JP7698181B2 - Titanium alloy material and its manufacturing method - Google Patents

Description

The present invention relates to a titanium alloy material and a method for manufacturing the same.

In recent years, as described in Non-Patent Document 1, a new manufacturing method called three-dimensional additive manufacturing technology has been developed and is being applied to metal materials. Three-dimensional additive manufacturing technology is generally known as 3D printer technology, but recently, together with various technologies for manufacturing practical components such as industrial products by sequentially adding material to obtain three-dimensional shapes, it has been collectively referred to as additive manufacturing (AM) technology and has attracted attention around the world.

The International Organization for Standardization's definition (ISO 52900) defines AM technologies as processes that combine materials to produce components from 3D model data, and classifies them into the following categories: Binder Jetting (BJT), Directed Energy Deposition (DED), Materials Extrusion (MEX), Material Jetting (MJT), Powder Bed Fusion (PBF), Sheet Lamination (SHL), and Vacuum Photo Polymerization (VPP). Of these, three methods are applied to metal materials: BJT (binder jet deposition), DED (directed energy deposition), and PBF (powder bed fusion).

These metal techniques are attracting attention because they require fewer manufacturing steps than conventional metal processing methods, are expected to improve yields, and are expected to shorten manufacturing times and reduce manufacturing costs.

When manufacturing metal components using AM technology (additive manufacturing technology), in any of the above methods, including PBF (powder bed fusion), the metal is ultimately melted and solidified to form the metal component, so the resulting metal component has a solidified structure. Such metal components are further processed as necessary by HIP treatment, forging, heat treatment, cutting, etc. to reduce internal defects, improve material properties, and increase shape precision, before being used as actual parts.

Titanium alloys, a type of metallic material, are lightweight, strong, and have good affinity with the human body, and are therefore used in the aircraft industry and in the medical field, such as for implants. Among titanium alloys, the Ti-6Al-4V alloy, an α+β type titanium alloy that has an excellent balance between high strength and ductility, is widely used.

In many cases, α+β titanium alloys such as Ti-6Al-4V are made by melting and solidifying a mixture of sponge titanium and mother alloys to produce an ingot, which is then processed by forging, rolling, and other processes to produce wrought materials such as plates, bars, and shapes, and then processed into the desired shape of the titanium alloy material. In this case, the titanium alloy material undergoes various processing and heat treatments in the manufacturing process to obtain an equiaxed crystal structure for applications requiring strength and ductility, and an acicular crystal structure for applications requiring fracture toughness and creep resistance.

However, as mentioned above, additive manufacturing technology is being used as a means of producing components made of metallic materials, and there are an increasing number of examples of producing components with a solidified structure that are shaped into a specified shape through melting and solidification. In addition, some applications have traditionally used cast materials with a solidified structure obtained by melting and solidifying raw materials that contain a mixture of titanium sponge and master alloys, or raw materials with a specified chemical composition. Titanium alloy materials produced through melting and solidification in this way have a solidified structure that is inferior in strength and toughness, because they have not been rolled or heat treated.

Regarding the metallographic morphology of titanium alloys produced by additive manufacturing techniques, U.S. Patent No. 5,399,433 describes a titanium alloy that can be formed using an additive manufacturing process to form an object having an equiaxed crystal structure (without a subsequent processing step to eliminate the columnar crystal structure). According to this, additively manufactured titanium alloys produce coarse columnar crystals resulting in undesirable anisotropic mechanical properties. However, by incorporating an effective amount of a β-eutectoid stabilizer, an equiaxed crystal structure can be produced when the titanium alloy is melted or sintered during the additive manufacturing process. Here, it is described that the β-eutectoid stabilizer can be selected from Fe, Ni, Cu, or a combination thereof.

In terms of the chemical composition of titanium alloys, Ti-Al-Fe-based titanium alloys containing inexpensive Al and Fe are known as an alternative to Ti-6Al-4V alloys (Patent Document 2). Also, Ti-Fe-Cu-based titanium alloys are known that have improved high-temperature strength over commercially pure titanium by adding inexpensive Cu (Patent Document 3).

Patent Document 4 describes a titanium alloy with controlled amounts of Al, O, Cu, Sn, and Si added, which has higher strength than existing alloys that can be rolled into coils, high cold rolling properties, and workability. Furthermore, Patent Document 5 describes a titanium alloy with controlled amounts of Al, Cr, Fe, Cu, Ni, and C added, which has strength, hot workability, and excellent machinability.

Patent Document 6 also describes a method for producing a titanium alloy with a tensile strength of 1000-1500 MPa and an elongation of 9-15% by mixing titanium alloy powder with Cu powder and using an elemental powder mixing method to increase the Cu content to a high concentration of 1-10%.

