Variable-density composite articles, preforms and methods

Abstract

A metal matrix composite article that includes at least first and second regions, first and second reinforcement materials, a metal matrix composite material occupying the second region of the body and comprising a metal matrix material and the second reinforcement component, a preform positioned in the first region of the body and infiltrated by at least the metal matrix material of the metal matrix composite material. The article further includes a transition region located proximate an outer surface of the preform that includes a distribution of the second reinforcement component comprising a density increasing according to a second gradient in a direction toward the outer surface of the preform.

Claims

1. A metal matrix composite article, comprising: a cast, reinforced body, the body comprising a first region and a second region, the first region having more reinforcement than the second region; a first reinforcement component; a second reinforcement component; a metal matrix composite material occupying the second region of the body and comprising a metal matrix material and the second reinforcement component; a preform positioned in the first region of the body and infiltrated by at least the metal matrix material of the metal matrix composite material, the preform comprising a first end, a second end, an outer surface, the first reinforcement component, the first reinforcement component comprising a density increasing between the first end of the preform and the second end of the preform according to a first gradient, and a porous structure configured to allow passage of the metal matrix material into the preform and to block or reduce passage of the second reinforcement component into the preform; and a transition region extending between the first and second region of the body and located proximate the outer surface of the preform, the transition region comprising a distribution of the second reinforcement component adjacent to the outer surface of the preform, the distribution of the second reinforcement component comprising a density increasing according to a second gradient in a direction toward the outer surface of the preform.

2. The metal matrix composite article of claim 1, wherein the metal matrix material comprises a metal or a metal alloy and wherein the second reinforcement component comprises a ceramic particle component and/or a ceramic fiber component.

3. The metal matrix composite article of claim 2, wherein the metal matrix material comprises aluminum, magnesium, or an alloy thereof.

4. The metal matrix composite article of claim 1, wherein the first reinforcement component comprises a ceramic particle component and/or a ceramic fiber component.

5. The metal matrix composite article of claim 1, wherein the transition region comprises a thickness corresponding to an amount of the metal matrix material infiltrated into the preform.

6. The metal matrix composite article of claim 1, wherein the distribution of the second reinforcement component at the outer surface of the preform comprises a first volume fraction that matches a volume fraction of the first reinforcement component at the outer surface of the preform.

7. The metal matrix composite article of claim 6, wherein the metal matrix composite material comprises the second reinforcement component at a minimum volume fraction, and wherein the volume fraction of the distribution of the second reinforcement component decreases linearly or substantially linearly from the first volume fraction to the minimum volume fraction.

8. The metal matrix composite article of claim 1, wherein the article is a cast brake rotor.

9. The metal matrix composite article of claim 8, wherein the first region comprises a friction face and the second region comprises a hub.

10. The metal matrix composite article of claim 8, further comprising a second preform positioned in the first region of the body adjacent to the first preform, the second preform infiltrated by at least the metal matrix material of the metal matrix composite material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following drawings illustrate some particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Some embodiments will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements.

(2) FIGS. 1A-1D depict a flow diagram illustrating a method for making a cast article, using a cavity and a preform, according to an embodiment of the invention.

(3) FIG. 2A is a partial cross-sectional image of a cast article depicting a material interface and a functional reinforcement gradient according to an embodiment of the invention.

(4) FIG. 2B is a magnified view of a portion of FIG. 2A.

(5) FIGS. 2C and 2D are magnified views of portions of FIG. 2B, illustrating portions of the functional reinforcement gradient.

(6) FIG. 2E is a magnified view of a portion of FIG. 2A.

(7) FIGS. 2F-2H are magnified views of portions of FIG. 2E, illustrating different material densities within the cast article of FIG. 2A according to an embodiment.

(8) FIG. 3 is a perspective view of a brake assembly according to an embodiment.

(9) FIG. 4A is a perspective view of a brake rotor according to an embodiment.

(10) FIG. 4B is a partial, sectional illustration of a brake rotor according to an embodiment.

(11) FIG. 4C is a partial, sectional illustration of a brake rotor illustrating changes in ceramic density according to an embodiment.

