Method for Preparing Nano-Sized Metal Powder Feedstock and Method for Producing Sintered Body Using the Feedstock

Abstract

A method for preparing a nano-sized metal powder feedstock comprises the steps of preparing a nano-sized metal powder, mixing the metal powder with a solution of an organic binder in a solvent, and wet-milling the mixture so that aggregates of the metal powder are uniformly formed. Further disclosed is a method for producing a sintered body using the feedstock.

Description

TECHNICAL FIELD

The present invention relates to a method for preparing a nano-sized metal powder feedstock. More particularly, the present invention relates to a method for preparing a feedstock suitable for the production of a sintered body of a nano-sized metal powder that can be completely compacted without deformation, such as twists and cracks, and a method for producing a sintered body using the feedstock.

BACKGROUND ART

Conventional methods for producing a sintered body using a micrometer-sized metal powder are commonly performed by powder injection molding. A molded body of the metal powder formed after removal of an organic binder has a degree of compaction as low as about 50%, and thus complete compaction and uniform shrinkage of the sintered body cannot be achieved.

A process for providing a large driving force for sintering, such as high temperature sintering, is required for complete compaction of a sintered body. The problems encountered with the process are that large particles are grown and thus undesirable modifications to characteristics of the raw materials are involved, deteriorating the physical properties of the sintered body. To solve the above problems, additional processing steps, e.g., addition of a small amount of an alloy element, pressurization during sintering, and remolding, have been employed.

To improve the mechanical properties of a sintered body of an Fe—Ni based powder, which is the most widely sintered body as a material for a powder metallurgical product, carburization and post-annealing after sintering are mainly used. However, such additional processing renders the overall procedure complex, entails considerable production costs, and causes poor corrosion resistance due to the presence of carbon added during carburization annealing. In conclusion, the conventional methods for producing a sintered body using a micrometer-sized metal powder have the problems that the molded body has a low density, the production procedure is complicated by the additional processing, and the physical properties of the sintered body are deteriorated.

In attempts to basically solve the problems of the conventional methods, methods for producing a sintered body using a nano-sized metal powder having a size of 100□ or less are now being actively undertaken. Since nano-sized metal powders have superior sinterability, they can be uniformly shrunken and completely compacted by low-temperature/atmospheric pressure sintering techniques. In addition, since nano-sized metal powders have highly uniform and fine crystalline structures, product characteristics are improved. Under these circumstances, application of nano-sized metal powders to metal injection molding techniques is actively under study.

However, despite such superior availability of nano-sized metal powders, production and sintering compaction techniques have not been established and thus application to the production of near-net sintered bodies is as yet insufficient.

Korean Patent No. 0366773 (Title: A method for producing a nano-sized metal powder feedstock for metal injection molding, patentee: Hanyang Educational Institute) suggests a method for producing a feedstock for metal injection molding by which explosive oxidation of the nano-sized metal powder can be controlled and complete compaction of a product can be achieved while maintaining the shape of the product during production. According to this method, the coating of a binder to the nano-sized metal powder inhibits explosive oxidation of the nano-sized metal powder and improves complete compaction of the product.

However, the method is limited to the production of a feedstock of the nano-sized metal powder, and fails to sufficiently consider the applicability to a near-net product. Specifically, since the nano-sized powder has a large interfacial energy, non-uniform pore distribution may arise inside the molded body after debinding. In addition, pores remain even after low-temperature/atmospheric pressure sintering, thus deteriorating the mechanical properties of a sintered body. Accordingly, the nano-sized powder should undergo high-temperature sintering at above 1,000° C. However, high-temperature sintering causes deteriorated physical properties of the sintered body, and makes it impossible to utilize the advantages of low-temperature/atmospheric pressure sintering, i.e. complete compaction and growth of particles. In addition, according to the method, five or six thermoplastic binders having different debinding temperatures are used in order to prevent the deformation of the molded body arising from rapid removal of the binder, which occupies 40˜60% of the total volume, during debinding. Accordingly, the method has problems of complicated procedure and increased production costs. Further, since an elevation in debinding temperature should be sufficiently slow, the overall processing time is lengthened.

Thus, there is a need in the art for a method for preparing a nano-sized metal powder feedstock practically applicable to the production of a near-net sintered body and suitable for low-temperature/atmospheric pressure sintering, and a method for producing a sintered body using the feedstock.

