DESCRIPTION

TECHNICAL FIELD

The present invention relates generally to a method of forming a metal matrix composite component, and particularly but not exclusively to forming a multi-phase metal matrix component.

BACKGROUND ART

Metal matrix composites (MMCs) are composed of a metal matrix and a reinforcement, or filler material, which confers excellent mechanical performance, and can be classified according to whether the reinforcement is continuous (monofilament or multifilament) or discontinuous (particle, whisker, short fibre or other). The principal matrix materials for MMCs are aluminium and its alloys. To a lesser extent, magnesium and titanium are also used, and for several specialised applications a copper, zinc or lead matrix may be employed. MMCs with discontinuous reinforcements are usually less expensive to produce than continuous fibre reinforced MMCs, although this benefit is normally offset by their inferior mechanical properties. Consequently, continuous fibre reinforced MMCs are generally accepted as offering the ultimate in terms of mechanical properties and commercial potential.

A basic process for casting fibre reinforced metals is described in U.K. patent specification GB 2115327. As a licensee under the patent, the present applicant developed the basic process into a full scale liquid pressure forming (LPF) process. In the LPF process, a pre-heated preform (fibres, short fibres, porous media or particulate) is placed in a heated die, which is closed and locked using a mechanical toggle system. The die and molten metal in a crucible housed in a pressure vessel are then subjected to a high vacuum. When the evacuation is complete, molten metal is transferred from the crucible into the die through a sprue fed by a riser tube by the introduction of nitrogen gas into the pressure vessel. The molten metal takes up the shape of the die, which can be complex, and largely infiltrates the preform. Once the die is filled with molten metal, a hydraulic compaction piston is used to seal the top of the riser tube and further consolidate the casting to encourage maximum infiltration of the preform and to consolidate the shrinking matrix during metal solidification. The resulting composite is then ejected from the die.

According to leading authorities in the field of materials’ science, the LPF process is one of the most efficient and cost-effective methods of manufacturing MMCs, and represents a significance technological advance in the commercialisation of these composite materials. In particular, achieving total cycle times in the range 2 to 5 minutes is one of many significant advantages over other fabrication routes for MMCs. Nevertheless, the present applicant has proposed an advanced liquid pressure forming (ALPF) process, as described in WO 2005/097377 (the entire contents of which are herein incorporated by way of reference), which provides various improvements to the LPF, including a way of increasing the degree of metal infiltration into the perform by applying pressure direct to the molten metal in the die cavity during solidification, and a way of decreasing cycle times using a duplex die.

US 9,186,723 discloses a method of producing a metal matrix composite made by placing a porous ceramic preform into a mould cavity and infiltrating with aluminium. The method uses a low volume fraction compressible ceramic fibre paper on the top and bottom layers of the preform to ensure a consistent uniform layer of encapsulating aluminium and help centre the preform within the mould cavity prior to metal infiltration.

The present applicant is seeking to develop a novel metal matrix composite component. Of particular interest are multi-phase metal matrix composite components. STATEMENT OF INVENTION

In accordance with a first aspect of the present invention, there is provided a method of forming a metal matrix composite component, comprising: providing a body defining a mould cavity; covering a first surface of the mould cavity with a first reinforcement material; restraining the first reinforcement material relative to the body to restrict movement of the first reinforcement material in the mould cavity; adding a second reinforcement material to the mould cavity, the second reinforcement material being in contact with the first reinforcement material; adding molten metal to the mould cavity such that the first reinforcement material and the second reinforcement material become embedded in a continuous metal matrix, and solidifying the molten metal in the mould cavity.

The resulting metal matrix composite component is a multi-phase component in the sense that it has at least two distinct regions: a first region or“skin” adjacent one external surface; and a second region or“core” spaced from said one external surface by the first region. The first region is reinforced by the first reinforcement material, and the second region is reinforced by the second reinforcement material. Despite the two distinct regions, a single metal matrix infiltrates and extends throughout both regions, providing a seamless transition from one region to the other without a boundary interface therebetween with an inherent weakness, caused for example by an oxide layer which may be found in a conventional physical joint. By restraining the first reinforcement material relative to the body, ensures the first region is uniform and for example avoids localized pockets which are aluminium rich compared with their surroundings.