Special Publication No. 2020-511597 Japanese Patent Application Publication No. 4-358036 JP 2005-298970 A JP 2014-001421 A JP 2016-183407 A JP 2013-112860 A

Fundamentals of 3D additive manufacturing technology for metal-based materials, Yuichiro Koizumi, Masahiko Chiba, Naoyuki Nomura, Takayuki Nakano, Materia, Vol. 56, No. 12 (2017), pp. 686-690

However, when additive manufacturing technology is used to produce titanium alloy materials with a solidified structure by melting and solidifying them, it is difficult to control the metal structure in the subsequent thermomechanical processing to improve strength and toughness. Therefore, there is a demand for titanium alloys that have excellent strength and toughness even in the solidified structure state.

To address such issues, the technology described in Patent Document 1 is a technology for reducing the anisotropy of mechanical properties by reducing the columnar crystals generated by melting or sintering to obtain an equiaxed crystal structure, but does not describe a method for improving strength or toughness.
Furthermore, the titanium alloy described in Patent Document 2 is an alloy intended to replace the expensive V raw material of the Ti-Al-V alloy with inexpensive Fe, and although there is a description of the tensile properties, there is no mention of the impact value.
Patent Document 3 relates to a heat-resistant titanium alloy plate containing inexpensive Cu, and describes the tensile properties at room temperature and high temperatures, but does not mention the impact value.
Patent Document 4 relates to a titanium alloy having good cold rolling properties, but makes no mention of a solidification structure having excellent strength and toughness.
Patent Document 5 relates to a titanium alloy having good cold rolling properties and a titanium alloy having excellent machinability, but is not a titanium alloy produced through melting and solidification.
Patent Document 6 describes a method for producing a titanium alloy containing a high concentration of Cu by an elemental powder mixing method, but does not mention the mechanical properties of the titanium alloy having a solidified structure formed by melting and solidifying.

In addition, not only additive manufacturing techniques but also titanium alloy materials having a solidified structure produced by general casting processes are required to have excellent strength and toughness, as well as an excellent balance between strength and toughness.
Furthermore, titanium alloy materials having a processed structure different from the solidified structure, which are produced through a casting process, a hot working process, a heat treatment process, etc., are also required to have excellent strength and toughness, and to have an excellent balance between strength and toughness.

The present invention was made in consideration of the above-mentioned circumstances, and aims to obtain a titanium alloy material that is superior in strength and toughness to conventional titanium alloys, and also has an excellent balance between the strength and toughness properties.

The present invention, which solves the above problems, has the following configuration .
[ 1 ] In mass%, it contains Al: 4.6% or more and 8.0% or less, Fe: 0.01% or more and 2.0% or less, Cu: 0.3% or more and 2.5% or less, O: 0.03% or more and 0.25% or less, and the balance: Ti and impurities;
A titanium alloy material satisfying the following formula (1):
0.4 × TS + CIS ≧ 370 … (1)
In formula (1), CIS is an impact value (unit: J/cm ² ) obtained by dividing the impact absorption energy determined in a Charpy test at 25° C. by the cross-sectional area of the test piece, and TS is tensile strength (unit: MPa).
[ 2 ] The titanium alloy material according to [1], further comprising, in place of a portion of Ti, one or more of the following, by mass%, Sn: 5.0% or less, Nb: 8.0% or less, Si: 1.0 % or less, and Mo: 5.0% or less.
[ 3 ] The titanium alloy material according to [1] or [2] , wherein a part or all of the metal structure is a solidification structure consisting of columnar crystal grains and equiaxed crystal grains.
[ 4 ] A method for producing a titanium alloy material according to [3], using a titanium alloy powder as a raw material, the titanium alloy material containing, by mass%, Al: 4.6% or more and 8.0% or less, Fe: 0.01% or more and 2.0% or less, Cu: 0.3% or more and 2.5% or less, O: 0.03 % or more and 0.25% or less, with the balance being Ti and impurities, by additive manufacturing technology.
[ 5 ] The method for producing a titanium alloy material according to [ 4 ], wherein the additive manufacturing technique is directed energy deposition or powder bed fusion.
[ 6 ] A first step of forming a powder bed by layering titanium alloy powder containing, by mass%, Al: 4.6% to 8.0%, Fe: 0.01% to 2.0%, Cu: 0.3% to 2.5%, O: 0.03% to 0.25%, and the balance being Ti and impurities, and irradiating a surface of the powder bed with a high-energy beam to melt and solidify a part of the powder bed;
a second step of laying the titanium alloy powder in a layer on the powder bed to form a new powder bed, and irradiating a surface of the new powder bed with a high-energy beam to melt and solidify a portion of the new powder bed;
A method for producing a titanium alloy material according to [ 3 ], comprising repeating the second step at least once after carrying out the first step.
[ 7 ] A method for producing a titanium alloy material according to [3], comprising: supplying a titanium alloy material consisting of a powder or a wire, the powder or wire containing, by mass%, Al: 4.6% or more and 8.0% or less, Fe: 0.01% or more and 2.0% or less, Cu: 0.3% or more and 2.5% or less, O: 0.03% or more and 0.25% or less, with the balance being Ti and impurities, onto a substrate; irradiating the titanium alloy material with a high-energy beam while supplying the titanium alloy material onto the substrate; melting and solidifying the titanium alloy material on the substrate ; and depositing solidified metal on the substrate.
[ 8 ] The method for producing a titanium alloy material according to any one of [4] to [7], wherein the titanium alloy powder further contains, in mass%, one or more of Sn: 5.0% or less, Nb: 8.0 % or less, Si: 1.0% or less, and Mo: 5.0 % or less, instead of a part of Ti.
[ 9 ] A method for producing a titanium alloy material, comprising subjecting a titanium alloy material obtained by the method for producing a titanium alloy material according to any one of [ 4 ] to [ 8 ] to one or more of hot working, cold working, straightening, and cutting.

