JP7724144B2 - Method for manufacturing a green compact and a sintered compact - Google Patents

Description

This invention relates to a method for producing a green compact containing titanium, aluminum, and other metal elements, and a method for producing a sintered compact.

Titanium alloys have been considered for use in various components due to certain excellent properties such as fatigue resistance, corrosion resistance, light weight and high specific strength.
However, the production of titanium alloy products generally requires a number of processes, such as melting by electron beam melting or vacuum arc melting, casting, and in some cases further forging, rolling, heat treatment, machining, welding, etc., which increases the manufacturing cost. Due to such high costs, it is difficult to say that the range of applications of titanium alloys has been sufficiently expanded.

In light of this, powder metallurgy has recently attracted attention as a so-called near-net-shape method. This involves filling a resin mold with raw material powder containing titanium and alloying elements, and then cold isostatically pressing the mold filled with the raw material powder to obtain a green compact of a desired shape. In powder metallurgy, after cold isostatic pressing, sintering and/or hot isostatic pressing may be performed as needed to form a sintered body and increase density.

One example of this type of technology is described in Patent Document 1, which discloses "a method for producing a metal powder compact having a recess, the method including a step of cold isostatically pressing raw material powder filled into a resin mold in a state in which a resin core material having a shape corresponding to the recess is positioned in a location of the resin mold that corresponds to the recess."

International Publication No. 2021/060363

Titanium alloys include Ti-Al alloys containing aluminum and other metal elements such as vanadium or iron. One example is a Ti-6Al-4V alloy, in which the other metal element is vanadium. When producing a Ti-Al compact or sintered compact, it is possible to use a raw material powder containing titanium powder and a master alloy powder containing aluminum and other metal elements.

The above-mentioned master alloy powders that contain more aluminum than other metal elements by mass are relatively expensive and difficult to obtain consistently on the market. Therefore, when producing Ti-Al-based green compacts or sintered compacts, it is preferable to use a master alloy powder in which the content of other metal elements is equal to or greater than the aluminum content, from the perspective of reducing costs and achieving stable production. In this case, depending on the composition of the green compact or sintered compact to be produced, the raw material powder may further contain aluminum powder in addition to the titanium powder and the master alloy powder described above.

It was discovered that when using ordinary aluminum powder, uneven distribution of the raw powder occurs within the mold when the raw powder is filled into the mold. This results in variations in the composition and components of the green compact and sintered compact, resulting in a loss of homogeneity.

The object of this invention is to provide a method for manufacturing a green compact and a method for manufacturing a sintered compact that can suppress variation in the composition of Ti-Al green compacts or sintered compacts.

After extensive research, the inventors discovered that by using pulverized powder for at least the master alloy powder and by using aluminum powder with an average aspect ratio of 1.38 or more and 3.00 or less, it is possible to suppress the uneven distribution of particles in the raw material powder within the mold.

Conventional aluminum powder has often been produced by methods such as atomization and is spherical in shape. If such aluminum powder is mixed with titanium powder and master alloy powder that is not a crushed powder to produce a raw powder, the aluminum powder, which is spherical and has a lower specific gravity than the titanium powder and master alloy powder, will tend to flow easily within the raw powder, causing uneven distribution.

In contrast, if at least the master alloy powder is pulverized and aluminum powder with an aspect ratio within the above range is used, it is believed that the flow of the aluminum powder within the raw material powder is suppressed, making it less likely for bias to occur within the raw material powder. As a result, it is possible to obtain a Ti-Al-based green compact or sintered compact with a uniform composition.

The method for producing a powder compact of this invention is a method for producing a powder compact containing titanium, aluminum, and other metal elements, and includes the steps of preparing raw material powder containing titanium powder, aluminum powder, and a master alloy powder containing aluminum and the other metal elements; filling the raw material powder into a resin mold; and cold isostatically pressing the mold filled with the raw material powder, wherein the content of the other metal elements in the master alloy powder is equal to or greater than the content of aluminum by mass, the master alloy powder is a pulverized powder, and the average aspect ratio of the aluminum powder is 1.38 or more and 3.00 or less.

