Abstract:
There is provided a method for fabricating a dual alloy structure that may in turn be machined to fabricate a rotary component for use in a gas turbine engine. The method provides a powder metal (PM) nickel based superalloy and a nickel aluminide intermetallic based alloy. The powder metal (PM) nickel based superalloy displays characteristics, such as improved strength, low cycle fatigue resistance, fracture toughness, and crack growth resistance. The nickel aluminide intermetallic based alloy displays characteristics, such as high temperature creep and oxidation resistance, suitable for use in the outer radial area of an impeller. A bore sub-element is fabricated from the powder metal (PM) nickel based superalloy. A body sub-element is fabricated from the nickel aluminide intermetallic based alloy. The bore sub-element and body sub-element are joined by inertia welding or diffusion bonding at a common mating surface.

Description:
TECHNICAL FIELD 
     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 alloy. 
     BACKGROUND 
     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. It has therefore been conceived that a dual alloy approach, combining a high strength bore alloy and a high temperature outer blade ring material, offers a viable solution. 
     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. 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. 
     BRIEF SUMMARY 
     The present invention provides a method and materials for fabricating a dual alloy gas turbine engine rotor. In particular, the method may be applied to dual alloy impellers characterized as withstanding operating temperatures in excess of approximately 1200° F. (650° C.). The method includes solid state diffusion bonding technology or inertia welding to obtain reproducible, high quality adhesion between impeller subcomponents and further control residual stresses and impart microstructures appropriate for the end use application. 
     In one embodiment, and by way of example only, there is provided a method for fabricating a machinable structure comprising the steps of: providing a bore sub-element comprising a γ/γ′ powder metal (PM) nickel based superalloy; providing a body sub-element comprising a nickel aluminide intermetallic based alloy, the nickel aluminide intermetallic based alloy having improved strength and oxidation resistance when exposed to temperatures in a range of between about 1250° F. to about 1500° F., that is greater than a strength of the γ/γ′ powder metal (PM)nickel based superalloy, when the γ/γ′ powder metal (PM) nickel based superalloy is exposed to temperatures in the range; contacting the bore sub-element and the body sub-element; joining the bore sub-element and the body sub-element; and machining to define the machinable structure. 
     In a further embodiment, still by way of example only, there is provided a method for fabricating a composite structure comprising: providing a nickel based superalloy with high strength properties; forming the nickel based superalloy into a bore sub-element having a mating surface; providing a nickel aluminide intermetallic based alloy with high creep resistance and oxidation resistance properties when exposed to temperatures in a range of between about 1250° F. to about 1500° F. that is more than the strength and oxidation resistance of the nickel based superalloy, when the nickel based superalloy is exposed to temperatures in the range; forming the nickel aluminide intermetallic based alloy into a body sub-element having a mating surface; joining the bore sub-element to the body sub-element so as to form an intermediate structure; and machining the intermediate structure. 
     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 sub-element wherein the bore sub-element comprises a powder metal (PM) nickel based superalloy with high strength properties; and a body sub-element wherein the body sub-element comprises a nickel aluminide intermetallic based alloy and wherein the bore sub-element and the body sub-element are joined at a mating interface. 
