EP1118404A1

EP1118404A1 - Alloy powder, alloy sintered compact and method for their production

Info

Publication number: EP1118404A1
Application number: EP99943375A
Authority: EP
Inventors: Katsuyoshi Kondoh; Ai Ito; Takatoshi Takikawa
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1998-09-24
Filing date: 1999-09-16
Publication date: 2001-07-25
Also published as: WO2000016937A1; JP2000160203A; EP1118404A4

Abstract

Primary powder particles 2 whose main component is aluminum and whose particle diameter is between 5 µm and 300 µm are bound together using an organic binder 3 to form secondary powder particles 1 having a particle diameter of 10 µm to 500 µm. Similarly, primary powder particles whose main component is iron and whose particle diameter is between 20 µm and 100 µm are bound together using an organic binder to form secondary powder particles having a particle diameter of 50 µm to 200 µm, Thus, an alloy powder having excellent fluidity can be obtained, and a sintered alloy material having high dimensional accuracy can be produced with these secondary powder particles.

Description

Technical Field

The present invention relates to an alloy powder, a sintered alloy material, and a method of producing the alloy powder and the sintered alloy material, and more specifically, to an aluminum alloy powder and a method of producing the aluminum alloy powder, an iron alloy powder and a method of producing a sintered iron alloy compact, a metal powder, and a sintered metal compact.
An aluminum alloy powder and the method of producing the aluminum alloy powder are related in particular to the aluminum alloy powder having high fluidity and being applicable for sintered aluminum alloy parts which require high dimensional accuracy such as a gear rotor and a side plate, and the method of producing such an aluminum alloy powder.
An iron alloy powder and the method of producing a sintered iron alloy compact are related in particular to the iron alloy powder having high fluidity which improves the dimensional accuracy of a green compact, which results in an improved dimensional accuracy of the sintered body, and the method of producing the sintered iron alloy compact.
A metal powder and a sintered metal compact are related in particular to a metal powder having high fluidity and a sintered body using such a metal powder.

Background Art

First, the description of an aluminum alloy powder will be given.
In recent years, lighter weight is required for drive system parts and valve system parts so that the application of an aluminum alloy instead of an iron-based material is being considered as a way of achieving such lighter weight. Among the aluminum alloys, in particular, sintered aluminum alloys having good mechanical characteristics such as high strength, high hardness, high rigidity, and high heat resistance show great promise. All parts require high dimensional accuracy, for instance, as represented by a gear rotor which is one example of drive system parts.
When compared with the iron-based powder, however, aluminum alloy powder that is a raw material powder has a lower density so that its force to drop downward in the vertical direction is small. In addition, the aluminum alloy powder is generally formed by the atomization method so that the particles are fine-grained. What is more, the powder surface has an irregular shape with uneven features as shown in Fig. 15. Thus, this results in a problem of the fluidity (flowability) of the aluminum alloy powder and its fillability into the die cavity being significantly inferior when compared with the case of the iron-based powder.
As a result, when conventional aluminum alloy powder is supplied into a die cavity to fill the die cavity, there may exist some voids within the filled powder due to bridging between the powder particles. Thus, if the aluminum alloy powder is compacted in this condition, the voids would be dispersed unevenly inside the compacted body. Consequently, the density of the compacted body becomes nonuniform, which may create a crack on the surface of the compacted body or degrade the dimensional accuracy of the compacted body. Moreover, when the green compact is sintered, the sintering will progress locally so that the dimensional accuracy of the sintered body can also be degraded.
To solve such problems, as described in Japanese Patent Laying-Open No. 04-154902 and as shown in Fig. 16, for instance, a dry granulation method is applied in which primary powder particles 2 of aluminum are mechanically joined by rolling, and are thereafter crushed to produce secondary powder particles 11 having a particle diameter larger than that of a primary powder particle 2. A secondary powder particle 11 obtained by this method has a flake-like structure in which primary powder particles 2 made flat by rolling are firmly attached to one another as shown in Fig. 17, and has a larger particle diameter than primary powder particle 2. In this way, by making the aluminum alloy powder coarse-grained, it was discovered that the fluidity of the aluminum alloy powder improved.
In the case of the secondary powder particles of aluminum formed by the dry granulation method, however, when the secondary powder particles undergo vibration or shock during transportation or transfer, the primary powder particles being mechanically attached to one another in the secondary powder particles would separate once again, which results in many fine powder particles existing among the secondary powder particles. As a result, the fluidity of the aluminum alloy powder changes with time, leading to the above-described problems.
Now, the description related to an iron alloy powder will be provided.
Such methods as a traditional casting method and cutting from steel sheets are employed as methods of manufacturing machine parts such as automobile parts and home electric appliances. Other methods include sintering in which a metal powder is filled into a die cavity, a pressure is applied to form a green compact, and the resulting green compact is treated by sintering in a sintering furnace to join the metal powder particles.
The advantages of the powder metallurgy method are that it allows for a greater production than the casting method and that it involves a lower manufacturing cost than the cutting method.
Poor fluidity of a raw material metal powder may cause choking within a hopper, which is a supply tank of the metal powder, or may result in the difficulty of the metal powder entering into the die cavity from a powder feed box (shoebox) to fill the die cavity. As a result, a portion having relatively more metal powder particles and a portion having less metal powder particles would be created within the die cavity.
Moreover, the metal powder particles do not move easily inside the die cavity after it is filled into the die cavity. Thus, when a green compact is formed by the application of pressure, density dispersion would result within the green compact. In addition, density variation was found among the individual green compacts.
When such a green compact is treated by sintering, the density dispersion of the sintered body produced by sintering was sometimes found to become greater due to the different shrinkage behavior owing to the difference in density. This sometimes resulted in the degradation in dimensional accuracy of the sintered body. Therefore, the produced sintered body could not be directly applied as a machine part but required further machining. As a result, the problem of increased manufacturing cost of the product arose.
On the other hand, in the techniques relating to powders for powder metallurgy, one or more kinds of metallic particles or non-metallic particles are added to the main component metal powder. At this time, the segregation of a component other than the metal powder cannot be avoided when the metal powder is supplied such that the segregation of the component inside the green compact results in degradation in the dimensional accuracy of the sintered body after sintering or leads to degradation in the mechanical strength of the sintered body.
In order to deal with such component segregation, techniques are proposed in which non-metallic particles are caused to adhere to the main component metallic particles (for instance, Japanese Patent No. 2898461, Japanese Patent Laying-Open No. 5-148505, etc.).
With the metal powder in which the component segregation is prevented, however, it was found to be difficult to fill the metal powder uniformly within the die cavity to keep the density within the green compact uniform so as to improve the dimensional accuracy of the green compact and the sintered body.
The present invention was made to solve the above-described problems, and an object of the present invention is to provide an aluminum alloy powder having good fluidity and good fillability and a method of producing such an aluminum alloy powder.
Moreover, another object of the present invention is to provide an iron alloy powder having high fluidity that allows uniform filling within a die cavity and reduction in the density dispersion within a green compact to improve the dimensional accuracy of a sintered body after sintering the green compact, and a method of producing a sintered iron alloy compact.
In addition, a further object of the present invention is to provide a metal powder that has excellent fluidity and that can be uniformly filled into a die cavity, and a sintered metal compact.

Disclosure of the Invention

The present inventors have invented an aluminum alloy powder having excellent fluidity and fillability and the method of producing such an aluminum alloy powder as a result of various experiments and consideration. First, the configuration will be illustrated below.
The aluminum alloy powder according to the first aspect of the present invention is an aluminum alloy powder formed of secondary powder particles produced by binding together using a binder primary powder particles whose main component is aluminum.
The aluminum alloy powder according to the second aspect of the present invention is an aluminum alloy powder formed of secondary powder particles produced by binding together primary powder particles whose main component is aluminum, where the secondary powder particles are aluminum alloy powder particles whose average value of the acicular ratio derived from the following equation is 2.0 or below, where a projected image is obtained by projecting upon a secondary powder particle:
Acicular ratio = maximum diameter in a projected image of one particle / a diameter of projected image in a direction orthogonal to maximum diameter.
Preferably, primary powder particles are rapidly solidified powder particles obtained by an atomization method.
Preferably, the particle diameter of a secondary powder particle is between 10 µm and 500 µm.
Preferably, the particle diameter of a primary powder particle is between 5 µm and 300 µm.
Among the secondary powder particles, the secondary powder particles having a particle diameter of at least 50 µm are preferably 25 percent by weight or below of the entire secondary powder particles.
The fluidity of secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 2.6 mm based on the Test of Determination of Flow Rate of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2502) is preferably 4.0 seconds/cm³ or below.
The fluidity of secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 4.0 mm based on the Test of Determination of Flow Rate of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2502) is preferably 2.5 seconds/cm³ or below.
The apparent density of secondary powder particles measured based on the Method of Determination of Apparent Density of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2504) is preferably between 80% and 100% of the tap density of the secondary powder particles measured based on the Method for determination of tap density of metal powders according to Japan Powder Metallurgy Association (JPMA P 08).
It is preferred that the average value of circularity derived from the following equation is 0.6 or greater, where the projected image is obtained by projecting upon a secondary powder particle:
Circularity = 4π × (area of projected image of one particle) / (length of outer periphery of projected image of one particle)².
It is preferred that the average value of the acicular ratio derived from the following equation is 2.0 or below, where the projected image is obtained by projecting upon a secondary powder particle:
Acicular ratio = maximum diameter in projected image of one particle / diameter of projected image in a direction orthogonal to maximum diameter.
The binder is preferably an organic binder.
The amount of the organic binder in the secondary powder particles is preferably between 0.05 and 0.5 percent by weight.
The decomposition temperature of the organic binder preferably is at most 400°C.
The organic binder preferably includes as the main component any one organic compound selected from the group consisting of polyvinyl alcohol, polyvinyl methyl ether, carboxymethylcellulose, and hydroxyethyl cellulose.
The method of producing an aluminum alloy powder according to the third aspect of the present invention includes a granulation step and a drying step. In the granulation step, primary powder particles having a particle diameter of 5 µm to 300 µm whose main component is aluminum are bound together using an aqueous organic binder solution to form secondary powder particles. In the drying step, the moisture contained in the secondary powder particles is removed.
Preferably, the primary powder particles are rapidly solidified powder particles obtained by the atomization method.
The granulation step preferably includes allowing primary powder particles to be suspended within a fluidized bed and spraying an aqueous organic binder solution onto the suspended primary powder particles.
The drying step preferably includes drying secondary powder particles inside the fluidized bed.
The drying step preferably includes introducing a gas of a prescribed temperature into the fluidized bed.
The temperature inside the fluidized bed during the drying step is preferably between 60°C and 120°C, and more preferably between 70°C and 90°C.
The concentration of the aqueous organic binder solution in the granulation step is preferably between 1% and 8%.
Now, the characteristics and the function and the effect of an aluminum alloy powder and the method of producing the aluminum alloy powder will be described.
Characteristics of Aluminum Alloy Powder Formed of Secondary Powder Particles
Particle Diameter of Primary Powder Particle (Raw material Powder)
As a starting raw material powder, an aluminum alloy powder whose particle diameter is between 5 µm and 300 µm and whose main component is aluminum is used. The raw material powder is a rapidly solidified powder obtained by the atomization method. The present inventors have found that the fluidity and the fillability of the powder into a die cavity, which are the conventional target problems to be solved, could be improved by employing secondary powder particles obtained by binding together primary powder particles serving as the starting raw material powder particles.
The secondary powder particles are formed, first, by allowing primary powder particles to be suspended within a fluidized bed, and by spraying an aqueous organic binder solution onto the primary powder particles to bind them together, and thereafter removing the moisture by drying. When the moisture is removed by drying, in the case in which the particle diameter of a primary powder particle is less than 5 µm, the specific surface area of a secondary particle powder becomes large so that oxidation during sintering progresses significantly, inhibiting the sinterability between the secondary powder particles. Moreover, a specific surface area refers to the sum of the surface areas of all powder particles contained within the powder of a unit quantity (volume).
On the other hand, when the particle diameter of a primary powder particle exceeds 300 µm, the particle diameter of the secondary powder particle exceeds 500 µm or 1 mm so that coarse secondary powder particles are formed, which also degrades the fluidity and fillability of secondary powder particles.
Thus, according to the present invention, the particle diameter of the primary powder particles of aluminum alloy which are the starting raw material powder particles is desirably between 5 µm and 300 µm, and the particle diameter of 40 µm to 200 µm is more preferable from the viewpoint of the ease of handling, efficient economy, and so on, of the primary powder particles.
Particle Diameter of Secondary Powder Particles (Composite Granulated Powder)
A secondary powder particle according to the present invention is a secondary powder particle 1 obtained by binding together primary powder particles 2 using an organic binder 3, as shown in Fig. 1. The particle diameter of the secondary powder particle is desirably between 10 µm and 500 µm. When the particle diameter of the secondary powder particle is less than 10 µm, the secondary powder particles, being fine grain powder particles, do not provide sufficient fluidity, and as a result, it becomes difficult to produce a green compact having uniform density distribution.
On the other hand, when the particle diameter of the secondary powder particle exceeds 500 µm, the secondary powder particles become coarse grain powder particles such that the fluidity and fillability of the secondary powder particles are degraded. Thus, according to the present invention, the particle diameter of a secondary powder particle is desirably between 10 µm and 500 µm. More preferably, the particle diameter of the secondary powder particle is between 60 µm and 250 µm in order to suppress the wedging of the particles into a gap of the die cavity and to stabilize the fluidity and the fillability of the secondary powder particles.
Moreover, in such secondary powder particles, it is desirable that the content of secondary powder particles having a particle diameter of 50 µm or below in the entire secondary powder particles is 25 percent by weight or below. When the content of the secondary powder particles having a particle diameter of 50 µm or below exceeds 25 percent by weight, the fluidity of the secondary powder particles, which is measured using a funnel-like orifice tube having a bore diameter of 2.6 mm based on the method of measuring the fluidity defined by the Test of Determination of Flow Rate of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2502 (established in 1958, revised in 1979)), exceeds 4.0 seconds/cm³ so that sufficient fluidity cannot be obtained.
Consequently, when using a die cavity having a thickness of 1 mm or below which makes it extremely difficult for the powder to be fed into the die cavity, for instance, the secondary powder particles cannot be filled uniformly with high speed so that it becomes difficult to produce a green compact having uniform density distribution. The fluidity of the secondary powder particles, however, when measured using a funnel-like orifice tube having a bore diameter of 4.0 mm based on the above-described Test of Determination of Flow Rate according to the Japanese Industrial Standards, is 2.5 seconds/cm³, which satisfies the appropriate range defined by the present invention.