The titanium powder is preferably a crushed powder.

The raw material powder preferably satisfies at least one condition selected from the group consisting of the following (1) to (4):
(1) The average particle size D50 of the titanium powder is 20 μm or more and 80 μm or less.
(2) The average particle size D50 of the master alloy powder is 10 μm or more and 80 μm or less.
(3) The particle size of 90% or more of the aluminum powder by mass is in the range of 20 μm or more and 90 μm or less.
(4) The aluminum powder has an average circularity of 0.60 or more and 0.75 or less.

It is preferable that the aluminum powder is produced by cutting foil or pressing powder.

The mold can be made of a thermoplastic resin with a Shore D hardness of 30 or more and 120 or less.

The mold can be one made using a three-dimensional modeling device.

The method for manufacturing a sintered body of the present invention includes the step of heating and sintering a powder compact manufactured by any of the above-described methods for manufacturing a powder compact.

This invention makes it possible to suppress variation in the composition of Ti-Al-based compacts or sintered bodies.

Hereinafter, an embodiment of the present invention will be described in detail.
A method for producing a powder compact according to one embodiment of the present invention produces a powder compact containing titanium, aluminum, and other metal elements, i.e., a so-called Ti-Al-based powder compact. This method includes the steps of preparing a raw material powder containing titanium powder, aluminum powder, and a master alloy powder containing aluminum and other metal elements, filling a resin mold with the raw material powder, and cold isostatic pressing the mold filled with the raw material powder. A method for producing a sintered body according to one embodiment also includes the step of sintering the powder compact produced by the method for producing a powder compact by heating, such as by sintering and/or hot isostatic pressing.

The raw material powder is obtained by mixing titanium powder, aluminum powder, and master alloy powder. The master alloy powder has a mass content of other metal elements equal to or greater than the aluminum content. Such master alloy powder is relatively inexpensive and readily available. However, because the mass content of other metal elements in this master alloy powder is equal to or greater than the aluminum content, when attempting to manufacture a Ti-Al-based compact or sintered body in which the aluminum content is higher than the other metal elements, such as a Ti-6Al-4V alloy, it may be necessary to add aluminum powder in addition to the titanium powder and master alloy powder, as in this embodiment, to adjust the composition to the desired level.

Aluminum powder has a lower specific gravity than titanium powder and is generally lower than master alloy powder. In particular, when such aluminum powder is mixed with raw material powder together with titanium powder and master alloy powder and then filled into a mold, it tends to flow and become unevenly distributed within the raw material powder within the mold. Uneven distribution of aluminum powder within the raw material powder within the mold leads to variations in composition in the green compact obtained by subsequent cold isostatic pressing, and in the sintered body obtained after sintering and/or hot isostatic pressing, etc.

In contrast, in this embodiment, at least the master alloy powder is pulverized, and the average aspect ratio of the aluminum powder is 1.38 or more and 3.00 or less. By using aluminum powder with a relatively large aspect ratio in combination with pulverized master alloy powder, uneven distribution of the aluminum powder is effectively suppressed even when the raw powder is filled into a mold and then tapped. As a result, it is possible to obtain Ti-Al-based green compacts and sintered compacts with excellent homogeneity of the metal elements.

(Raw material powder)
The raw material powder is obtained by mixing titanium powder, aluminum powder, and master alloy powder containing aluminum and other metal elements. The raw material powder may further contain other master alloy powders, alloy element powders, etc.

Using such raw material powders, it is possible to produce titanium alloy green compacts and sintered bodies made of titanium alloys such as Ti-5Al-1Fe, Ti-5Al-2Fe, Ti-6Al-4V, and Ti-3Al-2.5V. The numbers before the element symbols for the alloying elements indicate the content (mass %). For example, "Ti-6Al-4V" refers to a titanium alloy containing 6 mass % Al and 4 mass % V.

