METAL MEMBER AND MANUFACTURING METHOD THEREOF

TECHNICAL FIELD

The present invention is related to metal member using metal powder as material and manufacturing method thereof.

BACKGROUND ART

Metal powder injection molding (or metal injection molding) is a method of manufacturing a metal powder molded article by melting a molding material, which results by mixing fine metal powder and organic binder (for example, a mixture of a plurality of types of resins; hereinafter called “binder”) to perform injection molding thereof and then performing degreasing and incinerating thereof.

The fine metal powder used in metal powder injection molding is formed in a fine powder production process by a spray method, for example. There were cases where production of fine powder for metal powder injection molding is difficult because a “pouring blockage”, in which a nozzle is blocked in a process of producing fine powder, occurs when forming a nickel based alloy including titan with a high strength at high temperatures by the spray method. Patent literature 1 discloses an invention of setting a concentration of titan in the nickel based alloy equal to or less than 0.1 mass % and an invention of performing an adjustment to decrease a concentration of niobium in a case where the concentration of titan is more than 1 mass %, in order to prevent this “pouring blockage”.

CITATION LIST
Patent Literature

[Patent Literature 1] Japanese patent publication No. 2005-350710 A

SUMMARY OF THE INVENTION

A metal manufactured by metal powder injection molding has lower strength at high temperatures compared to a metal manufactured by casting or forging and could not be applied to members needing strength at high temperatures.

In view of the above circumstance, an objective of the present invention is to manufacture a metal member with high strength at high temperatures by metal powder injection molding.

Other objectives may be understood by following descriptions and explanation of embodiments.

To achieve the above objectives, a metal member related to a first aspect of the present invention is provided with crystal grains of a metal and a granular reinforcing substance formed at boundaries of the crystal grains. The reinforcing substance includes grains of a shape with a grain area equivalent grain size larger than 1/100 of a grain area equivalent grain size of the crystal grains.

The above mentioned reinforcing substance preferably includes grains with a grain area equivalent grain size smaller than ⅕ of the grain area equivalent grain size of the crystal grains.

The above mentioned reinforcing substance preferably includes grain of a shape so that a value of a length, in a first direction in which a length thereof is longest, divided by a length of a longest part in a direction orthogonal to the first direction is smaller than 5.

It is preferable that 95% or more of the above mentioned reinforcing substance is formed so that a value of a length, in a first direction in which a length thereof is longest, divided by a length of a longest part in a direction orthogonal to the first direction is smaller than 5.

It is preferable that the above mentioned reinforcing substance includes a plurality of types of substances and is formed to surround the crystal grains.

The above mentioned reinforcing substance preferably includes any of carbon, nitrogen or oxygen.

A manufacturing method of metal member by injection molding related to a second target of the present invention includes a mixing step of mixing metal powder, reinforcing powder and binder, an injection molding step of forming an injection molded body by injection molding of mixed powders, a degreasing step of removing the binder from the injection molded body and forming an intermediate molded body, and an incinerating step of incinerating the intermediate molded body. The reinforcing powder includes a reinforcing substance. A maximal grain size of the reinforcing powder is larger than 1/100 of a maximal grain size of the metal powder. The metal powder and the reinforcing powder both are mixed in a powder state in the mixing step.

The maximal grain size of the above mentioned reinforcing powder is preferably smaller than ⅕ of a maximal grain size of the metal powder.

It is preferable that a step of determining a carbon concentration of the metal powder in accordance with a mass of the reinforcing powder to be mixed is further included.

According to the present invention, a metal member with a high strength at high temperatures can be manufactured by metal powder injection molding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing processes of a manufacturing method related to the present invention.

FIG. 2 is structure photography by backscattered electron imaging in connection with a cross sectional surface of a metal member related to the present invention.

FIG. 3 is a mapping image of titan concentration by Electron Probe Micro Analyzer (EPMA) analysis in connection with a cross sectional surface of the metal member related to the present invention.

FIG. 4 is a mapping image of carbon concentration by EPMA analysis in connection with a cross sectional surface of the metal member related to the present invention.

FIG. 5 is a schematic drawing showing a structure of the metal member related to the present invention.

FIG. 6 is a graph showing tensile strengths of the metal member related to the present invention and a metal member manufactured by conventional injection molding.

