Abstract:
A process of fabricating a rotating component and components formed thereby. The process includes fabricating preforms corresponding to portions of the component. Each preform has an interface surface at which the preforms can be joined to locate a first of the portions in a radially outward direction from a second of the portions. The preforms are then inertia welded together to form a profile having a solid-state weld joint containing a finer-grained material than other portions of the profile. The profile is then forged with dies to produce a forging. At least one of the dies has a recess into which the finer-grained material from the weld joint is expelled during forging to purge a joint region of the forging between the forging portions of the finer-grained material. The joint region contains grains distorted in an axial direction of the forging.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to fabrication processes that include a joining operation. More particularly, this invention is directed to a technique for fabricating rotating hardware, as an example, rotating components of a turbomachine, joining techniques used in their fabrication, and the hardware formed thereby. 
     Components within the combustor and turbine sections of a gas turbine engine are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases produced in the combustor. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor components, including blades, spools, disks (wheels) and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, the rotating hardware such as compressor spools, compressor disks, and turbine disks are typically formed of superalloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a controlled grain structure and desirable mechanical properties. Notable superalloys for these applications include gamma prime (γ′) precipitation-strengthened nickel-base superalloys containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and/or vanadium as principal elements that combine with nickel to form the desirable gamma prime precipitate strengthening phase, principally Ni 3 (Al,Ti). Examples of gamma prime nickel-base superalloys include René 88DT (R88DT; U.S. Pat. No. 4,957,567) and René 104 (R104; U.S. Pat. No. 6,521,175), as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. Disks and other critical gas turbine engine components are often forged from billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Forging is typically performed on fine-grained billets to promote formability, after which a supersolvus heat treatment is often performed to cause uniform grain growth (coarsening) to optimize properties. 
     A turbine disk  10  of a type known in the art is represented in  FIG. 1 . The disk  10  generally includes an outer rim  12 , a central hub  14 , and a web  16  between the rim and hub  12  and  14 . The rim  12  is configured for the attachment of turbine blades (not shown) in accordance with known practice. A hub bore  18  in the form of a through-hole is centrally located in the hub  14  for mounting the disk  10  on a shaft, and therefore the axis of the hub bore  18  coincides with the axis of rotation of the disk  10 . The disk  10  is presented as a unitary forging of a single alloy, and is representative of turbine disks used in aircraft engines, including but not limited to high-bypass gas turbine engines such as the GE90® and GEnx® commercial engines manufactured by the General Electric Company. The weight and cost of single-alloy forgings have driven the desire to develop materials, fabrication processes, and hardware designs capable of reducing forging weight and costs for rotating hardware of gas turbines. One approach is prompted by the fact that the hubs and webs of compressor spools and disks and turbine disks have lower operating temperatures than their rims, and therefore can be formed of alloys with properties different from those required at the rims. Depending on the particular alloy or alloys used, optimal microstructures for the hub, web and rim can also differ. For example, a relatively fine grain size may be optimal for the hub and web to improve tensile strength and resistance to low cycle fatigue, while a coarser grain size may be optimal in the rim for improving creep, stress-rupture, and crack growth resistance. 
     Implementing a multi-alloy design generally entails separately fabricating the hub and rim of a disk from different materials and then joining the hub and rim by welding or another metallurgical joining process, as disclosed in U.S. Published Patent Application Nos. 2008/0120842 and 2008/0124210. Though a variety of joining techniques are available for producing multi-alloy disks, each has certain shortcomings. For example, electron beam (EB) welding creates a resolidified weld zone that is always weaker than the materials welded together, and joints formed by diffusion bonding (DB) and brazing are also weaker than the materials they join as a result of providing no mechanical work to the joint region. Solid-state welding processes such as inertia welding are disclosed in U.S. Pat. No. 6,969,238. While well suited for certain applications, weld joints formed by inertia welding are fine grained and therefore limit the high temperature operation of a disk. Furthermore, if the disk is heat treated to produce coarser grain size, the inertia weld joint is prone to cracking and critical grain growth during supersolvus heat treatment. 
