EP0431019A1

EP0431019A1 - Dual-alloy disk system.

Info

Publication number: EP0431019A1
Application number: EP89909656A
Authority: EP
Inventors: John M Hyzak; Thimoty E Howson; Wilford H Couts Jr; Steven H Reichman; Hugo E Delgado
Original assignee: Wyman Gordon Co
Current assignee: Wyman Gordon Co
Priority date: 1988-07-29
Filing date: 1989-07-28
Publication date: 1991-06-12
Anticipated expiration: 2009-07-28
Also published as: JPH04500040A; AU4180989A; AU4156593A; ATE107558T1; DE68916432D1; GB2239826B; GB9001813D0; EP0431019B1; WO1990002479A3; DE68916432T2; EP0431019A4; GB2239826A; JP2721721B2; WO1990002479A2

Abstract

Two pieces of metal are bonded together at a surface by placing the two pieces into contact at the surface and forging the two pieces in a die which causes substantial displacement of the metal originally at the surface in a direction parallel to and outwardly from the edges of the surface and into vents in the face of the die and within the die impressions. The strain and displacement fields are controlled by the die and vent geometry which is designed to concentrate the strain and displacement along the original bondline. In this way, up to 99.9 %+ of the defects which are potientially present at the original surface are efficiently displaced with moving metal away from the original contact between the two pieces of metal into sacrificial ribs that form in the vents and the remaining defects are subjected to significant strain. A portion of the displaced metal which contains many of the defects and which forms the sacrificial ribs is removed from the resulting bonded workpiece as the sacrificial ribs are removed from the workpiece. The result is a bond with superior properties and with a bond surface which can be located very precisely both in orientation and location (e.g., radial distance from the center of a disk). This system is particularly appropriate for forming dual-alloy high-pressure turbine disks for gas turbines in which an annular peripheral ring of a second superalloy is bonded to a central core of a first superalloy. The system is particularly effective if, prior to forging, surfaces to be bonded are closely shape-conforming, are very clean, and are diffusion-bonded using hot isostatic pressing while the surfaces are gas-free. The sacrificial ribs are formed by vents in the impressions of the forging dies. The vents are adjacent to the outer edges of the bond surface. The system may be accomplished by using one or more strikes in the same dies or same die geometry, or may include multiple strikes in which only one side of the bond is in proximity to a vent during each strike. Subsequent strikes would process the bondline resulting from the previous strikes, but with the rib removed. The die impressions would be so shaped that the resulting die cavity would closely conform to the original workpiece shape, except for the vents, so that metal displacement and strain would be concentrated at the bondline.

Description

PTO-ftLLQY PISK SYSTEM

BACKGROUND OF THE INVENTION

It is generally the case that metallic articles a called upon to have a combination of properties, a often the property requirements vary from one portion the article to another. In some cases a single materi can satisfy the various property demands throughout t article. In other cases, however, it is not possible achieve all material requirements in an article with single material. In such cases, it is known to u composite articles in which one portion of the article fabricated from one material and a second portion fabricated from another material. The various materia would be selected on the basis of the properties requir for the various portions of the article.

Occasionally, however, the use of composite articl involves serious practical problems. For example, in gas turbine engine, the disks which support the blad rotate . at a high speed in a relatively elevat temperature environment. The temperatures encountered the disk at its outer or rim portion are elevate perhaps on the order of 1500°F, whereas in the inner bo portion which surrounds the shaft upon which the disk i mounted, the temperature will typically be much lower less than 1000°F. Typically, in operation, a disk may b limited by the creep properties of the material in t high temperature rim area and by the tensile propertie of the material in the lower temperature bore region Since the stresses encountered by the disk are in larg measure the result of its rotation, merely to add mor material to the disk in areas where inadequate propertie are encountered is not generally a satisfactory solution since the addition of more material increases the weigh and resulting stresses in other areas of the disk. Ther have been proposals to make the rim and bore portions o the disk from different materials and to bond thes different materials together. This is not an attracti proposition, largely as a result of the difficulti encountered in bonding materials together in a fashi 5 that reliably resists cyclic high stresses.

Accordingly, it is an object of the invention provide a method of forming a metallic articl incorporating at least two portions, each formed of different alloy composition, the portions bei 0 effectively bonded together so that the article ha properties which vary from one portion of the article t another. It is a further object of the invention provide a method of forming a gas turbine disk having bore region formed of a first alloy, a peripheral ri 5 region formed of a second alloy, and an effectiv substantially defect free bond between the regions

Another object of the invention is to provide method of making an axisymmetric gas turbine disk havi optimum bore properties in its bore region and optim 0 rim properties in its rim region.

Another object of the invention is to provide method by which defects in the bond between two allo can be displaced to a zone which can be removed from t workpiece, the method also causing such strain at t 5 bondline that undesirable effects of any remaini defects are minimized.

Another object of the invention is to provide method by which defects in the bond between two all regions of a part can be displaced to a sacrificial zo

30 which can be removed from the finished part, t displacement occurring in a highly efficient manner which the minimum amount of alloy metal is displaced o of the part (into the sacrificial zone) while sti causing removal of up to 99.9+% of the original bondli

35. interface and associated defects, the method also causi such strain at the bondline that the undesirable effe of any remaining defects are minimized. With the foregoing and other objects in view, wh will appear as the description proceeds, the invent resides in the combination and arrangement of steps parts hereinafter described and claimed, it bei understood that changes in the precise embodiment of t invention herein disclosed may be made within the sco of what is claimed without departing from the spirit the invention.

