JP7457980B2 - Method for producing TiAl-based alloy - Google Patents

Description

The present disclosure relates to a method for manufacturing a TiAl-based alloy.

Metal injection molding (MIM) is known as a manufacturing method suitable for mass production of precision components.

The MIM method is a method of molding by injecting a mixture of a binder and metal powder into a mold. The MIM method includes injection molding and batch sintering.

Patent Document 1 discloses a technique for manufacturing a sintered body having a shape similar to a final product of a turbine wheel using the MIM method.

Japanese Patent Application Publication No. 2011-174096

TiAl-based alloys are inferior to other heat-resistant alloys in terms of castability and forgeability. In that respect, the MIM method is a promising manufacturing method for TiAl-based alloys.

On the other hand, since the MIM method includes a sintering process, removal of voids becomes an issue.

Compared to other heat-resistant alloys, the sintering temperature of TiAl-based alloys is close to the melting point. When sintering at a temperature close to the melting point, there is a concern that the TiAl-based alloy may be deformed. When a plurality of compacts are lined up and sintered in a sintering furnace, the temperature distribution within the furnace may become uneven. If the temperature inside the furnace is not uniform, the temperature locally increases close to the melting point, increasing the possibility that deformation will occur.

If the sintering temperature is lowered, deformation can be avoided, but voids tend to remain (=sintered density becomes lower). TiAl-based alloys with low sintered density tend to crack.

In Patent Document 1, deformations that occur during sintering are corrected by pressing.

Under these circumstances, it has been difficult to obtain a high-quality sintered body with a high sintering density using a TiAl-based alloy, and the yield rate in the sintering process is low.

The present disclosure has been made in view of such circumstances, and an object of the present disclosure is to provide a method for manufacturing a TiAl-based alloy that can more stably improve sinterability.

In order to solve the above problems, the method for manufacturing a TiAl-based alloy of the present disclosure employs the following means.

The present disclosure is a method for producing a TiAl-based alloy containing a β-stabilizing element using a metal powder injection molding method, in which the β+α/α transformation temperature is confirmed from a phase diagram of the TiAl-based alloy containing the β-stabilizing element. , the TiAl-based alloy is sintered at a temperature in the β phase existence temperature range equal to or higher than the β+α/α transformation temperature , and the β stabilizing element includes at least one of Nb and Cr .

Further, a reference aspect of the present disclosure provides a TiAl-based alloy manufactured by the above manufacturing method.

According to the present disclosure, by performing sintering in a temperature range where the volume fraction of the β phase is high, sinterability can be stably improved and a high-quality sintered body can be obtained.

It is a flowchart which shows the procedure of MIM method. This is a phase diagram of the Ti-Al-Nb-Cr quaternary system. This is a phase diagram of the Ti-Al-Cr ternary system. This is a phase diagram of the Ti-Al-Nb ternary system. FIG. 3 is a diagram showing the relationship between β phase volume fraction and relative density in Ti-44Al-4Cr. FIG. 1 is a diagram showing the relationship between the β-phase volume fraction and the relative density in Ti-47Al-5Nb. FIG. 3 is a diagram showing the relationship between relative density and temperature. It is a figure showing the diffusion coefficient of Ti and Al in each phase.

The method for producing a TiAl-based alloy according to the present embodiment can be applied to low-pressure turbine rotor blades of gas turbine engines, high-pressure compressor rotor blades of gas turbine engines, turbine wheels of turbochargers, and the like.

In this embodiment, the TiAl-based alloy is manufactured using the metal powder injection molding (MIM) method. The procedure of the MIM method is shown in FIG. 1. The MIM method includes the steps of (S1) kneading and granulation, (S2) injection molding, (S3) degreasing, and (S4) sintering. The method for manufacturing the TiAl-based alloy according to this embodiment may include a step of heat treating the sintered body (S5) after the above-mentioned (S4) sintering.

Metal powder and a binder are used as raw materials to manufacture TiAl-based alloys.

