COMPOSITE MATERIAL OF ZN-A1 ALLOY REINFORCED WITH SILICON CARBIDE POWDER

Abstract: A composite material is disclosed, which contains a matrix of Zn-Al alloy reinforced with silicon carbide powder, which powder has a granulometric distribution comprised within the range of from 1 to 200 µ.

Description: The present invention relates to a composite material of Zn-Al alloy reinforced with silicon carbide powder. In many applications, also structural, and, specifically, in the field of transports, whether by road, by railway or by aerospace means, materials are required, which are endowed with high mechanical properties, permanent also at medium-high temperatures, accompanied by light-weight characteristics. Among these materials, considerably interesting are the metal-matrix composites, reinforced with ceramic materials in the form of powders, whiskers, or long fibres, with possible unidimensional or planar orientations. A proper selection of the materials constituting the matrix and the reinforcer, of their relative amounts and arrangements, can supply a wide range of products with previously "designable" characteristics. The characteristics and the performance of a composite depend, of course, on the materials which constitute it, on their shapes and mutual arrangement, on their mutual interaction, and, finally on the methodologies used to manufacture them. It is hence necessary to correctly evaluate all these parameters, for the purpose of ensuring the desired properties to the end product. The mechanisms which control the efficacy of reinforcement on the matrix are several, but, among the main ones of them, those which cause the load to be transferred from the matrix to a reinforcer endowed with higher mechanical characteristics, or create discontinuities suitable for hindering the propagation of a fracture flaw can be regarded as fundamental. In the first case, an increase is obtained in the ultimate tensile strength and in the elastic modulus of the composite, relatively to the values of the corresponding properties of the matrix; in the second case, also an increase in toughness is obtained. An optimum adhesion between the matrix and the reinforcer is always required, in order to attain a good transfer of the stress from the one to the other. The manufacturing techniques for composite materials are many, and are different from each other, as a function of the type of metal used as the matrix, of the reinforcement type, or of the chacteristics which one wants to obtain. The reinforcer can be constituted by powders, by whiskers or by long fibres, whilst the metal can be in either solid or liquid phase. In case long fibres are used, composite materials can be obtained, which have anisotropic properties, but are reinforced in the desired direction, e.g., wires, flat rolled sections, bars, or, in determined cases, also more complex shapes, with directional characteristics. In case, on the contrary, reinforcers are used, which are constituted by powders or by whiskers, isotropic composites are generally obtained, i.e., composites endowed with homogeneous characteristics according to the various directions. With a reinforcer constituted by powders or whiskers, composites can be obtained by admixing the solid into the liquid, or by infiltration, under pressure, of the liquid metal into pre-formed articles formed by powders or fibres, or, finally, from blends of metal powders and ceramic reinforcers composites can be obtained by techniques of hot-pressing, extrusion, drawing, and, in general, of powder metallurgy. Studies are known on composites which are obtained by blending alloys of Al, Mg, or Zn reinforced with dispersed particles of Al₂O₃, SiO₂ or SiC, with a granulometry variable from some microns to some hundreds of microns, wherein the powder-matrix interfacing is accomplished by means of metal coatings applied to the powders. Other investigations relate to the high temperature-extrusion of blends of powders of aluminum and glass at a temperature higher than the softening point of glass (approximately 500°C), a composite reinforced by discontinuous fibres formed in loco , due to the plastic deformation of the glass particles being obtained. The need of having to secure an optimum bond between the fibres and the matrix is sometimes opposed by the fact that the commercial fibres have poor characteristics of wettability by the molten matrices, so that either the infiltration is made difficult, or it takes place regularly, but with the subsequent degradation of the mechanical properties. In these cases, in order to obtain a good adhesion, it is necessary to resort to contrivances, such as particular additions to the molten material, capable of varying the wettability of the metal on the reinforcer, or particular conditions of solidification of the matrix; or, as an alternative, it is necessary to resort to fibre coatings with materials which are wettable by the metal. In any case, a perfect control of the process parameters is always necessary in order to secure the efficacy of the reinforcement. US-A-2,793,949 discloses a method for preparing composite materials containing metal and non-metal materials, in which wetting agents are used, which are constituted by metalloids, as well as by alkali metals, or alkali-earth metals, to lower the mutual surface tensions. The Applicant knows that composites containing from 40 to 70% by volume of fibres have been prepared by using filaments of carbon or alumina (6-20 µm). In any case, these manufacturing methods, precisely denominated "in the liquid phase" have sometimes shown a decrease in properties because of the reaction with the molten alloy, so that the practice is limited to a small number of fibre-matrix combinations. Aluminum or magnesium alloys have been associated with carbides or oxides, preventing the agglomeration of these latter, and achieving the desired wettability, by adding, e.g., for the oxides, oxygen to the molten material. This system is claimed in US-A-3,468,658. Said patent limits the dimensions of the particles of the added materials within the range of from 10 nm to 1 µm. The present Applicant has surprisingly found now that by reinforcing the Zn-Al alloy with a SiC powder having a grit-size range of from 1 micrometre to 200 micrometres, it becomes possible to obtain a composite material having outstanding mechanical properties, which persist at medium-high temperatures and are not impaired by the drawbcks of the conventional composite materials referred to hereinabove. The present invention, therefore, provides a Zn-Al alloy based composite material having a modulus of elasticity, E, greater than 100 GPa, consisting of a reinforced Zn-Al alloy matrix containing from 73% to 96% by weight of zinc, and of a reinforcement of powdered abrasive-grade SiC having a grit size of from 1 micrometre to 200 micrometres. It is preferred that the maximum content of the powdered SiC reinforcement is 50% by volume. The composite material in question may additionally contain, if so desired, also whiskers selected from glass materials, metal oxides or steels. The procedure for preparing the composite material in question may be selected from the following: blending of ceramic powders or whiskers with metals or metal alloys in the liquid or the semi-solid state. infiltrating liquid metal into pre-formed articles of ceramic powders or fibres, and sintering metal powders blended with ceramic powders or whiskers. The ensuing Examples are intended to illustrate the invention more clearly from a practical standpoint. Example 1 By infiltration under pressure, a composite material was prepared from a Zn-Al alloy at 27% by weight of Al reinforced with SiC powder, the content of which is of 50% by volume, and whose granulometric distribution is comprised within the range of from 70 to 180 µm. For the infiltration under pressure, an equipment was used, which consisted of a pyrex glass tube, wherein the metal, placed in the upper position, is forced under pressure to enter the underlying pre-formed article. A composite material was obtained, which had zero porosity and a high abrasion resistance, with a good adhesion between the metal and the reinforcer, and which had a modulus of elasticity E = 145 GPa. Example 2 By infiltration under pressure, a composite material was prepared from a Zn-Al alloy at 12% by weight of Al reinforced with SiC powder, the content of which is of 50% by volume, and whose granulometric distribution is comprised within the range of from 40 to 70 µm. For the infiltration under pressure, the same equipment and the same procedure of Example 1 were used. A composite material was obtained, which had zero porosity and a high abrasion resistance, with a good adhesion between the metal and the reinforcer, and which had a modulus of elasticity E = 118 GPa. Example 3 By infiltration by blending, a composite material was prepared from a Zn-Al alloy at 8% by weight of Al reinforced with SiC powder, the content of which is of 50% by volume, and whose granulometric distribution is comprised within the range of from 20 to 60 µm. For the infiltration by blending, an equipment was used, which consisted of a temperature-controlled vessel, wherein the material was kept under strong stirring. A composite material was obtained, which had zero porosity and a high abrasion resistance, with a good adhesion between the metal and the reinforcer, and which had a modulus of elasticity E = 150 GPa. Example 4 By infiltration by blending, a composite material was prepared from a Zn-Al alloy at 4% by weight of Al reinforced with SiC powder, the content of which is of 50% by volume, and whose granulometric distribution is comprised within the range of from 5 to 25 µm. For the infiltration by blending, the same equipment and the same procedure of Example 3 were used. A composite material was obtained, which had zero porosity and a high abrasion resistance, with a good adhesion between the metal and the reinforcer, and which had a modulus of elasticity E = 155 GPa. Example 5 Example 4 was repeated, with the variant that the Zn-Al alloy contained 27% by weight of Al and SiC content was of 30% by volume, with a granulometric distribution comprised within the range of from 20 to 60 µm. The porosity was zero, the resistance to abrasion was high, the adhesion between the metal and the reinforcer was good, and the modulus of elasticity E = 140 Gpa.

