Patent Publication Number: US-6658715-B1

Title: Method of producing an element of composite material

Description:
The present invention relates to a method of producing elements of composite material, in particular, circular-geometry elements such as countershafts, turbine and compressor disks for turbomachines, etc. 
     BACKGROUND OF THE INVENTION 
     As is known from Italian Patent Application n. TO96A000979 filed on Dec. 3, 1996 by FIATAVIO S.p.A, composite-material elements of the above type are produced by forming a number of disks, each formed by winding a continuous reinforcing fiber about an axis to form a flat spiral; stacking the disks with the interposition of respective spacer sheets of metal material; and axially compacting the stack to form a metal matrix in which the various spirals of reinforcing fibers are embedded. 
     The physical characteristics of such composite-material elements depend mainly on the distribution of the reinforcing fibers inside the metal matrix; and the extent to which the fibers are distributed evenly depends on the extent to which the turns in each disk are equally spaced a predetermined distance apart, and the extent to which the freedom of movement of the various turns is restricted, especially at the compacting stage. 
     For which reason, the turns of reinforcing fiber are locked in place with respect to one another by fastening wires wound about each turn and extending spokefashion with respect to the axis of the spiral. 
     More specifically, the turns are equally spaced a given distance apart by forming, alongside formation of the spiral, a further two flat spirals of spacer wire, which are removed from the spiral of reinforcing fiber once the fastening wires are wound about the turns. 
     The method described briefly above involves several drawbacks. 
     In particular, producing composite-material elements using disks of reinforcing material and metal spacer sheets of given thicknesses means it is impossible to obtain any given desired distribution of the reinforcing fibers inside the metal matrix. 
     Moreover, the above method comprises various fairly complex, and therefore fairly high-cost, operations (weaving the spirals of reinforcing wire separately and fastening the relative turns; stacking the disks of ceramic material and spacer sheets; and placing the stacks inside a final container to form the composite-material elements). 
     In the case of a titanium metal matrix, the spacer sheets are not easy to procure in the form required by the methods described, i.e. of constant 0.1 mm thickness, and call for various dedicated machining operations (cutting, grinding, welding, etc.) which further increase the already high cost involved. 
     Finally, the fastening wires must be made of inert material, with respect to both the metal matrix and the reinforcing fibers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of producing an element of composite material, designed to eliminate in a straightforward, low-cost manner the aforementioned drawbacks typically associated with known methods. 
     According to the present invention, there is provided a method of producing an element of composite material comprising a metal matrix and a reinforcing structure, said method comprising the steps of: 
     forming a first distribution of first elements defining said matrix; 
     forming a second distribution of second elements defining said reinforcing structure; and 
     compacting said first and second elements to obtain a distribution of said reinforcing structure inside said matrix; 
     characterized in that said first elements are metal wires; and in that said step of forming said first distribution comprises the step of assigning each said second element an orderly distribution of said metal wires. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows a front view of an element of composite material formed in accordance with the present invention; 
     FIG. 2 shows an axial section of a supporting body with a ring of composite material, from which the FIG. 1 element is formed using the method according to the present invention; 
     FIG. 3 shows a larger-scale view of a detail of the FIG. 2 ring; 
     FIGS. 4 to  9  show partial axial sections of successive operating steps in the formation of the FIG. 1 element according to the method of the present invention; 
     FIG. 10 shows the FIG. 3 detail following application of the method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  1  in FIG. 1 indicates as a whole an element of composite material formed using the method according to the present invention—in the example shown, a rotary member, such as a compressor disk for turbomachines, to which the following description refers purely by way of example. 
     Element  1  is of circular annular shape with an axis of symmetry A, and comprises a central portion  2  in the form of a flat disk and defining a through hole  3  of axis A, and a substantially cylindrical peripheral portion  4  projecting axially in both directions with respect to central portion  2  and supporting externally a number of projecting radial blades  5 . 
     More specifically, central portion  2  is made of a composite material defined by a matrix of metal material —in the example shown, titanium alloy—and by a reinforcing structure of ceramic material—in the example shown, silicon carbide—and is coated externally with a thin layer of metal or so-called “skin”, preferably of titanium alloy. 
     Peripheral portion  4 , on the other hand, is made entirely of metal material, advantageously the same material as the matrix of central portion  2 . 
     Element  1  is formed by preparing and then compacting a toroidal base structure  6  (FIG. 6) of axis A. 