Fluidity of Secondary Powder Particles

The fluidity of the secondary powder particles according to the present invention measured using a funnel-like orifice tube having a bore diameter of 2.6 mm based on the above-described Test of Determination of Flow Rate according to the Japanese Industrial Standards is 4.0 seconds/cm³ or below. In addition, the fluidity of the secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 4.0 mm based on the above-described Test of Determination of Flow Rate according to the Japanese Industrial Standards is 2.5 seconds/cm³ or below.
The present inventors have discovered for the first time that the secondary powder particles that satisfy such fluidity requirement prove effective in economically producing a green compact for a complex-shaped part that requires high dimensional accuracy. In the case in which the fluidity of the secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 2.6 mm exceeds 4.0 seconds/cm³ and in the case in which the fluidity of the secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 4.0 mm exceeds 2.5 seconds/cm³, the time required for the secondary powder particles to be filled uniformly into a die cavity becomes long so that the productivity is significantly degraded.
In particular, improvement in the fillability of the secondary powder particles becomes even more important when the secondary powder particles are to be fed into a die cavity having a complex shape such as for a gear rotor or into a die cavity having a small wall thickness. In light of the above, the fluidity of the secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 4.0 mm is desirably 2.0 seconds/cm³ or below.
Fillability (Ratio of Apparent Density and Tap density) of Secondary Powder Particles
With regard to the fillability of the secondary powder particles according to the present invention, the apparent density (AD) of the secondary powder particles measured based on the Method of Determination of Apparent Density of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2504 (established in 1960, revised in 1979)) is from 80% to 100% of the tap density (TD) of the secondary powder particles measured based on the Method for determination of tap density of metal powders according to Japan Powder Metallurgy Association (JPMA P 08-1992).
The present inventors have discovered for the first time that the secondary powder particles satisfying such a ratio of the apparent density and the tap density are effective in economically producing a green compact for a complex-shaped part that requires high dimensional accuracy. The AD/TD value is approximately 60% to 70% for the primary particle powder of aluminum alloy serving as the starting raw material powder. Thus, in general, from the industrial point of view, it is devised to achieve a highly filled state closer to the tap density by reciprocating the powder feed box (shoe box) several times or by providing vibration when filling the aluminum alloy powder into a die cavity.
On the other hand, the secondary powder particles according to the present invention, when compared with the primary powder particles, have a significantly improved value of the ratio of the apparent density and the tap density (AD/TD) which is 80% or greater. Thus, the secondary powder particles according to the present invention can be filled more densely to achieve the filled state close to the tap density when feeding the secondary powder particles from the powder feed box or the like into a die cavity. As a result, the compaction time can be shortened.
When the ratio of the apparent density and the tap density of the secondary powder particles according to the present invention is less than 80%, no significant effect of improvement in the fillability of the secondary powder particles in comparison with the primary powder particles is derived so that it becomes difficult to achieve the effect of significant economy by the reduction in the compaction time. The ratio of the apparent density and the tap density in the secondary powder particles according to the present invention, more preferably, is between 85% and 100%. Moreover, the AD value never exceeds the TD value so that the value of AD/TD is 100% or below.

Shape (Circularity, Acicular Ratio) of Secondary Powder Particles

As shown in Fig. 2, in a projected image la obtained by projecting upon a secondary powder particle according to the present invention, the average value of circularity derived from equation (1) below is 0.6 or greater, where S represents the area per particle, and GL represents the length of the outer periphery of the projected image of one particle.
In addition, as shown in Fig. 3, the average value of the acicular ratio derived from equation (2) below is 2.0 or below, where Lmax represents the maximum diameter in projected image la of one particle, and L represents the diameter of the projected image in a direction orthogonal to the maximum diameter. The present inventors have discovered that the secondary powder particles satisfying the above-described conditions with respect to circularity and the acicular ratio are effective in improving the fluidity of the secondary powder particles. Circularity = 4π × (area S of projected image of one particle) / (length GL of outer periphery of projected image of one particle)2 Acicular ratio = maximum diameter Lmax in projected image of one particle / diameter L of projected image in a direction orthogonal to maximum diameter
The circularity and the acicular ratio defined by equations (1) and (2) both become indices representing the sphericity of the secondary powder particles. Thus, the greater the circularity and the smaller the acicular ratio (it must be at least 1.0, however), the shape of a secondary powder particle becomes closer to a sphere so that the fluidity of the secondary powder particles improves as a result.
When the secondary powder particles have a circularity that is less than 0.6 or an acicular ratio that exceeds 2.0, the secondary powder particles no longer satisfy the fluidity defined by the present invention. Further, as described above, for the fluidity of the secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 4.0 mm to be 2.0 seconds/cm³ or below, it is preferred that the average value of the circularity is at least 0.8 and that the average value of the acicular ratio is at most 1.5. In addition, for the purpose of comparison, conventional secondary powder particles shown in Fig. 11 have an acicular ratio of approximately 5 to 10.

Secondary Powder Particles Granulated Using an Organic Binder

As shown in Fig. 4, secondary powder particles 1 according to the present invention is formed by allowing primary powder particles 2 to be suspended within a fluidized bed 4 using a gas 5 such as nitrogen, and at the same time, by spraying an aqueous organic binder solution 7 from a nozzle 6 onto the suspended primary powder particles, thereby binding together primary powder particles 2.
The amount of the organic binder contained in the secondary powder particles is between 0.05 and 0.5 percent by weight of the weight of the secondary powder particles.
Conventionally, as shown in Fig. 10, it was possible to produce coarse secondary powder particles by mechanically joining the primary powder particles by subjecting the primary powder particles to plastic forming such as rolling and compression using a roller compactor, a press, or the like. A secondary powder particle produced by such techniques, however, is not spherical but is closer to a flat flake-like shape as shown in Fig. 11 so that it was difficult sufficiently to improve the fluidity of the secondary powder particles.
In addition, as described above, when the secondary powder particles produced by such mechanical joining undergo vibration or shock during transportation or transfer, the secondary powder particles, being mechanically attached to one another, would separate, producing many fine powder particles, such that the fluidity of the secondary powder particles changes with time.
In order to solve such problems, the present inventors have discovered that the use, as a binder, of an organic binder was effective that has a strong binding force, that allows the produced secondary powder particles to have a spherical shape, and that has such a characteristic as becoming decomposed in a heating process so that it does not remain in the sintered body when a green compact is sintered.
In particular, from the viewpoint of safety and efficient economy, an aqueous binder solution is used in which water and not an alcohol-based organic solvent is used as a solvent for the binder. The amount of the organic binder within the produced secondary powder particles is desirably between 0.05 and 0.5 percent by weight of the entire secondary powder particles. When the amount of the organic binder is less than 0.05 percent by weight, the primary powder particles are not sufficiently bound together, leaving behind fine-grained primary powder particles so that the fluidity of the secondary powder particles cannot be improved sufficiently.
On the other hand, the amount of the organic binder exceeding 0.5 percent by weight does not further improve the fluidity of the secondary powder particles, but instead, the secondary powder particles are further bound together to form coarser-grained powder whose particle diameter exceeds 500 µm such that the fluidity of the secondary powder particles is degraded. Moreover, the longer time required to remove the organic binder within the secondary powder particles by thermal decomposition leads to inefficient economy.
Therefore, the amount of the organic binder within the secondary powder particles according to the present invention is desirably between 0.05 and 0.5 percent by weight of the entire secondary powder particles, and more preferably, is between 0.15 and 0.4 percent by weight if the balance between the improvement in the fluidity of the secondary powder particles and the reduction of time required for organic binder removal is to be considered.

Characteristics and Types of Organic Binders

In order to remove the organic binder as described above, a preliminary heating step (binder removal step) in a temperature zone that is lower than the sintering temperatures is required before sintering a green compact. The reduction in the processing time of this step significantly contributes to the improvement of economy.
Generally, sintering of an aluminum alloy powder progresses in the temperature range of 430°C to 570°C. Moreover, since reductive decomposition of an aluminum oxide coating film by magnesium at the surface of the aluminum alloy starts from the temperature of 400°C in the method of producing AlN (aluminum nitride) by direct nitriding reaction discovered by the present inventors, and for other reasons, the decomposition temperature of the organic binder contained in the secondary powder particles according to the present invention is desirably 400°C or below.
When the decomposition temperature of the organic binder exceeds 400°C, some organic binder that does not decompose remains in the interface of the secondary powder particles so that the sintering among the secondary powder particles may be inhibited or the direct nitriding reaction between the aluminum and the nitrogen gas may be suppressed disadvantageously. More preferably, the application of an organic binder having a decomposition temperature of 350°C or below provides an advantage with respect to the production techniques such as the temperature control of the binder removal step.
Organic binders that satisfy the above-described characteristics and dissolve in water include polyvinyl alcohol (PVA), polyvinyl methyl ether (PVME), carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC) and the like. In addition to those given above, polyvinyl butyral (PVB), fatty ester, phenolic resin, polyvinyl ethyl ether (PVAE), polyvinyl isobutyl ether (PVIE) and the like have the effect of binding together primary powder particles and also have a decomposition temperature that is 400°C or below. These resins, however, either dissolve only in an alcohol-based organic solvent and not in water or do not readily dissolve in water so that they are not desirable for application to the production method according to the present invention.
Method of Producing Aluminum Alloy Powder Formed of Secondary Powder Particles
As described above, the method of producing an aluminum alloy powder according to the present invention allows the production of the desirable secondary powder particles described above by repeating a granulation step and a drying step, where the granulation step involves, as shown in Fig. 4, first allowing primary powder particles 2 whose main component is aluminum and whose particle diameter is from 5 µm to 300 µm already described as starting raw material powder particles to be suspended within fluidized bed 4, and then spraying the previously described aqueous organic binder solution 7 onto the suspended primary powder particles 2 to bind primary powder particles 2 together to produce secondary powder particles 1, and the drying step involves removing the moisture of aqueous binder solution 7 contained within secondary powder particles 1.

