A METAL MATRIX COMPOSITE AND METHOD FOR ITS PRODUCTION

Abstract: A metal matrix composite produced by squeeze forming comprises an implant material in a light metal alloy matrix wherein the implant material comprises a preform of silicon carbide, alumina or zirconia particulate together with glass, alumina or alumino-silicate material in filament, fibre or whisker form.

Description: A METAL MATRIX COMPOSITE AND METHOD FOR ITS PRODUCTION This invention relates to metal matrix composites of the type wherein an implant material is incorporated into a metal matrix " by a squeeze forming operation. In particular, the invention is concerned with components comprising a light metal alloy matrix such as an aluminium alloy having implant material incorporated therein at selected positions. For example, such components may comppise motor vehicle suspension components, engine cylinder liners, pistons and connecting rods and wheels. The term "squeeze forming" as us§d throughout this specification and claims will be understood to refer to a technique wherein liquid metal is introduced into a first part of a mould, the mould is closed under pressure so that the liquid metal is displaced by the mould closure to fill a cavity within the mould without entrapping air and the metal is maintained under pressure whilst solidification takes place so as to ensure that any shrinkage cavities which may occur are closed and filled; the mould then " being opened and the formed article removed. Certain implant materials for metal matrix composites have already been investigated and most of the metal matrix composites cxurrently being devised have limitations with regard to performance due largely to the mis-match in the mechanical and physical properties of the implant materials. Probably the best known current implant material used as a reinforcement in aluminium alloy metal matrices is a pol crystalline alumina based fibre; this material having an ultimate tensile strength of the order of 2,000 MPa and a Youngs Modulus of the order of 300 GPa. These properties compare quite favourably with the corresponding properties of most aluminium alloys, particularly at elevated temperatures. However, polycrystalline alumina based fibre material is quite expensive and some work is being devoted to the utilization of cheaper fibre material such as, for example, amorphous alumino-silicate fibre, which material has a cost of the order of one-quarter of that of polycrystalline alumina based fibre. The published properties of amorphous alumino-silicate fibre material are that it has an ultimate tensile strength of the order of 1,400 MPa and a Toungs Modulus of the order of 120 G-Pa. The reduced modulus of amorphous alumino-silicate fibre material could thus render it unsuitable for use in components where buckling or bending is to be resisted, for example, in vehicle suspension components or engine connecting rods. A further type of implant material is glass, specifically those types of glasses known as R-glass and polycrystalline ceramic glass. These glass materials are of a similar cost to that of alumino-silicate fibre materials, but offer a higher strength. The principal advantage of using glass is that it exists as continuous filament and can therefore be woven to produce any desired orientation. The properties of R-glass are that it has an ultimate tensile strength of the order of 4,400 MPa and a modulus of the order of 86 G-Pa whilst polycrystalline ceramic glass has an ultimate tensile strength of the order of 1,720 MPa and a modulus of the order of 1 2 G-Pa. Both of these glass materials offer high temperature stability and are therefore capable of withstanding the process conditions which would occur during their incorporation into a light metal alloy by the squeeze forming process. A further implant material is silicon carbide which, in whisker form, has an ultimate tensile strength of 3,000 to 4,000 MPa and a modulus of 400 to 700 GPa, the variations being dependant upon the method of whisker manufacture. Silicon carbide whiskers are, however, very expensive (approximately twenty times the cost of polycrystalline alumina based fibre materials) and there are potential health hazards due to the fineness of the whiskers. Silicon carbide particulate (grit) has the same modulus as the whisker form and is much cheaper to produce than any of the fibre reinforcements referred to above (approximately one-sixth of the cost of polycrystalline alumina based- fibre materials) but the particulate form of silicon carbide is unlikely to provide any strength enhancement. It is the object of the present invention to provide a higher performance (i.e. high strength and high modulus) cost-effective metal matrix composite produced by squeeze forming and to provide an improved implant material for incorporation by squeeze forming into a metal matrix. In accordance with the invention there is provided a metal matrix composite comprising a light metal alloy matrix having incorporated therein by a squeeze forming process an implant material comprising a preform of particulate high modulus material, together with a high strength material in filament, fibre or whisker form. The particulate high modulus material conveniently comprises silicon carbide, alumina or zirconia whereas the high strength material conveniently comprises glass, alumina or alumino-silicate. The light metal alloy conveniently comprises an aluminium or magnesium alloy. The invention also provides an implant material for incorporation by squeeze forming into a light metal alloy matrix wherein said implant comprises a preform of particulate silicon carbide, alumina or zirconia together with a glass, alumina or alumino-silicate material in filament, fibre or whisker form. A metal matrix composite in accordance with the invention conveniently comprises a component of aluminium or magnesium alloy having an implant material as described in the preceding sentence incorporated therein by squeeze forming at selected positions in the component. Also in accordance with the invention there is provided a method of manufacturing a metal matrix composite comprising locating an implant material as previously described in a mould part of a squeeze forming press prior to the introduction of molten metal into the mould, introducing molten metal into the mould, closing the mould under pressure so that molten metal is displaced by the mould closure to fill a cavity in the mould within which the implant material is located, maintaining the metal under pressure whilst solidification thereof takes place and subsequently opening the mould and removing the metal matrix composite therefrom. By way of example, one implant material in accordance with the invention may comprise a combination of polycrystalline alumina based fibres or amorphous alumino-silicate fibres with silicon carbide particulate as a preform incorporated into an aluminium alloy metal matrix by squeeze forming. To produce the preform, silicon carbide particulate is added to an aqueous suspension of polycrystalline alumina based fibres or amorphous alumino-silicate fibres prior to vacuum forming, thus ensuring an even dispersion of the silicon carbide particulate throughout the preform. The silicon carbide particulate size can be as small as one micron and may extend up to fifteen microns. Larger sizes of particulate offer no advantages and they may in fact prove to be extremely deleterious in introducing large fatigue crack initiation zones. Another example of implant material comprises the combination of silicon carbide particulate with glass fibre. The glass, in woven form as a mat, can be impregnated * with silicon carbide particulate in order to provide a high strength, high modulus implant. One specific example of a metal matrix composite in accordance with the invention contemplates the use of an aluminium alloy having the general composition 5 copper, 1.5$ nickel, 0.25 cobalt, 0.25$ antimony and 0.25$ manganese, with the balance being aluminium, infiltrated by a squeeze forming process into a preform of polycrystalline alumina based fibre material through which is dispersed silicon carbide particulate. The particulate size is 5 microns and the level of addition is 10$ by volume into a preform which would itself provide 24$ by volume of polycrystalline alumina based fibre material. The final metal matrix composite is of a composition by volume percentage of 2.4$ silicon carbide, 24$ polycrystalline alumina based fibre material and 73•6$ aluminium alloy.

