Titanium alloy sintered body

Abstract

This invention provides a titanium alloy sintered body with reduced oxygen content and increased fatigue strength, as well as a method for manufacturing this titanium alloy sintered body. [Solution] The composition is 5.50 to 6.50% by mass of aluminum, 3.50 to 4.50% of vanadium, 0.40% or less of iron, 0.2% or less of oxygen, 0.08% or less of carbon, 0.05% or less of nitrogen, and 0.015% or less of hydrogen, with the remainder being titanium, and the relative density is 97.0% or more.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a titanium alloy sintered body and a titanium alloy sintered body, and more particularly to a method for manufacturing a titanium alloy sintered body capable of achieving low oxygenation and a titanium alloy sintered body.

Background Art

[0002] Conventionally, titanium is the fourth most abundant metal element in the earth's crust after aluminum, iron, and magnesium, and is known to be a metal with excellent lightweight, high strength, corrosion resistance, and little adverse effects on the human body. However, titanium has a hexagonal close-packed structure at room temperature, making it difficult to process with deformation, and due to its high strength, machining is also not easy, so there has been a problem that it is difficult to suppress the manufacturing cost.

[0003] Therefore, in order to suppress the manufacturing cost, there is an increasing expectation for the production of titanium by metal powder injection molding (MIM) that can achieve near-net shape only by product molding without machining.

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

[0005] According to this method for manufacturing titanium alloy sintered bodies, titanium or titanium alloys with an average particle size of less than 25 μm in the metal powder used for green body production are used, and the sintering stage is carried out at a sintering temperature of up to 1100°C for a sintering time of 5 hours or less, in an environment with a reduced pressure compared to atmospheric pressure. This process can then be used to selectively influence the particle structure and material properties of the resulting material.

[0006] Furthermore, as described in Patent Document 2, a titanium alloy sintered body is known characterized by having an average crystal grain size on the surface greater than 30 μm and less than or equal to 500 μm, and a Vickers hardness on the surface of 300 or more and less than or equal to 800.

[0007] Such a titanium alloy sintered body prevents surface degradation even when exposed to harsh environments for extended periods, resulting in a titanium alloy sintered body with high mirror-like (aesthetic) properties. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Special Publication No. 2019-516021 [Patent Document 2] Japanese Patent Publication No. 2019-44225 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, it is known that the tensile strength and elongation properties of titanium alloy sintered bodies change depending on the oxygen content; increasing the oxygen content increases tensile strength and decreases elongation. In contrast, it is difficult to keep the oxygen content below 0.2 mass% in titanium alloy sintered bodies produced by conventional manufacturing methods, which presents a problem in increasing fatigue strength.

[0010] This invention has been made in view of the above circumstances, and aims to provide a method for manufacturing a titanium alloy sintered body with reduced oxygen content and increased fatigue strength, and a titanium alloy sintered body. [Means for solving the problem]

[0011] The titanium alloy sintered body according to the present invention is characterized in that, by mass%, it consists of 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, and 0.015% or less hydrogen, with the remainder being titanium, and has a relative density of 97.0% or more.

[0012] Furthermore, in the titanium alloy sintered body according to the present invention, it is preferable that the average grain size is 5.0 to 50.0 μm and the acicular ratio of the crystal structure is 3 or less. [Effects of the Invention]

[0013] According to the manufacturing method for a titanium alloy sintered body according to one embodiment, the sintering process is performed at a sintering temperature of 980°C and for a sintering time of 48 hours, thereby enabling the production of a low-oxygen titanium alloy sintered body. Furthermore, since the titanium alloy sintered body according to the present invention has a relative density of 97.0% or more and an oxygen content of 0.2% by mass or less, it is possible to provide a titanium alloy sintered body with high fatigue strength. [Brief explanation of the drawing]

[0014] [Figure 1] Flowchart of a method for manufacturing a titanium alloy sintered body according to an embodiment of the present invention. [Figure 2] The results of observing the microstructure are shown, where (A) is the titanium alloy sintered body according to this embodiment, and (B) is the observation result of a comparative example. [Figure 3] A graph showing the relationship between sintering time and relative density for the titanium alloy sintered body according to this embodiment and for a comparative example. [Figure 4] A graph showing the tensile strength test results of a titanium alloy sintered body according to an embodiment of the present invention and a comparative example.

Best Mode for Carrying Out the Invention

[0015] Hereinafter, preferred embodiments for carrying out the present invention will be described with reference to the drawings. Note that the following embodiments do not limit the invention according to each claim, and not all combinations of features described in the embodiments are essential for the solution means of the invention.

