Titanium alloy molding

Abstract

To provide a titanium alloy molding having effectively improved fatigue strength.SOLUTION: A titanium alloy molding comprises: a substrate 10 having a laminated structure of a titanium alloy; and a hardened layer 30 provided on a surface 20 of the substrate 10. The hardened layer 30 comprises: a first hardened layer 30a constituted of titanium oxide provided on the surface 20 of the substrate 10; a second hardened layer 30b constituted of an α phase provided on an inner layer of the first hardened layer 30; and a third hardened layer 30c constituted of an α'martensite phase and metastable β phase provided on an inner layer of the second hardened layer 30b.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a titanium alloy shaped article. [Background technology]

[0002] Conventionally, a high-frequency heat treatment method has been described in which an α+β type titanium alloy is heated for a short time of 1 to less than 600 seconds at a temperature of 800°C to 1200°C by high-frequency induction heating at a frequency of 1 kHz to 400 kHz, and then rapidly cooled to generate the α' phase, thereby improving the yield strength, tensile strength, and fatigue strength of the α+β type titanium alloy (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-80916 Summary of the Invention [Problem to be solved by the invention]

[0004] However, Patent Document 1 does not disclose a titanium alloy shaped article in which fatigue strength is effectively improved. Therefore, an object of the present invention is to provide a titanium alloy shaped article having effectively improved fatigue strength. [Means for solving the problem]

[0005] The titanium alloy shaped product according to the present invention comprises a substrate having a layered structure of a titanium alloy, and a hardened layer provided on a surface of the substrate, the hardened layer comprising: a first hardened layer formed on the surface of the substrate and made of titanium oxide; a second hardened layer formed on an inner layer of the first hardened layer and made of α phase; and a third hardened layer formed on the inner layer of the second hardened layer and made of α' martensite phase and metastable β phase. Effect of the Invention

[0006] According to the present invention, it is possible to provide a titanium alloy shaped article having effectively improved fatigue strength. [Brief description of the drawings]

[0007] [Figure 1] 1A and 1B are conceptual diagrams for explaining a titanium alloy shaped object according to an embodiment of the present invention, in which (a) is a conceptual diagram showing an example of the overall shape, (b) is a conceptual cross-sectional diagram when (a) is cut along line AA, and (c) is a conceptual cross-sectional diagram when region S in (b) is enlarged. [Diagram 2] 1A and 1B are schematic diagrams of test pieces used in the examples, where (a) is for EBSD analysis, X-ray diffraction, and cross-sectional hardness distribution measurement, and (b) is for fatigue testing. [Diagram 3] EBSD analysis results of the vicinity of the surface 20 in the examples and comparative examples. [Figure 4] 1 shows EBSD analysis results for the inside of a substrate 10 in an example and a comparative example. [Diagram 5] 1 shows the phase identification results by X-ray diffraction of the surface 20 in the examples and comparative examples. [Figure 6] 1 shows the cross-sectional hardness distribution results from the surface 20 toward the inner layer in the examples and comparative examples. [Figure 7] 4 is a graph showing fatigue test results of the surface 20 in the examples and comparative examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram for explaining a titanium alloy shaped object according to the present embodiment, in which (a) is a conceptual diagram showing an example of the overall shape, (b) is a conceptual cross-sectional diagram when (a) is cut along line AA, and (c) is a conceptual cross-sectional diagram when region S in (b) is enlarged.

[0009] 1, the titanium alloy shaped article 1 according to this embodiment is, for example, in the shape of a round bar and includes a substrate 10 having a laminated structure of a titanium alloy, and a hardened layer 30 provided on a surface 20 of the substrate 10. The hardened layer 30 includes a first hardened layer 30a made of titanium oxide provided on the surface 20 of the substrate 10, a second hardened layer 30b made of α phase provided in the inner layer of the first hardened layer 30a, and a third hardened layer 30c made of α' martensite phase and metastable β phase provided in the inner layer of the second hardened layer 30b.

[0010] The titanium alloy shaped article 1 according to this embodiment includes the hardened layer 30 as described above, and therefore effectively improves fatigue strength.

[0011] The substrate 10 having a titanium alloy laminated structure is laminated by three-dimensional additive manufacturing technology. Here, additive manufacturing refers to inputting three-dimensional data of the target product to be molded into a computer, slicing the three-dimensional data horizontally on the computer to create two-dimensional data, and then laminating materials such as metal powder and resin based on this data to create the product.