(12) FIGS. 5A-5F illustrate a method of making a metal matrix composite brake rotor according to an embodiment.

(13) FIGS. 6A-6D illustrate a method of making a metal matrix composite brake drum according to an embodiment.

(14) FIG. 7A is a collection of consecutive partial, cross-sectional images of a cast article incorporating a novel preform according to an embodiment.

(15) FIGS. 7B-7D are magnified views of portions of the images of FIG. 7A, illustrating different material densities within the cast article of FIG. 7A according to an embodiment.

(16) FIG. 8 is a partial, perspective view of a variable-density preform according to an embodiment.

(17) FIGS. 9A-9C illustrate a deposition method for making a variable-density preform according to an embodiment.

(18) FIG. 10 illustrates a spray-deposition method for making a variable-density preform according to an embodiment.

(19) FIG. 11A is a perspective view of a working surface for making a variable-density preform according to an embodiment.

(20) FIG. 11B is a perspective, cross-sectional view of the working surface in FIG. 11A according to an embodiment.

(21) FIG. 12 is a perspective view of a press step for a method for making a variable-density preform according to an embodiment.

DETAILED DESCRIPTION

(22) The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides some practical illustrations for implementing some embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of ordinary skill in the field of the invention. Those skilled in the art will recognize that many of the noted examples have a variety of suitable alternatives.

(23) Embodiments described herein are generally related and applicable to casting processes, including metal casting. Many examples described herein are related to or in various ways address the properties of metal matrix composites, including articles, components, and parts cast with a metal matrix composite (MMC). Examples are also related to methods for reinforcing such cast articles, and to particular structures used for reinforcement. For example, some embodiments discuss the use of casting preforms that may be placed in a mold cavity and infiltrated with a molten metal matrix composite to form an MMC article that is reinforced at least in part by the preform.

(24) Also, some embodiments discuss the formation of additional reinforcement members having tailored properties and structures. Some examples include the formation of distribution(s) of a reinforcement component within a casting preform, such as a reinforcement distribution that increases in density according to a particular gradient. Additional examples include the formation of distributions of reinforcement components within a portion of the MMC article next to, but not within, a preform. As will be discussed, in some embodiments such a distribution of a reinforcement component may form as a metal matrix composite infiltrates a preform.

(25) As will be appreciated as one reads further on, the disclosure provides a number of practical examples for embodying the concepts and features taught herein. One example is a lightweight metal matrix composite brake rotor. Another example is a MMC brake drum. As will be discussed, the novel structures within, and methods of forming, such components provided by embodiments of the invention can offer significant benefits over conventional cast iron brakes. For example, some embodiments provide the ability to tailor the structure and the properties of these types of components to meet certain variable and often severe thermal and mechanical loadings. In addition to the thermal and mechanical attributes, the MMC technology disclosed herein can also be employed in a cost-effective method for producing brake rotors. Of course, it should be appreciated that the embodiments described herein are examples of different products, articles, systems, and/or methods, and are not meant to limit the scope of possible embodiments or their application.

(26) Turning to FIGS. 1A-1D, a method 100 for making a cast article 102 is illustrated according to an embodiment of the invention. Referring to FIG. 1A, a die cavity 104 is schematically shown with a porous casting preform 106 inserted within the cavity 104 at one end. FIG. 1A also shows the remaining portion of the cavity 104 filled with a molten metal matrix composite material 108, in this example containing an aluminum effluent (Al) carrying a ceramic reinforcement component (C). Initial steps in the exemplary method include providing the porous preform 106, positioning the preform 106 within the die cavity 104, and introducing the molten metal matrix composite material 108 into the die cavity 104 about the preform 106.

(27) In general terms, the porous preform 106 in this example is formed with a porous structure made of a reinforcing material, such as a ceramic particles and/or ceramic fibers (e.g., continuous and/or discontinuous). Referring to FIG. 1A, the preform a first end 110, a second end 112, an outer surface generally indicated at 114 in this cross-sectional view. The reinforcing material, which is also referred to as a reinforcement component, is illustrated as a distribution of reinforcement particles 116, though it should be appreciated that fibers and/or other types of reinforcement components may also be used to make the preform 106.