Claims: Claims for the following Contracting States : BE, CH, DE, FR, GB, GR, LI, LU, NL, SE A Zn-Al-alloy-based composite material having a modulus of elasticity, E, greater than 100 GPa, consisting of a reinforced Zn-Al-alloy matrix containing from 73% to 96% by weight of zinc, and of a reinforcement of powdered abrasive-grade SiC having a grit size of from 1 micrometre to 200 micrometres. Composite material according to Claim 1, wherein the maximum content of the powdered SiC reinforcement is 50% by volume. Composite material according to Claim 1, further containing whiskers selected from glass materials, metal oxides or steels. Composite material according to Claim 1, wherein zinc is 88% by weight of the Zn-Al alloy. Composite material according to Claim 1, wherein zinc is 92% by weight of the Zn-Al alloy. Process for preparing the composite material defined in Claim 1, comprising the step of infiltrating the molten alloy under pressure into an SiC-reinforcement preformed article vertically positioned in a tubular mould. Process according to Claim 6, wherein the tubular mould is a Pyrex (Reg.trade Mark) glass mould. Process for preparing the composite material defined in Claim 1, comprising the step of blending SiC renforcement particles with the molten or semi-solid Zn-Al alloy concerned. Claims for the following Contracting States : AT, ES Process for preparing a Zn-Al alloy based composite material having a modulus of elasticity, E, greater than 100 GPa, consisting of a reinforced Zn-Al alloy matrix containing from 73% to 96% by weight of zinc, and of a reinforcement of powdered abrasive-grade SiC having a grit size of from 1 micrometre to 200 micrometres, comprising the step of blending SiC renforcement particles with the molten or semi-solid Zn-Al alloy concerned. Process for preparing the composite material defined in Claim 1, comprising the step of infiltrating the molten alloy under pressure into an SiC-reinforcement preformed article vertically positioned in a tubular mould. Process according to Claim 2, wherein the tubular mould is a Pyrex (Reg.Trade Mark) glass mould. Process according to Claim 1, wherein the maximum content of the powdered SiC reinforcement is 50% by volume. Process according to Claim 1, further containing whiskers selected from glass materials, metal oxides or steels. Process according to Claim 1, wherein zinc is 88% by weight of the Zn-Al alloy. Process according to Claim 1, wherein zinc is 92% by weight of the Zn-Al alloy.

Inventors: GUERRIERO, RENATO, TANGERINI, ILARIO
Applicants: NUOVA SAMIM S.P.A.
Application year: 1987
Main classification: C22C32
IPC codes: C22C32, B22F3, B22D19, C22C1