Abstract:
A forging die and process suitable for producing large forgings, including turbine disks and other rotating components of power-generating gas turbine engines, using billets formed by powder metallurgy. The forging die includes a backplate, and segments arranged in a radial pattern about a region on a surface of the backplate. Each segment has a backside facing the backplate and an interface surface facing away from the backplate, with the interface surface being adapted to engage the billet during forging. The segments are physically coupled to the surface of the backplate in a manner that enables radial movement of the segments relative to the backplate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to forging equipment and processes, including those used in the production of large forgings from metal powders. More particularly, this invention relates to a forging die equipped with radial segments that reduce the incidence of cracking during forging of powder metallurgy billets by promoting radial growth during forging. 
     Rotor components for power generation turbines have typically been formed of iron and nickel-based alloys with low alloy content, i.e., three or four primary elements, which permit their melting and processing with relative ease and minimal chemical or microstructural segregation. Recently, wheels, spacers, and other rotor components of more advanced land-based gas turbine engines used in the power-generating industry, such as the H and FB class gas turbines of the assignee of this invention, have been formed from high strength alloys such as gamma double-prime (γ″) precipitation-strengthened nickel-based superalloys, including Alloy 718 and Alloy 706. Typically processing of these components include forming ingots by triple-melting (vacuum induction melting (VIM)/electroslag remelting (ESR)/vacuum arc remelting (VAR)) to have very large diameters (e.g., up to about 90 cm), which are then billetized and forged. In contrast, rotor components for aircraft gas turbine engines are often formed by powder metallurgy (PM) processes, which are known to provide a good balance of creep, tensile and fatigue crack growth properties to meet the performance requirements of aircraft gas turbine engines. Powder metal components are typically produced by consolidating metal powders in some form, such as extrusion consolidation, then isothermally or hot die forging the consolidated material to the desired outline. 
     The use of powder metallurgy processes to produce large forgings suitable for rotor components of power-generating gas turbine engines provides the capability of producing more near-net-shape forgings, thereby reducing material losses. As more complex alloys such as Alloy 718 and beyond become preferred and the size of forgings continues to increase, the concerns of chemical and microstructure segregation, high material losses associated with converting large grained ingots to finish forgings, and limited industry capacity to process large, high strength forgings make the higher base cost PM alloys potentially more cost effective. However, problems encountered when forging powder metallurgy billets include high frictional forces that develop at the die-billet interface and impede free radial growth of the billet, resulting in cracks in the forging. These cracks, believed to be driven by tangential stresses, have been observed to be regularly spaced and in the radial direction at the Poisson-induced bugle in the forging during the upset process. Proposed solutions to this problem, including varying the forging die temperature, upset levels, and forging strain rates, have achieved only limited success. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a forging die and process suitable for producing forgings, including turbine disks and other large rotating components of power-generating gas turbine engines. The invention is particularly well suited for producing large forgings from billets formed by powder metallurgy techniques. 
     According to a first aspect of the invention, the forging die includes a backplate having a first surface, and a plurality of segments arranged in a radial pattern about a region on the first surface of the backplate. Each of the segments has a backside facing the backplate and defines an interface surface facing away from the backplate, with the interface surface being adapted to engage a billet during forging of the billet with the forging die. The segments are physically coupled to the first surface of the backplate in a manner that enables radial movement of the segments relative to the region of the backplate. 
     According to a second aspect of the invention, the forging process entails assembling a forging die by arranging a plurality of segments in a radial pattern about a region on a first surface of a backplate and physically coupling the segments to the first surface to enable radial movement of the segments relative to the region of the backplate. The segments are arranged and coupled to the backplate so that each segment has a backside facing the backplate and defines an interface surface facing away from the backplate, with the interface surface being adapted to engage a billet during forging of the billet with the forging die. A billet is then forged with the forging die by engaging and working the billet with the interface surfaces of the segments. 