Spraying of Aqueous Organic Binder Solution

Primary powder particles are bound together by the binding force of the aqueous organic binder solution. At this time, since coarse secondary powder particles would be formed if a large droplet comes into contact with the primary powder particles, it proves effective to apply the aqueous organic binder solution to the primary powder particles in the form of the minutest possible droplets. As a consequence, the present inventors have found that secondary powder particles having a prescribed particle diameter can be obtained by spraying the aqueous organic binder solution using the principles of a spray or an atomizer.
Particularly, from the viewpoint of effecting uniform application to the primary powder particles, the present inventors have discovered that it was effective to allow primary powder particles 2 to be suspended within fluidized bed 4 by allowing air or nitrogen gas 5 to flow in from the lower portion of fluidized bed 4, and spraying aqueous organic binder solution 7 onto the suspended primary powder particles 2 from an upper portion of fluidized bed 4 as shown in Fig. 4 when an organic binder is sprayed in the form of a mist. As a result, it was confirmed that secondary powder particles having a prescribed particle diameter can be uniformly produced without aggregation or segregation.
Conversely, when the organic binder is sprayed onto the primary powder particles without letting air or a nitrogen gas flow in from the lower portion of the fluidized bed, and thus without allowing the primary powder particles to be suspended, not only is it impossible to produce uniform secondary powder particles, but also coarse secondary powder particles would be formed.

Temperature within Fluidized Bed

In the method of producing the secondary powder particles described above, the temperature within the fluidized bed is between 60°C and 120°C, and more preferably, is between 70°C and 90°C. It is necessary to control the temperatures of the air or the nitrogen gas that flows within the fluidized bed in order to remove the moisture within the aqueous organic binder solution contained in the secondary particle powder.
If the temperature is below 60°C, the drying step to remove the moisture within the secondary powder particles and the secondary powder particles themselves would take a long time, resulting in inefficient economy. At the same time, the moisture would remain in the secondary particle powder for a long time in a high temperature so that the secondary powder particles would be oxidized. Moreover, the secondary powder particles would be oxidized if the secondary powder particles are stored with some moisture remaining in the secondary particle powder.
On the other hand, when the temperature within the fluidized bed exceeds 120°C, no significant drying effect is observed, while a problem arises relating to the ease of handling when the secondary powder particles are taken out after drying. In addition, when the temperature of an inflow gas exceeds 120°C, the moisture would evaporate from the aqueous organic binder solution when the solution is being sprayed from a spray, and the organic binder would solidify and be fixed at the tip of the spray nozzle, clogging the nozzle so that the organic binder can no longer be sprayed evenly. Moreover, the temperature within the fluidized bed is more preferably between 70°C and 90°C in order to achieve a good drying efficiency and to facilitate taking out of the secondary powder particles after drying.

Concentration of Aqueous Organic Binder Solution

In the method of producing the secondary powder particles described above, the concentration of the aqueous organic binder solution is desirably between 1% and 8%. When the concentration of the aqueous organic binder solution is less than 1%, sufficient binding force cannot be obtained so that it becomes difficult to produce the prescribed secondary powder particles defined by the present invention.
On the other hand, when the concentration of the aqueous organic binder solution exceeds 8%, the viscosity of the organic binder would be too great such that fine, mist-like droplets cannot be obtained when spraying the aqueous organic binder solution, which produces as a result coarse secondary powder particles whose particle diameter exceeds 500 µm.
Thus, the concentration of the aqueous organic binder solution employed in the present invention is desirably between 1% and 8%, and the effect of efficient economy such as shortening of the granulation step and the drying step due to the reduction in the amount of the aqueous binder solution can further be achieved with a concentration of at least 2%. Moreover, an effect of efficient economy such as the reduction of the spray pressure during spraying can be achieved with a concentration of the aqueous organic binder solution that is at most 5%. From the above, the concentration of the aqueous organic binder solution is more preferably between 2% and 5%.
In addition, the present inventors have invented an iron alloy powder having excellent fluidity and a method of producing a sintered iron alloy compact having high dimensional accuracy as a result of various experiments and considerations. The configuration will be illustrated below.
The iron alloy powder according to the fourth aspect of the present invention is an iron alloy powder formed of secondary powder particles produced by binding together using a binder iron-based primary powder particles whose main component is iron or, by binding together using a binder iron-based primary powder particles whose main component is iron along with primary powder particles of a non-ferrous component.
Preferably, the mean particle diameter of iron-based primary powder particles is between 20 µm and 100 µm, while the mean particle diameter of secondary powder particles is between 50 µm and 200 µm.
Preferably, among the secondary powder particles, the secondary powder particles having a particle diameter that is 45 µm or below are at most 10 percent by weight of the entire secondary powder particles.
Preferably, the value of the surface area of a secondary particle powder derived from the BET (Brunauer-Emmett-Teller) isothermal adsorption formula is 0.08 m²/g or below.
Preferably, a surface of a secondary powder particle is covered with a binder.
More preferably, the binder is an organic binder.
Still more preferably, the amount of the organic binder within the secondary powder particles is between 0.05 and 5 percent by weight.
More preferably, the organic binder includes as the main component any one organic compound selected from the group consisting of polyvinyl alcohol, polyvinyl ether, polyethylene oxide, methyl cellulose and carboxymethylcellulose.
Preferably, with a horizontal splitting cell for tensile strength measurement formed by a movable cell and a fixed cell, an iron alloy powder is filled into the cell, a prescribed load is applied from above the iron alloy powder to form an iron alloy powder bed having a porosity that is between 0.5 and 0.7, and the movable cell is pulled in the horizontal direction, causing the iron alloy powder bed to fail, at which point the tensile failure strength is derived which is 100 Pa or below.
Preferably, with a parallel plates cell for shear strength measurement formed by a fixed plate and a movable plate respectively provided with a notch, an iron alloy powder is inserted between the fixed plate and the movable plate, a pressure is applied from above the iron alloy powder to form an iron alloy powder bed having a porosity that is between 0.5 and 0.7, and the movable plate is pulled parallel to the fixed plate under a prescribed load to cause the iron alloy powder to be sheared, at which point the shear stress is derived, and the unconfined yield stress derived from the powder yield locus indicating the relation between the load and the shear stress is at most 300 Pa.
More preferably, the value of the ratio obtained by dividing the value of a major consolidating stress, derived from Mohr's stress circle that is in contact with the powder yield locus, by the value of the unconfined yield stress is 10 or above.
The method of producing a sintered iron alloy compact according to the fifth aspect of the present invention includes the following steps. Iron-based primary powder particles whose main component is iron are bound together using a binder, or iron-based primary powder particles whose main component is iron are bound together along with primary powder particles of a non-ferrous component using a binder, to form secondary powder particles. The secondary powder particles are compacted to form a green compact. The prescribed dimension of the green compact is measured. The green compact is sintered to form a sintered body. The prescribed dimension of the sintered body corresponding to the prescribed dimension of the green compact is measured. The value is 1.3 or below obtained by dividing the value that is six times the standard deviation of the prescribed dimension of the sintered body by the value which is six times the standard deviation of the prescribed dimension of the green compact.
Preferably, a step of adding a lubricant to the secondary powder particles is included between a granulation step for forming the secondary powder particles and a step of compaction.
Now, the characteristics and the function and effect of an iron alloy powder and a sintered iron alloy compact will be described.
Characteristics of Iron-Based Primary Powder Particles (Raw material Powder) and Characteristics of Secondary Powder Particles

Mean Particle Diameter, Particle Size, and Specific Surface Area

The mean particle diameter of secondary powder particles formed by binding together using a binder iron-based primary powder particles having a mean particle diameter that is between 20 µm and 100 µm was found to be between 50 µm and 200 µm. Moreover, the secondary powder particles whose mean particle diameter is within the above-described range were discovered to have high fluidity.
This is due to the smaller inter-particle forces resulting from reduction in the number of points of contact between the particles as a result of the particle diameter of the secondary powder particles becoming larger than the particle diameter of the iron-based primary powder particles. Moreover, it was found that the desirable ratio of the particles having a particle diameter of 45 µm or below (a screen of 325-mesh or below) included in the secondary powder particles was at most 10 percent by weight. This is because a ratio exceeding 10 percent by weight increases the frictional force between the particles caused by the fine particles, which results in the degradation of fluidity of the secondary powder particles.
Furthermore, it became apparent that the desirable specific surface area of the secondary particle powder obtained by the BET method was 0.08 m²/g or below. The value of the specific surface area obtained by the BET method is closely related to the particle diameter and the particle size distribution of a powder. If this value greatly exceeds 0.08 m²/g, it indicates that a relatively large amount of fine particles is contained in it. A large content of fine particles causes the degradation of the fluidity of the secondary powder particles as described above.
In the BET method, a surface area of a solid is derived based on the Brunauer-Emmet-Teller isothermal adsorption formula.
In addition, to the iron alloy powder, normally, particles of nickel (Ni), copper (Cu), carbon (C) or the like are added besides the iron powder which is the main component, and a solid lubricant such as zinc stearate or wax or the like is added in order to prevent the alloy powder from galling to a die cavity during compaction.
It was found that a better improvement in the fluidity was achieved in the case where these solid lubricants were added to the secondary powder particles obtained when compared with the case where similar solid lubricants were added to the iron-based primary powder particles.