Claims: CLAIMS 1. A metal matrix composite comprising a light metal alloy matrix having incorporated therein by a squeeze forming process an implant material comprising a preform of particulate high modulus material together with a high strength material in filament, fibre or whisker form. 2. An implant material for incorporation by squeeze forming into a light metal alloy matrix wherein said implant comprises a preform of particulate high modulus material together with a high strength material in filament, fibre or whisker form. 3. An implant material as claimed in Claim 2 wherein the particulate high modulus material is silicon carbide, alumina or zirconia. • 4- An implant material as claimed in either one of Claims 2 or 3 wherein the high strength material is glass, alumina or alumino-silicate material. 5. A method of manufacturing a metal matrix composite comprising locating an implant material as claimed in any one of Claims 2 to 4 in a mould part of a squeeze forming press prior to the introduction of molten metal into the mould, introducing molten metal into the mould, closing the mould under pressure so that molten metal is displaced by the mould closure to fill a cavity in the mould within which the implant material is located, maintaining the metal under pressure whilst solidi ication thereof takes place and subsequently opening the mould and removing the metal matrix composite therefrom. 6. A metal matrix composite comprising a component of aluminium alloy having implant material as claimed in any one of Claims 2 to 4 incoporated therein by squeeze forming at selected positions in the component.

Inventors: BARLOW JOHN, BARLOW, JOHN
Applicants: GKN TECHNOLOGY LTD, GKN TECHNOLOGY LIMITED
Application year: 1986
Main classification: B22D19
IPC codes: B22D19