The metal powder includes Ti powder and Al powder. The metal powder may include a powder of a β-stabilizing element as a third element, or as a third and fourth element. Examples of β-stabilizing elements include V, Nb, Cr, Mo, and Mn.

The metal powder may be a powder produced by a gas atomization method, a water atomization method, a rotating electrode method, or the like. It is preferable that the metal powder is a powder produced by a gas atomization method.

The particle size of the metal powder is preferably 45 μm or less, preferably 30 μm or less. When the upper limit is exceeded, the fluidity of the molding material during injection molding becomes low, and filling defects are likely to occur. In addition, the voids between the powders become larger and the density of the sintered body decreases.

For example, as the metal powder, a powder prepared by gas atomization and adjusted to have an average particle size of 45 μm or less by classification can be used.

The binder binds the powdered raw materials together. The binder can be any material that can impart fluidity (viscosity) to the injection molding material so that the inside of the mold is completely filled with the injection molding material during (S2) injection molding. The binder is a powder made of an organic material such as polyacetal, polypropylene, polyethylene, ethylene vinyl acetate, or acrylic resin.

The raw material may further include wax or other lubricants and the like.

(S1) Kneading and Granulation Raw materials (including metal powder and binder) are put into a kneader and kneaded while being pressurized and heated. The clay-like kneaded material is passed through a granulator to obtain pellets (injection molding material).

The amount of the binder added is preferably approximately 30 wt% or more and 45 wt% or less of the entire kneaded material. The amount of binder added is adjusted in accordance with the alloy composition, product shape, and mass production specifications, taking into account injection moldability and sinterability. At that time, the amount of oxygen contained in the final product varies depending on the alloy raw material, the type of binder, and manufacturing conditions. In this embodiment, a high oxygen alloy product of approximately 1000 ppm or more is molded.

(S2) Injection molding The injection molding material is placed in an injection molding machine, plasticized, and injection molded into the cavity of a mold, thereby obtaining a molded body (green body) of the desired shape.

(S3) Debinding The green body is placed in a furnace and heated under reduced pressure and in a gas atmosphere to volatilize the binder (thermal debinding). The furnace may be either a batch type or a continuous type. This results in a brown body from which the binder has been removed. If gas is used here, it may be argon, nitrogen, nitric acid gas, or the like.

Thermal degreasing is carried out at a temperature that does not bind the metal powders together. Although it depends on the binder component and the pressure, it can be carried out at a temperature at which the binder volatilizes, which is approximately 333 K or more and 873 K or less (60° C. or more and 600° C. or less). The heating is performed at a lower temperature than sintering.

Note that the degreasing may be performed by a known degreasing method using a solvent, or may be combined with heat degreasing. In that case, the green body is immersed in a solvent to extract the binder (solvent degreasing). Depending on the binder component to be extracted, the solvent may be an organic solvent or water. The operating temperature can be from room temperature to below the boiling point of the solvent.

(S4) Sintering The brown body is placed in a sintering furnace and sintered at a temperature equal to or higher than the β+α/α transformation temperature (T _β ). The β+α/α transformation temperature is confirmed in advance from the phase diagram of the TiAl-based alloy. If the phase diagram for the desired alloy component is unknown, estimate the β+α/α transformation temperature from the phase diagram of a similar alloy. The sintering time is appropriately set to a time at which a high relative density, such as a relative density of 95% or more, can be obtained by a sintering test or the like.

The β phase existence temperature range includes a β+α phase temperature range and a β single phase temperature range. Sintering is carried out in the β+α phase temperature range or the β single phase temperature range, depending on the use of the sintered alloy.