     Further examples of metallurgical joining techniques for fabricating multi-alloy disks and spools are disclosed in U.S. Pat. Nos. 5,106,012 and 5,161,950. These patents describe a technique termed forge enhanced bonding, by which separately formed regions of a disk can be bonded together during a forging operation. In a particular example, preforms of the rim region and the hub and web region of a disk are placed in a forging die and bonded together during forging as a result of material at the interface of the preforms being displaced into vents in the die halves. Potential defects originally present at the interface surfaces are displaced with the material that flows into the vents, forming sacrificial ribs that can be removed from the resulting bonded disk after forging, so that the portion of the bond line remaining in the finish part is of high integrity and substantially free from defects. While effective for bonding hub and rim preforms, the process requires producing the preforms so that their mating surfaces are very clean and closely shape-conforming, carefully assembling the preforms in a can while avoiding contamination, and hot isostatic pressing (HIP) the preforms prior to forging. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a process of fabricating rotating hardware, as an example, rotating components of turbomachines, joining techniques used in their fabrication, and rotating hardware formed thereby. 
     According to a first aspect of the invention, a process for fabricating a rotating component includes fabricating at least two preforms corresponding to at least two portions of the component. Each of the preforms comprises an interface surface at which the preforms can be joined to locate a first of the portions in a radially outward direction from a second of the portions. The preforms are then inertia welded together to form a profile, and such that the interface surfaces of the preforms form a solid-state weld joint located between portions of the profile corresponding to the portions of the component. The solid-state weld joint contains a finer-grained material relative to material in the portions of the profile and define joint surfaces located on opposite axial surfaces of the profile. The profile is then forged with dies to produce a forging containing forging portions corresponding to the portions of the component. The dies define first and second die cavities, of which at least one has a recess into which the finer-grained material from the solid-state weld joint is expelled during forging to purge a joint region of the forging between the forging portions of the finer-grained material. The joint region contains grains distorted in an axial direction of the forging. 
     Another aspect of the invention is a rotating component having a rotational axis and at least two portions that are welded together. A first of the portions is disposed in a radially outward direction from a second of the portions. A joint region is located between the portions of the component that is free of weld material and free of finer-grained material relative to material in the portions of the component. The joint region also contains grains distorted in an axial direction of the component. 
     A technical effect of the invention is the ability to produce a rotating component using a welding operation, but with finer-grained materials associated with a weld joint being expulsed from the component. This aspect is advantageous when producing, for example, a multi-alloy rotating component (such as a disk or spool) having rim and hub portions formed of different materials that can be tailored or otherwise particularly selected for the different operating conditions of the rim and hub. In addition, the joint interface between the rim and hub portions of a rotating component is capable of having improved properties without disadvantages associated with the prior art, including cracking and critical grain growth during supersolvus heat treatment. The process of this invention can potentially be applied to a wide variety of alloys, heat treatments, and forging conditions to achieve different grain sizes and structures within the rim and hub regions of the component. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a turbine disk of a type used in gas turbine engines. 
         FIGS. 2 through 5  represent steps performed in fabricating a rotating component, such as the disk of  FIG. 1 , by inertia welding a rim preform to a hub preform and then forging the welded assembly in accordance with an embodiment of the present invention. 
         FIG. 6  graphically represents the material flow that occurs in and around the weld joint of a disk during the forging operation of  FIG. 5 , in which the weld joint material is displaced into offset vents in accordance with an embodiment of the present invention. 
         FIG. 7  graphically represents the material flow that occurs in and around a weld joint of a disk produced by an alternative forging operation, in which the weld joint material is displaced into opposed vents during forging in accordance with an embodiment of the present invention. 
         FIG. 8  represents a fragmentary cross-sectional view of a multi-alloy disk that can be produced by a welding-forging process of this invention, and shows the appearance of the disk following removal of an annular flange produced by the forging process of either  FIG. 6  or  7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described with reference to rotating hardware of the type used in turbomachines, and particularly turbine and compressor disks and compressor spools of high-bypass gas turbine engines. For convenience, the invention will be described in particular reference to the turbine disk  10  represented  FIG. 1 , though it should be understood that the teachings and benefits of the invention are not limited to this particular disk  10  and can be adapted and applied to a wide range of rotating hardware. 