(28) According to some embodiments, the reinforcement component 116 has a density that increases between the first end 110 of the preform and the second end 112 of the preform according to a first gradient. FIGS. 1A-1D include simplified schematic representations of the preform 106, and thus do not depict a visible gradient of the reinforcement component's density, but it should be appreciated that the reinforcement component 116 is distributed throughout the preform 106 according to such a gradient in many (though not necessarily all) embodiments.

(29) According to some embodiments, the preform 106 and/or one or more additional preforms placed within a cast component, can be manufactured with a reinforcement distribution having a gradient according to the teachings of Applicant's co-owned U.S. Pat. No. 8,075,827 B2, titled Variable-Density Preforms, and issued Dec. 13, 2011, the content of which is hereby incorporated by reference in its entirety. As discussed in the '827 patent, Applicant often refers to such material gradients as Functional Reinforcement Gradients (FRG). Briefly referring to the teachings of the '827 patent, in some embodiments a Functional Reinforcement Gradient (e.g., gradient distribution of a reinforcement component) can be formed by establishing a flow of a ceramic slurry into a mold and then filtering the slurry so as to extract and remove the liquid, thereby creating (or leaving behind) a functional gradient of the ceramic reinforcement media contained in the slurry. In some embodiments, such processes can yield an FRG with a maximum reinforcement density of approximately 30% to 45% by volume on a friction surface of a preform.

(30) As discussed above, currently available technology for reinforcing castings with preforms can provide a highly reinforced section of a casting that includes the reinforcing preform. In infiltration casting, a metal or metal alloy is typically introduced into the die cavity to infiltrate the preform and also to fill other portions of the cavity to form unreinforced portions of the casting. As noted above, such state of the art examples of infiltration casting may provide a more integrated, infiltrated preform with a greater degree of contact between reinforcement and matrix materials. Even so, the materials still exhibit abrupt and problematic differential coefficients of thermal expansion (CTE) between the matrix material and reinforcement member. Such abrupt transitions in CTE at the matrix-reinforcement interface boundaries can give rise to residual stress during the forming process (e.g., residual stress-concentration), and also manifest in stress fractures during thermal cycling of the reinforced components during service.

(31) According to some embodiments of Applicant's invention, MMC components and articles are provided with one or more variable-density reinforcement components that provide improved thermal and mechanical performance when compared with previous reinforcement techniques. As just one example, in some cases Applicant's reinforcing techniques can provide cast article in which the volume fraction of reinforcement can range from a very high reinforcement level (e.g., 35-45% vol. on the frictional surface discussed above) to a very low or nearly 0% reinforcement level in other areas of the casting. Providing a casting with a wider range of changing reinforcement densities along a wider or longer dimension of a casting, and especially in the vicinity of material interfaces, can thus reduce, limit and/or minimize the CTE differences between materials, thus also reducing the residual stress and stress fractures due to thermal cycling.

(32) As will be discussed in greater detail further herein, applications of embodiments of the invention are well suited for, but not limited to, automotive and airplane applications, including rotor brake systems for heavy trucks and/or trailers. Embodiments disclosed herein provide one or more advantages or features over past practices. For example, two-piece rotors have been used in industry (e.g., Brembo, Performance Friction) to date because it is extremely difficult to maintain rotor flatness during a transient braking event. As noted above, the instant application discloses embodiments that overcome this and/or other deficiencies. In one example, the use of a functional reinforced gradient can assist in overcoming these types of problems.

(33) Returning to FIGS. 1A-1D, an improved functional reinforcement gradient can be built up while the metal matrix composite material 108 is infiltrating the preform 106. In this case the molten metal matrix composite material 108 includes at least a liquid component or effluent 120, which in this example is illustrated as aluminum (Al). The MMC material 108 also includes a reinforcement component 122 carried by the liquid component 120. As shown in FIGS. 1B and 1C, the method 100 includes pressurizing 124 the molten MMC material 108 (e.g., through squeeze casting). As the material 108 is pressurized, the liquid component 120 infiltrates into the porous preform 106, while passage of the reinforcement component 122 is limited and/or blocked by the structure of the preform 106. As more of the liquid component 120 infiltrates into the preform 106, the preform continues to filter out the reinforcement component 122 in the molten MMC, thus creating a buildup of the reinforcement component 122 starting at the outer surface 114 of the preform (i.e., the interface between the preform 106 and the MMC material 104). The buildup forms a distribution of the reinforcement component that extends away from the material-preform interface 130. According to the operation of this embodiment, the distribution of the reinforcement component 122 forms with a density that increases according to a gradient 132 in the direction toward the interface 130 and the outer surface of the preform. For example, in some cases the density of the reinforcement material 122 may increase at a substantially linear or linear rate as it approaches the material interface 130.