[0012] There are various methods for this additive manufacturing, but the method used to manufacture the substrate 10 having the titanium alloy layered structure according to this embodiment is not particularly limited. For example, additive manufacturing may use the selective laser melting method (SLM method). In this selective laser melting method, a laser is irradiated to the metal powder, which is the raw material of the target product, spread in a laser melting device based on the data of the target product to be manufactured, and the metal powder in the irradiated area is melted. Then, by lowering the base on which the metal powder is spread, the metal powder is repeatedly melted and solidified, making it possible to manufacture any shape. By using additive manufacturing methods such as selective laser melting, it is possible to produce a product with precise dimensions even if the product has a complex shape. Therefore, by using additive manufacturing methods such as selective laser melting, it is possible to produce titanium alloys into complex shapes that are impossible to produce using other processing methods such as cutting, and they can be used favorably for biomaterial products such as artificial bones that are adapted to the skeletal shape of each user. The thickness of the substrate 10 can be appropriately set depending on the application of the titanium alloy shaped article.

[0013] The first hardened layer 30a is made of titanium oxide and provided on the surface 20 of the substrate 10, as shown in FIG. The thickness of the first hardened layer 30a is, for example, about 1 μm.

[0014] The second hardened layer 30b is formed of an α phase and is provided in the inner layer of the first hardened layer 30a on the surface 20 of the substrate 10 as shown in FIG. The thickness of the second hardened layer 30b is, for example, about 10 μm to 30 μm.

[0015] As shown in FIG. 1, the third hardened layer 30c is an inner layer of the first hardened layer 30a, and is further composed of an α' martensite phase and a metastable β phase provided in the inner layer of the second hardened layer 30b. The third hardened layer 30c has a thickness of, for example, about 120 μm to 140 μm.

[0016] As described above, in the titanium alloy object 1 according to this embodiment, the above-mentioned first hardened layer 30a, second hardened layer 30b, and third hardened layer 30c are laminated in this order from the surface 20 of the substrate 10 toward the inner layer. It is believed that the presence of the first hardened layer 30a, the second hardened layer 30b and the third hardened layer 30c generates high compressive residual stress, and the fatigue strength is effectively improved due to the suppression of crack propagation caused by the stress-induced transformation of the metastable β phase contained in the third hardened layer 30c. In addition, since the first hardened layer 30a is made of titanium oxide, it is believed that the wear resistance of the surface 20 of the substrate 10 is improved.

[0017] Next, an example of a method for producing a titanium alloy shaped article according to this embodiment will be described. In order to manufacture the titanium alloy shaped article according to this embodiment, a base material having a laminated structure of titanium alloy is subjected to high-frequency induction heating. For this high-frequency induction heating, a high-frequency generator that generates high-frequency waves and a heating coil that generates an induction magnetic field are prepared. However, in this embodiment, the structures of the generator and the heating coil are not limited, and known devices can be used. The frequency of the high frequency generated by the generator is 1 kHz or more. A high frequency current of this frequency is applied to a heating coil to generate an induction magnetic field, thereby rapidly raising the temperature of the substrate. The frequency is preferably within the range of 1 kHz to 400 kHz.

[0018] In the above-mentioned high-frequency induction heating, the maximum temperature is preferably 998°C (β transformation point) or higher. By setting the temperature at such a level, it is possible to achieve diffusion of oxygen and nitrogen from the surface of the substrate 10 having a laminated structure of titanium alloy in a short period of time. In addition, in the case of heating by high-frequency induction, a heating time of at least 1 second is sufficient to form a hardened layer effective for improving wear resistance and fatigue strength. After the above heating, the material to be treated is rapidly cooled by water cooling or the like, as in the conventional method.

[0019] The heating by high frequency induction is performed in an air atmosphere. In this way, by performing heating in an air atmosphere, oxygen and nitrogen can be supplied to the surface and the surface layer of the substrate having a laminated structure of titanium alloy. Therefore, a first hardened layer made of titanium oxide, a second hardened layer made of α phase provided in the inner layer of the first hardened layer, and a third hardened layer made of α' martensite phase and metastable β phase provided in the inner layer of the second hardened layer can be formed on the surface of the substrate having a laminated structure of titanium alloy. In addition, since the heating is performed in the air, there is no need to install a new atmosphere supply device, and the heating can be performed using an existing high-frequency induction heating device, thereby preventing an increase in manufacturing costs. EXAMPLES

[0020] FIG. 2 is a conceptual diagram of a test piece used in the examples, (a) for EBSD analysis, X-ray diffraction, and cross-sectional hardness distribution measurement, and (b) for fatigue testing. As a substrate with a titanium alloy laminated structure, a Ti-6Al-4V alloy having a cylindrical shape as shown in Figure 2(a) and (b) was fabricated by three-dimensional additive manufacturing technology using the selective laser melting method (hereinafter, both Figures 2(a) and (b) are referred to as the "processed material"). In addition, a high-frequency generator capable of outputting a high-frequency voltage of 200 kHz was prepared.