FIG. 7 is a graph showing elongations of the metal member related to the present invention and the metal member manufactured by conventional injection molding.

FIG. 8 is structure photography by backscattered electron imaging in connection with a cross sectional surface of a metal member manufactured by a conventional injection molding.

FIG. 9 is a mapping image of titan concentration by EPMA analysis in connection with a cross sectional surface of the metal member manufactured by a conventional injection molding.

FIG. 10 is a mapping image of carbon concentration by EPMA analysis in connection with a cross sectional surface of the metal member manufactured by a conventional injection molding.

FIG. 11 is a schematic drawing showing a structure of the metal member manufactured by a conventional injection molding.

FIG. 12 is structure photography by secondary electron imaging in connection with a cross sectional surface of a metal member manufactured by casting.

FIG. 13 is a mapping image of titan concentration by EPMA analysis in connection with a cross sectional surface of the metal member manufactured by casting.

FIG. 14 is a mapping image of carbon concentration by EPMA analysis in connection with a cross sectional surface of the metal member manufactured by casting.

FIG. 15 is a schematic drawing showing a structure of the metal member manufactured by casting.

FIG. 16 is a graph showing tensile strengths of the metal members with different carbon concentrations manufactured by a conventional injection molding.

FIG. 17 is a graph showing elongations of the metal members with different carbon concentrations manufactured by a conventional injection molding.

DESCRIPTION OF EMBODIMENTS
(Strength of Metal)

Causes of lower strength at high temperatures of metal manufactured by metal powder injection molding compared to metal manufactured by casting, forging or the like will be explained.

Although an influence of crystal grain boundaries on a tensile strength of a metal is usually limited in a temperature range near room temperature, the higher the temperature is, the stronger the influence of the crystal grain boundaries on the tensile strength of the metal becomes. In order to improve metal strength, in general, a metal is manufactured by including reinforcing substance including a substance such as carbon, oxygen, nitrogen or the like. This reinforcing substance also reinforces crystal grains boundaries.

In metal powder injection molding, grain surfaces are molten and grains are bonded to each other to be molded as a member. That is, in metal powder injection molding, grains are not entirely molten. For this reason, a metal manufactured by metal powder injection molding has weaker strength at boundaries of crystal grains, compared to metals manufactured by casting, forging or the like. The applicant found out that a cause thereof is that, not only voids are likely to enter at boundaries of crystal grains, but substance reinforcing boundaries of crystal grains is held inside metal grains.

Here, a metal has been manufactured by metal powder injection molding, by use of reinforcing substance including metal powder of nickel based alloy including titanium carbide. As shown in FIGS. 8 to 10, it is understood that titanium carbide of a reinforcing substance is distributed throughout a cross sectional surface of the metal. That is, the reinforcing substance is held inside metal grains and is not precipitated from inside metal grains to boundaries of crystal grains. By showing schematically, the reinforcing substance 120 is distributed inside crystal grains 110, however is not precipitated at boundaries of crystal grains 110, as shown in FIG. 11. In other words, reinforcing influence by the reinforcing substance 120 is small at boundaries of crystal grains 110. For this reason, the crystal grains themselves are hardly divided due to reinforcing substance and strength of metal incinerated bodies is high at a low temperature environment. On the other hand, since the reinforcing substance is not precipitated at boundaries of crystal grains, the crystal grains are easily divided at boundaries and strength of the metal member at high temperatures is low.

For comparison, a metal has been manufactured by casting, by use of reinforcing substance including metal powder of nickel based alloy including titanium carbide. As shown in FIGS. 12 to 14, titanium carbide of the reinforcing substance is precipitated at boundaries of crystal grains. That is, it is understood that since grains are entirely molten in casting, titanium carbide precipitates from inside crystal grains to boundaries of crystal grains. By schematically showing, as shown in FIG. 15, since grains are entirely molten, the reinforcing substance 220 inside the crystal grains 210 precipitates at boundaries of crystal grains 210 as reinforcing substance 230 For this reason, there is a few amount of reinforcing substance 220 inside crystal grains 210. On the other hand, there is a large amount of reinforcing substance 230 at boundaries of crystal grains 210. As a result, the strength of metal members manufactured by casting or the like is high at a high temperature environment. On the other hand, compared to metal members manufactured by metal powder injection molding, the strength at a low temperature environment is low.