     Significant advantages of the forging die and process of this invention include the ability to forge powder metallurgy billets to produce large disks and other large articles with a lower incidence of cracking and the ability to achieve more uniform properties in such articles. Reduced incidence of cracking is able to achieve a corresponding reduction in scrappage, while reduced variance in properties results in higher design allowable properties, hence more efficient article designs. The die and process also enable the forging of large articles from alloys that might otherwise have been previously unsuited or otherwise difficult to forge. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation showing a plan view of a forging die in accordance with an embodiment of the present invention. 
         FIGS. 2 and 3  are schematic representations showing views along lines  2 - 2  and  3 - 3 , A-A and B-B, respectively, of  FIG. 1 . 
         FIG. 4  is a schematic representation corresponding to the view in  FIG. 2 , and shows the forging die of  FIGS. 1 through 3  prior to initiating a forging operation on a billet. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to the manufacture of components formed by forging, a particular example being the forging of large billets to form rotor components of land-based gas turbine engines, though other applications are foreseeable and within the scope of the invention. In a preferred embodiment, the billets are formed by a powder metallurgy process, such as by consolidating (e.g., hot isostatic pressing (HIP) or extrusion consolidation) a metal alloy powder. A variety of alloys can be used for this purpose, including low-alloy iron and nickel-based alloys, as well as higher strength alloys such as gamma double-prime precipitation-strengthened nickel-based superalloys including Alloy 718 and Alloy 706. 
       FIGS. 1 through 4  represent a forging die  10  made up of an assembly of individual components, including a backplate  12  and segments  14  arranged in a radial pattern about a central region  16  of the backplate  12 . The surfaces  20  and  22  of the segments  14  and central region  16 , respectively, cooperate to define an interface surface  18  with which material forged by the die  10  is deformed. As seen in  FIG. 3 , the surface  22  of the central region  16  is substantially flush with the surrounding surfaces  20  of the individual segments  14 , though it is foreseeable that these surfaces  20  and  22  might not be coplanar. The segments  14  are seen in  FIG. 1  as being essentially identical in size and having essentially identical wedge shapes, though different sizes and shapes are also within the scope of the invention. The radially innermost extent of each segment  14  is shown as abutting the central region  16 , while the radially outermost extent of each segment  14  is shown as coinciding with the radially outermost extent of the backplate  12 . As evident from  FIG. 2 , a radial gap  32  exists between the adjacent radial edges of each adjacent pair of segments  14 . 
     As more readily evident from  FIGS. 2 and 3 , the segments  14  are coupled to the backplate  12  but adapted for radial movement relative to the backplate  12  as a result of the backplate  12  and segments  14  having complementary guide features. In the embodiment shown, the surface  24  of the backplate  12  facing the segments  14  has radially-oriented rails or splines  26  that extend between the central region  16  and perimeter of the backplate  12 . The splines  26  can be integrally-formed raised features on the surface  24  of the backplate  12 , or separately manufactured and installed on the backplate  12 . As evident from  FIG. 2 , the splines  26  are sized and shaped to be individually received in grooves  28  defined in the backside  30  of each segment  14 . The splines  26  and grooves  28  are shown as having complementary-shaped dovetail cross-sections that prevent the segments  14  from being removed from the backplate  12  in a direction normal to the surface  24  of the backplate  12 , yet permit free radial movement of the segments  14  on the backplate  12  such that the splines  26  serve as radial guides for the segments  14 . While dovetail cross-sections are shown for the splines  26  and grooves  28 , other interlocking cross-sections could also be used and are within the scope of this invention. 
     The backplate  12  is also preferably constructed of individual components in the form of concentric bands  34  surrounding the central region  16  of the backplate  12 . The bands  34  are secured together by radial pins  36  inserted through holes in the outermost band  34 , through aligned holes in the inner band(s)  34 , and into the central region  16  of the backplate  12 . While each of the bands  34  is represented as having an annular or ring shape, other shapes are also within the scope of the invention. With this construction, each band  34  is preferably manufactured or otherwise equipped to carry a portion of each spline  26 , and proper circumferential alignment of the bands  34  results in individual aligned splines  26 , each made up of the spline portions on the bands  34 . 