According to the present invention, it is possible to obtain a titanium alloy material which is superior in strength and toughness to conventional titanium alloys, and further has an excellent balance between the strength and toughness properties.
The present invention is not limited to a titanium alloy material having a solidified structure, but may be a titanium alloy part having a processed structure. In the case where the titanium alloy material has a processed structure, a titanium alloy material capable of achieving a higher level of balance between the properties of strength and toughness can be obtained.
The titanium alloy material of the present invention can be used for components for a wide variety of applications manufactured by additive manufacturing. Furthermore, the titanium alloy material of the present invention is a titanium alloy material with an excellent balance between strength and toughness, and can contribute to energy and resource conservation by taking advantage of its lightweight and high-strength properties.

The inventors have investigated chemical compositions that are superior in strength and toughness to existing titanium alloys and also have an excellent balance between strength and toughness in various metal structure states such as solidified structure and processed structure, particularly in the solidified structure state, and have found a component system that provides good properties. In this specification, the term solidified structure is used to refer to a metal structure obtained by going through the processes of melting and solidification. The term processed structure is used to refer to other metal structures other than solidified structures such as cast materials, specifically, metal structures such as processed structures and annealed structures obtained by further hot working or annealing.

In addition, the solidification structure can be exemplified by a structure composed of columnar crystal grains and equiaxed crystal grains. The processed structure can be exemplified by a structure composed of wrought crystal grains and equiaxed crystal grains. These are classified according to the difference in the shape of the crystal, as opposed to the classification in the manufacturing process. Wrought crystal grains are formed by stretching or shrinking equiaxed crystal grains formed by recrystallization during annealing in a specific direction by deformation, or by a combination of these. In addition, it includes an acicular structure (β phase) formed by heat treatment, specifically heat treatment accompanied by transformation, as described below, and precipitates of various shapes (including equiaxed grains). When observing an arbitrary cross section of a titanium alloy, the aspect ratio of the equiaxed crystal grains is approximately 1, while the aspect ratio of the wrought crystal grains is a value other than 1. When the equiaxed crystal grains after annealing are spherical, the equiaxed crystal grains are perfectly circular, and the wrought crystal grains after processing are elliptical. The processed structure is not limited to the above.

As an index of toughness of titanium alloy, impact value (CIS) obtained by dividing impact absorption energy obtained by Charpy test at 25°C by cross-sectional area of test piece was used. As a result of the study by the inventors, it was found that the relationship between impact value (CIS, unit: J/cm ² ) and tensile strength (TS, unit: MPa) of conventional titanium alloy such as Ti-6Al-4V is in the range of 0.4×TS+CIS<370. That is, it was found that the total value of 0.4 times the value of impact value (J/cm ² ) and the value of strength (MPa) is less than 370 at the maximum. Therefore, even if the balance between strength and toughness is achieved in conventional titanium alloy, the strength and toughness themselves are not high. Therefore, various studies were conducted to simultaneously achieve the relationship between toughness and strength of 0.4×TS+CIS≧370, CIS≧30 for toughness, and TS≧620, which is equal to or greater than that of existing Ti-3Al-2.5V alloy. As a result, by appropriately controlling the amounts of Al, Fe, Cu, and O added, a titanium alloy that is excellent in both impact value and strength even in a solidified structure state was discovered.
Hereinafter, a titanium alloy material and a manufacturing method thereof according to an embodiment of the present invention will be described.

The titanium alloy material of this embodiment has a chemical composition containing, by mass%, Al: 4.6% to 8.0%, Fe: 0.01% to 2.0%, Cu: 0.3% to 2.5%, O: 0.03% to 0.25%, and the balance being Ti and impurities. The titanium alloy material of this embodiment is an α+β type titanium alloy material.

The chemical composition of the titanium alloy material of this embodiment will be described below. In the following description, the "%" for the content of each element in the chemical composition means "mass %". Furthermore, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits. Note that when "more than" or "less than" is added to the numerical values written before and after "~", the numerical range does not include these numerical values as the lower or upper limit.

Al is an α-phase stabilizing element and mainly strengthens the α-phase. If the Al content is less than 4.6%, it becomes difficult to secure a tensile strength of 620 MPa or more, which corresponds to Ti-3Al-2.5V (ASTM grade 9) of α+β-type titanium alloy. On the other hand, if the Al content exceeds 8.0%, the ductility decreases significantly and the toughness decreases. Also, as described later, in order to enjoy the effect of the combined addition of Al and Cu, an Al content of less than 4.6% is insufficient, and the effect of the combined addition can be obtained by making it 4.6% or more. Therefore, the Al content is 4.6% or more and 8.0% or less. The Al content may be 5.0% or more, 5.5% or more, 7.0% or less, or 6.5% or less.

