JP2000144208A - Compression molding method of powder for powder metallurgy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which is capable of efficiently increasing the density of a formed body, and further improving the mechanism characteristic and the magnetic characteristic of a final formed body in compressing powders for powder metallurgy such as iron powders and iron-based powders. SOLUTION: In compressing powders for powder metallurgy using a forming die, the forming pressure is set to >=5 tons/cm2, the vibration is applied to the forming die, and the amplitude without pressure is set to 0.002 to 0.10 mm. The amplitude with pressure of 5 tons/cm2 is set to >=20% of the amplitude without pressure. Compression forming is achieved by filling powders for powder metallurgy including <=0.2 wt.% (including 0%) lubricant in the forming die to an inner wall surface of which the lubricant is preferably applied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compressively molding powder for powder metallurgy such as iron powder or iron-based alloy powder. And a method capable of improving magnetic characteristics and the like.

[0002]

2. Description of the Related Art It is effective to increase the density as much as possible in order to enhance the mechanical and magnetic properties of a molded article formed by powder metallurgy. It is important to obtain the highest possible density.

[0003] Therefore, a method of increasing the compaction by applying vibration to the powder for powder metallurgy in the compression molding stage has been adopted (for example, Japanese Patent Publication No. 3-25278, Japanese Patent Publication No. 41-6549, and Japanese Patent Publication No. 54-1979). No. 14781, Japanese Patent Publication No. 54-41523).

[0004] However, these conventional vibration molding methods focus on the enhancement of rearrangement of powder for powder metallurgy, and are effective when molding at low pressure such as tiles and ceramic powders. It is not necessarily a sufficient method for applications in which the powder is plastically deformed under high pressure to consolidate.

[0005] In the conventional powder metallurgy method, the fluidity of the powder is increased by previously mixing a lubricant with the powder to be molded, and the friction between the powder and the powder or between the powder and the molding die is reduced. That is being done. The main purpose of this is to reduce the resistance when the molded body is removed from the mold and to prevent seizure of the molded mold. The amount of the conventionally used lubricant is generally about 0.2 to 10% by weight based on the powder to be molded (for example, Japanese Patent Application Laid-Open No. 2-156002).
powder report "Vol. 42, no.
11, p. 78-786 (1987) also report that the maximum compaction density is obtained when the amount of the lubricant added is 0.5%.

[0006] Further, the amount of the lubricant currently in practical use is in the range of 0.5 to 1.0% by weight.
Even if the molding pressure is increased to increase the density of the compression molded body, the lubricant is filled in the voids between the powders and hinders the improvement of the density. On the other hand, if the amount of the lubricant is reduced, the friction between the powder and the mold becomes large, so that it is not possible to achieve high-pressure densification, and there is also a problem that the life of the mold is shortened.

In order to further increase the density at the time of compression molding, there has been proposed a method in which powder for powder metallurgy or a molding die is heated to a temperature lower than the melting point of a lubricant (usually about 70 to 120 ° C.) to carry out compression molding. (See U.S. Pat. No. 4,9555,798)
JP-A-5-271709 discloses a temperature lower than the temperature at which the lubricant is completely melted (specifically, 37 ° C.).
A method of heating to about 0 ° C. or lower and performing pressure molding is disclosed. These techniques are all based on the finding that when the lubricant is melted, the flow of the powder is significantly inhibited.However, even in these methods, the lubricant remains in the molded body at the usual amount of the lubricant added. This is not a fundamental improvement.

On the other hand, it is well known that if a lubricant is applied to the inner surface of a molding die, friction between the metallurgical powder and the molding die is reduced, but no lubricant is blended in the metallurgical powder. Therefore, the fluidity of the powder is poor, and the purpose of high density cannot be achieved.