SUMMARY OF THE INVENTION

As a general matter, the present invention can used in two modes. The first mode, which shall be call forge bonding, involves the application of the prese forging method to pieces of metal which are simply physical contact or have been bonded together in only limited way such as tack welding, or encapsulati welding. In this mode, the forge bonding provides t primary means by which the two pieces of metal beco bonded. In the second mode, which shall be called for enhanced bonding, the two pieces of metal are bonded other means prior to the application of the forgi technique of this invention. In a situation which particularly appropriate for the application of t second mode of this invention, the two pieces of met are nickel-based superalloys formed from fine-grain powder metal, and, prior to forge enhanced bonding, ha been diffusion-bonded together using the method of h isostatic pressing. When practical, the forging accomplished under conditions which allow enhanc plastic flow or superplastic flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a turbine disk workpiece incorporating t principles of the present^invention,

FIG. 2 is a workpiece in which a section has be removed,

FIG. 3 is a workpiece in which a sacrificial rib been removed,

FIG. 4 is a process flow sheet, FIG. 5 is a process flow sheet,

FIGS. 6-17 are diagrammatic views in cross-secti of various process steps, and

FIG.18 is a computer-simulated cross-sectional vi of a workpiece before processing; FIG.19 is a computer-simulated cross-sectional vi of a workpiece after processing,

FIG. 20 is a computer-simulated, cross-sectio view of a workpiece and symmetric, equidistant vent p before processing, FIG. 21 is a computer-simulat cross-sectional view of a workpiece and symmetr equidistant vent pair after processing, FIG. 22 a computer-simulated, cross-sectional view of a workpi and symmetric, equidistant vent pair before processi

FIG. 23 is a computer-simulated, cross-sectio view of a workpiece and symmetric, equidistant vent p after processing,, FIG. 24 is a computer-simulat cross-sectional view of a workpiece and asymmetr offset vent pair before processing,

FIG. 25 is a computer-simulated, cross-sectio view of a workpiece and asymmetric, offset vent p after processing,

FIG. 26 is a cross-sectional view of a workpiece a asymmetric, offset vent pair before processing,

FIG. 27 is a cross-sectional view of a workpi and asymmetric, offset vent pair after processing, an

FIG. 28 is.a. generalized cross-sectional view of arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. l shows a graphic representation of a forg workpiece which will be machined into a gas turbine di The workpiece 10 is shown to still bear the sacrific rib 11 which is positioned adjacent the bond between bore or plug 13 and the rim 15.

FIG. 2 shows a cut-away view of a workpiece a particularly, shows a section of the sacrificial ribs and 17 which are adjacent the bondline 16. The bondl 16 is, of course, in fact, a surface of revolution wh represents the contact between the bore section 13 the rim section 15. in FIG. 3, the disk is shown after the sacrifici rib 11 has been machined away from the disk.

While the present invention may be applied to ma situations involving the bonding of two or more pieces metal, the invention is particularly appropriate for u in the geometry shown schematically in Fig. 28. Th particularly appropriate geometry represents a method forming a disk 150 having a disk axis 151, a first di face 152, a second disk face 153, and an annular out edge 171, which defines the outermost extent of t workpiece. The disk also has a central portion 1 formed of a first alloy and an annular peripheral porti 155 formed of a second alloy. The boundary 156 betwe the central and peripheral portions is a surface revolution 157 about the disk axis 151 and is defin by a generatrix 158 having a first end 159 and a seco end 160. The surface 157 has a first circular edge 1 at the first face 152 of the disk 150 , generated by t first end 159 of the generatrix 158, and a seco circular edge 162 at the second face 153 of the disk 15 generated by the second end 160 of the generatrix 158 Essentially, the.process comprises three steps. The fir step involves placing the disk 150 between a first d 163 having a first die face 164 and a second die 1 having a second die face 166. Each die face has concave impression 173 and 174 and the two impressio form a forging cavity 175. At least one of said di must have an annular vent 167 formed in its die face a on the surface of its impression, said vent 167 havin two concentric vent edges 168 and 169 at the die face The second step involves causing the dies 163 and 165 t approach one another along a forging axis 170 which i parallel to the disk axis 151 so that the vent edges 16 and 169 straddle one of the edges of the surface, or, i some applications, straddle the location where the ed is desired. This die movement causes forging actio which is conducted in a manner to cause some of the firs alloy and some of the second alloy, along with th material at the original bondline to flow into the ven along a line of movement substantially parallel to th forging axis to form a rib in the vent. The third ste involves removing the rib from the disk. FIG. 4 shows a flow chart of a typical applicatio of forge enhanced bonding (mode 2). In steps 21 and 2 respectively, the bore and rim sections would be forme preferably by extrusion techniques, from fine-graine powdered metal into a billet. In steps 23 and 24, t bore and rim would be forged into preform shapes preferably without causing grain growth. In steps 25 a 26, the parts are machined, and in particular, the mati surfaces are machined so that they are shape conformi to one another as the rim section fits peripherally abo the bore section. In steps 27 and 28, the mati surfaces are cleaned, as, for example, electro-polishing. Although this discussion will foc on bondlines which are parallel to the forge axis and t axis of an axisymmetric workpiece, it should understood that the designer may elect to give t bondline a draft angle (make it non-parallel to t workpiece axis) , for ease of assembly. This will, course, make the boundary a conic surface rather than cylindrical surface. in step 29, the bore and rim pieces are placed contact and encapsulated in a vacuum environment. Th encapsulation can be accomplished by electron-be welding simply at the outer edges of the bond surface, electron-beam brazing in the same way, or encapsulating the entire disk in a can. The purpose to keep the mating surfaces clean during the bond cyc (step 30) .

In step 30, the two pieces are diffusion bonded exposing the workpiece to hot isostatic pressing.

In step 31, the encapsulation is removed and in st 32, the bond is inspected. Step 33 is where the workpiece is exposed to t forge enhanced bonding which will be discussed in deta subsequently. In step 34, the sacrificial rib is remov and inspected in step 35.

In step 36, the bond within the workpiece itself i inspected. The workpiece is machined to appropria shape in step 37.

In step 38, the workpiece is solution heat treated either employing monotonic or differential heat treatmen or a combination thereof. in step 39, the workpiece is aged (employin monotonic or differential heat treatment or a combinatio thereof), and in step 40 the workpiece is inspected.

