Method of producing titanium-based sintered material, and titanium-based sintered material

Abstract

A method of producing a titanium-based sintered material includes: producing a powder mixture by mixing a first powder and a ceramic powder; producing a raw material powder by mixing the powder mixture and a second powder; producing a molded powder compact by performing pressure-molding onto the raw material powder; and producing the titanium-based sintered material by sintering the molded powder compact, wherein one of the first powder and the second powder is a powder composed of titanium or a titanium alloy, and the other is a powder including an element that is able to be alloyed with titanium.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of producing a titanium-based sintered material and the titanium-based sintered material.

[0002] The present application claims priority based on Japanese Patent Application No. 2023-006375 filed on Jan. 19, 2023, the entire contents of which are incorporated herein by reference.BACKGROUND ART

[0003] A titanium alloy including titanium has an excellent mechanical property. Therefore, a titanium-based ingot, a titanium-based sintered material, or the like, each of which is composed of such a titanium alloy, is used for components of various machines.

[0004] PTL 1 discloses a method of producing a powder-sintered impeller by performing pressure-molding onto a raw material powder including a metal and a ceramic powder so as to produce a molded material and sintering the molded material so as to produce the powder-sintered impeller.CITATION LISTPatent Literature

[0005] PTL 1: Japanese Patent Laying-Open No. 8-120307SUMMARY OF INVENTION

[0006] A method of producing a titanium-based sintered material according to the present disclosure includes: producing a powder mixture by mixing a first powder and a ceramic powder; producing a raw material powder by mixing the powder mixture and a second powder; producing a molded powder compact by performing pressure-molding onto the raw material powder; and producing the titanium-based sintered material by sintering the molded powder compact, wherein a combination of the first powder and the second powder is (A) or (B) below.

[0007] (A) A combination in which the first powder is a powder composed of titanium or a titanium alloy and the second powder is a powder including an element that is able to be alloyed with titanium.

[0008] (B) A combination in which the second powder is a powder composed of titanium or a titanium alloy and the first powder is a powder including an element that is able to be alloyed with titanium.BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a flowchart showing steps of a method of producing a titanium-based sintered material according to an embodiment.

[0010] FIG. 2 is a graph showing a test result of a sample No. 21 described in a test example 2.

[0011] FIG. 3 is a graph showing a test result of a sample No. 22 described in test example 2.

[0012] FIG. 4 shows a cross sectional photograph of a titanium-based sintered material of a sample No. 31 described in a test example 3.

[0013] FIG. 5 shows a cross sectional photograph of a titanium-based sintered material of a sample No. 32 described in test example 3.

[0014] FIG. 6 shows a cross sectional photograph of a titanium-based sintered material of a sample No. 33 described in test example 3.

[0015] FIG. 7 shows a cross sectional photograph of a titanium-based sintered material of a sample No. 34 described in test example 3.

[0016] FIG. 8 shows a cross sectional photograph of a titanium-based sintered material of a sample No. 35 described in test example 3.

[0017] FIG. 9 is a schematic view showing a test piece in a tensile test described in a test example 4.DETAILED DESCRIPTIONProblem to be Solved by the Present Disclosure

[0018] The powder-sintered impeller described in PTL 1 includes a structure called a metal matrix composite (hereinafter, MMC). The MMC includes: a matrix including a metal; and a plurality of precipitates dispersed in the matrix. Each of the precipitates is a compound derived from a ceramic powder.

[0019] In the MMC, it is difficult to uniformly disperse the fine precipitates in the matrix. A variation of the precipitates in the matrix causes decreased mechanical property of the MMC.

[0020] It is one of objects of the present disclosure to provide a method of producing a titanium-based sintered material, by which a titanium-based sintered material in which a plurality of precipitates are uniformly dispersed in a matrix can be produced.Advantageous Effect of the Present Disclosure

[0021] According to the method of producing the titanium-based sintered material according to the present disclosure, a titanium-based sintered material in which a plurality of precipitates are uniformly dispersed in a matrix can be produced.DESCRIPTION OF EMBODIMENTS

[0022] First, embodiments of the present disclosure will be listed and described.

[0023] <1> A method of producing a titanium-based sintered material according to the present disclosure includes: producing a powder mixture by mixing a first powder and a ceramic powder; producing a raw material powder by mixing the powder mixture and a second powder; producing a molded powder compact by performing pressure-molding onto the raw material powder; and producing the titanium-based sintered material by sintering the molded powder compact, wherein a combination of the first powder and the second powder is (A) or (B) below.

[0024] (A) A combination in which the first powder is a powder composed of titanium or a titanium alloy and the second powder is a powder including an element that is able to be alloyed with titanium.

[0025] (B) A combination in which the second powder is a powder composed of titanium or a titanium alloy and the first powder is a powder including an element that is able to be alloyed with titanium.

[0026] By mixing the ceramic powder with the raw material powder, there is obtained the titanium-based sintered material composed of the metal matrix composite (hereinafter, MMC) including the structure in which the compound derived from the ceramic powder is precipitated in the matrix of the titanium alloy. The MMC is light in weight and is excellent in heat resistance and wear resistance.

[0027] In the above production method, the raw material powder is composed of the plurality of types of powders, and the mixing is performed in the two stages. By mixing the plurality of types of powders in stages as described above, the plurality of fine precipitates are uniformly dispersed in the matrix of the titanium-based sintered material. Further, by producing the molded powder compact using the raw material powder obtained by mixing the plurality of types of powders in stages, it is possible to dramatically improve the relative density of the titanium-based sintered material obtained by sintering the molded powder compact.

