JPH0649505A - Method for improving dimensional accuracy in sintering - Google Patents

Abstract

PURPOSE:To produce a sintered compact with the dimensional accuracy remarkably improved by incorporating a specified jig for preventing the contraction of a green compact into the recess of the green compact to be contracted in sintering and then sintering the green compact. CONSTITUTION:A jig with the outer-diameter shape almost similar to the inner shape of the bored or recessed part of the green compact except locally different, with the size smaller than the inner size of the green compact and with the size equal to or smaller than the dimensional contraction amt. of the green compact in sintering is incorporated into the green compact consisting of a metal (e.g. SUS 430 atomized alloy powder), and the green compact is sintered. At this time, the thermal expansion coefficient of the jig is made equal to or larger than that of the green compact, and the difference is controlled to <=5X10<-6>/ deg.C. Consequently, the green compact is solid-phase-sintered with good dimensional accuracy, and sizing, etc., are not needed for correction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for solid-phase sintering a powder compact, which is obtained by solidifying a metal powder and has a recess inside, which shrinks or expands during sintering with high dimensional accuracy. It is about. Sintering methods are used to fabricate metal articles of various shapes. In this method, metal powder is put into a mold and pressure is applied to form a powder compact, which is heated in a furnace.

Powder compacts are intermediate, in that metal powders are simply solidified. The metal powders are bonded to each other by sintering. The object of the present invention is one having a recess inside. That is, it has a shape such as a boring or a hollow. The outer shape is arbitrary.

[0003]

2. Description of the Related Art Conventionally, a powder compact, even if it has a recess therein, was placed in a furnace in a free state and restrained by heating without restraining the powder compact. Depending on the material of the metal powder, there are those that expand and contract due to sintering. When such a material is used, since it allows free expansion and contraction, the dimensions will change due to sintering. That is, the inner and outer diameters of the powder compact and the sintered body differ. Moreover, the dimensional change is anisotropic.

As a conventional similar sintering method, as shown in FIG. 9, there is an example of a method of manufacturing a metal filter such as bronze or stainless powder, in which an outer die, a middle die and a bottom die are filled with powder and sintered. is there. This is a method in which a sintered body is made from metal powder all at once (powder filling method). The step of once forming a powder compact and sintering this is not taken. This is to prevent the powder from flowing out and losing its shape before the powder is combined. There is a possibility that the powder will flow out because we do not make a powder compact once, and a medium-sized mold is put in to prevent this. The purpose is different from the present invention.

In the conventional method, a powder compact, even if it has a boring, is formed in a free state and then sintered. It does not add any restraint to the powder compact.

[0006]

Conventionally, in this type of sintered product, the powder compact and the sintered compact are not completely the same in shape and dimension, so post-processing has been essential. The cause of the difference in shape and size between the powder compact and the sintered compact is that the powder density partially differs during powder compaction. When this is sintered, contraction and expansion will occur depending on the difference in powder density. The powder compact is heated in the sintering furnace and the sintered compact is cooled,
The coefficient of thermal expansion between the powder compact, the sintered compact and the object with which they come into contact, such as a refractory plate, is different. For this reason, uneven stress is applied to the powder compact or the sintered body from the contacting body, which causes variations in the dimensions of the inner and outer peripheries of the sintered body.

Due to the above reasons, the sintered body and the powder compact are not similar to each other. Moreover, this difference in shape and size differs depending on the product. It is not enough to design the shape and size of the powder compact by considering the deformation from the beginning.

Therefore, in order to ensure the dimensional tolerance accuracy of the product, it was necessary to add steps such as (a) machining (cutting, polishing, etc.) (b) sizing for straightening after sintering.

Such a phenomenon that the powder compact and the sintered compact do not have a similar shape but differ from each other in shape is not a serious problem in the hydrostatic compact. In the case of hydrostatic molding,
This is because there is no partial difference in density during powder molding described in 1. and no difference in shrinkage or expansion occurs due to sintering.