FIG. 5 shows a flow sheet for the application of th present invention to forge bonding (mode 1). Essentiall the preliminary activities are similar to those shown i FIG. 4 until step 59. In step 59, the bore and rim ar placed in contact. At this point, the process may simpl continue to the next step of forge bonding. This i particularly acceptable where the two pieces ar force-fit together by designing the bondline with a appropriate draft.angle or by using thermal expansion an contraction to form a very tight fit. However, it may b necessary, in appropriate circumstances, to tack weld th pieces together or to encapsulate the pieces in order t protect the clean surface from contamination.

The remainder of the steps are essentially the sam as those described in connection with FIG. 4. FIGS. 6 through 11, demonstrate the steps of a application of the present invention in which vents 8 and 86 are simultaneously positioned at each end of th bondline during the forging process. FIGS. 12 through 17 show a similar processin sequence in which the venting at one side is done in on strike and then the venting at the other side is done i the other strike. This will be called asymmetric ventin as opposed to the symmetric venting of the process i FIGS. 6 through 11. This flexibility to adjust the ven shape and location allows the designer to control t metal flow into the vent and in so doing to control th displacement and straining of defects in the origina bondline. in FIGS. 6 through 17, it should be understood tha the disk, which is shown in cross-section, is made up o a bore and a rim (which appears in two places). Th heavy dark line which appears at the bond line represents potential defects which, as will be seen, ar progressively moved out of the body of the workpiece an into the sacrificial ribs. Defects can be contaminate such as oxides, dirt, dust, voids, or inclusions in t metal. In addition, a defect can also be a grain, zone or region of metal at or adjacent to the bondline whic has a microstructure from the diffusion bond step no appropriate for service (depleted zone). FIG.6 sho the disk, or workpiece 70, in cross-section through i center, or axis. The workpiece 70 is made up of central bore or plug 71 and a rim 72, which appears the drawing in two places. The bore 71 and rim 72 are i contact at a -bond surface 73 which is shown in t drawing as bondline 74 and bondline 75. At bondline and bondline 75 can be bodies of defects shown as hea dark lines 76 and 77. The forging die 78 itself is ma up of an upper die 79 and a lower die 81. T impressions of both the upper die 79 and the lower die include rib-forming vents 85 and 86 positioned at each the ends of the bond lines. It should be understood tha these vents are, in fact, circular grooves in the face o the die.

The forging dies of the present invention ar normally shaped to closely conform to the initial shap of the workpiece or preform, except, of course, for th vents. In this way, the forge bonding process cause little change in the shape of the workpiece an relatively small strains (metal flow) within th workpiece. The exception is flow of the metal adjacent t the initial bondline. That metal flows toward th bondline and then flows parallel to and with the bondlin outwardly from the ends of the bondline into the vents These large displacements and strains are concentrate almost entirely at and adjacent the bondline and at th region at the mouth of the vent. The minimization o metal flow in the rest of the workpiece increases th predictability of the flow at the bondline and reduce flash as the dies close. Furthermore, the proces minimizes strain gradients in the workpiece and therefor minimizes microstructural changes that would result fro strain gradients.

However, in some limited instances, it might b advantageous to intentionally design the dies so tha they do not exactly fit the initial shape of th workpiece in a local area, but rather the dies are shape so that they form a receptacle with the initia workpiece. The receptacle (local gap between the prefor and the die surface) is designed to passively accep metal flow during the process, and thereby to control th flow during -.the process. This approach may b particularly useful in two situations. The firs situation occurs when the volume of one alloy is muc greater than that of the other. Applying the process to this situation can sometimes result in curving and radial displacement of the bondline. These results can be kept within acceptable limits by providing a receptacle to accept metal flow from the more plentiful alloy. In t second situation, it has been found that when an initi workpiece with a very short bondline (and, therefor ostensibly, few defects) is processed in dies with receptacle between one end of the bondline and a ven the bondline can sometimes be made to extend into t receptacle and vent. This allows elongation of t bondline length within the die cavity before the bondli enters the vent. FIG. 6 shows the position of the workpiece and di before the forging step.

In FIG. 7, the forging step has been carried out a it can be seen that material from the workpiece h flowed into the vents to form ribs on each side of t workpiece. It should be noted that the defect materia shown as dark lines, has been broken up, stretched ou and displaced outwardly from the bondline and into t area of the sacrificial ribs. The dynamic movement the metal during the forging operation causes effecti displacement of defect material from the area of the bo lines and exposes any defect material left at t original bondline to very high levels of strain. It important to note that the straining and displacement material at the bond lines is caused by gener displacement induced in the bulk metal by the forgi pressure. It is not merely the result of movement of t bore with respect to the rim as the dies close.

The forging operation is normally designed to carried out at elevated temperature to lower the fl stress of the metal. In the particular case superalloys, the .forge process is designed to be carri out under isothermal conditions, that is, condition which the workpiece and dies are at nominally the sa temperature during forging and in which superplastic enhanced plasticity deformation of the metal enhanc metal flow to the bondline and into vent. The process designed so that the whole workpiece is heated to t same temperature during forging rather than the case o local heating of just the bondline region. This help maintain microstructural uniformity throughout each allo in the workpiece. It is also important to note that th die vents have been designed to effect a controlled an efficient displacement of the original bondline an associated defects. Normally, the dies would be designe so that they closely fit the contour of the workpiec preform prior to forging. As a result, the large scal deformation is concentrated at the bondline. Analytica simulations have shown that, via this general cavit design and loading situation (loading parallel to th metal movement into the vents) , virgin metal is force from both the bore and rim preforms to the bondline an the original bondline metal and defects are thereb forced out of the part geometry into the sacrificia ribs. The vents are designed to remove the maximu amount of bondline metal for the least amount of tota metal expelled into the sacrificial rib. It is als important to note that the forge bonding concept ha shown excellent results in precise location of the fina bondline. This ability to reproducibly predict th location of the bondline is imperative in turbine engin applications. FIG. 8 shows the workpiece after the removal of th sacrificial ribs on each side of the workpiece. It ca be noted that substantially all of the defect materia (theoretically 99.9%+) has been displaced into th sacrificial ribs leaving little or no defect materia within the remaining body of the workpiece once th sacrificial ribs.have been removed. Because it has bee noted that the exposure of defect materials to hig strain within the workpiece significantly reduces th deleterious effect of the defect materials on th properties of workpieces, it is often appropriate t accept the very low level of defect material whic remains in the workpiece at FIG. 8 and continue th processing of the workpiece in the conventional way.