Dynamic Characteristics

There is a plurality of indices indicating the fluidity of a powder. Among them, a method of measuring the fluidity of a powder by measuring the tensile strength and the shearing strength of a powder bed to which a pre-compression load is applied is widely known. The fluidity of a powder is governed by the interaction between the particles. Two kinds of interaction between the particles are adhesive force and frictional force. It is said that the tensile strength of a powder bed is related to the adhesive force, and the shearing force is related to the frictional force. Especially, the smaller the tensile strength of the powder bed, the smaller the adhesive force and the higher the fluidity.
As shown in Fig. 9, the tensile strength (breaking force) of a powder bed was measured using a horizontal splitting cell for tensile strength measurement 21 (a powder bed tester produced by Sankyo Piotech Co. Ltd.) formed by a movable cell 22 and a fixed cell 23.
Specifically, an iron alloy powder was filled into cell 21, and a pre-compression load was applied to the iron alloy powder to form an iron alloy powder bed 24 having a porosity of 0.5 to 0.7. Then, movable cell 22 was pulled in the horizontal direction, and the tensile strength at which iron alloy powder bed 24 broke was measured. As a result, it became apparent that the iron alloy powder demonstrated high fluidity when the value of the tensile strength is 100 Pa or below.
In addition, as shown in Fig. 10, the flow function of the powder was derived using a parallel plates cell for shear strength measurement formed by a fixed plate and a movable plate respectively provided with notches.
Specifically, first, the powder bed whose porosity was adjusted to 0.5 to 0.7 by a pre-compression load was inserted between the fixed plate and the movable plate. While a load a was exerted from above onto the movable plate, a shearing force was applied to the movable plate parallel to the fixed plate. Then, the shearing force τ was derived at the time at which sliding occurred in the powder bed and the powder bed collapsed. The respective shearing forces τ were derived using three different loads.
From a set of values of shearing force τ and load σ obtained, a cohesion strength c and a shear index n were calculated with the following Warren-Spring formula to derive the powder yield locus PYL: (τ/ c)n = 1 + (σ / σT) τ: shear stress, σ: load, σ_T: tensile stress, c: cohesion strength, n: shear index
As shown in Fig. 11, this powder yield locus was plotted in the σ-τ coordinates.
A Mohr's stress circle A passing through the origin of the σ-τ coordinates and being in contact with the powder yield locus was derived. The value (F_c) of the stress at the point of intersection of Mohr's stress circle A and σ axis was derived. The stress at this point of intersection corresponds to the stress required for breaking down a cylindrical green compact having a unit area, and is referred to as an unconfined yield stress.
Then, a value σ₁ of the stress at the point of intersection of the σ axis and a Mohr's stress circle B being in contact with a point E in the powder yield locus was derived. This stress σ₁ is specifically referred to as a major consolidating stress or a major compression stress. Moreover, the point E in the powder yield locus was the point at which the shear fracture occurred to the powder bed with no change in the porosity of the powder bed.
A value was derived by dividing the value of major consolidating stress σ₁ by the value of unconfined yield stress F_c. This value is in general referred to as a flow function. It is known that the fluidity of a powder is high when the value of the flow function is greater than 10.
In addition, porosity refers to the proportion of void within the bulk volume of a powder. Specifically, a filling function is derived by dividing the weight of the powder by the product of the true density of the powder times the bulk volume of the powder, and the subtraction of this value of filling function from 1 produces the value of porosity.

Organic Binder

An iron alloy powder formed of secondary powder particles is produced by binding together using a binder iron-based primary powder particles whose main component is iron, or binding together iron-based primary powder particles whose main component is iron along with primary powder particles of a non-ferrous component. The method of producing an iron alloy powder is the same as the method of producing an aluminum alloy powder described above.
The amount of organic binder within the secondary powder particles is desirably in the range of 0.05 to 5 percent by weight. When the amount of organic binder is less than 0.05 percent by weight, sufficient granulation of the secondary powder particles does not take place so that many fine primary powder particles remain. Consequently, the fluidity of the secondary powder particles does not improve as much as the primary powder particles. Moreover, due to the weak binding force of the secondary powder particles, the secondary powder particles might break up when conveyed.
On the other hand, when the amount of the organic binder exceeds 5 percent by weight, the component of the organic binder might remain even when heat treatment is performed to remove the organic binder after the green compact is formed. As a result, carbon (C), existing in the organic binder, may cause voids to be generated in the sintered body, which reduces the mechanical strength of the sintered body. In addition, when the amount of the organic binder is more than 5 percent by weight, the secondary powder particles may further be bound together to form coarse powder particles.
A coating film of organic binder is formed on a surface of a secondary powder particle. Thus, the stress can be reduced that is produced when ejecting from a die cavity a green compact produced by filling secondary powder particles into a die cavity and compacting the secondary powder particles. Consequently, the amount of a solid lubricant such as zinc stearate and wax normally added in order to prevent the galling to the die cavity can be reduced.
Such solid lubricants are known to degrade the fluidity of a powder due to their nature and the fact that they are in general fine-grained. Thus, the fluidity of secondary powder particles can be improved by limiting the added amount of such solid lubricants.
In the case of the present secondary powder particles, it was found that the degradation of the fluidity of secondary powder particles can still be prevented even when a solid lubricant is added. It is surmised that this is due to the frictional force between the fine powder and the lubricant being reduced when the amount of fine powder is reduced by granulating the secondary powder particles, and due to the electrostatic force of attraction acting between the organic binder and the solid lubricant trapping the lubricant.
A desirable organic binder is one the contains as a main component at least one or more kinds of organic compounds from polyvinyl alcohol, polyvinyl ether, polyethylene oxide, methyl cellulose, and carboxymethylcellulose. These organic binders are water-soluble, have strong adhesion, and easily decompose at 500°C or below so that they are suitable for the granulation of the iron alloy powder formed of the present secondary powder particles.

Method of Producing Sintered Iron Alloy Compact

In the method of producing a sintered iron alloy compact according to the present invention, secondary powder particles are produced by binding together using a binder iron-based primary powder particles whose main component is iron, or binding together using a binder iron-based primary powder particles whose main component is iron along with primary powder particles of a non-ferrous component. A pressure is applied to the secondary powder particles to form a green compact, and the dimension of one green compact particularly in a direction of compression is measured in four locations. Then, the green compact is sintered to form a sintered body. The dimension of the produced sintered body is measured in the corresponding four locations.
When an iron alloy powder formed of the present secondary powder particles is employed, the value is 1.3 or below obtained by dividing the value that is six times the standard deviation value of the dimension of the sintered body by the value that is six times the standard deviation value of the dimension of the green compact.
The present secondary powder particles having high fluidity are uniformly filled into a die cavity such that the density dispersion within the green compact and the density variation among the green compacts are significantly reduced.
When sintering such green compacts, the shrinkage behavior becomes substantially the same due to the small density dispersion, and thus the density dispersion within a sintered body and the density variation among the sintered bodies are reduced.
As a result, the value becomes 1.3 or below which is derived by dividing the value that is six times the standard deviation value of a prescribed dimension of a sintered body by the value that is six times the standard derivation value of a prescribed dimension of a green compact.
In addition, as described above, solid lubricants such as zinc stearate and wax added to prevent the galling to the die cavity are known normally to degrade the fluidity of a powder.
In the case of the present secondary powder particles, however, it was discovered that the degradation of the fluidity of the secondary powder particles could be prevented even when such solid lubricants were added. As a result, it was confirmed that a sintered body having a high dimensional accuracy could be produced with the present secondary powder particles with the addition of a solid lubricant.
Further, the present inventors have invented a metal powder having high fluidity and a sintered metal compact having a high dimensional accuracy. The configuration will be illustrated below.
A metal powder according to the fifth aspect of the present invention is formed of secondary powder particles produced by binding together using a binder primary powder particles whose main component is metal.
Preferably, the binder is an organic binder.
A sintered metal compact according to the sixth aspect of the present invention is obtained by sintering a green compact formed of secondary powder particles produced by binding together using a binder primary powder particles whose main component is metal.
Now, the characteristics and the function and effect of the metal powder and the sintered metal compact will be described.

Metal Powder and Sintered Metal Compact

Besides an aluminum alloy powder and an iron alloy powder, the fluidity of a metal powder of some other metal such as copper, for instance, can be improved by producing secondary powder particles by binding primary powder particles together using a binder. As a result, the uniform filling of the powder into a die cavity can be effected, which improves the accuracy of the dimension of a sintered metal compact produced by sintering the green compact obtained.

Brief Description of the Drawings

Fig. 1 is a schematic diagram of a cross sectional structure of a secondary powder particle according to an embodiment of the present invention.
Fig. 2 is a diagram illustrating the circularity of a secondary powder particle according to an embodiment of the present invention.
Fig. 3 is a diagram illustrating an acicular ratio of a secondary powder particle according to an embodiment of the present invention.
Fig. 4 is a diagram showing a process of producing secondary powder particles according to an embodiment of the present invention.
Fig. 5 is a cross sectional view showing how secondary powder particles according to an embodiment of the present invention are filled into a die cavity.
Fig. 6 is a scanning electron micrograph showing the appearance of secondary powder particles according to the eighth embodiment of the present invention.
Fig. 7 is a scanning electron micrograph showing the appearance of one secondary powder particle for comparison in the same embodiment.
Fig. 8 is a scanning electron micrograph showing the appearance of another secondary powder particle for comparison in the same embodiment.
Fig. 9 is a schematic cross sectional view of a horizontal splitting cell for tensile strength measurement used for measuring a tensile strength of secondary powder particles in the ninth embodiment of the present invention.
Fig. 10 is a schematic cross sectional view of a parallel plates cell for shear strength measurement used to measure a flow function of secondary powder particles in the same embodiment.
Fig. 11 is a diagram showing a Mohr's stress circle and a powder yield locus for deriving a flow function of secondary powder particles in the same embodiment.
Fig. 12 is a scanning electron micrograph showing the appearance of secondary powder particles according to the thirteenth embodiment of the present invention.
Fig. 13 is a scanning electron micrograph showing the appearance of secondary powder particles for comparison in the same embodiment.
Fig. 14 is a scanning electron micrograph showing the appearance of other secondary powder particles for comparison in the same embodiment.
Fig. 15 is a schematic outside drawing of a primary powder particle of aluminum obtained by an atomization method.
Fig. 16 is a diagram illustrating the steps of producing conventional secondary powder particles of aluminum.
Fig. 17 is a schematic diagram showing a cross sectional structure of a conventional secondary powder particle.

Best Modes for Carrying Out the Invention

First, the embodiment relating to an aluminum alloy powder will be described below.

First Embodiment

An aluminum alloy powder whose particle diameter shown in Table 1 and obtained by the atomization method was prepared as a primary particle powder (starting raw material powder). The aluminum alloy powder was filled into a cylindrical tumble-type fluidized bed granulator, and air was allowed to flow in from the bottom. While the aluminum alloy powder particles were suspended within the fluidized bed, an aqueous polyvinyl alcohol (PVA) solution having a concentration of 2% at 10 percent by weight on the weight of the aluminum alloy powder (the amount of PVA being 0.2 percent by weight calculated on the aluminum alloy powder) was sprayed from above with a spray to form secondary powder particles.
The temperature within the fluidized bed was set at 75°C. Moreover, the spraying time for the PVA solution as a binder was set to 15 minutes, and the drying time was set to 10 minutes. Other conditions also satisfied the appropriate range defined by the present invention in forming the secondary powder particles.

The fluidity and the oxygen content of the secondary powder particles obtained were measured and analyzed. The fluidity of the secondary powder particles was determined by employing, as an evaluation index of fluidity, the value derived by first measuring the time required for 25 grams of secondary powder particles to complete their flow using one of a funnel-like orifice tube having a bore diameter of 2.6 mm and a funnel-like orifice tube having a bore diameter of 4.0 mm based on the method for measuring fluidity defined by Test of Determination of Flow Rate of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2502), then taking this value and dividing this value by the volume of secondary powder particles corresponding to 25 grams of secondary powder particles, the volume being converted by an apparent density (AD) of secondary powder particles measured based on Method of Determination of Apparent Density of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2504). The results are shown in Table 1. Further, no flowing was observed in any of these primary particle powders in the above-described measuring method.