[0009] Further, JP-A-9-272901 discloses that
Disclosed is a method of obtaining a high-density molded body by applying a lubricant to an inner surface of a molding die, heating the metallurgical powder containing no lubricant and the molding die to 150 ° C. to 400 ° C., and compression molding. Have been. However, in this method, since no lubricant is contained in the metallurgical powder, the fluidity of the powder and the reduction of friction between powder particles are insufficient, and it is difficult to obtain a satisfactory high-density compression molded body. . In addition, since friction between metallurgical powders cannot be reduced, density unevenness is likely to occur inside the compression-molded product, which causes dimensional variations in the subsequent sintering process and the like.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems in the prior art, and has been described in connection with the problem of poor fluidity of metallurgical powder during compression molding, and the problem with molding dies. It is an object of the present invention to solve the problem of friction and to establish a technology capable of efficiently compacting metallurgical powder at high density.

[0011]

Means for Solving the Problems A molding method according to the present invention which can solve the above-mentioned problems is that, when compressing powder for powder metallurgy using a molding die, the molding pressure is 5 ton / cm ² or more. And apply vibration to the mold,
The gist is that the amplitude at the time of no pressurization is set to 0.002 to 0.10 mm and the amplitude at the time of pressurization of 5 tons / cm ² is set to 20% or more of the amplitude at the time of no pressurization. .

In carrying out this method, a lubricant is applied to the inner wall surface of the mold, and the powder for powder metallurgy is made to contain 0.2% by weight or less (including 0%) of a lubricant. For example, the density of the compression molded body can be more effectively increased, and the preferable frequency of the vibration applied to the mold is 5 Hz to 20 kHz, more preferably 5 Hz to 20 kHz.
It is in the range of -200 Hz.

The temperature during compression molding is 80 to 500 ° C.
In particular, adjusting the molding temperature to the range of not less than the melting point (Tm) of the lubricant contained in the powder for powder metallurgy and not more than (Tm × 3) can reduce the density of the compact. More effective in raising.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted various studies on metal powders for powder metallurgy, particularly iron powders, from various angles in order to increase the density in compression molding. as a result,
The inventor has found that the object can be easily achieved by setting the above conditions, and the present invention has been made.

Hereinafter, embodiments of the present invention will be described in detail. The powder for powder metallurgy used in the present invention is a general term for powders used to obtain various molded products by compressing the powder into a predetermined shape and, if necessary, sintering or the like. In this specification, the powder for powder metallurgy includes a powder mixed with a lubricant or the like for reducing friction between a molding die and powder and friction between powders. Examples include metal powder and ceramic powder.
In particular, the present invention can be used very effectively for metal powders that undergo plastic deformation during compression molding.

Typical examples include pure iron powder (or one containing a small amount of C, Mn, Si, P, S, Cr, O, N, etc. as impurities), Iron-based alloy powder (prealloy type, diffusion type, hybrid type thereof, etc.) to which Ni, Mo, Mn, Cr, Si and other elements are intentionally added to secure the strength are included. However, it should be noted that if the addition amount of the alloying element is too large, the iron powder hardens and the compressibility decreases, which may be a factor that hinders the high density of the powder metallurgy product.

Further, in order to enhance the properties after sintering, various alloying elements such as graphite, Cu, Ni, Mo and the like may be used alone or in combination of two or more kinds. A composite powder in which graphite or the like is adhered to the surface of an iron powder using the above method may be used.

What is very important in the present invention is the control of the amplitude of vibration applied during compression molding. In order to reduce the friction between powders and increase the density of the powders by vibration during compression molding of the powders, it is necessary to maintain the amplitude of the vibrations to a certain level even during pressing. However, when plastically deforming powder, such as iron powder, is compression-molded at high pressure, the conventional vibration molding method provides vibration with sufficient amplitude when no pressure is applied, but it is attenuated when pressurized. Was not fully effective.

However, the present inventors have confirmed that if the control is performed so that the amplitude at the time of pressurization of 5 tons / cm ² is 20% or more, more preferably 50% or more of the amplitude at the time of no pressurization. It was confirmed that even at the time of high-pressure molding of 5 ton / cm ² or more, the effect of reducing friction between powders due to vibration and the effect of reducing friction between powder and a molding die were sufficiently exhibited, and the density of the compact could be greatly increased. Was.

