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Casting Technology of Large-Size Turbine Blade

Posted: 2014-11-18 02:20:51  Hits: 2142
Abstract: The article summaries the processing status of investment casting technology of large-size turbine blades. Dies designing and making, wax dimension controlling, techniques of shell, techniques of direction blade, single-crystal blade and other processing ways are introduced as examples. The article also discusses the future trend of those key process methods.
 
Keywords: Large-size Blade, Investment Casting, Lost Wax Casting, Orientation And Single Crystal Alloy, High Temperature Alloy
 
Gas turbine is the third generation of machinery power which is the most market application prospect in the 21st century. it is the core technology of the power system. With the further development of gas resources, the development of the storage and transportation technology and the expansion of the application field, especially after the advanced technology of transplant aviation gas turbine in nineties of the 20th century, the unit capacity and efficiency of gas turbine have been greatly ascend, but the development of new type gas turbine combined cycle unit with large capacity and high efficiency, making the gas turbine won the unprecedented rapid development in the power generation field. In recent years, electric power system reform and strengthening environmental protection makes the power generation assembly low cost of installation, low pollutant emission, high thermal efficiency, fast start-stop, short construction period, high reliability and availability, small footprint and low water consumption. It has been got more and more attention all over the world.
 
There are 21000 units gas turbine above 1 mw power in the world. The total capacity is 1 million megawatts, close to one fifth of the world's total electricity generation. By the end of 2010, China's gas turbine capacity has more than 34000 mw, accounting for the total installed capacity of 3.5%. But China's gas turbine development ability is weak. Technology of material and core parts, especially hot components manufacturing technology is difficult to bring in, so the manufacturing technology of high temperature parts is the key to the research and development of domestic gas turbine localization. Turbine blades is the key core components for the gas turbineFig.1. Because of poor working conditions, we usually use high temperature alloy with high-temperature and corrosion resistance to make it. High temperature alloy initially was applied to the hot end components manufacturing of the aero-engine. With the development of the aerospace engine technology continuous, precision casting with non allowance becomes the main method of manufacturing high temperature alloy turbine blade in the nineteen sixties. The study of high temperature alloy material has accumulated enough in the 1970s. Since the 1980 s, new technology has become the main way to improve the high temperature alloy material performance. Compared with the aircraft engine blade, heavy duty gas turbine blade is heavier, bigger, and longer. Thus it also puts forward more requirements on investment casting process.
 
 Image of large-size turbine blade casting
Fig 1. Image of large-size turbine blade casting

1. The development of investment casting process of gas turbine engine turbine blade
1.1 Technology of mold design and manufacturing
In 1950s, precision casting mold was designed mainly by the designers’ two-dimensional serial by hand in the drawings. It had high requirements, long period and frequent repeat. In the late 1960s, with the development of computer hardware and software technology, there appeared some mould CAD design software, such as Auto CAD etc. Designers could use the computer to CAD mould design. It greatly improved the efficiency of mould design, and shortened the cycle of mold design. In 1990s, there appeared CAD/CAE/CAPP/CAM powerful 3D engineering software such as UG PRO/E CATIA etc. Designers could use it for secondary development, and finished mold design and manufacturing process. It took the precision mold manufacturing cycle to a new level.
 
Precision casting mold basically has the following two methods of manufacture. One is composite machining method. Combined machining is a technique that using a variety of energy together to remove the material, in order to improve the efficiency of processing or get a high dimensional accuracy, shape precision and surface integrity. Composite processing method which was commonly used in precision casting mould manufacturing was supplemented by electric pulse processing when using machining at the same time. Another is a method of rapid prototyping (RP). Rapid prototyping technology was a new type of integrated manufacturing technology and quickly developed in the 1980s. RP technology combined mechanical engineering, CAD, CAM, CNC, detection technology, precision servo drive technology and new materials. Process a series of thin layer section and get section profile for each layer according to 3D CAD design model on the computer.
 
Use rapid-prototyping machine to heap up the material layer-by-layer, and finally generate three-dimensional entity model or parts according to two-dimensional data of each layer cross section shape. Rapid prototyping method has the characteristics of short cycle and wide applicability, but the precision and intensity of wax models still has some problems at present. As a result, it is not suit for mass production, and the main way of precision casting mould is composite processing method at present.
 
