Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Consolidation of powder prior to sintering
Reexamination Certificate
1999-11-03
2002-02-05
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Consolidation of powder prior to sintering
C419S054000
Reexamination Certificate
active
06344169
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for the compaction of powders for powder metallurgy.
2. Description of the Prior Art
In order to enhance mechanical and magnetic characteristics of compacted products obtained by powder metallurgy, it is effective to increase the density as high as possible. To this end, it is important to obtain a green density, which is as high as possible, at a compacting stage prior to sintering.
Accordingly, there have been adopted methods of promoting compaction of powders for powder metallurgy to which vibrations are applied at a compacting stage (see, for example, Japanese Patent Publication Nos. Hei 3-25278, Sho 41-6549, Sho 54-14781, Sho 54-41525 and the like).
However, these known vibration compactions are a method which has the primary object of promoting rearrangement of powders for powder metallurgy. This method may be effective in the case where compaction is performed at low pressure such as for tile or pottery powders, but is not always a satisfactory one when applied to a field where powders such as iron powders are subjected to plastic deformation at a high compression or compaction pressure and thus, are compacted.
In conventional powder metallurgy, a lubricant is pre-mixed with powders to be compacted to increase the fluidity of the powders so as to reduce the mutual friction between the powders and the friction between the powder and a compacting die or mold. The use of a lubricant is mainly for the purposes of reducing a friction caused on ejection of a green compact from the die and preventing the die from galling.
The formulating amount of a lubricant is generally in the range of from 0.2 to 10 wt % based on the powders to be sintered (see, for example, Japanese Laid-open Patent Application No. Hei 2-156002). In Metal Powder Report, Vol. 42, No. 11, pp. 781-786 (1987), it is stated that a maximum compaction density is obtained when the amount of a lubricant is at 0.5%. In currently employed instances, the amount is, in most cases, in the range of 0.5 to 1.0 wt %.
In this connection, however, if a compacting pressure is increased so as to increase a green density, a lubricant is filled in voids or spaces among starting powders to impede the increase of the density, thus placing the inevitable limitation on high density compaction. Nevertheless, if the amount of a lubricant is reduced, a great friction is brought between the powders and the compaction die, with the attendant problem that high density compaction is disenabled along with a lowering in life of the compaction die.
On the other hand, it is well known that when a lubricant is applied onto the inner wall surfaces of a compaction die, the friction between the powders and the die is reduced. However, because any lubricant is not formulated in starting powders, the powders degrade in fluidity and packing property. Thus, it is difficult to obtain a high density green compact when compacted at high pressure.
Furthermore, U.S. Pat. No. 4,955(5),798 discloses press compaction by heating starting powders at a temperature not higher than the melting point of a lubricant (usually, at approximately 70° C. to 120° C.) in order to increase the density of a green compact. In Japanese Laid-open Patent Application No. Hei 5-271709, it is stated to carry out press compaction by heating to a temperature lower than a temperature at which a lubricant is completely melted (particularly, at temperatures of approximately 370° C. or below). These methods are both based on the finding that if a lubricant melts, the fluidity of powders lowers considerably.
In this connection, however, with an ordinarily employed amount of a lubricant, the lubricant remains within the resultant green compact, and thus, such methods as mentioned above are not ones which fundamentally ensure high density compaction.
In Japanese Laid-open Patent Application No. Hei 9-272901, there is proposed a method of increasing a green density wherein lubricant-free powders are used and a lubricant is applied onto the inner wall surfaces of a compaction die, followed by heating the die to 150 to 400° C. and press compaction. However, any lubricant is not formulated in the starting powders in this method with poor fluidity of the powders. In addition, the powders being compacted are unlikely to cause rearrangement, and a satisfactory high density does not result. Moreover, because the effect of reducing the friction among the powders cannot be obtained in this method, density irregularity is liable to occur inside the resultant green, thereby causing a dimensional variation after sintering.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for the compaction of powders for powder metallurgy which overcomes the problems of the prior art techniques discussed hereinbefore.
It is another object of the invention to provide a method for the compaction of powders for power metallurgy which can overcome the problems on shortage in fluidity of powders at the time of compaction and also on the friction with a die so as to reliably obtain a green compact of high density.
The above objects can be achieved, according to the invention, by a method for the compaction of powders for powder metallurgy, which comprises packing powders for powder metallurgy formulated with a lubricant in a compacting die whose inner wall surfaces are applied with a lubricant, and subjecting the packed powders to warm or hot compaction wherein the lubricant is present in the powders in an amount of 0.2 wt % or below (non-inclusive of 0 wt %) based on the total of the powders and the lubricant.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
We have made intensive studies from various angles in order to attain a high density of green compacts, and as a result, found that powders for powder metallurgy formulated with a lubricant are packed in a compaction die applied with a lubricant on the inner wall surfaces thereof and are subsequently subjected to warm or hot compaction wherein when the lubricant formulated in the powders for powder metallurgy is added to in a slightly reduced amount, the above objects can be successfully attained. The invention has been accomplished based on this finding. The preferred embodiments of the invention are described in detail.
The term “powders for powder metallurgy” used herein generically means powders which are used for the manufacture of green compacts of desired forms by subjecting the powders to press compaction to a required contour, followed by sintering, if necessary. Moreover, in the present specification, those powders, which are formulated with lubricants and the like in order to reduce the friction between a die and powder and the mutual friction of the powders, may also be called powders for powder metallurgy.
Specific examples of the powders include metallic powders and ceramic powders. In particular, the method of the invention is very effective when applied to metallic powders which undergo plastic deformation at the time of compaction. Most typical ones include pure iron powders (including those iron powders containing, as impurities, small amounts of C, Mn, Si, P, S, Cr, O, N and the like), alloy powders to which Ni, Mo, Mn, Cr, Si and other elements are purposely added in order to improve strength after sintering (e.g. those powders of the pre-alloy type, diffusion type, hybrid type thereof and the like), or metallic powders undergoing various surface treatments for improving characteristics in magnetic fields, particularly, soft magnetic powders.
When alloy powders are used, care should be paid to the fact that when the amounts of alloy elements are in excess, iron powders become hardened to lower compacting properties, thereby impeding high densification as a powder-metallurgical product.
Various types of alloying elements such as, for example, graphite, Cu, Ni, Mo and the like may be formulated singly or in admixture of two or mores in order to enhance characteristic properties after sintering. Additionally, composite powde
Akagi Nobuaki
Sato Masa-aki
Sawayama Tetsuya
Seki Yoshikazu
Tsuchida Takehiro
Jenkins Daniel J.
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd).
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