Incidentally, when the amplitude at the time of pressurization of 5 ton / cm ² is less than 20% of the amplitude at the time of no pressurization, the above-mentioned friction reducing effect by vibration is greatly reduced, and the density of the compression-molded body is sufficiently improved. become unable.

When vibration is applied to the molding die, the density of the compact can be most effectively increased by transmitting the vibration to the powder through the upper and lower punches. It is also effective to apply vibration to the die or to apply vibration to the die and to combine the vibration with the punch. The timing for applying the vibration is an essential condition that the vibration is applied when pressure is applied, and the vibration can be freely applied at the time of filling the powder or at the time of demolding after compression molding.

There is no particular limitation on the device for giving the vibration, and any vibration generating device may be used as long as the vibration can be controlled to the above-mentioned amplitude.

The fundamental frequency of the applied vibration is usually 5 Hz to 20 kHz so as to reduce the friction between the powders.
selected from the range of z, more preferably from 5 to 20
The range is 0 Hz. By the way, if the fundamental frequency is less than 5 Hz, the friction between powders cannot be reduced sufficiently,
This is because excessive energy is required to maintain an amplitude exceeding Hz at the time of pressurization, which is not suitable for implementation on a practical scale. However, in the case where the amplitude of the frequency corresponding to an integral multiple of those is synthesized in the vibration generator, it can be put to practical use without any problem.

Regarding the amplitude, the amplitude when no pressure is applied is 0.0
A range of 02 to 0.10 mm is preferable because a sufficient amplitude can be obtained even when the amplitude at the time of pressurization of 5 ton / cm ² is set to 20% of that at the time of no pressurization. When the amplitude is less than 0.002 mm, the amplitude at the time of pressurization tends to be insufficient, and it is difficult to effectively exert the effect due to vibration.
When the amplitude is excessively larger than the above, excessive energy is required to maintain the amplitude during pressurization, and it becomes substantially difficult to maintain the amplitude during pressurization.

According to the present invention, a high density can be achieved even when the pressure during compression molding is low, but in the present invention, a higher degree of consolidation can be achieved by plastically deforming the powder during compression molding. Therefore, the molding pressure should be 5 ton / cm ² or more. However, the density improvement effect is 1
It saturates at about 5 ton / cm ² , and even if the pressure is further increased, no further effect of increasing the density can be obtained, which is economically wasteful.

Next, the reasons for limiting the amount of lubricant added will be clarified. As in the conventional example, when a lubricant is added in an amount of about 0.5 to 10% by weight to effectively exert a lubricating effect, the lubricant occupies most of the voids between the powders as the density of the compression molded body increases. Therefore, there is a limit in increasing the density. However, when vibration is applied during the compression molding, the amount of the lubricant used can be reduced to 0.2% by weight or less, and accordingly, the amount of the lubricant entering the voids between the powders decreases, and the compression molding is performed. It is possible to greatly increase body density.

At this time, even if no lubricant is added, a certain degree of high density can be achieved by the effect of promoting rearrangement by vibration, but from the viewpoint of securing the fluidity of the powder, desirably 0.01% by weight or more. The content is preferably 0.20% by weight or less, more preferably 0.01% by weight or more and 0.10% by weight or less.

If the amount of the lubricant is reduced to the above-mentioned level, the friction between the molding die and the powder becomes large, making it difficult to increase the density. Therefore, when practicing the present invention, it is necessary to apply a lubricant to the inner wall surface of the mold before filling the powder into the mold. The method of applying the lubricant to the inner wall surface of the mold is not particularly limited. However, usually, a method of applying the lubricant in a solid state, a method of brushing or spraying a solution dissolved or dispersed in a solvent, a method of applying a lubricant, Is applied by heating and melting.

Preferred examples of the lubricant to be added to the powder for compression molding include metal salts of higher fatty acids such as stearic acid and wax-based lubricants. Good. When heating at the time of compression molding and adding 0.5 to 10% by weight of a lubricant as in the prior art, heating the powder to a temperature higher than the melting point of the added lubricant significantly reduces the fluidity of the powder. As described above, since the amount of the lubricant added is largely suppressed as described above, the fluidity does not decrease even if heated slightly. In addition, since the lubricating action between the molding die and the powder is maintained even at a temperature higher than the melting point of the lubricant, the high density is not hindered.