In the process of mold design and machining, the shrinkage rate of mold design is an important parameter. It largely decides dimension precision of casting. Because of the big size turbine blade casting structure is complicated, wall thickness differs greatly. The restriction of up and down marginal plate etc, casting is relatively limited during solid-state contraction. Casting’s final total contraction is comprehensively combined wax models alloy contraction with a certain amount of shell expanding. Usually, shell expansion and wax mold cooling contraction are fairly, and reduced scale of mold design mainly depends on the shrinkage rate of alloy. In China, mold profile design mainly use profiles scaling method to compensate at present. It is divided into 4 kinds of uniform scaling method, namely chord length shrinkage method, mean camber line scaling method and shrinkage center scaling method. This method is easy but obviously has some shortcomings. First, the contraction of the casting is approximate uniform, with the assumption that the shrinkage value is the same in different parts. Second, the volume shrinkage ratio of leaves is approximate, ignoring the geometric characteristics such as deformation of blade bending and twisting, curvature of leaf area, etc.   
 
1.2  Size control technology of wax mold 
The quality of wax pattern directly determines the final dimension precision of casting. High quality mould and reasonable mould technology is the key to ensure the quality of wax pattern. If the gas turbine blade size is bigger, part of the blade’s length is more than 400 mm, and wax mold size is big with slow cooling, the wax mold shrinkage will be large. That is because the leaf blade’s body and channel using precision casting with non-allowance, blade body parts is complex curved surface and thickness is uneven. Thus the shrinkage brings great difficulty in size precision control in production. In order to solve this problem, we usually use false core of wax mould technology to control the wax pattern size in the process of gas turbine blade production. Reduce the wall thickness of wax mould through suppression of wax mould’s false core in advance. On the one hand, it ensures the completeness of filling wax models. On the other hand, it reduces the contraction of the wax models, preventing shrinkage and dimension change sharply. We can effectively solve size problems caused large volume shrinkage when wax injection through the adjustment and cooperation of suppress parameter.
 
In the actual production of the blade wax models, in order to improve the production efficiency, wax models are not fully cooling after completion of the suppression of wax models, and wax mold shrinkage is mainly produced after module delivery within 0.5 h. Large size blade wax models is easy to cause the irregular deformation of blade body parts when the unrestricted contraction. This deformation is hard to observe and control, which can cause the final size deviation, and result in leaf blade scrap. To solve this problem, wax pattern correction module technology become the necessary tools for the precision casting of large gas turbine‘s turbine blade, wax models’ moldings bed making by blade shape. It can be used to calibrate the wax mold size in the process of cooling after leaf blade wax models delivery, and reduce the size deviation brought by the wax models contraction deformation in the process of cooling.
 
1.3 Shell manufacture process
The casting mould of Investment casting often referred to as shell. Manufacturing of high quality shell is necessary if you want get casting with smooth surface, corner angle clear, and precise dimension. High quality shell should meet series of performance requirements, including strength, permeability, thermal conductivity, linear change and thermal shock resistance, thermal chemical stability and shelling etc. In the preparation of shell, good process performance can accurately reproduce the casting shape and ensure accurate size and smooth surface. Shell should have enough normal and high temperature strength and necessary rigidity in order to avoid the deformation, cracks and breakage under the complex stress function in the process of dewaxing, roasting, pouring etc.
 
Blade casting usually applies silica sol shell. Silica sol is a high-quality adhesive commonly used for investment casting, which is convenient and good stability coatings. Shell manufacturing is simple without chemical hardening. The shell has good heat performance, high ability of high temperature strength and high temperature resistance to deformation, but the silica sol coating on the wet ability of wax is poor. It needs to be added surfactant to improve smearing performance of the paint coating. Otherwise, colloidal silica drying speed is slow, shell wet strength is low, and shell making cycle is long. Silica sol is a water-based adhesive without air pollution in producing, so it has become the main shell making binder for high temperature alloy precision casting.
 
The bad quality of the silica sol shell strength or surface in production is mostly caused by bad control in shell dry link. Shell’s performance and quality is directly related to dry hardening. Shell’s strength is good if it dries thoroughly, and vice versa. The main factors influencing the dry shell including environment humidity, wind speed (air velocity), environment temperature, types of refractory etc. Assay methods of shell dry consist gravimetric method, development process, resistivity method, γ-ray attenuation method etc.
 
Dewax after shell dry and form the mould cavity. Clean the shell after dewaxing in order to prevent dirt residues in the shell. In order to improve the shell’s low temperature and high temperature strength and increase the shell cold strength and shell's damage ability to fight metal liquid when high temperature casting, shell needs to be roasted. For silica sol shell, the calcination temperature generally is 970 ~ 1030 ℃. Controlling the calcination temperature is a key factor for shell quality. If the shell roasting temperature is too low, shell reaction is not completely. If the shell high temperature strength is not enough, cracks can be easily generated in pouring process. If the shell roasting temperature is too high, it will easily make the shell deformation, which is bad for the dimension precision of casting.
 
