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Powder metallurgy methods and compositionsRelated Patent Categories: Powder Metallurgy Processes, Powder Metallurgy Processes With Heating Or Sintering, Post Sintering Operation, Subsequent Heat Treatment (e.g., Annealing, Etc.)Powder metallurgy methods and compositions description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070077164, Powder metallurgy methods and compositions. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of Invention [0002] The present invention relates to methods and compositions for use in pressed powder metallurgy. [0003] 2. Description of Related Art [0004] In pressed powder metallurgy, a substantially dry metal powder composition is charged into a die cavity of a die press and compressed to form a green compact. Pressing causes the metal powder particles in the metal powder composition to mechanically interlock and form cold-weld bonds that are strong enough to allow the green compact to be further processed. After pressing, the green compact is removed from the die cavity and sintered at a temperature that is below the melting point of the major metallic constituent of the metal powder composition, but sufficiently high enough to strengthen the bond between the metal powder particles, principally through solid-state diffusion. Some metal powder compositions include minor amounts of other metals and/or alloying elements that melt during sintering to facilitate liquid phase sintering of the non-melting major constituent of the metal powder composition. This increases the bonding strength between the metal powder particles and typically increases the final density of the sintered part. [0005] In most pressed powder metallurgy applications, it is necessary to add a lubricant to the dry metal powder composition before it is pressed to form the green compact. The most commonly used lubricants in pressed powder metallurgy are ethylene bis-stearamide wax and zinc stearate, but other lubricants are also sometimes used. The lubricant helps the individual metal powders flow into all portions of the die cavity, allows for some particle to particle realignment during pressing and also serves as a release agent that facilitates removal of the green compact from the die cavity after pressing. The least amount of lubricant necessary to obtain good flow and release is used. [0006] The lubricant is conventionally removed from the green compact by gradually heating the green compact at a relatively low heating rate (e.g., .about.15.degree.F./min) until the lubricant melts, boils and/or decomposes. This "delubing" is typically accomplished during an initial heating or preheating stage at the beginning of the sintering process. This can be accomplished in a batch furnace or in a continuous furnace. In a continuous furnace, the green compact is placed on a conveyor that moves the part slowly into and through a sintering oven. The slow movement of the conveyor allows the temperature of the green compact to increase at a slow rate, allowing the lubricant to melt and then boil and then gas off. Most of the remaining lubricant residue is decomposed and burned out as the temperature of the green compact increases. Some small quantity of the lubricant may diffuse into the base metal and contribute carbon to the final part. The lubricant is completely removed from the green compact at a temperature that is substantially lower than the final sintering temperature. In a batch furnace, the temperature is gradually increased to remove the lubricant prior to sintering that may be programmed to run at different conditions. [0007] To maximize the opportunity for the individual metal particles to bond to each other, it has long been the practice to sinter the green compact at a peak sintering temperature for a significant amount of time, typically on the order of 30 minutes or more. Allowing the part to soak or dwell at the peak sintering temperature for this period of time is believed to increase the likelihood that individual metal particles will bond via solid-state diffusion. The slow movement of the conveyor or the temperature profile in a batch furnace insures that the green compact receives a lengthy soak or dwell time in the hot zone of the sintering oven. [0008] Ideally, the sintered density of a final part would be 100% of the theoretical density of the metallic constituents of the metal powder composition used to form the part. However, the sintered density of parts formed from most metal powder compositions does not approach 100% of theoretical density. Using conventional carbon or low alloy steel metal powder compositions and pressed powder metallurgy methods, a sintered density of about 93% to 94% of theoretical density can be achieved. BRIEF SUMMARY OF THE INVENTION [0009] The present invention provides metal powder compositions for pressed powder metallurgy and methods of forming metal parts using the metal powder compositions. In one embodiment of the invention, the metal powder composition comprises a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Preferably, the primary metal particles comprise iron, which may be alloyed with other metals/elements such as, for example, carbon, copper, manganese, phosphorus, silicon, sulfur, nickel, chromium, bismuth, cobalt, niobium, molybdenum, tungsten, tin, aluminum and titanium. The liquid phase forming materials are preferably selected from the group consisting of Fe--C--Mn, Fe--C, Fe--C--Si, Fe--Mn, Fe--P, Fe--S, Co--C, Mo--C, Mn--C, Ni--C, Fe--B and Fe--Cr, or precursors thereof selected from the group consisting of graphite, ferro phosphorous, copper phosphorous, boron, silica, manganese sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum. [0010] A first method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature at a heat up rate of 60.degree. F./min or higher in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a "clean" surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. The rapid heating rate during the sintering step ensures that the liquid phase formers have adequate time to create liquid phase bonds between the primary metal particles before the constituents of the liquid phase diffuse into the particles. [0011] In a second embodiment of the invention, the metal powder composition comprises primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Preferably, the