[0028] <2> In the method of producing the titanium-based sintered material according to <1>, the first powder may be composed of an alloy including aluminum and vanadium, and the second powder may be composed of titanium.

[0029] According to the method of producing the titanium-based sintered material according to <2>, the matrix of the titanium-based sintered material is composed of a titanium alloy including titanium (Ti), aluminum (Al), and vanadium (V). The titanium alloy is, for example, Ti-6Al-4V, which is so-called 64 titanium. The 64 titanium has very excellent strength.

[0030] <3> In the method of producing the titanium-based sintered material according to <1>, the first powder may be composed of titanium, and the second powder may be composed of an alloy including aluminum and vanadium.

[0031] According to the method of producing the titanium-based sintered material according to <3>, the matrix of the titanium-based sintered material is composed of a titanium alloy including Ti, Al, and V. The titanium alloy is, for example, 64 titanium. The 64 titanium has very excellent strength.

[0032] <4> In the method of producing the titanium-based sintered material according to any one of <1> to <3>, the ceramic powder may be composed of titanium diboride.

[0033] The titanium diboride (TiB2) is precipitated as titanium boride (TiB) in the titanium-based sintered material. The titanium-based sintered material including the titanium boride as the precipitates is light in weight and is excellent in wear resistance and heat resistance.

[0034] <5> In the method of producing the titanium-based sintered material according to <1> to <4>, the first powder and the ceramic powder may be mixed by a ball mill, an attritor, or a jet mill.

[0035] The ball mill, the attritor, and the jet mill can mix the first powder and the ceramic powder while exerting large stress to the ceramic powder. Therefore, the ceramic powder is facilitated to be uniformly dispersed in the powder mixture in a fine state. As a result, the precipitates derived from ceramic are facilitated to be uniformly dispersed and disposed in the matrix of the titanium-based sintered material.

[0036] <6> In the method of producing the titanium-based sintered material according to any one of <1> to <5>, the powder mixture and the second powder may be mixed by a V-type mixer.

[0037] The V-type mixer can uniformly mix the powder mixture and the second powder. The titanium alloy of the matrix is formed by the first powder included in the powder mixture and the second powder. Therefore, by uniformly mixing the powder mixture and the second powder, a homogeneous matrix is facilitated to be obtained.

[0038] <7> In the method of producing the titanium-based sintered material according to any one of <1> to <6>, in the producing the molded powder compact, the molded powder compact may be produced by cold isostatic press.

[0039] Since a mold in the cold isostatic press is not a metal, the titanium included in the raw material powder is not burned to adhere to the mold. Therefore, pressure in the press is facilitated to be increased, with the result that the density of the molded powder compact is facilitated to be high.

[0040] <8> The method of producing the titanium-based sintered material according to any one of <1> to <7> may further include compressing the titanium-based sintered material by hot isostatic press.

[0041] By the hot isostatic press, the titanium-based sintered material has a higher density, thereby improving the strength of the titanium-based sintered material.

[0042] <9> A titanium-based sintered material according to the present disclosure includes: a matrix including titanium; and a plurality of precipitates each composed of a compound derived from a ceramic powder, the plurality of precipitates being dispersed in the matrix, wherein an average value R0, a maximum value R1, and a minimum value R2 of a plurality of ratios of existence respectively obtained from a plurality of different measurement ranges, which are 50 or more measurement ranges, in a cross section satisfy the following formulas.R⁢1≤R⁢0+4.5,andR⁢2≥R⁢0-4.5.

[0043] Each of the plurality of ratios of existence is an area ratio of the plurality of precipitates when a total area of the matrix and the plurality of precipitates in a corresponding one of the plurality of measurement ranges is regarded as 100%.

[0044] The titanium-based sintered material according to the present disclosure is typically an MMC including a structure in which precipitates each composed of ceramic are precipitated in a matrix of a titanium alloy. The MMC is light in weight and is excellent in heat resistance and wear resistance.

[0045] The unit of each of average value R0, maximum value R1, and minimum value R2 is % (percent). That is, R1≤R0+4.5 means that maximum value R1 is equal to or less than average value R0+4.5%, and R2≥R0−4.5 means that minimum value R2 is equal to or more than average value R0−4.5%. For example, when average value R0 is 10%, maximum value R1 is 14.5% or less, and minimum value R2 is 5.5% or more. Maximum value R1 and minimum value R2 of the ratios of existence of the precipitates satisfying the above formulas mean that the ratio of existence in each of the measurement ranges is not deviated greatly from average value R0. That is, the provisions of the above formulas indicate that a variation in the ratios of existence of the precipitates among the plurality of measurement ranges is small. Therefore, in the titanium-based sintered material according to the present disclosure, the plurality of precipitates are uniformly dispersed in the entire region of the titanium-based sintered material. Therefore, the mechanical property of the whole of the titanium-based sintered material is uniform. Such a titanium-based sintered material is less likely to be locally damaged due to use and has a long life.

[0046] <10> In the titanium-based sintered material according to <9>, the matrix may be composed of an alloy including titanium, aluminum and vanadium.

[0047] The titanium alloy including Ti, Al, and V has excellent strength. Such a titanium alloy is, for example, 64 titanium. The 64 titanium has very excellent strength.

[0048] <11> In the titanium-based sintered material according to <9> or <10>, each of the precipitates may be composed of titanium boride.