     Other independent features and advantages of the method to fabricate a dual alloy 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figure, wherein: 
         FIG. 1  is a schematic view of a prior art impeller; 
         FIG. 2  is a side view of an impeller cross section illustrating dual alloys according to an embodiment of the present invention; 
         FIG. 3  is a side view of an impeller cross section illustrating dual alloys according to an embodiment of the present invention; 
         FIG. 4  is a side view of an impeller cross section illustrating dual alloys according to an embodiment of the present invention; 
         FIG. 5  is a side view of an impeller cross section illustrating dual alloys according to an embodiment of the present invention; 
         FIG. 6  is a perspective view of a bore sub-element and body sub-element according to an embodiment of the present invention; and 
         FIG. 7  is a flow chart depicting an exemplary method for forming an impeller structure according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Referring now to  FIG. 1  there is shown a representation of a typical impeller suitable for use with the present invention. An impeller  10  includes a plurality of impeller airfoils  11  attached to a central core  12 . The impeller  10  has a generally radial structure and, as shown in  FIG. 1 , a central bore area  13 . In some designs, the impeller  10  is fabricated as a unitary piece with an axle and would not have an open bore area though it would have the corresponding bore region. The central bore area  13  is aligned along an imaginary central axis  14  that runs through the central bore area  13  in an axial direction. In operation, the impeller  10  is disposed on a central axle (not shown) at the central bore area  13  and rotates thereon or rotates with the axle. The plurality of impeller airfoils  11  extend from the central bore area  13  in an outwardly radial and axial direction. The impeller  10  further defines an upstream position  15  and a downstream position  16 . The upstream position  15  and the downstream position  16  correspond to the fluid path flow through and across the impeller  10 . Fluid, air, first enters the impeller  10  at the upstream position  15  (inducer). As air passes the impeller  10  it exits in the downstream position  16  (exducer). Air passing across the impeller  10  is pressurized such that the air exiting the impeller  10  is at a higher temperature and pressure relative to the air entering the impeller  10 . The direction of an air flow  17  is across the face of the impeller  10 , the face being that portion of the impeller  10  which is exposed to air flow. In operation, the impeller  10  is disposed within a housing or structure (not shown) which, by close proximity to the plurality of impeller airfoils  11 , assists in placing the air under pressure. 
     In the impeller configuration as shown in  FIG. 1 , the plurality of impeller airfoils  11  press against air as the impeller  10  rotates. The plurality of impeller airfoils  11  act to compress the air. The rotation of the impeller  10  during this compression imparts high tensile stresses in the central bore area  13 . Simultaneously, air that exits the impeller  10  at the downstream position  16  (exducer) is typically at a much higher temperature than compared to the air entering in the upstream position  15  (inducer). Temperatures in excess of 1000° F. (537.8 degree Celsius) can be experienced at the downstream position  16  (exducer). Thus, the structure in the downstream position  16  and on a back face  24  ( FIG. 2 ) are particularly subject to high temperature creep and fatigue. 
     It has now been discovered that an impeller can be designed and manufactured so that the impeller is comprised of multiple alloys. In one preferred embodiment, dual alloys are joined to form an intermediate structure that may itself be further processed into a finished impeller. The finished impeller thus incorporates the dual alloys of the intermediate structure. 
     The combination of materials to create the intermediate structure is selected so that material performance is optimized given the location of the material in the final product. The material that will be proximate to the bore of the impeller is selected for suitable strength properties. Similarly, the material placed in the area of the fluid exit is chosen for suitable high temperature properties. Referring now to  FIGS. 2 ,  3 ,  4 , and  5  there are illustrated exemplary embodiments of the material selection in a silhouette of an impeller cross-section. In each illustration, a region  20  represents a bore sub-element, and a region  22  represents a body, or rim, sub-element. As shown, the bore sub-element  20  and the body sub-element  22  can be fashioned so that a chosen material extends to a finished location on the impeller. 
     The back face  24  of the impeller cross section is indicated in  FIG. 2 . 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 composition of the region of the back face  24  is considered with respect to creep resistance. 
     A preferred method to manufacture the dual alloy impeller is further illustrated in  FIGS. 6 and 7 . More specifically,  FIG. 6  illustrates a bore sub-element  20  and a body sub-element  22 . The bore sub-element  20  and the body sub-element  22  are configured so that they may be brought together in close mating alignment. In a preferred embodiment, the bore sub-element  20  and the body sub-element  22  are brought into mating alignment along corresponding mating surfaces  34 , thus forming a mating interface (described presently) when joined. In a preferred embodiment illustrated in  FIG. 6 , the bore sub-element  20  is generally conical in shape. The mating surface  34  of the bore sub-element  20  follows the line of the conical shape. Similarly, the body sub-element  22  has a hollow that is in conical form. The mating surface  34  on the body sub-element  22  also follows the conical shape. Subsequent to joining of the mating surfaces  34 , a mating interface  36  is formed, wherein the slope of the mating interface  36 , in various embodiments, is illustrated in  FIGS. 2 ,  3 ,  4 , and  5 . Each of these figures illustrates the mating interface  36  having a different slope and/or position. When brought into close mating alignment, the bore sub-element  20  and the body sub-element  22  are in substantial contact along the mating interface  36 . Preferably, the contact is sufficient so that the bore sub-element  20  and the body sub-element  22  may be joined at the mating interface  36  through diffusion bonding techniques or inertia welding techniques, as described below. 