[0016] FIG. 1 is a flowchart of a method for manufacturing a titanium alloy sintered body according to an embodiment of the present invention, FIG. 2 is an observation result of a microstructure, (A) is a titanium alloy sintered body according to the present embodiment, and (B) is an observation result of a comparative example. FIG. 3 is a graph showing the relationship between the sintering time and the relative density of the titanium alloy sintered body according to the present embodiment and the comparative example, and FIG. 4 is a graph showing the tensile strength test results of the titanium alloy sintered body according to the embodiment of the present invention and the comparative example.

[0017] As shown in FIG. 1, the method for manufacturing a titanium alloy sintered body according to the present embodiment includes a step (S101) of manufacturing a kneaded product of metal powder and a binder, a step (S102) of injection-molding the kneaded product to manufacture a molded body, and a step of degreasing the molded body to remove the binder. (S103), a step (S104) of sintering the molded body from which the binder has been removed to obtain a titanium alloy sintered body, and a step (S105) of performing post-processing and inspection on the titanium alloy sintered body.

[0018] The step (S101) of manufacturing a kneaded product of metal powder and a binder kneads the metal powder and the binder to manufacture a kneaded product. As the metal powder, conventionally well-known pure titanium or a titanium alloy is preferably used, and a low-oxygen powder having an oxygen content of 0.13% by mass or less is more preferably used. For example for Ti-6Al-4V material, it is preferably a powder equivalent to ASTM grade23 (Extra-Low Interstitial).

[0019] The binder is an additive that imparts the fluidity necessary for injection molding described later, and a binder composed of a general-purpose synthetic resin with a lubricant and a plasticizer added thereto is preferably used. Note that the ratio of the metal powder to the binder can be appropriately adjusted according to the properties, shape, etc. of the titanium alloy sintered body to be manufactured. For example, a ratio of the metal powder to the binder of 60 vol%:40 vol% is preferable.

[0020] The production of the kneaded material is carried out by adding a binder to the metal powder, heating, pressurizing and mixing, and then pulverizing and granulating the cooled and solidified kneaded material to obtain a kneaded material having fluidity.

[0021] The step (S102) of manufacturing a molded body by injection molding the kneaded material is to inject the kneaded material into a mold and then cool and solidify it to manufacture a molded body having a predetermined shape. As the mold used for injection molding, a mold corresponding to the shape of a conventionally well-known molded body can be used.

[0022] The step (S103) of degreasing the molded body to remove the binder is a step of removing the binder contained in the molded body prior to sintering described later to obtain a degreased body. A heat degreasing treatment in which the molded body is heated under an inert gas flow to evaporate and thermally decompose the binder, or a solvent degreasing treatment in which the binder is extracted with an organic solvent is performed.

[0023] The step (S104) of sintering the molded body from which the binder has been removed to obtain a titanium alloy sintered body is to heat and sinter the degreased body at 800 to 995 °C, more preferably about 980 °C, for 6 to 200 hours, more preferably 48 hours, under a vacuum of 1 × 10−3 Pa or less. Note that the remaining binder contained in the degreased body is removed in the step of being heated by sintering. Since the binder is removed from the molded body through degreasing and sintering in this way, the sintered body shrinks by about 10 to 20% compared to the molded body.

[0024] Further, in the step (S104) of obtaining a titanium alloy sintered body, a zirconia setter is placed in a molybdenum container, the degreased body is placed on the setter, the container is closed with a molybdenum lid, and then the inside of the container is evacuated and sintering is performed.

[0025] The process of post-processing and inspection of the titanium alloy sintered body (S105) is a process in which the titanium alloy sintered body obtained by sintering is post-processed and inspected, and specifically the titanium alloy sintered body Heat treatment is performed, and polishing is carried out to ensure dimensional accuracy. [Examples]

[0026] Next, the present invention will be described in more detail with reference to examples.

[0027] The method for manufacturing a titanium alloy sintered body involved a step (S101) to prepare a mixture of metal powder and a binder. The titanium alloy powder used consisted of 6.22% aluminum, 4.04% vanadium, 0.2% iron, 0.091% oxygen, 0.004% carbon, 0.012% nitrogen, 0.002% hydrogen, with the remainder being titanium, and had an average particle size of 27.3 μm. The binder used was the one described in Japanese Patent No. 5163596, which was mixed and kneaded with the titanium alloy powder at a ratio of 40% by volume. Subsequently, the mixture was injection molded to produce a molded body (S102), and the molded body was degreased to remove the binder (S103), both of which were carried out by heat degreasing.