Fe is a β-phase stabilizing element, and compared to other elements, its amount of solid solution in the α-phase is small. When the amount of solid solution in the α-phase is exceeded, the titanium alloy is transformed into two phases, α and β, to improve strength. In the titanium alloy material of the present invention, Cu, which is also a β-stabilizing element, is added at the same time, so the effect can be obtained if Fe is 0.01% or more. On the other hand, when Fe exceeds 2.0%, the influence of grain boundary segregation becomes large and toughness begins to decrease. Furthermore, when the solid solubility limit of Fe is exceeded and compounds with Ti are precipitated, toughness decreases significantly. Therefore, the Fe content is set to 0.01% or more and 2.0% or less. The Fe content may be 0.2% or more, or 1.7% or less.

Cu is a β-stabilizing element, and like Fe, it improves strength by forming two phases, α and β. At the same time, Cu can dissolve in the α phase up to a maximum of about 2.5%, improving the strength of titanium alloy materials. Furthermore, even when dissolved in the α phase, it does not suppress twin deformation, which is one of the deformation mechanisms of the α phase, so it hardly impairs the ductility and toughness of the α phase, making it an essential additive element for improving the balance between strength and toughness. If Cu exceeds 2.5%, the amount of solid solution in the α phase will saturate, and Ti and Cu compounds will precipitate coarsely, reducing toughness. On the other hand, if Cu is less than 0.3%, the effect of improving the balance between strength and ductility will be poor.

The maximum amount of Cu dissolved in the α phase increases with increasing Al content, is hardly affected by the Fe content, and decreases with increasing O content. Therefore, in the titanium alloy material according to the present invention, which contains 4.6% or more Al, the effect of Cu can be enjoyed to a greater extent. As mentioned above, when Al exceeds 8.0%, the strength increases, but the toughness decreases.

Therefore, the Cu content is 0.3% or more and 2.5% or less. The Cu content may be 0.5% or more and 2.0% or less.

O is an α-phase stabilizing element that strengthens the α-phase. However, as the O content increases, toughness decreases, so it is preferable that the O content be 0.25% or less. On the other hand, O is an element that is difficult to remove in titanium smelting, and keeping it below 0.03% would result in excessive cost increases. Therefore, the lower limit is preferably 0.03% or more.

In addition to the above elements, the present invention may contain one or more of the following elements in place of a portion of Ti: Sn: 5.0% or less, Nb: 8.0% or less, Si: 1.0% or less, and Mo: 5.0% or less.

Sn is an element that dissolves in the α and β phases to strengthen both phases. When dissolved in the α phase, it can strengthen the material without impairing ductility and toughness, just like Cu, but the effect is smaller than that of Cu. If the Sn content exceeds 5.0%, the toughness decreases. Therefore, the Sn content is set to 5.0% or less. If Sn is contained, it may be 0.2% or more, 0.5% or more, or 1.0% or more.

Nb is a β-stabilizing element, but some of it dissolves in the α-phase to strengthen it. However, its strengthening ability per mass percent is smaller than that of Fe, Cu, and Sn. When the Nb content exceeds 8.0%, the effect saturates. Therefore, the Nb content is set to 8.0% or less. When Nb is contained, it may be 0.2% or more, 0.5% or more, or 1.0% or more.

Si is a β-stabilizing element, but it also dissolves in the α-phase, and is highly effective in improving the strength of the alloy. However, if the Si content exceeds 1.0%, Ti and Si compounds precipitate coarsely, reducing toughness. Therefore, the Si content is set to 1.0% or less. If Si is included, it may be 0.2% or more, or 0.3% or more.

Mo is a β-stabilizing element, and its strengthening ability per mass percent is greater than that of Cu, but if it exceeds 5.0%, the α-phase that precipitates in the β-phase becomes too fine, causing a significant decrease in toughness. Therefore, the Mo content is set to 5.0% or less. If Mo is contained, it may be 0.2% or more, 0.5% or more, or 1.0% or more.

The balance in the chemical composition may be Ti and impurities. Specific examples of impurities include Cl, Na, Mg, and Ca that are mixed in during the refining process, and Zr, Ta, and V that are mixed in from scrap. If these impurities are contained, the content is, for example, 0.1% or less for each, and there is no problem if the total amount is 0.3% or less.

Cr or Ni, which are typical eutectoid β-stabilizing elements, are sometimes used as a partial or complete replacement for Fe, because, like Fe, they stabilize the β-phase and contribute to improving strength by forming a two-phase structure. However, if either or both of Cr and Ni are contained in the Ti-Al-Fe-Cu-O system, which is the basic component system of the present invention, the amount of Cu dissolved in the α-phase is reduced, and the effect of improving the balance between strength and ductility due to Cu cannot be obtained. Therefore, it is preferable to keep Cr or Ni at the level contained as an impurity in titanium raw materials such as sponge titanium and titanium scrap. The level is preferably 0.1% or less for each of the Cr and Ni concentrations.