[0021] Next, the workpiece was surrounded by a heating coil, and a high-frequency voltage generated by the high-frequency generator was applied to the heating coil to generate an induction magnetic field, and the workpiece immediately after three-dimensional additive manufacturing by the selective laser melting method was high-frequency induction heated for 60 seconds in an air atmosphere. During this heating, the output of the high-frequency generator was controlled to test so that the heating temperature of the workpiece was 1050 ° C. and the heating rate to the heating temperature was 100 ° C. / sec. After the heating time, the workpiece was quenched so that the cooling rate was 100 ° C. / sec (Example). EBSD analysis, X-ray diffraction, cross-sectional hardness distribution measurement, and fatigue test were performed on the obtained treated material. As a comparative example, EBSD analysis, X-ray diffraction, cross-sectional hardness distribution measurement, and fatigue testing were similarly performed on the workpiece immediately after additive manufacturing, which had not been subjected to the above-mentioned high-frequency induction heating.

[0022] FIG. 3 shows EBSD analysis results near the surface 20 in the examples and comparative examples. FIG. 4 shows EBSD analysis results within the substrate 10 in the examples and comparative examples. FIG. 5 shows phase identification results by X-ray diffraction of the surface 20 in the examples and comparative examples. FIG. 6 shows cross-sectional hardness distribution results in the direction from the surface 20 to the inner layer in the examples and comparative examples. FIG. 7 is a graph showing fatigue test results of the surface 20 in the examples and comparative examples.

[0023] As shown in Fig. 3, it can be seen that in the Example, a second hardened layer 30b (α phase in the phase map) composed of α phase is formed in the vicinity of the surface 20 within a depth range of about 10 μm to 30 μm, compared to the Comparative Example. Also, as shown in Fig. 3, it can be seen that in both the Example and the Comparative Example, a third hardened layer 30c composed of α' martensite phase (linear α phase in the phase map) and metastable β phase (part other than the linear α phase in the phase map) is formed in the inner layer of the second hardened layer 30b. On the other hand, as shown in FIG. 4, it can be seen that within the substrate 10 in the example, there is no particular difference compared to the comparative example.

[0024] 5, it can be seen that the peaks of TiO2 and TiO appear on the surface 20 in the Example, compared to the Comparative Example. From the above, it can be seen that the first hardened layer 30a made of titanium oxide is formed on the surface 20 in the Example.

[0025] As shown in FIG. 6, it can be seen that in the embodiment, compared to the comparative example, the hardness of the surface 20 in the inward direction is a gradient hardness layer (hardened layer 30) in which the hardness decreases from about 1330HT115 near the surface 20 to the hardness of the substrate 10 (about 450HT115).

[0026] 3, the third hardened layer 30c has linear α phase (α type titanium alloy) in both the example and the comparative example. On the other hand, in FIG. 6, the third hardened layer 30c (for example, at a distance (μm) from the surface of 50 μm to 100 μm) has a higher hardness than the substrate 10 and the comparative example. The reason for this increase in hardness is believed to be that during the high-frequency induction heating described above, oxygen and nitrogen contained in the atmospheric air diffuse from the surface 20 of the workpiece toward the inner layer to the third hardened layer 30c, and the rapid cooling effect forms the third hardened layer 30c composed of α' martensite phase and metastable β phase. It is presumed that the second hardened layer 30b, which is the outer layer of the third hardened layer 30c, contains a large amount of the diffused oxygen and nitrogen, and therefore the α phase is formed.

[0027] Furthermore, as shown in Fig. 7, the fatigue strength (number of cycles to fracture: 10 7 It can be seen that the stress amplitude (MPa) is improved by more than five times, from 80 MPa (Comparative Example) to 440 MPa (Example). Therefore, it was confirmed that a titanium alloy shaped article having effectively improved fatigue strength could be obtained. [Explanation of symbols]

[0028] 1 Titanium alloy molding 10 Base material 20 surface 30 Hardened layer 30a 1st hardened layer 30b 2nd hardened layer 30c 3rd hardened layer

Claims

[Claim 1] A substrate having a laminated structure of a titanium alloy; A hardened layer provided on the surface of the substrate, The titanium alloy shaped article has: a first hardened layer made of titanium oxide provided on a surface of the base material; a second hardened layer made of α phase provided on an inner layer of the first hardened layer; and a third hardened layer made of α' martensite phase and metastable β phase provided on an inner layer of the second hardened layer.

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