In situations in which it is particularly importan to minimize the potential presence of defects at th bondline, it has been found effective to essentially d 5 a reforging (restrike) of the workpiece after removal o the sacrificial rib, and thereby to carry out the defec displacement again on the bondline that resulted from th previous strike. This is a very useful aspect of th bonding process of the present invention because i

10 allows continuation of the process in multiple increment (restrikes) without degrading the properties of th workpiece and without the need for cutting out th resultant bondline from the previous strike. This can b particularly important when the initial bonding operatio

1.5 does not achieve a sufficient level of bondline quality In that event, the process can be carried out agai without cutting out the unacceptable bond and wastin metal. This works well in the case of a two-part (bor and rim) disk, in which the idea of cutting out a

20 unacceptable bond creates serious practical problems Other bonding techniques, such as inertia welding an friction welding do not allow reprocessing of a bondline Instead, the unacceptable bond must be cut out (an scrapped) and the process repeated from the beginning. I

25 the case of the two-part disk, this may require scrappin the entire workpiece, because the workpiece may not hav sufficient metal to make up for the cut-out piece. A will be known to those in the art, the intention to carr out this restriking capability should be considered i

30 designing the die and entire forging process.

FIGS. 9 .through 11 show the sequence of th restrike. As can be seen by noting the location of th dark spots in the workpiece, they are displaced outwar from the body of the workpiece into the sacrificial rib

35 where they are removed in FIG. 11.

FIGS. 12 through 17 show a process in which the rib are formed in an asymmetric manner. This technique ha been found to be very effective in various circumstanc because there is no point along the bondline where t displacement reaches an essential equilibrium (ze displacement). As a result, the displacement whi occurs at every point along the bondline, at one or t other of the two forging steps, effectively displace the defects away from the body of the workpiece. FIG. 1 shows the unprocessed workpiece 100 and the othe elements which correspond roughly to those shown in FIG 12. Note, however, that the lower die does not have th rim-forming vents.

Thus, as shown in FIG. 13, the forging operatio causes displacement of material from the area of th bondline upwardly into the vents of the upper die. Thi very effectively moves the material in this specific cas from approximately the upper 90% of the bondline upwar into the sacrificial rib area; the remaining 10% i highly strained and stretched over the thickness of th disk. in FIG. 14, the workpiece is shown after removal o the upper sacrificial rib.

Since the amount of defect material which remains i the part in FIG. 14 may not be acceptable, thi embodiment of the invention probably requires the furthe processing which is shown in FIG. 15. In that case, new set of dies, in which there is no vent in the uppe die, but there is a vent in the lower die, is used. I is also possible, in some applications, to design th workpiece so that, after the rib is removed from on side, the workpiece can be simply inverted and reforged essentially reusing the original dies and vents.

FIG. 16 shows the second forging step in whic displacement of the material at the bondline occur downwardly into the vents in the lower die. This ver effectively removes 90% of the remaining defects whic were stretched across the bondline and essentially ha removed 99% of the defects from the main body of th workpiece in two operations. The remaining defects hav been stretched in two directions, thus significantl reducing their effect on properties.

FIG. 17 shows the removal of the lower sacrificia rib and shows that the defects have been effectivel removed from the body of the workpiece. It should b kept in mind that any of the defects which remain in th body of the workpiece have been exposed to ver significant strain, thereby, reducing their deleteriou effects.

It has been found that this process can shift 99+ of the original bondline and associated defects, out o the final shape or volume and into the sacrificial rib This can be done in one or more forge operation depending on vent type (symmetric (in both dies) asymmetric (in one die)), vent offset from axis, ven profile shape, and vent volume or cross-sectional area Typically one strike removes 80-90%, of the origina bondline, and the second strike removes all but less tha 1%. Since, normally, the defects, if present, ar distributed along the original bondline, the amount o bondline removed correlates with the amount of defec removal. Furthermore, any remaining defects are deforme by 350% or more, thus substantially reducing thei contribution to low cycle fatigue failure. The amount o bondline which is displaced can be changed (increased) b modifying the vent geometry. For example, it is possibl to remove 99% of the bondline in a single operation usin an enlarged cavity. The defects in question may includ trapped dirt, oxides and voids, metallurgical defects an undesired interface alloys, and carbide precipitates, an gamma prime depleted zones. In essence, new metal fro the body of the alloys is presented to the bondline.

The preferred embodiment of the present inventio involves a series of process steps for forming dual-alloy disk suitable to be formed into rotors, suc as those used in gas turbine engines. The technica approach is centered on technology best described "forge bonding" or "forge enhanced bonding". As will clear from the context, the term "forge bonding" sometimes alternatively used generically to denomina the forging operation itself which is the focus of bo modes. in experiments, the feasibility of thi technology for producing a dual-alloy disk with a hi integrity bond has been demonstrated.

The concept of forge bonding powdered meta superalloys includes four basic steps:

1. Isothermal forging of bore and rim preforms.

2. HIP diffusion bonding of bore and rim preforms.

3. Isothermal finish forge operations to locally defor the bondline. 4. Heat treating the forge bonded disk to optimize th properties in the bore, rim and across the bondline.