Next, the solidification structure of the titanium alloy material of this embodiment will be described.
The titanium alloy material according to the present embodiment has a chemical composition generally called an α+β titanium alloy, and its metal structure contains 70% or more of α phase and 30% or less of β phase at room temperature. Depending on the elements contained and the manufacturing history, compounds of Ti and Cu, compounds of Ti and Fe, and compounds of Ti and Si may be present in a total amount of 3% or less.

When a titanium alloy having the chemical composition according to this embodiment is solidified from a molten state, columnar crystals are formed continuously in the direction of solidification, but the metal phase first forms a β phase (bcc structure), and as the temperature decreases, an α phase (hcp structure) is formed at the grain boundaries and within the grains of the β phase.

The grain size of the β phase formed during solidification varies depending on the size of the titanium alloy material being manufactured and the cooling method, but for components with sides measuring no more than a few tens of mm, the grain size of the β phase when observed in a cross section perpendicular to the solidification direction is 2 mm or less. For components with sides measuring no more than a few hundred mm, the grain size of the β phase when observed in a cross section perpendicular to the solidification direction is approximately 50 mm.

In addition, in the solidification structure, the α phase formed at the grain boundaries of the β phase is generally called grain boundary α, and has a thickness of 5 μm or less. When the cooling rate is fast, grain boundary α is not formed. The α phase formed within the grains is generally called acicular α, and has a thickness of 5 μm or less and an aspect ratio of 3 or more. Except when the cooling rate is fast, the acicular α often forms a colony structure in which grains having approximately the same crystal orientation are formed in layers. The size of the colony structure generally has a positive correlation with the crystal grain size of the β phase.

The β phase remains even at room temperature, and some of it may form the β" phase or ω phase, or form compounds of Ti and Cu, Ti and Fe, or Ti and Si. The larger the crystal grain size of the β phase, or the larger the size of the α phase colonies formed within the β phase, the more likely it is that strength and toughness will decrease. Furthermore, the ω phase and compounds of Ti and Fe can significantly reduce toughness. To reduce the crystal grain size of the β phase and the colony size of the α phase, it is advantageous to increase the cooling rate when solidifying the molten metal.

The titanium alloy material of this embodiment may have a solidification structure consisting of columnar and equiaxed crystal grains, as described above.

The metal structure of the titanium alloy material of this embodiment is not limited to the solidified structure described above, but may have a processed structure. Examples of processed structures include elongated grains obtained by subjecting a member having a solidified structure such as a cast material to processing involving deformation such as hot rolling, equiaxed grains obtained by further annealing, acicular grains obtained by heat treatment involving transformation as described above, and structures containing precipitates of various shapes (including equiaxed grains).

The titanium alloy material of this embodiment has excellent toughness and strength, whether the metal structure is a solidified structure, a processed structure, or other structure. That is, the titanium alloy material of this embodiment has a relationship between the impact value (CIS, unit: J/cm ² ) obtained by dividing the impact absorption energy obtained by the Charpy test at 25°C by the cross-sectional area of the test piece and the tensile strength (TS, unit: MPa) of 0.4×TS+CIS≧370, the toughness is CIS≧30, and the strength is TS≧620. Such toughness and strength can be achieved even if the titanium alloy material of this embodiment has a solidified structure. In addition, the above-mentioned toughness and strength can be achieved even if the titanium alloy material of this embodiment has a solidified structure, which is generally considered to have a toughness inferior to that of a processed structure.

Next, a method for producing the titanium alloy material according to this embodiment will be described.
There is no particular limitation on the manufacturing method of the titanium alloy material according to this embodiment, and it can be manufactured by a general casting solidification method, or by additive manufacturing technology. Furthermore, it can also be manufactured by a method of obtaining plate material, bar material, tube material, etc. by performing hot rolling or the like on a cast piece.

When the titanium alloy material of this embodiment is manufactured by a general casting solidification method, metallic raw materials such as sponge titanium, aluminum, iron, and copper are mixed and, if necessary, compression molded, and then manufactured into an ingot by a consumable electrode vacuum arc melting method (VAR), an electron beam melting method (EBR), a plasma arc melting method (PAM), or the like. The raw materials may be not only pure metals, but also master alloys and scrap. The raw materials may be in the form of lumps, plates, powders, etc. The molten raw materials are generally cooled and solidified using a water-cooled copper mold.

In addition, when manufacturing the titanium alloy material of this embodiment using additive manufacturing technology, it can be manufactured using, for example, directed energy deposition or powder bed fusion.

In the case of powder bed fusion, titanium sponge, aluminum, iron, copper and other metal raw materials are mixed and, if necessary, compression molded, then melted using consumable electrode vacuum arc melting (VAR), electron beam melting (EBR), plasma arc melting (PAM) or other methods, and then titanium alloy powder is produced by means of, for example, gas atomization.

