Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Patent
1997-11-28
1999-11-23
Mai, Ngoclan
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
B22F 100
Patent
active
059893044
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an iron-based powder composition for powder metallurgy in which lubricant, graphite powder, copper powder and the like are added and mixed beforehand, and more particularly to an iron-based powder composition for powder metallurgy which in normal handling undergoes little segregation of the additive materials and dust generation and has excellent flowability and compactibility in a wide temperature range over the order of the room temperature to 473 K.
BACKGROUND ART
Hitherto, iron-based powder compositions for powder metallurgy have been produced by a mixing method in which alloying powders such as copper, graphite, and iron phosphide powders, are mixed with an iron powder, and according to the necessity, in addition to the powders for improving the machinability a lubricant such as zinc stearate, aluminium stearate, and lead stearate is mixed. Such a lubricant has been adopted in view of a homogeneous mixing with a metal powder, an easy decomposition and a removability at the time of sintering.
Recently, as a requirement of higher strength for sintering manufactures is increased, as disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei. 2-156002, Japanese Patent Publication (Kokoku) Hei. 7-103404, U.S. Pat. No. 5,256,185 and U.S. Pat. No. 5,368,630, there is proposed a warm compaction technology which permits higher density and higher strength of compacts by means of performing a compaction while metal powders are heated. It is considered for the lubricant used in such a compaction procedure that a lubricity at the time of heating is important as well as the homogeneous mixing with a metal powder, the easy decomposition and the removability at the time of sintering.
Specifically, a mixing of mixtures of a plurality of lubricants having mutually different melting points with metal powders serves, at the time of a warm compaction, to melt part of the lubricants, uniformly spread the lubricants between iron and/or alloying metal particles, and decrease frictional resistances among the particles and between a compact and dies, so that a compactibility is improved.
However, such a metal powder composition involves the following drawbacks. First, a raw material mixture undergoes segregation. Regarding the segregation, since the metal powder composition contains powders having different sizes, shapes and densities, segregation occurs readily during transport after mixing and upon charging the powder composition into hoppers, or upon discharging the powder composition from the hoppers or during molding treatments. For example, it is well known that segregation of a mixture of iron-based powder and graphite powder occurs within a transport vehicle owing to vibrations during trucking, so that the graphite powder rises to the top. It is also known as to graphite charged into a hopper that the concentration of graphite powder differs at the beginning, middle, and end of the discharging operation from the hopper owing to segregation within the hopper.
These segregations cause fluctuations in the composition of products of the powder metallurgy; fluctuations in dimensional changes and strength become large, and this causes the production of inferior products.
The flow rate of the powder composition increases as a result of the increased specific surface area of the mixture, since graphite and other powders are fine powders. Such increases in flow rate is disadvantageous because it decreases the production speed of green compacts by decreasing charging speed of the powder composition into die cavities for compaction.
As technologies for preventing segregation of such a powder composition, there are known methods based on selection of an appropriate binder as disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Sho. 56-136901 and Japanese Patent Publication (Kokoku) Sho. 58-28321. However, these methods involve such a drawback that if the quantity of binder added is increased so that segregation of the powder composition is sufficiently
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Ogura Kuniaki
Ozaki Yukiko
Uenosono Satoshi
Kawasaki Steel Corporation
Mai Ngoclan
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