The first reinforcement material may be continuous (e.g. may comprise monofilaments or multi-filaments) and may have a planar profile. The first reinforcement material may comprise inter-engaging fibres which are entangled, entwined or woven into a sheet-like structure. Alternatively, the first reinforcement material may comprise at least one elongate member, such as a filament or a braid, wrapped around a mandrel or a former such as an open frame to form a raft-like structure, with parts of the at least one elongate member aligned in a single orientation (i.e. unidirectional) and arranged side-by-side. The planar profile of the first reinforcement material may have a thickness of at least lmm, perhaps at least l .5mm, and possibly even at least 2mm.

Restraining the first reinforcement material relative to the body may comprise bonding the first reinforcement material to the body, for example with an adhesive. Alternatively or additionally, restraining the first reinforcement material relative to the body may comprise clamping the first reinforcement material between a first part of the body and a second part of the body. Restraining the first reinforcement material relative to the body may even comprise placing the first reinforcement material under tension in the mould cavity. Such restraining helps maintain the first reinforcement material adjacent the first surface of the mould cavity even when adding molten metal to the mould cavity under pressure, e.g. during liquid pressure forming. In the resulting metal matrix composite component, uniformity in positioning of the first reinforcement material may be important to structural integrity: localized variations in depth of the first reinforcement material from one external surface may cause localized weaknesses in strength and/or stiffness.

The second reinforcement material may comprise loose fibres, particles and/or whiskers. The particles may be hollow or porous particles, e.g. ceramic particles. In this way, the second region or core of metal matrix composite component is a metal matrix syntactic foam. As known in the art, metal matrix syntactic foams (hereinafter“MMSF”) are a class of composite materials consisting of a continuous metal matrix with hollow or porous particles embedded therein. MMSF have potential for many applications, particularly in structural applications where either weight reduction or impact energy absorption is desired. Various combinations of matrix materials and hollow or porous particles have been proposed. Aluminium and magnesium alloy matrices are generally used, although syntactic foams with steel or titanium matrices have also been developed. The hollow or porous particles are typically ceramic particles (e.g. silica- and alumina-based), but hollow particles of glass and metal have also been proposed. The hollow particles are sometimes referred to as“microballoons” because they are hollow and approximately spherical.

Adding molten metal to the mould cavity may be performed under elevated pressure. A vacuum may even be applied to the mould cavity before molten metal is added. The molten metal may be forced in to the mould cavity through the first reinforcement material. For example, molten metal may be pressed in to the mould cavity to infiltrate both the first reinforcement material (e.g. in the form of a woven, sheet-like structure) and the second reinforcement structure (e.g. in the form of loosely packed ceramic particles). For example, the metal matrix composite component may be formed using a LPF process (for example, as described in GB 2115327) or an ALPF process (for example, as described in W02005/097377). The advantages of these fabrication processes include good reproducibility, and good interfacial bonding between the metal matrix and the first and second reinforcement materials. The molten metal may be aluminium or an aluminium alloy.

The method may further comprise covering a second surface of the mould cavity with a third reinforcement material. The second surface of the mould cavity may be opposite the first surface of the mould cavity. The third reinforcement material may be in contact with the second reinforcement material. The method may further comprise restraining the third reinforcement material relative to the body to restrict movement of the third reinforcement material in the mould cavity. The third reinforcement material may be the same as the first reinforcement material, both in composition and structure.

The resulting metal matrix composite component has three distinct regions: a core region sandwiched between opposing skin regions. The skin regions may be configured to maximise stiffness and strength, whereas the core region may be configured primarily to space the two skin regions away from a notional median plane in the metal matrix composite component whilst minimising weight.

The method may further comprise removing a solidified metal matrix composite structure from the mould cavity and cutting the metal matrix composite component from the solidified metal matrix composite structure such that the first reinforcement material is adjacent one external surface of the metal matrix composite component.

In accordance with a second aspect of the present invention, there is provided a metal matrix composite component comprising a body with an external surface, the body having a surface region adjacent the external surface and a core region spaced from the external surface by the surface region, with a metal matrix extending continuously through the surface region and the core region, wherein the surface region comprises a reinforcement material embedded in the metal matrix, and the core region has a density which is less than that of the surface region. For example, the core region may be 10% less dense than the surface region, possibly at least 15% less dense than the surface region, and possibly even at least 20% less dense than the surface region. The surface region may be configured to provide more stiffness and strength to the metal matrix composite component than the core material. Whilst contributing to the strength and stiffness of the component, the core region may primarily be configured to space the surface region from the central axis of the component to increase its second moment of area. With this in mind, providing the core region with a lower density than the surface region helps to keep the weight of the metal matrix composite component to a minimum for a given stiffness and/or strength.