Although titanium powder may inevitably contain impurities, it primarily contains titanium, and it is preferable that the titanium content be 99% by mass or more. Specific examples of titanium powder include hydrogenated/dehydrogenated titanium powder (so-called HDH powder), which is obtained by hydrogenating sponge titanium, pulverizing it, and then dehydrogenating it, as well as hydrogenated titanium powder that is not dehydrogenated after pulverization. Titanium powder may contain hydrogen at 5% by mass or less. If the titanium powder contains hydrogen, it can be dehydrogenated before sintering the green compact, or dehydrogenation and sintering can be performed simultaneously in a single heat treatment. The hydrogen can also be removed during the heating process for sintering.

The master alloy powder contains aluminum and other metal elements. Here, the other metal elements may be, for example, at least one selected from the group consisting of vanadium and iron. The master alloy powder typically contains aluminum and vanadium, or aluminum and iron, but is not limited to these.

The master alloy powder has a mass content of other metal elements equal to or greater than the mass content of aluminum. Master alloy powders with a higher aluminum content than the mass of other metal elements can be difficult to obtain or produce, and can be difficult to prepare consistently. For example, the master alloy powder may have an aluminum content of 15% by mass or more and 50% by mass or less, preferably 40% by mass or more and 50% by mass or less, and a mass content of other metal elements of 50% by mass or more and 85% by mass or less, preferably 50% by mass or more and 60% by mass or less.

Of the titanium powder and master alloy powder described above, at least the master alloy powder is a pulverized powder. The particles of pulverized powder are often polyhedral. Using pulverized master alloy powder, coupled with the relatively large aspect ratio of the aluminum powder described below, effectively suppresses uneven distribution of each powder within the raw powder within the mold. The titanium powder may be composed of spherical particles produced by a method other than pulverization, but it is preferable that the titanium powder also be a pulverized powder. A pulverized powder is a powder produced by pulverizing a lump or the like. For example, the aforementioned HDH titanium powder and titanium hydride powder are examples of pulverized titanium powder. The pulverized master alloy powder is sometimes produced by pulverizing an ingot.

The aluminum powder is primarily composed of aluminum, and preferably has an aluminum content of 99% by mass or more. The aluminum powder may contain impurities, but the impurity content is preferably less than 1% by mass.

The aluminum powder has an average aspect ratio (also referred to as the "average aspect ratio") of 1.38 or more and 3.00 or less. If the average aspect ratio of the aluminum powder is less than 1.38, there is a concern that uneven distribution of the powder particles may not be sufficiently suppressed when the raw material powder is filled into a mold. On the other hand, if the average aspect ratio of the aluminum powder exceeds 3.00, there is a risk that the effect of suppressing uneven distribution of the alloy component powder may not be fully exerted. From this perspective, the average aspect ratio of the aluminum powder is preferably 1.40 or more and 2.00 or less, and more preferably 1.42 or more and 1.50 or less. In such aluminum powder, each particle often has a flake shape, such as a scale shape.

The average aspect ratio of aluminum powder is determined as follows. First, a particle shape image analyzer PITA-04 (manufactured by Seishin Enterprise Co., Ltd.) is used to obtain a projected image of aluminum powder particles dispersed in isopropyl alcohol (IPA) as a dispersant. The pump speed can be set to 2000 Hz. The maximum length between two points on the outline of the projected image is then measured as the major axis L, and the minimum length perpendicular to the major axis is measured as the minor axis d. The aspect ratio of aluminum powder particles is calculated as L/d by dividing the major axis L by the minor axis d. The aspect ratio (L/d) is calculated for approximately 10,000 aluminum powder particles, and the average value is taken as the average aspect ratio.

The average circularity of aluminum powder may be, for example, 0.60 or more and 0.75 or less. Circularity is the ratio of the perimeter of a circle with an area equal to the area of a projected image of a particle measured using an electron microscope to the perimeter of the projected image. The average circularity is calculated from the perimeter P and projected area A of the projected image of the particle by 4πA/ ^P2 . The average circularity is determined by flowing particles together with a carrier liquid into a cell, capturing a large number of particle images with a CCD camera, and calculating the above circularity (4πA/ ^P2 ) for each particle from approximately 10,000 individual particle images, and then determining the average circularity of each particle.