[0049] The titanium boride improves the heat resistance and wear resistance of the titanium-based sintered material.DETAILS OF EMBODIMENTS OF THE PRESENT DISCLOSURE

[0050] Hereinafter, a specific example of the method of producing the titanium-based sintered material according to the present disclosure will be described with reference to figures. In each of the figures, the same reference characters denote the same or corresponding portions. The size of a member shown in each of the figures is illustrated for clarity of description and does not necessarily represent an actual dimension thereof. It should be noted that the present invention is not limited to these examples, is defined by the scope of claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.First Embodiment

[0051] A method of producing a titanium-based sintered material according to an embodiment includes the following steps as shown in a flowchart of FIG. 1.

[0052] A first mixing step

[0053] A second mixing step

[0054] A step of performing pressure-molding onto a raw material powder

[0055] A step of cutting a molded powder compact

[0056] A step of removing a lubricant

[0057] A step of sintering the molded powder compact

[0058] A step of compressing the titanium-based sintered material by hot isostatic press

[0059] A step of performing finishing

[0060] Hereinafter, each of the steps will be described in detail.<<First Mixing Step>>

[0061] In the first mixing step, a powder mixture is produced by mixing a first powder and a ceramic powder for the raw material powder, which is composed of a plurality of types of powders. The first powder may be a powder composed of pure titanium. The first powder may be a powder composed of a titanium alloy. The first powder may be a powder including a first element that can be alloyed with titanium. The first element is, for example, aluminum (Al), vanadium (V), tin (Sn), chromium (Cr), molybdenum (Mo), or zirconium (Zr). The first element is appropriately selected in accordance with the composition of the titanium-based sintered material to be produced. One example of the first powder including the first element is an Al—V powder composed of a compound of aluminum and vanadium. The ceramic powder is, for example, titanium diboride (TiB2).

[0062] The first mixing step of mixing the first powder and the ceramic powder can be performed by, for example, a ball mill, an attritor, or a jet mill. By performing the mixing using the mixing method by which high energy is applied to the two or more types of powders including the ceramic powder, the ceramic powder is facilitated to be finely dispersed.<<Second Mixing Step>>

[0063] In the second mixing step, the powder mixture obtained in the first mixing step is mixed with a second powder so as to produce the raw material powder. The second powder may be a powder composed of pure titanium. The second powder may be a powder composed of a titanium alloy. The second powder may be a powder including the first element. Here, when the first powder is a powder composed of titanium or a titanium alloy, the second powder is a powder including the first element. When the first powder is a powder including the first element, the second powder is a powder composed of titanium or a titanium alloy.

[0064] The second mixing step of mixing the powder mixture and the second powder can be performed by, for example, a V-type mixer. The V-type mixer has an excellent capability of mixing powders. Therefore, the powder mixture and the second powder are uniformly mixed. The mixing method using the V-type mixer is a mixing method in which energy applied to the raw material powder is relatively low.

[0065] The particle size of the raw material powder, i.e., each of the particle size of the first powder, the particle size of the second powder, and the particle size of the ceramic powder is, for example, 0.1 μm or more and 100 μm or less. When performing pressure-molding onto a fine raw material powder, air is likely to be included in a clearance between particles of the molded powder compact, with the result that the oxygen concentration of the molded powder compact is likely to be high. The oxygen included in the molded powder compact may cause decreased mechanical property of the titanium-based sintered material obtained by sintering the molded powder compact. When the particle size of the raw material powder is 0.1 μm or more, aggregation thereof can be suppressed at the time of mixing. When the particle size of the raw material powder is 100 μm or less, the density of each of the molded powder compact and the titanium-based sintered material is facilitated to be high. The particle size of the raw material powder may be, for example, 0.5 μm or more and 90 μm or less.

[0066] In the second mixing step, the lubricant may be further mixed with the powder mixture and the second powder. The lubricant improves cuttability of the molded powder compact. The lubricant does not need to be included in the molded powder compact when the molded powder compact is not to be cut. The lubricant is, for example, stearic acid, zinc stearate, stearamide (stearic acid amide), or ethylene bis stearamide.

[0067] A mixing amount of the lubricant with respect to the raw material powder is, for example, 0.05 mass % or more and 0.5 mass % or less when the raw material powder is regarded as 100 mass %. When the mixing amount of the lubricant with respect to 100 mass % of the raw material powder is 0.05 mass % or more, the molded powder compact can be facilitated to be cut in the below-described step of cutting the molded powder compact. When the mixing amount of the lubricant with respect to 100 mass % of the raw material powder is 0.5 mass % or less, the amount of the lubricant with respect to the raw material powder does not become too large, and the density of the molded powder compact is likely to be high. The mixing amount of the lubricant may be 0.2 mass % or more and 0.5 mass % or less, or may be 0.3 mass % or more and 0.4 mass % or less.<<Step of Performing Pressure-Molding onto Raw Material Powder>>

[0068] The pressure-molding is performed, for example, by cold isostatic press. A molding temperature is 0° C. or more and 50° C. or less. A mold in the cold isostatic press is composed of a non-metallic elastic body such as urethane rubber, an acrylic resin, an acrylic resin containing elastomer, or a polylactic acid (PLA) resin, for example. Since the mold in the cold isostatic press is not a metal, the titanium is not burned to adhere to the mold. The molded powder compact obtained by the cold isostatic press has a relatively simple shape. For example, the shape of the molded powder compact is a circular columnar shape or a cylindrical shape.