     Structure  6  is formed from a substantially annular main body  7  (FIGS. 2,  4 - 9 ) comprising a through hole  8  of axis A defining hole  3  of element  1 , and a disk-shaped portion  9 , from a flat end surface  10 , perpendicular to axis A, of which projects axially a cylindrical tubular portion  11  having an outside diameter smaller than the outside diameter of disk-shaped portion  9 . 
     Hole  8  is defined at portions  9  and  11  by respective cylindrical surfaces  12 ,  13  having different diameters and connected to each other by a flat intermediate surface  14  perpendicular to axis A and extending along an extension of end surface  10 . More specifically, cylindrical surface  12  is larger in diameter than cylindrical surface  13 . 
     Main body  7  also comprises an annular projection  15 , of axis A, projecting inside hole  8  from intermediate surface  14  and having a right-triangular section with the hypotenuse facing cylindrical surface  13 . 
     Base structure  6  is formed as follows. 
     First of all, a first distribution of metal wires  20  defining the metal matrix of element  1 , and a second distribution of fibers  21  of ceramic material defining the reinforcing structure of element  1  are positioned coaxially on main body  7 . 
     An important characteristic of the present invention is that the first distribution is formed by assigning each fiber  21  an orderly distribution of metal wires  20 . Wires  20  and fibers  21  together define a composite-material ring  16  (FIG. 2) woven on a known winding machine not shown. In the example shown, wires  20  and fibers  21  are annular with a circular section (FIG. 3) and are made respectively of titanium alloy and silicon carbide. 
     More specifically, ring  16  is positioned coaxially about tubular portion  11  of main body  7 , and rests on end surface  10  of disk-shaped portion  9 . 
     Wires  20  and fibers  21  are advantageously combined in a weave pattern (FIG. 3) in which two wires  20  are interposed between each pair of fibers  21 . More specifically, in the weave pattern, each fiber  21  is surrounded by six wires  20  forming the vertices of a hexagon, and occupies the barycenter of the hexagon. 
     Ring  16  is defined externally by a radially outer and radially inner cylindrical lateral surface  22   a ,  22   b , and by two opposite flat annular end surfaces  22   c ,  22   d ; which surfaces  22   a ,  22   b ,  22   c ,  22   d  are made exclusively of metal wires  20  for ensuring, after the compacting step, the structural continuity of ring  16 , main body  7  and the other metal parts of structure  6  described in detail later on. 
     Wires  20  and fibers  21  have the same diameter and together define a number of hexagonal base cells  18  (shown by the dash lines in FIG.  3 ); and each base cell  18  is defined by a central fiber  21  and by respective 120° angular portions of the six wires  20  surrounding central fiber  21 , so that the volume of the reinforcing structure is 33% that of the matrix. 
     Structure  6  is completed by fitting main body  7  coaxially with two annular closing elements  23 ,  24  (FIGS. 4 and 5) and a cover  25  (FIG.  6 ), which, together with main body  7 , define a closed seat for ring  16 . 
     With particular reference to FIGS. 4-9, closing element  23  is the same axial height as tubular portion  11  of main body  7 , while the axial height of closing element (or piston ring)  24  equals the difference between the axial heights of tubular portion  11  and ring  16 . 
     Closing element  23  is fitted onto the radially outer surface  22   a  of ring  16  so as to rest on end surface  10  of disk-shaped portion  9  of main body  7 ; and, similarly, closing element  24  is inserted between tubular portion  11  of main body  7  and closing element  23  so as to rest on end surface  22   d  of ring  16 , on the opposite side to disk-shaped portion  9 . 
     Cover  25  comprises a circular, annular, disk-shaped wall  28 , from the radially inner and outer peripheral edges of which project respective concentric inner and outer cylindrical walls  29 ,  30 . 
     Cover  25  is assembled by positioning disk-shaped wall  28  facing respective free axial ends of closing elements  23 ,  24  and tubular portion  11  of main body  7 , and by inserting cylindrical wall  29  inside hole  8  so that the end rests on projection  15 , and by fitting cylindrical wall  30  on the outside of closing element  23  so that the end rests on a peripheral annular shoulder  31  of disk-shaped portion  9  of main body  7  (FIG.  6 ). 
     Cover  25  is then fixed to main body  7  by spot welding the portions contacting projection  15  and shoulder  31 . 
     At this point, the air inside structure  6  is extracted using a known molecular pump (not shown) and a known muffle furnace (not shown) for heating structure  6  to a temperature of about 600° C. 