DISCLOSURE OF INVENTION

Technical Problem

Therefore, the present invention has been made in view of the above problems of the prior art, and it is an object of the present invention to provide a method for preparing a nano-sized metal powder feedstock suitable for the production of a sintered body that can be completely compacted by preventing the occurrence of coarse pores during subsequent debinding through the structural control of the nano-sized metal powder.

It is another object of the present invention to provide a method for producing a sintered body that is completely compacted and has a uniform grain size by debinding for a shorter period of time using the feedstock.

Technical Solution

In accordance with one aspect of the present invention for achieving the above objects, there is provided a method for preparing a nano-sized metal powder feedstock comprising the steps of preparing a nano-sized metal powder, mixing the metal powder with a solution of an organic binder in a solvent, and wet-milling the mixture so that aggregates of the metal powder are uniformly formed.

Preferably, the mixing step and the wet-milling step are simultaneously carried out to simplify the procedure of the method.

According to the method of the present invention, since pores can be uniformly distributed during the subsequent formation of a molded body, desired debinding can be carried out without deformation of the molded body, despite mixing of only one or two organic binders with the metal powder.

It is preferred that the organic binder is a water-soluble binder and the solvent is distilled water or alcohol. The water-soluble organic binder may be stearic acid.

For improved coating effects, the viscosity of the binder solution is preferably 2 Pa□s or lower at 100˜200° C., and more preferably 1 Pa□s or lower. For sufficient coating effects, there can be preferably used a binder solution having a viscosity of 0.002 Pa□s.

The nano-sized metal powder is an Fe-based alloy powder and contains at least one metal selected from the group consisting of Ni, Cu, Mo and W. A representative nano-sized metal powder is an Fe—Ni powder whose Ni content is 2˜80 wt %.

The mixing step may further include the sub-step of adding a surfactant to the mixture. At this step, the surfactant is preferably added in an amount not exceeding 2 wt %.

The mixing step and the wet-milling step are preferably carried out in a state where atmospheric air is blocked. Specifically, the steps can be carried out in an inert gas or protective gas atmosphere.

In accordance with another aspect of the present invention, there is provided a method for producing a sintered body using a nano-sized metal powder. The method comprises the steps of preparing the nano-sized metal powder feedstock, molding the nano-sized metal powder feedstock into a desired shape, debinding the molded body, and sintering the debound body.

In a specific embodiment, the molding step can be carried out by injection molding or extrusion molding. The debinding step is carried out by heating the molded body to about 300° C. to about 500° C. at a rate 3˜10° C./min., thus shortening the debinding time to about 2 hours.

The sintering step can be carried out by rapidly heating the debound body to about 500˜1,000° C. at a rate of 300° C./min. or higher. It is preferred that the sintering step is carried out consecutively after the debinding step.

The sintered body thus produced has a grain size of 200□ or less and a degree of compaction of 95% or higher.

Hereinafter, various features of the present invention and effects thereof will be explained in more detail.

The present invention is characterized in that the size of the aggregates of the nano-sized metal powder is uniformly controlled so that the aggregates can be applied to low-temperature/atmospheric pressure sintering. Specifically, in the method for preparing a nano-sized metal powder feedstock according to the present invention, the nano-sized metal powder is mixed with the organic binder in a solution state and is wet-milled, thereby maintaining the size of the aggregates at a uniform level.

The use of the binder solution allows the binder to be more effectively coated on the surface of the powder particles so that oxidation of the particles is prevented. Accordingly, even when a small amount of the binder is added during molding, the viscosity of the binder solution is lowered for sufficient coating, thus providing a nano-sized metal powder feedstock that can be stored in air for a prolonged period of time without oxidative contamination.

The binder is commonly mixed in an amount of from about 2% to about 50%. In the case where the binder is added in a relatively small amount, e.g., in bi-directional compression molding, sufficient coating effects cannot be attained by the method disclosed in Korean Patent No. 0366773. In contrast, the use of the binder solution and wet-milling process in the present invention ensures uniform distribution of the aggregates and more effective coating of the binder.

The solvent used to form the binder solution is not especially limited to distilled water or alcohol. Any solvent can be used so long as it forms a binder solution having a sufficiently low viscosity. Various known solvents can be used depending on the particular kind of the binder. At this time, the binder solution preferably has a viscosity not higher than about 2 Pa□s at from about 100° C. to 200° C.