In addition, carbon, nitrogen, oxygens and the like in reinforcing substance are also brittle materials. For this reason, the higher the concentration in the reinforcing substance is, the smaller the elongation of metal member becomes. This is because reinforcing substance prevents transitions inside metal crystals. A metal member A has been manufactured by metal powder injection molding by using metal powder of nickel based alloy with a carbon concentration of 0.02 mass % with respect to the entire metal powder. Similarly, a metal member B has been manufactured by using metal powder of nickel based alloy with a carbon concentration of 0.06 mass %. The carbon concentration in the metal member A after manufacturing was 0.06 mass % with respect to the entire metal member A. On the other hand, the carbon concentration in the metal member B was 0.12 mass %.

As shown in FIG. 16, the tensile strength of the metal member A is approximatively 470 MPa. On the other hand, the tensile strength of the metal member B is approximatively 5.50 MPa: the metal member B with a higher carbon concentration has a higher tensile strength. In addition, the elongation of the metal member A is, as shown in FIG. 17, approximatively 5%. On the other hand, the elongation of the metal member B is approximatively 3%: the metal member A with a lower carbon concentration has a higher elongation. That is, in general, the higher the carbon concentration is, the higher the tensile strength becomes and the smaller the elongation becomes.

From the above, a high strength can be kept at high temperature environment by, arranging reinforcing substance at boundaries of crystal grains when manufacturing metal members by metal powder injection molding. In addition, the elongation of metal members can be prevented to become smaller by keeping a concentration of reinforcing substance equal to or lower than a certain value.

(Manufacturing Method of Metal Member)

Based on the above characteristics, a manufacturing method 1 of forming a metal member with a high strength at a high temperature environment by metal powder injection molding will be explained. In particular, the manufacturing method 1 manufactures a metal member in which reinforce substance is arranged at boundaries of crystal grains even by using metal powder injection molding.

At first, as shown in FIG. 1, in the step S10, a powder manufacturing step of manufacturing metal powder and reinforcing powder is performed. The method of manufacturing the metal powder includes a method of melting the metal once and then manufacturing, a method of mechanically pulverizing and then manufacturing, a method of chemically manufacturing and the like, and any method can be chosen. For example, as a method of melting once and then manufacturing, there is an atomization method. The atomization method is a method of manufacturing a powder by blowing a gas to the molten metal which flows out. A maximal grain size of this metal powder is, for example, 20 μm (micrometer). This metal powder is, for example, a fine powder classified by openings of sieve net compliant to standard of JIS Z8801 or ASTM E11. In order to verify maximal grain size, grain size distribution may be measured by a laser diffraction-scattering method compliant to JIS Z8825-1 standard. In addition, fine powder classified by airstream classification may be used.

Here, by including a certain amount of reinforcing substance including carbon or the like to the molten metal, the strength of the metal member to be manufactured is ensured. For example, 0.12 mass % of carbon is added related to the entire metal to be molten. Here, in order to mix reinforcing powder and then mold, it is preferable to determine carbon concentration in metal powder in accordance with mass of reinforcing powder to mix. In other words, mass of carbon to add is adjusted in accordance with a mass of the reinforcing powder to mix. In particular, it is preferable that the carbon to add is of an amount larger than 5 mass % and smaller than 90 mass % related to carbon concentration of the metal member to manufacture. In particular, in a case where carbon concentration of the metal member to manufacture is 0.2 mass %, then a mass of the carbon to add is preferably larger than 0.01 mass % and smaller than 0.18 mass % related to the entire metal to melt. For example, in a case where 6 parts by mass of reinforcing powder is added to 1000 parts by mass of metal powder, an amount of carbon to add is 0.01 mass % related to the entire metal to melt. It should be noted that any metal such as nickel based alloy, cobalt based alloy, titanium alloy, tungsten alloy, stainless steel, tool steel, aluminum alloy, copper alloy and the like can be used as the metal.