(34) Continuing with reference to FIGS. 1A-1D, in one non-limiting exemplary embodiment, the effluent 120 is a light metal alloy (e.g., aluminum, magnesium, or silicon) that contains a percentage of ceramic reinforcement component 122. As shown in this example, the MMC material is formed from a mixture of 90% volume aluminum and 10% volume ceramic reinforcement (e.g., short fibers and/or particulate). Such a material composition may be provided by the commercially available material Duralcan, which contains 10% SiC particulate. In accordance with the embodiment, the preform 106 is then used to filter or hold back/retain the particles as the effluent passes through. This creates an accumulation of the particulate as it begins to dam against the outer surface 114 of the preform 106. According to the illustrated embodiment, the porous preform 106 has an average of approximately 40% ceramic reinforcement and approximately 60% void that is infiltrated by the effluent.

(35) According to one non-limiting embodiment, the preform 106 thickness may be designed for use as the friction surface of a brake rotor. In one example, the preform thickness is set at approximately 0.234 thick (approximately 6 mm) for a square inch. This is approximately 0.234 cubic inches of volume, of which approximately 0.1404 cubic inches (approximately 60%) is void (0.1404 cubic inches=0.60*0.234 cubic inches). In this example, the volume fraction of reinforcement 122 at the interface 130 will build to equal the preform reinforcement volume fraction of approximately 40% and decrease linearly away from the interface 130 as the alloy 120 is pressed from the stir cast MMC material 108 into the preform 106. The volume fraction of reinforcement 122 then decreases until it is approximately the same as reinforcement level of the original MMC stir cast volume fraction (i.e., approximately 10% in this example). The thickness of the transition zone at the interface can then be determined by a slope determined according to the volume of alloy lost to the preform and the velocity of the shot.

(36) According to some embodiments, the process by which the Functional Reinforcement Gradient 132 forms has some similarities with Applicant's novel process for forming Functional Reinforcement Gradients within variable-density preforms, as taught in Applicant's co-owned U.S. Pat. No. 8,075,827 B2. As in the example of casting a preform, embodiments of the instant invention employ a filtering mechanism provided by a configured, porous media, which in this case is a preform disposed in the die cavity. Accordingly, instead of requiring additional or external filtering materials to build the functional reinforcement gradients within a cast article, embodiments disclosed herein advantageously incorporate a filtering mechanism designed into the reinforcement preform that will already be used for reinforcing a portion of the cast article.

(37) Turning now to FIGS. 2A-2H, several partial images of an exemplary MMC cast article 200 that includes a body formed from a MMC material 202 and a preform 204 at least partially infiltrated by the MMC material 202. FIG. 2A is a partial cross-sectional image of the cast article 200 depicting a material interface 206 located between the preform 202 and the MMC material 204. According to some embodiments, the cast article 200 can be considered as having multiple material regions of different compositions. For example, referring to FIGS. 2A and 2B, the article body includes a first region 210 generally located in the vicinity of the infiltrated preform. The article also includes a second region 212 generally located opposite the interface 206 from the preform 204 in an area of the article formed from the MMC material 202. Accordingly, the first region 210 (e.g., by the preform) typically has more reinforcement than the second region 212, which does not have the additional reinforcement from the preform 204.

