Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-01-25
2004-02-10
Jenkins, Daniel J. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C419S036000
Reexamination Certificate
active
06689188
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to metallurgical powder compositions and methods for using the same. More particularly, the invention relates to metallurgical powder compositions that include an improved lubricant for enhancing green densities and sintered densities while reducing stripping and sliding pressures.
BACKGROUND
The powder metallurgy industry has developed metal-based powder compositions, generally iron-based powders, that can be processed into integral metal parts having different shapes and sizes for uses in various industries, including the automotive and electronics industries. One processing technique for fabricating parts made from metal-based powder composition involves charging a die cavity with metal-based powder composition and compacting the metal-based powder composition under high pressure to form a “green” compact. The green compact is then removed from the die cavity and sintered to form the finished part.
Metallurgical powder compositions are traditionally provided with a lubricant to reduce internal friction between particles during compaction, to permit easier ejection of the compact from the die cavity, to reduce die wear, and/or to allow more uniform compaction of the metallurgical powder composition. The internal friction forces that must be overcome to remove a compacted part from the die are measured as “stripping” and “sliding” pressures. Internal friction forces increase as the pressure of compaction increases.
Lubricants are classified as internal (dry) lubricants or external (spray) lubricants. Internal lubricants are admixed with a metal-based powder prior to adding the metal-based powder composition to the die. External lubricants are sprayed onto the interior walls of the die cavity prior to adding the metal-based powder composition. Common lubricants include metallic stearates or synthetic waxes.
Most known internal lubricants reduce the green strength of the compact. It is believed that during compaction the internal lubricant is exuded between iron and/or alloying metal particles such that it fills the pore volume between the particles and interferes with particle-to-particle bonding. As a result some shapes cannot be pressed using known internal lubricants. Tall, thin-walled bushings, for example, require large amounts of internal lubricant to overcome die wall friction and reduce the required ejection force. Such levels of internal lubricant, however, typically reduce green strength to the point that the resulting compacts crumble upon ejection. Also, internal lubricants such as zinc stearate often adversely affect powder flow rate and apparent density, as well as green density of the compact, particularly at higher compaction pressures. Moreover, excessive amounts of internal lubricants can lead to compacts having poor dimensional integrity, and volatized lubricant can form soot on the heating elements of the sintering furnace. To avoid these problems, it is known to use an external spray lubricant rather than an internal lubricant. However, the use of external lubricants increases the compaction cycle time and leads to less uniform compaction. An example of an external lubricant is set forth in U.S. Pat. No. 5,518,639 issued to Luk, assigned to Hoeganaes Corporation.
Accordingly, there exists a need in the art for metallurgical powder compositions that can be used to fabricate strong green compacts that are easily ejected from die cavities without the need for an external lubricant. Prior solutions to this problem are described in U.S. Pat. Nos. 5,498,276, 5,290,336, 5,154,881, and 5,256,185 issued to Luk, assigned to Hoeganaes Corporation. The U.S. Pat. No. 5,498,276 patent discloses use of a polyether as lubricant for the metallurgical powder composition that provides improved strength and ejection performance of the green compact while maintaining equivalent or superior compressibility relative to the use of other lubricants. The U.S. Pat. No. 5,290,336 patent discloses use of a binder/lubricant comprising a dibasic organic acid and one or more additional polar components that provides enhanced physical properties to the powder composition such as apparent density, flow, compressibility, and green strength. The U.S. Pat. No. 5,154,881 patent discloses use of an amide lubricant that is admixed with iron-based powders that permits compaction of the powder composition at higher temperatures without significant die wear and improves green strength and density.
SUMMARY
The metallurgical powder compositions of the present invention contain metal-based powders and solid lubricants. The solid lubricants contain polyalkylene-polyalkylene oxide (PAO) block copolymer lubricants or a combination of polyalkylene-polyalkylene oxide block copolymer lubricants and at least one additional lubricant.
The polyalkylene-PAO block copolymer lubricants include at least one block A of a linear or branched polyalkylene chain having from about 5 to about 500 carbon atoms, and at least one block B of an alkylene oxide chain having a formula
—[O(CH
2
)
r
]
m
—,
or
[(CH
2
)
r
O]
m
—
where r is from about 1 to about 7, m is from about 1 to about 350, and the copolymer has a weight average molecular weight of less than about 20,000. The polyalkylene-PAO block copolymer lubricants can have a formula represented as A-B, A-B-A, or B-A-B.
The additional lubricants include polyamides, C
10
to C
25
fatty acids, metal salts of C
10
to C
25
fatty acids, metal salts of polyamides, or combinations thereof. These additional lubricants have a melting range beginning at a temperature of at least about 30 degrees Centigrade.
The solid lubricant contains polyalkylene-PAO block copolymer lubricants, or a mixture of the polyalkylene-PAO block copolymer lubricants and at least one additional lubricant. Preferably, the mixture of lubricants is in the form of discrete particles of each, or the polyalkylene-PAO block copolymer lubricants and the at least one additional lubricant are a melt blend of both forming a homogeneous combination thereof.
The present invention also includes methods for preparing the solid lubricants. The solid lubricants can be prepared by, for example, preparing and then atomizing polyalkylene-PAO block copolymer lubricants, or admixing discrete particles of polyalkylene-PAO block copolymer lubricants and at least one additional lubricant. Alternatively, the solid lubricant can be prepared by blending polyalkylene-PAO block copolymer lubricants and at least one additional lubricant as a melt. The melt is subsequently solidified and atomized.
The present invention also includes methods for preparing metallurgical powder compositions. Metallurgical powder compositions are prepared by admixing the solid lubricant with a metal-based powder.
The present invention also includes methods of making metal parts. Metal parts are prepared by providing a metallurgical powder composition of the present invention, charging the metallurgical powder composition into a die, and compressing the metallurgical powder composition at a pressure of at least about 5 tsi to form a metal part.
DETAILED DESCRIPTION
The present invention relates to improved metallurgical powder compositions, methods for the preparation of those compositions, methods for using those compositions to make compacted parts, methods for making solid lubricants for use in metallurgical powder compositions, and the solid lubricants themselves. Ejection properties, such as stripping pressure and sliding pressure, of compacted parts can be improved by using the solid lubricants.
Metallurgical powder compositions that include solid lubricants are easily removed from a compaction die as shown by reduced stripping and sliding pressures associated with removal of a compacted part from a die. Strip pressure measures the static friction that must be overcome to initiate ejection of a compacted part from a die. Slide pressure is a measure of the kinetic friction that must be overcome to continue the ejection of the part from the die cavity.
Green properties, such as green den
Luk Sydney
Poszmik George
Hoeganes Corporation
Jenkins Daniel J.
Woodcock & Washburn LLP
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