The aluminum powder described above can be purchased commercially and used. Aluminum powder can be produced, for example, by finely cutting a relatively thin aluminum foil, or by flattening aluminum powder produced by an atomization method or the like through pressing. Aluminum powder produced simply by an atomization method such as gas atomization or water atomization tends to be substantially spherical, with an average aspect ratio outside the above-mentioned range. However, the aluminum powder is not limited to a specific production method, as long as the average aspect ratio falls within the specified range.

The average particle size D50 of the titanium powder is preferably 20 μm or more and 80 μm or less. If the average particle size D50 of the titanium powder is 20 μm or more, it is expected that an increase in the oxygen content can be reduced, and if it is 80 μm or less, it is expected that a decrease in packing density can be suppressed.
The average particle size D50 of the master alloy powder is preferably 10 μm or more and 80 μm or less. If the average particle size D50 of the master alloy powder is 10 μm or more, it is possible to reduce an increase in the oxygen content and to suppress the occurrence of uneven distribution due to extreme particle size differences, while if it is 80 μm or less, it is thought that local concentration of components due to insufficient diffusion of coarse particles can be prevented.
It is also preferable that the particle size of 90% or more of the aluminum powder by mass is 20 μm or more and 90 μm or less. When the particle size of the aluminum powder is 20 μm or more, it is thought that the effect of suppressing uneven distribution of alloying components can be exerted, and when it is 90 μm or less, it is thought that local concentration of components due to insufficient diffusion of coarse particles can be suppressed.

Note that the average particle size D50 refers to the particle size D50 (median diameter) of the particle size distribution (volume basis) obtained by laser diffraction scattering. Furthermore, particle size refers to the size of the openings of a "metal mesh sieve" as specified in JIS Z8801-1:2006. A particle size in the range of 20 μm or more and 90 μm or less means that the particles pass through a sieve with 20 μm openings and are undersized on a sieve with 90 μm openings. If 90% or more of the particles pass through a sieve with 20 μm openings and are undersized on a sieve with 90 μm openings, by mass, the particle size is considered to be "in the range of 20 μm or more and 90 μm or less."

(mold)
The mold into which the raw material powder is filled has an internal space having a shape corresponding to the outer shape of the green compact or sintered body to be finally produced. When producing a green compact or sintered body having a recess such as a through-hole, a core or a core having a shape corresponding to the recess may be placed in the mold before or after filling the mold with the raw material powder. The shapes of the mold and core can be changed as appropriate depending on the shape of the green compact or sintered body.

The raw material powder can be filled into the mold through the opening by tapping, etc. After the raw material powder is filled, the mold opening is closed and cold isostatic pressing, as described below, is performed.

In this embodiment, at least the master alloy powder is pulverized, and the average aspect ratio of the aluminum powder is 1.38 or more and 3.00 or less. This effectively prevents the powder particles from becoming unevenly distributed within the mold when the raw material powder is filled into the mold.

Here, a resin mold is used. More specifically, the mold is preferably made of a thermoplastic resin, and particularly preferably made of acrylic resin, acrylic resin containing elastomer, polylactic acid (PLA) resin, or the like.

To ensure the required strength and maintain its shape when filled with raw material powder, the mold is preferably made of a thermoplastic resin with a Shore D hardness of 30 or more and 120 or less, or alternatively, a thermoplastic resin with a Shore D hardness of 30 or more and 85 or less. Shore D hardness can be measured using a test method conforming to JIS K7215-1986. From the same perspective, the thickness of the resin mold is preferably 0.5 mm or more and 2.0 mm or less.

Resin molds can be made using a variety of methods, but are preferably made using a three-dimensional modeling device (a so-called 3D printer). This makes it easy to create molds of various shapes. There are no particular restrictions on the modeling method used for the three-dimensional modeling device, and it can be any of the following: optical modeling, inkjet modeling, inkjet powder layering, powder sintering layering, fused deposition modeling, or powder bonding.

