Powder metallurgy methods and compositions

Abstract

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, the metal powder composition according to the invention includes 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. During a “delubing” step, the organic acid reacts with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes into a base metal or a metal-carbide during sintering.

1 . A method of forming a metal part comprising the steps of: 
 (i) providing a metal powder composition comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof 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° 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.    
   
   
       2 . The method according to  claim 1  wherein the metal powder composition further comprises a lubricant.  
   
   
       3 . The method according to  claim 1  wherein the heating steps (iv) and (v) are conducted in a reducing atmosphere.  
   
   
       4 . The method according to  claim 1  wherein heating step (v) is conducted in a vacuum furnace, a continuous furnace or a microwave furnace.  
   
   
       5 . The method according to  claim 1  wherein the organic acid is free of sulfur, halogens, nitrogen and phosphorous.  
   
   
       6 . The method according to  claim 1  wherein the organic acid is a fatty acid.  
   
   
       7 . The method according to  claim 1  wherein the organic acid is present in the metal powder composition at a loading of from about 0.1% to about 4.0% by weight.  
   
   
       8 . The method according to  claim 1  wherein the primary metal particles comprise one or more metals selected from the group consisting of iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten.  
   
   
       9 . The method according to  claim 1  wherein the liquid phase forming materials are 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.  
   
   
       10 . The method according to  claim 1  wherein the metal powder composition comprises precursors of liquid phase forming materials 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.  
   
   
       11 . The method according to  claim 1  wherein the primary metal particles comprise iron and the organic acid comprises citric acid.  
   
   
       12 . The method according to  claim 3  wherein the reducing atmosphere comprises a mixture of hydrogen and nitrogen or vacuum with or without partial pressure.  
   
   
       13 . A metal part formed according to the method of  claim 1 .  
   
   
       14 . The metal part according to  claim 13  wherein the metal part comprises a carbon or low-alloy steel having a sintered density of greater than 95% of theoretical density.  
   
   
       15 . A metal powder composition for use in pressed powder metallurgy comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof 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.  
   
   
       16 . The metal powder composition according to  claim 15  further comprising a lubricant.  
   
   
       17 . The metal powder composition according to  claim 15  wherein the organic acid is free of sulfur, halogens, nitrogen and phosphorous.  
   
   
       18 . The metal powder composition according to  claim 15  wherein the organic acid is a fatty acid.  
   
   
       19 . The metal powder composition according to  claim 15  wherein the organic acid is present in the metal powder composition at a loading of from about 0.1% to about 4.0% by weight.  
   
   
       20 . The metal powder composition according to  claim 15  wherein the primary metal particles comprise one or more metals selected from the group consisting of iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten.  
   
   
       21 . The metal powder composition according to  claim 15  wherein the liquid phase forming materials are 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.  
   
   
       22 . The metal powder composition according to  claim 15  wherein the metal powder composition comprises precursors of liquid phase forming materials 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.  
   
   
       23 . The metal powder composition according to  claim 15  wherein the primary metal particles comprise iron and the organic acid comprises citric acid.  
   
   
       24 . The metal powder composition according to  claim 15  wherein the reducing atmosphere comprises a mixture of hydrogen and nitrogen or vacuum with or without partial pressure.  
   
   
       25 . A metal part formed of the metal powder composition according to  claim 15 .  
   
   
       26 . The metal part according to  claim 25  wherein the metal part comprises a carbon or low-alloy steel having a sintered density of greater than 95% of theoretical density.  
   
   
       27 . A 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 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.  
   
   
       28 . The method of forming a metal part according to  claim 27  wherein the metallic elements having relatively low viscosity when molten in the primary metal particles are selected from the group consisting of copper and aluminum.  
   
   
       29 . The method according to  claim 27  wherein the metal powder composition further comprises a lubricant.  
   
   
       30 . A metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten 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.  
   
   
       31 . The metal powder composition according to  claim 30  wherein the metallic elements having relatively low viscosity when molten in the primary metal particles are selected from the group consisting of copper and aluminum.  
   
   
       32 . The metal powder composition according to  claim 30  further comprising a lubricant.