Method For Producing Continuous Lengths Of Metal Matrix Fiber Reinforced Composites

Abstract

A method for producing metal matrix fiber reinforced tape by directing a plurality of arranged fibers through a stable meniscus of molten metal and effecting rapid solidification of the molten metal through the use of moving heat extracting members so as to retain the desired mechanical properties of the composite material by minimizing the exposure of the fibers to high temperature molten metal.

Description

BACKGROUND OF THE INVENTION

The present invention is directed toward a method of making a metal matrix fiber reinforced structure by passing a plurality of indeterminate length fibers through a stable meniscus and effecting a solidification of the metal at a rate that would prevent the molten metal fiber thermokinetic interaction from seriously degrading the properties of the resultant composite product.

Methods in the prior art for producing fully dense metal matrix fiber reinforced materials may be broadly divided between those which involve solid state diffusion processes at some point in the sequence and those methods based on casting. Examples of the former category include plasma spraying of metallic powders into the interstices of a filament-wound array of fibers backed with metal foil and direct consolidation of fibers and metal foils or powders by static or dynamic hot pressing methods. However, only the casting methods are germane to the present invention and discussion will be limited to this category of prior art.

Two methods have been used in prior art to produce metal matrix fiber reinforced composites by casting. One is a batch process based on vacuum injection casting of molten metal into the interstices of a pre-arranged array of fibers. The other is a continuous process, based on drawing one or more fibers through a pot of molten metal.

The vacuum injection casting method is limited to very small pieces due to the thermokinetic problems which exist with most composite systems of interest. Chemical interactions between fibers and molten metal degrade the engineering properties of the composite. In order to insure complete infiltration of closely spaced fiber arrays, the metal must be superheated to have sufficiently low viscosity. Kinetic difficulties can thus be controlled only by rapid cooling; a condition which is inconsistent with end product sizes of practical interest.

There are three basic problems with prior art in continuous casting of metal matrix fiber reinforced composites. The first problem is one of thermokinetics, since the fiber is drawn through an appreciable path length of molten metal in the crucible. Chemical interaction between the fiber and molten matrix material remains a problem with these path lengths unless protective coatings are placed on the fibers, thereby increasing the cost of the end product. The second problem with prior art continuous casting is that the lower limit on fiber volume fraction is controlled by the surface tension of the molten metal and the relative surface energies of the two constituents. Accordingly, the lower threshold of fiber loading for a continuous product made by prior art casting is on the order of 60 to 70 volume percent. Below this threshold, the product tends to be a plurality of individual coated fibers rather than a continuous composite product. The high volume percent fiber loadings resulting from prior art continuous casting processes exceed the critical limit on fiber loading for most systems of practical interest, yielding a brittle material of limited structural value. Finally, maintaining critical fiber spacing through a significant path of molten metal presents some difficulty. In prior art this problem is either solved by drawing a closely packed array of filaments through a shape-defining orifice, depending thereby on nearest-neighbor fiber interactions for self-spacing, or by applying a significant tensile load on the collimated array as it passes through the crucible. In the former case, the spacing is effective only because of an impractically high fiber volume fraction. Applying effectively high tensile loads to the moving fiber array as it passes through the melt and handling apparatus increases the probability of fracturing the brittle filaments used in systems of interest.

The present invention has numerous advantages over these prior art methods. The advantages over injection casting lie principally in the capability of making a continuous product while minimizing thermokinetic interaction between the constituents. Advantages over prior art continuous casting methods lie in minimizing time at temperature and hence chemical interaction, applicability to fiber loading fractions of engineering interest and ease of maintaining fiber collimation. These advantages are attained through use of a stable meniscus of molten metal as a matrix supply and the incorporation of moving heat extracting members as an integral part of the casting process.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method of producing a metal matrix composite structure of an indeterminate length. This is accomplished by passing a plurality of fibers arranged so as to have a desired spacing and density through a stable meniscus of molten metal. The molten metal infiltrates the arranged fibers and the entire structure is subjected to rolling contact with a moving heat-extracting surface disposed both to draw the structure from the meniscus and to extract heat from the molten metal matrix at a sufficiently high rate that the heat from the matrix does not detrimentally affect the properties of the resultant product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus employing the present invention with the molten metal meniscus in contact with a rotating cylindrical heat-extracting member. FIG. 2 is a schematic view of an appratus having a belt-like heat-extracting member.