[0069] A molding pressure is appropriately selected in accordance with the material of the raw material powder and the density of the molded powder compact. For example, the molding pressure is 200 MPa or more. The molding pressure may be 350 MPa or more or 500 MPa or more. The upper limit of the molding pressure depends on the capacity of a facility. For example, the upper limit of the molding pressure is 800 MPa. As the molding pressure is higher, the density of the molded powder compact is facilitated to be higher.<<Step of Cutting Molded Powder Compact>>

[0070] The cutting is performed, for example, by a lathe or a machining center. The cutting may be continuous cutting using a bite or the like, or may be intermittent cutting using a rotary tool or the like. Since the molded powder compact includes the lubricant, the cutting tool is less likely to be damaged during the cutting. Since the cutting tool is less likely to be damaged, cutting cost involving replacement of the cutting tool can be suppressed from being increased. Moreover, cutting precision is less likely to be decreased, thereby improving a surface property of the molded powder compact having been cut.

[0071] As the content of the lubricant in the molded powder compact is larger, the cutting tool is less likely to be damaged and the surface property of the molded powder compact is facilitated to be improved. The content of the lubricant with respect to the raw material powder in the molded powder compact may be regarded as being the same as the mixing amount of the lubricant with respect to the raw material powder at the time of mixing the raw material powder and the lubricant.

[0072] The step of cutting the molded powder compact is not essential. The molded powder compact may be sintered without being cut.<<Step of Removing Lubricant>>

[0073] In the step of removing the lubricant, heat treatment is performed onto the molded powder compact in an inert atmosphere at 400° C. or less. The inert atmosphere is, for example, a nitrogen atmosphere or an argon atmosphere. Under the inert atmosphere at 400° C. or less, the titanium included in the molded powder compact is less likely to be nitrided. When titanium nitride is generated in the molded powder compact, the mechanical property of the titanium-based sintered material obtained by sintering the molded powder compact may be decreased because the titanium nitride is brittle. Moreover, the titanium nitride may inhibit sintering of the molded powder compact to cause decreased mechanical property of the titanium-based sintered material. Here, even when the heat treatment is performed onto the molded powder compact in the argon atmosphere, the titanium nitride can be generated under the argon atmosphere at more than 400° C. because the molded powder compact may include air.

[0074] The stearic acid is vaporized at 270° C. or more. The molded powder compact including the stearic acid as the lubricant is subjected to the heat treatment in the inert atmosphere at, for example, 270° C. or more and 380° C. or less. When the temperature range of the inert atmosphere is 270° C. or more and 380° C. or less, most of the stearic acid is removed from the molded powder compact. Moreover, in the above temperature range, the titanium nitride can be effectively suppressed from being generated in the molded powder compact.

[0075] A time during which the molded powder compact is maintained in the above temperature range, i.e., a heat treatment time, is, for example, 10 minutes or more and 8 hours or less. When the heat treatment time is 10 minutes or more, the temperature of the central portion of the molded powder compact becomes sufficiently high, with the result that the lubricant is facilitated to be removed from the whole of the molded powder compact. When the heat treatment time is 8 hours or less, a production time for the titanium-based sintered material does not become too long. The heat treatment time may be 3 hours or more and 6 hours or less. The molded powder compact after the heat treatment is cooled to an ordinary temperature in the furnace.

[0076] When the molded powder compact includes no lubricant, the step of removing the lubricant is not necessary.<<Step of Sintering Molded Powder Compact>>

[0077] In the step of sintering the molded powder compact, the molded powder compact is sintered in a vacuum atmosphere. An atmospheric pressure is, for example, 0.1 Pa or less. The atmospheric temperature is appropriately selected in accordance with the material of the raw material powder. For example, the atmospheric temperature is 1100° C. or more and 1400° C. or less.

[0078] A sintering time is, for example, 1 hour or more and 25 hours or less. When the sintering time is 1 hour or more, the whole of the molded powder compact is facilitated to be sufficiently sintered. When the sintering time is 25 hours or less, the production time for the titanium-based sintered material does not become too long. The sintering time may be 8 hours or more and 18 hours or less. The titanium-based sintered material is cooled to the ordinary temperature in the furnace.

[0079] Even in the vacuum atmosphere, a slight amount of air is included. Therefore, nitrogen in the air may react with the titanium included in the molded powder compact during the sintering of the molded powder compact. The amount of the titanium nitride in the titanium-based sintered material is decreased by embedding the molded powder compact in a zirconia ball and further disposing a getter, which is composed of a titanium piece, on the zirconia ball.

[0080] The relative density of the titanium-based sintered material is, for example, 95 volume % (percent) or more. The relative density in the present example is a volume ratio of a substantial portion in the volume of the measurement target. The relative density of the titanium-based sintered material is changed when a different method of producing a molded powder compact is employed, even though the relative density of the molded powder compact before the sintering is the same. For example, when a molded powder compact constituted of a raw material powder composed of a pre-alloy powder is sintered, the relative density of the titanium-based sintered material is less likely to be high. The pre-alloy powder is a powder having the same composition as that of the titanium alloy of the titanium-based sintered material. On the other hand, when a molded powder compact constituted of a raw material powder obtained by mixing a plurality of types of powders is sintered, the relative density of the titanium-based sintered material is likely to be high.<<Step of Compressing Titanium-Based Sintered Material By Hot Isostatic Press>>

[0081] The relative density of the titanium-based sintered material becomes higher by the hot isostatic press. The relative density of the titanium-based sintered material to be subjected to the hot isostatic press is 95% or more. It is difficult to improve, by the hot isostatic press, the density of a titanium-based sintered material having a relative density of less than 95%.