(38) Both the metal matrix composite material 202 and the preform material 204 include reinforcement components, which may be the same or different depending upon the particular embodiment. In this example, the MMC material 202 includes an AlSi metal alloy that carries an 8% volume SiC reinforcement component. The preform 204 is formed from an approximately 40% volume aluminum hybrid composite, thus also containing a reinforcement component. The preform 204 is configured to allow passage of the AlSi metal alloy into the preform and to block or reduce passage of the SiC reinforcement component. During casting, as the SiC reinforcement component was blocked, a distribution of the SiC reinforcement component built up adjacent to the outer surface of the preform, with the density of the distribution increasing according to a gradient toward the material interface 206. As shown in the magnified view of FIG. 2B, the functional gradient of the SiC reinforcement component increases through a transition region 220 of the material until it reaches the material interface 206.

(39) FIGS. 2C and 2D are magnified views of portions of FIG. 2B, illustrating portions of the functional reinforcement gradient within the transition region 220. As shown, the SiC reinforcement component has built up to a density of about 22% volume at the 0 mark, which is close to the material interface 206. The density of the SiC steadily decreases further away from the material interface 206 according to the functional gradient until the density of the reinforcement component reaches an equilibrium with the level of SiC (e.g., 8% volume) within the original MMC material 202.

(40) FIG. 2E is a magnified view of a portion of FIG. 2A, showing a closer view of the material interface 206 and identifying the locations of the magnified views in corresponding FIGS. 2F, 2G, and 2H. FIG. 2F provides a closer view of the original composition of the MMC material, which includes the AlSi alloy carrier component and the 8% volume SiC reinforcement component. As can be seen in FIG. 2G, the density of the SiC reinforcement component is much higher at points nearer to the material interface 206.

(41) FIG. 3 is a perspective view of a brake assembly 300 according to an embodiment of the invention. As discussed above, embodiments of the invention can be useful in automotive applications, and particularly suited to automotive, heavy truck, and/or trailer brake rotor systems. As noted above, embodiments of Applicant's invention can be useful for limiting and/or overcoming problems associated with different coefficients of thermal expansion (CTE) in adjacent materials.

(42) As is generally known in the art, when a brake is used for slowing and/or stopping a moving object, the motion of the object gets transformed into heat and transferred to the brake disc in order to stop the moving vehicle. Upon application of the brake actuator the disk is clamped between the brake pads and rotational drag is created. This clamping causes energy transformation to take place and heat is generated. Under certain operating conditions, the heat energy does not get transferred to the disk uniformly over the whole surface due to the differences in tangential (or linear) velocities along the radius of the brake pad interface. The tangential speed of the brake rotor at any radial distance, r, from the rotor center is defined as the product of 2r and the rotational speed (i.e., ((2r)(RPM)). As will be apparent, a higher velocity found at the outer positions corresponds to a higher thermal energy state (i.e. higher temperatures). Therefore, the rotor thermal load carrying abilities at the outer locations needs to handle the higher heat energy generated there.

(43) In accordance with an embodiment, higher heat capacity/handling can be accomplished through placement of a higher concentration of ultra-high temperature ceramic material at specific locations on the rotor. In some embodiments, the coefficient of thermal expansion (CTE) is tailored with the changing of the ceramic component in the MMC content such that the rotor will expand and contract while remaining in-plane. Control of expansion and contraction is not possible if a monolithic material such as cast iron is used in a brake rotor. This is one of the reasons why brake jitter is noticed in a monolithic cast iron automotive braking system.

(44) Accordingly, some embodiments disclosed herein provide the ability to control the expansion and contraction of the rotor during braking (e.g., due to heating and cooling), enabling embodiments of the rotor(s) of the invention disclosed in the instant disclosure to be hard mounted to the hub. While other material options may contain better material properties in terms of maximum operation temperatures, the ability to tailor (or customize) the FRG material properties as taught herein allows for effective mounting of the friction surfaces directly to the hub section of the rotor and more effective thermal management to avoid brake fade (i.e., reduced breaking action).