FIG. 3 is a schematic view of an apparatus employing the present invention where the heat-extracting surface is separated from the molten metal meniscus.

FIG. 4 is a schematic view of an apparatus disposed to produce a composite member by solidifying an additional layer of composite material on the surface of a previously formed composite member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is directed to the embodiment of the present invention where the molten metal meniscus is in direct contact with the heat-extracting surface at the point where the fibers are introduced to the molten metal matrix. The form of the apparatus shown with the exception of the means to introduce the fibers is described in U.S. Pat. No. 3,605,863, Derek King. The apparatus as used in the present invention utilizes a supply of molten metal 1 fed through a nozzle or orifice 2 with the orifice and the rate of supply controlled to produce a meniscus 3 of molten metal protruding from the orifice 2. Directly adjacent to the orifice 2 and separated therefrom is a heat-extracting member 5 which has the capability of moving in relation to the orifice 2. As shown in FIG. 1, the heat-extracting surface is a rotating cylindrical drum having an axis of rotation perpendicular to the axis of the orifice 2.

A source 8 of individual fibers 10 is disposed to supply a plurality of fibers to the meniscus 3 with the source 8 having means 12 to adjust the position of the fibers 10 as they enter the meniscus. The source 8 may also include means of controlling the tension in the fiber supplied. Between the source of the fibers and the meniscus 3, there may be a means for arranging the fibers into the configuration desired to be introduced to the molten metal meniscus with the means hereinafter called a collimator 20. The collimator 20 may also have the capability of adjusting the position of the fibers 10 relative to the heat-extracting surface 6. The collimator 20 has the principle function of arranging the fibers 10 into a configuration that will yield the desired fiber spacing and density in the final product where the fiber bundle is composed of an array from a supply of individual fibers.

The present invention may also be utilized with fibers already in the desired density and spacing as for example an open woven or felt-like structure that would allow the matrix material to infiltrate the pores in the structure. With this type of fiber supplied to the meniscus, a collimator would not be required.

The process is best illustrated by example using a configuration of elements as shown in FIG. 1. In initiating the process, the heat-extracting member 5 would be disposed a distance from the meniscus 3 so as not to be in contact with the meniscus. A collimator 20 and the source 8 of a plurality of single fibers as well as the adjusting means 12 could be arranged to move with the heat-extracting member 5 thereby maintaining a constant geometric relationship between the fibers 10 and the heat-extracting surface 6. When the collimator 20 has been made to direct the fibers into the proper arrangement and density, and the position of the collimator 20 and the adjusting means 12 are correct in relation the the heat-extracting surface 6, then the fibers may be passed through the collimator and attached temporarily (as for instance with adhesive tape) to the stationary heat-extracting surface 6. The source of molten metal would then be made to deliver a stable meniscus 3 of the metal at the orifice 2. When the molten metal meniscus protrudes from the orifice 2, the heat-extracting member 5 would then be rotated and the surface 6 of the member 5 having the fibers 10 arranged thereon would be brought into contact with the meniscus 3. The heat-extracting surface 6 would solidify a portion of the molten metal meniscus 3 around the fibers 10 and move the resulting composite structure 11 out of contact with the meniscus. Upn the production of a sufficient length of the composite structure 11, the temporary attachment of the fibers 10 to the heat-extracting surface could be removed and the fiber 10 would be drawn from the fiber source 8 and arranging means 12 through the collimator 20 and into the meniscus 3 by the movement of the preceding fiber 10 now within the structure 11.

After the process has been initiated, then the collimator 20 and fiber adjusting means 12 can be further adjusted to present the fibers 10 to the meniscus at the optimum geometric relationship. The distance from the heat-extracting surface 6 to the orifice 2 may be changed to alter the shape or behavior of the meniscus 3. The temperature of the molten metal may be altered during the process to affect both the process and the properties of the final product.

Irrespective of the configuration of the heat-extracting surface, the present invention has the advantage over the prior art due to the very short exposure of the fibers to the heat and chemical effects of the molten metal. The fibers enter the meniscus in an arranged pattern either from the collimator 20 or as a structure of arranged fibers and are free to remain so arranged while the molten metal infiltrates the spaced fibers. The contact with the heat-extracting surface 6 promotes rapid solidification, and, therefore, the fibers maintain the desired properties substantially unaffected by the momentary encounter with the molten metal. Additionally, use of the heat-extracting surface 6 allows preparation of lower volume percent fiber end products than would be possible otherwise.