Note that preliminary sintering may be performed before sintering (main sintering) in the β phase existing temperature range. In pre-sintering, metal particles are loosely bonded together (necked). Preliminary sintering is performed at a temperature higher than the heating for degreasing and lower than the β-transformation start temperature. For example, the preliminary sintering temperature may be 1323 K or higher and 1523 K or lower (1050° C. or higher and 1250° C. or lower). Although it depends on the alloy components, if the preliminary sintering temperature is less than 1323K, necking will be insufficient and the shape will not be retained. If the sintering temperature is higher than 1523K, there is a concern that sintering will proceed partially and unexpected shrinkage deformation will occur. Preliminary sintering may be performed for a holding time of 30 minutes or more and 2 hours or less.

(S5) Heat treatment After (S4), the sintered body is heat treated by hot isostatic pressing (HIP), if necessary. The HIP treatment is preferably carried out at a temperature lower than the sintering temperature by about 50° C. or more and about 200° C. or less, and at a high pressure of about 100 MPa or more and about 180 MPa or less. This eliminates internal defects such as voids and provides a healthier sintered body.

After the HIP treatment, the sintered body may be held at the HIP treatment temperature or lower for a predetermined period of time.

The effects of the method for manufacturing a TiAl-based alloy according to this embodiment will be explained below.

(State diagram)
2 to 4 show examples of phase diagrams of TiAl-based alloys. FIG. 2 is a phase diagram of a Ti-Al-Nb-Cr quaternary system (44 at.% Al). In the figure, the horizontal axis indicates the Nb and Cr contents (at.%), and the vertical axis indicates the temperature (K). FIG. 3 is a phase diagram of a Ti-Al-Cr ternary system (44 at.% Al). In the figure, the horizontal axis indicates the Cr content (at.%), and the vertical axis indicates the temperature (K). FIG. 4 is a phase diagram of a Ti-Al-Nb ternary system (47 at.% Al). In the figure, the horizontal axis indicates the Nb content (at.%), and the vertical axis indicates the temperature (K).

In any of the phase diagrams, there is a β single-phase region in the highest temperature region, a (β+α) two-phase region below it, and an α single-phase region further below that. The β+α/α transformation temperature is the boundary between the α single-phase region and the (β+α) two-phase region, and it is possible to understand the temperature at the boundary from these phase diagrams based on the alloy components.

In FIGS. 2 to 4, the region shown as β+α,β is the temperature region where β phase exists. In the above embodiments, sintering is performed at a temperature within this temperature range. By sintering at a higher temperature, the volume fraction of the β phase can be increased.

Although Nb and Cr are β-stabilizing elements, their phase stabilizing abilities are different. Therefore, in FIGS. 2 to 4, the β+α/α transformation temperature range also differs depending on the phase stabilizing ability of the β stabilizing element included.

In FIG. 2, the Nb and Cr contents are 6 at. The plot shown on the dashed line extending along the vertical axis through % is illustrative of typical heat treatment temperatures. 〇 indicates high temperature heat treatment, ● indicates medium temperature heat treatment, and ■ indicates low temperature heat treatment. By performing the heat treatment at a temperature in the β phase existence temperature range, the sintered density can be more efficiently increased.

Note that Figures 2 to 4 are based on phase diagrams that do not take into account the oxygen content. As a result of extensive research by the inventors, it has been discovered that the transformation temperature shifts as the oxygen content increases. In Figure 2, the upper line of the α single phase region when an oxygen content of 1.0 at. % is taken into account is shown by a dashed line.

Oxygen content 1.0 at. %, the upper line of the α single phase region shifts about +40K toward the upper right of the figure, compared to the case where the oxygen content is not considered. This result shows that even if the Nb and Cr contents and temperature conditions are the same, the β+α/α transformation temperature changes depending on the presence or absence of oxygen content. Therefore, when checking the β+α/α transformation temperature, it is desirable to consider the oxygen content. Thereby, the volume fraction of the β phase can be increased more reliably during sintering. Oxygen content can be measured by soft X-ray spectroscopy using a soft X-ray spectrometer.

(Relationship between β phase volume fraction and relative density)
FIG. 5 shows the relationship between the volume fraction of the β phase and the relative density in Ti-44Al-4Cr. FIG. 5 was created based on the phase diagram of the Ti-Al-Cr ternary system and the results of the sintering test. In the figure, the vertical axis is relative density (%), and the horizontal axis is β phase volume fraction (%).