     With the above construction, the segments  14  are free to move in the radial direction (relative to the region  16 ) to coincide with and accommodate the radial motion of a material being deformed during a forging process in which the die  10  is used. In other words, during a forging cycle in which a material, such as a billet ( 40  in  FIG. 4 ), is being deformed by the die  10 , radially outward flow of the deformed material is automatically assisted by the simultaneous radially outward travel of the segments  12 , with the result that the incidence of cracking of the forging can be reduced by promoting—instead of frictionally inhibiting—radial growth of the billet material during forging. Because forging operations are typically performed in stages (i.e., partial upsets/stages), with each successive stage further deforming the material to increase its width or diameter, the concentric bands  34  of the backplate  12  can be added and removed as necessary to accommodate the increasing size of the forging. Multiple sets of segments  14  can be provided to match the different diameters of the backplate  12  achieved by varying the number of bands  34 . 
     From the foregoing, it should be understood that the forging die  10  is not limited to installation on any particular type of forging ram, but is generally intended to be adapted for installation on a wide variety of forging equipment. In use, the forging die  10  is first assembled to contain the desired number of bands  34  for the backplate  12  and segments  14  of appropriate number and size for the particular material to be forged. As is well understood by those skilled in the art, dimensions and physical and mechanical properties required for the die  10  and its components will also depend on the material being forged. In general, suitable materials for the backplate  12  and segments  14  include conventional tool steels and nickel alloys for improved durability, though other materials are also possible. When forging nickel-based alloys to produce turbine disk forgings, tool steels and nickel alloys are both suitable as materials for the backplate  12  and segments  14 . 
     Billets suitable for forging a turbine disk can be produced according to various known practices. In a particular embodiment of the invention, in which the billet  40  is produced by powder metallurgy, the starting powder material can be produced from a melt whose chemistry is that of the desired alloy. This step is typically accomplished by VIM processing, but could also be performed by adaptation of ESR or VAR processes. While in the molten condition and within chemistry specifications, the alloy is converted into powder by atomization or another suitable process to produce generally spherical powder particles. The powder is then placed and sealed in a can, such as a mild steel can, whose size will meet the billet size requirement after consolidation. Thereafter, the can and its contents are consolidated at a temperature, time, and pressure sufficient to produce a dense consolidated billet  40 . Consolidation can be accomplished by hot isostatic pressing (HIP), extrusion, or another suitable consolidation method. 
     Prior to forging, the interface surface  18  of the die  10  is preferably lubricated with a high temperature lubricant, such as a glass slurry of a type known in the art, for example, a slurry containing molybdenum disulfide (MoS 2 ), to promote sliding between the interface surface  18  and the billet  40 . The same or different lubricant may also be applied between the splines  26  and grooves  28  to facilitate movement of the segments  14  on the backplate  12 . The billet  40  can then be forged with the die  10  of this invention according to known procedures, such as those currently utilized to produce disk forgings for large industrial turbines, though possibly modified to take advantage of the radial movement of the segments  14  during each forging stage, as well as any adjustments to the size of the die  10  made possible by the concentric bands  34  of the backplate  12 . In general, the forging operation is preferably performed at temperatures and under loading conditions that allow complete filling of the finish forging die cavity, avoid fracture, and produce or retain a uniform desired grain size within the material. For this purpose, forging is typically performed under superplastic forming conditions to enable filling of the forging die cavity through the accumulation of high geometric strains. 
     While the invention has been described in terms of particular processing parameters and compositions, the scope of the invention is not so limited. Instead, modifications could be adopted by one skilled in the art, such as altering the configuration of the die  10 , using the die  10  to forge billets formed by various processes and from various alloys, substituting other processing steps, and including additional processing steps. Accordingly, the scope of the invention is to be limited only by the following claims.