However, the dissimilarity phenomenon between the powder compact and the sintered compact is likely to occur in the compact which is formed by applying an axial force in the vertical direction. This is because the load when the powder is solidified has anisotropy and a strong load is applied in the axial direction, but only a weak load is applied in the axial vertical direction (transverse direction). Since the molding is performed by a uniaxial force, the lateral stress is remarkably different between the part near the mold surface and the part far from the mold surface.

In particular, among the uniaxially molded powder compacts, (a) a thin, hollowed-out ring-shaped product, (b)
This is a problem that can occur with inner and outer diameters that have complicated entrances and exits, and (c) long hollow bodies. This is because the variation of stress in the lateral direction is particularly remarkable and the degree of freedom of deformation in the lateral direction is high. Here, such a shape is referred to as a shape having a hollow or a hollow. Or simply referred to as a shape having a recess inside.

[0012]

The present invention is to solve the above problems. If the powder compact shrinks due to sintering, sintering is performed with the jig placed inside the powder compact to prevent inward deformation of the inner periphery, and the powder compact is baked. When the powder compact is expanded by binding, the powder compact is sintered with the jig fitted to the outer periphery of the powder compact to prevent outward deformation of the outer periphery. The jig prevents free expansion and contraction of the powder compact and sinters with high dimensional accuracy.

That is, according to the method for improving the dimensional accuracy of sintering of the present invention, a powder compact having a boring or hollow shape, which is made of a material that shrinks by sintering, has an outer diameter shape of the hollow boring middle hollow. It is almost similar except for the difference between the inner peripheral shape and the local shape, and the dimension of each part of the similar shape is smaller than the inner peripheral dimension of the powder compact, and the dimensional difference is the amount of dimensional shrinkage during sintering of the powder compact. A jig which is equal to or less than the above is incorporated into the inner circumference of the powder compact and sintered.

Alternatively, a powder compact which is a material that expands due to sintering and has a hollow or a hollow is substantially similar in inner diameter except for the difference in the outer peripheral shape and the local shape of the compact. The feature is that each jig of the similar shape is larger than the outer circumference of the compact, and the jig whose difference in size is equal to or less than the dimensional expansion amount at the time of sintering of the compact is incorporated and sintered in the outer periphery of the compact. And

Regarding the difference in the coefficient of thermal expansion between the powder compact and the jig, (a) in the case of a material that contracts due to sintering, the coefficient of thermal expansion is equal to or higher than that of the material of the powder compact, and A jig having a difference of 5 × 10 ⁻⁶ / ° C. or less is incorporated into the inner circumference of the powder compact and sintered. (A) If the material is one that expands due to sintering, a jig having a coefficient of thermal expansion equal to or less than the coefficient of thermal expansion of the material of the powder compact and having a difference of 5 × 10 ⁻⁶ / ° C. or less is powder molded. It is built into the outer circumference of the body and sintered.

Since it is difficult to understand, it is simply expressed by the following formula. [In case of material that shrinks by sintering] Insert a jig in the inner circumference. Let A be the outer diameter of the outer periphery of the jig, B be the inner diameter of the inner periphery of the powder compact, and X be the amount of shrinkage of the inner periphery due to sintering.

BX ≦ A <B (1)

That is, If the coefficients of thermal expansion of the jig and powder compact are Σ _a and Σ _b ,

[0019] _{0 ≦ Σ a -Σ b ≦ 5} × 10 -6 / ℃ (2)

That is, However, the outer diameter A and the inner diameter B are abstract here. In some cases, the outer diameter and inner diameter cannot be defined because the inner circumference is not necessarily circular. Here, (BA) / 2 means the size of the void.

[In the case of a material that expands by sintering] A jig is fitted on the outer peripheral portion. Let C be the inner diameter of the jig, D be the outer diameter of the outer peripheral portion of the powder compact, and Y be the expansion amount of the outer peripheral portion due to sintering.