     The resulting structure  6  is compacted in a conventional autoclave (not shown) for HIPping (Hot Isostatic Pressing) processing with automatic temperature and pressure control. 
     At the first stage, lasting about two hours, the temperature of the autoclave, initially at ambient conditions, is increased to the superplasticity temperature of the titanium alloy—in the example described, about 900° C. 
     The temperature in the autoclave is then maintained constant long enough to enable the entire mass defining structure  6  to reach a uniform temperature. This period of time—two hours on average—is calculated bearing in mind that heat transmission at this stage is slowed down by the absence of air inside structure  6 , and by the fact that the contact area between wires  20  of surfaces  22   a ,  22   b ,  22   c ,  22   d  of ring  16  and main body  7  is extremely small and therefore permits very little heating by conduction of wires  20 . At the same time, the pressure inside the environment housing structure  6  and defined by the autoclave is increased to such a threshold value—in the example described, 900 Kg/cm2—as to permanently deform disk-shaped wall  28  of cover  25  in a direction parallel to axis A (FIG.  7 ). More specifically, disk-shaped wall  28  of cover  25  flexes so as to come to rest on closing element  24 , which in turn presses against composite-material ring  16  to act as a pressure equalizer and transmitter. Once disk-shaped wall  28  of cover  25  is so deformed as to enable closing element  24  to axially stress composite-material ring  16 , metal wires  20  are deformed so as to fill the gaps formerly present between wires  20  and fibers  21 . At this stage, composite-material ring  16  contracts along axis A, while the position of fibers  21  with respect to axis A remains constant to ensure uniform distribution of the reinforcing structure inside the metal matrix. 
     At this point, the pressure inside the autoclave is increased further to such a threshold value—in the example shown, about 1300 Kg/cm2—as to collapse the whole of structure  6 , which is also compacted crosswise to axis A (FIG.  9 ). More specifically, cylindrical walls  29 ,  30  of cover  25  adhere respectively to a radially outer surface of closing element  24  and to surface  13  defining hole  8 , while composite-material ring  16  adheres along metal peripheral surfaces  22   a ,  22   b ,  22   c ,  22   d  to disk-shaped and tubular portions  9 ,  11  of main body  7  and to closing elements  23  and  24 . 
     The compacted structure  6  is then cooled by so reducing the temperature and pressure as to minimize the residual stress produced in the portion derived from composite-material ring  16  by the different coefficients of thermal expansion of the metal matrix and reinforcing fibers  21 . 
     The portion of element  1  derived from ring  16  assumes the FIG. 10 configuration, in which fibers  21  are evenly distributed inside the metal matrix, are equally spaced in a direction perpendicular to axis A, and are separated by varying distances in a direction parallel to axis A. 
     Finally, the compacted structure  6  may be subjected to mechanical machining or similar to obtain the finished contour of element  1 . In particular, blades  5  are formed from the part of compacted structure  6  derived from disk-shaped portion  9  of main body  7 . 
     Using metal wires  20  to form the matrix of composite-material element  1  therefore provides, by appropriately selecting the diameter of wires  20  and fibers  21 , for obtaining any desired distribution of the reinforcing structure inside the metal matrix. 
     In particular, by appropriately selecting the type of distribution of metal wires  20  relative to each reinforcing fiber  21 , e.g. by adopting the hexagonal distribution described previously, the freedom of movement of fibers  21  can be limited during compaction to maintain the positions of fibers  21  with respect to axis A. 
     Moreover, unlike known methods, the method described provides for forming composite-material element  1  by weaving wires  20  and fibers  21  directly onto parts (main body  7 ) eventually forming part of the metal matrix of element  1 , thus eliminating the need for producing separate disks of reinforcing wire, fastening the turns of each disk, the long, complicated process of stacking the disks with respective metal spacer sheets in between, and placing the stacks inside containers for producing elements  1 . 
     The spacer sheets, which are particularly expensive when titanium-based, and the work involved in preparing the sheets may therefore be eliminated with considerable saving. 
     Finally, contraction of structure  6  at the compacting stage is less than that of stacks of ceramic disks and metal spacer sheets using the known methods described previously. 
     Clearly, changes may be made to the method described and illustrated herein without, however, departing from the scope of the accompanying Claims. 
     In particular, reinforcing fibers  21  may be made of different materials, including metal. 
     Main body  7 , closing elements  23 ,  24  and cover  25  may be made of different metal materials from each other and from the material of wires  20 . 
     Finally, once formed, composite-material ring  16  may even be extracted from structure  6  and used to form different composite-material elements.