METHOD FOR PRODUCING TITANIUM ALLOY SINTERED PART, AND TITANIUM ALLOY SINTERED PART

Abstract

There is provided a titanium alloy sintered part in which the oxygen content is reduced and the fatigue strength is enhanced, and a method for producing the titanium alloy sintered part. The method for producing a titanium alloy sintered part by a metal injection molding method includes a mixing process of producing a compound of a metal powder and a binder, an injection process of subjecting the compound to injection molding to produce a green part, a degreasing process of degreasing the green part to remove the binder, and a sintering process of sintering the green part from which the binder was removed to obtain a sintered body, and the sintering process is performed at a sintering temperature of 800 to 995? C. for a sintering time of 6 to 200 hours.

Claims

1. A method for producing a titanium alloy sintered part by a metal injection molding method, comprising: a mixing process of producing a compound of a metal powder and a binder; an injection process of subjecting the compound to injection molding to produce a green part; a degreasing process of degreasing the green part to remove the binder; and a sintering process of sintering the green part from which the binder is removed to obtain a sintered body, wherein the sintering process is performed 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 part according to claim 1, wherein the sintering process is performed under vacuum, and in the vacuum, an atmospheric pressure during sintering is 1?10.sup.?3 Pa or less.

3. The method for producing a titanium alloy sintered part according to claim 1, wherein a low-oxygen metal powder is used as the metal powder.

4. A titanium alloy sintered part, consisting of, in 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, and hydrogen: 0.015% or less, with the balance being titanium, and having a relative density of 97.0% or more.

5. The titanium alloy sintered part according to claim 4, wherein a mean grain size is 5.0 to 50.0 ?m, and a aspect ratio of a crystal structure is 3 or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a flowchart of a method for producing a titanium alloy sintered part according to an embodiment of the present invention;

[0017] FIG. 2 is a multiple view drawing showing results of microstructure observation, in which (A) is a view showing an observation result with respect to a titanium alloy sintered part according to the present embodiment, and (B) is a view showing an observation result with respect to a Comparative Example;

[0018] FIG. 3 is a graph illustrating the relation between sintering time and relative density with respect to the titanium alloy sintered part according to the present embodiment and the Comparative Example; and

[0019] FIG. 4 is a multiple view drawing showing graphs that show tensile strength test results obtained with respect to the titanium alloy sintered part according to an embodiment of the present invention and the Comparative Example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] Hereinafter, a preferred embodiment for implementing the present invention will be described using the accompanying drawings. Note that the following embodiment does not limit the invention according to each claim, and not all combinations of features described in the embodiment are necessarily essential for the solution of the invention.

[0021] FIG. 1 is a flowchart of a method for producing a titanium alloy sintered part according to an embodiment of the present invention. FIG. 2 is a multiple view drawing showing results of microstructure observation, in which (A) is a view showing an observation result with respect to a titanium alloy sintered part according to the present embodiment, and (B) is a view showing an observation result with respect to a Comparative Example. FIG. 3 is a graph illustrating the relation between sintering time and relative density with respect to the titanium alloy sintered part according to the present embodiment and the Comparative Example. FIG. 4 is a multiple view drawing showing graphs that show tensile strength test results obtained with respect to the titanium alloy sintered part according to an embodiment of the present invention and the Comparative Example.

[0022] As illustrated in FIG. 1, the method for producing a titanium alloy sintered part according to the present embodiment includes: a step of producing a compound of a metal powder and a binder (S101); a step of subjecting the compound to injection molding to produce a green part (S102); a step of degreasing the green part to remove the binder (S103); a step of sintering the green part from which the binder is removed to obtain a titanium alloy sintered part (S104); and a step of subjecting the titanium alloy sintered part to post processing and inspection (S105).

[0023] In the step of producing a compound of a metal powder and a binder (S101), a metal powder and a binder are kneaded together to produce a compound. As the metal powder, pure titanium or a titanium alloy which is conventionally known is preferably used, and a low-oxygen powder in which the oxygen content is 0.13% by mass or less is more preferably used. For example, with respect to the Ti-6Al-4V alloy, a powder corresponding to ASTM grade 23 (Extra Low-Interstitial) is preferable.

