment casting, a substantial improvement in the gas turbine blade manufacturing process can be expected. However, this process needs to be able to compete with conventional investment casting from a dimensional accuracy view of point. The purpose of this paper is to investigate the manufacture of gas turbine blades via two indirect rapid tooling (RT) technologies, namely epoxy (EP) resin tooling and silicon rubber molding.Design/methodology/approachThe second stage blade of a Ruston TA 1750 gas turbine (rated at 1.3 MW) was digitized by a coordinate measuring machine. The aluminum‐filled EP resin and silicon rubber molds were fabricated using StereoLithography master models. Several wax patterns were made by injection in the EP resin and silicone rubber molds. These wax patterns were utilized for ceramic shell fabrication and blade casting.FindingsDimensional inspection of cast blades showed that silicone rubber molding was not a suitable approach for production of blade wax patterns. The maximum deviation for the final cast blade made using the silicone rubber mold was +0.402 mm. The maximum deviation for the final cast blade made using the EP resin mold was lower at −0.282 mm. This showed that EP resin tooling could enable new cost‐effective solutions for small batch production of gas turbine blades.Practical implicationsThe research results presented will give efficient industrial approach and scientific insight of the gas turbine blade manufacturing by use of rapid technologies.Originality/valueThere are some general research works related to utilization of rapid technologies for manufacturing of gas turbine blade. However, this paper presents a unique procedure of integrated reverse engineering and RT technologies for rapid investment casting of gas turbine blade through presenting comprehensive comparison between two techniques from dimensional accuracy view of point.

Distortion Behavior of a Heavy Hydro Turbine Blade Casting During Forced Air Cooling in Normalizing Treatment Process

Distortion behavior of blade castings in heat treatment process determines their geometrical accuracy, and improper control of it may result in additional repair, shape righting, or even rejection. This article presents a novel approach for discovering the distortion behavior of heavy blade castings during heat treatment process in production. Real-time measurements of distortion and temperature field of a heavy hydro turbine blade casting weighted 17 ton during forced air cooling in normalizing treatment process were carried out by using deformation measurement instruments and an infrared thermal imaging camera. The distortion processes of the typical locations of blade and the temperature field of the blade were obtained. One corner on the blade outlet edge side exhibits variation of distortion with two peaks and a valley. The range reaches 97 mm and the final distortion value is 76 mm. The maximum temperature difference on blade surface reaches 460 °C after 80 min of cooling. Influences of thermal stress and phase transformation stress on the distortion of the blade were elucidated and discussed. The results are of great significance for the understanding and control of the distortion behavior of hydro turbine blades in heat treatment.

Distortion behavior of hydro turbine blade castings during heat treatment processes under variation of seasonal temperature

Distortion often appears at the corners of heavy turbine blade castings during heat treatment processes, so a great machining allowance is generally set in production which directly results in cost increase. In this paper, a novel real-time measuring technology is developed for non-contact measuring the deformation behavior of heavy steel castings in heat treatment process. It was employed to measure the distortion and the temperature field of a batch of heavy turbine blade castings at cooling stage in normalizing process. Three inflection points appear in the distortion-time curves, and the residual distortion is affected by the regional area of not finished martensite transformation when the second inflection point appears. When the mean air temperature falls into the range of 10°C–20°C, the residual distortion is small; when it is lower than 10°C, positive distortion appears; when it is higher than 20°C, negative distortion appears. The distortion varies with seasonal temperature, which is directly responsible for the great machining allowance given in production.

Modeling of grain selection during directional solidification of single crystal superalloy turbine blade castings

Single crystal superalloy turbine blades are currently widely used as key components in gas turbine engines. The single crystal turbine blade casting’s properties are quite sensitive to the grain orientation determined directly by the grain selector geometry of the casting, A mathematical model was proposed for the grain selection during directional solidification of turbine blade casting. Based on heat transfer modeling of the directional withdrawing process, the competitive grain growth within the starter block and the spiral of the grain selector were simulated by using the cellular automaton method (CA). Validation experiments were carried out, and the measured results were compared quantitatively with the predicted results. The model could be used to predict the grain morphology and the competitive grain evolution during solidification, together with the distribution of grain orientation of primary dendrite growth direction, with respect to the longitudinal axis of the turbine blade casting.

Analytical evaluation of the kinetics of pore removal in cast turbine blades under hot isostatic pressing

The method of scanning electron microscopy is used to study the structure of refractory alloy ZhSBU-VI forming in the process of hot isostatic pressing. The stress-strain state of the metal is analyzed in the zone of a spherical pore in an approximation of the solution to the Lame problem for a void thick-wall sphere under the action of external hydrostatic pressure. An analytical dependence of the creep rate of the refractory alloy on the temperature-and-force parameters of the loading is used to perform a kinetic evaluation of the process of densification of turbine blade castings due to hot isostatic pressing.

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Gravity and centrifugal investment casting processes of low-pressure turbine blades with high Nb–TiAl alloy were simulated by Procast software. Actual blade components were poured by vacuum induction suspended furnace with Ar protection. The experimental verification indicated that the simulation results were in good agreement with the experimental results. Comparative results had shown that the surface of centrifugal casting blade was more complete than that of gravity casting one. In gravity casting process, molten metal filled the thinnest trailing edge at last, resulting in the generation of misrun defects. Furthermore, the shrinkage porosity and crack defects of gravity casting were much more and dispersive. The internal and external quality of centrifugal casting was much better than that of gravity casting. Microstructures from edge to center of gravity casting blade had no significant change. The microstructure for centrifugal casting blade is finer than that for gravity casting blade, however, a large number of dentritic γ segregation appeared in the blade edge of centrifugal casting, which resulted from the fast cooling rate of centrifugal casting surface.