primary metal particles comprise copper or aluminum, which may be alloyed with conventional alloying elements. [0012] A second method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a "clean" surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. Because no liquid phase forming materials or precursors thereof are present in the composition, the heating rate during sintering is not critical. [0013] Metal parts formed using the metal powder compositions and methods according to the invention exhibit a substantially higher sintered density than metal parts formed from metal powder compositions that do not comprise an organic acid, and such higher densities can be reached in less time and at lower energy costs. For example, it is possible to form carbon steel or low alloy steel metal parts that have a sintered density that approaches 100% of theoretical density. Subsequent heat treatment of metal parts formed from the metal powder compositions and methods of the invention substantially improve the mechanical properties of the parts, which in some cases are better than can be achieved using non-powder metallurgical processes such as forging and casting. [0014] The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed. DETAILED DESCRIPTION OF THE INVENTION [0015] In a first embodiment of the invention, metal powder compositions according to the present invention comprise a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Throughout the instant specification and in the accompanying claims, the term "primary metal particles" refers to the principal metal powder component of the metal powder composition by weight. The primary metal particles can comprise a single metallic element, or can be alloys, agglomerations or blends of two or more metallic elements. Suitable metals include, for example, iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten. Iron is a particularly preferred metal and is the major constituent of steel. [0016] The primary metal particles tend to have surfaces that are oxidized, typically as a result of contact with oxygen in the atmosphere or with water vapor. Primary metal particles comprising iron, which are frequently used in pressed powder metallurgy to form steel parts, have surfaces that are oxidized to form iron oxide. Applicants believe that metal oxides on the surface of primary metal particles may interfere with solid-state diffusion bonding between such particles during sintering. The metal oxides on the surface of the primary metal particles may also inhibit the formation of liquid phase alloys, which can be used to solder, weld or otherwise bind the individual metal particles together. [0017] A variety of organic acids are known to react with metal oxides to produce organic metal salts. For example, acetic acid will react with iron oxide to form ferrous acetate. Similarly, citric acid will react with iron oxide to form ferrous citrate. Lactic acid will react with iron oxide to ferrous lactate. And, malic acid, tartaric acid, oxalic acid, oleic acid, and stearic acid will react with iron oxide to form ferric malate, ferrous tartrate, ferrous oxalate, ferric oleate and ferrous stearate, respectively. [0018] Organic acids suitable for use in the invention are those which are strong enough to react with metal oxides on the surface of the primary metal particles to produce metal salts, and which are compatible with the mixing, filling and compaction and sintering steps of the pressed powder metallurgy process. Preferably, the organic acid or acids used in the invention do not leave undesirable residues or by-products when decomposed during delubing and sintering. Accordingly, organic acids that are free of, or contain very little, sulfur, nitrogen, phosphorous and halogens are preferred. [0019] Fatty acids are particularly suitable organic acids for use in the invention. A non-exhaustive list of fatty acids is set forth in Section 7-28 ("Properties of Selected Fatty Acids") of the CRC Handbook of Chemistry and Physics, 76th Edition (1995), which is hereby incorporated by reference. It will be appreciated that other organic acids can also be used. Many organic acids are listed in Section 8-45 to 8-55 ("Dissociation Constants of Organic Acids and Bases") of the CRC Handbook of Chemistry and Physics, 76th Edition (1995), which is also hereby incorporated by reference. The organic acids identified in that list that are compatible with pressed powder metallurgy and which are free of, or contain very little, sulfur, nitrogen, phosphorous and halogens can be used. [0020] Citric acid is the presently most preferred organic acid for use with metal powder compositions for low alloy steel and carbon steels as well as stainless steel, copper and aluminum. Other particularly useful organic acids include acids that have a pKA value low enough to react with metal oxides and which are solids at press conditions (typically .about.140.degree. F. and higher). Examples of suitable alternative acids to citric acid include, for example, oxalic acid, tartaric acid, malic acid and low-melting acids that are partially solublized in higher melting acids or other organic materials that decompose into constituents that are similar to citric acid or the other acids identified above. [0021] The amount of organic acid present in the metal powder composition will depend on the amount of metal oxide to be removed, and the ability of the organic acid to remove the metal oxide during the delubing/sintering cycle(s). Loadings from about 0.1% by weight to about 4% by weight are typically sufficient. Ideally, a stoichiometric amount of acid would be added relative to the oxides on the surface of the metal particles, plus an excess of about 10 mole percent, if press conditions would allow it. To insure adequate distribution of the organic acid in the metal powder composition, it is preferable that the organic acid be micronized to an average particle size of about 30 .mu.m or less (e.g., via milling). When used neat (i.e., not blended with other materials), it is preferable for the organic acid to be mirconized close in time prior to use so that the micronized particles do not have an opportunity to degrade upon exposure to atmospheric moisture. Continue reading about Powder metallurgy methods and compositions... 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