In addition, the reinforcing powder, which includes reinforcing substance including a plurality of type of substances such as carbon, nitrogen and the like, is similarly manufactured as well. In particular, the reinforcing powder is titanium carbide powder, silicon carbide powder, titanium nitride powder, silicon dioxide powder or the like. A maximal grain size of the reinforcing powder is preferably smaller than ⅕ of a maximal grain size of the metal powder. Further, the maximal grain size of the reinforcing powder is preferably smaller than 1/7 of the maximal grain size of the metal powder. In addition, the maximal grain size of the reinforcing powder is preferably larger than 1/100 of the maximal grain size of the metal powder. For example, in a case where the maximal grain size of the metal powder is 20 μm, the maximal grain size of the reinforcing metal is 3 μm. Reinforcing powder is, for example, fine powder classified by a sieve net. In order to verify the maximal grain size, grain size distribution may be measured by the laser diffraction-scattering method compliant to JIS Z8825-1 standard. In addition, fine powder which results of reinforcing powder classified by airstream classification may be used.

Next, in the step S20, a mixing step of mixing the metal powder, the reinforcing powder and a binder is performed. A mixing drum or the like is used to mix the metal powder and the reinforcing powder, both in powder state when mixing. An amount of the reinforcing powder to mix is preferably larger than 1 part of mass and smaller than 50 parts of mass related to 1000 parts of mass of the metal powder. Additives may be mixed as needed. As the binder, for example, a mixture of one or more types in each of organic compounds, such as paraffin wax, carnauba wax, fatty acid ester and the like, and thermoplastic resins with relatively low melting point, such as polyethylene (PE), polypropylene (PP), ethylene vinyl acetate (EVA) copolymer and the like, can be used. The metal powder and the binder may be mixed when manufacturing the powder in the step S10. In particular, the binder is kneaded in a molten state together with the molten metal to be granulated into metal powder which is then used.

In the step S30, an injection molding step of molding an injection molded body by injection molding of the mixed powder. In particular, the mixed powder is provided to an injection molding apparatus. The provided powder is heated and molten, then pumped into a metal mold to be injection molded. Later, the metal mold is cooled down and opened to take the injection molded body therefrom.

In the step S40, a degreasing step of degreasing the injection molded body that has been taken out and removing the binder from the injection molded body is performed. For example, the injection molded body is heated to 500° C. to remove the binder therefrom. As a result, an intermediate molded body, from which the binder is removed, is formed. In addition to this, various methods can be used, such as degreasing by irradiating a light beam, degreasing by immersing in a solvent such as water or organic solvent, or the like, in accordance with characteristics of the binder.

In the step S50, an incinerating step of incinerating the intermediate molded body from which the binder has been removed to form an incinerated body is performed. In particular, bonding of metal powder is grown by heating in vacuum or inert gas. Incineration temperature is, for example, 1200° C. or more and 1300° C. or less.

In the step S60, a pressurizing step of pressurizing the incinerated body to remove voids in the incinerated body is performed. By this pressurization, a metal member with an incinerated density of 90% or more and 100% or less is molded. The incinerated density may be 95% or more. In addition, the incinerated density may be 97% or less.

As described above, by using a powder in which the metal powder is added with a reinforcing powder, a metal member with a reinforcing substance arranged at boundaries of crystal grains can be manufactured.

EXAMPLE

A nickel based alloy member has been manufactured by the manufacturing method 1. In particular, a nickel based alloy powder has been prepared. This nickel based alloy powder has a maximal grain size of 20 μm and a carbon concentration of 0.01 mass %. This carbon concentration is a value in accordance with an amount of reinforcing powder to be mixed, because the carbon concentration of the metal member after manufacturing is controlled to 0.2 mass %. In addition, a titanium carbide powder has been prepared as reinforcing powder. The maximal grain size of this titanium carbide powder is 3 μm. An amount of the titanium carbide is 0.66 mass % related to the nickel based alloy powder.

As a result of using above powder, carbon concentration of manufactured metal member has been 0.22 mass %. As shown in FIGS. 2 to 4, in the manufactured metal member, the titanium carbide of the reinforcing powder is formed in granular state so as to surround crystal grains of the nickel based alloy. This is a characteristic obtained by mixing reinforcing powder to a powder for injection molding. Actually, as shown in FIGS. 8 to 10, no reinforcing substance is precipitated at boundaries of crystal grains of an injection molded metal member in general.