(cold isostatic pressing)
After the raw material powder is filled into the mold, the mold is subjected to cold isostatic pressing (CIP). More specifically, the mold filled with the raw material powder is placed inside a cold isostatic pressing device, where the raw material powder inside the mold is compressed by isostatically pressing the mold from the outside. By cold isostatic pressing, the raw material powder in the molding space of the mold is compressed and compacted into a green compact.

The pressure applied to the mold during cold isostatic pressing is, for example, 300 MPa or more, preferably 400 MPa or more, and more preferably 450 MPa or more. A pressure of 300 MPa or more sufficiently compresses the raw material powder, making it easier to obtain a green compact with the desired shape precision. The pressure may be, for example, 600 MPa or less, typically 500 MPa or less. The holding time at such a pressure may be, for example, 0.5 to 30 minutes.

In cold isostatic pressing, the mold is isotropically (hydrostatically) pressed by the surrounding fluid. Therefore, cold isostatic pressing allows the use of molds of various shapes. Furthermore, since the resin mold deforms appropriately when subjected to the isotropic pressure of the fluid, green compacts of a variety of shapes can be produced.

(heat sintering)
When producing a sintered body, a step of heating the green compact after cold isostatic pressing to sinter the particles that make up the green compact is included, thereby producing a sintered body.

In this process, the green compact can be heated without pressure at a temperature of, for example, 1200°C or higher and 1300°C or lower for 1 hour or longer and 12 hours or shorter. Alternatively, the green compact can be subjected to hot isostatic pressing (HIP), in which an isostatic pressure of approximately 100 MPa or higher and 200 MPa or lower is applied to the green compact using a pressure medium such as argon gas at a temperature of, for example, 800°C or higher and 1000°C or lower for 30 minutes or longer and 150 minutes or shorter.

Whether pressureless heating or hot isostatic pressing is used, sintering of the green compact progresses by exposing it to a relatively high temperature. When hot isostatic pressing is used, gas pressure is applied in a temperature range where the material's resistance to deformation is low, thereby crushing any remaining pores in the material and increasing its density. Both pressureless heating and hot isostatic pressing may also be used. When both pressureless heating and hot isostatic pressing are used, the order is not particularly important, but for example, hot isostatic pressing can be performed after pressureless heating.

Next, the methods for manufacturing a green compact and a sintered compact according to the present invention were experimentally implemented, and their effects were confirmed. These are described below. However, the descriptions here are merely illustrative and are not intended to be limiting.

The raw material powders, including titanium powder, aluminum powder, and master alloy powder containing aluminum and vanadium, shown in Tables 1 and 2, were filled into a resin mold, and the mold was subjected to cold isostatic pressing to produce a green compact. The green compact was then removed from the mold, heated under vacuum without pressure, and then subjected to further hot isostatic pressing to produce a sintered body.

In Examples 1 to 12, as shown in Table 2, aluminum powder from Table 1 with an average aspect ratio of 1.45 (the shape of this aluminum powder is sometimes described as flaky) was used, and pulverized powder was used from the master alloy powder from Table 1. In Comparative Examples 1 to 36, aluminum powder with an average aspect ratio of 1.09 was used, and/or atomized powder was used as the master alloy powder. In other words, in Comparative Examples 1 to 36, at least one of the aluminum powder and the master alloy powder was atomized powder. Note that 90% or more of the flaky aluminum powder was in the range of 20 μm or more and 90 μm or less by mass.

The mold described above was 1.0 mm thick, had internal dimensions of φ20 mm x 120 mm, was made of PLA (polylactic acid), and had a Shore D hardness of 30 or more and 85 or less. Cold isostatic pressing (hydrostatic pressing) was performed with a pressure of 490 MPa for 1 minute. The end face of the green compact obtained in the mold after cold isostatic pressing, facing the opening filled with the raw material powder, is referred to as the "Top," and the end face facing the bottom is referred to as the "Bottom." During subsequent heat sintering, the green compact was heated to 1200°C without pressure and held at that temperature for 480 minutes. This was followed by hot isostatic pressing at 900°C and 178 MPa for 120 minutes. This resulted in a sintered rod-shaped compact measuring approximately φ15 mm x 100 mm.