Next, the obtained titanium alloy powder is spread in layers to form a powder bed, and the first step is performed in which a part of the powder bed is melted and solidified by irradiating the surface of the powder bed with a high-energy beam.

Next, a layer of titanium alloy powder is laid on top of the powder bed to form a new powder bed, and a second process is carried out in which a high-energy beam is irradiated onto the surface of the new powder bed to melt and solidify part of the new powder bed.

After carrying out the first step, the second step can be repeated at least once to produce a titanium alloy material having a desired shape.

In the case of directed energy deposition, metal raw materials such as titanium sponge, aluminum, iron, and copper are mixed and compression molded as necessary, then melted by consumable electrode vacuum arc melting (VAR), electron beam melting (EBR), plasma arc melting (PAM), or the like, and then titanium alloy powder is produced by means of, for example, gas atomization. Alternatively, after melting, the mixture is solidified to form a slab, which is then hot rolled to produce titanium alloy wire. These titanium alloy powders or titanium alloy wires are used as titanium alloy materials.

Next, while supplying the powdered or wire-shaped titanium alloy material onto a substrate, the titanium alloy material is irradiated with a high-energy beam to melt and solidify the titanium alloy material on the substrate, thereby depositing the solidified metal on the substrate, thereby producing a titanium alloy material having a predetermined shape.

Titanium alloy materials produced using the above-mentioned directed energy deposition method or powder bed fusion method have a solidified structure.

Titanium alloy materials manufactured by typical casting solidification methods or additive manufacturing techniques may be further subjected to typical processing and heat treatments such as HIP treatment, forging, extrusion, rolling, and cutting in order to reduce internal defects and control the shape.

Therefore, the titanium alloy material according to this embodiment may have a solidification structure containing the columnar and equiaxed crystal grains, as well as a structure containing the compounds described above, such as elongated grain crystals, equiaxed grains, elongated grains, or rectangular shapes. Furthermore, the titanium alloy material having the solidification structure described above may be further processed and heat-treated as in general titanium alloys, such as forging, rolling, extrusion, bending, cutting, and heat treatment, to produce elongated materials such as plates, bars, and tubes.

Furthermore, when manufacturing the titanium alloy material of this embodiment by subjecting a cast piece to hot rolling or the like, for example, a cast material manufactured by the above-mentioned general casting solidification method is used as a raw material, and hot rolling is performed on this raw material. Furthermore, after hot rolling, annealing, cold rolling, final annealing, etc. may be performed. In this way, a titanium alloy material having a processed structure can be obtained.

Next, the present invention will be described in more detail with reference to examples. However, the examples described below are merely examples of the embodiments of the present invention and do not limit the present invention.

Titanium alloy materials were manufactured using a general melting and solidification method. That is, metal raw materials with the composition adjusted to each chemical composition shown in Table 1 were vacuum arc melted to produce molten metal, and this molten metal was charged into a water-cooled copper mold to manufacture disk-shaped titanium alloy materials (Nos. 1 to 34) with a diameter of 60 mm, a thickness of 10 mm, and a weight of approximately 100 g. Blanks in Table 1 indicate that no alloying elements were intentionally included.

Titanium alloy materials No. 1 to 34 were cooled from below using a water-cooled copper mold, forming a solidification structure consisting of columnar and equiaxed crystal grains extending from the bottom to the top of the disk. Tensile test pieces with a parallel section diameter of 3 mm and a parallel section length of 20 mm were taken from these titanium alloy materials, with the test piece's longitudinal direction parallel to the bottom surface. In addition, 2 mm V-notch subsize Charpy test pieces 5 mm thick and 10 mm wide were taken with the test piece's longitudinal direction parallel to the bottom surface and the notch in the thickness direction.

The tensile test was performed at room temperature with a crosshead displacement rate of 0.3 mm/min. The impact test was performed at room temperature (25° C.) using a 300 J Charpy impact tester. The impact test results were evaluated as the impact value (J/cm ² ) obtained by dividing the absorbed energy by the cross-sectional area of the test piece.

Table 2 shows the results of the tensile strength and impact value corresponding to each chemical composition in Table 1. The tensile strength was determined to be 620 MPa, which is the tensile strength of the existing alloy Ti-3Al-2.5V (JIS 61 type), and 620 MPa or more was considered to be acceptable. In addition, the impact value was determined to be 30 J/cm ² or more. Furthermore, as an index of the balance between the properties of tensile strength and impact value, the criteria was whether the following formula (1) was satisfied, and the case where the following formula (1) was satisfied was considered to be acceptable. In the judgment column of Table 2, the case where the left side of formula (1) is 370 or more is represented as ◯ (achieved), and the case where the left side of formula (1) is less than 370 is represented as × (not achieved).

0.4 x TS + CIS ≥ 370 … (1)

In addition, in the notes section of Tables 1 and 2, the example corresponding to claim 1 is indicated as Example 1, and the example corresponding to claim 2 is indicated as Example 2.