The process is inherently simple so as to lend itself to variations on the process. Some of these variations are inherent in the means to supply the molten metal to the meniscus such as the orifice size, wettability of the orifice, and the flow rate of the molten metal. The previously cited King patent uses simply the removal of material from the meniscus by solidification on the heat-extracting surface to control flow. While this means of controlling flow makes operation simple, the subject invention may also be used where the metal is induced to flow through the orifice by other means. The elevation of the molten metal supply in relation to the orifice or gas pressurization of the molten metal supply could be used to induce flow to the orifice.

The surface of the heat-extracting member must have sufficient adherence to the solidified product to draw the product, and hence the fiber, through the meniscus. Where the surface of a desirable heat-extracting member is not sufficiently adherent, then an additional drive roll 35 may be provided having its surface velocity equal that of the heat-extracting surface with the composite product disposed therebetween and driven by the combined action of the opposing surfaces. This additional roll may also serve to improve the surface finish of the side of the product otherwise solidifying in contact only with the surrounding atmosphere. The placement of such a roll can be selected to engage the product at any point where it is supported on its opposite side, and with the roll located close to the meniscus the temperature of the product may be elevated an amount to allow substantial reduction in cross section of the composite product if such a treatment is desired.

It it is desired that the composite structure not undergo any bending or deformation associated with production on a curved heat-extracting surface, then the heat-extracting member may be of the configuration shown in FIG. 2. The heat-extracting member is comprised of an outer belt-like member 50 rotating on at least two cylindrical members 51 and 52. The cylindrical member 51 may also act as a heat-extracting member if molten metal is directed onto 50 at a point on 50 that is close to 51. The molten metal supplying orifice 60 may direct the metal to any portion of the member 50; however, if the composite product 11 is to be formed and removed with minimal bending, then molten metal would be introduced to the surface of the member 50 at a point between the centers of members 51 and 52. The member 50 may be constructed of a material having sufficient thermal capacity to completely solidify the molten metal in contact with the member 50. The member 50 not having sufficient thermal capacity may be cooled by a spray of coolant 60 or equivalent means on the side opposite the heat-extracting surface 55 of the member 50. The collimator 20 and the fiber supply 8 with the adjusting means 12 could then provide the fiber 10 to molten metal meniscus 30 in the same line as the final form of the composite product 11.

A process of this type has the advantage of exposing the fiber to an environment of molten metal for a minimal time. Where the molten metal is known to chemically degrade the fiber, an appropriate coating on the fiber may be utilized with the performance of the coating less critical than that required of the prior art due to the nonabrasive exposure of the fiber to molten metal.

Where the solidification rate of the metal need not be accelerated by an immediately adjacent heat-extracting member, a fiber reinforced composite structure may be formed in the manner shown in FIG. 3. In this embodiment the fibers are arranged in the desired spacing and density and then passed through a protruding stable meniscus 30 of molten metal. The fibers are passed through the meniscus at a rate that will allow the molten metal to infiltrate the array of fibers 10. The composite product 11 is then drawn between two rolls 41 and 42 disposed both to drive the product 11 (and hence the fibers 10) as well as impart an improved surface finish to the product 11. In this embodiment, the most desirable configuration would have the fiber supply 8, the fiber adjusting means 12, the collimator 20 if required, and the two rolls 41 and 42 constructed to move as a unit, separate from the molten metal supply 1. In this way the process is easily initiated by passing the fibers through the collimator 20 (if required) to the rolls 41 and 42 without contacting the molten metal meniscus 30. The production of the structure 11 is begun by placing the moving fibers 10 in contact with the molten metal meniscus 30. The relative position of the fibers 10 within the meniscus 30 may vary by moving the aforementioned unit consisting of the drive rolls 41 and 42, the collimator 20, and the fiber adjusting means 12.

The present invention may also be used to produce thick sections of composite material by reintroducing a formed composite product to a molten meniscus and introducing additional arranged fibers to the meniscus. In this manner a thin composite product could be successively thickened by several additional layers of matrix material containing the arranged fibers.