[0082] The temperature of the hot isostatic press is appropriately selected in accordance with the composition of the titanium-based sintered material. For example, the temperature of the hot isostatic press is 800° C. or more and 1100° C. or less. A treatment time is, for example, 30 minutes or more and 6 hours or less. When the treatment time is 30 minutes or more, the relative density of the titanium-based sintered material becomes sufficiently high. When the treatment time is 6 hours or less, the production time for the titanium-based sintered material does not become too long. The treatment time may be 1 hour or more and 4 hours or less. The titanium-based sintered material is cooled to the ordinary temperature in a hot isostatic press apparatus. A pressure is, for example, 150 MPa or more.<<Step of Performing Finishing>>

[0083] In the step of performing the finishing, for example, a surface of the titanium-based sintered material is ground. By the grinding, the size of the titanium-based sintered material becomes a desired size and the surface of the titanium-based sintered material becomes smooth. Moreover, an impurity concentrated in the surface of the titanium-based sintered material is removed from the surface of the titanium-based sintered material by the grinding. Examples of the impurity include titanium oxide, titanium carbide, and titanium nitride. With the removal of the impurity from the surface of the titanium-based sintered material, the mechanical property of the titanium-based sintered material is improved.<<Titanium-Based Sintered Material>>

[0084] As shown in FIG. 4, a titanium-based sintered material 1 of the present example is an MMC including a matrix 2 and a plurality of precipitates 3 dispersed in matrix 2. FIG. 4 shows a cross sectional photograph of a titanium-based sintered material produced in a test example 3 described later. The MMC is excellent in tensile strength, heat resistance, and wear resistance.

[0085] Matrix 2 includes titanium. Matrix 2 is composed of, for example, a titanium alloy. The titanium alloy is, for example, Ti-5Al-2.5Sn, Ti-6Al-4V, Ti-3Al-2.5V, Ti-6Al-4V-2Sn, Ti-15V-3Cr-3Sn-3Al, Ti-13V-11Cr-3Al, or Ti-3Al-8V-6Cr-4Mo-4Zr. In particular, Ti-6Al-4V is excellent in strength.

[0086] Each of precipitates 3 is composed of ceramic. Precipitate 3 is, for example, TiB. Precipitate 3 derived from the ceramic powder contributes to improvement in tensile strength, wear resistance, and heat resistance of the titanium-based sintered material.

[0087] In titanium-based sintered material 1 of the present example, the plurality of precipitates 3 are uniformly dispersed in the entire region of titanium-based sintered material 1, and the mechanical property of the whole of titanium-based sintered material 1 is uniform. Therefore, titanium-based sintered material 1 of the present example is less likely to be locally damaged due to use and has a long life.

[0088] The homogeneity of titanium-based sintered material 1 can be evaluated by a variation in ratios of existence of precipitates 3 among a plurality of locations in titanium-based sintered material 1. In the present example, an indicator for the evaluation is such that an average value R0, a maximum value R1, and a minimum value R2 of the plurality of ratios of existence satisfy the following formulas. R⁢1≤ R⁢0+4.5,andR⁢2≥R⁢0-4.5

[0089] Each of the plurality of ratios of existence is an area ratio of the plurality of precipitates when a total area of matrix 2 and the plurality of precipitates 3 in a corresponding one of the plurality of measurement ranges is regarded as 100%. The plurality of measurement ranges are obtained from a cross section of titanium-based sintered material 1. The number of the measurement ranges is 10 or more.

[0090] Maximum value R1 and minimum value R2 of the ratios of existence of precipitates 3 satisfying the above formulas mean that the ratio of existence in each of the measurement ranges is not deviated greatly from average value R0. That is, the provisions of the above formulas indicate that the variation in the ratios of existence among the plurality of measurement ranges is small.TEST EXAMPLESTest Example 1

[0091] In a test example 1, an influence of existence or non-existence of the lubricant and an amount of the lubricant over cuttability of the molded powder compact was examined. Samples prepared in test example 1 are as follows.[Sample No. 1]

[0092] A first powder composed of an Al—V alloy was prepared. The particle size of the first powder was 20 μm or more and 90 μm or less. The first powder was obtained by sieving a commercially available Al—V alloy powder.

[0093] A second powder composed of pure titanium was prepared. The particle size of the second powder was 20 μm or more and 45 μm or less. The second powder was obtained by sieving a commercially available titanium powder.

[0094] A ceramic powder composed of titanium diboride (TiB2) was prepared. The particle size of the ceramic powder was 0.7 μm or more and 10 μm or less.

[0095] 1221 g of the first powder and 77.9 g of the ceramic powder were mixed by a ball mill so as to produce a powder mixture composed of the first powder and the ceramic powder. A container of the ball mill was composed of tungsten carbide. Further, each of pulverization balls introduced in the container of the ball mill was composed of tungsten carbide. The diameter of the pulverization ball was 10 mm, and the number of the pulverization balls was 50. A mixing condition was 300 rpm for 1 hour. The “rpm” is the number of rotations per minute.

