TECHNICAL FIELD

This disclosure relates to a powder mixture for powder metallurgy, and particularly to a powder mixture for powder metallurgy which can be ejected from a die with less force at the time of compaction and suppresses die galling. In addition, this disclosure relates to a method of manufacturing the powder mixture for powder metallurgy.

BACKGROUND

Powder metallurgy includes forming a raw material powder mainly composed of an iron-based powder into a green compact using a die, and subjecting the green compact to sintering to obtain a sintered part. In order to improve the formability at the time of compaction, it is usual to add a lubricant to the raw material powder and to make the lubricant adhere to the surface of the die used in the compaction. If no lubricant is used, the iron-based powder contained in the raw material powder is in direct contact with the die, which increases friction. As a result, a desired green density cannot be obtained at the time of compaction, and a large force is required for ejecting the green compact from the die after the compaction.

For these reasons, various lubricants are used in green compacting. Examples of the lubricant include metal soaps such as lithium stearate and zinc stearate, and amide-based lubricants such as ethylenebis stearamide.

Furthermore, JP 2005-330547 A (PTL 1) proposes using a graphite powder to improve the lubricity. When the surface of the iron-based powder is covered with graphite, the lubricity of the surface of the iron-based powder is improved. In addition, direct contact between the iron-based powder and the die is avoided by interposing graphite therebetween, so that die galling is prevented.

CITATION LIST

Patent Literature

PTL 1: JP 2005-330547 A

SUMMARY

Technical Problem

As proposed in PTL 1, using an iron-based powder which is covered with a graphite powder can reduce friction at the time of compaction and reduce the force required for ejecting the compact from the die. However, it was found that the powder mixture of PTL 1 has the following problems.

PTL 1 uses fluid dispersion, in which graphite and a binder are dispersed in water or an organic solvent, to cover the surface of an iron-based powder with a graphite powder, and therefore it requires production equipment capable of handling liquid raw materials. In particular, it is necessary to provide an apparatus for recovering and processing spent solvent.

In addition, PTL 1 uses a binder to adhere the graphite powder to the iron-based powder. As a result of investigating the powder mixture obtained with the above-described method, however, it was found that the binder is also present on the surface of the graphite powder adhering to the iron-based powder. Due to the presence of the binder on the powder surface, the flowability of the powder mixture cannot be sufficiently improved.

It could thus be helpful to provide a powder mixture for powder metallurgy which is extremely excellent in flowability, can be ejected from a die with less force, and suppresses die galling at the time of compaction. In addition, it could be helpful to provide a method of manufacturing the powder mixture for powder metallurgy without using any solvent.

Solution to Problem

We engaged in intensive studies on the above problems and made the following discoveries.

(1) In the case where a raw material powder, a graphite powder, and a binder are simultaneously mixed, the surface of the graphite powder is also covered with the binder, and it is impossible to uniformly cover the outermost surface of the raw material powder with graphite.

(2) By covering the surface of a raw material powder with a binder and then with a fine graphite powder, it is possible to prevent the surface of the graphite powder from being covered with the binder. In this way, it is possible to obtain a powder mixture for powder metallurgy which is extremely excellent in flowability, can be ejected from a die with less force at the time of compaction, and suppresses die galling.

The present disclosure is based on the above discoveries and has the following primary features.

1. A powder mixture for powder metallurgy, comprising a raw material powder, a binder, and a graphite powder, wherein

the raw material powder contains an iron-based powder in a content of 90 mass % or more of the raw material powder,

the graphite powder has an average particle size of less than 5 μm,

a ratio of mass of the binder (mb) to the sum of mass of the raw material powder (mr) and mass of the graphite powder (mg), expressed as [mb/(mr+mg)×100], is 0.10 mass % to 0.80 mass %,

a ratio of mass of the graphite powder (mg) to the sum of mass of the raw material powder (mr) and mass of the graphite powder (mg), expressed as [mg/(mr+mg)×100], is 0.6 mass % to 1.0 mass %,

surface of the raw material powder is covered with at least a part of the binder, and

surface of the binder covering the surface of the raw material powder is covered with at least a part of the graphite powder.

2. The powder mixture for powder metallurgy according to 1., wherein the binder is at least one selected from the group consisting of fatty acid amide, copolymerized polyamide, polyurethane, and polyethylene.

3. The powder mixture for powder metallurgy according to 1. or 2., wherein the raw material powder contains at least one auxiliary raw material selected from the group consisting of alloying powder and machinability improvement powder.

4. A method of manufacturing a powder mixture for powder metallurgy, comprising:

first mixing where a raw material powder and a binder are mixed at a temperature equal to or higher than a melting point of the binder to obtain a powder covered with binder, and

second mixing where the powder covered with binder and a graphite powder having an average particle size of less than 5 μm are mixed at a temperature equal to or higher than the melting point of the binder to obtain a powder mixture for powder metallurgy, wherein

the raw material powder contains an iron-based powder in a content of 90 mass % or more of the raw material powder,

a ratio of mass of the binder (mb) to the sum of mass of the raw material powder (mr) and mass of the graphite powder (mg), expressed as [mb/(mr+mg)×100], is 0.10 mass % to 0.80 mass %, and

a ratio of mass of the graphite powder (mg) to the sum of mass of the raw material powder (mr) and mass of the graphite powder (mg), expressed as [mg/(mr+mg)×100], is 0.6 mass % to 1.0 mass %.

5. The method of manufacturing a powder mixture for powder metallurgy according to 4., wherein the binder is at least one selected from the group consisting of fatty acid amide, copolymerized polyamide, polyurethane, and polyethylene.

6. The method of manufacturing a powder mixture for powder metallurgy according to 4. or 5., wherein the raw material powder contains at least one auxiliary raw material selected from the group consisting of alloying powder and machinability improvement powder.

Advantageous Effect

The powder mixture for powder metallurgy of the present disclosure has extremely excellent flowability. Therefore, it can be ejected from a die with less force at the time of compaction, and at the same time, the compaction can be performed continuously without causing die galling. In this way, the yield rate of green compacts is improved, and high productivity can be realized. In addition, according to the manufacturing method of the present disclosure, the powder mixture for powder metallurgy can be manufactured without using any solvent.