According to FIG. 5, it was confirmed that the higher the volume fraction of the β phase, the higher the relative density. If the volume fraction of the β phase is 56% or more, a relative density of 95% or more can be achieved. The higher the sintering temperature, the higher the relative density of the TiAl-based alloy. According to the volume fraction of the β phase and the phase diagram, the sintering temperature at which the relative density of 95% can be achieved in FIG. 5 is 1631 K (1358° C.) or higher. According to FIG. 5, it can be confirmed that a high relative density of about 98% can be obtained when the β phase volume fraction is 68% (sintering temperature 1653K).

FIG. 6 shows the relationship between the volume fraction of the β phase and the relative density in Ti-47Al-5Nb. FIG. 6 was created based on the phase diagram of the Ti-Al-Nb ternary system and the results of the sintering test. In the figure, the vertical axis is relative density (%), and the horizontal axis is β phase volume fraction (%).

As seen in Figure 6, as in Figure 5, it was confirmed that the higher the volume fraction of the β phase, the higher the relative density. If the volume fraction of the β phase is 94% or more, a relative density of 95% or more can be achieved. According to the volume fraction of the β phase and the phase diagram, the sintering temperature at which a relative density of 95% can be achieved in Figure 6 is 1756K (1483°C).

(Relationship between temperature and relative density)
FIG. 7 shows the relationship between relative density and temperature. In the figure, the vertical axis is relative density (%), the horizontal axis (bottom) is the reciprocal of temperature (1/T) (10 ⁻⁴ K ⁻¹ ), the horizontal axis (top) is temperature (K), and ■ is Ti -44Al-4Cr, ◆ is Ti-47Al-5Nb.

According to Figure 7, for both Ti-44Al-4Cr and Ti-47Al-5Nb, the higher the temperature, the higher the relative density.

According to FIG. 7, Ti-44Al-4Cr can have the same relative density as Ti-47Al-5Nb even at a lower sintering temperature than Ti-47Al-5Nb. That is, when sintering is performed at the same temperature, the relative density of Ti-44Al-4Cr can be increased in a shorter time than that of Ti-47Al-5Nb.

The dashed line in FIG. 7 is the β+α/α transformation temperature of Ti-47Al-5Nb. The β+α/α transformation temperature in Ti-47Al-5Nb is around 1703 K (see FIG. 4). When sintering at a temperature lower than the β+α/α transformation temperature, there is no β phase in Ti-47Al-5Nb. Therefore, the relative density of Ti-47Al-5Nb is lower on the lower temperature side than the dashed-dotted line in FIG. On the other hand, when the β+α/α transformation temperature is exceeded, the relative density of Ti-47Al-5Nb increases rapidly. The reason for this is an increase in the volume fraction of the β phase.

Cr has a higher β-phase stabilizing ability than Nb, and as shown in FIGS. 3 and 4, more β-phase exists in the Cr-added alloy at lower temperatures, and the sinterability is more likely to be improved. Therefore, in Figure 7, there is a difference in the temperature required to obtain the same relative density between Ti-44Al-4Cr and Ti-47Al-5Nb, but it is said that the increase in β phase improves sinterability. The points show a common effect.

(Diffusion coefficient of Ti and Al in each phase)
FIG. 8 shows the diffusion coefficients of Ti atoms and Al atoms in each phase (α, γ, β). In the figure, the vertical axis is the diffusion coefficient (m ² /s), the horizontal axis (bottom) is the reciprocal of temperature (1/T) (10 ⁻⁴ K ⁻¹ ), and the horizontal axis (top) is the temperature (K), ▲ (solid line) is the diffusion of Ti atoms in the α phase, △ (broken line) is the diffusion of Al atoms in the α phase, ■ (solid line) is the diffusion of Ti atoms in the γ phase, and □ (broken line) is the diffusion of Ti atoms in the α phase. Diffusion of Al atoms in the γ phase, ● (broken line) is the diffusion of Ti atoms in the β phase, and ○ (solid line) is the diffusion of Al atoms in the β phase.