Preheating is needed before pouring, to remove the residual moisture in the shell and get ready for hot pouring in investment casting. Under the same condition of the pouring temperature and modeling, higher shell temperature can effectively reduce the alloy melt and mold temperature, reduce the speed of alloy melt temperature, and improve the ability of alloy filling. With the increase of shell temperature, alloy melt filling capacity is improved significantly. But if the shell temperature is too high, it is difficult to form a larger temperature gradient during casting solidification. It is easy to cause loose organization and coarse grains. The general preheating temperature is 950 ~ 1050 ℃.
 
In the process of dewaxing and roasting, shell has microcracks because of the expansion of the wax pattern, which will affect the casting quality. In order to prevent this situation, shell microcracks can be prevented by permeability methyl blue test. We can clearly understand the degree of shell’ crack according to the seepage situation. We can use reinforce and repair methods to solve the problem of shell micro cracks.
 
1.4 Manufacturing technology of mold core
With the efficiency of gas turbine constant improving continually, the working temperature of gas turbine blades also improve continually. So, a new type of gas turbine blades usually has internal cooling channels that allow cold air flowing through. Because of the cooling requirement, most of these internal cooling channel’s structure is complex and in small size. In the process of precision casting, the core cavity of these narrow complex castings can be produced if using the process of conventional dip coating, stucco etc. It only forms through the ceramics core. That requests the production of ceramic core to be sufficient strength and stability at high temperature. According to different materials, ceramic cores can be divided into silica-based ceramic core and aluminum ceramic core. Silicon-based ceramic core’s high temperature active, so it is easy to produce porosity and adhering sand on the interface of ceramics and metal. Aluminum matrix ceramic core has a problem that it is difficult to emerge from the casting. With the continuous development of gas turbine blade, the demands of advanced ceramic core requirements are constantly improve. It requires that the advanced ceramic core has characteristics with high refractoriness, low thermal expansion rate, high temperature and high strength, good chemical stability and easy removal. Therefore, on the base of the research of ceramic core manufacturing methods, constantly further improve the performance of silica-based ceramic core through the study of different types of enhancer for silicon ceramic core, and more effective and simpler stripping method is in study for aluminum ceramic core.
 
1.5 Technology of orientation and single crystal blade
China began to research the precision casting technology of directional column crystal alloy and hollow non allowance blade in the late 1960s. After 20 years of efforts, we have successfully developed DZ4 DZ22 directional alloy, etc, and mass production now. We also started to use precision casting technology to produce single crystal alloys and single crystal blades in the late 1980s. Our country has developed the single crystal blade used for aviation engine at present. Orientation and single crystal blades also become the development trend of gas turbine blades. Since gas turbine blade size is bigger than the aircraft engine blade size, manufacturing difficulty of orientation and single crystal turbine blade also greatly increases. International advanced heavy-duty gas turbine blades have been adopted by the directional blades now, and our country has begun to research the large size of directional gas turbine blade. From the developing trend of gas turbine, with the improving of the gas turbine performance, orientation and single crystal alloy will be important materials for the future gas turbine.
 
2. Summary and outlook
Gas turbine is aero-engine’s technical extension in the field of non-aero. It can be widely used in ships power, panzer tank vehicle dynamics, electricity and gas/oil transportation. It is high technology products with a great development prospect, which is related to national economy and people's livelihood national such as defense, energy, transportation, environmental protection etc. With the continuous development of modern science and technology, there are higher requirements on gas turbine. In order to meet the characteristics of high power, high gas initial temperature, high pressure ratio, high efficiency, long life of modern heavy duty gas turbine, the material and casting process requirements of precision casting blades for gas turbine also continue to improve. There will be more advanced precision casting technology being applied to the production of gas turbine blades in the future, and the precision casting technology of gas turbine blade will be further improved.
 
From the domestic economic and technological level, the key technology development of large size turbine blade casting is as the following:
A. Research the practical application of mould non-uniform scale design, wax moulds composite forming technology, predeformation technology, providing high precision wax models that can meet the technical.
B. Speed up the study of core material and its forming process, providing technical support to the design and manufacturing of hollow turbine blade that can resist higher temperature.
C. Use directional solidification of alloy and the next generation single crystal alloys as well as its casting process to develop new and large size gas turbine blades.

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