Abstract:
An article having a hollow cavity formed therein and a method for forming the same. The article includes a hollow structure having an open end and a body portion that is surrounded by a powdered material. The article is processed in, for example, a hot isostatic pressing operation, to permit a pressurized fluid to consolidate the powdered material. The pressurized fluid is permitted to pass through the open end of the hollow structure and into the body portion to thereby prevent the body portion from collapsing while the powdered material is being consolidated.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the formation of articles with powdered materials and more particularly to an article formed with a powdered material to include a hollow cavity formed therein and a method for forming the same. 
     BACKGROUND OF THE INVENTION 
     Background Art 
     Turbine disks and blades are commonly subject to high cycle fatigue failures due to high alternating stresses as a result of resonant vibration and/or fluid-structural coupled instabilities. Turbine disks are typically designed to avoid standing wave diametrical mode critical speeds within the operating speed range. High dynamic response occurs when the backward traveling diametrical mode frequency is equal to the forward speed diameteral frequency which results in a standing wave form with respect to a stationary asymmetric force field. Turbine blades are designed to avoid having any of the blade natural frequencies from being excited by the stationary nozzle forcing frequencies in the operating speed range. 
     In conventional turbine wheel assemblies, conventional blade dampening techniques are typically employed to reduce the fluid-structure instabilities that results from the aerodynamic forces and structural deflections. Accordingly, it is common practice to control both blade and disk vibration in the gas turbine and rocket engine industry by placing dampers between the platforms or shrouds of individual dovetail or fir tree anchored blades. Such blade dampers are designed to control vibration through a non-linear friction force during relative motion of adjacent blades due to tangential, axial or torsional vibration modes. Blade dampers, in addition to the blade attachments, also provide friction dampening during vibration in disc diametral modes. 
     Integrally bladed turbine disks (blisks) are becoming increasingly common in the propellant turbopumps of liquid fueled rocket engines and gas turbines. While the elimination of separate turbine blades reduces fabrication costs, the monolithic construction of integrally bladed turbine disks eliminates the beneficial vibration damping inherent in the separately bladed disk construction. Accordingly, the above-mentioned damping mechanism is not heretofore been feasible for integrally bladed turbine disks unless radial slots were machined into the disk between each blade to introduce flexibility to the blade shank. The added complexity of the slots would increase the rim load on the turbine blade and defeat some of the cost, speed and weight benefits of the blisk. Consequently, the lack of a blade attachment interface had resulted in a significant reduction in damping and can result in fluid-structure instabilities at speeds much lower than the disk critical speed and at minor blade resonances. 
     Other dampening mechanisms have been proposed that typically require multiple machining operations followed by the use of external fastener attachments. These machining operations tend to be rather expensive, thereby negating many of the cost advantages of the integrally-bladed turbine disk. Furthermore, there is a general desire to reduce or eliminate the use of any fasteners which, if over stressed, could possibly break loose and cause damage. Accordingly, there remains a need in the art for an improved vibration dampening mechanism that is cost-effectively integrated into an integrally-bladed turbine disk such that the dampening mechanism is housed within a cavity formed into the integrally-bladed turbine disk. 
     SUMMARY OF THE INVENTION 
     In one preferred form, the present invention provides a method for forming a hollow cavity in an article. The method includes the steps of providing a preformed article; positioning a hollow structure having an open end and an inside wall at a predetermined position relative to the preformed article; filling a space around at least a portion of the hollow structure with a powdered material, the space abutting the preformed article; and exposing the hollow structure and the powdered material to a pressurized fluid such that the pressurized fluid compacts the powdered material and simultaneously exerts a resisting force onto the inside wall of the hollow structure. 