In the method for preparing a feedstock of the present invention, the step of mixing the nano-sized metal powder and the binder solution and the wet-milling step for uniform size control of the aggregates can be simultaneously carried out to simplify the procedure of the method. For example, the binder solution and the nano-sized metal powder are charged into a milling machine and the mixture is milled. Mixing and grinding in the milling machine enables both the coating of the binder solution and size control of the aggregates. These processing steps are preferably carried out in a state where atmospheric air is blocked. More specifically, it is preferred that the processing steps are carried out in clean equipment filled with an inert or protective gas.

For better uniform distribution, a small amount of a surfactant can be optionally used as a dispersant. The surfactant is preferably added in an amount not exceeding 2 wt %, based on the weight of the mixture, so as not to deteriorate the characteristics of the sintered body. For sufficient effects of the surfactant, it is preferred to add the surfactant in an amount of 0.5 wt % or higher.

In the case where the nano-sized metal feedstock is used to produce a sintered body, aggregates are uniformly distributed in the feedstock and thus occurrence of coarse pores is prevented. Accordingly, deformation arising from separation of the binder can be minimized during the subsequent debinding. Accordingly, unlike in conventional methods (where five or more binders are used depending on temperature gradients), one or two binders can be used in the present invention, thus simplifying the procedure of the method. In addition, the debinding is conducted by heating the molded body to 300˜500° C. at a rate 3˜10° C./min., thus shortening the debinding time to about 2 hours.

Furthermore, since the debound body has a uniform particle size without occurrence of coarse pores, low-temperature/atmospheric pressure sintering at a temperature range of 500˜1,000° C. can be applied to the debound body to manufacture a nano-sized metal product having a grain size of 200□ or less and a degree of compaction of 95% or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a scanning electron micrograph (SEM) of a nano-sized Fe—Ni alloy powder that can be used in the present invention.

FIG. 2 is a graph showing the results of the particle size analysis of a nano-sized Fe—Ni alloy powder feedstock prepared in Example 1 of the present invention using a laser particle size analyzer.

FIGS. 3 and 4 are scanning electron micrographs of the broken side of a debound body of a nano-sized Fe—Ni alloy powder obtained in Example 2 of the present invention at different magnifications (200× and 20,000×, respectively).

FIG. 5 shows photographs of a molded body and a sintered body of a nano-sized Fe—Ni alloy powder produced in Example 2 of the present invention.

FIG. 6 is an optical micrograph (200×) of a sintered body of a nano-sized Fe—Ni alloy powder produced in Example 2 of the present invention.

FIG. 7 is a scanning electron micrograph of an overetched surface of the sintered body shown in FIG. 6.

MODE FOR THE INVENTION

Preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. The advantages and effects of the present invention will be better understood by the embodiments.

Example 1

First, a nano-sized Fe—Ni alloy powder was prepared as a nano-sized metal powder in accordance with the following procedure. Specifically, an Fe oxide and an Ni oxide, each of which had an average particle size of 1 m, were mixed together to have a weight ratio of 92:8, and were then subjected to high-energy ball milling in a steel attritor for 10 hours to finely pulverize the mixture to a size of 10˜20□.

Thereafter, the pulverized mixture was dried and reduced under a hydrogen atmosphere at 450° C. for 40 minutes to prepare a nano-sized Fe-8 wt % Ni alloy powder. As shown in FIG. 1, particles having a size of about 70□ gathered to form aggregates having a size of about 5 m to tens of micrometers.

Next, a binder solution and a surfactant were added to the nano-sized Fe-8 wt % Ni alloy powder. The binder solution was prepared by mixing 5 g of stearic acid (CH₃(CH₂)COOH) and 35 ml of ethanol as a solvent. As the surfactant, 0.5 mol of octanol (C₈H₁₈O) was used.

In this example, the mixing was conducted together with wet-milling using a three-dimensional mixer. Specifically, the milling was conducted using 40 g of steel balls at 60 rpm for 9 hours. The milled mixture was dried until the loading rate of the nano-sized Fe-8 wt % Ni alloy powder reached 50%, to prepare a nano-sized metal powder feedstock.