(45) In accordance with some embodiments of the invention, the volume fraction of ceramic in the brake rotor changes across the braking surface extending between the inside and the outside diameters of the rotor. For instance, in a non-limiting exemplary embodiment, a location or section proximate the outermost extent (for example proximate the circumference) of the brake disc can include approximately 40% to 45% by volume of ceramic reinforcement and approximately 55% to 60% by volume of one or more light alloy; whereas at some location other than the outermost extent, e.g., proximate the hub or at a section or location between the hub and the circumference, the brake disc can include approximately 30% to 35% by volume of ceramic reinforcement and approximately 65% to 70% by volume of one or more light alloy. A method for making such changes in volume fraction is taught by Applicant's granted patent, U.S. Pat. No. 8,075,827 B2, titled Variable-Density Preforms issued Dec. 13, 2011.

(46) In some cases distinct zones and interfaces between zones are provided. For example, certain embodiments of brake rotors include at least three functional zones, viz., a) friction interface (heating zone), b) venting (cooling zone) and c) mounting hub (torque transfer zone). In some cases these zones should or must have specific material attributes for the rotor, as a whole, to function properly. They also must have the proper interfaces between the zones. High ceramic on the rotor's breaking surface, for example proximate the outermost extent, must be graded to a lower ceramic content in the radial direction extending between the circumference and the hub, and through the thickness toward the internal venting.

(47) In some cases an exemplary design according to some embodiments involves the use of two preforms to cast an article such as a blade or brake rotor. In some cases the two preforms are functionally graded in the radial direction. In some embodiments, the blades are infiltrated during the casting process that creates all of the rotor geometry such as, but not limited to, the hub, venting, and blade section, etc.

(48) In certain embodiments, the effluent (or slurry) can be, but is not limited to, an aluminum or magnesium alloy containing ceramic particles and/or fibers.

(49) In some embodiments, the at least one exit end or orifice can be, but is not limited to, a functionally graded porous preform.

(50) In certain embodiments, a secondary gradient can be developed at the metal-preform interface of a cast article.

(51) In some embodiments, the functional gradient in the interface regions can serve, but is not limited to, one or more of the following: (1) to provide a continually changing ceramic structure so as to minimize the stress riser at the interface from the mechanical loading; and/or (2) to grade the differences in the CTE at the interface to reduce the thermal stresses developed at the interface. In a braking event, the thermal load is just as important if not more important than that of the mechanical load.

(52) In some embodiments of the invention, the thickness of the FRG can be altered by changing one or more of the density or the thickness or the composition of the preform in the die cavity. In certain embodiments, the preform and the incoming ceramic carrying aluminum can be tailored such that a predetermined FRG is achieved at the one or more interfaces. In some embodiments, the fraction of the fiber and/or particle combination can be altered to increase or decrease the thickness of the transition region.

(53) Accordingly, in some embodiments, the preform can contain ceramic particles or continuous ceramic fibers or discontinuous ceramic fibers or any combination thereof in an amount ranging between approximately 5% to approximately 70% by volume. In certain embodiments, the incoming alloy or slurry can include ceramic particles and/or ceramic fibers in an amount ranging between approximately 5% to 40% by volume. In some embodiments, vibration can be induced during the molding process to initiate and/or enhance the dispersion of the ceramic for providing a predetermined gradient.

(54) As is well known in the art, different materials have different coefficients of thermal expansion (CTE) and therefore each expands at a different rate when heated. Accordingly, if different material having different CTE are attached to one another, the thermal stress at the interface and in the vicinity thereof can be substantially, and in some cases significantly, high when heated. Accordingly, providing or creating an FRG can reduce, minimize, and/or eliminate such thermal stresses as may arise due to the differences (or mismatch) in the CTE of the material used in the rotor.

(55) Thermal stress in a constrained material is defined by =ET where is thermal stress, is the material's coefficient of thermal expansion (CTE), E is the material's Young's modulus and T is the change in temperature. The CTE () of unreinforced aluminum is approximately 22.9 m/m- C. (in the temperature range of approximately 20 C. to approximately 300 C.); whereas for a 40% SiC particulate reinforced aluminum, the CTE () is approximately 11 m/m- C. Accordingly, at approximately 300 C., the stress in the aluminum and at the interface between the aluminum and the MMC, respectively, would be
.sub.Al-MMC=((22.911 m/m C.)*124E9 N/m2*300 C.)=443.39 MPa
(64.26 ksi).sub.Al=((22.911 m/m C.)*69E9 N/m2*300 C.)=246.33 MPa (36 ksi)

(56) This indicates that failure would occur in the MMC as the stress in each material is at the yield point of many alloys at room temperature and exceeds the yield of almost all reinforced and unreinforced aluminum alloys at 300 C. This is why a functional gradient can be desirable at this interface in some cases. If an incremental change in volume fraction is implemented, the thermal stress can be effectively managed.