Nos. 1 and 2 are titanium alloy materials with a conventional chemical composition of the Ti-Al-V system, and do not satisfy formula (1) showing the relationship between strength (TS) and impact strength (CIS), and therefore do not have desirable characteristics. In addition, No. 2 also had low tensile strength and did not reach the acceptable range.

No. 3 is a Ti-Al-Fe alloy that does not contain Cu, and does not satisfy formula (1) showing the relationship between strength and impact value, so it failed the test.

No. 4 had an Al content below the range of the present invention, did not satisfy the strength and formula (1) showing the relationship between strength and impact value, and was therefore unsuccessful.

Nos. 5 to 15 have a chemical composition of Al: 4.6 to 8.0%, Fe: 0.01 to 2.0%, Cu: 0.3 to 2.5%, O: 0.03 to 0.25%, with the remainder being Ti and impurities. They meet the ranges of the present invention, and all of the strength, impact value, and formula (1) showing the relationship between strength and impact value are satisfied, making them pass the test.

No. 16 had an Al content that exceeded the range of the present invention, and the impact value was unacceptable.

No. 17 had an Fe content that exceeded the range of the present invention, and the impact value was unacceptable.

No. 18 had a Cu content that exceeded the range of the present invention, and did not satisfy formula (1) showing the relationship between strength and impact value, so it was a failure.

No. 19 had an O content below the range of the present invention, did not satisfy the strength and formula (1) showing the relationship between strength and impact value, and was therefore unsuccessful.

No. 20 had an O content that exceeded the range of the present invention, and the impact value was unacceptable.

Nos. 21 to 23 are examples of Ti-Al-Fe-Cu alloys further containing Sn. Nos. 21 and 21 meet the chemical composition of the present invention, and pass the tests in terms of strength, impact value, and formula (1) showing the relationship between strength and impact value. On the other hand, No. 23 has an Sn content that exceeds the range of the present invention, and fails the impact value test.

Nos. 24 to 26 are examples of Ti-Al-Fe-Cu alloys that further contain Si. Nos. 24 and 25 meet the chemical composition of the present invention, and all of the strength, impact value, and formula (1) showing the relationship between strength and impact value were passed. On the other hand, No. 26 had a Si content that exceeded the range of the present invention, and the impact value was unsatisfactory.

Nos. 27 to 29 are examples of Ti-Al-Fe-Cu-Sn alloys containing Si, Nb, and Mo in combination. All of these satisfy the chemical composition of the present invention, and all of the strength, impact value, and formula (1) showing the relationship between strength and impact value are met, making them acceptable.

Nos. 30 and 31 are examples of Ti-Al-Fe-Cu alloys further containing Nb. No. 30 met the chemical composition of the present invention, and met the strength, impact value, and formula (1) showing the relationship between strength and impact value, so it passed. On the other hand, No. 31 had a Nb content that exceeded the range of the present invention, and the impact value failed.

Nos. 32 to 34 are examples of Ti-Al-Fe-Cu alloys further containing Mo. Nos. 32 and 33 meet the chemical composition of the present invention, and pass the tests in terms of strength, impact value, and formula (1) showing the relationship between strength and impact value. On the other hand, No. 34 has a Mo content that exceeds the range of the present invention, and fails the impact value test.

The metal structure of No. 1 to No. 34 was a solidification structure composed of columnar grains and equiaxed grains.

Next, titanium alloy materials were manufactured by the directed energy deposition method (WAAM method (wire and arc-based additive manufacturing)) using titanium alloy wire as a material. Specifically, titanium alloy wire (φ1.6 mm) with the chemical composition shown in Table 3 was prepared as the titanium alloy material. A plasma welding torch was used for additive manufacturing, with a current value of 200 A. The substrate was a titanium alloy plate with a thickness of 12.7 mm. A pre-set molding area on the substrate was surrounded by a chamber and shielded with Ar gas at a flow rate of 15 L/min. While supplying titanium alloy wire at a speed of 200 mm/min, the wire was melted by the plasma welding torch and further solidified on the substrate. By performing the additive manufacturing process in which such operations were repeated multiple times, a titanium alloy material consisting of an additive manufacturing sample with a width of 18 mm, a length of 100 mm, and a thickness of 15 mm was manufactured.

As described above, tensile test pieces and 2 mm V-notch subsize Charpy test pieces were taken from the obtained titanium alloy material, and tensile tests and Charpy impact tests were performed at room temperature (25°C). Table 4 shows the results of tensile strength and impact value corresponding to each chemical composition in Table 3.

Nos. 35 to 37 are examples of Ti-Al-Fe-Cu-O alloys. No. 38 is an example of a Ti-Al-Fe-Cu-O alloy that further contains Si. All of them meet the chemical composition of the present invention, and satisfy the strength, impact value, and formula (1) showing the relationship between strength and impact value, and were therefore successful.

The metal structure of No. 35 to No. 37 was a solidification structure composed of columnar grains and equiaxed grains.

