JP2008507623A - A method for preparing a feedstock of nano-sized metal powder and a method for producing a sintered body using the feedstock. - Google Patents

Abstract

A method for preparing a feedstock of nano-sized metal powder and a method for producing a sintered body using the feedstock.
Disclosed is a method for producing a nano-sized metal powder feedstock. The method includes providing a nano-sized metal powder, mixing the metal powder with a solution of an organic binder in a solvent, and wet milling the mixture until an agglomerate of the metal powder is uniformly formed. It consists of stages. Further disclosed is a method for producing a sintered body using the feedstock.
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Description

TECHNICAL FIELD The present invention relates to a method for preparing a feedstock of nano-sized metal powder. More particularly, the present invention provides a method for preparing a feedstock suitable for the manufacture of a sintered body of nano-sized metal powder that can be fully densified without deformation such as strain and cracks, and uses the feedstock. The present invention relates to a method for manufacturing a sintered body.

BACKGROUND ART A conventional method for producing a sintered body using a metal powder having a micrometer size is generally performed by powder injection molding. The compact of the metal powder formed after the removal of the organic binder has only a density of about 50%, and for this reason, complete densification and uniform shrinkage of the sintered body cannot be achieved. .

  A process that provides a large driving force for sintering, such as high temperature sintering, is required for complete densification of the sintered body. The problem with the process is that large particles are produced, thus accompanied by undesirable deformations to the material properties, which degrades the physical properties of the sintered body. In order to solve the above mentioned problems, further process steps have been used, for example addition of small amounts of alloying elements, pressing during sintering and reshaping.

  Carburization and post-annealing after sintering are mainly used to improve the mechanical properties of the sintered body of Fe-Ni based powder, which is the most popular sintered body as a material for powder metallurgy products. used. However, such additional processes complicate the overall procedure, require significant manufacturing costs, and reduce corrosion resistance due to the presence of carbon added during the carburizing anneal. As a result, the conventional method of manufacturing a sintered body using a metal powder of micrometer size has a low density of the molded body, the manufacturing procedure is complicated by an additional process, and the physical body of the sintered body There is a problem that the characteristics deteriorate.

  In order to fundamentally solve the problems of the conventional methods, methods for producing a sintered body using nano-sized metal powder having a size of 100 nm or less are being actively studied. Nano-sized metal powders have excellent sinterability and can be uniformly shrunk and fully densified by low temperature / atmospheric pressure sintering techniques. Furthermore, since the nano-sized metal powder has a high uniformity and a fine crystal structure, the product properties are improved. Under these circumstances, the application of nano-sized metal powder to metal injection molding technology has been actively researched.

  However, despite the excellent availability of nano-sized metal powders, manufacturing and sintering filling techniques have not been established and, therefore, their application to the production of substantially reticulated sintered bodies is still insufficient. .

Patent Document 1 (Title of Invention: Method of producing a feedstock of nano-sized metal powder for metal injection molding, Patent holder: Hanyang Educational Institute
Institute)) is a feedstock for metal injection molding where the explosive oxidation of nano-sized metal powders can be controlled while maintaining the shape of the product during manufacture, and complete densification of the product can be achieved Has proposed a method of manufacturing. According to the method, the coating of the binder on the nano-sized metal powder suppresses the explosive oxidation of the nano-sized metal powder and improves the complete densification of the product.

  However, the method is limited to the production of nano-sized metal powder feedstocks and does not fully consider applicability to substantially reticulated products. In particular, since nano-sized powder has a large interfacial energy, a non-uniform pore distribution can occur in the molded body after debinding. Further, even after sintering at low temperature / atmospheric pressure, pores remain, which deteriorates the mechanical properties of the sintered body. Therefore, the nano-sized powder should be subjected to a high temperature sintering process at 1000 ° C. or higher. However, high temperature sintering causes a reduction in the physical properties of the sintered body and makes it impossible to take advantage of the low temperature / atmospheric pressure sintering, i.e. complete densification of the particles and particle growth. . Furthermore, according to the method, 5 or 6 thermoplastic binders with different degreasing temperatures are used during degreasing to prevent deformation of the molded body caused by rapid removal of the binder occupying 40-60% of the total volume. Is done. The method therefore has the problem of complicated procedures and high production costs. Furthermore, since the increase in the degreasing temperature should be sufficiently slow, the overall process time is lengthened.