[0096] 154 g of the powder mixture and 846 g of the second powder were mixed by a V-type mixer so as to produce a raw material powder. Pressure-molding was performed onto this raw material powder by cold isostatic press, thereby producing a molded powder compact having a cylindrical shape. A molding pressure was 390 MPa, and a holding time was 30 seconds. The molded powder compact had an outer diameter of 40 mm, an inner diameter of 20 mm, and a height of 30 mm. The molded powder compact includes no lubricant.[Samples No. 2 to No. 5]

[0097] Each of molded powder compacts of samples No. 2 to No. 5 includes a lubricant. Each of samples No. 2 to No. 5 is different from sample No. 1 only in terms of existence or non-existence of the lubricant. The lubricant was stearic acid. The lubricant was introduced into the V-type mixer together with the powder mixture and the second powder. A mixing amount of the lubricant in sample No. 2 was 0.05 mass % with respect to 100 mass % of the raw material powder. A mixing amount of the lubricant in sample No. 3 was 0.1 mass %, a mixing amount of the lubricant in sample No. 4 was 0.3 mass %, and a mixing amount of the lubricant in sample No. 5 was 0.5 mass %.[Sample No. 100]

[0098] A sample No. 100 is a sintered material obtained by performing heat treatment onto a molded powder compact, which has been produced by the same production method as that for sample No. 3, at 380° C. for 4 hours in a nitrogen atmosphere and sintering the molded powder compact at 1300° C. for 12 hours in a vacuum atmosphere.[Cutting Test]

[0099] A surface of each of the sintered material of sample No. 100 and the molded powder compacts of samples No. 1 to No. 5 was cut by a lathe. The cutting was dry cutting. A peripheral speed S was 180 m / min, a feed F was 0.2 mm / rev (millimeter per revolution), and a depth of cut D was 0.5 mm / rev. A wear amount of the cutting tool after the cutting test was measured by a measurement instrument provided in a microscope. As a result, the wear amount of the cutting tool used in the cutting of each of samples No. 2 to No. 5 was smaller than the wear amount in each of samples No. 1 and No. 100. Moreover, as the mixing amount of the lubricant was larger, the wear amount of the cutting tool was smaller. However, there was substantially no difference in wear amount between sample No. 4 in which the mixing amount of the lubricant was 0.3 mass % and sample No. 5 in which the mixing amount of the lubricant was 0.5 mass %.Test Example 2

[0100] In a test example 2, an influence of the method of mixing the raw material powder over the relative density of the titanium-based sintered material was examined. Samples prepared in test example 2 are as follows.[Sample No. 21]

[0101] First, a molded powder compact of a sample No. 21 was produced by using the same material and the same production method as those for sample No. 3. Therefore, the molded powder compact of sample No. 21 includes the powder composed of the pure titanium, the powder composed of the Al—V alloy, the ceramic powder composed of the TiB2, and the lubricant.

[0102] The molded powder compact was placed in an inert oven furnace, and the stearic acid was removed from the molded powder compact by heat treatment. An atmosphere in the inert oven furnace was a nitrogen atmosphere, a heat treatment temperature was 380° C., and a heat treatment time was 4 hours. A rate of increasing the temperature in the inert oven furnace was 5° C. / min.

[0103] The molded powder compact from which the lubricant had been removed was placed in a sintering furnace, and a titanium-based sintered material was produced by sintering. An atmosphere of the sintering furnace was a vacuum atmosphere of 0.1 Pa or less, a sintering temperature was 1300° C., and a sintering time was 12 hours. A rate of increasing the temperature in the sintering furnace was 6.7° C. / min until 1290° C., and was 1° C. / min in a range of 1290° C. to 1300° C. The titanium-based sintered material was an MMC including a matrix including titanium and a plurality of precipitates dispersed in the matrix. The composition of the matrix was a Ti-6Al-4V alloy, which is a so-called 64 titanium alloy. The composition of each of the precipitates derived from the TiB2 was titanium boride (TiB).

[0104] In test example 2, the relative density of the molded powder compact was measured before sintering the molded powder compact, and the relative density of the titanium-based sintered material was measured. The relative density is a volume ratio of a substantial portion in the volume of the measurement target. The unit of the relative density is volume %. The relative density was measured by the Archimedes' method.

[0105] A relation between the relative density of the molded powder compact and the relative density of the titanium-based sintered material is shown in a graph of FIG. 2. Each of plots of white circles indicates the relative density of the molded powder compact, and each of plots of black circles indicates the relative density of the titanium-based sintered material. In FIG. 2, the horizontal axis represents the molding pressure, and the vertical axis represents the relative density. The unit of the molding pressure is MPa (megapascal). In FIG. 2, the relative density of the molded powder compact and the relative density of the titanium-based sintered material under a molding pressure of 100 MPa, 200 MPa, 300 MPa, or 390 MPa are shown together.

[0106] As shown in FIG. 2, it was found that as the molding pressure is higher, the relative density of the molded powder compact is higher. Moreover, the relative density of the titanium-based sintered material was increased by 15 volume % or more with respect to the relative density of the molded powder compact before the sintering. The relative density of each of the titanium-based sintered materials shown in FIG. 2 was 95 volume % or more. In view of these, it was found that a titanium-based sintered material having a relative density of 95 volume % or more can be produced by sintering even when the relative density of the molded powder compact is low. On the other hand, when cutting the molded powder compact, if the relative density of the molded powder compact is low, the molded powder compact may be chipped or cracked. From the viewpoint of suppressing the cracking or chipping of the molded powder compact, the relative density of the molded powder compact is preferably 75 volume % or more. A molding pressure for obtaining a molded powder compact having a relative density of 75 volume % or more is, for example, 300 MPa or more.[Sample No. 22]

[0107] A molded powder compact of a sample No. 22 was obtained by performing pressure-molding onto a mixture of a pre-alloy powder, a ceramic powder, and stearic acid. The pre-alloy powder is a powder composed of a Ti-6Al-4V alloy. A molding pressure is the same as that in sample No. 21. The lubricant was removed from the molded powder compact by heat treatment, and a titanium-based sintered material was produced by sintering. A heat treatment condition and a sintering condition are the same as those in sample No. 21. The titanium-based sintered material of sample No. 22 was also an MMC in which a plurality of precipitates were dispersed in a matrix.

