Manufacturing method of titanium alloy sintered body and titanium alloy sintered body

Abstract

To provide a titanium alloy sintered body with enhanced fatigue strength while reducing an oxygen content, and also to provide a manufacturing method of the titanium alloy sintered body.SOLUTION: A manufacturing method of a titanium alloy sintered body using a metal powder injection molding method includes: a mixing step of preparing a mixture of metal powder and binder; an injection step of injection-molding the mixture to produce a molded body; a defatting step of defatting the molded body to remove the binder; and a sintering step of sintering the molded body with the binder removed to obtain a sintered body. The sintering step is conducted at a sintering temperature of 800 to 995°C and with a sintering time from 6 to 200 hours.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a titanium alloy sintered body and a titanium alloy sintered body, and more particularly to a method for producing a titanium alloy sintered body capable of achieving low oxygen content and a titanium alloy sintered body. [Background technology]

[0002] Titanium is known to be the third most abundant metal in the earth's crust after aluminum, iron and magnesium, and is lightweight, strong and corrosion resistant, with few adverse effects on the human body. However, titanium has a close-packed hexagonal crystal structure at room temperature, which makes it difficult to process it, and because it is strong, it is difficult to machine it, making it difficult to reduce manufacturing costs.

[0003] For this reason, in order to reduce manufacturing costs, there are high hopes for the production of titanium using metal injection molding (MIM), a process that allows titanium to be produced in a near-net shape simply by forming the product without any machining.

[0004] Various methods and forms of such a titanium alloy sintered body and a titanium alloy sintered body by metal powder injection molding are known. For example, as described in Patent Document 1, a method for manufacturing a titanium or titanium alloy part by powder metallurgy in which a green body is formed using a metal powder composed of titanium or a titanium alloy, and this green body is compressed and solidified in a sintering step is known in which a metal powder composed of titanium or a titanium alloy and having an average particle size of less than 25 μm as measured using laser light scattering in accordance with ASTM standard B822-10 is used to form the green body, and the sintering step is carried out at a sintering temperature of up to 1100° C. for a sintering time of 5 hours or less in an atmosphere of reduced pressure relative to normal pressure.

[0005] According to this method for producing a sintered titanium alloy body, titanium or titanium alloy metal powders having an average particle size of less than 25 μm are used for producing the green body, and the sintering step is carried out at a sintering temperature of up to 1100° C. for a sintering time of less than 5 hours in an environment of reduced pressure relative to atmospheric pressure, which can have a targeted influence on the grain structure and also on the material properties of the material thus obtained.

[0006] Also, as described in Patent Document 2, a titanium alloy sintered body is known that is characterized in that the average crystal grain size on the surface is more than 30 μm and not more than 500 μm, and the Vickers hardness on the surface is 300 or more and 800 or less.

[0007] Such a titanium alloy sintered body does not deteriorate on the surface even when exposed to a harsh environment for a long period of time, and as a result, a titanium alloy sintered body with high specularity (designability) can be provided. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 2019-516021 [Patent Document 2] JP 2019-44225 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, it is known that the tensile strength and elongation properties of titanium alloy sintered bodies change depending on the oxygen content, and it is known that an increase in the oxygen content increases the tensile strength and decreases the elongation. In contrast, it is difficult to suppress the oxygen content to 0.2 mass% or less in titanium alloy sintered bodies manufactured by conventional manufacturing methods for titanium alloy sintered bodies, and there is a problem that it is difficult to increase the fatigue strength.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a titanium alloy sintered body having a reduced oxygen content and improved fatigue strength, and a titanium alloy sintered body. [Means for solving the problem]

[0011] The method for producing a titanium alloy sintered body according to the present invention is a method for producing a titanium alloy sintered body by metal powder injection molding, comprising the steps of: a kneading step for producing a kneaded mixture of metal powder and a binder; an injection step for injection molding the kneaded mixture to produce a molded body; a degreasing step for degreasing the molded body to remove the binder; and a sintering step for sintering the molded body from which the binder has been removed to obtain a sintered body, characterized in that the sintering step is performed at a sintering temperature of 800 to 995°C for a sintering time of 6 to 200 hours.