Thus, conventionally, a method for preparing a nano-sized metal powder feedstock that is practically applicable to the production of a substantially reticulated sintered body and that is suitable for sintering at a low temperature / atmospheric pressure, and the feedstock There is a need for a method of producing the used sintered body.
Korean Registered Patent No. 0366773 Specification

DISCLOSURE OF THE INVENTION Technical Problem Therefore, the present invention has been made in view of the above-mentioned problems of the prior art, and the object of the present invention is to control coarse pores during subsequent degreasing by structural control of nano-sized metal powder. It is an object of the present invention to provide a method for preparing a nano-sized metal powder feedstock suitable for producing a sintered body that can be completely densified by suppressing generation.

  Another object of the present invention is to provide a method for producing a sintered body having a uniform crystal grain size which is completely densified by degreasing using the feedstock for a shorter period of time.

TECHNICAL SOLUTION According to one aspect of the present invention to achieve the above object, the steps of providing a nano-sized metal powder, mixing the metal powder with a solution of an organic binder in a solvent, and the metal powder There is provided a method for preparing a nano-sized metal powder feedstock comprising the step of wet milling the mixture so that agglomerates of the same are formed uniformly.

  Preferably, the mixing step and the wet milling step are performed at the same time to further simplify the procedure of the method.

  According to the method of the present invention, the pores can be uniformly distributed during the subsequent formation of the molded body. Therefore, even if only one or two organic binders are mixed with the metal powder, the desired degreasing is a deformation of the molded body. Can be done without.

  The organic binder is preferably a water-soluble binder, and the solvent is preferably distilled water or alcohol. The water-soluble organic binder can be stearic acid.

For improved coating effect, the viscosity of the binder solution is preferably 2 Pa · s or less, and more preferably 2 Pa · s or less, at 100 to 200 ° C. For a sufficient coating effect, a binder solution having a viscosity of 0.002 Pa · s is preferably used.

  The nano-sized metal powder is an Fe-based alloy powder and includes at least one selected from the group consisting of Ni, Cu, Mo, and W. A typical nano-sized metal powder is an Fe—Ni powder having a Ni content of 2 to 80% by mass.

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

  The mixing step and the wet milling step are preferably performed in a state where the atmosphere is shut off. In particular, the step can be carried out in an inert gas or protective gas atmosphere.

  According to another aspect of the present invention, a method for producing a sintered body using nano-sized metal powder is provided. The method comprises the steps of providing a feed stock of nano-sized metal powder, forming the nano-sized metal powder feed stock into a predetermined shape, degreasing the formed body, and sintering the degreased body. It consists of stages.

  In a specific embodiment, the molding step may be performed by injection molding or extrusion molding. The degreasing step is performed by heating the molded body to approximately 300 ° C. to approximately 500 ° C. at a rate of 3 to 10 ° C./min, whereby the degreasing time is shortened to approximately 2 hours.

  The sintering step may be performed by rapidly heating the degreased molded body to 500 to 1000 ° C. at a rate of 300 ° C./min or more. The sintering step is preferably performed after the degreasing step.

  The sintered body thus produced has a crystal grain size of 200 nm or less and a density of 95% or more.

FIG. 1 is a scanning electron micrograph (SEM photograph) 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 particle size analysis of the feedstock of nano-sized Fe—Ni alloy powder prepared in Example 1 of the present invention using a laser particle size analyzer. FIG. 3 is a scanning electron micrograph (200 ×) of a fracture surface of a degreased body of nano-sized Fe—Ni alloy powder obtained in Example 2 of the present invention. FIG. 4 is a scanning electron micrograph (20000 times) of a fracture surface of a degreased body of nano-sized Fe—Ni alloy powder obtained in Example 2 of the present invention. FIG. 5 shows photographs of a compact and sintered body of 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 nano-sized Fe—Ni alloy powder produced in 2 of the present invention. FIG. 7 is a scanning electron micrograph of the over-etched surface of the sintered body shown in FIG.