[0108] A relation between the relative density of the molded powder compact and the relative density of the titanium-based sintered material is shown in a graph of FIG. 3. The graph of FIG. 3 is viewed in the same manner as in the graph of FIG. 2. As shown in FIG. 3, it was found that as the molding pressure is higher, the relative density of the molded powder compact is higher. However, the relative density of the titanium-based sintered material is increased only by about 5 volume % with respect to the relative density of the molded powder compact before the sintering. The relative density of each of the titanium-based sintered materials shown in FIG. 3 was 85 volume % or less.Conclusion

[0109] In view of the result of test example 2, it was found that in order to produce an MMC having a relative density of 95 volume % or more, it is effective to produce a molded powder compact by using a raw material powder composed of a plurality of types of powders.Test Example 3

[0110] In a test example 3, an influence of a method of mixing the raw material powder over a structure of the titanium-based sintered material was examined. In test example 3, samples No. 31 to No. 35 were produced by different methods of mixing the raw material powder. In the production of each of samples No. 31 to No. 34, the raw material powder was mixed in two stages. In the production of sample No. 35, the raw material powder was mixed at one time. A mixing condition of a ball mill in each sample was 300 rpm for 1 hour, and a mixing time by a V-type mixer was 1 hour.[Sample No. 31]

[0111] In the first stage of mixing, a powder composed of an Al—V alloy and a ceramic powder composed of TiB2 were mixed by a ball mill so as to produce a powder mixture. In the second stage of mixing, a raw material powder was produced by mixing the powder mixture, a powder composed of pure titanium, and stearic acid by a V-type mixer. A mixing amount of the stearic acid was 0.3 mass %.[Sample No. 32]

[0112] In the first stage of mixing, a powder composed of pure titanium and a ceramic powder composed of TiB2 were mixed by a ball mill so as to produce a powder mixture. In the second stage of mixing, a raw material powder was produced by mixing the powder mixture, a powder composed of an Al—V alloy, and stearic acid by a V-type mixer.[Sample No. 33]

[0113] In the first stage of mixing, a powder composed of an Al—V alloy and a powder composed of pure titanium were mixed by a ball mill so as to produce a powder mixture. In the second stage of mixing, a raw material powder was produced by mixing the powder mixture, a ceramic powder composed of TiB2, and stearic acid by a V-type mixer.[Sample No. 34]

[0114] In the first stage of mixing, a powder composed of an Al—V alloy and a powder composed of pure titanium were mixed by a ball mill so as to produce a powder mixture. In the second stage of mixing, a raw material powder was produced by mixing the powder mixture, a ceramic powder composed of TiB2, and stearic acid by a ball mill.[Sample No. 35]

[0115] A powder composed of an Al—V alloy, a powder composed of pure titanium, and a ceramic powder composed of TiB2 are mixed by a V-type mixer so as to produce a raw material powder. Sample No. 35 does not include stearic acid.

[0116] A titanium-based sintered material was produced from each of the raw material powders of samples No. 31 to No. 35. A molding pressure in cold isostatic press, a condition of heat treatment for removing the lubricant, and a condition of sintering were the same among the samples. The molding pressure was 390 MPa, the heat treatment condition was a nitrogen atmosphere at 380° C. for 4 hours, and the sintering condition was a vacuum atmosphere at 1300° C. for 12 hours.

[0117] A cross section of each sample was observed to measure an area ratio of the precipitates each composed of TiB. Cross sectional photographs of titanium-based sintered materials 1 of samples No. 31 to No. 35 are shown in FIGS. 4 to 8, respectively. A gray portion in each of the cross sectional photographs is a matrix 2 composed of 64 titanium, and a black portion is precipitates 3 each composed of the TiB. In comparison among FIGS. 4 to 8, it was found that precipitates 3 of the TiB are uniformly dispersed in each of samples No. 31 and No. 32. On the other hand, it was found that precipitates 3 are unevenly dispersed in each of samples No. 33 to No. 35 because a large number of regions each with a small number of precipitates 3 in black are recognized in the form of spots.

[0118] Next, a dispersion state of the TiB was quantitatively evaluated. An observation visual field of 13.2 mm×9.5 mm was divided into 64, and matrix 2 and the TiB in each divided visual field were distinguished from each other by binarization processing. Then, image analysis software was used to determine the area ratio of the TiB when the total area of matrix 2 and precipitates 3 in each divided visual field was regarded as 100%. The area ratio of the TiB is a ratio of existence of the TiB, and the unit thereof is %. An average value R0, a maximum value R1, and a minimum value R2 of a plurality of ratios of existence respectively obtained from the divided visual fields were determined. Average value R0, maximum value R1, and minimum value R2 in each sample are shown in Table 1. A sample satisfying R1≤R0+4.5 and R2≥R0−4.5 was evaluated as “A”, and a sample not satisfying R1≤R0+4.5 and R2≥R0−4.5 was evaluated as “B”. The sample satisfying R1≤R0+4.5 and R2≥R0−4.5 is determined as a sample in which precipitates 3 of the TiB are uniformly dispersed in matrix 2.TABLE 1Sample No.3132333435Average Value10.110.08.49.18.7R0 (%)Maximum Value13.914.513.414.113.3R1 (%)Minimum Value7.17.71.71.74.7R2 (%)R1 − R03.84.55.05.04.6R0 − R23.02.36.77.44.0DeterminationAABBB

[0119] As shown in Table 1, in each of samples No. 31 and No. 32, the TiB was uniformly dispersed in matrix 2. Each of these samples is a sample in which the ceramic powder was mixed by the ball mill in the first mixing step.

