JPH04285102A - Production of sintered body - Google Patents

Abstract

PURPOSE:To efficiently produce the sintered bodies having complicated shapes with high precision. CONSTITUTION:The powders having different particle diameters and density distributions are appropriately mixed to prepare a raw powder having >=40% tap density, and the raw powder is kneaded with <=38vol.% of a binder. The kneaded material is injection-molded, and the molded body is calcined in an atmosphere contg. 1-30% oxygen, degreased and then sintered in a reducing atmosphere.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a sintered body. More specifically, the present invention relates to an injection molding sintering method for producing sintered parts with complex shapes by molding powder by injection molding and sintering the molded body.

In the injection molding sintering method, metals or their various alloys as raw materials, and various ceramic powders including nitrides, borides, etc. are kneaded with an organic substance called a binder to prepare a so-called kneaded body. A sintered part is manufactured by molding these using injection molding, degreasing the binder by thermally decomposing it to release it, and then sintering it. According to this method, parts with complex shapes can be obtained using any metal, its various alloys, or ceramics, making it possible to integrally mold parts that previously relied on machining or joining, and reducing the number of parts. It has become possible to significantly reduce the amount of energy used, and the role it plays in improving the performance and reducing costs of many devices, including various OA equipment, is immeasurable.

0003

[Prior Art] In the injection molding sintering method, carbonyl powder or atomized powder having an average particle size of 20 μm or less is used as a raw material powder, as described in, for example, Japanese Patent Application Laid-Open No. 61-210101. . In addition, as a binder for injection molding, for example, Japanese Patent Publication No. 61-4
8563, a mixture of several types of compatible organic substances is used, and the amount added ranges from 40 to 50% by volume based on the raw material powder. In particular, the above-mentioned 2
When using powder with an average particle size of 0 μm or less, the amount of binder needs to be 45% by volume or more.

On the other hand, in the so-called degreasing step in which the binder in the molded body is removed by firing after injection molding, in order to prevent cracks and blisters that occur due to the decomposition of a large amount of binder and the scattering of the decomposed products, for example, Japanese Patent Publication No. 58-1893
02, pyrolysis in a non-oxidizing atmosphere such as nitrogen or argon has been carried out for many hours.

[0005]

[Problems to be Solved by the Invention] This conventional method has the following problems. 1. Carbonyl powder is Fe or Ni as raw material powder.
The disadvantage is that the degree of freedom in materials is limited. On the other hand, although atomized powder has a high degree of material freedom, the particle size distribution and surface condition of the powder differ from lot to lot, as well as the oxidation state of the surface, so the type and amount of binder must be adjusted for each lot obtained. There is a need. Furthermore, if the powder properties are not suitable for injection molding, there is a problem that it cannot be used as a raw material powder. Furthermore, in the atomization method, 20μ
Since the recovery rate of fine powder having an average particle diameter of m or less is as low as 10% or less of one charge, there is a drawback that the cost of the raw material powder becomes extremely high.

2. As a large amount of binder is used, as much as 40 to 50% by volume, when manufacturing complex, large or tall parts, the shape of the molded object cannot be maintained due to the softening of the binder. Many defects occur during degreasing. Furthermore, since sintering shrinkage increases, dimensional accuracy decreases, and there is a problem that it is difficult to produce precise parts with a near net shape.

3. In degreasing, a non-oxidizing atmosphere is used to thermally decompose the organic binder.
Since the decomposition reaction is slow and the temperature is relatively high, it takes a long time and has the disadvantage of reducing mass productivity. Furthermore, in degreasing in a non-oxidizing atmosphere, even if an attempt is made to shorten the time, there is a problem in that degreasing increases deformation and cracks and blisters are more likely to occur.

[0008]

[Means for Solving the Problems] According to the present invention, in order to solve the above problems, a raw material powder having a tap density of 40% or more is prepared by appropriately mixing powders having different average particle sizes and particle size distributions, This is kneaded with a binder in an amount of 38% by volume or less and injection molded, and the obtained molded body is degreased by firing in an atmosphere containing 1 to 30% oxygen, and then sintered in a reducing atmosphere. A method for manufacturing a sintered body is provided.