     In another preferred form, the present invention provides an article having a first article portion, a second article portion and a hollow structure. The hollow structure has an endless body portion with an inside wall and a stem portion that intersects the body portion and has an open end. The body portion is positioned around a portion of first article portion. The second article portion is formed from a powdered material. The second article portion abuts the first article portion and surrounds the body portion of the hollow structure. The second article portion is consolidated and diffusion bonded to the first article portion in a hot isostatic pressing operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a portion of an integrally-bladed turbine disk constructed in accordance with the teachings of the present invention; 
     FIG. 2 is a perspective cross-section of a portion of the integrally-bladed turbine disk of FIG. 1 illustrating the first disk portion; 
     FIG. 3A is a perspective view of a portion of the integrally-bladed turbine disk of FIG. 1 illustrating the hollow structure in partial cross-section; 
     FIG. 3B is a perspective view similar to that of FIG. 3A but illustrating the end of an alternately constructed hollow structure; 
     FIG. 4A is an exploded view illustrating the fabrication of the integrally-bladed turbine disk of FIG. 1; 
     FIG. 4B is a partial top perspective view illustrating the fabrication of the integrally-bladed turbine disk of FIG. 1; 
     FIG. 5 is a cross-sectional view illustrating the fabrication of the integrally-bladed turbine disk of FIG. 1; 
     FIG. 6A is a cross-sectional view of an autoclave illustrating the fabrication of the integrally-bladed turbine disk of FIG. 1; 
     FIG. 6B is partial cross-sectional view of an autoclave similar to that of FIG. 6A but illustrating the hollow structure as filled with an incompressible fluid; 
     FIG. 6C is a partial cross-sectional view of an autoclave similar to that of FIG. 6A but illustrating the hollow structure as coupled to a secondary pressure source; 
     FIG. 7 is a cross-sectional view of the integrally-bladed turbine disk of FIG. 1 illustrating the rim portion after the completion of the HIP operation; 
     FIG. 8A is a perspective view in partial cross-section of the integrally-bladed turbine disk of FIG. 1 illustrating the severing of the rim portion into segments; 
     FIG. 8B is a perspective view similar to that of FIG. 8A but illustrating the severing rim portion segments and the dampening members; 
     FIG. 9 is a perspective view in partial cross-section of the integrally-bladed turbine disk of FIG. 1 illustrating the insertion of the dampening members into the dampening channels; 
     FIG. 10 is a cross-sectional view of the body portion of a hollow structure formed in accordance with the teachings of an alternate embodiment of the present invention; 
     FIG. 11 is a cross-sectional view taken along the line  11 — 11  of FIG. 10; and 
     FIG. 12 is a perspective view in partial cross-section illustrating an integrally-bladed turbine disk constructed with the hollow structure of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1 of the drawings, an integrally-bladed turbine disk constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Turbine disk  10  is shown to include a preformed turbine disk or first disk portion  12 , a second disk portion  14 , a pair of hollow dampening channels  16  and a plurality of dampening members  18 . The first disk portion  12  includes a hub portion  20  and a plurality of blades  22  that are coupled to the hub portion  20  at their proximal end. The first and second disk portions  12  and  14  cooperate to define a rim portion  24  that is coupled to the distal end of the blades  22 . The rim portion  24  is cut at regular intervals to divide it into a plurality of segments  26 , with each of the segments being coupled to a predetermined quantity of the blades  22 . In the particular example illustrated, each of the segments  26  is coupled to one of the blades  22 . 
     The dampening channels  16  are tubes that are disposed within the rim portion  24 . In the particular embodiment illustrated, the dampening members  18  are wires  30  that are disposed within the hollow cavity  32  of the dampening channels  16 . Preferably, each of the wires  30  overlaps a plurality of adjacent segments  26  and frictionally engages the inside wall  34  of its associated dampening channel  16  to absorb vibrational energy that is transmitted between the blades  22  and the rim portion  24 . Those skilled in the art will understand that while the dampening members  18  are illustrated to be metallic wires  30 , the dampening members  18  may, however, be fabricated from any suitable material, including a non-metallic and/or non-conductive material. 
     In FIG. 2, the first disk portion  12  is illustrated in greater detail. The first disk portion  12  may be formed through any process that may be employed to form an internally-bladed turbine disk, including forging, casting, machining or net-shape hot isostatic pressing (HIP). In the particular embodiment illustrated, the first disk portion  12  is shown to include a continuous annular flange  40  that is interconnected to all of the blades  22 . The annular flange  40  includes an axially extending portion  42  that is coupled to the blades  22  at its proximal end and a pair of radially outwardly extending portions  44  that are spaced axially apart from one another and coupled to the distal surface of the axially extending portion  42 . In the particular example provided, the first disk portion  12  is formed in via net-shape HIP and thereafter machined to precisely control the dimensioning of the annular flange  40 . 