As shown in FIG. 8, the diffusion rate of Ti atoms and Al atoms in the β phase is much faster than in the other phases (α, γ). From this, it can be seen that the sintering phenomenon is promoted by increasing the volume fraction of the β phase and performing sintering. If the diffusion rate is faster, the sintering time can be shortened accordingly. The sintered density can be improved by increasing the diffusion rate.

According to FIG. 8, the diffusion rate in each phase decreases as the temperature decreases. On the other hand, if the volume fraction of the β phase, which has a fast diffusion rate, can be increased, a sufficient amount of Ti and Al atoms will be diffused even when the sintering temperature is lowered, and sintering will proceed efficiently due to the promoted diffusion. do.

<Additional Notes>
The TiAl-based alloy and the manufacturing method thereof according to the above-described embodiment can be understood, for example, as follows.

In the method for producing a TiAl-based alloy according to the present disclosure, a metal powder injection molding method is used, the β+α/α transformation temperature (T _β ) is confirmed from the phase diagram of the TiAl-based alloy, and the temperature is equal to or higher than the β+α/α transformation temperature. Sinter.

Within the temperature range where the β phase exists, the volume fraction of the β phase increases as the temperature increases. Among the constituent phases (α, γ, β) of a TiAl-based alloy in a high temperature range, the β phase has a large diffusion coefficient. Sintering relies on diffusion within the grains and on grain boundaries. The faster the diffusion rate, the higher the sinterability. That is, the sintering phenomenon is promoted by performing sintering in a temperature range where the volume fraction of the β phase becomes high. This makes it possible to lower the sintering temperature or shorten the sintering time when trying to obtain a sintered body with the same relative density.

In one embodiment of the above disclosure, it is desirable to add a β stabilizing element.

When a β stabilizing element is added, the region where the β phase exists expands to the low temperature side. As a result, the volume fraction of β can be increased at a lower temperature, making it easier to promote the sintering phenomenon. Furthermore, by lowering the sintering temperature, the temperature difference between the melting point and the melting point can be widened, so deformation during sintering can be suppressed.

In one aspect of the above disclosure, if only sinterability is considered, it is desirable to sinter at a temperature within the β single phase temperature range. In order to obtain an appropriate structure depending on the use of the sintered alloy, sintering may be performed within the (β+α) two-phase region.

Within the β single-phase temperature range, it is composed only of the β phase with a fast diffusion rate, so by sintering at a temperature within the β single-phase temperature range, it is possible to further improve the sinterability. A relative density of 95% or more can be easily achieved.

In one aspect of the above disclosure, after the sintering, the sintered body may be held at a temperature lower than the sintering temperature and higher than the β+α/α transformation temperature for a predetermined period of time.

By holding the sintered body at the above temperature, diffusion of Ti atoms and Al atoms can be promoted and sinterability can be improved.

The TiAl-based alloy manufactured by the manufacturing method according to the above disclosure becomes a high-quality sintered body with high sintering density.

Claims (3)

A method for producing a TiAl-based alloy containing a β-stabilizing element using a metal powder injection molding method, the method comprising:
Confirm the β+α/α transformation temperature from the phase diagram of the TiAl-based alloy containing the β stabilizing element ,
Sintering at a temperature in the β phase existence temperature range that is higher than the β + α / α transformation temperature ,
The method for producing a TiAl-based alloy in which the β stabilizing element includes at least one of Nb and Cr . 2. The method for producing a TiAl-based alloy according to claim 1, wherein the sintering is performed at a temperature in a β single phase existence temperature range. 3. The method for producing a TiAl-based alloy according to claim 1, wherein after the sintering, the sintered body is held for a predetermined period of time at a temperature that is lower than the sintering temperature and is in the β phase existence temperature range.

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