(57) Taking the same temperature change as above but with an incremental CTE difference (directly related to volume fraction of ceramic in the MMC by rule of mixtures), the stress at the interface is computed as:
.sub.Al=((22.920 m/m C.)*69E9 N/m2*300 C.)=60 MPa (8.7 ksi)
.sub.Al-MMC=((22.920 m/m C.)*124E9 N/m2*300 C.)=108 MPa (15.7 ksi)

(58) The resultant thermal stress is more manageable in both materials and hence will be able to handle the repeated loadings it will experience during braking events.

(59) Accordingly, in an embodiment having the interface modification in accordance with an embodiment of the invention, the high volume fraction reinforcement can be graded to a low or no reinforcement smoothly.

(60) Machining attributesSiC reinforced alloys are known to be very difficult to drill and tap. The ability of putting high levels of reinforcement only on the braking surface helps reduce machining time and cost. Historically, the rotors that have been in production in automotive applications have been fully reinforced all the way to the hub.

(61) In some embodiments, the friction face may contain up to approximately 45% ceramic while the hub of the rotor is >approximately 5% ceramic.

(62) FIG. 4A is a perspective view of a brake rotor 400 according to an embodiment. As shown in the figure, the rotor 400 includes an outer diameter 402, an inner diameter 404, a hub portion 406, and at least one friction face 408. FIG. 4B is a partial, sectional illustration of a somewhat similar brake rotor 420 according to an embodiment of the invention. In this example, the brake rotor 420 includes two disc-shaped preforms 422, 424, which form the friction faces of the rotor. The figure also illustrates the rotor's hub 428 and venting features 426. FIG. 4C is a partial, sectional illustration of a somewhat similar brake rotor 450 illustrating changes in ceramic density according to an embodiment. As shown in the illustration, in this example the brake rotor 450 is formed with a preform 452 in a first reinforcement region 454. The preform 452 is formed with a first functional reinforcement gradient 456 that decreases in density at points moving away from a first end 460 of the preform. At a second end of the preform, the rotor 450 includes a transition region 470, which begins at a macro interface between the preform region of the rotor and a less reinforced region 472 of the rotor connecting the first region 454 with a hub region 474. As explained above with respect to other embodiments, the transition region 470 includes a second functional reinforcement gradient that decreases in density of reinforcement material as it moves away from the interface with the preform region. This smoothing of the changes in reinforcement between regions of the rotor can significantly enhance both thermal and mechanical wear properties of the rotor as discussed above.

(63) FIGS. 5A-5F illustrate a method of making a metal matrix composite brake rotor 500 according to an embodiment. As shown in the figures, two preforms 502 are press fit within a rotor die 504 as an initial step. In some cases, the preforms 502 may include multiple material layers, including, for example, SiC 506 and an alumina fiber layer 508. A metal matrix composite material 510 is injected into the die 504 and pressurized with a piston 512 until the material infiltrates the preforms 502, thus forming functional reinforcement gradients 520 extending from the outside surface of the preforms in a manner discussed above. As shown in this case, a multi-dimensional functional reinforcement gradient scheme can thus be achieved.

(64) FIGS. 6A-6D illustrate a method of making a metal matrix composite brake drum 600 according to an embodiment similar in some respects to the example shown in FIGS. 5A-5F. FIGS. 6B and 6C illustrate the preform 602 press fit around the die 604 in a cross-sectional view.

(65) FIG. 7A is a collection of consecutive partial, cross-sectional images of a cast article 700 incorporating a novel preform 702 built according to a preform spray/deposition method according to an embodiment. FIGS. 7B-7D are magnified views of portions of the images of FIG. 7A, illustrating different material densities within the cast article of FIG. 7A according to an embodiment.