As shown in FIGS. 2 to 4, it is understood that the titanium carbide precipitated at boundaries of those crystal grains is greater than the maximal grain size of the reinforcing powder. It is understood by this as well that the titanium carbide at boundaries of crystal grains results of the mixed reinforcing powder which has melted and precipitated. That is, the grain area equivalent grain size of the reinforcing substance precipitated at boundaries of crystal grains is larger than 1/100 of the grain area equivalent grain size of crystal grains. In addition, in a case where the maximal grain size of the reinforcing powder is smaller than ⅕ of the maximal grain size of the metal powder, the reinforcing powder includes a one with a size smaller than ⅕ of the grain area equivalent grain size of the crystal grains. In a case where the maximal grain size of the reinforcing powder is smaller than ⅛ of the maximal grain size of the metal powder, the reinforcing powder includes a one with a size smaller than ⅛ of the grain area equivalent grain size of the crystal grains. In addition, most of titanium carbide have a shape so that a value of a length, in a direction in which the length thereof is the longest, divided by a length of a longest part in a direction orthogonal to this direction, is smaller than 5. In particular, 90% or more of granular titanium carbide has a shape in which this value is smaller than 5. Further, this value may be smaller than 3.

On the other hand, in a metal member manufactured by casting, as shown in FIG. 15, reinforcing substance 230 is precipitated along boundaries of crystal grains 10. This is because, since a metal completely melts in casting, reinforcing substance inside crystal grains 10 precipitates at boundaries of crystal grains. As a result, the reinforcing substance 230 precipitating at boundaries of the crystal grains 10 precipitates along the boundaries. In addition, in the metal member manufactured by casting, most of the reinforcing substance 230 precipitated at boundaries have a shape so that a value of a length in a direction in which the length thereof is the longest, divided by a length of a longest part in a direction orthogonal to this direction, is greater than 5.

The manufacturing method 1 manufactures by injection molding. For this reason, it is understood that the entire grain is not molten and that titanium carbide of reinforcing substance is distributed inside the crystal grains too. By schematically showing, as shown in FIG. 5, since reinforcing powder is mixed, the reinforcing substance 30 is formed in granular state so as to surround crystal grains 10. In addition, the reinforcing substance 20 is included inside crystal grains 10 too. On the other hand, as shown in FIGS. 12 to 14, reinforcing substance is hardly verified inside crystal grains of metal members manufactured by casting.

As described above, metal members manufactured by the manufacturing method 1 has a structure different from ones manufactured by conventional metal powder injection molding, casting or the like.

Next, tensile strengths and elongations will be compared between a metal member manufactured by the manufacturing method 1 and a metal member manufactured by a general metal powder injection molding. For comparison, a metal member C has been manufactured by use of metal powder of nickel based alloy with a carbon concentration of 0.12 mass %, by metal powder injection molding. A maximal grain size of this powder is 20 μm. The carbon concentration of the manufactured metal member was 0.20 mass %. That is, the carbon concentration is of a same level than the one of the metal member manufactured by manufacturing method 1.

As shown in FIG. 6, the tensile strength of this metal member C was approximatively 585 MPa. On the other hand, the tensile strength of the metal member manufactured by the manufacturing method 1 was approximatively 620 MPa, which is a higher strength compared to the metal member C. Further, by comparing their elongations, while the one of the metal member C is approximatively 2%, the one of the metal member by the manufacturing method 1 is approximatively 6% which is larger. That is, it is understood that the tensile strength and the elongation of a metal member manufactured by the manufacturing method 1 are higher than ones of metal member having a carbon concentration of a same level.

Further, as shown in FIG. 17, the elongation of the metal member A, of which the carbon concentration after manufacturing is 0.06%, is approximatively 5%. The elongation of the metal member by the manufacturing method 1 is approximatively 6%: although the carbon concentration thereof after manufacturing is higher, this is larger than the elongation of the metal member A. That is, the metal member manufactured by the manufacturing method 1 has both larger tensile strength and larger elongation. Thus, the metal member manufactured by the manufacturing method 1 has an advantageous effect on both tensile strength and elongation compared to a metal member manufactured by conventional metal powder injection molding.

The present invention has been explained above by use of embodiments. However, various modifications can be made to the embodiments. The above described embodiments can be arbitrary combined unless technical contradiction occurs. Such modifications and combinations are included in the present invention.

The present application claims priority based on Japanese patent application No. 2017-085858 filed on Apr. 25, 2017 and the entire disclosure thereof is incorporated by reference herein.

METAL MEMBER AND MANUFACTURING METHOD THEREOF

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