Preferred lubricants applied to the inner wall of the mold include metal salts of higher fatty acids such as stearic acid, waxes, molybdenum disulfides, BNs, graphites, and the like.
Other general lubricants can be used, and these may be used alone or in combination of two or more. In this case, it is desirable to select and use an optimal lubricant according to the heating temperature during compression molding.

The temperature during compression molding may be room temperature, but it is desirable to lower the plastic deformation resistance by heating the powder in order to achieve higher density molding. As the heating means, the powder itself may be preheated to an appropriate temperature, or may be heated by utilizing heat transfer from the mold after filling the mold, but if the temperature of the mold is low, heating is performed. Since the temperature of the powder decreases during compression and the compressibility decreases, it is preferable that the temperature of the mold can be appropriately maintained.

The heating temperature of the mold naturally depends on the type of powder, but when the most typical iron powder is used, the temperature is preferably in the range of 80 to 500 ° C. By the way, 8
If the temperature is lower than 0 ° C., the deformation resistance of the powder is large, so that it is difficult to increase the density. If the temperature is excessively higher than 500 ° C., the molding die undergoes thermal distortion or shortens the life. A more preferred lower limit of the heating temperature is 100 ° C., and a more preferred upper limit is 250 ° C. in consideration of the density of the obtained molded body, the cost required for heating, and the life of the mold.

At this time, with respect to the melting point (Tm) of the lubricant mixed into the powder, the temperature of the mold was set to [Tm or more, Tm × 3
It is preferable to adjust the temperature to the following value because the density of the compression molded body can be further increased. The reason is that when the lubricant is melted at the time of compression molding, the lubricant oozes out on the surface of the molded product, the lubricant is naturally removed from the gap between the powders, and the lubricating effect between the molding die and the powder is also enhanced. is there.

In the prior art, as mentioned above, it was recommended to mold at a temperature lower than the melting point of the lubricant from the viewpoint of ensuring the fluidity of the powder. However, in the present invention, the amount of the lubricant added is greatly reduced as described above. Therefore, there is no problem of a decrease in fluidity due to heating. However, the molding temperature is the melting point of the lubricant (Tm)
On the other hand, if it exceeds [Tm × 3], the thermal deterioration of the lubricant becomes so severe that a problem such as seizure to a molding die occurs, so that the temperature should be kept below that.

As a method for heating the mold, any method such as external heater heating, Joule heating by energization, high-frequency heating, and infrared heating can be adopted. Also, since it takes some time before the powder filled in the mold is warmed by the heated mold, to complete the compression molding in a shorter time, before filling the powder into the mold, Preheating to a predetermined temperature is also effective. The preheating temperature at that time is not particularly limited as long as the powder is not oxidized or sintered.

[0036]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. However, the following Examples are not intended to limit the present invention, and should be appropriately performed within a range that can conform to the purpose described above and below. Modifications can be made and implemented, all of which are included in the technical scope of the present invention.

Example Using a V-type mixer, raw material powders having the composition shown in Tables 1 and 2 are mixed for 30 minutes. About 20 g of each obtained mixed powder
Each mold was weighed and heated to a predetermined temperature (diameter 31.5
mm × 12.5 mm in depth) and press-molded under the conditions shown in Tables 1-4. In this molding process, vibration is applied to the mold using a vibration generator (vibration board unit manufactured by Daiichi Co., Ltd.), and the amplitude A when no pressure is applied and the amplitude B when 5 tons / mm ² are applied are varied. An experiment was performed. The density of each obtained compact was measured by the following method, and the results shown in Tables 3 and 4 were obtained.
In addition, the density of the compact was calculated from the volume and weight of the compact.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

The following can be considered from Tables 1 to 4. No.
1 to 7 are vibration damping rates during pressure molding (5 tons /
(amplitude B at the time of pressurization of cm ² and the width A of the plate at the time of no pressurization).
o. Nos. 1 to 5) are Nos. 1 to 5 in which the amplitude attenuation rate during pressure molding is less than 20%. A higher compact density is obtained as compared with 5,6.