FIG. 4 illustrates one possible method of carrying out the reintroduction of the composite member to the molten meniscus and forming an additional layer of composite material on one surface. An apparatus similar to that shown in FIG. 2 may be used with the previously formed composite member 11 introduced tangentially to the surface 55 of the belt-like heat-extracting member 50. A molten meniscus of matrix material 3 is introduced to the side of the initial composite 11 opposite the surface 55. In this embodiment a woven structure of arranged fibers 10' is introduced to the meniscus 3 to be infiltrated by the matrix material and therefore a collimator is not required since the woven structure has the fibers in the desired arrangement. The molten matrix material in the meniscus 3 infiltrates the fibers 10' and solidifies. With proper control of the temperature of the molten matrix material and the surface of the member 11 in contact with the meniscus 3, the new composite structure should bond to the member 11 forming a thicker composite member 11'. The process may be repeated until the member 11' reaches the desired thickness or until the thermal properties of the member 11 prevent effective heat removal from the meniscus 3 so as to affect solidification or the properties of the composite member so produced.

The above description of the preferred embodiments of the present invention does not confine the invention to those specific embodiments and those skilled in the art may make alterations, modifications, or combinations of those embodiments and remain within the scope of the specification and the appended claims.

Claims

We claim:

1. A method of producing a metal matrix fiber reinforced composite member comprising the steps of:

a. supplying a stable meniscus of molten metal protruding from an orifice;

b. moving a heat-extracting surface in contact with said meniscus;

c. introducing a plurality of prearranged fibers to said meniscus in a direction substantially tangent to said heat-extracting surface;

d. solidifying said molten metal containing said fibers into a metal matrix fiber reinforced composite member; and

e. removing said member from said heat-extracting surface.

2. The method of claim 1 where said heat-extracting member is cylindrical and possesses sufficient thermal capacity to solidify said molten metal.

3. The method of claim 2 where a cylindrical roll bears on the exposed surface of said composite member at a point prior to removal of said member from said heat-extracting surface.

4. The method of claim 1 where said fibers pass through a means for arranging said fibers into a preferred spacing prior to the introduction of said fibers to said molten metal meniscus.

5. The method of claim 4 where said fibers are supplied to said means for arranging said fibers from a supply of said fibers having a means to control the tension in said fibers supplied to said molten metal meniscus.

6. The method of claim 1 where said heat-extracting surface comprises an endless belt-like member rotating on two cylindrical rolls with said belt-like member disposed to solidify said molten metal without imparting curvature to said composite member.

7. The method of claim 6 where said belt-like member having insufficient intrinsic thermal capacity to solidify said molten metal is provided a means for cooling the surface of said belt-like member opposite the surface on which said molten metal is introduced.

8. The method of claim 6 where a cylindrical roll bears on the exposed surface of said composite member at a point prior to removal of said member from said heat-extracting surface.

9. The method of claim 6 where said fibers pass through a means for arranging said fibers into a preferred spacing prior to the introduction of said fibers to said molten metal meniscus.

10. The method of claim 6 where said fibers are supplied to said means for arranging said fibers from a supply of said fibers having a means to control the tension in said fibers supplied to said molten metal meniscus.

11. The method of claim 1 where said prearranged fibers are supplied to said meniscus in the form of a woven structure.

12. The method of claim 1 where said prearranged fibers are supplied to said meniscus in the form of a felt-like structure.

13. A method of producing a metal matrix fiber reinforced composite member comprising the steps of:

a. supplying a stable meniscus of molten metal protruding from an orifice;

b. introducing a plurality of prearranged fibers to said meniscus so as to completely infiltrate said fibers with molten metal so as to form a fully dense composite member; and

c. withdrawing said member from said meniscus by the action of two opposed drive rolls with the engagement of said rolls with said composite member being in substantially a straight line with said composite member and said prearranged fibers.

14. The method of claim 13 where said fibers are maintained at a predetermined tension between said drive rolls and the means of supplying said fibers.

15. The method of claim 13 where said prearranged fibers are supplied to said meniscus in the form of a woven structure.

16. The method of claim 13 where said prearranged fibers are supplied to said meniscus in the form of a felt-like structure.

17. A method of producing a metal matrix fiber reinforced composite member comprising the steps of:

a. supplying a stable meniscus of molten metal protruding from an orifice;

b. moving a solid member in relation to said meniscus having at least one surface of said member in contact with said meniscus;

c. providing a heat-extracting surface in contact with said member moving at substantially the same velocity as said member in relation to said meniscus;

d. introducing a plurality of prearranged fibers to said meniscus prior to the solidification of said molten metal; and

e. removing the resulting member from said heat-extracting surface.

18. The method of claim 17 where said solid member is a composite having the same matrix material as is supplied to said meniscus.