Patent Application
20100233504
The present invention relates to methods and materials for manufacturing gas turbine engine components. More particularly the invention relates to improved methods and materials with which to manufacture impellers and impeller-like rotating components comprising more than one microstructure.
In an attempt to increase the efficiencies and performance of contemporary jet engines, and gas turbine engines generally, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently projected place increased demands on engine components and materials. Indeed the gradual change in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine.
The compressor stage of the gas turbine engine is one area that has seen increased demands placed on it. For example, increasing performance and reliability demands for gas turbine engines require both high compression ratios and reduced compression stages. Relatively higher compression ratios in turn result in high compressor discharge temperatures. A reduced number of compression stages to accomplish higher compression ratios results in higher compressor stage tip speeds and higher bore stresses. These combined demands have made it very difficult to utilize monolithic alloy impellers for high pressure compressor (HPC) stages of gas turbine engines. It would thus be desirable to develop a high pressure impeller that can withstand the increased pressures and temperatures associated with gas turbine engines. It is also desired that the impeller design be suitable to relatively smaller gas turbine engines.
A rotary compressor such as an impeller undergoes differing stresses at differing locations. Typically a central opening or bore defines an axis about which the rotor spins. In the case of an HPC impeller, multiple airfoils extend radially outward from a bore and axially along the length of the bore. Additionally impellers wrap tangentially, from an inducer section near the inner diameter to the exducer near the impeller outer diameter. In operation, an impeller receives a fluid, such as air, at an upstream axial position. Due to the rotational movement of the impeller, the air is compressed. Typically, a given volume of air that is being compressed is passed from an upstream position to a downstream position in the impeller. As the air exits the impeller, at an outwardly radial position, it is at a relatively higher pressure and temperature than it was when the air first contacted the impeller.
It should be noted that this general structure of a gas turbine impeller is also true of other rotary devices such as turbines found in turbochargers and turbopumps. The principles of the invention described herein are thus applicable to these devices as well.
As mentioned, an impeller is characterized by differing stresses at different impeller locations. Stresses due to rotation are greatest in the bore section. These stresses arise as a result of the high centrifugal forces that develop during high RPM operation. It is this area where cracks tend to develop and propagate. Hence, it is an important design criterion that materials in this area of the impeller have relatively high strength characteristics.
Differences in temperature also occur at different points in an operating impeller. As previously noted, air enters an individual impeller at a relatively lower temperature and pressure. When this same air exits the impeller it is at a relatively higher temperature and pressure. Thus, the upstream leading edge of an impeller airfoil at the inducer experiences relatively lower temperatures; and the outer radial edge of an impeller, the area where compressed gas exits, the exducer, experiences relatively higher temperatures. As a consequence, materials used in the gas exiting region must be selected to withstand these high temperatures.
Hence there is a need for an improved impeller design and method to manufacture the same. The improved design should take advantage of material characteristics that provide high strength and high temperature performance. It is desired that the impeller, and the method of manufacturing the impeller, provide improved strength performance in bore regions while also providing improved high temperature performance in the outward radial positions. It has therefore been conceived that a dual microstructure approach, combining a high strength bore region having a fine grain size and a high temperature outer blade ring microstructure having a coarse grain size, offers a viable solution. There is a need that the improved impeller design maintains advantageous weight performance of materials. The present invention addresses one or more of these needs.
The present invention provides a method and materials for fabricating a dual microstructure gas turbine engine rotor. In particular, the method may be applied to dual microstructure impellers characterized as withstanding operating temperatures in excess of approximately 1350° F. (732 degree Celsius). The method includes steps to fabricate a dual microstructure element capable of withstanding high operating temperatures.
In one embodiment, and by way of example only, there is provided a method for fabricating a dual microstructure machinable element comprising: providing an intermediate structure including a bore region comprising a nickel based superalloy having a grain size that is finer than ASTM 10.0 and a body region comprising a nickel based superalloy having a grain size that is coarser than ASTM 7.0, the bore region and the body region defining a microstructure interface; and machining the intermediate structure to define the dual microstructure machinable element.
In a further embodiment, still by way of example only, there is provided a method for fabricating a dual microstructure element comprising: providing a nickel based superalloy with high strength properties; atomizing the nickel based superalloy to form an atomized nickel based superalloy powder; forming the atomized nickel based superalloy powder into a bore region having a grain size finer than ASTM 10.0 and a body region having a grain size coarser than ASTM 7.0, the bore region and the body region defining an intermediate structure having a microstructure interface; and machining the intermediate structure to define the dual microstructure element.
In a further embodiment, still by way of example only, there is provided a structure suitable for processing into a turbine impeller comprising: a bore region wherein the bore region comprises a nickel based superalloy with high strength properties having a fine grain size of ASTM 10.0 or finer; and a body region wherein the body region comprises a nickel based superalloy having a coarse grain size of ASTM 7.0 or coarser. The bore region defines a first microstructure and the body region define a second microstructure, the first microstructure and the second microstructure defining a dual microstructure interface.