[0012] In the method for producing a titanium alloy sintered body according to the present invention, the sintering step is carried out under a vacuum, and the vacuum is an atmospheric pressure during sintering of 1×10 -3 It is preferable that the pressure is less than or equal to Pa.

[0013] In the method for producing a titanium alloy sintered body according to the present invention, it is preferable that the metal powder is a low-oxygen metal powder.

[0014] The titanium alloy sintered body according to the present invention is characterized by the following components, by mass%, aluminum: 5.50 to 6.50%, vanadium: 3.50 to 4.50%, iron: 0.40% or less, oxygen: 0.2% or less, carbon: 0.08% or less, nitrogen: 0.05% or less, hydrogen: 0.015% or less, the balance being titanium, and having a relative density of 97.0% or more.

[0015] In the titanium alloy sintered body according to the present invention, it is preferable that the average crystal grain size is 5.0 to 50.0 μm and the acicular ratio of the crystal structure is 3 or less. Effect of the Invention

[0016] According to the manufacturing method of the titanium alloy sintered body of the present invention, the sintering temperature is 980°C and the sintering time is 48 hours in the sintering step, so that a low-oxygen titanium alloy sintered body can be obtained. In addition, the titanium alloy sintered body of the present invention has a relative density of 97.0% or more and an oxygen content of 0.2 mass% or less, so that it is possible to provide a titanium alloy sintered body with high fatigue strength. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a flow diagram of a method for producing a titanium alloy sintered body according to an embodiment of the present invention. [Diagram 2] 1 shows the results of observation of the microstructure, where (A) is the titanium alloy sintered body according to the present embodiment, and (B) is the result of observation of a comparative example. [Diagram 3] 3 is a graph showing the relationship between sintering time and relative density of the titanium alloy sintered body according to the embodiment and the comparative example. [Figure 4] Graph showing the results of tensile strength tests of the titanium alloy sintered body according to the embodiment of the present invention and the comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Preferred embodiments for carrying out the present invention will be described below with reference to the drawings. Note that the following embodiments do not limit the inventions according to the claims, and all of the combinations of features described in the embodiments are not necessarily essential to the solution of the invention.

[0019] FIG. 1 is a flow diagram of a method for producing a titanium alloy sintered body according to an embodiment of the present invention. FIG. 2 shows the results of observation of the microstructure, where (A) is the titanium alloy sintered body according to the present embodiment and (B) is the observation result of a comparative example. FIG. 3 is a graph showing the relationship between sintering time and relative density of the titanium alloy sintered body according to the present embodiment and the comparative example. FIG. 4 is a graph showing the results of a tensile strength test of the titanium alloy sintered body according to the embodiment of the present invention and the comparative example.

[0020] As shown in FIG. 1, the method for producing a titanium alloy sintered body according to this embodiment includes a step (S101) of producing a kneaded product of a metal powder and a binder, a step (S102) of injection molding the kneaded product to produce a green body, a step (S103) of degreasing the green body to remove the binder, a step (S104) of sintering the green body from which the binder has been removed to obtain a titanium alloy sintered body, and a step (S105) of post-processing and inspecting the titanium alloy sintered body.

[0021] In the step (S101) of producing a kneaded product of metal powder and binder, the metal powder and binder are kneaded to produce a kneaded product. The metal powder is preferably a conventionally known pure titanium or titanium alloy, and more preferably a low-oxygen powder having an oxygen content of 0.13 mass% or less. For example, for Ti-6V-4Al material, it is preferable to use a powder equivalent to ASTM grade 23 (Extra-Low Interstitial).

[0022] The binder is an additive that provides the necessary fluidity for injection molding, which will be described later, and is preferably a binder made of a general-purpose synthetic resin to which a lubricant and a plasticizer have been added. The ratio of the metal powder to the binder can be adjusted appropriately according to the properties and shape of the titanium alloy sintered body to be manufactured, but for example, a ratio of 60 vol%:40 vol% is preferable.

[0023] The kneaded product is produced by adding a binder to metal powder, mixing the mixture under heating and pressure, and then pulverizing and granulating the kneaded product that has cooled and solidified to obtain a kneaded product having flowability.

