METHOD FOR PRODUCING ALUMINUM ALLOY

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2024-182405 filed on Oct. 18, 2024. The disclosure of the above-identified application, including the specification, drawings, and claims, is incorporated by reference herein in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a method for producing an aluminum alloy, and more particularly to, a method for producing an aluminum alloy for high-pressure gas storage.

2. Description of Related Art

In recent years, hydrogen has been attracting attention as a clean energy source. Hydrogen has the property of embrittling metals such as iron and aluminum. Therefore, efforts have been made to develop metal alloy materials for safely and easily storing hydrogen particularly at high pressure.

For example, Japanese Unexamined Patent Application Publication No. 2002-348631 (JP 2002-348631 A) describes an aluminum-zinc-magnesium based aluminum alloy for casting and forging that has excellent castability and forgeability and includes, in mass %, 3% to 5% of zinc, 1% to 3% of magnesium, 0.20% to 1.0% of copper, 0.15% to 0.30% of titanium, 0.10% to 0.40% of zirconium, 0.30% or less of silicon, 0.50% or less of iron, and the balance being aluminum and unavoidable impurities.

Japanese Unexamined Patent Application Publication No. 2014-101541 (JP 2014-101541 A) describes an aluminum alloy material for a high-pressure hydrogen gas container. The aluminum alloy material is composed of an aluminum alloy containing 0.6 mass % to 1.5 mass % of Si, 0.6 mass % to 1.6 mass % of Mg, 0.1 mass % to 1.0 mass % of Cu, and 0.05 mass % to 0.4 mass % of Fe, with restrictions that Mn is 0.9 mass % or less, Cr is 0.3 mass % or less, Zr is 0.15 mass % or less, V is 0.2 mass % or less, Zn is 0.25 mass % or less, and Ti is 0.1 mass % or less. The balance is Al and unavoidable impurities. The total content of the Mn, the Cr, the Zr, and the V is 0.05 mass % or more. The aluminum alloy material satisfies formula (1) [S≤−10.46×E+801] and formula (2) [S≥−25×E+1296] related to a yield strength S (MPa) and an electrical conductivity E (IACS %). The yield strength S is 270 MPa or more, and the electrical conductivity E is 36 IACS % or more.

SUMMARY

High-pressure gas storage tanks are used for hydrogen stations, for transportation, and on vehicles. In particular, on-board tanks are required to be lightweight. Therefore, aluminum alloys that are lighter than iron have been considered as candidates for metal alloy materials for manufacturing on-board tanks. Aluminum alloys for such applications are required to have not only hydrogen embrittlement resistance, intergranular corrosion resistance, and stress corrosion cracking (SCC) resistance, but also resistance to stress corrosion cracking in an atmosphere that may cause hydrogen embrittlement of the aluminum alloys, that is, a hydrogen atmosphere containing moisture as an impurity (resistance to humid gas (HG)-SCC).

Therefore, an object of the present disclosure is to provide a method for producing an aluminum alloy that has sufficient HG-SCC resistance, specifically, that can pass the HG-SCC test (HPISE 103: 2018) established by the High Pressure Institute of Japan in 2018 (hereinafter also simply referred to as “HG-SCC test”).

The inventors have studied various means for achieving the above object. As a result, in a method for producing an aluminum alloy material containing copper (Cu) and silicon (Si) by hot working (hot casting), it has been found that an aluminum alloy that can pass the HG-SCC test can be produced by setting the Cu content in a range of 0.15 mass % or more and 0.40 mass % or less and the Si content in a range of 0.65 mass % or more and 0.80 mass % or less when the draft in the hot working is 40% or more, the Si content in a range of 0.65 mass % or more and (0.6002×Cu content (mass %)+0.5606) mass % or less when the draft in the hot working is less than 40%, or the Si content in a range of (0.6002× Cu content (mass %)+0.5606) mass % or more and 0.80 mass % or less when the draft in the hot working is 40% or less (excluding a composition in which the Cu content is 0.15 mass % and the Si content is 0.80 mass %). Thus, the present disclosure have been completed.

The summary of the present disclosure is as follows.

