Patent Publication Number: US-6698645-B1

Title: Method of producing fiber-reinforced metallic building components

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of producing fiber-reinforced metallic building components, i.e. structural components, with a complicated three-dimensional geometry. 
     BACKGROUND INFORMATION 
     The extraordinary strength properties of SiC fibers are known. These properties in combination with their thermal stability has predestined ceramic SiC fibers for use as reinforcing elements for metallic materials. With regard to an intimate, load-transferring connection between the ceramic fibers and the metallic matrix, the fiber must first be provided with a well-adhering surface coating of a metal that is identical or at least “related” to the material of the building component from the standpoint of the subsequent diffusion bonding or diffusion welding. The fiber coating is usually provided by the PVD method, specifically by magnetron sputtering. The fiber-reinforced metallic building components ultimately produced are also known as MMCs (metal matrix composites). SiC fibers are produced as long fibers or continuous fibers with lengths of up to approximately 40 km, but fractions or sections 150 meters in length, for example, are usually used in construction practice. A preferred fiber diameter is approximately 100 μm. A certain disadvantage of the rigid SiC fiber is its susceptibility to kinking, which is why it can be bent only with a relatively large radius of bending. The minimum bending radius for said 100 μn fibers is approximately 2.5 cm. Due to the great length of the fiber, it is possible to apply it to building components that are to be reinforced by the winding technique to advantage, of course taking into account the fiber-specific minimum bending radius. Concrete applications so far have been mainly relatively simple rotor elements, e.g., in the form of rotationally symmetrical shafts, disks and rings or combinations of these elements. They should usually be produced by winding a metal-coated SiC long fiber around metallic carriers having a contour that corresponds at least mostly to the final form, covering the fiber windings with the metal, and producing a bonded monolithic structure, i.e., consolidating the resulting prefabricated unit in vacuo under the influence of pressure and temperature, the latter preferably by the HIP method (hot isostatic pressing). In addition to contoured components such as covers, sleeves, pipes, disks, etc., flexible and free-flowing elements such as films, wires, powders and the like may also be used as the covering for the fibers. Because of the favorable strength/weight ratio, titanium and its alloys have a preferred position among the materials to be reinforced. In this regard, see German Patent 4,324,755, for example. 
     For higher use temperatures, metals such as nickel and cobalt are recommended as matrix materials. Because of the great strength of the SiC fiber and its relatively low density (approx. 3.9 g/cm 3 ) SiC-fiber-reinforced building components practically always permit lighter constructions than corresponding building components made only of metal. This again predestines MMCs with SiC reinforcement for use in high-speed rotors of all types. The fiber content that is currently feasible in the area of reinforcement is approx. 40 vol %. 
     The problem of production of MMC building components with SiC fiber reinforcement in complex, three-dimensional geometric shapes, e.g., in the form of blades for motors, has not been solved satisfactorily so far. First, it is practically impossible to cover a metal carrier—as a building component precursor—having a complex three-dimensional shape with the “unmanageable” SiC fibers in a defined manner, and definitely not by the preferred winding technique. On the other hand, consolidated SiC fibers, whose metallic surfaces have already formed bonds cannot be deformed permanently without destruction and/or breakage of the fibers. 
     Against this background, the object of this invention is to provide a method of producing SiC fiber-reinforced metallic building components which makes it possible to produce a defined fiber reinforcement in a reproducible and economical manner especially with the more complex three-dimensional geometric shapes, thus making the use of MMC technology for building components having complex shapes truly possible for the first time. 
     This object is achieved by process steps A through C characterized in Patent claim  1  in combination with the generic features in the introductory clause. 