	Primary powder particles	Secondary powder particles
No.	Particle diameter (µm)	Amount of oxygen (wt%)	Fluidity (sec./cm³)	Amount of oxygen (wt%)
	Mean	Maximum value	Minimum value		 2.6mm	 4.0mm
1	9	15	5	0.42	3.8	2.3	0.43
2	28	41	12	0.39	3.6	2.2	0.38
3	68	84	19	0.35	3.1	1.8	0.36
4	96	126	25	0.33	2.9	1.6	0.33
5	124	157	39	0.29	2.7	1.5	0.28
6	154	188	34	0.28	2.6	1.4	0.28
7	192	234	38	0.26	3.2	2.0	0.25
8	245	276	49	0.24	3.7	2.2	0.25
9	105	126	88	0.28	2.7	1.4	0.28
10	268	288	225	0.20	3.4	2.1	0.22
11	1	4	0.4	0.57	No flow	No flow	1.07
12	3	4	1	0.52	No flow	4.2	0.81
13	328	345	311	0.18	No flow	3.2	0.19
Present inventive examples; 1∼10, Comparative examples; 11∼13

With present inventive examples nos. 1 to 10, the use, as the starting raw material, of primary powder particles having an appropriate particle diameter as defined by the present invention produced the secondary powder particles having good fluidity, and no significant oxidation occurred during the granulation and drying steps.
In particular, with present inventive examples nos. 3 to 6 and 9, the particle diameter of primary powder particles is between 40 µm and 200 µm so that primary powder particles could be transported easily without the primary powder particles flying about when they are filled into the fluidized bed granulator. Moreover, in this case, the amount of fine powder particles contained is small, which leads to a lower cost, and the fluidity of secondary powder particles is more strikingly improved as seen from Table 1. Thus, it is more preferable to use a primary particle powder having a particle diameter of between 40 µm and 200 µm as the starting raw material powder.
On the other hand, comparative examples nos. 11 to 13 not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 11, a particle diameter of primary powder particles was less than 5 µm, and particularly because fine powder particles having a diameter smaller than 1 µm were in existence, significant oxidation occurred during the granulation and drying steps, while the secondary powder particles obtained failed to show fluidity in the fluidity measurement using funnel-like orifice tubes having bore diameters of 2.6 mm and 4.0 mm.
With comparative example no. 12, a particle diameter of primary powder particles was less than 5 µm so that significant oxidation occurred during the granulation and drying steps, and the secondary powder particles obtained failed to show fluidity in the fluidity measurement using a funnel-like orifice tube having a bore diameter of 2.6 mm. Moreover, in the fluidity measurement using a funnel-like orifice tube having a bore diameter of 4.0 mm, it was found that the secondary powder particles obtained failed to satisfy the appropriate range defined by the present invention.
With comparative example no. 13, a particle diameter of primary powder particles exceeded 300 µm so that the secondary powder particles obtained failed to show fluidity in the fluidity measurement using the funnel-like orifice tube having a bore diameter of 2.6 mm. Moreover, in the fluidity measurement using the funnel-like orifice tube having a bore diameter of 4.0 mm, it was found that the obtained secondary powder particles failed to satisfy the appropriate range defined by the present invention.

Second Embodiment

A primary particle powder having a minimum particle diameter of 7 µm, a maximum particle diameter of 180 µm, and a mean particle diameter of 57 µm obtained by the atomization method was employed as the starting raw material powder. From these primary powder particles, secondary powder particles having a particle diameter shown in Table 2 were formed using a tumble-type fluidized bed granulator by variously changing the added amount of an aqueous PVA solution binder (3% concentration) which was sprayed onto the primary powder particles.

In relation to the secondary powder particles obtained, a content (based on weight) of secondary powder particles whose particle diameter is 50 µm or below is also shown in Table 2. With regard to the fluidity of secondary powder particles, the time required for 25 grams of secondary powder particles to complete their flowing was measured using each of a funnel-like orifice tube having a bore. diameter of 2.6 mm and a funnel-like orifice tube having a bore diameter of 4.0 mm based on Japanese Industrial Standards mentioned earlier, and at the same time, this value was divided by the volume of secondary powder particles corresponding to 25 grams of secondary powder particles, the volume being converted by an apparent density (AD) of secondary powder particles measured based on the above-mentioned Japanese Industrial Standards, to produce a value employed as an evaluation index of fluidity. The results are shown in Table 2.

No.	Particle diameter (µm)	Content of particles having diameter of 50 µm or below (wt%)	Fluidity (sec./cm³)
	Mean	Maximum value	Minimum value		 2.6mm	 4.0mm
1	78	235	37	8	3.6	2.2
2	104	288	44	3	3.4	2.0
3	88	140	72	0	2.7	1.3
4	144	182	96	0	2.8	1.4
5	166	296	95	0	3.2	1.8
6	52	82	31	22	3.8	2.2
7	52	76	9	38	No flow	4.2
8	44	64	28	56	No flow	3.8
9	385	725	220	0	No flow	No flow
Present inventive examples; 1∼6 Comparative examples; 7∼9

With present inventive examples nos. 1 to 6, secondary powder particles having appropriate diameters defined by the present invention were found to have good fluidity. In particular, with present inventive examples nos. 3 and 4, a particle diameter of secondary powder particles was in the range of 60 µm to 250 µm so that the secondary powder particles were found to have particularly good fluidity when compared with other examples. Thus, it can be concluded that the particle diameter of secondary powder particles that is between 60 µm and 250 µm is more preferable.
On the other hand, the comparative examples nos. 7 to 9 not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 7, the content of powder whose secondary powder particle diameter was 50 µm or below exceeded 25 percent by weight such that the secondary powder particles lacked good fluidity.
With comparative example no. 8, the content of powder whose secondary powder particle diameter was 50 µm or below exceeded 25 percent by weight such that the secondary powder particles lacked good fluidity. With comparative example no. 9, the secondary powder particles formed a coarse powder whose particle diameter exceeded 500 µm so that the secondary powder particles lacked good fluidity.

Third Embodiment

A primary particle powder having a minimum particle diameter of 8 µm, a maximum particle diameter of 150 µm, and a mean particle diameter of 46 µm obtained by the atomization method was employed as the starting raw material powder. From these primary powder particles, secondary powder particles having a shape (circularity, acicular ratio) shown in Table 3 were formed using a tumble-type fluidized bed granulator by variously changing the added amount of an aqueous PVA solution binder (2% concentration) which was sprayed onto the primary powder particles.

With regard to fluidity of the secondary powder particles, the time required for 25 grams of secondary powder particles to complete their flowing was measured using each of a funnel-like orifice tube having a bore diameter of 2.6 mm and a funnel-like orifice tube having a bore diameter of 4.0 mm based on the above-mentioned Japanese Industrial Standards, and at the same time, this value was divided by the volume of secondary powder particles corresponding to 25 grams of secondary powder particles, the volume being converted by an apparent density (AD) of secondary powder particles measured based on the above-mentioned Japanese Industrial Standards, to produce a value employed as an evaluation index of fluidity. The results are shown in Table 3. Moreover, the circularity and the acicular ratio were calculated using the following equations as described in relation to Figs. 2 and 3: Circularity = 4π × (area of projected image of one particle) / (length of outer periphery of projected image of one particle)2 Acicular ratio = maximum diameter in projected image of one particle / diameter of projected image in a direction crossing maximum diameter

No	Circularity	Acicular ratio	Fluidity (sec./cm³)
.			 2.6mm	 4.0mm
1	0.68	1.87	3.8	2.3
2	0.77	1.62	3.6	2.2
3	0.92	1.44	3.1	1.8
4	0.96	1.18	2.7	1.4
5	0.52	1.94	No flow	2.8
6	0.63	2.24	No flow	2.9
7	0.44	2.48	No flow	No flow
8	0.41	2.70	No flow	No flow
Present inventive examples; 1∼4 Comparative examples; 5∼8

With present inventive examples nos. 1 to 4, secondary powder particles having an appropriate shape (circularity, acicular ratio) defined by the present invention were found to have good fluidity. In particular, with present inventive examples nos. 3 and 4, the circularity was 0.8 or above and the acicular ratio was 1.5 or below so that the secondary powder particles were found to have even better fluidity when compared with other examples.
On the other hand, the comparative examples nos. 5 to 8 not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 5, the circularity of secondary powder particles was less than 0.6 so that the secondary powder particles lacked good fluidity. With comparative example no. 6, the acicular ratio of secondary powder particles exceeded 2.0 such that the secondary powder particles lacked good fluidity.
With comparative example no. 7, the circularity of secondary powder particles was less than 0.6 and the acicular ratio exceeded 2.0 so that the fluidity of the secondary powder particles was dramatically degraded. With comparative example no. 8, the circularity of secondary powder particles was less than 0.6 and the acicular ratio exceeded 2.0 so that the fluidity of the secondary powder particles was dramatically degraded.

Fourth Embodiment

A primary particle powder of aluminum alloy having an appropriate particle diameter defined by the present invention (a minimum particle diameter of 6 µm, a maximum particle diameter of 215 µm, and a mean particle diameter of 65 µm) obtained by the atomization method was employed as the starting raw material powder. In a tumble-type fluidized bed granulator, an aqueous organic binder solution (binder concentration of 2%) was sprayed onto the primary powder particles to bind together the primary powder particles and secondary powder particles having a particle diameter shown in Fig. 4 were formed.

Moreover, the amount of binder to be sprayed onto the primary powder particles was variously changed in forming the secondary powder particles. The evaluated results of the content of the binder within the secondary powder particles obtained, of the particle diameter, and of fluidity are shown in Table 4. With regard to the fluidity of the secondary powder particles, the time required for 25 grams of secondary powder particles to complete their flowing was measured using each of a funnel-like orifice tube having a bore diameter of 2.6 mm and a funnel-like orifice tube having a bore diameter of 4.0 mm based on the above-mentioned Japanese Industrial Standards, and at the same time, this value was divided by the volume of secondary powder particles corresponding to 25 grams of secondary powder particles, the volume being converted by an apparent density (AD) of secondary powder particles measured based on the above-mentioned Japanese Industrial Standards, to produce a value employed as an evaluation index of fluidity.

No.	Organic binder	Particle diameter (µm)	Ratio of particles having diameter of 50µm or below (%)	Fluidity (sec./cm³)
	Type	Content (wt%)	Mean Mean	Maximum value	Minimum value		 2.6mm	 4.0mm
1	A	0.065	84	178	54	0	3.7	2.4
2	A	0.150	94	194	72	0	3.5	2.2
3	A	0.300	112	224	88	0	3.0	1.8
4	A	0.450	181	327	97	0	3.4	2.3
5	B	0.200	106	208	81	0	3.1	1.8
6	B	0.350	122	251	96	0	3.0	1.9
7	C	0.200	114	197	78	0	2.9	1.7
8	D	0.250	124	238	91	0	3.0	1.9
9	A	0.015	37	76	6	33	No flow	No flow
10	A	0.030	53	96	6	14	N0 flow	4.4
11	A	0.600	277	588	125	0	No flow	4.0
12	B	0.025	48	88	6	29	No flow	4.2
13	C	0.800	385	1200	185	0	No flow	No flow
Present inventive examples; 1∼8, Comparative examples; 9∼13
Type of binder A; polyvinyl alcohol, B; polyvinyl methyl ether, C; carboxymethylcellulose, D; hydroxyethyl cellulose

With present inventive examples nos. 1 to 8, secondary powder particles that were formed using an aqueous solution binder defined by the present invention contained appropriate amount of binder so that the secondary powder particles were found to have good fluidity.
In contrast, comparative examples nos. 9 to 13 not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 9, the content of the binder was as low as 0.015 percent by weight so that the content of powder whose particle diameter was 50 µm or below exceeded 25%, resulting in poor fluidity of the secondary powder particles. With comparative example no. 10, the content of the binder was as low as 0.030 percent by weight so that the particle diameter became less than 10 µm, resulting in poor fluidity of the secondary powder particles.
With comparative example no. 11, the content of the binder was as high as 0.60 percent by weight so that coarse powder particles having a particle diameter exceeding 500 µm were formed, which resulted in poor fluidity of the secondary powder particles. With comparative example no. 12, the content of the binder was as low as 0.025 percent by weight such that the content of powder whose particle diameter was 50 µm or below exceeded 25%, resulting in poor fluidity of the secondary powder particles.
With comparative example no. 13, the content of the binder was as high as 0.80 percent by weight so that coarse powder particles having a particle diameter exceeding 500 µm were formed, which resulted in poor fluidity of the secondary powder particles.

Fifth Embodiment

A primary particle powder of aluminum alloy having an appropriate particle diameter defined by the present invention (a minimum particle diameter of 7 µm, a maximum particle diameter of 148 µm, and a mean particle diameter of 37 µm) obtained by the atomization method was employed as the starting raw material powder. In addition, an organic binder having a decomposition temperature as shown in Table 5 was diluted with distilled water to prepare an aqueous organic binder solution having a concentration of 3%. The starting raw material powder particles were suspended within a tumble-type fluidized bed granulator (held at a temperature between 70°C and 80°C), and the binder was sprayed from above so as to bind together a plurality of primary powder particles to form secondary powder particles.