FIG. 2 is a graph showing the results of the particle size analysis of the nano-sized Fe—Ni alloy powder feedstock using a laser particle size analyzer (LPA). As described above, the nano-sized Fe—Ni alloy powder feedstock was prepared by adding 0.5 mol of octanol as a surfactant to the Fe-8 wt % Ni nano-sized metal powder in 35 ml of ethyl alcohol, and wet-milling the mixture using 40 g of steel balls for 9 hours. The laser particle size analysis indicates that the powder particles with a size of tens of micrometers were efficiently pulverized and dispersed by wet-milling to form aggregates with an average size of 700□.

Example 2

In this example, a cylindrical sintered body was produced using a nano-sized metal powder feedstock.

First, the nano-sized metal powder feedstock prepared in Example 1 was injected into a cylindrical mold under 100 MPa at 100° C. to produce a cylindrical molded body. The cylindrical molded body thus produced was compacted to about 52% (see the left hand side of FIG. 5).

Thereafter, to protect the injection-molded nano-sized Fe-8 wt % Ni alloy powder against occurrence of cracks by oxidation, the molded body was subjected to debinding by heating to 400° C. at a rate of 5° C./min.

FIG. 3 is a scanning electron micrograph (200×) of the broken side of the sample obtained after debinding, and FIG. 4 is a scanning electron micrograph (20,000×) of the broken side of the debound body. As shown in FIGS. 3 and 4, no coarse pores (micrometer-scale pores) adversely affecting the subsequent sintering process were observed, and instead, a fine structure consisting of uniform particles with a size not larger than 100□ was observed. This is because the aggregates wet-milled in Example 1 were uniformly filled into pores between unpulverized aggregates.

Based on the uniform distribution of the aggregates, only one binder could be used to prevent the occurrence of coarse pores arising from separation of the binder. Unlike in conventional methods where five or more binders are used, the debinding time could be shortened to 2 hours.

Subsequently to the debinding, the debound body was heated to 700° C. at a rate of 300° C./min., and was sintered for from 30 minutes to 4 hours to produce a cylindrical sintered body having a degree of compaction of 95% (see the right hand side of FIG. 5).

Compared to the cylindrical molded body having a degree of compaction of 52% shown in FIG. 5, no deformation, such as twists and cracks, was observed in the cylindrical sintered body even after debinding and sintering, and the shape of the cylindrical sintered body was unchanged during molding. As is evident from this example, the sintered body having a degree of compaction of 95% or higher could be produced from the debound body having a degree of compaction of 52% (after debinding) even at a sintering temperature as low as 700° C. Therefore, the sintered body can be useful in the manufacture of a complicated near-net sintered product.

FIG. 6 is an optical micrograph (200×) showing the fine structure of the cylindrical Fe-8 wt % Ni sintered body. As shown in FIG. 6, the sintered body has a completely compacted structure (degree of compaction: 95% or higher). FIG. 7 is a scanning electron micrograph (5,000×) showing the grain of the cylindrical Fe-8 wt % Ni sintered body after overetching. As shown in FIG. 7, the sintered body has a fine structure wherein the grains having a size of about 300□ are uniformly distributed.

To evaluate the mechanical properties of the Fe-8 wt % Ni sintered body, the micro-hardness values at ten or more sites of the sintered body were measured with a load of 200 g by means of a micro Vickers hardness tester, and averaged. The obtained average value was compared with the standard hardness values of commercially available injection molded sintered bodies. The results are shown in Table 1 below.

	TABLE 1
	Sintered body	Composition	Hardness (Hv)
	Example 2	Fe-8 wt % Ni	298
	MIM-2200	Fe-2 wt % Ni	85
	MIM-2700	Fe-7 wt % Ni	130
	MIM-4605	Fe-2 wt % Ni-0.5 wt % C	110
	MIM-Fe2Ni	Fe-2 wt % Ni-0.6 wt % C	300
	MIM-Fe8Ni	Fe-8 wt % Ni-0.6 wt % C	340

The sintered bodies (MIM-2200, MIM-2700 and MIM-4605) according to the standard specification adopted by the Metal Powder Industry Federation (MPIF) have a Vickers hardness of 85˜130. In addition, since the injection molded metal powder sintered bodies according to the Japanese standard specification were subjected to carburization and annealing in order to improve the mechanical properties of the injection molded Fe—Ni powders, they have a high Vickers hardness of 300 (in the case of 2 wt % Ni) and 340 (in the case of 8 wt % Ni). The Fe-8 wt % Ni sintered body produced in the present invention has a Vickers hardness of 298, which is greater than two times that specified in the U.S standard specification. In addition, the Vickers hardness of the Fe-8 wt % Ni sintered body produced in the present invention is comparable to that specified in Japanese standard specification without involving additional carburization and annealing for improving the mechanical properties of the sintered body.