       FIGS. 2 through 5  and  8  represent steps involved in fabricating the disk  10  using an inertia welding technique. A first step represented in  FIG. 2  is to prepare rim and hub preforms  22  and  24 , which are then inertia welded together in  FIG. 3  and then machined in  FIG. 4  to yield a disk profile  40  in preparation for forging. The disk profile  40  is then placed in dies  42  and  44  of a forge press that substantially fit the profile  40  everywhere except at the weld joint  28  shown in  FIG. 3 .  FIG. 5  represents the result of the forging operation, during which material flows from the weld joint  28  into cavities or vents  52  and  54  of the dies  42  and  44 . Finally,  FIG. 8  depicts the result of removing annular flanges  69  from each axial face of the forging  60  produced in  FIG. 5 , after which finish processing of the disk (for example, heat treatment, sonic inspection, machining to final shape, etc.) can be performed. These steps are discussed in greater detail below. 
     In  FIG. 2 , portions of the rim preform  22  and hub preform  24  are represented in cross-section. It should be appreciated that, because of the axisymmetric configuration of the disk  10 , there is a diametrically opposite portion of the disk  10  that is not shown in  FIG. 2 . The preforms  22  and  24  can be produced by a variety of known processes, including billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. The preforms  22  and  24  preferably are fine-grained to promote their forgeability. The outlines of rim and hub profiles  32  and  34  are shown in  FIGS. 2 and 3 , and illustrate that the hub and rim preforms  22  and  24  could be forged or otherwise fabricated prior to inertia welding to produce a disk profile  40  ( FIG. 4 ) that more closely corresponds to the desired geometries of the rim  12 , hub  14  and web  16  in the final disk  10 . 
     The preforms  22  and  24  can be produced from a wide variety of materials chosen on the basis of the operating conditions to which the rim  12 , hub  14  and web  16  will be subjected when the disk  10  is installed in a turbomachine, such as a gas turbine engine. Nonlimiting examples of suitable materials include the aforementioned gamma prime nickel-base superalloys R88DT and R104, as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. Importantly, the rim and hub preforms  22  and  24  can be produced from different alloys, so that the disk  10  is a multi-alloy component whose rim  12 , hub  14  and web  16  can be formed of materials better tailored for different operating conditions to which the rim  12 , hub  14  and web  16  will be subjected. Also, as will be noted below, the rim and hub preforms  22  and  24  can be produced from different alloys that enable the final article to respond to a mono-temperature heat treatment with different grain growth responses, or to enable the use of a dual heat treatment method to achieve a range of desired structures between the bore  14  and rim  12 . 
     The preforms  22  and  24  are shown in  FIG. 2  as having two machined interface surfaces  26 , at which joining occurs by inertia welding in  FIG. 3 . The interface surfaces  26  are represented as being oriented at an angle other than parallel to the axis  20  of the eventual disk  10 , providing a contact (draft) angle that facilitates assembling and mating of the annular-shaped rim preform  22  within the hub preform  24 , as indicated by the arrows in  FIG. 2 . Consequently, the resulting weld joint  28  shown in  FIG. 3  is also inclined at the same angle. However, it is foreseeable that the interface surfaces  26  of the rim and hub preforms  22  and  24  could be parallel to the disk axis  20 . To further facilitate assembly and contact between the preforms  22  and  24 , the surfaces  26  are preferably conformably shaped so that they readily slide into contact with each other. 