[0024] In the step (S102) of injection-molding the kneaded mixture to produce a molded product, the kneaded mixture is injection-molded into a mold, and then cooled and solidified to produce a molded product of a predetermined shape. The mold used for injection molding can be a conventional mold that corresponds to the shape of the molded product.

[0025] The step (S103) of degreasing the green body to remove the binder is a step of removing the binder contained in the green body to obtain a degreased body prior to sintering, which will be described later. A thermal degreasing process in which the green body is heated under an inert gas flow to evaporate and pyrolyze the binder, or a solvent degreasing process in which the binder is extracted with an organic solvent, etc., are carried out.

[0026] The step (S104) of sintering the compact from which the binder has been removed to obtain a titanium alloy sintered compact is carried out by sintering the degreased compact at a temperature of 1×10 -3 The compact is heated and sintered at 800 to 995°C, more preferably at about 980°C, in a vacuum of 0.1 Pa or less for 6 to 200 hours, more preferably for 48 hours. Note that any binder remaining in the degreased compact is removed in the heating process by sintering. As the binder is removed from the compact through degreasing and sintering in this way, the sintered compact shrinks by about 10 to 20% compared to the compact.

[0027] In addition, the step (S104) of obtaining the titanium alloy sintered body includes placing a zirconia setter in a molybdenum container, placing a degreased body on the setter, closing the container with a molybdenum lid, and then creating a vacuum in the container to perform sintering.

[0028] The step (S105) of post-processing and inspecting the titanium alloy sintered body is a step of post-processing and inspecting the titanium alloy sintered body obtained by sintering. Specifically, the titanium alloy sintered body is subjected to a heat treatment and polished to ensure dimensional accuracy. EXAMPLES

[0029] Next, the present invention will be described in more detail with reference to examples.

[0030] In the manufacturing method of the titanium alloy sintered body, the step (S101) of preparing a kneaded product of metal powder and binder used titanium alloy powder with an average particle size of 27.3μm and with 6.22% aluminum, 4.04% vanadium, 0.2% iron, 0.091% oxygen, 0.004% carbon, 0.012% nitrogen, 0.002% hydrogen, and the remainder titanium, in mass %. The binder used was the one described in Patent No. 5163596, which was mixed and kneaded at a ratio of 40 volume % to the titanium alloy powder. Then, the step (S102) of injection molding the kneaded product to produce a molded body and the step (S103) of degreasing the molded body to remove the binder were carried out by heating and degreasing.

[0031] The step (S104) of sintering the compact from which the binder has been removed to obtain a titanium alloy sintered compact is carried out by sintering the degreased compact at a temperature of 1×10 -3 The temperature was raised to 980°C under a vacuum of 100 Pa or less, and the compact was sintered at this temperature for 48 hours. The binder was removed from the compact through debinding and sintering, so the sintered compact shrunk by about 15% compared to the compact. The relative density of the titanium alloy sintered compact was 97.5%.

[0032] In addition, the step (S104) of obtaining the titanium alloy sintered body involved placing a zirconia setter in a molybdenum container, placing the degreased body on the setter, closing the container with a molybdenum lid, and then creating a vacuum in the container to perform sintering.

[0033] In the step (S105) of post-processing and inspection of the titanium alloy sintered body, the titanium alloy sintered body obtained by sintering was cut and polished into fatigue test pieces. The tensile test pieces were only inspected and no post-processing was performed.

[0034] First, a grain size observation test was carried out on the titanium alloy sintered body according to the present embodiment and the comparative example. The comparative example is a sintered body that is kneaded using a normal metal powder with a higher oxygen content than the low-oxygen powder used in the titanium alloy sintered body according to the present embodiment, and sintered at a sintering temperature of 1100°C and a sintering time of 6 hours. In the grain size observation test, a photograph of the surface of the example and the comparative example taken at 400 times was printed, the outline of the granular structure was traced by hand, and the image was imported into a computer, and then the circle equivalent diameter, absolute maximum length, diagonal width, and acicular ratio were measured using measurement software (Winroof). In addition, the average circle equivalent diameter, average absolute maximum length, average diagonal width, and average acicular ratio were calculated by excluding those with a circle equivalent diameter of less than 5 μm from the measurement data. The average equivalent circular diameter is the diameter of an equivalent circle with the same area as the area of ​​an object, the absolute maximum length is the longest length of an object, the diagonal width is the shortest distance between two lines parallel to the maximum absolute length when the object is sandwiched between them, and the acicular ratio is the absolute maximum length divided by the diagonal width.