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

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

  The use of a binder solution allows the binder to be more effectively applied to the surface of the powder particles, so that particle oxidation can be prevented. Therefore, even when a small amount of binder is added during molding, the viscosity of the binder solution is reduced for sufficient coating, thereby allowing nano-sized that can be stored in the atmosphere for extended periods without oxidative contamination. A metal powder feedstock can be provided.

  The binder is generally mixed in an amount of about 2% to about 50%. When the binder is added in a relatively small amount, for example, in the two-way compression molding, the method disclosed in Patent Document 1 cannot achieve a sufficient covering effect. In contrast, in the present invention, the use of a binder solution and a wet milling process ensures a uniform distribution of aggregates and a more effective coating of the binder.

  The solvent used to form the binder solution is not particularly limited to distilled water or alcohol. Any solvent can be used as long as it forms a binder solution having a sufficiently low viscosity. Various known solvents can be used depending on the type of binder. In this case, the binder solution preferably has a viscosity of about 2 Pa · s or less at about 100 ° C. to 200 ° C.

  In the method for producing the feedstock of the present invention, the step of mixing the nano-sized metal powder and the binder solution and the step of wet milling to control the uniform size of the agglomerates are performed at the same time. Can be further simplified. For example, a binder solution and nano-sized metal powder are put into a milling machine and the mixture is milled. Mixing and grinding in a milling machine allows both binder solution coating and agglomerate size control. These process steps are preferably performed with the atmosphere shut off. More specifically, the process steps are preferably performed in a clean apparatus filled with an inert or protective gas.

  For a more uniform distribution, a small amount of surfactant can be used as a dispersant if desired. The surfactant is preferably added in an amount not exceeding 2% by weight based on the weight of the mixture so as not to degrade the properties of the sintered body. In order to obtain a sufficient effect of the surfactant, it is preferable to add the surfactant in an amount of 0.5% by mass or more.

  In the case where nano-sized metal feedstock is used to produce a sintered body, the agglomerates are uniformly distributed in the feedstock, thereby suppressing the generation of coarse pores. Therefore, deformation resulting from binder separation during subsequent degreasing is minimized. Therefore, unlike the conventional method (when more than 5 types of binders are used depending on the temperature step gradient), one or two types of binders are used in the present invention, thereby simplifying the method procedure. I can do it. Further, the degreasing is performed by heating the molded body to 300 ° C. to 500 ° C. at a rate of 3 to 10 ° C./min, whereby the degreasing time can be shortened to about 2 hours.

  Furthermore, since the degreased body has a uniform particle size without the generation of coarse pores, low temperature / atmospheric pressure sintering in a temperature range of 500 to 1000 ° C. is applied to the degreased body to obtain crystals of 200 nm or less. Nano-sized metal products having a particle size and a density of 95% or more can be produced.

Aspects of the Invention Preferred aspects of the invention are described in greater detail in conjunction with the accompanying drawings. The advantages and advantages of the present invention are better understood by the embodiments.

Example 1
First, an Fe—Ni alloy powder was prepared as a nano-sized metal powder according to the following procedure. That is, Fe oxide and Ni oxide each having an average particle diameter of 1 μm are mixed so as to have a mass ratio of 92: 8, and then high energy ball milling is performed for 10 hours in a steel grinder. The mixture was pulverized to a size of 10-20 nm.

  Thereafter, the finely pulverized mixture was dried and reduced at 450 ° C. for 40 minutes under a hydrogen atmosphere to prepare nano-sized Fe-8 mass% Ni alloy powder. As shown in FIG. 1, particles having a size of about 70 nm gathered to form an aggregate having a size of about 5 to several tens of μm.