     The axially extending portion  42  and the radially outwardly extending portions  44  cooperate to define a cover pocket  45  that will be discussed in greater detail, below. A pair of dampening grooves  46  are formed into an outer portion of the axially extending portion  42  and intersect the cover pocket  45 . A cross-hole  47  extends through each lateral face  48  of the annular flange  40  and intersects an associated one of the dampening grooves  46 . In the particular embodiment illustrated, the dampening grooves  46  are rectangular in cross-section and have heavily chamfered sidewalls  49 . Those skilled in the art will understand, however, that the cross-section of the dampening grooves  46  may be constructed in any desired manner. 
     In FIG. 3A, a hollow structure  50  that is employed to form one of the dampening channels  16  is illustrated. In the particular embodiment provided, the hollow structure  50  includes a stem portion  52  and a body portion  54 , both of which are formed from identically sized hollow cylindrical tubing. The body portion  54  is endless, having a hollow cavity  32  of a substantially uniform cross-section over its entire length. As the body portion  54  will become the dampening channel  16 , the body portion  54  is sized and shaped in a predetermined manner, which in the example provided, corresponds to a generally circular shape having a diameter that is sized to fit around the axially extending portion  42  of the annular flange  40 . Those skilled in the art will understand, however, that the body portion  54  may alternatively be constructed with a different cross-section (e.g., rectangular) or to have a varying wall thickness. The stem portion  52  is fixedly coupled to the body portion  54  at its outer circumference, extending axially outwardly therefrom in a direction parallel to the axis of the body portion  54 . A first end  56  of the stem portion  52  is open and the opposite end  58  intersects the body portion  54 , thereby providing a flow path between the stem and body portions  52  and  54  that permits fluids to enter the hollow structure  50  through the open end  56  and travel into the hollow cavity  32  of the body portion  54 . 
     The term “endless” has been used to describe the body portion  54  to emphasize that the hollow cavity  32  is substantially continuous over the entire length of the body portion  54 . Those skilled in the art will understand that various design criteria for a particular application will dictate the characteristics of the body portion  54 , including its shape and whether the body portion  54  is constructed in an “endless” manner or includes one or more closed ends  59  (FIG.  3 B). 
     Referring back to FIG. 3A, the body portion  54  is shown to be formed from a single length of tubing that is first bent to a desired radius and thereafter welded together. A hole is formed through the body portion  54  and the stem portion  52  is welded to the body portion  54 . Those skilled in the art will understand that any welds mentioned herein are employed to seal the joint between two structures (e.g., the joint between the stem and body portions  52  and  54 ) as well as to withstand the substantial forces that will be exerted onto these structures at later points in the fabrication process. 
     In FIGS. 4A through 5, a pair of the hollow structures  50  are shown to be fitted to the first disk portion  12  such that the body portion  54  of each of the hollow structures  50  encircles the axially extending portion  42  of the annular flange  40  so as to lie in the dampening groove  46  and abut an inward one of the sidewalls  49 . Positioning of each of the hollow structures  50  in a predetermined manner (e.g., into abutment with an inward one of the sidewalls  49 ) may be controlled as desired by any one of numerous positioning means, including the geometry of the dampening channel (e.g., the size of the dampening groove  46 , the incorporation of special protrusions or barbs that secure the hollow structure  50  within the dampening groove  46 , etc.) and mechanical fastening mechanisms, including welds, that are well known in the art and need not be discussed in detail herein. 
     A pair of sleeves  150 , which are preferably fabricated from the same material as that of the hollow structure  50 , each have an inner diameter  152  that is sized to slip fit the stem portion  52  and an outer diameter  154  that is sized relatively larger than the cross-hole  47 . Each of the sleeves  150  are slipped over one of the stem portions  52  and into abutment with an associated one of the lateral faces  48  of the annular flange  40  where the sleeves  150  are welded into place. The relatively thin-walled stem portions  52  are then sealingly welded to the inside diameter  152  of one of the sleeves  150 . The sleeves  150  thus prevent fluid communication through the lateral face  48  of the annular flange  40  and into an associated dampening groove  46 . 