In all of Examples 1 to 12 and Comparative Examples 1 to 36, the relative density of the green compact after cold isostatic pressing was 88% or more, and the relative density after heating without pressure was 95% or more. Furthermore, the relative density of the sintered body obtained after hot isostatic pressing was 100%. The relative density is calculated by dividing the measured density of the sintered body by the theoretical density and expressing this as a percentage. To obtain the measured density of the sintered body, the volume of the sintered body was determined by Archimedes' method. The theoretical density was 4.43 g/ ^cm3 for Ti-6Al-4V and 4.48 g/ ^cm3 for Ti-3Al-2.5V.

To confirm the homogeneity of the sintered body's components, samples measuring approximately 15 mm in diameter and 10 mm in length were cut from three locations on the rod-shaped sintered body: position T, approximately 10 mm from the top; position B, approximately 10 mm from the bottom; and position M, the center between positions T and B. Each sample was then embedded in resin and polished in a cross section perpendicular to the longitudinal direction of the rod-shaped sintered body. X-ray fluorescence analysis (XRF) was then performed to examine the aluminum and vanadium contents. For this analysis, the X-ray irradiation area was approximately 10 mm in diameter. The maximum and minimum differences between the analytical values for aluminum and vanadium contents at positions T, M, and B were calculated, and the maximum difference divided by the minimum difference was used as the homogeneity index (maximum difference/minimum difference). The results are shown in Table 2. The smaller the homogeneity index, the more homogeneous the sample. A homogeneity index of 1.10 or less was considered acceptable, and 1.04 or less was considered excellent.

The results in Table 2 show that the homogeneity index of the sintered bodies in all Examples 1 to 12 was sufficiently small. This is thought to be because the master alloy powder used as the raw material powder was a pulverized powder, and the average aspect ratio of the aluminum powder was relatively large, which prevented uneven distribution within the raw material powder when it was filled into the mold.

On the other hand, in Comparative Examples 1 to 36, the master alloy powder was not a pulverized powder, and/or the average aspect ratio of the aluminum powder was small, resulting in a high homogeneity index for at least one of the aluminum content and vanadium content.

From the above, it has been found that this invention can suppress variation in the composition of Ti-Al-based compacts or sintered bodies.

Claims (7)

A method for producing a powder compact containing titanium, aluminum, and other metal elements, comprising:
preparing raw material powders including titanium powder, aluminum powder, and master alloy powder containing aluminum and the other metal elements;
a step of filling the raw material powder into a resin mold;
and applying cold isostatic pressing to the mold filled with the raw material powder,
a content of the other metal element in the master alloy powder that is equal to or greater than the content of aluminum by mass, the master alloy powder being a pulverized powder, and the aluminum powder having an average aspect ratio of 1.38 or more and 3.00 or less. The method for producing a powder compact according to claim 1, wherein the titanium powder is a pulverized powder. 3. The method for producing a powder compact according to claim 1, wherein the raw material powder satisfies at least one condition selected from the group consisting of the following (1) to (4):
(1) The average particle size D50 of the titanium powder is 20 μm or more and 80 μm or less.
(2) The average particle size D50 of the master alloy powder is 10 μm or more and 80 μm or less.
(3) The particle size of 90% or more of the aluminum powder by mass is in the range of 20 μm or more and 90 μm or less.
(4) The aluminum powder has an average circularity of 0.60 or more and 0.75 or less. The method for manufacturing a powder compact according to any one of claims 1 to 3, wherein the aluminum powder is produced by cutting foil or by press-molding powder. The method for manufacturing a powder compact according to any one of claims 1 to 4, wherein the mold is made of a thermoplastic resin having a Shore D hardness of 30 or more and 120 or less. The method for manufacturing a powder compact according to any one of claims 1 to 5, wherein the mold is fabricated using a three-dimensional modeling device. A method for producing a sintered body, comprising the step of heating and sintering a powder compact produced by the method for producing a powder compact described in any one of claims 1 to 6.