     The inertia welding process represented by the steps of  FIGS. 2 and 3  is a solid-state welding technique accomplished by rotating the rim preform  22  and/or hub preform  24  about the disk axis  20 . As a matter of convenience, the rim preform  22  may be held stationary and the hub preform  24  rotated. While relative rotation is occurring, the rim and hub preforms  22  and  24  are moved together parallel to the axis  20  until the interface surfaces  26  of the preforms  22  and  24  come in contact. As relative rotation continues, the contacting surfaces  26  generate frictional heating, and increasing the application of force in the axial direction increases the temperatures of the regions underlying the surfaces  26  of the rim and hub preforms  22  and  24  to a temperature approaching the incipient melting temperatures of the materials from which the preforms  22  and  24  are made. The axial force, relative rotational speeds and input rotational energy at initiation of welding, and required relative displacements necessary to inertia weld the preforms  22  and  24  will vary, depending on the size, mass and materials of the preforms  22  and  24  and the surface area of their interface surfaces  26 . The preforms  22  and  24  are held in contact under these conditions for a period of time sufficient to cause them to bond together along their contacting surfaces  26  as the rotational speed decays to zero, forming a solid-state weld joint  28  that contains fine-grained material as a result of the temperatures sustained during inertia welding. 
     The disk preform  30  produced by the welded preforms  22  and  24  can be forged or machined after welding to acquire a disk profile  40  represented in  FIG. 4 , whose geometry is preferably suitable for a forging operation represented in  FIG. 5 . Alternatively, and as noted above, the preforms  22  and  24  could have been forged or machined prior to welding as indicated by the outlines of the rim and hub profiles  32  and  34  in  FIGS. 2 and 3 , such that the welding operation approximately yields the disk profile  40  of  FIG. 4 .  FIG. 5  represents a forging  60  produced by subjecting the disk profile  40  of  FIG. 4  to a forging operation within two die halves  42  and  44 . Die cavities  46  and  48  are defined in the mating surfaces  50  of the die halves  42  and  44  that closely correspond to the final geometry desired for the disk  10 , yielding the forging  60  with rim, hub and web portions  62 ,  64  and  66  corresponding to the rim  12 , hub  14  and web  16  of the final disk  10 . However, the die cavities  46  and  48  diverge from the desired profile of the disk forging  60  as a result of the presence of two annular-shaped cavities or vents  52  and  54  defined in their surfaces. The vents  52  and  54  are represented as coaxial but having different diameters, so that the vents  52  and  54  are not axially aligned in the axial direction of the disk axis  20  but instead are radially offset from each other. The offset is selected so that the exposed surfaces  58  ( FIG. 4 ) of the solid-state weld joint  28  at each axial surface of the disk profile  40  will face one of the die cavity vents  52  and  54  when forging is initiated, and during forging the exposed surfaces  58  will be displaced or expelled into the vents  52  and  54 . 
     The effect of this offset is graphically represented in a model prediction shown in  FIG. 6 , which indicates a very large degree of metal flow and grain distortion within a joint region  68  of the disk forging  60  where the weld joint  28  of the disk profile  40  was originally present. As evident from  FIG. 6 , grain distortion within the joint region  68  of the forging  60  is largely in the axial direction of the forging  60 , roughly coinciding with the contact angle of the interface surfaces  26  of the preforms  22  and  24  and the angle of the weld joint  28  in the disk profile  40  from which the forging  60  was produced. The effect of this distortion is to purge the forging  60  of the weld joint  28  and the fine-grained material that was present there. As evident from  FIG. 5 , the vents  52  and  54  are filled with material that was within and immediately adjacent the weld joint  28 , resulting in the creation of an annular flange  69  at each of the axial faces of the forging  60 . The forging operation is ideally performed so that the flanges  69  contain the fine-grained material originally present within the weld joint  28 . This result may be achieved with a single or multiple strokes during the forging operation. Furthermore, it is foreseeable that the disk profile  40  could undergo forging in two steps, such that one of the flanges  69  is first formed with a first set of dies in which a single vent  52  or  54  is present, and then the other flange  69  is formed with a second set of dies in which the other vent  52  or  54  is present. The flanges  69  are then removed during final machining of the forging  60  to produce the desired profile of the disk  10 , as shown in  FIG. 8 . 