     Each of the bore sub-element  20  and the body sub-element  22  may be formed through known methods of powder metallurgy, extrusion, forging, and machining (described presently). The mating surfaces  34  on the both bore sub-element  20  and the body sub-element  22  may be formed through these known methods. The bore sub-element  20  and the body sub-element  22  may include flanges, thrust faces, and other shapes that assist in the manufacture process. The body sub-element  22  may include the airfoils described in  FIG. 1  or material from which such airfoils may subsequently be formed. 
     Referring again to  FIG. 6 , the bore sub-element  20  and the body sub-element  22  can be joined at the mating interface  36  through solid state diffusion bonding techniques or inertia welding techniques. The solid state diffusion bonding techniques may include a vacuum braze followed by a hot isostatic pressing (HIP) diffusion cycle. Upon completion of the joining operation, what remains is an intermediate structure that includes both the original bore sub-element  20  and the body sub-element  22 . The joint between the two pieces is sufficiently strong and secure that the intermediate structure can be further machined and formed to create the finished impeller shape. Further, the bond is sufficiently strong to allow the joined pieces, when machined, to operate as an impeller. 
     In a preferred embodiment, dual alloy combinations are selected from high strength alloys and superalloys. Alloys that may be utilized for the bore sub-element  20  include a powder metal (PM) nickel (Ni) based superalloy such as an atomized powder metal (PM) alloy  10 . The bore sub-element materials are chosen due to their inherent low cycle fatigue (LCF) and tensile properties at bore conditions, typically at or near 1250° F. (676.7 degree Celsius). In one particular embodiment, the bore sub-element  20  is formed of a γ/γ′ PM nickel based superalloy. Alloys that may be utilized for the body sub-element  22  include those with excellent oxidation and creep/stress rupture properties at or near 1450° F. (787.8 degree Celsius). More specifically, the body sub-element  22  is preferably formed of a Ni 3 Al intermetallic based alloy or any enhanced version of this alloy having a strength 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 a strength and oxidation resistance of the PM nickel based superalloy that forms the bore sub-element  20 , when the PM nickel based superalloy is exposed to temperatures in the same range. Furthermore, alloys that may be utilized for the body sub-element  22  include derivations of alloy Ni 3 Al, where chemistry modifications, or novel manufacturing methods, enable improved mechanical properties and oxidation resistance of the material in the thermal range of interest (1200 F to 1500 F). 
     A preferred embodiment has been described as a method to fabricate an intermediate structure from two pieces. However, multiple pieces may be used to fabricate the intermediate structure. Further, the finished impeller may be fabricated of multiple regions having different compositions. Likewise, it is preferred that the mating interface  36  be linear in cross section. However, other shapes for the mating surfaces  34  and mating interface  36  may be employed. For example, in cross section, the mating interface  36  may include composite lines of differing angles, curves, or other complex shapes. 
     As illustrated in  FIG. 6 , the bore sub-element  20  and the body sub-element  22  contain excess material, material that will ultimately be machined away in order to yield a finished impeller shape. Both the bore sub-element  20  and the body sub-element  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. Thus, the body sub-element  22  need not have an outer shape in the form of a cylinder, but may take other shapes. The bore sub-element  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  FIG. 1 . Alternatively, the bore sub-element  20  may be formed with an integral axle. 
     Turning now to  FIG. 7 , an exemplary method for forming an impeller structure is outlined in a flow diagram. In a preferred embodiment, a bore sub-element and a body sub-element are separately formed prior to joining. For purposes of explanation, the following description outlines formation of a bore sub-element first, but it should be understood that in an alternate embodiment a body sub-element may be formed first, or simultaneously with the formation of the bore sub-element. Accordingly, provided as step  100  is an atomized nickel based superalloy material for fabricating a bore sub-element, similar to the bore sub-element described in  FIG. 6 . In one embodiment, the atomized nickel based superalloy undergoes hot isostatic pressing (HIP) as step  102  to a super solvus condition. More specifically, the PM nickel based superalloy material is subjected to elevated temperatures and pressures to form a fully dense compacted billet. In a preferred embodiment, the material undergoes HIP processing to a grain size in a range of ASTM 6.0-8.0, and preferably a grain size of ASTM 7.5. HIP processing allows for the formation of the bore sub-element of uniform grain size and fully dense billet adequate for inspection using available technologies. The consolidated material is next machined into a specified configuration, as step  108  and definition of the final bore sub-element. 