The focus of the forge bond approach is Step #3, th finish forge operation. The purpose of this operation i to highly deform the original bondline and to displac the original bondline material with inherent defect outside of the finish machined part. A schematic of bonded preform in a set of dies is shown in FIG. 6. Th dies are designed such that the deformation in the finis forge operation is concentrated at the bondline. Figure 18 and 19 show the results of an analytical simulation o the forge enhanced bonding operation. The simulation wa carried out using the ALPID (Analysis of Large Plasti Incremental Deformation) finite element, meta deformation computer program and appropriate meta property data.

Figure 18-shows one quarter section in profile of workpiece in a die with symmetrically-cross-sectioned equally-radially-spaced, forge enhanced bonding vents Thus, this case is for the symmetric (top and botto cavities of the same size,same symmetric profile and sam distance from the disk axis) die vents. Only one quarte section needs to be modelled because of geometri symmetry. The line pattern in Figure 18 on the workpi represents a finite element grid or mesh. Each l intersection represents a point of metal and each clo figure represents a zone of metal. Thus, one can fol both the displacement and the strain before (Fig. 18) after (Fig. 19) the process. Because the Fig. 18-19 c is relatively simple, the result is relativ quantitatively accurate. Figures 20 -27 involve m complex cases, and the finite element grids portray grid distortion as a result of metal flow for the l 20% of the process cycle, cumulative patterns are available because a " remeshing " process is required these complex cases. Thus, while these figures do quantitatively portray the process, they do genera represent the qualitative metal flow pattern genera by the vent geometry.

Figure 19 shows the displacement of the grids af the forge enhanced bonding operation. The displacem and strain are concentrated at the bondline, resulting efficient removal of the original bondline and defec It should be noted in this example that seven of eight zones of metal adjacent to the bondline in fig 18 have been displaced into the vent (out of the part) figure 19 as a result of one forging operation. addition, the fine spacing of the vertical lines at bondline shows the movement of virgin metal from the b of the forging to the bondline to replace the origi bondline material, which has been forced into the ven

It should be noted that similar metal flow can accomplished with an original bondline joint that has angle with respect to the loading axis, as will discussed below.

Finite element modeling of bondline displacements subscale forgings has shown that strains of up to 350% the bondline and displacements of as much as 98% of original bondline to a position outside of the fin part can be obtained with the cavity and vent geometr tested. These results have been verified by experiments Larger strains and greater displacements are achievabl with different die cavity and vent designs.

The strains and displacements are effective i removing defects from the original bondline. This ha been demonstrated in forging of subscale, plane strai coupons. In the extreme, highly oxidized, unbonde interfaces have been dramatically improved by forg bonding. In one test of two Rene' 95 superallo preforms, forge bonding caused 200% strain and 85 bondline displacement out of the part final shape

Cutting off the top and bottom "ribs" and reforgin increased the bondline strain to 350% and the bondlin displacement to 98% out of the final shape. The bondlin which remained in the final shape was substantiall defect free.

Similar results have been demonstrated usin unbonded couples of dissimilar alloys. There was significant improvement in bond cleanliness as a resul of forge bonding.

The demonstrated results of forging "dirty" unbonde preforms support the concept of forge bonding. Th finish forge operation removes the original bondlin interface and associated defects. As the productio process is envisioned, however, preforms will b diffusion bonded prior to the finish forge operation. Prior to the diffusion bond operation, the matin surfaces will be scrupulously cleaned to produce a hig integrity bond. Consequently, the forge bond operatio (mode 2) will only further improve the bondlin properties, especially in fatigue, where defec population is so critical. This forge bonding process is ideally suited for use with the procedure for making a "clean" diffusion bond between dissimilar powder metal superalloys by electropolishing mating surfaces and hot isostatic pressing (HIP).

Besides providing bond strength and bond cleanliness, the forge bond approach to producing a dua alloy disk also gives exceptional control of the bondlin position. The original diffusion bond location can b controlled to machining tolerances (plus or minu 0.002"). Subsequent forging in the finish dies is als a controllable process since the deformation i concentrated in the area of the bondline, and flow i from both sides of the bondline toward the center an then outward parallel to and along the bondline. Meta flow is predictable using finite element modeling. Thi standard situation is shown in Figures 18 and 19. Because the flow of metal in the process has been foun to be consistent and predictable, the process can b refined for specific special problems. For example, th vent shape can be used to normalize the effect o differing flow characteristics of the two alloys. Thi aspect of this invention involves the shape of th cross-section of the vent and/or the position of the ven edges in relation to the edge of the bondline. When th flow characteristics (especially flow stress) of the tw alloys are similar, the cross-sectional shape of the ven would be symmetric on each side of the bondline. Whe the flow characteristics are different, however, th shape of the vent can be skewed to open up the sid adjacent the alloy with the greater flow resistance i order to normalize the net flow of each alloy into th vent and, thereby, stabilize the bondline. Note tha this vent profile is shown in Figure 24 although tha figure also involves a different aspect of the inventio (vent offset from axis).