       FIG. 7  is a graph plot similar to  FIG. 6 , but showing a model prediction of a forging  70  produced from preforms (not shown) whose preform surfaces and resulting weld joint were parallel to the disk axis  20 , and then forged with a die (not shown) in which the vents were axially aligned with each other instead of being radially offset as shown in  FIG. 5 . As evident from  FIG. 7 , the model predicts that the flanges  79  formed within the vents contain material that was previously within and immediately adjacent the weld joint, though a significant amount of the weld joint material is still within the joint region  78  between the rim and hub forgings  72  and  74 . According to this prediction, the offset vents  52  and  54  of  FIG. 5  are expected to be more effective in purging a forging of the fine-grained material originally present within the weld joint  28 . In view of the above, the contact angle of the preform surfaces  26  ( FIG. 2 ) and the offset of the vents  52  and  54  are considered together to optimize the forging process. A particularly suitable range for the contact angle is believed to be about zero to about forty-five degrees to the disk axis  20 , and a preferred range is believed to be about seven to about thirty degrees. However, it is expected that an optimal contact angle will be determined by various factors, including the material(s) of the preforms  22  and  24  and the sizes of the rim and hub preforms  22  and  24  (or the rim and hub profiles  32  and  34 ). As such, contact angles of as much as sixty degrees and even up to about ninety degrees could possibly be used with the invention. 
     Suitable forging and heat treatments conditions will depend on the particular materials and sizes of the preforms  22  and  24  or profiles  32  and  34  and are generally within the knowledge and capability of those skilled in the art, particularly in view of the following discussion as well as the teachings of U.S. patent publications cited below, and therefore will not be discussed in any detail here. In most cases, the desire will be to obtain a smoothly varying grain size across the joint region  68 / 78 , while avoiding the fine-grained inertia weld zone associated with conventional inertia welding. 
     The forging operation performed on the disk profile  40  can be carried out using controlled strain and strain rates to achieve a desired final grain size throughout the forging  60 / 70 , including the joint region  68 / 78  between the rim and hub portions  62 / 72  and  64 / 74  corresponding to the original location of the weld joint  28  within the disk profile  40 . The forging parameters are preferably controlled so that the material flow into the vents  52  and  56  within the die cavity is accomplished at controlled strain rates, generally within the regime of superplastic deformation (but for certain alloys possibly outside the region of superplasticity), so that subsequent supersolvus heat treatment of the entire joint region  68 / 78  in and around the joint  28  of the disk forging  60 / 70  can be performed without critical grain growth. For example, see the teachings of U.S. Pat. Nos. 4,957,567 to Krueger et al., 5,529,643 to Yoon et al., 5,584,947 to Raymond et al., and 5,759,305 to Benz et al., and U.S. Published Patent Application No. 2009/0000706 to Huron et al. Typically the desire will be to supersolvus heat treat the entire forging  60 / 70  to produce a metallurgically clean, fully supersolvus disk  10  having a substantially uniform grain size, including the joint region  68 / 78  encompassing the original location of the weld joint  28 . 
     Grain sizes within the rim  12 , hub  14 , and web  16  can be further controlled and, if desired, modified by the manner in which the disk profile  40  was produced. For example, the rim and hub profiles  32  and  34  can be separately forged prior to welding, and the rim profile  32  can undergo relatively slower forging at higher temperatures than the hub profile  34  to yield a coarser grain size in the rim profile  32  and, subsequently, a coarser grain size in the rim  12 . In addition or alternatively, a dual heat treatment can be performed on the forging  60 / 70 , in which the rim  12  and hub  14  are subjected to different supersolvus and/or different stabilization/aging temperatures to optimize grain size and properties within the rim  12  and hub  14 . Examples of dual heat treatment techniques are disclosed in U.S. Pat. Nos. 4,820,358, 5,527,020, 5,527,402 and 6,478,896. 
     It should also be noted that the alloys chosen for the rim  12  and bore  14  can be optimized via their major element chemistry composition (for example, to influence gamma-prime solvus composition and content) and their minor element chemistry composition (for example, to influence degree of grain refinement). In addition or alternatively, the rim and hub preforms  22  and  24  can be produced from different alloys that enable or cause the final article to respond to controlled and even mono-temperature heat treatments to achieve different grain growth responses in the rim  12  and bore  14 . 
     While the invention has been described in terms of a specific embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.