     The above object has been achieved according to the invention in a method of producing a fiber-reinforced metallic building component or structural component. The principle of this invention is that metal-coated SiC fibers forming the fiber reinforcement are applied to a metallic sectional piece having a simple geometry and are held without being restrained thereon by means of a metallic counterpart piece, next the unit of the sectional piece, fibers and counterpart piece is plastically deformed and shaped into the complex final shape whereby the fibers are still “loose” and unbonded, and only then the unit is consolidated into a monolithic part by diffusion bonding. The steps of plastic deformation or shaping and consolidation take place at least mostly separately and in succession in the same device or within the same mold, with the process parameters of pressure, temperature and time being controlled appropriately. After consolidation, the part is still not a finished building component, so additional manufacturing steps such as cutting or joining must then follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is explained in greater detail below on the basis of the drawings, showing in simplified schematic diagrams: 
     FIG.  1 : a cross section through a sectional piece covered with fibers and a counterpart piece, 
     FIG.  2 : a section through two molds with a unit to be shaped, 
     FIG.  3 : a diagram showing the pressure and temperature over time in shaping and consolidation and a sectional view comparable to that in FIG. 2, showing a shaped and consolidated part, 
     FIG.  4 : a rotating carrier with several sectional pieces wrapped with fiber, 
     FIG.  5 : two consolidated parts to be combined to a hollow paddle or blade, and 
     FIG.  6 : the blade assembled by joining the parts according to FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS 
     The geometrically simple metallic sectional piece  1  in FIG. 1 is formed by a U-shaped section having a planar base face and low vertical legs. It is already covered with metal-coated SiC fibers  4 —to be more precise, with pieces of one or a few long SiC fibers—and it is to be “sealed” by the metallic counterpart piece  7  like a cover, the latter being secured on the legs of the sectional piece or member  1  by spot welding, for example. The counterpart piece  7  should hold the SiC fibers  4  in their desired position as smoothly as possible so that metallic fiber surfaces still remain displaceable in length relative to one another and relative to the adjacent sectional surfaces with little friction, which is important for the subsequent shaping. The hollow spaces between the fibers can be filled—at least in part—with a metal powder (not shown), so the subsequent consolidation may be facilitated and improved. 
     FIG. 2 shows a planar unit  10  of sectional piece  2 , SiC fibers  5  and counterpart piece  8  inserted between two mold halves  12 ,  13  having similar convex and concave curvatures for the contact surfaces. Mold halves  12 ,  13  belong to a hot press (not shown) whose working space can be evacuated and heated (“T” stands for temperature). The arrows above and below the mold halves  12 ,  13  including the symbol “p” represent the press pressure, with at least one mold half being designed to move in the direction of the arrow and vice versa. The contact faces of mold halves  12 ,  13  (shown here with a simple curvature for the sake of simplicity) are usually more complicated, three-dimensional shapes in reality, such as those required for gas turbine blades, for example. 
     FIG. 3 shows at the left a diagram showing curves for pressure (p) and temperature (T) over time for the two process steps “shaping” and “consolidation” which take place in chronological succession in the same device. The curves for pressure and temperature tend to be similar, although that need not always be the case. As an example, the plastic deformation is carried out at a temperature of approximately 800° C. and the consolidation is carried out at a temperature of approximately 950° C. 
     Starting with the condition illustrated in FIG. 2 with mold halves  12 ,  13  still opened and after reaching a mold temperature and workpiece temperature at which the metal parts of unit  10  can undergo plastic deformation with no problem, mold halves  12 ,  13  are moved toward one another at a defined pressure and/or a defined force until unit  10  has undergone complete plastic deformation, i.e., it is in full contact with the contact faces of mold halves  12 ,  13 . During this deformation process, the metal coated SiC fibers  5  must not bond and/or weld to one another or to the adjacent parts  2 ,  8  because the resulting high shear stresses would interfere with shaping and/or would lead to fiber breakage. Therefore, the pressure p and temperature T must not be too high here. In the p-T-time diagram, this shaping step can be seen in the form of the two small lower plateaus. 
     After the end of plastic shaping, i.e., after the movable mold half has come to a standstill at an unchanged pressure, the pressure and temperature are increased further to initiate the process step of consolidation, where a monolithic part which is largely free of hollow spaces and has an integrated, load-bearing fiber reinforcement is obtained with further densification of the structure through diffusion bonding and/or welding of the inside metal surfaces. This condition with the finally compressed, consolidated part  11  is shown at the right in FIG.  3 . In the pressure-temperature-time diagram, the consolidation corresponds to the two broad upper plateaus. 