The secondary powder particles obtained were compacted and solidified at surface pressure area of 7t/cm², and thereafter, was heated for one hour in an nitrogen gas atmosphere controlled to be at 400°C in an attempt to remove the binder within the green compact. Then, from this sample, a transverse test piece was formed and its flexural strength was measured. Table 5 also shows the content of a binder within secondary powder particles, the amount of the binder within a green compact after heating, and the results of flexural strength measurement.

No.	Decomposition temperature of binder (°C)	Amount of binder in secondary powder particles (wt%)	Amount of binder in compacted body after heating (wt%)	Flexural strength of green compact (kgf/mm²)
1	280	0.20	<0.01	11.8
2	245	0.15	<0.01	10.2
3	360	0.30	<0.01	12.4
4	280	0.40	<0.01	12.6
5	440	0.20	0.09	4.6
6	440	0.40	0.27	3.5
Present inventive examples; 1∼4, Comparative examples; 5∼6

With present inventive examples nos. 1 to 4, by employing secondary powder particles formed by using a binder having a decomposition temperature defined by the present invention, it was found that the binder within the respective green compacts can be sufficiently decomposed and removed by a heating treatment at 400°C and that the amount of the remaining binder was below measurement limits (< 0.01%). In addition, it was confirmed that the strength of a green compact was not reduced by a thorough removal of the binder. Moreover, the amount of the remaining binder was measured by TG-MS method.
On the other hand, with both comparative examples nos. 5 and 6 formed using a binder not satisfying the appropriate range defined by the present invention, it was found that the decomposition temperature of the binder was as high as 440°C such that the binder remained within a green compact even after it was subjected to a heating treatment and the binder was not sufficiently removed. As a result, the sintering among the secondary powder particles was inhibited, and the strength of the green compact was reduced.

Sixth Embodiment

A primary particle powder of aluminum alloy having an appropriate particle diameter defined by the present invention (a minimum particle diameter of 7 µm, a maximum particle diameter of 146 µm, and a mean particle diameter of 41 µm) obtained by the atomization method was employed as the starting raw material powder. After filling the primary powder particles into a tumble-type fluidized bed granulator, a nitrogen gas was allowed to flow in at a flow rate of 50 liter/hour from the bottom of the granulator to keep the primary powder particles suspended within the granulator, and an aqueous PVA binder solution was sprayed from above with a spray to bind together the primary powder particles, thereby forming secondary powder particles.

The conditions of a concentration of an aqueous PVA binder solution and a temperature of the fluidized bed were as indicated in Table 6. The particle diameters of secondary powder particles obtained and the results of oxygen content measurement are also shown in the same Table 6.

No.	Binder concen-	Temperature within	Particle diameter of secondary powder particles (µm)	Amount of oxygn (wt%)	Abnormality in production process found/not found
	tration (wt%)	fluidized bed(°C)	Mean	Maximum value	Minimum value
1	2.0	80	85	196	48	0.27	Not found
2	4.0	85	97	212	56	0.25	Not found
3	2.0	95	84	204	45	0.26	Not found
4	2.0	110	88	208	53	0.24	Not found
5	0.5	85	48	104	16	0.29	Not found
6	2.0	50	75	184	44	1.08	Not found
7	2.0	125	102	245	77	0.25	Clogged nozzle
8	10.0	90	326	875	190	0.31	Not found
Present inventive examples; 1∼4, Comparative examples; 5∼8

With present inventive examples nos. 1 to 4, it was confirmed that secondary powder particles produced by controlling the binder concentration and the temperature within the fluidized bed as defined by the present invention had an appropriate particle diameter and formed a good powder without significant oxidation.
On the other hand, comparative examples nos. 5 to 8 formed under manufacturing conditions not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 5, the binder concentration was as low as 0.5 percent by weight so that the binding between primary powder particles did not progress sufficiently, and as a result, secondary powder particles having an appropriate particle diameter could not be produced. With comparative example no. 6, the temperature within the fluidized bed was as low as 50°C so that the drying of the powder was insufficient, and the secondary powder particles were oxidized during the granulation and drying processes.
With comparative example no. 7, the temperature within the fluidized bed was as high as 125°C such that the binder, when being atomized, solidified at the tip of a nozzle, causing the clogging of the nozzle, and the binder could not be stably sprayed. With comparative example no. 8, the binder concentration was as high as 10 percent by weight so that the viscosity of the binder was increased and minute droplets of the binder were not formed upon spraying, and as a result, coarse secondary powder particles having a particle diameter exceeding 500 µm were formed.

Seventh Embodiment

A primary particle powder of aluminum alloy having an appropriate particle diameter defined by the present invention (a minimum particle diameter of 6 µm, a maximum particle diameter of 98 µm, and a mean particle diameter of 34 µm) obtained by the atomization method was employed as the starting raw material powder. After filling the primary powder particles into a tumble-type fluidized bed granulator, air was allowed to flow in at a flow rate of 50 liter/hour from the bottom of the granulator to keep the primary powder particles suspended within the granulator, and an aqueous PVA binder solution was sprayed from an upper portion of the granulator with a spray to bind together the primary powder particles, thereby forming secondary powder particles.
At this time, secondary powder particles having apparent densities (AD) and tapped densities (TD) as those shown in Table 7 were formed by changing the concentration of an aqueous binder solution and the sprayed amounts thereof. Moreover, as described previously, the AD value and the TD value were respectively measured based on a method described in the Method of Determination of Apparent Density of a Metal Powder (JIS Z 2504) according to the Japanese Industrial Standards and on a method described in the Method for determination of tap density of metal powders according to Japan Powder Metallurgy Association (JPMA P 08). Further, comparative example no. 7 in Table 7 is the starting raw material powder (primary powder particles).

Then, as shown in the schematic representation of Fig. 5, secondary powder particles 1 obtained were filled from above into a powder feed box (shoe box) 8 that is 100 mm long, 100 mm wide, and 50 mm deep, and this powder feed box 8 was moved back and forth or reciprocated over a ring-shape die cavity fill 10 having an outside diameter of 40 mm and an inside diameter of 30 mm formed within a die cavity 9 in order to feed and fill secondary powder particles 1 into die cavity fill 10. The relation between the number of reciprocating passes of powder feed box 8 (each reciprocation counted as one pass) and the amount of powder filled is shown in Table 7.

No.	Fillability of secondary powder particles (g/cm³)	Relation between no. of reciprocating passes of powder feed box and amount filled (g)
	AD	TD	AD/TD(%)	One	Two	Three	Four	Five	Ten
1	0.97	1.08	89.8	5.91	5.94	5.94	5.94	5.94	5.94
2	0.88	1.01	87:1	5.50	5.53	5.55	5.55	5.55	5.55
3	0.78	0.94	83.0	5.11	5.14	5.16	5.17	5.17	5.17
4	1.01	1.21	83.5	6.55	6.59	6.63	6.65	6.66	6.66
5	0.92	1.24	74.2	5.39	5.71	5.98	6.34	6.54	6.79
6	0.78	1.04	75.0	4.29	4.48	4.75	4.95	5.39	5.68
7	1.01	1.44	70.1	5.23	5.53	5.86	6.14	6.56	7.14
Present inventive examples; 1∼4, Comparative examples; 5∼7

With present inventive examples nos. 1 to 4, secondary powder particles having an AD/TD value defined by the present invention were obtained, and two to four reciprocating passes of the powder feed box resulted in a filled condition close to the tap density. Thus, it was found that the secondary powder particles could be sufficiently filled into a die cavity in a short time.
On the other hand, comparative examples nos. 5 to 7 not satisfying the appropriate range defined by the present invention involved the following problems. With comparative example no. 5, the AD/TD value was as small as 74.2% so that it was discovered that the filling of secondary powder particles close to the tap density with only five times of reciprocating passes of powder feed box was difficult and that it required more time to feed the powder into the die cavity than in present inventive examples nos. 1 to 4. In addition, in this case, ten reciprocating passes of powder feed box could finally ensure the prescribed amount of filled powder.
With comparative example no. 6, the AD/TD value was as small as 75.0% so that it was found that the filling of secondary powder particles close to the tap density with five reciprocating passes or so of powder feed box was difficult, and that it required more time to feed the powder into the die cavity than in present inventive examples nos. 1 to 4. In addition, in this case, ten reciprocating passes of powder feed box could finally ensure the prescribed amount of powder filled.
With comparative example no. 7, the powder to be filled into the die cavity was a raw material powder having poor fluidity, and the AD/TD value was as small as 70.1% such that it was found that the filling of the powder close to the tap density with five reciprocating passes or so of powder feed box was difficult, and that it required ten reciprocating passes of powder feed box to ensure the prescribed amount of filled powder.

Eighth Embodiment

A primary particle powder of aluminum alloy having an appropriate particle diameter defined by the present invention (a minimum particle diameter of 6 µm, a maximum particle diameter of 175 µm, and a mean particle diameter of 58 µm) obtained by the atomization method was employed as the starting raw material powder. By spraying an aqueous PVA binder solution onto the primary powder particles in a tumble-type fluidized bed granulator, the primary powder particles were bound together to form secondary powder particles. Fig. 6 is a micrograph showing the appearance of the secondary powder particles produced as observed by a scanning electron microscope (SEM).
The secondary powder particles shown in Fig. 6 are those produced by spraying 150 grams of aqueous PVA solution having a concentration of 2% onto 1 kilogram of primary powder particles (amount of binder being 0.3 percent by weight). These secondary powder particles had a mean particle diameter of 221 µm, a maximum particle diameter of 340 µm, and a minimum particle diameter of 52 µm so that it was confirmed that the fluidity defined by the present invention was satisfied. Moreover, the secondary powder particles were all found to have a quasi-spherical shape.
On the other hand, for the purpose of comparison, the SEM micrograph showing the appearance of secondary powder particles produced by spraying 250 grams of aqueous PVA solution having a concentration of 4% onto 1 kilogram of primary powder particles (amount of binder being 1.0 percent by weight) is shown in Fig. 7. These secondary powder particles had a mean particle diameter of 340 µm, a maximum particle diameter of 327 µm, and a minimum particle diameter of 143 µm so that it was confirmed that neither the appropriate range of a particle diameter nor the fluidity defined by the present invention was satisfied. Moreover, due to the excessive amount of binder, the secondary powder particles were found further to bind together (secondary binding) forming coarse particles.
In addition, for the purpose of comparison, the SEM micrograph showing the appearance of secondary powder particles produced by spraying 100 grams of aqueous PVA solution having a concentration of 10% onto 1 kilogram of primary powder particles (amount of binder being 1.0 percent by weight) is shown in Fig. 8. Since the binder concentration was as great as 10%, these secondary powder particles had a mean particle diameter of 441 µm, a maximum particle diameter of 685 µm, and a minimum particle diameter of 152 µm, thus confirming that neither the appropriate range of particle diameter nor the fluidity defined by the present invention was satisfied.
Now, the embodiments relating to an iron alloy powder will be described below.