Next, the binder solution and the surfactant were added to the nano-sized Fe-8 mass% Ni alloy powder. The binder solution was prepared by mixing 5 g of stearic acid (CH ₃ (CH ₂ ) COOH) and 35 ml of ethanol as a solvent. Octanol (C ₈ H ₁₈ as surfactant)
O) 0.5 mol was used.

  In this example, the above mixing was performed together with wet milling using a three-dimensional mixer. Specifically, the milling was performed for 9 hours at 60 rpm using 40 g of steel balls. The milled mixture was dried until the loading rate of 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 particle size analysis of a feedstock of nano-sized Fe—Ni alloy powder using a laser particle size analyzer (LPA). As described above, the feedstock of nano-sized Fe—Ni alloy powder is obtained by adding 0.5 mol of octanol as a surfactant to Fe-8 wt% Ni nano-sized metal powder in 35 mL of ethyl alcohol, and the mixture Was prepared by wet milling for 9 hours using 40 g of steel balls. Laser particle size analysis showed that powder particles having a size of several tens of μm were efficiently ground and dispersed by wet milling to form aggregates having an average size of 700 nm.

(Example 2)
In this example, a cylindrical sintered body was manufactured using a feed stock of nano-sized metal powder.
First, the nano-sized metal powder feedstock prepared in Example 1 was injected into a cylindrical mold at 100 ° C. under a pressure of 100 MPa to produce a cylindrical formed body. The cylindrical formed body thus produced was densified to approximately 52% (see the left side of FIG. 5).

  Thereafter, in order to prevent the injection-molded Fe-8 mass% Ni alloy powder against the occurrence of cracks due to oxidation, the molded body is heated to 400 ° C. at a rate of 5 ° C./min. went.

FIG. 3 is a scanning electron micrograph (200 times) of the fracture surface of the sample obtained after degreasing, and FIG. 4 is a scanning electron micrograph (20,000 times) of the fracture surface of the degreased body. As shown in FIGS. 3 and 4, no coarse pores (micrometer sized pores) that adversely affect the subsequent sintering process are observed and instead consist of uniform particles having a size not exceeding 100 nm. A fine structure was observed. This is because the wet milled aggregates in Example 1 are uniformly filled with pores between the aggregates that have not been finely pulverized.

  Based on the uniform distribution of aggregates, only one binder could be used to suppress the generation of coarse pores resulting from binder separation. Unlike the conventional method using 5 or more types of binders, the degreasing time could be shortened to 2 hours.

  Following the degreasing step, the degreased body was heated to 700 ° C. at a rate of 300 ° C./min and sintered for 30 minutes to 4 hours to produce a cylindrical sintered body having a density of 95% ( See right side of FIG.

  Compared with the cylindrical formed body having a density of 52% shown in FIG. 5, even after the above degreasing and sintering, deformation such as distortion and cracking is not observed, and the cylindrical sintered body The shape did not change during molding. As is clear from this example, a sintered body having a density of 95% or more can be produced from a degreased body having a density of 52% (after degreasing) even at a sintering temperature of about 700 ° C. It was. Therefore, the sintered body can be beneficial in the manufacture of complex, substantially mesh-sintered products.

  FIG. 6 is an optical micrograph (200 ×) showing the microstructure of a cylindrical Fe-8 mass% Ni sintered body. As shown in FIG. 6, the sintered body had a completely densified structure (density: 95% or more). FIG. 7 is a scanning electron micrograph (5000 magnifications) showing particles of a cylindrical Fe-8 mass% Ni sintered body after over-etching. As shown in FIG. 7, the sintered body had a fine structure in which particles having a particle size of about 300 nm were uniformly distributed.

In order to evaluate the mechanical properties of the Fe-8 mass% Ni sintered body, the micro hardness of the sintered body was measured at a load of 200 g with a micro Vickers hardness meter and averaged. Priced. The average value obtained was compared with the standard hardness value of a commercially available injection-molded sintered body. The results are shown in Table 1 below.