D + Y ≧ C> D (3)

That is, If the coefficients of thermal expansion of the jig and powder compact are Σ _c and Σ _d ,

0 ≦ Σ _d −Σ _c ≦ 5 × 10 ⁻⁶ / ° C. (4)

1 to 4 show the shape of a jig to be put inside the powder compact when it shrinks due to sintering. The jig shown in FIG. 1 is a jig having a straight column shape at the top and bottom. This jig has various shapes such as a cylindrical shape and a prismatic shape, corresponding to the shape of the powder compact. There is a narrow gap between the jig and the powder compact. The powder compact is shown as a simple ring, but this is for simplifying the drawing and can be applied to more complicated powder compacts.

The jig shown in FIG. 2 has a flange at the lower end of the jig. The one shown in FIG. 3 is a hollow cylindrical jig. FIG. 4 shows a skewered one. A long pillar jig
It penetrates the hole of the molded body. It is capable of processing several powder compacts of the same shape at one time. There are various other possibilities. An appropriate jig is combined depending on the material and shape of the powder compact.

Of course, the jig does not cause fusion or mutual reaction with the molded body composition, and it is necessary to use a jig whose shape uniformly changes between the sintering temperature and room temperature. Therefore, it is necessary to determine the material and dimensions of the jig according to the material of the product to be sintered and the sintering shrinkage rate. It is most preferable if similar or identical materials can be selected.

[0028]

The temperature of the furnace changes with time as shown in FIG. When the predetermined sintering temperature is reached, this temperature is maintained for an appropriate time. When the sintering is completed, it is gradually cooled. When the powder compact is heated below the melting point, the particles are diffused and densified into a sintered compact.

As shown in FIG. 5 (b), the dimensions of the jig and the molded body placed in the furnace in a free state change with temperature. Initially, the powder compact has a larger inner diameter. This difference
E) is shown in the graph. Since the jig undergoes reversible thermal expansion, it exhibits the same dimensional change as the temperature change (i.e. The molded body undergoes simple thermal expansion (hohe) when the temperature rises. However, it starts shrinking during sintering (heat). Shrinks smaller than the jig size when sintering is completed (G). Thereafter, the sintered body shrinks (torch) due to the usual temperature decrease. Finally (r), the shrinkage due to sintering becomes smaller than the jig. This is a deformation due to heat and sintering in the free state.

Therefore, if a jig is inserted into the recess of the powder compact, the deformation of the sintered body is limited by the jig. FIG. 6 shows the dimensional changes of the jig, the powder compact, and the sintered compact in the combined state. The jig changes are the same as in FIG. As the temperature changes, it changes from one to another. Since the molded body is larger than the jig at the beginning, it is non-contact and thermally expands to the hole. Since the sintering starts at, the contraction starts. It touches the jig with a tool and cannot shrink any more. However, sintering is still in progress. In the case of Nur, sintering is performed while dimensional change is prohibited. This is important.

Sintering is completed and a cooling period begins. Since the coefficient of thermal expansion of the sintered body is lower than the coefficient of thermal expansion of the jig, a void is generated between the sintered body and the jig (row and honey). That is, due to the difference in shrinkage, a clearance is generated between the outer circumference of the jig and the inner circumference of the sintered body. When cooling is completed, the sintered body becomes a work piece, which is larger than the jig (d). It is easy to remove the sintered body from the jig because it is not in close contact.

If the restraint zone (null) is long, the dimensional stability is increased. This is because the shrinkage of the sintered body occurs evenly, so that the entire surface comes into contact with the jig and the dimensions are completely defined by the jig. However, on the other hand, if the restraint time (null) is long, the clearance (one) during cooling becomes narrow. A large clearance is recommended to facilitate removal from the jig. The difference in the coefficient of thermal expansion greatly contributes to the clearance.

On the contrary, if the difference in the coefficient of thermal expansion is large, it is necessary to take an excessively large initial void (why) because the jig does not come into contact with the powder compact when the temperature is raised. There is a fear that the restraint of the body will be incomplete. Therefore, the difference in expansion coefficient is preferably 5 × 10 ⁻⁶ / ° C. or less.