Ninth Embodiment

An iron alloy powder (iron-based primary particle powder and primary particle powder of a non-ferrous component) having a mean particle diameter shown in Table 8 and obtained by the atomization method was prepared. The iron alloy powder was filled into a tumble-type fluidized bed granulator. Air of a prescribed temperature was allowed to flow in from the bottom of the tumble-type fluidized bed granulator, and while the iron alloy powder particles were suspended within the fluidized bed, an aqueous polyvinyl alcohol (hereinafter referred to as PVA) solution having a concentration of 5% was sprayed from a nozzle attached to the upper portion. The solid content of PVA was controlled to be 2 percent by weight of the iron alloy powder, and the temperature within the fluidized bed was held at about 60°C. After the spraying of PVA, drying was effected within the same fluidized bed for about 15 minutes or so to produce secondary powder particles.
The secondary powder particles obtained were filled into a powder feed box (shoe box) having a dimension of about 150 mm × 100 mm × 75 mm (height). The powder feed box was moved back and forth or reciprocated in order to feed the secondary powder particles into a die cavity fill having an outside diameter of 45 mm and an inside diameter of 35 mm. The number of reciprocating passes of the powder feed box was three. The secondary powder particles, after being fed into the die cavity fill, were compacted to produce a green compact. Twenty green compacts were successively produced.
The thickness of a green compact in the direction of compression was measured in four locations per green compact. The standard deviation was calculated from these measured values, and the value (6σ) that was six times this standard deviation value (σ) was taken as a dimensional accuracy. Then, the green compacts were treated by sintering to produce sintered bodies. Like the green compacts, each sintered body obtained was measured in four locations, and a dimensional accuracy 6σ was similarly calculated.
The fluidity of the secondary powder particles was evaluated using a powder bed tester produced by Sankyo Piotech Co. Ltd., as described above. First, a flow function of the secondary powder particles was evaluated using a parallel plates cell for shear strength measurement. The procedure will be briefly described. A pre-compression load was applied in advance to a powder bed such that a prescribed porosity of 0.5 to 0.7 was achieved. Then, three levels of vertical loads were applied to the powder bed to perform a shear test. The powder yield locus was derived by plotting on a graph a vertical load (σ) and a shear stress (τ) of the shear test (see Fig. 11).
Then, a critical Mohr's stress circle A was derived that passes through the origin of the σ-τ coordinates and in contact with the powder yield locus. The stress at the point of intersection of the critical Mohr's stress circle A and σ axis, i. e. an unconfined yield stress F_c was derived. Then, the stress at the point of intersection of the σ axis and a Mohr's stress circle B in contact with a point E in the powder yield locus, i. e. a major compaction stress σ₁, was derived. Major compaction stress σ₁ was divided by uniaxial fracture stress F_c, producing a value that was the flow function (F.F) of the powder. The powder was determined to have high fluidity when this value was 10 or greater.
Then, a tensile strength of secondary powder particles was evaluated using a horizontal splitting cell for tensile strength measurement. The procedure will be described briefly. As in the case of the shear test, a pre-compression load was applied to a powder bed such that a prescribed porosity of 0.5 to 0.7 was achieved. Then, the movable cell was pulled in the horizontal direction to apply the tensile stress. The load at which the powder bed collapsed was read in order to calculate the tensile strength.
Moreover, the weight proportion of particles having a particle diameter of 45 µm or smaller contained in the secondary powder particles was evaluated by a sieving method.
The specific surface area of a secondary particle powder was measured by a gas adsorption method according to the BET method.
The results are shown in Table 8.
As shown in inventive examples (nos. 1 to 4) of Table 8, a flow function (F.F) of secondary powder particles produced where the mean particle diameter of the iron-based primary powder particles was in the range of 20 µm to 100 µm became 10 or greater, indicating high fluidity.
In addition, as shown in inventive examples (nos. 5 and 6), the flow function (F.F) of secondary powder particles that were produced by binding together iron-based primary powder particles along with primary powder particles of copper (Cu) or carbon (C) as a non-ferrous component also became 10 or greater, indicating high fluidity.
The characteristics of secondary powder particles indicating such high fluidity were that they had a mean particle diameter of 50 µm or greater and that the ratio of particles whose mean particle diameter was 45 µm or smaller was 10 percent by weight.
The mean particle diameter of secondary powder particles exceeding 200 µm is not desirable since the compactibility would be degraded. Thus, the mean particle diameter of the secondary powder particles is preferably in the range of 50 µm to 200 µm. A green compact was produced using the secondary powder particles, and the green compact was treated by sintering to produce a sintered body.
It was discovered that the value became 1.3 or below that was derived by dividing a prescribed dimensional accuracy of a sintered body by a prescribed dimensional accuracy of a green compact (dimensional accuracy of a sintered body / dimensional accuracy of a green compact), which kept small the variation in dimensional accuracy of the sintered body. This is probably due to the even filling, into a die cavity, of secondary powder particles having high fluidity, which resulted in a green compact of uniform density.
On the other hand, the results of the fluidity evaluation of iron-based primary powder particles are shown with comparative examples (nos. 7 to 10). As shown in the respective examples, iron-based primary powder particles include a high ratio of fine particles whose particle diameter is 45 µm or smaller, and also have a large specific surface area value so that the fluidity is seen to be low.
Moreover, upon evaluation of dimensional accuracy respectively of the green compact molded using the iron-based primary powder particles and of the sintered body, the value derived by dividing the dimensional accuracy of the sintered body by the dimensional accuracy of the green compact became greater than 1.3 which indicated that the dimensional accuracy of the sintered body had degraded.
Further, the cases of secondary powder particles having a mean particle diameter not in the appropriate range are shown in the comparative examples (nos. 11 and 12). In these cases, the secondary powder particles were produced using the same techniques as an inventive example (no. 1) except for the mean particle diameter of the iron-based primary powder particles. When the mean particle diameter of the iron-based primary powder particles was 18 µm, the mean particle diameter of the secondary powder particles obtained was 30 µm, and the ratio of particles having a mean particle diameter of 45 µm or below was 70 percent by weight. As a result, the flow function (F.F) was 3.4, indicating low fluidity.
On the other hand, when the mean particle diameter of the iron-based primary powder particles exceeded 100 µm, the mean particle diameter of secondary powder particles became greater than 200 µm so that, although the fluidity was relatively high, it was difficult to obtain a high molded density.
In addition, it was discovered that secondary powder particles demonstrated high fluidity when the specific surface area value (BET value) was 0.08 m²/g or less. Furthermore, from the tensile strength and the unconfined yield stress of the secondary powder particles derived, it was revealed that the flow function (F.F) was in the appropriate range when the tensile strength was 100 Pa or below and the unconfined yield stress was 300 Pa or below.

Tenth Embodiment

Secondary powder particles were produced from iron-based primary powder particles by the procedure according to the ninth embodiment. To the secondary powder particles produced, carbon (C), copper (Cu) powder, or as a solid lubricant, paraffin wax or zinc stearate powder, was added at a composition ratio shown in Table 9. The secondary powder particles were mixed for about thirty minutes by a V-type mixer to form a mixed powder.
The mixed powder obtained was compacted to produce a green compact, which was then sintered to form a sintered body. The prescribed dimensional accuracy of the green compact and of the sintered body, respectively, was evaluated using the procedure according to the ninth embodiment. Moreover, a flow function (F.F), a tensile strength, and an unconfined yield stress of the secondary powder particles were measured. The results are shown in Table 9.
It was found that degradation in the dimensional accuracy of the sintered body was low even in the case of a mixed powder having carbon or a metal powder such as copper, or a non-metallic powder such as paraffin wax or zinc stearate added to the secondary powder particles.
In addition, it was discovered that the flow function (F.F) of the mixed powder was 10 or greater, indicating high fluidity. Thus, the secondary powder particles obtained were also found to be effective for practical use as a mixed powder.
Moreover, the observation by SEM of the mixed powder obtained revealed that much of the added solid lubricant was adhered to the surface of secondary powder particles.
In a comparative example (no. 5), fluidity was evaluated of a mixed powder produced by adding 0.8 percent by weight carbon powder and 0.7 percent by weight paraffin wax powder to iron-based primary powder particles. As a result, the flow function (F.F) value was 6.3, which indicated that the dimensional accuracy would be degraded significantly in the sintered body formed using these primary powder particles.
In addition, the observation by SEM revealed that the iron alloy powder and the solid lubricant existed independently in the comparative example (no. 5).
From the above, it was concluded that the high fluidity of the secondary powder particles was brought about by the reduction of fine iron alloy particles and by the solid lubricant adhering to the surface of secondary powder particles, resulting in the reduction of solid lubricant existing independently.
Further, it was concluded that the dimensional accuracy of the sintered body could be improved as a result of a mixed secondary particle powder of high fluidity being formed and filled uniformly into a die cavity.

Eleventh Embodiment

Secondary powder particles were produced from iron-based primary powder particles by the procedure according to the ninth embodiment. At this time, the type and the added amount of an organic binder were controlled as shown in Table 10. As with the ninth and tenth embodiments, a flow function (F.F), a tensile strength, a unconfined yield stress, a ratio of particles 45 µm or smaller, and a prescribed dimensional accuracy were measured with respect to the secondary powder particles obtained. The results are shown in Table 10.
With inventive examples (nos. 1 to 3), the amount of PVA was varied in the range of 0.05 to 5 percent by weight on the weight of the secondary powder particles. It was discovered that the flow function (F.F) of the secondary powder particles exceeded 10 and the fluidity was good when the amount of PVA was within this range.
The tensile strength and the unconfined yield stress of the secondary powder particles were evaluated. Moreover, a green compact was formed using the secondary powder particles obtained, and the green compact was further sintered to produce a sintered body. Then, the prescribed dimensional accuracy was evaluated of the green compact and of the sintered body. As a result, both these values were found to satisfy the prescribed range.
Moreover, although the iron-based primary particle powder having a mean particle diameter of 74 µm was employed in this evaluation, it was found that the same result could be produced employing an iron-based primary particle powder having a mean particle diameter of 20 µm to 100 µm as long as the amount of PVA was within the range of 0.05 to 5 percent by weight.
Further, in the inventive examples (nos. 4 to 7), polyvinyl ether, polyethylene oxide, methyl cellulose, and carboxymethylcellulose were respectively used as an organic binder. Then, secondary powder particles were formed with the amount of the each organic binder being 1.0 percent by weight. It was discovered that the same result as that achieved using PVA was produced in these inventive examples and that the above-mentioned organic binders were also effective as binders.
On the other hand, the cases in which the amount of an organic binder was not within the appropriate range were as also evaluated. The results are indicated in the comparative examples (nos. 8 to 11). It was found that, when the concentration of the organic binder was 0.02 percent by weight, the binding force was small such that the particle diameter of secondary powder particles did not become large, and thus the fluidity could not be improved.
Moreover, it was found that low concentration of an organic binder was not desirable, since the secondary powder particles tended easily to break up upon handling of the secondary powder particles. On the other hand, it was discovered that, when the concentration of the organic binder exceeded 5 percent by weight, the particle diameter of the secondary powder particles became too large, resulting in a degraded compactibility.

Twelfth Embodiment

An iron-based primary particle powder having a mean particle diameter of 75 µm was prepared. The iron-based primary particle powder were treated by the same method as that of the ninth embodiment to produce a secondary particle powder having a mean particle diameter of 92 µm. PVA was used as an organic binder in the amount of 2 percent by weight. 0.8 percent by weight carbon powder and 0.8 percent by weight zinc stearate powder were further added to and mixed with the secondary particle powder thus obtained to form a mixed powder.
Then, the secondary particle powder or the mixed secondary particle powder produced was filled into the powder feed box, and the filled mixed powder was fed into a prescribed die cavity fill. At this time, the reciprocating passes of the powder feed box were three. Pressure was applied to the secondary powder particles or the mixed secondary particle powder after being fed into the die cavity fill, and a green compact was produced. Twenty green compacts were formed successively. Then, the weights of even-numbered green compacts and the weight variation thereof were evaluated. The green compacts were further treated by sintering, and the prescribed dimensional accuracy of the sintered bodies and the green compacts were compared. The results are shown in Table 11.
On the other hand, for comparison, the same evaluation was conducted on an iron-based primary particle powder and on a mixed powder produced by adding 0.8 percent by weight carbon powder and 0.8 percent by weight zinc stearate to the iron-based primary particle powder as comparative examples (nos. 3 and 4).
As shown in the inventive examples (nos. 1 and 2) and the comparative examples (nos. 3 and 4), the variation in weight of the green compacts was found to be relatively smaller when employing a secondary particle powder and a mixed secondary particle powder. In addition, the dimensional accuracy of the sintered body was found to increase when a secondary particle powder or a mixed secondary particle powder was used as opposed to the use of an iron-based primary particle powder or of a mixed iron-based primary particle powder. In other words, it was confirmed that the high fluidity of the secondary particle powder that allowed even filling of the secondary powder particles into a die cavity improved the dimensional accuracy of the sintered body.
Upon successive feeding of a powder, the weight variation of the green compacts could be kept low in the case where a secondary particle powder was employed as the powder in comparison with the case where an iron-based primary particle powder was employed. It can be concluded that this is due to the fluidity of the secondary powder particles being higher than that of the iron-based primary powder particles.