[0009]

[Operation] In the method of the present invention, first, powders having different average particle sizes and particle size distributions are mixed as appropriate, and this is used as a raw material powder. In addition, the mixed powder is controlled by tap density, and the tap density is 40% or more, preferably 55%.
% or more.

It is possible to change the average particle size and particle size distribution of the powder by suitably mixing powders with different average particle sizes and particle size distributions. This eliminates differences in powder properties between obtained lots, and provides a powder that is not obtainable using normal powder production methods and has a particle size distribution with excellent fluidity. In addition, the reason why such mixed powders are managed by tapped density is that tap density changes depending on powder properties such as average particle size, particle size distribution, particle shape, and surface condition, so it should be considered as a comprehensive value of powder properties. This is because it is possible. In this case, injection molding is possible if the tap density is 40% or more, and 55%
If this is the case, the amount of binder required can be significantly reduced.

[0011] Next, this raw material powder is
The mixture is kneaded with a binder in an amount of 8% by volume or less and then injection molded. In this case, it is preferable to use two or more types of binders, at least one of which is incompatible with the other binders or has a different softening temperature. Moreover, preferably, when kneading the raw material powder and these binders, kneading is performed at least twice or more. By coating the powder with the binder with the highest softening temperature during the first kneading, and performing the second and subsequent kneading at a temperature and pressure that does not destroy the coating layer around the powder, the temperature and pressure during injection molding can be reduced. Therefore, when the powders come into contact with each other, the coating layer acts as a sliding layer to prevent the powders from agglomerating,
This is because high liquidity can be secured. According to this method, the kneaded body only needs to have fluidity that allows injection molding, so the amount of binder used is 38% by volume of the raw material powder.
It becomes possible to greatly reduce the amount of heat to below, and it becomes possible to significantly improve the dimensional accuracy of the obtained sintered body.

The obtained molded body is then degreased by firing in an atmosphere containing 1 to 30% oxygen. When oxygen is present in the degreasing atmosphere, the decomposition reaction due to the cutting of the main chain of the organic binder is accelerated in a chain manner as radicals are generated. Therefore, the decomposition start temperature of the binder is lower, and the end temperature is also approximately 100°C lower than when a non-oxidizing atmosphere is used, making it possible to keep the degreasing temperature low and promoting the decomposition of the binder. Ru. On the other hand, rapid decomposition of the binder in a non-oxidizing atmosphere causes defects such as cracks and blisters in the degreased body, but in the presence of oxygen, the oxide film formed on the surface of the metal particles firmly binds the powders together. Because of this bond, the above-mentioned defective degreasing is suppressed, and the oxide film also greatly improves the handling properties of the degreased body. Therefore, this makes it possible to shorten the time required for the degreasing step, which previously took a lot of time.

[0013] Here, if the amount of oxygen is 1% or less, the effect due to the presence of oxygen as described above cannot be exhibited because the amount of oxygen is too small. In addition, if the oxygen content is 30% or more, the oxidation of the raw material powder will progress too much, and it will not be possible to reduce it sufficiently in the next sintering process in a reducing atmosphere, resulting in a decrease in product properties and decomposition of the binder. Because the reaction is too rapid, there is a problem in that cracks and blisters occur, which can lead to poor degreasing in large, complex-shaped parts.

The method of the present invention can be carried out, for example, as follows. Average particle size is 8μm and 20μm respectively
m, 47 μm three types of powder (A to C) at 15:70
:15 to obtain powder D. Alternatively, powder A (
Powder E (average particle size: 10 μm) instead of powder E (average particle size: 8 μm)
) and powder E and C at 20:67:13 respectively.
A powder having the same particle size distribution as Powder D can be obtained by mixing with Powder D. Similarly, a powder with the same particle size distribution as Powder D can be obtained by replacing Powders B and C with other powders having different average particle sizes and changing the mixing ratio.