If the forge, bonding is done with symmetric vent equidistant from the disk axis, even a bond surface wit a draft angle (for fit-up) will predictably becom parallel to the axis during the forge enhanced bondin operation. Fit-up angles of up . to 457 have bee analytically modelled and found to be capable of bein eliminated using this invention. Figures 20 and 21 sho before and after forging models of a small draft an and figures 22 and 23 show before and after models o large (approx. 45 degrees) draft angle. This tendency that venting configuration to transform an initial b surface with a draft angle to a bond surface with draft angle (parallel to the disk axis), can be used significant advantage. For assembly and bondi purposes, it is sometimes desirable to form the separa disk portions so that they mate with a draft angle. Th allows the mating surface machining tolerance to be le critical (because conic sections are self-adjusting and, if the surface of the inner element is slight oversized, allows an enhanced degree of pressure to occ at the bondline at various points in the proces However, it is sometimes desirable to eliminate th draft angle during the forge bonding step so that t radial location of the bondline is uniform across t thickness of the disk. The present invention provides effective method for removing the draft angle. on the other hand, it may be desired to maintain establish a bondline with an angle (draft angle) to t loading or forging axis and centerline or axis of t disk after the forge enhanced bonding operation. Th may be done to improve nondestructive inspectability the bondline. It has been shown analytically that t die vents can be so designed in shape and locati (location of the top and bottom vents relative to ea other) to accomplish this. It is, therefore, importa to note that the forge enhanced bonding concept provid precise and predictable control of the finished forg bondline location, and shape (especially draft angle More specifically, in one approach to establishing maintaining the draft angle during the forge bondi step, the vents in the upper and lower dies should be s at different distances from the disk axis, i.e. , with t edges of each vent straddling the locations where t edges of the surface are desired. As can be seen Figures 24 (before forging) and 25 (after forging), t non-equal radius (offset) vent arrangement will cause t draft angle to be formed where none previously existe Figures 26 (before) and 27 (after) show how an existi draft angle can be maintained.

A number of geometric relationships are significa in optimizing the method of this invention and the effect must be considered both for each strike a cumulatively over a multistrike application. The fir factor is the cross-sectional area of the ven especially in relation to the bondline length. Oth important factors are, second, the cross-sectional sha of.the vents, third, the relationship between the heig and the mouth width of the vents, and ,fourth, t relationship between the vent mouth widths and the di thickness. It should be understood that references cross-sectional areas and to dimensions in the cros sectional plane (which, in the axisymmetric cas includes the axis) relate directly to and incorporate t three-dimensional geometry of the specific application workpiece. Thus, for example, the cross-sectional area a vent relates directly to the volume of the ven although the relationship is not always simple.

The cross-sectional shape and cross-sectional ar of the vents play an important role in optimizing th invention. For example, the cross-sectional area of t vent will determine how much metal is moved out of t workpiece by the vent. Likewise, the total metal mov from the workpiece by a particular application of th invention will be roughly equal to the tot cross-sectional area of the vents used, with each reu of a vent considered a separate use.

As a practical minimum (for the symmetric ve shape) , this invention requires a total movement of met out of the workpiece equivalent to the initial thickne of the disk (the thickness dimension) at the initi bondline times one quarter (25%) that dimension. As preferred minimum, this invention requires a movement o metal out of the workpiece equivalent to the initia thickness of the disk (the thickness dimension) at th bondline times one half (50%) the thickness dimension. A an optimal value, the invention requires a movement o metal out of the workpiece equivalent to the thicknes dimension times 100% of the thickness dimension. Th optimization balances increasing defect removal agains increasing waste of metal. The metal removal may b accomplished in one or in more than one operation depending on engineering considerations such as di strength, forge press capabilities, etc. In application where freedom from bondline defects is not a critica requirement (e.g., where the presence of 20% of initia defects is tolerable) less metal removal than describe may be appropriate.

The cross-sectional shape of the vents can take man forms. The preferred shape would be roughly that of triangle with a base side initially adjacent th workpiece and forming the mouth of the vent and a heigh line defining the distance between the base side and th farthest vent point from the base side. In practice, th inside and outside corners would be rounded. The two ven profiles shown in figures 20 (a balanced or symmetri profile) and 24 (unbalanced or asymmetric profile) hav been found to be particularly effective, not only i operation but also in analytical computer modeling. Thes vents might be described as bell-shaped. They can b characterized by a height (H) , a radius of curvature (RC) at the crown (closed end), a draft angle (Al and A2) fo each side, and entrance radii of curvature (ER1 and ER2). The width (W) (mouth) of the vent is defined by th intersection of the vent wall (along the draft angle) with the continuation of the die impression (die face). The entrance radii are not involved in defining the ven widt .

The relationship between the width of the vent mout and the disk thickness or initial bondline length is significant. A narrow mouth or width tends to concentrate flow at the bondline and therefore removes the maximum original bondline for the minimum total metal displaced into the vent. This represents the theoretically most efficient process with the least wasted metal. However, a narrow width restricts metal flow due to frictional effects along the vent wall, and this restriction of flow is undesirable. A wider mouth has the opposite effects. The ratio between the vent width and initial bondline length should be two or less, preferably between 2.0 and 0.1, and optimally between 1.0 and 0.2. These values apply to the symmetric cross section. Appropriate adjustment must be made for asymmetrical profile cases. in a typical one-strike application, the total cross-sectional vent area of both vents will be equal to or greater than the average width of the vents times the initial length of the bondline. The cross-section of th vent will be substantially triangular with a base sid against the workpiece, the width (W) of the vent bein the length of the base side, and the height being th length of a height line which is a line representing the distance between the base side and the vent poin farthest from base side. The cross-section may b symmetric on both sides of the height line, or it may be asymmetric, i.e., the portion of the base side on one side of the height line is greater than the portion o the other side.

In order to achieve maximum bondline transfer into the vent with the minimum metal transfer, the width o the vent should be small compared to the height of th vent. As a practical extreme, the height of a symmetri vent profile should be equal to or greater than one-hal the width of the vent . It is preferred that the heigh of the vent is at least the width of the vent, an optimally at least twice the width of the vent. Applyin these principles to the general cases of multiple vent and/or multiple strikes, the total cross-sectional are of the vents employed in the method equals approximatel the average vent width of all of the vents employed i the method times the initial thickness of the disk. I should be understood that each edge of the vent will b curved, but that the vent width will be determined as i curves (entrance radii) were not present.