     In an alternate embodiment, the atomized PM nickel based superalloy may undergo an extruding process as step  104  and isothermal forging as step  106  to a grain size in a range of ASTM 10.0-12.0, and preferably a grain size of ASTM 11.5. If necessary, the material may be machined to a specified configuration as step  108 , thereby defining a final bore sub-element. 
     As previously stated, a body sub-element may be formed subsequent to, or simultaneously with, the formation of the bore sub-element. Accordingly, provided as step  200  is a NiAl intermetallic based alloy material, and more particularly a Ni 3 Al intermetallic based alloy material, for fabricating a body sub-element, similar to the body sub-element described in  FIG. 6 . In one embodiment, the Ni 3 Al intermetallic based alloy initially undergoes atomization, as step  200 , by powder metal manufacturing techniques and then hot isostatic pressing (HIP) as step  202 . More specifically, the Ni 3 Al intermetallic based alloy material is subjected to elevated temperatures and pressures to fabricate a fully dense billet capable of inspection using available technologies. As previously described, hot isostatic processing (HIP) allows for the formation of consolidated material. The consolidated material is next machined into a specified configuration, as step  208  and definition of the final body sub-element. 
     In an alternate embodiment, compaction of the Ni 3 Al powder may be achieved by extrusion, as step  204 , to fabricate an inspectable billet. The extruded material undergoes an isothermal forging process as step  206 , prior to machining of the forged material into a specified configuration, as step  208  and definition of the final body sub-element. 
     The body sub-element and the bore sub-element are next pressed together, as step  300 , through application of force on one or both of them, thereby contacting the bore sub-element and the body sub-element at their mating surfaces described with regard to  FIG. 2 . The bore sub-element and the body sub-element undergo a joining process, as step  302 , to join the two sub-elements and define a joined assembly. 
     The joining process preferably utilizes diffusion bonding techniques that are an advantageous method of joining the bore sub-element to the body sub-element. Alternatively, the bore sub-element and the body sub-element may be joined using inertia welding techniques. During a diffusion bonding process, the bore sub-element and the body sub-element undergo a brazing process followed by a HIP diffusion cycle. During an inertia welding process, stored rotational kinetic energy provides the energy needed to make the weld. Generally, in inertia welding, a first work piece is connected to a flywheel. A separate work piece, the one to which the first work piece is to be joined, is restrained from rotating. The flywheel is accelerated to a desired rotational speed. The two pieces are then forced into contact. The kinetic energy stored in the rotating work piece and the flywheel is dissipated as heat through friction at the weld interface. If desired, during a diffusion bonding process or an inertia welding process, the two work pieces may be pressed together through application of force on one or both of them. The machines employed in either a diffusion bonding process or inertia welding process are versatile in that they can accommodate a wide range of part shapes and sizes. During diffusion bonding, standardized cleaning processes ensure the mating surfaces are clean and free from contaminants present on the mating surfaces prior to bonding. A joint that results from diffusion bonding may be free from voids, inclusion, an extensive heat affected zone, deleterious materials, uniform, precisely located, and repeatedly fabricated. During inertia welding, all defects present at the initial mating interface are eliminated with the result that defects in the bond are minimized. A joint that results from inertia welding may be free from voids, inclusion, deleterious materials, fabricated from as-machined surfaces, and repeatedly fabricated. Finally, as the processes are machine-controlled, they may minimize variations that occur through the human element. 
     After joining, the joined assembly may be machined in a step  304 , using a combination of conventional and non-conventional machining processes, for further definition of a final composite structure. Conventional machining processes may employ, but are not limited to, turning, milling, hole drilling, chemical etch, broach, grinding, hand finish, and shot peening. Non-conventional machining processes may employ, but are not limited to, electrochemical machining (ECM) and electro discharge machining (EDM), and laser shock peening. 
     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.