Other independent features and advantages of the method of fabricating a dual microstructure impeller will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The present invention will hereinafter be described in conjunction with the following drawing figure, wherein:
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In addition, all grain sizes are given in accordance with known methods for determining average grain size standards as set forth by the American Society for Testing and Materials (ASTM), and in particular ASTM E112.”
Referring now to
In the impeller configuration as shown in
It has now been discovered that an impeller can be designed and manufactured so that the impeller is comprised of dual microstructures, wherein a microstructure of a bore region is different than the microstructure of a body region. In one preferred embodiment, dual microstructures form an intermediate forging that may itself be further machined into a finished impeller. The finished impeller thus incorporates the dual microstructure of the intermediate forged structure.
The material properties resulting from the dual microstructure are selected so that material performance is optimized given the location of the material in the final product. The fine grain material properties in the area of the impeller bore are optimized for low cycle fatigue resistance and burst strength. Similarly, the coarse grain material properties in the area of the fluid exit are optimized for high temperature creep resistance. Referring now to
In a preferred embodiment, the bore region 20 is fabricated having a specific fine grain microstructure, and the body region 22 is fabricated having a specific coarse grain microstructure that is different than the microstructure of the bore region 20. The differing microstructures of the bore region 20 and the body region 22 define a microstructure interface 36. It should be understood that a slope of the microstructure interface 36, as illustrated in
Each of the bore region 20 and the body region 22 may be formed through known methods of powder metallurgy, extrusion, isothermal forging, heating, and machining (described presently). The bore region 20 and the body region 22 may further include flanges, thrust faces, and other shapes (not shown) that assist in the manufacture process and ultimately be machined away in order to yield a finished impeller shape. The body region 22 may include the airfoils described in
The back face 24 is an area of an impeller where the elevated temperature properties of the material are important. Although the temperature is higher at the blade tip, the stress is also lower at the tip. It has been discovered that the back face 24 is generally an area where the stress and temperature combination becomes more critical. Thus, in a preferred embodiment, the properties of the region of the back face 24 are considered with respect to creep resistance.
In a preferred embodiment, the material used in the fabrication of the dual microstructure impeller is a high strength superalloy. Superalloys that may be utilized to fabricate the dual microstructure include a nickel (Ni) based superalloy such as an atomized powder metal (PM) alloy 10 or other similar material. The material is chosen due to its inherent low cycle fatigue (LCF) and tensile properties at bore conditions, typically at or near 1050° F. (565.6 degree Celsius) and excellent oxidation and creep/stress rupture properties at body or rim conditions, typically at or near 1350° F. (732 degree Celsius) and above. The microstructure of the body region 22 is preferably formed having improved creep resistance when exposed to temperatures in a range of between about 1250° F. (676.7 degree Celsius) to about 1500° F. (815.6 degree Celsius) that is greater than the creep resistance of the bore region 20, when exposed to temperatures in the same range.
A preferred embodiment has been described as a method to fabricate an intermediate structure including dual microstructure regions. The finished impeller may be fabricated of more than two regions having different microstructures. It is preferred during the fabrication process that the microstructure interface 36 be linear in cross section. However, other shapes for the microstructure interface 36 may be formed. For example, in cross section, the microstructure interface 36 may include composite interfaces of differing angles, curves, or other complex shapes.
As previously stated, both the bore region 20 and the body region 22 may themselves be cast, forged or formed by powder metallurgy techniques or otherwise machined so as to minimize the material that must be removed in order to create the impeller. The body region 22 need not have a typical outer shape in the form of a cylinder, but may take other shapes. The bore region 20 may initially be formed so that it has a hollow axial area (not shown) that corresponds to where a central bore area would appear, if such an area is part of the design of a finished impeller such as the central bore area 13 of
Turning now to
In a preferred embodiment the impeller manufacturing process may include heat treatments that are designed to control stresses and optimize the microstructure of the structure, as steps 110 and 112. It will be understood by those skilled in the art that a particular heat treatment may be tailored depending on the desired resultant microstructure, and more particularly desired grain size of each of the bore region 20 and the body region 22. Accordingly, preferred heat treatments can be defined in terms of the microstructure that results from the treatment. As previously described, a nickel based superalloy is preferred for both the bore region 20 and the body region 22. When these materials are used, the following described heat treatments are preferred.
As best illustrated by step 110 in
In an alternate method, as illustrated in
Following step 212, the intermediate structure 60 undergoes a direct aging treatment as step 214. During this treatment step, the intermediate structure 60 is heated to approximately 1400° F. The goal of this step is to impart optimum mechanical properties for the application. As a final step, the intermediate structure 60 may be machined to a specified configuration in a step 216, using a combination of conventional and non-conventional machining processes as detailed with respect to
It will be understood by those skilled in the art that the target microstructures in the bore region 20 and the body region 22 may be achieved while deviating from the above-described specific heating temperatures due to heating times. For example a material may be heated at a slightly higher temperature for a shorter time period, or, heated at a slightly lower temperature for a longer period of time. Thus, it is still within the invention to deviate from the specific heating schedule while achieving the finished dual microstructures.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