By employing the methods of the present invention it is possible to approach complete removal (greater tha 99.9%) of the bondline material and bondline defects

Yet this result can be achieved with high efficiency not only in terms of energy and equipment expense, bu also in terms of metal waste. When the process i applied to turbine disks formed of expensive superalloys, minimization of metal waste is particularly important

Because the present process causes the metal which flow into the sacrificial rib to be primarily from the zone at or adjacent to the bond line, as opposed to being fro other zones of the workpiece, the process tends to wast the minimum metal necessary to achieve the extremel clean bondline in the workpiece. ^"

This desirable tendency for the waste metal t particularly come from the zone at or adjacent the bon line can sometimes be enhanced even more by employin dies that are slightly cooler than the workpiece. The chilling of the workpiece in contact with the dies tends to favor flow at the workpiece surface straddled by the edges of the vent and at the interior of the workpiece, since these regions are less cooled. Thus metal flow at mid-thickness toward the bondline and then outward, parallel to the-bond line, into the vents, is encouraged.

The method set out in this description is particularly useful when applied to superalloys and when applied under conditions that allow the metal flow to occur in an enhanced plasticity mode. More specifically, to achieve enhanced plasticity, certain alloys must have been previously processed to develop and maintai extremely fine grain size. Then, the process of th present invention is carried out at a temperatur approaching the recrystallization temperature but belo the grain-coarsening temperature of the alloys an employing low strain rates. This normally requires tha both the dies and workpiece be heated to approximatel the recrystallization temperature of the workpiece. Th metal of the workpiece flows far more readily than woul be observed at lower and significantly highe temperatures and faster strain rates, and this results i effective and predictable flow of metal from along th length of the bond line and outward into the vents. Thi allows the use of forge enhanced bonding vents wit greater height-to-width ratios which increase th efficiency of the bondline removal. By employing th present method under conditions which allow enhance plasticity, the process can be effectively employed o alloy pairs which would otherwise not be suitabl choices.

For the purpose of this discussion, the ter enhanced plasticity shall be used to address the genera regime in which the flow stress of a workpiece is lowere by isothermally forging at elevated temperature and lo strain rate while maintaining fine grain structure Superplasticity refers to the portion of this regime i which strain rate sensitivity is 0.35 or greater Subsuperplasticity refers to the portion of the regime i which strain rate sensitivity is less than 0.35. Another important part of a dual-alloy turbine dis concept is the.need for non-destructive evaluation. Thi will be critical to the ultimate success of a dual allo disk. Regarding non-destructive evaluation, the forg bond concept does provide a unique non-destructive mean of "testing" the quality of the bondline. The materia that is forged into the vents represents over 99% of th original bondline. That material can be removed from th forging as a destructible "test ring", and examined. will provide a check on the quality of the origin diffusion bond, especially on its cleanliness. It wi also be a check on the forging of the bondline; t bondline should be present in the rib and in predictable orientation.

It is sometimes possible, in the forge bo approach, to "restrike". If the bondline displaced in the vent is not of the cleanliness required, the part c be forged again, displacing additional bondline into t vents without degrading the microstructure in t workpiece and without cutting out the unacceptab bondline. This material can again be removed a metallographically examined. Another potential application of the restri capability would involve machining and sonic inspecti of just the bondline region after forging. Again, there was a defect present, the part could be reforged the forge enhanced bonding die to remove that bondli defects and then reinspected.

The ability of the forge enhanced bonding concept precisely control the location and orientation of t bondline after forging may be critical to the success non-destructive inspection, especially sonic inspection The invention having been thus described, what i claimed as new and desired to secure by Letters Pate is:

Claims

1. A method of forming a disk having a disk axis, a first disk face and a second disk face and an annula outer edge which defines the outermost extent of th workpiece, the disk having a central portion formed of first alloy and an annular peripheral portion formed o a second alloy, and the boundary between the central an peripheral portion being a surface of revolution abou the disk axis and being defined by a generatrix having first end and a second end, said surface having a firs circular edge at the first face of the disk and generate by the first end of the generatrix, and a second circula edge at the second face of the disk and generated by th second end of the generatrix, and the disk als comprising material initially present at the boundary comprising the steps of:

(a) placing the disk between a first die having a firs die face and a second die having a second die face a least one of said dies having an annular vent formed i its die face, said vent having two concentric vent edge at the die face,

(b) causing the dies to approach one another along forging axis which is parallel to the disk axis so tha the vent edges straddle a circular line on a face of th disk, said circular line being the desired location o one of the circular edges of the surface, and thereby t cause some of the first alloy and some of the secon alloy, along with, a substantial amount of the materia that was present at the boundary, to flow into the ven along a line of movement substantially parallel to th forging axis to form a rib in the vent, and

(c) removing the rib from the disk.

2. A method as recited in claim 1, wherein at leas one of the alloys is a superalloy.

3. A method as recited in claim 1, wherein the firs and second alloy are superalloys.

4. A method as recited in claim 1, wherein the dis is a gas turbine disk.

5. A method as recited in claim 1, wherein th generatrix is a curved line.

6. A method as recited in claim 1, wherein th generatrix is a straight line.

7. A method as recited in claim 6, wherein, befor the method, the generatrix is parallel to the disk axi and, after the method, the generatrix is parallel to th disk axis.

8. A method as recited in claim 6, wherein, befor the method, the generatrix is parallel to the disk axi and, after the method, the generatrix has a draft angl with respect to the disk axis.

9. A method as recited in claim 6, wherein, befor the method, the generatrix has a draft angle with respec to the disk axis and, after the method, the generatrix i parallel to the disk axis.

10. A method as recited in claim 6, wherein, befor the method, the.generatrix has a draft angle with respec to the disk axis and, after the method, the generatri has a draft angle with respect to the disk axis.

11. A method as recited in claim 1, wherein th distance between every point on the surface of revolutio and the disk axis is less than the distance between th 28 outer edge of the disk and the disk axis.

12. A method as recited in claim 1, wherein the ven is present in only one of the die faces.

13. A method as recited in claim 12, wherein afte step c, the workpiece is inverted and the method step are repeated.

14. A method as recited in claim 12, wherein, afte step c, the workpiece is placed in a second pair o forging dies in which the vent is in the other die fac and the method is repeated.