[0028] In the process (S104) of obtaining a titanium alloy sintered body by sintering a molded body from which the binder has been removed, the degreased body is heated under a vacuum of 1 × 10⁻³ Pa or less, and then heated to 980°C for 48 hours to sinter. Because the binder is removed from the molded body through degreasing and sintering, the sintered body shrunk by about 15% compared to the molded body. The relative density of the titanium alloy sintered body was 97.5%.

[0029] Furthermore, in the process of obtaining a titanium alloy sintered body (S104), a zirconia setter was placed inside a molybdenum container, the degreased material was placed on the setter, the container was sealed with a molybdenum lid, and then the container was sintered under vacuum.

[0030] In the post-processing and inspection step (S105) of the titanium alloy sintered body, the titanium alloy sintered body obtained by sintering was cut and polished to form a fatigue test specimen. The tensile test specimen was only inspected and no post-processing was performed.

[0031] First, particle size observation tests were conducted on the examples and comparative examples of titanium alloy sintered bodies according to this embodiment. Here, the comparative example was a sintered body prepared using a mixture of ordinary metal powders with a higher oxygen content than the low-oxygen powder used in the titanium alloy sintered body according to this embodiment, and sintered at a temperature of 1100°C for 6 hours. For the particle size observation test, photographs of the surface of the examples and comparative examples were taken at 400x magnification, printed, the outline of the granular structure was traced by hand, and the images were imported into a computer. Then, the equivalent circle diameter, absolute maximum length, diagonal width, and needle-like aspect ratio were measured using measurement software (Winroof). Furthermore, particles with an equivalent circle diameter of less than 5 μm were excluded from the measurement data, and the average equivalent circle diameter, average absolute maximum length, average diagonal width, and average needle-like aspect ratio were calculated. The mean circle equivalent diameter is the diameter of an equivalent circle with the same area as the object; the absolute maximum length is the length of the longest part of the object; the diagonal width is the shortest distance between two lines parallel to the maximum absolute length when the object is placed between them; and the needle-shape ratio is the absolute maximum length divided by the diagonal width.

[0032] As shown in Figure 2, the titanium alloy sintered body obtained by the manufacturing method of the titanium alloy sintered body according to this embodiment exhibits a microstructure with a generally rounder grain size compared to the conventional comparative example. As shown in Figure 2(A), it can be confirmed that the needle-like aspect ratio of the crystalline structure in the microfibers of the titanium alloy sintered body according to this embodiment is 3.0 or less. In contrast, as shown in Figure 2(B), the conventional comparative example has a generally elongated crystalline structure with a needle-like aspect ratio of 3.0 or more, confirming that the titanium alloy sintered body according to this embodiment has a generally finer and rounder crystalline structure compared to the comparative example.

[0033] The particle size observation results for the examples are as follows. [Table 1]

[0034] Next, as shown in Figure 3, it was confirmed that the relative density of the titanium alloy sintered body using low-oxygen powder could be obtained at a relative density of 98% or higher, equivalent to that of the comparative example, by sintering for 48 hours.

[0035] Furthermore, as shown in the table below, the results of the oxygen, nitrogen, and carbon analysis of the titanium alloy sintered body according to this embodiment were found to have nitrogen and carbon content equivalent to that of the comparative example, and an oxygen content of 0.18%, which was significantly lower than that of the comparative example. This satisfies the conditions of JIS standard 60 and ASTM standard Gr5 for molten materials. [Table 2]

[0036] Next, tensile strength tests and fatigue strength tests were performed on the titanium alloy sintered body according to this embodiment and the comparative example. For the tensile strength test, the scoring distance was set to 15 mm. As shown in Figure 4, the tensile strength of the embodiment was equivalent to that of the comparative example, and it was confirmed that the elongation of the embodiment was higher than that of the comparative example.

[0037] The fatigue strength test was conducted under the following 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 × 10⁷ cycles (6) Frequency: 10Hz

[0038] The fatigue strength test results showed that the fatigue strength after 1.0 × 10⁷ cycles was 350 MPa for the example and 280 MPa for the comparative example.

[0039] Thus, it has been confirmed that the method for manufacturing a titanium alloy sintered body according to this embodiment makes it possible to obtain a titanium alloy sintered body with reduced oxygen content and increased fatigue strength.

Claims

1. A titanium alloy sintered body characterized by comprising, by mass percent, 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, and 0.015% or less hydrogen, with the remainder being titanium, and having a relative density of 97.0% or more.

2. In the titanium alloy sintered body according to claim 1, The average crystal grain size is 5.0 to 50.0 μm. A titanium alloy sintered body characterized by having a cucleate ratio of 3 or less in its crystalline structure.

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