     A powdered material  60 , which is employed to form the second disk portion  14 , is packed to a predetermined density around the perimeter of the first disk portion  12  and secured in place by a sheet metal cover  62 . More specifically, the cover  62  is fitted so as to lie in the cover pocket  45  and abut the inner edge of the radially outwardly extending portions  44 . With the cover  62  fitted to the outer perimeter of the annular flange  40 , it is then welded to the radially outwardly extending portions  44  of the annular flange  40 . As the cover  62  is formed from a strip of material, the ends of the cover  62  are also welded to one another to thereby encase the powdered material  60  in a sealed cavity. The powdered material  60  may be a powdered metal, a ceramic material, or a mixture of powdered metal and ceramic materials and is preferably a material that will diffusion bond with the material that forms the first disk portion  12  during a subsequent HIP operation that will be discussed in detail below. 
     Alternatively, the hollow structure  50  may be configured such that the stem portion  52  extends radially outwardly from the body portion  54  and through a stem aperture (not shown) formed through the cover  62 . The stem portion  52  is then welded around its perimeter to the cover  62  to fixedly secure the stem portion  52  to the cover  62  as well as to seal the joint between the stem portion  52  and the cover  62 . 
     An evacuation tube  66  extends through an evacuation aperture  68  in the cover  62  and into the powdered material  60 . A weld extends around the perimeter of the evacuation tube  66  to secure the evacuation tube  66  to the cover  62  as well as to seal the joint between the evacuation tube  66  and the cover  62 . A vacuum source  70 , shown in FIG. 5, is coupled to the evacuation tube  66  and employed to evacuate interstitial gases  72  from the powdered material  60 . Once the interstitial gases  72  have been removed from the powdered material  60 , the evacuation tube  66  is sealed (e.g., crimp welded) and the vacuum source  70  is removed. 
     In FIG. 6A, the assembly  74  that consists of the first and second disk portions  12  and  14 , the hollow structures  50 , the powdered material  60 , the cover  62  and the sealed evacuation tube  66  is placed into an autoclave  76  where the assembly  74  is subjected to a pressurized fluid  80 , such as argon, nitrogen or helium, and heat  82  in a HIP operation. The heat  82  in combination with the force that is extorted by the pressurized fluid  80  through the cover  62  and onto the powdered material  60  operates to consolidate and solidify the powdered material  60 . The pressurized fluid  80  enters the hollow structure  50  through the open end  56  of the stem portion  52  and also acts on the inside wall  34  of the body portion  54  to prevent the hollow cavity  32  of the body portion  54  from collapsing due to the force that is exerted by the pressurized fluid  80  onto the cover  62  and the powdered material  60 . 
     Those skilled in the art will understand that collapse of the hollow cavity  32  may be prevented in other ways including the filling of the hollow structure  50  with an incompressible fluid  86  or a pressurized fluid and thereafter sealing the open end  56  of the stem portion  52  prior to placing the assembly  74  in the autoclave  76  as illustrated in FIG.  6 B. Alternatively, the hollow structure  50  may be coupled to a secondary pressure source  88  as illustrated in FIG.  6 C. This arrangement is advantageous in that the magnitude of the pressurized fluid  80 ′ that is delivered by the secondary pressure source  88  may be controlled independently of the magnitude of the pressurized fluid  80  that is delivered to the autoclave  76 . Accordingly, the magnitude of the pressure of pressurized fluid  80 ′ may be controlled so as to be greater than the magnitude of the pressure of pressurized fluid  80  to thereby expand the body portion  54  of the hollow structure  50  while simultaneously consolidating the powdered material  60 . 
     After the HIP operation is completed, the cover  62 , evacuation tube  66  and sleeves  150  are removed from the assembly  74  as shown in FIG.  7 . In the example provided, the powdered material  60  that was employed to form the second disk portion  14  has diffusion bonded to the first disk portion  12  and as such, the interface between the first and second disk portions  12  and  14  is imperceptible. The assembly  74  is thereafter machined as illustrated in FIG. 8A to form the rim portion  24  in a desired manner, as well as to sever a predetermined portion of the stem portion  52  from each of the hollow structures  50 . Those skilled in the art will understand that the cover  62  may also be diffusion bonded to the first and second disk portions  12  and  14  and as such, the step of removing the cover  62  may be performed substantially simultaneously with the step of machining the assembly  74 . In the particular example illustrated, any welds which had been employed to secure the cover  62  and the sleeve  150  to the axially extending portion  42  of the annular flange  40  are advantageously removed during the machining operation so as to minimize or eliminate the weld of heat-effected zones in the assembly  74 . 