No. Nos. 8 to 14 are obtained by changing the amplitude of the vibration. Higher molded body density is obtained as compared with 8,14.

No. 15-20 are the ones with different molding pressures
But the molding pressure is 5 ton / cm ^Two (No. 1)
In 5), the density of the compact is low. And increase the molding pressure
The density of the compact increases as the^Two About ascending
The density is saturated, and even if the pressure is increased further,
It is not economically wasteful because it does not rise anymore
You.

No. 21 to 27 are obtained by changing the frequency of the vibration, and when the frequency is set within a suitable range of the frequency (20 to 200 kHz) (Nos. 22 to 26), a high compact density is obtained. No. 28 to 33 are obtained by changing the amount of lubricant in the mixed powder after performing mold lubrication.
A high molded article density is obtained when the amount of the lubricant added is 0.2% by weight or less, and a high molded article density is particularly obtained when the amount of the lubricant is 0.01% by weight or more and 0.1% by weight or less.

No. Nos. 34 to 41 are examples in which Li stearate is used as an additive lubricant, and molybdenum disulfide is used as a mold coating lubricant, and the molding temperature is changed. The molding temperature is preferably 80 ° C or higher. As the density increases, the density of the compact increases, but the density increases by about 500 ° C.
, The temperature is preferably 500 ° C. or less. No. Nos. 42 to 49 use Zn stearate as an additive lubricant and B as a mold coating lubricant.
This is an example in which the N-type lubricant was used and the molding temperature was similarly changed. In this case, too, it is understood that the molding temperature is particularly preferably in the range of 80 to 500 ° C.

[0047]

According to the present invention, the amount of the lubricant compounded in the metallurgical powder is suppressed to 0.2% or less, and the lubricant is preferably applied to the inner wall surface of the mold. At the same time, by applying vibration to the mold and specifying the amplitude attenuation rate particularly during pressure molding, a high compact density can be stably secured.

Continued on the front page (72) Inventor Yoshikazu Seki 2-3-1, Shinhama, Arai-machi, Takasago City, Hyogo Prefecture Inside Kobe Steel, Ltd. Takasago Works (72) Inventor Masaaki Sato 2-3-1, Shinama, Araimachi, Takasago-shi, Hyogo Prefecture Kobe Steel, Ltd. Takasago Works (72) Inventor Noriaki Akagi 2-3-1, Shinama, Arai-machi, Takasago City, Hyogo Prefecture Inside Kobe Steel Works, Takasago Works (72) Inventor Tetsuya Sawayama 2-chome, Araimachi, Takasago City, Hyogo Prefecture No.3-1 F-term in Kobe Steel, Ltd. Takasago Works (reference) 4K018 AA24 CA02 CA05 CA07 CA16

Claims (5)

[Claims] When compressing powder for powder metallurgy using a molding die, the molding pressure is set to 5 ton / cm ² or more, vibration is applied to the molding die, and the amplitude when no pressure is applied is set to 0.
002 to 0.10 mm and 5 tons / cm
⁽²⁾ A compression molding method of powder for powder metallurgy, wherein the amplitude at the time of pressurization is 20% or more of the amplitude at the time of no pressurization. 2. A compact having a lubricant applied to an inner wall thereof is filled with a powder for powder metallurgy containing 0.2% by weight or less (including 0%) of a lubricant, and compression molding is performed. 2. The molding method according to 1. 3. The molding method according to claim 1, wherein the temperature during compression molding is 80 to 500 ° C. 4. The temperature at the time of compression molding is not less than the melting point (Tm) of the lubricant compounded in the powder for powder metallurgy and (Tm ×
3) The molding method according to claim 3, which is controlled as follows. 5. The molding method according to claim 1, wherein the frequency of the vibration is 5 Hz to 20 kHz.