[0120] In each of samples No. 33 and No. 34, the dispersion state of the TiB in matrix 2 was varied. Each of these samples is a sample in which the ceramic powder was mixed in the second mixing step.

[0121] In sample No. 35, the dispersion state of the TiB in matrix 2 was varied. This sample is a sample in which all the raw materials for the titanium-based sintered material were mixed at one time by the ball mill.

[0122] In view of these results, it was found that in order to uniformly disperse precipitates 3 in matrix 2, it is important to mix the raw materials for the titanium-based sintered material in a plurality of times. Moreover, it was found that it is important to mix the ceramic powder by the mixing method in which high stress is applied to the ceramic powder in the first mixing step.Test Example 4

[0123] In a test example 4, a mechanical property of an MMC was examined. Samples prepared in test example 4 are as follows.[Sample No. 40]

[0124] A sample No. 40 is a melt-produced material composed of 64 titanium alloy.[Samples No. 41 to No. 44]

[0125] Samples No. 41, No. 42, No. 43, No. 44, and No. 45 are titanium-based sintered materials having the same configurations as those of samples No. 31, No. 32, No. 33, No. 34, and No. 35 of test example 3, respectively.[Tensile Test]

[0126] A test piece composed of each of the samples was produced and was subjected to a tensile test. FIG. 9 is a schematic view showing a shape of a test piece 5. Test piece 5 includes a first grip portion 51, a second grip portion 52, and an intermediate portion 50. The diameter of intermediate portion 50 was 6.35 mm (millimeters), and an inter-reference-point distance d between a first reference point 50A and a second reference point 50B each set in intermediate portion 50 was 25.4 mm. A temperature in the tensile test was room temperature, a strain rate until 0.2% proof stress was reached was 1.2 mm / min, and a strain rate after the 0.2% proof stress was 12.8 mm / min. The unit of the tensile strength was MPa. Test results are shown in TableTABLE 2Sample No.404142434445Material64Ti64Ti + TiB64Ti + TiB64Ti + TiB64Ti + TiB64Ti + TiBTensile Strength98011101050960970980(MPa)

[0127] As shown in Table 2, the tensile strength of the titanium-based sintered material of each of samples No. 41 to No. 45 was comparable to or equal to or more than the tensile strength of the melt-produced material composed of 64 titanium. In view of these results, it was found that the titanium-based sintered material of the present example is useful as a material of a component of a machine such as an automobile, for example.REFERENCE SIGNS LIST1 titanium-based sintered material

[0129] 2 matrix

[0130] 3 precipitate

[0131] 5 test piece

[0132] 50 intermediate portion

[0133] 50A first reference point

[0134] 50B second reference point

[0135] 51 first grip portion

[0136] 52 second grip portion

[0137] d inter-reference-point distance

Claims

1. A method of producing a titanium-based sintered material, the method comprising:producing a powder mixture by mixing a first powder and a ceramic powder;producing a raw material powder by mixing the powder mixture and a second powder;producing a molded powder compact by performing pressure-molding onto the raw material powder; andproducing the titanium-based sintered material by sintering the molded powder compact, whereina combination of the first powder and the second powder is a combination in which the first powder is a powder composed of titanium or a titanium alloy and the second powder is a powder including an element that is able to be alloyed with titanium, or a combination in which the second powder is a powder composed of titanium or a titanium alloy and the first powder is a powder including an element that is able to be alloyed with titanium.

2. The method of producing the titanium-based sintered material according to claim 1, whereinthe first powder is composed of an alloy including aluminum and vanadium, andthe second powder is composed of titanium.

3. The method of producing the titanium-based sintered material according to claim 1, whereinthe first powder is composed of titanium, andthe second powder is composed of an alloy including aluminum and vanadium.

4. The method of producing the titanium-based sintered material according to claim 1, wherein the ceramic powder is composed of titanium diboride.

5. The method of producing the titanium-based sintered material according to claim 1, wherein the first powder and the ceramic powder are mixed by a ball mill, an attritor, or a jet mill.

6. The method of producing the titanium-based sintered material according to claim 1, wherein the powder mixture and the second powder are mixed by a V-type mixer.

7. The method of producing the titanium-based sintered material according to claim 1, wherein in the producing the molded powder compact, the molded powder compact is produced by cold isostatic press.

8. The method of producing the titanium-based sintered material according to claim 1, further comprising compressing the titanium-based sintered material by hot isostatic press.

9. A titanium-based sintered material comprising:a matrix including titanium; anda plurality of precipitates each composed of a compound derived from a ceramic powder, the plurality of precipitates being dispersed in the matrix, whereinan average value R0, a maximum value R1, and a minimum value R2 of a plurality of ratios of existence respectively obtained from a plurality of different measurement ranges, which are 50 or more measurement ranges, in a cross section satisfy the following formulas:R⁢1≤R⁢0+4.5,andR⁢2≥R⁢0-4.5,each of the plurality of ratios of existence is an area ratio of the plurality of precipitates when a total area of the matrix and the plurality of precipitates in a corresponding one of the plurality of measurement ranges is regarded as 100%.

10. The titanium-based sintered material according to claim 9, wherein the matrix is composed of an alloy including titanium, aluminum and vanadium.

11. The titanium-based sintered material according to claim 9, wherein each of the precipitates is composed of titanium boride.

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