Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making porous product
Reexamination Certificate
1999-04-16
2002-06-04
Koehler, Robert R. (Department: 1775)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making porous product
C075S236000, C075S240000, C075S244000, C075S246000, C075S252000, C164S091000, C164S097000, C164S061000, C164S066100, C419S005000, C419S013000, C419S018000, C419S027000, C419S065000, C428S545000
Reexamination Certificate
active
06399018
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of making products by infiltrating a porous, three-dimensional, interconnected structure with a molten metal, and the products produced therefrom.
BACKGROUND
The field of rapid tooling deals with methods to reduce the time required for producing a tooling component compared to traditional machining. Within this context, tooling or tools refer to mold cavities as well as machine elements used in manufacturing. Most injection molding, die-casting, stamping, and other industrial molding processes have high costs associated with tooling production. The field of rapid tooling has brought increased speed to producing prototype parts and molds, but has been unable to attain the high surface finishes, tolerances, and mechanical properties of their machined counterparts.
Ideally, manufacturers, engineers and tool builders would have a technique that could take them from a computer model to production tooling with a reduction in the number of steps. Such a technology could significantly reduce the time needed to get a new product to market, as well as drive the price of tooling down. It has been estimated in recent reports that total profits on new products are reduced by as much as 60% by the inability to get the product to market quickly. Implicitly, the cost savings of such a method have the ability to go beyond simply the cost of tooling production having considerable impact on new product success and profit potential. Any savings in time will greatly improve the profit potential of industries such as fashion, automotive, toy, cosmetics and consumer electronics, which produce products that are time sensitive. Previous attempts to produce tooling using powdered metals have had problems with speed of production and final properties such as thermal conductivity, hardness, surface finish, porosity, wear resistance, and dimensional precision. Tooling is traditionally produced by machining with extremely expensive equipment. This approach utilizes highly qualified personnel, typically requiring up to 14 weeks to produce a quality tool. The purpose of the present invention, therefore, is to enable industry to make production grade tooling for a wide variety of manufacturing techniques in a short amount of time with minimal investment in equipment. The significance of the present invention lies in the fact that the expenses, time, and equipment investments are much less than prior art methods.
Current methods of tool production include the machining of steel or tool steel stock (or other non-ferrous based metals available in block shapes), epoxy (filled and un-filled) molds, nickel spray formed molds, several varieties of powdered metal rapid tooling, and molds containing metals, cermets or ceramic compounds. For example, U.S. Pat. Nos. 3,929,476, 4,073,999, 4,314,399, 4,327,156, 4,455,354, 4,491,558, and 5,507,336 generally describe the use of ceramic, carbide, and metal powders including but not limited to tungsten carbide, steels, tool steels and stainless steels. The conventional methods described in the prior art have been limited by their inability to produce near net complex shapes.
U.S. Pat. No. 4,024,902 to Baum, for example, the teachings of which are herein incorporated by reference, describes composites consisting of sintered tungsten carbide agglomerates in a matrix of steel alloy. However, this reference is directed to adding a molten steel alloy that dissolves the tungsten carbide material. Accordingly, the process is not well suited for the production of precision molded articles.
Similarly, U.S. Pat. No. 4,554,218 to Gardner et al., the teachings of which are also incorporated by reference, describes an infiltrated metal composite article comprising multiple metal powders. However, this reference describes sinterable metal powders without describing such powders in combination with a particulate ceramic material such as sintered tungsten carbide agglomerates. Such a particulate ceramic material can help improve the hardness and wear properties in the final part. Additionally, the advantage of using specific volume fractions of the constituents to form a rigid, percolated microstructure with interconnected phases is not exploited in these systems.
A percolated microstructure implies that there is a communication across the structure, such as thermal or electrical conductivity, resulting from interconnected phases. For powder processing, the percolation limit, which is defined as the amount below which communication across the structure is interrupted, is above approximately 16 volume percent of a phase in mixed powders. Therefore, when a structure contains a component in amounts less than approximately 16 volume percent, a percolated phase for that component is not formed.
Gardner describes less than 15 volume percent of a relatively soft refractory phase (preferably tungsten) that is encapsulated by a sinterable hard phase (preferably tool steel). Such encapsulation, coupled with less than 15 volume percent of the refractory phase, does not allow the formation of an interconnected, percolated microstructure, but leads to the formation of discrete “islands” in the microstructure.
Furthermore, the method described by Gardner requires the mechanism of volume diffusion during sintering. Bonding via volume diffusion can be recognized by a change in shape of the sintering particle surface, which can provide distortion in an object fabricated by the process. This is in contrast to the bonding mechanism of surface diffusion during sintering which is characterized by a lack of change in the shape of the sintering particle surface.
A need exists for a process and an object that can overcome the problems of shrinkage and distortion traditionally associated with forming large is objects with intricate shapes via powdered metals. In particular, the possibility of achieving low temperature sinter bonding of powders in the absence of volume diffusion can provide an important route to minimizing distortion in the metal object. At the same time, the process should also be capable of resulting in an object that possess the mechanical properties desirable in production tooling at greatly reduced times compared to traditional methods. Accordingly, the inventors have developed a method that provides improvements in the combination of speed of production, and production of high quality, multiple molds or parts from a single prototype. These improvements are achieved by a combination of the method and the composition contained herein.
SUMMARY OF THE INVENTION
The current invention achieves the end goal of producing metallic objects by means of a novel multi-step forming, debinding, sintering and infiltrating process, using a metal-ceramic composition. Metallic object or metallic tool is understood to mean a metal-based object or tool such as a cermet. Additionally, a metallic tool further encompasses a die and a mold. The term particulate ceramic material is understood to mean monolithic ceramic powders, cermets, agglomerated ceramic powders, ceramic powders bonded together with metallic or non-metallic constituents, mixtures of these components or sintered compositions comprising of any of these materials.
To make such metallic objects with the present process, a mixture comprising a binder, a powdered first metal and particulate ceramic material is used. In one embodiment of the invention, the sintered, bonded granular ceramic is composed of a material selected from the group consisting of: silicon carbide, boron nitride, and tungsten carbide. In another embodiment of the invention the bonding material is a sintering activator for the particulate ceramic or cermet material. In a further embodiment of the invention, the particle sizes of the powdered first metal and the particulate ceramic or cermet material are in a multi-modal distribution in the ratio from greater than 1:1 to about 1:10. In a preferred embodiment of the invention, the particle sizes of the powdered first metal and the powdered ceramic are in a bi-modal distribution in
Atre Sundar V.
German Randall M.
Griffo Anthony
Thomas Julian A.
Weaver Timothy J.
Koehler Robert R.
Ohlandt Greeley Ruggiero & Perle L.L.P.
The Penn State Research Foundation
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