     It may be sufficient to increase only one of the parameters p or T for the transition from plastic deformation to consolidation. Experimental investigations are definitely indispensable in this regard. 
     It should be pointed out that as a rule, part  11  is still not a finished building component even after being removed from mold halves  12 ,  13 . 
     FIG. 4 shows an especially economical method of providing a fiber covering for several sectional pieces  3 . However, this presupposes a unidirectional fiber orientation—at the beginning. The trick is to arrange several sectional pieces  3  on the periphery of a wheel-shaped rotating carrier  14  in such a way that the theoretical fiber direction of each profile piece  3  runs tangentially. Sectional pieces  3  may be planar or they may have a relatively simple curvature. By rotating the carrier  14  and winding at least one long tangentially supplied SiC fiber  6  around it, the desired coverage is achieved after a certain number of revolutions and a controlled lateral displacement of the fiber feed, i.e., a helical winding, optionally in multiple layers. Then the metallic counterpart pieces  9  are applied and secured so that the SiC fibers are held securely. This condition, with carrier  14  stationary—is shown in FIG. 4 (therefore, the arrow indicating rotation about the axis of the carrier is indicated only with dotted lines). Now the exposed fiber strands between the sectional pieces  3  can be severed and cut back to the ends of the building components so that the units of sectional pieces, fibers and counterpart pieces can be removed separately from the carrier  14 . Then each unit undergoes plastic deformation and consolidation as explained above. 
     It is also conceivable to design the mold halves from FIGS. 2 and 3 so that several prefabricated units, each consisting of sectional piece, fibers and counterpart piece, undergoes plastic deformation and consolidation together, possibly also being bonded together, with the units being arranged side by side/in succession and/or one atop the other between the mold halves. 
     FIGS. 5 and 6 concern in particular the production of hollow titanium blades for gas turbines in the axial design. 
     FIG. 5 shows two separate, shaped and consolidated parts  11  and  15  made of titanium or titanium alloy with integrated SiC fiber reinforcement. The fiber orientation and coverage are adapted to the subsequent operating condition, with the fiber direction being unidirectional or with several orientations. With rotor blades, the fibers run primarily in the direction of centrifugal force, i.e., radially, but with turbine guide vanes, other fiber orientations or multiple fiber orientations may be advantageous, e.g., to counteract vibration modes. The plate-shaped parts  11 ,  15  have different curvatures to form a hollow flow profile after they are joined. 
     Reference letter R with an arrow indicates that in the simplest case, the curvature may follow an arc of a circle. Depending on the technical flow requirements, however, three-dimensional curves of almost any shape may be implemented. Parts  11  and  15  have metallic surfaces which can be bonded together in various ways, in particular by soldering and welding. In the meantime, solders and soldering methods have been developed for titanium and its alloys, permitting joints with a strength equal to that of the material of the building component. 
     In this sense, FIG. 6 shows a hollow blade  16  which is joined by soldering the two parts  11  and  15 . The soldered spots are located in the area of the leading edge and the trailing edge of the blade and are labeled as  17  and  18 . A longitudinal axis of the blade, preferably the axis of the stack running through the centers of gravity of the section, can be seen here as vertical arrow Z. In a gas turbine using blade  16 , the axis Z runs at least mostly radially, starting from the longitudinal center axis of the gas turbine which may also be an aircraft engine. It would be clear to those skilled in the art that the blade  16  shown here is not yet ready for installation. It has no connection elements or function elements, such as footing with or without a platform, an internal and external shroud segment in the case of a turbine guide vane, a wear-resistant blade tip, etc. These elements are made entirely or partially of a comparable metal, in particular a titanium alloy, and they may contain ceramic fibers and/or particles. The elements may consist of different alloys that are adapted to the local operating conditions in the best possible way. Criteria such as the fire resistance of titanium, wear resistance, etc. play a role here. The material integration is preferably achieved by soldering. 
     This hollow blade design can of course also be used with other fiber-reinforced metals, e.g., those based on iron, nickel or cobalt (Fe, Ni, Co).