[0035] As shown in Fig. 2, the titanium alloy sintered body obtained by the manufacturing method of the titanium alloy sintered body according to the present embodiment has a microstructure with a rounder grain size overall compared to the conventional comparative example. As shown in Fig. 2(A), it can be confirmed that the acicular ratio of the crystal structure in the microfiber of the titanium alloy sintered body according to the present embodiment is 3.0 or less. In contrast, as shown in Fig. 2(B), the conventional comparative example has a crystal structure that is elongated overall, with an acicular ratio of 3.0 or more, and it can be confirmed that the titanium alloy sintered body according to the present embodiment has a crystal structure that is finer and rounder overall compared to the comparative example.

[0036] The particle size observation results for the examples are as follows: [Table 1]

[0037] Next, as shown in FIG. 3, it was confirmed that the titanium alloy sintered body using the low-oxygen powder had a relative density of 98% or more, equivalent to that of the comparative example, by sintering for 48 hours.

[0038] In addition, the results of oxygen, nitrogen and carbon analysis, as shown in the table below, show that the titanium alloy sintered body according to this embodiment has the same nitrogen and carbon contents as the comparative example, and has an oxygen content of 0.18%, which is significantly lower than the comparative example. This satisfies the conditions of JIS Class 60 and ASTM Gr5 for ingot materials. [Table 2]

[0039] Next, the titanium alloy sintered body according to the present embodiment and the comparative example were subjected to a tensile strength test and a fatigue strength test. The tensile strength test was performed with a rating distance of 15 mm. As shown in FIG. 4, the tensile strength of the example was equivalent to that of the comparative example, and it was confirmed that the elongation of the example was higher than that of the comparative example.

[0040] The fatigue strength test was carried out under the following test conditions. (1) Test temperature: Room temperature (2) Standard: ASTM E466 (3) Stress ratio: R = 0.1 (4) Waveform: Sine wave (5) Termination cycle: 1.0 × 10 7 cycle (6) Frequency: 10Hz

[0041] The fatigue strength test result was 1.0×10 7 The fatigue strength in cycles was 350 MPa in the example and 280 MPa in the comparative example.

[0042] In this way, it was confirmed that the method for producing a titanium alloy sintered body according to this embodiment makes it possible to obtain a titanium alloy sintered body having a reduced oxygen content and improved fatigue strength.

Claims

1. A kneading step for producing a kneaded mixture of metal powder and a binder; an injection step of injection molding the kneaded mixture to produce a molded body; a degreasing step of degreasing the compact to remove the binder; and a sintering step of sintering the compact from which the binder has been removed to obtain a sintered body, The method for producing a titanium alloy sintered body is characterized in that the sintering step is carried out at a sintering temperature of 800 to 995°C for a sintering time of 6 to 200 hours.

2. The method for producing a titanium alloy sintered body according to claim 1, The sintering step is carried out under vacuum, The vacuum is an atmospheric pressure during sintering of 1×10 -3 4. A method for producing a titanium alloy sintered body, wherein the compressive strength is 10 Pa or less.

3. The method for producing a titanium alloy sintered body according to claim 1 or 2, A method for producing a titanium alloy sintered body, wherein the metal powder is a low-oxygen metal powder.

4. A titanium alloy sintered body having, by mass%, 5.50 to 6.50% aluminum, 3.50 to 4.50% vanadium, 0.40% or less iron, 0.2% or less oxygen, 0.08% or less carbon, 0.05% or less nitrogen, 0.015% or less hydrogen, the balance being titanium, and having a relative density of 97.0% or more.

5. The titanium alloy sintered body according to claim 4, The average crystal grain size is 5.0 to 50.0 μm; A titanium alloy sintered body, characterized in that the acicular ratio of the crystal structure is 3 or less.

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