Although the present invention has been described herein with reference to the foregoing embodiments and the accompanying drawings, the scope of the invention is defined by the claims that follow. Accordingly, those skilled in the art will appreciate that various substitutions, modifications and changes are possible, without departing from the technical spirit of the present invention as disclosed in the accompanying claims, and such substitutions, modifications and changes are within the scope of the present invention.

INDUSTRIAL APPLICABILITY

As apparent from the foregoing, according to the method of the present invention, the application of wet-milling in the presence of the binder solution allows the binder to be more effectively coated on the surface of the powder particles and enables uniform control of the size of the aggregates. Since the nano-sized metal powder feedstock prepared by the methods of the present invention can maintain the internal structure of the molded body uniform and fine, completely compacted near-net nanostructured products can be manufactured without deformation, such as twists and cracks, even after sintering using the nano-sized metal powder feedstock. Therefore, according to the method of the present invention, simplification of the production procedure and reduction in production costs can be expected.

Claims

1. A method for preparing a nano-sized metal powder feedstock, comprising the steps of:

preparing a nano-sized metal powder;

mixing the metal powder with a solution of an organic binder in a solvent; and

wet-milling the mixture so that aggregates of the metal powder are uniformly formed.

2. The method according to claim 1, wherein the mixing step and the wet-milling step are simultaneously carried out.

3. The method according to claim 1, wherein the metal powder is mixed with one or two organic binders.

4. The method according to claim 1, wherein the organic binder is a water-soluble binder and the solvent is distilled water or alcohol.

5. The method according to claim 4, wherein the water-soluble organic binder is stearic acid.

6. The method according to claim 1, wherein the binder solution has a viscosity of 2 Pa·s or lower at 100˜200° C.

7. The method according to claim 1, wherein the nano-sized metal powder is an Fe-based alloy powder and contains at least one metal selected from the group consisting of Ni, Cu, Mo and W.

8. The method according to claim 7, wherein the nano-sized metal powder is an Fe—Ni powder whose Ni content is 2˜80 wt %.

9. The method according to claim 1, wherein the mixing step further includes the sub-step of adding a surfactant to the mixture.

10. The method according to claim 9, wherein the surfactant is added in an amount not exceeding 2 wt %.

11. The method according to claim 1, wherein the mixing step and the wet-milling step are carried out in a state where atmospheric air is blocked.

12. The method according to claim 11, wherein the mixing step and the wet-milling step are carried out in an inert gas or protective gas atmosphere.

13. A method for producing a sintered body using a nano-sized metal powder, comprising the steps of:

preparing a nano-sized metal powder feedstock prepared by the method according to claim 1;

molding the nano-sized metal powder feedstock into a desired shape;

debinding the molded body; and

sintering the debound body.

14. The method according to claim 13, where the molding step is carried out by injection molding or extrusion molding.

15. The method according to claim 14, where the debinding step is carried out by heating the molded body to about 300° C. to about 500° C. at a rate 3˜10° C./min.

16. The method according to claim 14, where the sintering step is carried out by sintering the debound body at 500˜1,000° C.

17. The method according to claim 16, where the sintered body has a grain size of 200 nm or less and a degree of compaction of 95% or higher.

18. A method for producing a sintered body using a nano-sized metal powder, comprising the steps of:

preparing a nano-sized metal powder feedstock prepared by the method according to claim 7;

molding the nano-sized metal powder feedstock into a desired shape;

debinding the molded body; and

sintering the debound body.

19. A method for producing a sintered body using a nano-sized metal powder, comprising the steps of:

preparing a nano-sized metal powder feedstock prepared by the method according to claim 8;

molding the nano-sized metal powder feedstock into a desired shape;

debinding the molded body; and

sintering the debound body.

20. A method for producing a sintered body using a nano-sized metal powder, comprising the steps of:

preparing a nano-sized metal powder feedstock prepared by the method according to claim 12;

molding the nano-sized metal powder feedstock into a desired shape;

debinding the molded body; and

sintering the debound body.