Abstract:
A metal matrix composite includes a plurality of fibers having an average diameter of about eight micrometers with a coating, and a metal or alloy distributed with said fibers, a fibers-to-metal or alloy ratio has a range is has a range of about 9:1 to less than about 1:1.

Description:
TECHNICAL FIELD 
     The present invention relates generally to metal-matrix composites, and more particularly, to a metal-matrix composite suitable for rapid prototyping. 
     BACKGROUND 
     In the automotive industry, it is desirable to bring a product to market in a short amount of time. This reduces the overall cost of the vehicle. One manner in which automakers have reduced product development time is by using rapid prototyping and tooling methods such as stereolithography. Stereolithography or other rapid prototyping systems in conjunction with a computer aided design system can be used to form a prototype part based upon drawings generated by the CAD system. 
     One drawback to parts developed in known rapid prototyping systems is that the techniques use layers of paper and plastic or plastic powders to construct the parts. As a result, the part can only be used in low-temperature prototype environment. Also, additional processing must take place in order to make metal dies. 
     It would therefore be desirable to provide a rapid prototyping system that forms parts having a higher melting point than those known rapid prototyping systems. This allows the part to be used under higher temperature processing such as the formation of metal dies. 
     SUMMARY OF THE INVENTION 
     The present invention provides suitable metal-matrix composites for use in rapid prototyping systems. The metal-matrix composites have higher melting temperatures than materials used in prior known rapid prototyping systems. 
     In one aspect of the invention, a metal-matrix composite comprises a plurality of fibers having an average diameter of about eight micrometers with a coating, and a metal or alloy distributed with said fibers, a fibers-to-metal or alloy ratio by volume has a range is has a range of about 9:1 to less than about 1:1 
     In a further aspect of the invention, a method of forming a part includes the steps of forming a composite material by mixing a molten metal or alloy with a bundle of fibers; heating the composite material to a melting point of the composite material to from a molten material; dispensing the molten material at least at said melting point into a shape of a part; and solidifying the molten material. 
     One advantage of the invention is that by eliminating additional processing in prototypes, the cost and time of producing a product is reduced. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described with respect to a metal-matrix composite for use in a rapid prototyping system. However, the metal-matrix composite of the present invention may also be suitable for other applications both inside and outside of the automotive industry. For example, the aerospace industry may also benefit from the metal-matrix composite. 
     Metal-matrix composites are metals or alloys strengthened with tiny inclusions of another material which inhibit crack growth and increase performance. In the preferred embodiment, the metal is a Sn alloy reinforced with carbon fiber or fibers. The carbon fibers may be continuous or discrete fibers. The preferred aspect ratio of discrete fibers is greater than 10:1. Carbon fibers having a diameter of about eight micrometers have been used. 
     Preferably, the carbon fibers have a coating to increase wetability. The coatings may be formed from Ni, Au and/or In. The thickness of the coating preferably ranges from about 0.3 micrometers to 1.5 micrometers. 
     The metal-matrix is preferably a Sn alloy that has a melting temperature above 130° C. but below 300° C. The composition of the metal is preferably eutectic or near-eutectic. Alloys of Sn include the mixture of Ag, Sb, Cu, Zn, Bi. Fe, Co, In, Ni, or Pb. Preferred compositions include by weight Ag from 0 to 5%, Sb from 0 to 7%, Cu from 0 to 5%, Zn from 0 to 15%, Bi from 0 to 60%, In from 0 to 60%, Fe, Co, Ni from 0 to 1%, and Pb from 0 to 50%. Suitable compositions include Sn-3.5 Ag, Sn-5 Sb, Sn-0.7 Cu, Sn-9 Zn, Sn-58 Bi, or Sn-62 Pb. For many applications, however, the lead alloy may not be preferred due to its lead content. The ratio of fiber to metal by volume preferably extends from about nine to below one. However, the preferred range is between about nine and about seven. 
     A goal of the present invention is to have comparable, equal or better stiffness yield strength mechanical strength than pure aluminum and aluminum alloys. Pure aluminum has a tensile of 90 MPa. Sn alloy has a tensile strength of 30 MPa. A composite material formed according to the present invention using a continuous fiber and an alloy with 63% Sn and 37% Pb was formed having a tensile strength of 270 MPa. Using non-woven carbon fiber and the same alloy, a tensile strength of 95 MPa has been achieved. 
     Another desirable feature of the composite of the present invention is the melting temperature. Known rapid prototyping systems have processing temperatures around 280° C. The present invention has a melting temperature below a desired 250° C. One composite material had a melting temperature below 230° C. In contrast, aluminum and its alloys have a melting temperature near or above 600° C. so processing temperatures are typically above 650° C. To form the composite material, a capillary flow may be used to flow the molten metal alloy into the bundle of fibers. The molten metal flows into the gaps between the fibers to form the composite. Little or no pressure has to be used. 
     A flux may be used to first wet the fibers before applying the molten Sn alloy. The wetting process may be manual or may be performed by dipping the fibers into the flux material. An automatic system feeding the fiber into the flux bath then drawing them out is also contemplated. Another method for applying flux to the fibers is dispensing the metal flux directly onto the fibers. A squeegee may be used to remove excess molten material from the fiber bundle. 
     To form a rapid prototype part, the composite material is deposited into the desired shape by rapid prototyping system. Typically, such systems use an XYZ coordinate system for directing movement. The composite material of the present invention is particularly suitable for rapid prototyping because the composite material solidifies within seconds to form a solid structure. 
     Another manner for manufacturing the composite material is vacuum infiltration with pressurized molten Sn-based alloy. 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.