Next, titanium alloy materials were produced by a general melting and solidification method, and then forged and hot rolled into plate shapes. That is, metal raw materials with the composition adjusted to obtain each of the chemical compositions shown in Table 5 were vacuum arc melted to produce molten metal, and this molten metal was charged into a water-cooled copper mold to produce cylindrical titanium alloy materials (Nos. 41 to 44) with a diameter of 430 mm, a length of 300 mm, and a weight of approximately 200 kg. Blanks in Table 5 indicate that alloy elements were not actively included.

Each titanium alloy material was hot forged at a heating temperature of 1100°C to a thickness of 220 mm and width of 100 mm, after which the scale was removed and hot rolled at a heating temperature of 1050°C to produce a plate with a thickness of 5 mm.

Tensile test pieces were prepared with the same thickness as the plate, a width of the parallel part of 6 mm, and a length of 30 mm perpendicular to the rolling direction. The tensile test was performed at a crosshead displacement rate of 0.75 mm/min. Impact test pieces were also prepared with the same thickness as the plate, a length of 55 mm in the rolling direction of the plate, a width of 10 mm in the plate width direction, and a 2 mm V-notch in the plate thickness direction. Both tensile tests and impact tests were performed at room temperature (25°C). Table 6 shows the results of tensile strength and impact value corresponding to each chemical composition in Table 5.

No. 41 was a Ti-Al-Fe-O based Ti alloy that did not contain Cu, and had an impact value below 30 J/ ^cm2 , did not satisfy formula (1) showing the relationship between strength and impact value, and was therefore unsuccessful.

Nos. 42 and 43 are Ti alloys containing Cu, and meet the composition range of the present invention. They also meet the impact value and formula (1) showing the relationship between strength and impact value, and were therefore accepted.

No. 44 is an example that further contains Si, and satisfies the composition range of the present invention, and satisfies the impact value and formula (1) showing the relationship between strength and impact value, so it was a pass.

The metal structure of No. 41 to No. 44 was a processed structure consisting of wrought grains and equiaxed grains.

Claims (9)

In mass%, the alloy contains Al: 4.6% or more and 8.0% or less, Fe: 0.01% or more and 2.0% or less, Cu: 0.3% or more and 2.5% or less, O: 0.03% or more and 0.25% or less, and the balance is Ti and impurities;
A titanium alloy material satisfying the following formula (1):
0.4 × TS + CIS ≧ 370 … (1)
In formula (1), CIS is an impact value (unit: J/cm ² ) obtained by dividing the impact absorption energy determined in a Charpy test at 25° C. by the cross-sectional area of the test piece, and TS is tensile strength (unit: MPa). Further, instead of a part of Ti, the titanium alloy material according to claim 1 contains, by mass%, one or more of Sn: 5.0% or less, Nb: 8.0% or less, Si: 1.0% or less, and Mo: 5.0 % or less. 3. The titanium alloy material according to claim 1, wherein a part or all of the metal structure is a solidification structure consisting of columnar grains and equiaxed grains. A method for producing a titanium alloy material according to claim 3, using a titanium alloy powder as a raw material, the titanium alloy powder containing, by mass%, Al: 4.6% to 8.0%, Fe: 0.01% to 2.0%, Cu: 0.3% to 2.5 %, O: 0.03% to 0.25%, and the balance being Ti and impurities, by additive manufacturing technology. The method of claim 4 , wherein the additive manufacturing technique is directed energy deposition or powder bed fusion. A first step of forming a powder bed by spreading titanium alloy powder containing, by mass%, Al: 4.6% to 8.0%, Fe: 0.01% to 2.0%, Cu: 0.3% to 2.5%, O: 0.03% to 0.25%, and the balance being Ti and impurities, in a layered form, and irradiating a surface of the powder bed with a high-energy beam to melt and solidify a portion of the powder bed;
a second step of laying the titanium alloy powder in a layer on the powder bed to form a new powder bed, and irradiating a surface of the new powder bed with a high-energy beam to melt and solidify a portion of the new powder bed;
A method for producing a titanium alloy material according to claim 3 , comprising repeating the second step at least once after carrying out the first step. 4. A method for producing a titanium alloy material according to claim 3, comprising the steps of: supplying a titanium alloy material consisting of a powder or a wire material containing, by mass%, Al: 4.6% to 8.0%, Fe: 0.01% to 2.0%, Cu: 0.3% to 2.5%, O: 0.03% to 0.25%, and the balance being Ti and impurities, onto a substrate; irradiating the titanium alloy material with a high-energy beam while supplying the titanium alloy material onto the substrate; melting and solidifying the titanium alloy material on the substrate ; and depositing solidified metal on the substrate. The method for producing a titanium alloy material according to any one of claims 4 to 7, wherein the titanium alloy powder further contains, in mass%, one or more of the following elements in place of a part of Ti : Sn: 5.0% or less, Nb: 8.0% or less, Si: 1.0% or less, and Mo: 5.0% or less. A method for producing a titanium alloy material, comprising the steps of: subjecting a titanium alloy material obtained by the method for producing a titanium alloy material according to any one of claims 4 to 8 to one or more of hot working, cold working, straightening, and cutting.