In this way, even if the average particle size and particle size distribution of the powders used for mixing change, by changing the mixing ratio, the mixed powder before the average particle size and particle size distribution of the powders used for mixing change. It is possible to reproduce the particle size distribution of This indicates that it is possible to eliminate differences between lots of available powder. Furthermore, by appropriately changing the raw material powder and its mixing ratio, it is possible to produce powder having a particle size distribution that cannot be produced by normal atomization.

The powder obtained as described above is kneaded with a binder whose main components are polymethyl methacrylate (PMMA) and wax, and then injection molded. As for the amount of binder required for injection molding, the larger the tap density, the smaller the amount of binder required for injection molding, and when the tap density is 40% or more, the amount of binder required for injection molding is smaller than the amount of binder used in general injection molding sintering method. Injection molding is possible.

[0017] In the above, wax and PMMA are not compatible, and the softening temperature is 80°C for wax and 80°C for PMMA.
It is different from 140℃. After kneading the powder and PMMA at 180℃, the temperature was lowered to 100℃ and wax was added.
Knead again. The obtained kneaded body is injection molded.

The injection molded article is then degreased by heat treatment in a nitrogen atmosphere containing 1 to 30% oxygen. moreover,
This is sintered at 1400° C. for 1 hour in a hydrogen atmosphere.

[0019]

[Examples] The present invention will be further explained below with reference to Examples.

Example 1 Three types of Fe-6.5%Si alloy powders with average particle diameters of 8 μm, 20 μm, and 47 μm, respectively, were mixed at a ratio of 2:6:2, and the tap density was 55%. A powder was prepared. This powder was kneaded with a binder (binder A) containing polymethyl methacrylate as a main component to form a coating layer of binder A around the powder. Furthermore, a binder (binder B) mainly composed of this kneaded body and polyethylene
was kneaded. The total amount of binder was 35% by volume. This kneaded body was injection molded to produce a molded body having a shape as shown in FIG.

[0021] This molded body was degreased under the following conditions. Atmosphere: Nitrogen (mixed with 0 to 100% oxygen) Maximum temperature: 300 to 450°C Total degreasing time: 8 to 48 hours Heating and cooling were performed according to the curve shown in FIG. 2.

Next, these degreased bodies were heated in a hydrogen atmosphere for 14 hours.
Sintering was carried out at 00°C for 1 hour. Reduce the amount of oxygen in the degreasing atmosphere to 0~
FIG. 3 shows the amount of oxygen in the degreased body and the sintered body when it is changed to 100%. The amount of oxygen in the degreased body when the amount of oxygen in the degreased atmosphere is 1% or more is greater than the amount of oxygen when the amount of oxygen in the degreased atmosphere is 0%, indicating that the degreased body is oxidized. ing. However, the amount of oxygen in the sintered body is almost the same in the range of 0 to 30% oxygen in the degreasing atmosphere. From these facts, if oxygen exists in the degreasing atmosphere, the degreased body will be oxidized, but if the amount of oxygen in the degreasing atmosphere is 30% or less, it can be reduced by sintering in a hydrogen atmosphere (reducing atmosphere). It is recognized that

[0023] The amount of oxygen in the degreasing atmosphere is 0% and 20%.
FIG. 4 shows the relationship between the maximum temperature and the amount of binder remaining in the degreased body. To reduce the amount of binder remaining, remove the degreased body in nitrogen for 60 minutes.
It was calculated from the weight loss when heated to 0° C. and all the binder components were scattered. When the amount of oxygen in the degreasing atmosphere is 0%, the remaining amount of binder becomes 10% at the maximum temperature of 440° C., and the process can proceed to the sintering process. Generally, when the remaining amount of binder reaches 0%, it becomes difficult to handle the degreased body.
In the injection molding sintering method, 5-10% of the binder is added to the degreased body.
% remain. Furthermore, if the residual amount of binder in the degreased body exceeds 10%, defects are likely to occur due to sudden decomposition of the binder during sintering, so it is necessary to suppress the residual amount of binder to 10% or less. On the other hand, if oxygen is mixed into the degreasing atmosphere, the maximum temperature will be 340° C. and the remaining amount of binder will be 10%, making it possible to lower the maximum temperature during degreasing. In addition,
When the oxygen content in the degreasing atmosphere ranged from 1 to 100%, the same effect as when the oxygen content was 20% was obtained.