Thirteenth Embodiment

With an iron-based primary particle powder obtained by the atomization method serving as the starting raw material powder, secondary powder particles were formed by binding the iron-based primary powder particles together by spraying an aqueous PVA binder solution onto the iron-based primary powder particles. The appearance of the secondary powder particles produced was observed with a scanning electron microscope (SEM).
First, the appearance of secondary powder particles according to the inventive example is shown in Fig. 12. The secondary powder particles shown in Fig. 12 are secondary powder particles with a PVA content of 1.0 percent by weight. It was found that these secondary powder particles satisfied the fluidity and the range of appropriate particle diameter defined by the present invention and that the desired dimensional accuracy improved of the sintered body obtained by compacting and sintering these secondary powder particles.
On the other hand, as comparative examples, the outer appearances of the secondary powder particles with PVA content of 0.02 percent by weight and of the secondary powder particles with PVA content of 6 percent by weight are respectively shown in Figs. 13 and 14. It was discovered that the secondary powder particles shown in Fig. 13 or 14 did not satisfy the fluidity or the range of appropriate particle diameter defined by the present invention so that a sintered body having a desired high dimensional accuracy could not be produced.
Particularly, with the secondary powder particles shown in Fig. 14, it was found that coarse particles were formed by further binding (secondary binding) of the secondary powder particles due to an excessive amount of PVA.
The concept of producing secondary powder particles having excellent fluidity by binding primary powder particles together using a binder is applicable not only to an aluminum alloy powder or an iron alloy powder described in the above embodiments but also to other metal powders such as that of copper. Moreover, such metal powder can be compacted and sintered to produce a metallic sintered body having a high dimensional accuracy.
The embodiments disclosed herein are in every respect set forth by way of illustration and are to be construed as non-limiting. The scope of the present invention is indicated not by the description given above but by the scope of claims, and is intended to encompass the scope of the claims, its equivalents, and all modifications within the scope of claims.
By employing the secondary alloy powder particles of aluminum according to the present invention, fluidity of and fillability into a die cavity of secondary powder particles are improved such that, for instance, an internal rotor set having an inner periphery portion or an outer periphery portion shaped like a tooth profile based on one of a trochoid curve, an involute curve, and a hypo-cycloid curve can be produced with high dimensional accuracy. As a result, such effects as improved economy due to reduction in machining cost and improved pump performance due to reduction in the chip clearance between the tooth profile portions of a rotor can be achieved, while similar effects can be expected for other drive system parts or valve system parts.
Moreover, by employing the secondary powder particles of iron alloy according to the present invention, fluidity of and fillability into a die cavity of secondary powder particles are improved such that machine parts or the like formed of sintered iron alloy compacts having high dimensional accuracy can be produced. As a result, additional machining of a sintered body is no longer required, which allows the reduction in the manufacturing cost of the machine parts and the like.

Claims

An aluminum alloy powder, formed of secondary powder particles

(1) produced by binding together using a binder (3) primary powder particles (2) whose main component is aluminum.
An aluminum alloy powder, formed of secondary powder particles (1) produced by binding together primary powder particles (2) whose main component is aluminum,
said secondary powder particles (1) having an average value of acicular ratio that is at most 2.0 derived from following equation, where a projected image is obtained by projecting upon one of said secondary powder particles (1): acicular ratio = maximum diameter in a projected image of one particle / a diameter of projected image in a direction orthogonal to maximum diameter.
The aluminum alloy powder according to claim 1, wherein said primary powder particles (2) are rapidly solidified powder particles obtained by an atomization method.
The aluminum alloy powder according to claim 1, wherein a particle diameter of said secondary powder particles (1) is between 10 µm and 500 µm.
The aluminum alloy powder according to claim 1, wherein a particle diameter of said primary powder particles (2) is between 5 µm and 300 µm.
The aluminum alloy powder according to claim 1, wherein, among said secondary powder particles (1), secondary powder particles having a particle diameter of at least 50 µm are at most 25 percent by weight of entire secondary powder particles (1).
The aluminum alloy powder according to claim 1, wherein fluidity of said secondary powder particles measured using a funnel-like orifice tube having a bore diameter of 2.6 mm based on a Test of Determination of Flow Rate of a Metal Powder according to Japanese Industrial Standards (JIS Z 2502) is at most 4.0 seconds/cm³.
The aluminum alloy powder according to claim 1, wherein fluidity of said secondary powder particles (1) measured using a funnel-like orifice tube having a bore diameter of 4.0 mm based on the Test of Determination of Flow Rate of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2502) is at most 2.5 seconds/cm³.
The aluminum alloy powder according to claim 1, wherein an apparent density of said secondary powder particles (1) measured based on a Method of Determination of Apparent Density of a Metal Powder according to the Japanese Industrial Standards (JIS Z 2504) is between 80% and 100% of a tap density of said secondary powder particles (1) measured based on a Method for determination of tap density of metal powders according to Japan Powder Metallurgy Association (JPMA P 08).
The aluminum alloy powder according to claim 1, wherein an average value of circularity derived from following equation is at least 0.6, where a projected image is obtained by projecting upon one of said secondary powder particles (1): Circularity = 4π × (area of projected image of one particle) / (length of outer periphery of projected image of one particle)2.
The aluminum alloy powder according to claim 1, wherein an average value of acicular ratio derived from following equation is at most 2.0, where the projected image is obtained by projecting upon one of said secondary powder particles (1): Acicular ratio = maximum diameter in projected image of one particle / diameter of projected image in a direction orthogonal to maximum diameter.
The aluminum alloy powder according to claim 1, wherein said binder (3) is an organic binder.
The aluminum alloy powder according to claim 12, wherein amount of said organic binder in said secondary powder particles (1) is between 0.05 and 0.5 percent by weight.
The aluminum alloy powder according to claim 12, wherein a decomposition temperature of said organic binder is at most 400°C.
The aluminum alloy powder according to claim 12, wherein said organic binder includes as a main component any one organic compound selected from the group consisting of polyvinyl alcohol, polyvinyl methyl ether, carboxymethylcellulose, and hydroxyethyl cellulose.
A method of producing an aluminum alloy powder, comprising:

a granulation step in which primary powder particles (2) having a particle diameter of 5 µm to 300 µm whose main component is aluminum are bound together using an aqueous organic binder solution to form secondary powder particles (1); and

a drying step for removing moisture contained in said secondary powder particles (1).
The method of producing an aluminum alloy powder according to claim 16, wherein said primary powder particles (2) are rapidly solidified powder particles obtained by an atomization method.
The method of producing an aluminum alloy powder according to claim 16, wherein said granulation step includes allowing said primary powder particles (2) to be suspended within a fluidized bed (4) and spraying an aqueous organic binder solution on the suspended primary powder particles (2).
The method of producing an aluminum alloy powder according to claim 18, wherein said drying step includes drying said secondary powder particles (1) inside said fluidized bed (4).
The method of producing an aluminum alloy powder according to claim 18, wherein said drying step includes introducing a gas of a prescribed temperature into said fluidized bed (4).
The method of producing an aluminum alloy powder according to claim 19, wherein a temperature inside said fluidized bed (4) during said drying step is between 60°C and 120°C, and more preferably, between 70°C and 90°C.
The method of producing an aluminum alloy powder according to claim 16, wherein a concentration of said aqueous organic binder solution in said granulation step is between 1% and 8%.
An iron alloy powder, formed of secondary powder particles produced by binding together using a binder iron-based primary powder particles whose main component is iron or, by binding together using a binder iron-based primary powder particles whose main component is iron along with primary powder particles of a non-ferrous component.
The iron alloy powder according to claim 23, wherein a mean particle diameter of said iron-based primary powder particles is between 20 µm and 100 µm, while a mean particle diameter of said secondary powder particles is between 50 µm and 200 µm.
The iron alloy powder according to claim 23, wherein, among said secondary powder particles, secondary powder particles having a particle diameter that is at most 45 µm form at most 10 percent by weight of entire secondary powder particles.
The iron alloy powder according to claim 23, wherein a value of surface area of a secondary particle powder derived from BET isothermal adsorption formula is at most 0.08 m²/g.
The iron alloy powder according to claim 23, wherein a surface of one of said secondary powder particles is covered with a binder.
The iron alloy powder according to claim 27, wherein said binder is an organic binder.
The iron alloy powder according to claim 28, wherein amount of said organic binder within said secondary powder particles is between 0.05 and 5 percent by weight.
The iron alloy powder according to claim 28, wherein said organic binder includes as a main component any one organic compound selected from the group consisting of polyvinyl alcohol, polyvinyl ether, polyethylene oxide, methyl cellulose, and carboxymethylcellulose.
The iron alloy powder according to claim 23, wherein, using a horizontal splitting cell for tensile strength measurement (21) formed by a movable cell (22) and a fixed cell (23), an iron alloy powder is filled into the cell, a prescribed load is applied from above the iron alloy powder to form an iron alloy powder bed (24) having a porosity that is between 0.5 and 0.7, and
movable cell (22) is pulled in horizontal direction, breaking said iron alloy powder bed (24), at which point a tensile breaking strength is derived, the tensile breaking strength being at most 100 Pa.
The iron alloy powder according to claim 23, wherein, using a parallel plates cell for shear strength measurement (25) formed by a fixed plate (27) and a movable plate (26) respectively provided with a notch, an iron alloy powder is inserted between fixed plate (27) and movable plate (26), a pressure is applied from above the iron alloy powder to form an iron alloy powder bed (24) having a porosity that is between 0.5 and 0.7, and

movable plate (26) is pulled parallel to fixed plate (27) under a prescribed load to cause the iron alloy powder to be sheared, at which point a shear stress is derived,

an unconfined yield stress derived from a powder yield locus indicating relation between the load and the shear stress being at most 300 Pa.
The iron alloy powder according to claim 32, wherein a value of ratio is 10 or above which is obtained by dividing a value of a major consolidating stress derived from Mohr's stress circle (B) that is in contact with said powder yield locus by a value of said unconfined yield stress.
A method of producing a sintered iron alloy compact, comprising:

a granulation step for forming secondary powder particles by binding together using a binder iron-based primary powder particles whose main component is iron or, by binding together using a binder iron-based primary powder particles whose main component is iron along with primary powder particles of a non-ferrous component;

a step of forming a green compact by compacting said secondary powder particles;

a step of measuring a prescribed dimension of said green compact;

a step of sintering said green compact to form a sintered body; and

a step of measuring a prescribed dimension of said sintered body corresponding to the prescribed dimension of said green compact, wherein

a value is at most 1.3 which is obtained by dividing a value which is six times standard deviation of said prescribed dimension of said sintered body by a value which is six times standard deviation of said prescribed dimension of said green compact.
The method of producing a sintered iron alloy compact according to claim 34, comprising:
a step of adding a lubricant to said secondary powder particles between the granulation step for forming said secondary powder particles and the step of forming said green compact.
A metal powder formed of secondary powder particles produced by binding together using a binder primary powder particles whose main component is metal.
The metal powder according to claim 36, wherein said binder is an organic binder.
A sintered metal compact obtained by sintering a green compact formed of secondary powder particles produced by binding together using a binder primary powder particles whose main component is metal.