In the metal matrix composite component, the reinforcement material may comprise inter-engaging fibres which are entangled, entwined or woven into a sheet-like structure. The sheet-like structure may be planar. Alternatively, the reinforcement material may comprise at least one elongate member, such as a filament or a braid, with lengths arranged side-by-side to form a raft-like structure.

In the metal matrix composite component, the reinforcement material may have a thickness of at least lmm, perhaps at least l .5mm, and possibly even at least 2mm, with the thickness being measured transverse to the external surface and towards the core region.

In the metal matrix composite component, the core region may comprise hollow or porous particles, e.g. ceramic particles. Alternatively, and or additionally, the core region may have a space frame structure comprising struts with channels therebetween. The channels may extend parallel to the surface region, and may be open on opposite lateral sides of the body.

The body of the metal matrix composite component may have a second surface region adjacent the external surface of the body, the second surface region being spaced from the first mentioned surface region by the core region, the second surface region comprising a further reinforcement material embedded in the metal matrix. The second surface region may be on an opposite side of the body to the first mentioned surface region. The further reinforcement material in the second surface region may be the same as the reinforcement material in the first mentioned surface region.

The metal matrix composite component may further comprise a member protruding from either the surface region or the core region of the body, the member comprising a metal. The member may further comprise fibre reinforcement. The member may be integrally formed with the metal matrix of the body, i.e. the metal matrix extends continuously through the surface region, the core region and the member

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows schematically a cross-sectional view of a metal matrix composite component made in accordance with the present invention;

Figure 2 is a flowchart illustrating schematically a method embodying the present invention of forming the metal matrix composite component of Figure 1;

Figure 3 illustrates schematically some of the parts used in the method of Figure 2; Figure 4 illustrates schematically an alternative form of parts used in Figure 3; and Figures 5A-5B are perspective illustrations of a kinematic linkage embodying the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

Figure 1 illustrates schematically a cross-sectional view of a metal matrix composite component 10 made in accordance with an embodiment of the present invention. The metal matrix composite component 10 is a multi-phase product comprising a core region 12 sandwiched between a pair of skin regions 14, 16. A single metal matrix 18 extends continuously through the core region 12 and skin regions 14, 16. The core region 12 is a metal matrix syntactic foam and comprises hollow or porous particles 20 embedded in the metal matrix 18. The skin regions 14, 16 comprise woven fibre sheets 22 also embedded in the metal matrix 18.

Figure 2 is a flowchart illustrating schematically a method 100 embodying the present invention of manufacturing the metal matrix composite component 10 of Figure 1, and Figure 3 illustrates schematically some of the parts used in the method 100. The method 100 comprises the step 102 of providing a body 200 defining at least part of a mould cavity 202. The next step 104 comprises covering a first surface 204 of the mould cavity 202 with a first reinforcement material 206 which is in the form of a woven sheet of ceramic fibres 208, and which is larger than the body 200. At step 106, the first reinforcement material 206 is restrained relative to the body 200, using adhesive to bond the fibrous woven sheet 208 to the body 200. Alternatively, or additionally, the woven sheet of ceramic fibres 208 may be placed under tension by pulling a pair of opposing peripheral edges 210 of the woven sheet of ceramic fibres 208 which overlap the body 200.

The method further comprises the step 108 of adding a second reinforcement material 212 in the form of hollow or porous ceramic particles 214 to the mould cavity 202, on top of the first reinforcement material 206. The ceramic particles occupy a high volume fraction of available space, perhaps 40-60% per unit volume. The next step 110 comprises covering a second surface 216 of the mould cavity 202, opposite the first surface 204, with a third reinforcement material 220, which may also be in the form of a woven sheet 222 of ceramic fibres. The third reinforcement material 220 is in contact with the ceramic particles 214 of the second reinforcement material 212, and is restrained at step 112 relative to the body 200 in the same way as the first reinforcement material 206. The resulting assemblage or“cassette” is clamped between a pair of plates or within a net shape cavity and heated prior to molten metal being forced under pressure into the mould cavity 202 at step 114, to infiltrate and embed the first, second and third reinforcement materials in a continuous metal matrix.