FIG. 5 shows the relationship between the amount of oxygen in the degreasing atmosphere and the shortest total degreasing time to obtain a defect-free degreased body. When the amount of oxygen in the degreasing atmosphere was 0 and the total degreasing time was less than 24 hours, no defect-free degreased body could be obtained no matter what temperature profile was used. When the amount of oxygen in the degreasing atmosphere was 1 to 30%, a degreased body with no defects was obtained with a total degreasing time of 12 hours or more. When the amount of oxygen in the degreasing atmosphere was 50%, a degreased body with no defects was obtained with a total degreasing time of 24 hours or more. The amount of oxygen in the degreasing atmosphere is 1
In the case of 00%, even if the total degreasing time was 48 hours, a defect-free degreased body could not be obtained. These results show that the degreasing time can be shortened by mixing 1 to 30% oxygen into the degreasing atmosphere.

The handleability of the degreased body was evaluated based on the height at which the degreased body would not chip or crack when dropped onto a wooden stand. The relationship between the amount of oxygen in the degreasing atmosphere and this height is shown in FIG. Note that the binder remaining amount in the degreased body was 10%. When the amount of oxygen in the degreasing atmosphere is 0%,
If the height of the drop onto the wooden stand was 1 cm or more, the degreased body would chip or crack. The amount of oxygen in the degreasing atmosphere is 1%.
In the above case, no chipping or cracking occurred in the degreased body even if it was dropped from a height of 10 cm or more. This is considered to be because the oxide film of the powder in the degreased body firmly binds the powder. In addition, when all the binder is scattered from the degreased body, the amount of oxygen in the degreased atmosphere is 0%.
In this case, the degreased body disintegrated just by touching it, but when the amount of oxygen in the degreasing atmosphere was 1% or more, the remaining amount of binder was 10%.
The results were similar to those in the case of .

Example 2 Fe-6.5%Si alloy powder in Example 1 was replaced with Fe-6.5%Si alloy powder.
Example 1 was repeated, with almost the same results as Example 1, except that 50% Co alloy powder was used.

The above embodiment is considered to be applicable to most materials as long as they are metals that can be reduced in hydrogen. For example, it can be applied to tool materials such as high speed steel and cemented carbide, mechanical component materials such as stainless steel, and magnetic component materials such as Fe-50%Co alloy, Fe-Si alloy, and pure iron.

[0028]

[Effects of the Invention] The method of the present invention makes it possible to shorten the degreasing time, and is expected to reduce costs and increase production. Furthermore, by improving the handling properties of the degreased body, it is possible to carry out transportation, which has been difficult until now due to the degreased body, and it is possible to improve the yield. Furthermore, since the atmosphere can be used as the degreasing atmosphere, running costs can also be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape and dimensions of a molded article produced in an example. a is a plan view, b is a sectional view taken along the line A-A, and the units of numbers are mm.

FIG. 2 is a graph showing heating and cooling curves used in degreasing in Examples.

FIG. 3 is a graph showing the amount of oxygen in the degreased body and the sintered body when the amount of oxygen in the degreased atmosphere is varied from 0 to 100%.

FIG. 4 is a graph showing the relationship between the maximum temperature and the amount of binder remaining in the degreased body when the oxygen content in the degreased atmosphere is 0% and 20%.

FIG. 5 is a graph showing the relationship between the amount of oxygen in the degreasing atmosphere and the shortest total degreasing time to obtain a defect-free degreased body.

[Figure 6] The relationship between the amount of oxygen in the degreasing atmosphere and this height when the handleability of the degreased body was evaluated by the height at which the degreased body would not chip or crack when dropped onto a wooden stand. This is a graph showing.

Claims (1)

[Claims] Claim 1: A raw material powder having a tap density of 40% or more is prepared by suitably mixing powders with different average particle sizes and particle size distributions, and this is kneaded with a binder in an amount of 38% by volume or less and injection molded. A method for producing a sintered body, which comprises firing the obtained molded body in an atmosphere containing 1 to 30% oxygen to degrease it, and then sintering it in a reducing atmosphere.