[0034]

【Example】

[Example] In the case of a material that is dimensionally contracted by sintering The metal powder is SUS430 sprayed alloy powder, the shape of the powder compact is φ60 mm × φ80 mm × 10 mm, and the compact density is 6.5 g / cm ^3. did. This was sintered in a 1130 ° C. vacuum sintering furnace. The molding, the jig, and the clearance before sintering were performed under the following conditions.

Jig a. Single product sintering (sintering in free state without using jig) b. Jig having the shape shown in FIG. 1 (simple cylindrical jig) c. Jig with the shape shown in Fig. 2 (Jig with flange on the bottom) There are 5 types of clearance before sintering between the molded body and the jig. 0.1 mm b. 0.2 mm c. 0.3 mm d. 1.5mm e. 2.0 mm

Powder compact SUS430 atomized alloy powder.
Powder compact shape 60 mm x 80 mm x 10 mm ring. The density of the compact is 6.5 g / cm ³ . Sintering Sintered at 1130 ° C in a vacuum furnace.

Experiments were carried out on the above-mentioned powder-sintered compacts by simple sintering, on five types of clearances (a) to (e) using the jig shown in FIG. 1, and on the clearances using the jig shown in FIG. I am doing about things. After sintering, it was cooled and taken out from the furnace, and the strain amount of the inner diameter and the outer diameter was measured. The results are shown in Table 1.

[0038]

[Table 1]

As shown in Table 1, in simple sintering of a, there is nothing that restrains the strain, so that the strain amount (maximum-minimum) is large.

Using the jig shown in FIG. 1, b-i having a clearance before sintering of 0.1 mm has the best dimensional accuracy of 0.010 mm in molding strain value. The inner diameter strain of the molded body was 0.037 mm. It can be said that the high accuracy of b-a was corrected by being strongly restrained by the jig during sintering.

In the case of b-ro having a clearance before sintering of 0.2 mm using the jig shown in FIG. 1, the inner diameter strain amount is 0.012.
mm is excellent and good. In the case of b-ha having a clearance of 0.3 mm, the inner diameter strain is 0.027 mm, which gives good results. In addition, the clearance of 0.
The c-a made 1 mm is also excellent in the inner diameter strain of 0.011 mm. Such a material has little distortion and the distortion is constant, so that post-processing after sintering is not necessary.

However, the b-d having a clearance of 1.5 mm and the b-e having a clearance of 2.0 mm have inner diameter strains of 0.072 mm and 0.142 mm, which are not preferable. This is because there is almost no restraint effect by the jig.

[Example] In the case of a material that expands by sintering As shown in FIGS. 7 and 8, a grooved cylindrical molded body is put into a cylindrical jig and sintered while restraining expansion. After cooling, the dimensional accuracy (strain amount) was measured. Molded material Fe-2Cu (expanded by sintering) Molded shape φ48mm × φ60mm × 7mm Ring (grooved product at 4 places) Molded density 6.7g / cm ³ Clearance before sintering 0.1mm Sintering condition 1130 ° C vacuum Sintering furnace

Strain amount of 0.128 after simple sintering (without jig)
It was mm. As shown in FIGS. 7 and 8, when the jig of the present invention was used, the amount of strain was 0.015 mm. This is a significant improvement. Of course, post-processing after sintering is unnecessary.

[0045]

As described above, in the material which shrinks by sintering, a jig is incorporated in the inner peripheral portion of the molded body and sintered. Since the sintered body cannot shrink beyond the outer periphery of the jig, it is sintered while maintaining the shape and dimensions along the jig. Due to the difference in thermal expansion in the cooling process, a clearance is created between the sintered body and the jig, and the jig can be removed. By selecting the optimum shape, material and thermal expansion coefficient of the jig, the dimensional accuracy is significantly improved. That is, if the restraining process with the jig is long, the dimensional accuracy of sintering closer to the accuracy of the jig can be obtained.

When a jig is provided with a flange as shown in FIG. 2, the resistance to the sintered refractory plate is removed, the dimensional difference between the upper and lower parts of the product is reduced, and the accuracy is further improved.

