Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-08-12
2004-01-20
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C419S037000, C419S038000
Reexamination Certificate
active
06679935
ABSTRACT:
FIELD OF INVENTION
The present invention concerns a lubricant system. More particularly, the present invention concerns a lubricant system for use in powder metal compositions that are used in the production of metal parts.
BACKGROUND OF THE INVENTION
The use of powder metal compositions to produce metal products is well-known in the prior art. Powder metals (powder metallurgy) is commonly employed in applications wherein casting, forging or other metal processing techniques is not cost effective. The fabrication of parts using powder metals includes the steps of placing the metal composition in the cavity of a mold, pressing the composition to form a green part, removing the green part from the cavity, and firing the green part.
During the pressing operation, it is generally important that a lubricant be employed in order to facilitate the removal of the green part from the cavity and allow for the slippage of particles against each other so that forces are spread uniformly and density can be made to be as uniform as possible in the cross-section of the pressed part. Generally, in the prior art there are two separate approaches to the use of lubricants. One approach is to apply the lubricant to the wall of the mold cavity prior to adding the metal composition to the cavity, with the metal composition having a relatively low level of lubricant. The downside to this approach is that it is time-consuming to apply a uniform coating of a liquid lubricant to the cavity walls. The second approach is to incorporate a relatively higher level of lubricant into the powder metal composition. However, the use of prior art lubricants results in several adverse effects. Specifically, such lubricants reduce the flow of the powder metal into the mold cavity thereby slowing the pressing operation. Lubricants can also detrimentally impact green density and result in the evolution of undesirable effluents during preheat and the sintering operation. Lubricants can also contribute to low final density in parts, protracted furnace time, and the formation of cracks and blisters during firing.
The present invention provides a lubricant system that overcomes the deficiencies of the prior art lubricant systems.
SUMMARY OF THE INVENTION
The present invention provides a new and improved lubricant system for use in powder metallurgy. The lubricant system is solid at ambient conditions, but upon application of press pressure (forming pressure and stress) it transforms to a liquid phase. The lubricant system provides an excellent lubricant for use in powder metals for it results in good flow of the powder composition, low loading requirements, shorter furnace times, ease of removal of the green part from the mold cavity and the formation of minimal effluents during heating. In addition to the lubricant system, the present invention also provides a metal mixture and method of using the same. The lubricant system of the present invention may also be used in connection with the pressing of ceramic powders.
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
The lubricant system of the present invention is a solid at ambient conditions. Thus, the lubricant system is a solid when it is mixed with the powder metal. However, when pressure and stress is applied to the lubricant system during the pressing of the metal composition, it must be capable of transforming at least in part to a liquid phase along the walls of the mold cavity.
There are various compositions that will work in accordance with the invention. Preferably, the major components of the lubricant system display solubility with one another in the melt state. The melt points of the components of the lubricant system are depressed during melting, possibly forming some type of eutectic.
It is normal for a solid material to have an increase in melting point as pressure is applied. There are other exceptions however. The Clapeyron-Clausius thermodynamic equation predicts that when the density of a material in its solid state is less than the density of a substance in its liquid state, then the melting point of that substance will decrease when pressure is increased. Applicant believes that this equation may be used to predict material systems suitable for use in the present invention.
Applicant has found that materials that work flow under pressure and shear, flow better with increased shear and temperature, are partially crystalline at room temperature. Transformation of a lubricant system to a liquid phase, preferably takes place at about 4 tons per square inch at room temperature, a very low end of the working pressure of a press. The faster a press runs, the more shear is generated and temperature due to particle-to-particle friction. Both of these actions reduce the viscosity of a lubricant system. Normal press operations impart to a part a temperature of about 90° to about 140° F. Thus, applicant has found that a lubricant system that displays a viscosity range of from about 1000 to about 6000 poise at a shear rate of 1000 1/second and a temperature of 100° F., performs well. By taking advantage of the shear thinning properties of the lubricant system, non-dusting metal mixes can be made without the use of solvents, thereby also resulting in metal mixes with reduced segregation of components, and the loss and segregation of minor ingredients. Also, preferably there is a strong attraction by the lubricant system to the surface of the metal particles. Further, preferably the lubricant system cleanly burns during the firing or sintering of the green part, with no formation of undesired residual metals or undesired reduced metals. Additionally, the lubricant system of the present invention permits the operation of presses at much greater loads leading to improved green densities and parts free of defects such as blisters and delaminations.
One lubricant system that performs in accordance with the requirements of the present invention comprises a fatty acid material. The lubricant system may also include a wax, and in one preferred embodiment the lubricant system comprises a guanidine material. The wax may be synthetic or natural. One preferred synthetic wax is an amide wax.
The fatty acid material of the present invention comprises a carboxylic acid derived from or contained in an animal or vegetable fat or oil. Preferably, the fatty acid material comprises an unsaturated fatty acid or a mixture of such acids, and salts thereof such as lithium stearate. Examples of suitable unsaturated fatty acids include butyric acid, lauric acid, palmitic acid and stearic acid. More preferably, the fatty acid material comprises a mixture of lauric acid, palmitic acid and stearic acid. The fatty acid may also comprise a fatty acid ester such as, for example, glycerol monostearate or butyl stearate.
In one embodiment, the guanidine material is a reaction product of guanidine and an acid selected from a fatty acid, an organic acid, or a stronger acid. The guanidine material is a reaction product which may be an amide or actually may be more in the nature of a hydrated salt. For example, according to the CRC Handbook of Chemistry and Physics, 74th Ed., guanidine acetate has the formula (H
2
N)2,C═NH.CH
3
COOH, rather than an amide-type formula such as H
2
N—C═NH(NH)COCH
3
, as would be expected for an amide. This is due to the fact that guanidine is a very strong base, and is much more likely to simply abstract a proton from a relatively weak organic acid, rather than react with the organic acid in a “standard” amidization reaction forming an amide with concomitant loss of H
2
O. However, in some cases, the reaction of guanidine and the acid may yield an amide in the “standard” manner. For this reason, the guanidine material of the pre
Apex Advanced Technologies, LLC
Mai Ngoclan
Rankin, Hill Porter & Clark LLP
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