Sintered bodies (MIM-2200, 2700, 4605) according to the standards adopted by the Metal Powder Industry Federation (MPIF) have a Vickers hardness value of 85 to 130. Furthermore, the sintered body of injection molded metal powder according to Japanese standard is subjected to carburizing and annealing to improve the mechanical properties of injection molded Fe-Ni powder, so 300 (in the case of 2 mass% Ni) And a high Vickers hardness value of 340 (in the case of 8 mass% Ni). The Fe-8 mass% Ni sintered body produced in the present invention had a Vickers hardness value of 298 which is more than twice the value determined by the US standard. Furthermore, the Vickers hardness of the Fe-8 mass% Ni sintered body produced in the present invention is stipulated in Japanese standards without including further carburizing and annealing to improve the mechanical properties of the sintered body. It was comparable to the value made.

  While the invention has been described with reference to the above-described embodiments and accompanying drawings, the scope of the invention is defined by the appended claims. Accordingly, those skilled in the art can make various modifications, improvements, and alterations within the scope of the technical idea of the present invention disclosed in the appended claims, and such substitutions, improvements, and modifications. Will also be recognized as within the scope of the present invention.

INDUSTRIAL APPLICABILITY As is apparent from the above, according to the method of the present invention, by applying wet milling in the presence of a binder solution, the binder can be efficiently coated on the surface of the powder particles and Allows uniform control of dimensions. The nano-sized metal powder feedstock prepared by the method of the present invention can maintain the internal structure of the compact uniformly and finely, so that it can be used after sintering using the nano-sized metal powder feedstock. However, a fully densified, substantially reticulated nanostructured product can be produced without deformation such as strain and cracks. Therefore, according to the method of the present invention, simplification of the manufacturing procedure and reduction of the manufacturing cost can be expected.

Claims (17)

Providing nano-sized metal powder;
Mixing the metal powder with a solution of an organic binder in a solvent; and wet milling the mixture until an aggregate of the metal powder is uniformly formed;
A method for preparing a nano-sized metal powder feedstock comprising: The method of claim 1, wherein the mixing and wet milling are performed simultaneously. The method of claim 1, wherein the metal powder is mixed with one or two organic binders. The method according to claim 1 or 3, wherein the organic binder is a water-soluble binder, and the solvent is distilled water or alcohol. The method according to claim 4, wherein the water-soluble organic binder is stearic acid. The method according to claim 1, wherein the binder solution has a viscosity of 2 Pa · s or less at 100 to 200 ° C. The method of claim 1, wherein the nano-sized metal powder is an Fe-based alloy powder and includes at least one metal selected from the group consisting of Ni, Cu, Mo and W. The method according to claim 7, wherein the nano-sized metal powder is Fe-Ni powder having a Ni content of 2 to 80 mass%. The method of claim 1, wherein the mixing further comprises a sub-step of adding a surfactant to the mixture. The method of claim 9, wherein the surfactant is added in an amount not exceeding 2% by weight. The method according to claim 1, wherein the mixing and the wet milling are performed in a state where the atmosphere is shut off. The method of claim 11, wherein the mixing and wet milling are performed in an inert gas or protective gas atmosphere. Providing a feedstock of nano-sized metal powder prepared by the method of any one of claims 1-12;
Forming the nanosized metal powder feedstock into a predetermined shape;
Debinding the shaped body; and sintering the degreased body;
A method for producing a sintered body using nano-sized metal powder. The method according to claim 13, wherein the forming is performed by injection molding or extrusion. The method according to claim 14, wherein the degreasing step is performed by heating the shaped body to approximately 300 ° C to approximately 500 ° C at a rate of 3 to 10 ° C / min. The method according to claim 14, wherein the sintering is performed by sintering the degreased compact at 500 to 1000 ° C. The method according to claim 16, wherein the sintered body has a crystal grain size of 200 nm or less and a density of 95% or more.

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