[0024] The binder is an additive agent that provides fluidity that is necessary for injection molding which is described later, and a material obtained by adding a lubricant and a plasticizer to a binding agent consisting of a general-purpose synthetic resin is preferably used. Note that, the ratio between the metal powder and the binder can be appropriately adjusted according to the properties and shape or the like of the titanium alloy sintered part to be produced, and for example it is suitable to make the ratio between the metal powder and the binder a ratio of 60 vol %:40 vol %.

[0025] Production of the compound is performed by adding the binder to the metal powder, performing heating, pressurizing, and mixing, and thereafter subjecting the cooled and solidified compound to pulverization and granulation to obtain a compound having fluidity.

[0026] In the step of subjecting the compound to injection molding to produce a green part (S102), the compound is injection-molded into a mold, and thereafter is caused to cool and solidify to produce a green part having a desired shape. A mold corresponding to the shape of a green part that is conventionally known can be used as the mold to be utilized for the injection molding.

[0027] The step of degreasing the green part to remove the binder (S103) is a step in which, prior to sintering which is described later, the binder contained in the green part is removed to obtain a degreased body. In this step, a thermal degreasing treatment in which the green part 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 using an organic solvent or the like is performed.

[0028] In the step of sintering the green part from which the binder was removed to obtain a titanium alloy sintered part (S104), the degreased body is heated and sintered at a temperature in a range from 800 to 995? C., more preferably a temperature of about 980? C., under vacuum of 1?10.sup.?3 Pa or less for 6 to 200 hours, and more preferably for 48 hours. Note that, any remaining binder contained in the degreased body is removed in the step in which the degreased body is heated by sintering. Because the binder is removed from the green part by performing degreasing and sintering in this way, the sintered body shrinks by about 10 to 20% compared to the green part.

[0029] Further, in the step of obtaining a titanium alloy sintered part (S104), a setter made of zirconia is arranged inside a case made of molybdenum, the degreased body is placed on the setter, the case is sealed with a lid made of molybdenum, and thereafter the inside of the case is evacuated and sintering is performed.

[0030] The step of subjecting the titanium alloy sintered part to post processing and inspection (S105) is a step in which post processing and inspection or the like of the titanium alloy sintered part obtained by sintering is performed, and specifically, in this step the titanium alloy sintered part is subjected to a heat treatment, and polishing or the like to secure the dimensional accuracy is performed.

Example

[0031] Next, the present invention is described in further detail with reference to an Example.

[0032] In the method for producing a titanium alloy sintered part, as the titanium alloy powder in the step of producing a compound of metal powder and a binder (S101), a titanium alloy powder was used which consisted of, in mass %, aluminum: 6.22%, vanadium: 4.04%, iron: 0.2%, oxygen: 0.091%, carbon: 0.004%, nitrogen: 0.012%, and hydrogen: 0.002%, with the balance being titanium, and which had a mean particle size of 27.3 A binder described in Japanese Patent No. 5163596 was used as the binder, and was mixed at a ratio of 40 vol % with respect to the titanium alloy powder and kneaded. Thereafter, the step of subjecting the compound to injection molding to produce a green part (S102) was performed, and the step of degreasing the green part to remove the binder (S103) was performed by thermal degreasing.

[0033] In the step of sintering the green part from which the binder was removed to obtain a titanium alloy sintered part (S104), under vacuum of 1?10.sup.?3 Pa or less, the temperature of the degreased body was increased to 980? C. by heating, and sintering was performed at 980? C. for 48 hours. Because the binder was removed from the green part by performing the degreasing and sintering, the sintered body shrunk by about 15% compared to the green part. Further, the relative density of the titanium alloy sintered part was 97.5%.

[0034] Further, in the step of obtaining a titanium alloy sintered part (S104), a setter made of zirconia was arranged inside a case made of molybdenum, the degreased body was placed on the setter, the case was sealed with a lid made of molybdenum, and thereafter the inside of the case was evacuated and sintering was performed.

[0035] In the step of subjecting the titanium alloy sintered part to post processing and inspection (S105), the titanium alloy sintered part obtained by sintering was subjected to cutting and polishing to obtain a fatigue test specimen. A tensile test specimen was only inspected, and was not subjected to post processing.