(66) In accordance with an embodiment of the invention, a similar resultant FRG MMC structure can be obtained by a spray application of the preform. A non-limiting exemplary embodiment includes a process by which a high concentration of particles is used in a preform slurry and sprayed thru a nozzle (e.g., a venturi nozzle) onto a heated surface or plate on which vacuum is applied (to drive off the excess water/effluent). In some embodiments of the invention, the slurry composition can then be altered after the desired build up of ceramic structure to a higher fiber containing mix to create a less dense ceramic structure. In certain embodiments of the invention, such build up of an FRG can provide a structure similar to that previously described without the need to cast with a ceramic containing alloy. In accordance with an embodiment of the invention, the preform can be dried and fired, as previously described, to dry out moisture and/or burn out organic binders and/or sinter the inorganic binders to yield a preform ready for casting. In accordance with an embodiment of the invention, the structure can then be cast using a pressure infiltration method (e.g., squeeze casting). The figure below illustrates a non-limiting exemplary embodiment of a structure within a casting manufactured in accordance with an embodiment of the invention.

(67) In some embodiments, the preform mix progresses from all fibers to a combination of low fibers and high particles to all particles. The light area, in an embodiment of the invention, is the alloy that has infiltrated the mix.

(68) In some embodiments, the preform mix progresses from all fibers to a combination of high fibers and low particles to a combination of low fibers and high particles to all particles.

(69) In certain embodiments, the disclosed spraying process of the instant invention could be used to spray an existing preform (e.g., a preform having high volume of SiC particles) with a fiber-particle layer and then a fiber layer prior to squeeze casting so as to minimize stress concentrations that may arise due to differences in, and not limited to, the CTE and stiffness at one or more interfaces of mating materials.

(70) In some cases a process by which a high concentration of particles is used in a preform slurry and sprayed or deposited thru a venturi nozzle or otherwise deposited onto a heated or unheated surface or plate on which vacuum and/or a centrifugal force is applied (to drive off the excess water/effluent). The slurry composition can then be altered after the desired build up of ceramic structure to a higher or lower fiber ceramic containing density mix to create a less/or more more or less dense ceramic structure. This build up of an FRG obtains a similar structure to that above without the need to cast with a ceramic containing alloy. This preform is dried and fired in the same common practice as before spelled out in authors' patent, but the drying process may also be used in between compositional layer changes as well. To This drying process driesy the final moisture out, burns out the organic binders and to sinters the inorganic binders to obtain a preform ready for casting.

(71) This structure can then be cast using a pressure infiltration method (i.e. squeeze casting). The final structure of the casting is seen below. The preform mix moves from all fiber to low fiber-high particle to all particle mix. The light area is the alloy that has infiltrated the mix. According to some embodiments, the spray or deposition method can be carried out according to the teachings of Applicant's co-owned U.S. Patent Application Publication No. US 2013/0169901, published Oct. 17, 2013, and filed Mar. 15, 2013, the content of which is hereby incorporated herein by reference in its entirety.

(72) FIG. 8 is a partial, perspective view of a variable-density preform 800 according to an embodiment. FIGS. 9A-9C illustrate a deposition method for making a variable-density preform according to an embodiment. FIG. 10 illustrates a spray-deposition method for making a variable-density preform according to an embodiment.

(73) FIG. 11A is a perspective view of a working surface for making a variable-density preform according to an embodiment.

(74) FIG. 11B is a perspective, cross-sectional view of the working surface in FIG. 11A according to an embodiment.

(75) FIG. 12 is a perspective view of a press step for a method for making a variable-density preform according to an embodiment. This would be a hydraulic press, the preform would be put on the bottom platen and the top platen would move down and squeeze the preform to shape, squeezing some of the moisture out of the preform. This provides a very even gradient. The preform can be cut to shape after it has been pressed, in the case of a rotor.

(76) Thus, embodiments of the invention are disclosed. Although the present invention has been described in considerable detail with reference to certain disclosed embodiments, the disclosed embodiments are presented for purposes of illustration and not limitation and other embodiments of the invention are possible. One skilled in the art will appreciate that various changes, adaptations, and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.