15. A method as recited in claim 1, wherein th first die face is provided with a first vent and th second die face is provided with a second vent.

16. A method as recited in claim 15, wherein th first vent and second vent are equidistant from the dis axis during the method.

17. A method as recited in claim 16, wherein th cross-sectional profile of the vents are symmetric abou the height line.

18. A method as recited in claim 16, wherein th cross-sectional profile of the vents are asymmetric abou the height line.

19. A metho as recited in claim 15, wherein th first and second vents are different distances from th disk axis during the method.

20. A method as recited in claim 19, wherein th cross-sectional profile of the vents are symmetric abou the height line.

21. A method as recited in claim 19, wherein th cross-sectional profile of the vents are asymmetric abou the height line.

22. A method as recited in claim 1, wherein th method is carried out so that the workpiece deforms wit enhanced plasticity.

23. A method as recited in claim 22, wherein th workpiece deforms subsuperplastically.

24. A method as recited in claim 22, wherein th workpiece deforms superplastically

25. A method as recited in claim 1, wherein th method is carried out with the entire workpiece a approximately the same elevated temperature.

26. A method as recited in claim 1, wherein th method is carried out with the dies and the entir workpiece at approximately the same elevated temperature

27. A method as recited in claim 1, wherein th method is carried out with the dies and the entir workpiece at approximately the same elevated temperatur and in such a way that workpiece grain growth i suppressed.

28. A method as recited in claim 1, wherein substantially all of the material originally present a the bondline is caused to move into whatever vents are provided.

29. A method as recited in claim 1, wherein the method is carried out in such a way as to cause bulk flow within substantially the entire workpiece.

30. A method as recited in claim 1, wherein th cross-sectional vent area is equal to or greater than th width of the mouth of the vent times the initial lengt of the bondline.

31. A method as recited in claim 1, wherein th cross-section of the vent is substantially triangula with a base side against the workpiece, the width of th mouth of the vent being the length of the base side, an the height being to length of a height line which is line representing the distance between the base side an the vent point farthest from base side.

32. A method as recited in claim 31, wherein th cross-section is symmetric on both sides of the heigh line.

33. A method as recited in claim 31, wherein th portion of the base side on one side of the height lin is greater than the portion on the other side.

34. A method as recited in claim 1, wherein th height of the cross-section of the vent is equal to o greater than the width of the mouth of the cross-section.

35. A method as recited in claim 1, wherein th height of the cross-section of the vent is at least twic the width of the..mouth of the cross-section.

36. A method as recited in claim l, wherein th total cross-sectional area of the vents employed in th method equals approximately the average mouth width o all of the vents employed in the method times the initia thickness of the disk.

37. A method as recited in claim 1, wherein no pa of the rib extends farther from the disk axis than do the outer edge.

38. A method as recited in claim 1, wherein, durin step b, the edges of the vent are all closer to the dis axis than the outer edge of the disk.

39. A method as recited in claim l, wherein, eac die face is provided with a forging impression whic includes the vents, and, except for the vents, the shape of the impressions of the forging dies define a cavit which closely conforms to the initial shape of th workpiece.

40. A method as recited in claim 1, wherein, eac die face is provided with a forging impression whic includes the vents, and, except for the vents, the shape of the impressions of the forging dies define a cavit which closely conforms to the initial shape of th workpiece, so that, except for the ribs at the vents there is little change in the shape of the workpiec during the process and the displacements and strains i the workpiece are concentrated along the boundary a metal at and adjacent the boundary flows into the vents.

41. A method as recited in claim 1, wherein following step c, the process is repeated on th bondline that results from the previous application o the process.

42. A method as recited in claim 1, wherein the sai substantial amount is substantially all of the materia initially present at the boundary.

43. A method as recited in claim 1, wherein the sai substantial amount is at least 80% of the materia initially present at the boundary.

44. A method as recited in claim 1, wherein the sai substantial amount is at least 90% of the materia initially present at the boundary.

45. A method as recited in claim 1, wherein the sai substantial amount is at least 95% of the materia initially present at the boundary.

46. A method as recited in claim 1, wherein the sai substantial amount is at least 99% of the materia initially present at the boundary.

47. A preform suitable for forming a gas turbin disk, the preform comprising,

(a) a disk having a disk axis, a first disk face and second disk face and an annular outer edge which define the outermost extent of the disk, the disk having central portion formed of a first alloy and an annula peripheral portion formed of a second alloy, and th boundary between the central and peripheral portion bein a surface of revolution about the disk axis and bein defined by a generatrix having a first end and a secon end, said surface having a first circular edge at th first face of the disk and generated by the first end o the generatrix, and a second circular edge at the secon face of the disk and generated by the second end of th generatri ,

(b) a sacrificial rib attached to a face of the disk an adapted to be removed therefrom, the rib being entirel within the outer edge and having the boundary passin through it so that it contains some of each alloy, and

(c) defects which exist at the boundary, substantiall all of which exist at that portion of the boundary whic is within the rib and the remainder of which have bee highly strained.

48. A pair of forging dies adapted for forming disk having a disk axis, a first disk face and a secon disk face and an annular outer edge which defines th outermost extent of the workpiece, the disk having central portion formed of a first alloy and an annula peripheral portion formed of a second alloy, and th boundary between the central and peripheral portion bein a surface of revolution about the disk axis and bein defined by a generatrix having a first end and a secon end, said surface having a first circular edge at th first face of the disk and generated by the first end o the generatrix, and a second circular edge at the secon face of the disk and generated by the second end of th generatrix, comprising :

(a) a first die having a first die face and a firs impression formed in its die face and adapted to engag the disk, said first die having an annular vent formed i its die face and in the impression, said vent having tw concentric vent edges at the die face both of which ar entirely within the cavity, and positioned to straddle desired circular edge of the disk, and

(b) a second die having a second die face and a secon impression adapted to engage the disk.