[0036] First, the Example of the titanium alloy sintered part according to the present embodiment and a Comparative Example were subjected to a grain size observation test. Here, the Comparative Example was a sintered body obtained by forming a compound using a normal metal powder having a large oxygen content in comparison to the low-oxygen powder used in the titanium alloy sintered part according to the present embodiment, and performing sintering at a sintering temperature of 1100? C. for a sintering time of six hours. In the grain size observation test, photographs obtained by photographing the surface of the Example and the Comparative Example at a magnification of ?400 were printed, an contour of the granular structure was traced by hand on the respective photographs, and each photograph was then scanned as an image into a computer, and thereafter the equivalent circular diameter, absolute maximum length, diagonal width, and aspect ratio were measured using measurement software (Winroof). Further, grains having an equivalent circular diameter of less than 5 ?m were excluded from the measurement data, and the mean equivalent circular diameter, mean absolute maximum length, average diagonal width, and average aspect ratio were calculated. For the mean equivalent circular diameter, the value of the diameter of an equivalent circle having the same area as the area of the object was determined. For the absolute maximum length, the value of the length of the longest portion among the lengths of the object was determined. For the diagonal width, the value of the shortest distance between two straight lines when the object was sandwiched between the two straight lines in parallel to the absolute maximum length was determined. For the aspect ratio, a value obtained by dividing the absolute maximum length by the diagonal width was used.

[0037] As shown in FIG. 2, in the titanium alloy sintered part obtained by the method for producing a titanium alloy sintered part according to the present embodiment, the grains in the microstructure were observed as being round overall in comparison to the conventional Comparative Example. As shown in FIG. 2(A), it can be confirmed that the aspect ratio of the crystal structure in the microfibers of the titanium alloy sintered part according to the present embodiment is 3.0 or less. In contrast, as shown in FIG. 2(B), in the conventional Comparative Example, the crystal structure is elongated overall, indicating that the aspect ratio is 3.0 or more, and it can be confirmed that, the crystal structure of the titanium alloy sintered part according to the present embodiment is fine and round overall in comparison to the Comparative Example.

[0038] The results of the grain size observation with respect to the Example were as follows.

TABLE-US-00001 TABLE 1 Equivalent Absolute Circular Maximum Diagonal Diameter (?m) Length (?m) Width (?m) Aspect Ratio Average 15.2 21.7 14.4 1.6 Maximum 40.1 59.1 45.4 4.5 Minimum 5.1 7.2 3.3 1.0

[0039] Next, as shown in FIG. 3, with regard to the relative density, it was confirmed that in the titanium alloy sintered part which used a low-oxygen powder, a relative density of 98% or more that was on an equal level to that of the Comparative Example was obtained by sintering for 48 hours.

[0040] Further, as the result of analyzing the contents of oxygen, nitrogen, and carbon, as shown in the following table, it was found that in the titanium alloy sintered part according to the present embodiment, the nitrogen content and carbon content were on an equal level to the nitrogen content and carbon content in the Comparative Example, and it was found that the oxygen content was 0.18% which was lower by a large margin in comparison to the oxygen content in the Comparative Example. This satisfies the conditions for a wrought material of Grade 60 of the JIS standard and Grade 5 of the ASTM standard.

TABLE-US-00002 TABLE 2 Oxygen Content Nitrogen Content Carbon Content (%) (%) (%) Example 0.18 0.045 0.072 Comparative 0.30 0.059 0.083 Example

[0041] Next, a tensile strength test and a fatigue strength test were conducted on the titanium alloy sintered part according to the present embodiment and the Comparative Example. In the tensile strength test, the gauge length was set to 15 mm. As shown in FIG. 4, it can be confirmed that the tensile strength of the Example was on an equal level to the tensile strength of the Comparative Example, and the elongation of the Example was higher than the elongation of the Comparative Example.

[0042] The fatigue strength test was conducted under the following test conditions. [0043] (1) Test temperature: normal temperature [0044] (2) Standard: ASTM E466 [0045] (3) Stress ratio: R=0.1 [0046] (4) Waveform: sine wave [0047] (5) Cut-off cycle: 1.0?10.sup.7 cycles [0048] (6) Frequency: 10 Hz

[0049] The results of the fatigue strength test showed that the fatigue strength at 1.0?10.sup.7 cycles was 350 MPa in the Example, and was 280 MPa in the Comparative Example.

[0050] Thus, it was confirmed that according to the method for producing a titanium alloy sintered part according to the present embodiment, a titanium alloy sintered part in which the oxygen content is reduced and the fatigue strength is enhanced can be obtained.

[0051] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

[0052] The entire disclosure of Japanese Patent Application No. 2022-151959 filed on Sep. 22, 2022 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.