Contrary to the material that expands, a jig is fitted into the outer peripheral portion of the molded body and sintered. Since the sintered body cannot expand beyond the inner circumference of the jig, it is sintered while maintaining the shape and dimensions along the jig. Due to the difference in thermal expansion in the cooling process, a clearance is created between the sintered body and the jig, and the jig can be removed. Even with the material that expands, the accuracy was improved because the product was restrained by the jig during sintering.

As described above, since the accuracy of the sintering is improved, it is possible to eliminate the steps such as machining and sizing for straightening after the sintering. This is a major factor in simplifying the process and contributing to productivity.

[Brief description of drawings]

FIG. 1 is a sectional view showing a combination of a simple columnar jig and a powder compact used in the present invention.

FIG. 2 is a sectional view showing a combination of a jig with a flange and a powder compact used in the present invention.

FIG. 3 is a sectional view showing a combination of a hollow columnar jig and a powder compact used in the present invention.

FIG. 4 is a cross-sectional view showing a combination of a jig for a long object used in the present invention and a plurality of powder compacts.

FIG. 5 is a graph showing changes over time in temperature and dimensions in the sintering and cooling process. (A) is a time change figure of the temperature of a furnace. (B) is a time change diagram of the inner diameter of the powder compact sintered body and the outer diameter of the jig in the free state.

FIG. 6 is a graph showing changes over time in the inner diameter of the powder compact sintered body and the outer diameter of the jig when the jig is combined with the powder compact sintered body.

FIG. 7 is a plan view showing a state in which a powder compact sintered body that expands due to sintering is fitted into a jig and sintered.

FIG. 8 is a cross-sectional view showing a state in which a powder compact sintered body that expands by sintering is fitted into a jig and sintered.

FIG. 9 is a cross-sectional view of a known metal filter sintering manufacturing method (powder filling method).

Claims (4)

[Claims] 1. A powder compact having the shape of a hollow or a hollow, which is made of a material that shrinks due to sintering, and the outer diameter of the powder compact is the phase of the inner peripheral shape of the hollow hollow and the local shape. Except for the differences, the jigs that are similar to each other, each dimension of the similar shape is smaller than the inner peripheral dimension of the powder compact, and the dimension difference is equal to or less than the dimensional shrinkage amount at the time of sintering of the powder compact A method for improving the dimensional accuracy of sintering, characterized by incorporating and sintering the molded body. 2. A powder compact having the shape of a hollow or a hollow, which is made of a material that shrinks by sintering, and the outer diameter of the powder compact is a phase of the inner peripheral shape and the local shape of the hollow. Except for the differences, they are almost similar, the size of each part of the similar shape is smaller than the inner circumference of the powder compact, and the difference in size is equal to or less than the amount of dimensional shrinkage during sintering of the powder compact, and its coefficient of thermal expansion. Is equal to or higher than the coefficient of thermal expansion of the powder compact material, and the difference is 5 × 1
A method for improving dimensional accuracy of sintering, which comprises incorporating a jig having a temperature of 0 ^-6 / ° C or less into an inner circumference of a powder compact and sintering the jig. 3. A powder compact having the shape of a hollow or a hollow, which is made of a material that expands due to sintering, and has an inner diameter that is substantially similar to that of the outer periphery of the compact except a difference in local shape. , Each dimension of the similar shape is larger than the outer peripheral dimension of the molded body,
A method for improving the dimensional accuracy of sintering, comprising incorporating a jig, the size difference of which is equal to or less than the dimensional expansion amount during sintering of the molded body, into the outer periphery of the molded body and sintering the jig. 4. A powder compact having a shape that is expandable by sintering and has a hollow or a hollow, and has an inner diameter that is substantially similar to that of the outer periphery of the compact except a difference in local shape. , Each dimension of the similar shape is larger than the outer peripheral dimension of the molded body,
The dimensional difference is equal to or less than the dimensional expansion amount of the compact during sintering, the thermal expansion coefficient is equal to or less than the thermal expansion coefficient of the powder compact material, and the difference is within 5 × 10 ⁻⁶ / ° C. A method for improving sintering dimension accuracy, which comprises incorporating a jig into the outer periphery of a molded body and sintering the jig.