Figure 4 illustrates an alternative way in which the first and third reinforcement materials 206 and 220 are formed from a continuous fibre 300 (monofilament or multifilament) wrapped tightly around the body 200 which acts as open frame former. As the continuous fibre 300 is wrapped around the body 200, the first and second surfaces 204 and 216 of the mould cavity 202 are covered simultaneously. Each surface 204 and 216 is covered by parts of the continuous fibre 300, which are aligned and side-by-side in a raft-like structure. A small gap 302 is left uncovered by the continuous fibre 300 at one end of the body 200 so that the second reinforcement material 212 may be added to and fill the mould cavity 202. As before, the resulting assemblage or“cassette” is clamped between a pair of plates and heated prior to molten metal being forced under pressure into the mould cavity 202, to infiltrate and embed the first, second and third reinforcement materials in a continuous metal matrix. The metal matrix composite component 10 may be cut out from the body 200 once the metal has solidified.

Advanced liquid pressure forming nay be used to infiltrate the reinforcement materials 206, 212, 220, although other liquid metal infiltration processes could be utilised including, but not limited to, direct or indirect squeeze casting. If the second reinforcement material 212 comprises hollow spheres (e.g. formed from glass), a low pressure processes could be utilised. An important part of the infiltration process is presenting a quiescent melt front to construction and designing the casting process so that directional solidification is achieved so to prevent porosity in the core or skin.

The assembled cassette is thus preheated to above the liquidus of the molten metal used to form the matrix. This could be done in-situ or external to the casting machine die. If external, this is then moved into the casting machine die and the liquid metal poured into the die. If using primary aluminium, the casting temperature range could be between 700 to 820°C. The same levels of superheat could be applied to any aluminium alloy or other low melting point metal (and its alloys).

If using fibre outer layers with syntactic core a suitable pressure is applied after pouring so not to“crush” or damage the hollow spheres. This would typically be within the range of 5 to 25MPa metal pressure. For other reinforcement cores, much higher pressure could be used, up to 450MPa metal pressure (and all ranges in between to suit the core and skin combination). Pressure is generally modified to suit the material, with higher pressure used to create“higher strength” (but higher density) materials. Lower pressure is used where very low density, high modulus materials are required (such as syntactic foams & foam cores). The density of these materials can be as low as l .6g/cc - if using hollow spheres core, continuous fibre skins and aluminium matrix, but lower densities could be achieved by changing to matrix to a lower density metal such as magnesium.

The cassette is held at pressure during solidification before ejection from the die. Cooling could be ambient or quenched. After ejecting from the die and cooling, depending on the cassette design, the material can either be removed from the cassette (if using net shape) or extracted so that the cassette is removed and that none of the extended fibres form part of the final material. It is anticipated that some final machining will be required.

The thickness of the core region 12 and skin regions 14 and 16 can be tailored to suit a specific need, as can the matrix, with temperatures and pressure modified to suit. It is also possible to design the cassette so that metal rich surfaces are present if a coating or polished surface is required. The outer skin could be a single fibre type or could be a co-mingle, or in the case of woven or tape layup a multilayer stack, consisting of different fibres. This would be utilised if a surface ply or different surface finish was required to the“load bearing” fibre in the skin region(s) 14 and 16. In the case of a net shape cassette, the skin regions may additionally include short fibre, whisker or particle reinforcement, although there would likely be some mixing of these with the second reinforcement material 212 at the interface between the core region 12 and skin regions 14 and 16.

Figures 5A and 5B illustrate schematically two embodiments of a metal matrix composite component (a kinematic linkage) 400, each comprising a body 402 with an external surface 404. The body 402 has a surface region 406 adjacent the external surface 404, and a core region 408 spaced from the external surface by the surface region 406, in a direction perpendicular to the external surface 404. The body 402 comprises a metal matrix 410 extending continuously through the surface region 406 and the core region 408, infiltrating a reinforcement material (not shown) in the surface region 406. The core region 408 has a density which is less (e.g. at least 10 % less) than that of the surface region 406. In the case of Figure 5 A, the core region 408 is a metal matrix syntactic foam, with hollow or porous particles embedded in the metal matrix. In the case of Figure 5B, the core region 108 includes integrally formed struts 410 with open channels 412 therebetween and which extend through the body 402 parallel to the surface region 406. The struts 410 and open channels 412 space the surface region 406 from an opposing surface region 414.

In Figures 5A and 5B, the reinforcement material may include a continuous fibre(s) wound around opposing end lugs 420,422 of the body 402, with adjacent lengths of fibre aligned side-by-side in a single orientation to form a raft-like structure. Woven plies of fibres may also be included to enhance transverse and shear properties of the metal matrix composite component 400. Also in Figures 5A and 5B, the metal matrix composite component 400 has an upper lug 424 protruding from the surface region 406, and which is integrally formed with the metal matrix of the body 402.