     The assembly  74  is placed into an electro-discharge machine (EDM)  100  and an electrode  102  that has been shaped in a predetermined manner is employed to form a cut  104  that severs the rim portion  24  at predetermined intervals to form the plurality of segments  26  discussed above. In the particular example provided, the electrode  102  is a strip of copper that has been shaped to sever the rim portion  24  such that the distance between two adjacent blades  22  along the cut  104  is equal. 
     As shown in FIG. 9, insertion holes  90  are formed into the rim portion  24  to intersect (i.e., breach) the body portion or dampening channels  16  such that the axis of the insertion hole  90  is tangent or gradually sloped relative to the dampening channel  16 . In the embodiment illustrated, four insertion holes  90  intersect each of the dampening channels  16 , with each of the insertion holes  90  being spaced circumferentially about the diameter of the rim portion  24  at equal intervals (i.e., spaced apart at 90° intervals). As illustrated, the insertion holes  90  that intersect one dampening channel  16  are offset from the insertion holes  90  that intersect the other one of the dampening channels  16  (i.e., in the example shown, the amount of the offset is 45°). Each insertion hole  90  is sized to receive a dampening member  18  that is inserted therethrough and into the hollow cavity  32  of the dampening channel  16 . In the particular embodiment illustrated, the dampening member  18  is a wire  30  that is sized to frictionally engage the inside wall  34  of the dampening channel  16  in response to the transmission of vibrations between the blades  22  and the rim portion  24 . 
     Those skilled in the art will understand that the wires  30  may alternatively be installed prior to the cutting of the rim portion  24  via the electrode  102  as illustrated in FIG.  8 B. The electrode  102  may then be controlled to cut around the wires  30  while severing the rim portion  24  or may alternatively be controlled to cut the wires  30  into wire pieces  30 ′ when the rim portion  24  is severed. Depending upon the desired orientation of the wire pieces  30 ′ relative to the cut  104 , the wire pieces  30 ′ be repositioned after the cut  104 , as when it is desirable to have each of the wire pieces  30 ′ extend through one of the cuts  104 . In this regard, it may be beneficial to simultaneously insert the wire  30  and make the cuts  104  so that the wire  30  can be employed to reposition each wire piece  30 ′ after each of the cuts  104  has been made. The insertion holes  90  may be plugged, if desired, by welds  106  or via other mechanical means, including threaded plugs or staking. Unlike the other prior mentioned welds that were employed to seal a joint, the welds  106  are employed to inhibit the wire pieces  30  from being expelled from the dampening channels  16  during the operation of the integrally-bladed turbine disk  10 . 
     While the present invention has been described thus far in a manner wherein wires  30  are inserted to the dampening channels  16  after the rim portion  24  has been fully formed, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the hollow structure  50  may be formed as shown in FIGS. 10 and 11. In this arrangement, the body portion  54   a  is shown to include a plurality of crimps  120  that constrict a portion of the inside diameter of the body portion  54   a  at regular intervals. The crimps  120  define a plurality of cells  122  into which is received a dampening member  18 , such as a wire piece  30 ′. As illustrated, the crimps  120  do not completely close off the cells  122 , thereby permitting the pressurized fluid  80  flow around each of the dampening members  18  and into all of the cells  122 . In the embodiment illustrated, the body portion  54   a  is positioned in the manner described above and also rotated about the perimeter of the first disk portion  12  such that each of the crimps  120  is positioned between a pair of blades  22  in the area where the cut  104  will be made to form the segments  26  in the rim portion  24 . As mentioned above, the electrode  102  may then be controlled to cut around the wires  30  while severing the rim portion  24  or may alternatively be controlled to cut the wires  30  into wire pieces  30 ′ when the rim portion  24  is severed. Construction in this manner is advantageous in that it eliminates the subsequent step of inserting the wires  30  into the dampening channel  16  and provides each segment  26  with its own dampening member  18 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.