• • (1) A method for producing an aluminum alloy for high-pressure gas storage, the method including: (i) an adjustment step of adjusting a composition of a raw material of the aluminum alloy such that the raw material of the aluminum alloy contains, when an entirety of the raw material of the aluminum alloy is 100 mass %, Cu in a range of 0.15 mass % or more and 0.40 mass % or less, Si in a range of 0.65 mass % or more and 0.80 mass % or less, Al, and unavoidable impurities; (ii) a continuous casting step of casting the raw material of the aluminum alloy having the composition adjusted in the step (i) to produce an ingot; and (iii) a hot working step of performing hot working on the ingot produced in the step (ii) such that a draft is 40% or more.
  • (2) The method according to (1), in which a Cu content and a Si content in the adjustment step (i) are in a range surrounded by three points (x, y)=(0.15, 0.65), (0.15, 0.80), and (0.40, 0.80) when an x-y graph is created by taking the Cu content on an x-axis and the Si content on a y-axis.
  • (3) A method for producing an aluminum alloy for high-pressure gas storage, the method including: (i) an adjustment step of adjusting a composition of a raw material of the aluminum alloy such that the raw material of the aluminum alloy contains, when an entirety of the raw material of the aluminum alloy is 100 mass %, Cu in a range of 0.15 mass % or more and 0.40 mass % or less, Si in a range of 0.65 mass % or more and (0.6002×Cu content (mass %)+0.5606) mass % or less, Al, and unavoidable impurities; (ii) a continuous casting step of casting the raw material of the aluminum alloy having the composition adjusted in the step (i) to produce an ingot; and (iii) a hot working step of performing hot working on the ingot produced in the step (ii) such that a draft is less than 40%.
  • (4) A method for producing an aluminum alloy for high-pressure gas storage, the method including: (i) an adjustment step of adjusting a composition of a raw material of the aluminum alloy such that the raw material of the aluminum alloy contains, when an entirety of the raw material of the aluminum alloy is 100 mass %, Cu in a range of 0.15 mass % or more and 0.40 mass % or less, Si in a range of (0.6002×Cu content (mass %)+0.5606) mass % or more and 0.80 mass % or less, Al, and unavoidable impurities, excluding a composition in which the Cu content is 0.15 mass % and a Si content is 0.80 mass %; (ii) a continuous casting step of casting the raw material of the aluminum alloy having the composition adjusted in the step (i) to produce an ingot; and (iii) a hot working step of performing hot working on the ingot produced in the step (ii) such that a draft is 40% or less.
  • (5) The method according to any one of (1) to (4), in which the aluminum alloy passes a humid gas stress corrosion cracking test established by the High Pressure Institute of Japan in 2018.
  • (6) An aluminum alloy for high-pressure gas storage, the aluminum alloy being produced by the method according to any one of (1) to (5).

The present disclosure provides the method for producing the aluminum alloy that has sufficient HG-SCC resistance, specifically, that can pass the HG-SCC test.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

FIG. 1 is a diagram schematically showing hot (forging) working of a continuously cast bar to describe a draft;

FIG. 2 is a diagram schematically showing the relationship among a Cu content, a Si content, and a hot working rate in aluminum alloy castings of examples of the present disclosure and comparative examples; and

FIG. 3 is a diagram schematically showing a test piece prepared in “2. HG-SCC Test,” in which the unit is “mm.”

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment of the present disclosure will be described in detail.

In this specification, features of the present disclosure will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shape of each part are exaggerated for clarity and are not accurately depicted in accordance with the actual dimensions and shape. Therefore, the technical scope of the present disclosure is not limited to the dimensions and shapes of the parts shown in the drawings. The method for producing the aluminum alloy according to the present disclosure is not limited to the following embodiment, and can be embodied in various forms incorporating modifications and revisions that can be made by those skilled in the art without departing from the gist of the present disclosure.

In the present disclosure, the expression “a range of a numerical value (lower limit) or more to a numerical value (upper limit) or less” indicates a range including the lower limit and the upper limit.

The method for producing the aluminum alloy according to the present disclosure includes: (i) an adjustment step of adjusting the composition of a raw material of the aluminum alloy to a specific composition; (ii) a continuous casting step of casting the raw material of the aluminum alloy having the composition adjusted in step (i) to produce an ingot; and (iii) a hot working step of performing hot working on the ingot produced in step (ii) such that a draft is appropriate according to the composition of the aluminum alloy.

Steps (i) to (iii) will be described below.

In the adjustment step (i), the composition of the raw material of the aluminum alloy is adjusted. Examples of the raw material of the aluminum alloy include those in the form of powder, molten metal, and casting (e.g., aluminum alloy ingot).

As the raw material of the aluminum alloy, an aluminum ingot etc. can be used.

As the raw material of the aluminum alloy, a commercially available material with a known composition may be used. When the composition of the raw material of the aluminum alloy is unknown, the composition of the raw material of the aluminum alloy can be analyzed.

The composition of the raw material of the aluminum alloy, in particular, the content of Cu, Si, Mg, Zn, Fe, and Mn, can be analyzed by, but not limited to, optical emission spectroscopy, X-ray fluorescence analysis (XRF), etc.

By analyzing the raw material of the aluminum alloy, it is possible to prepare the raw material of the aluminum alloy having the same composition as the aluminum alloy to be produced.

Next, the raw material of the aluminum alloy is adjusted to contain, when the entire raw material of the aluminum alloy is 100 mass %, Cu in a range of 0.15 mass % or more and 0.40 mass % or less, Si in a range of 0.65 mass % or more and 0.80 mass % or less, 0.65 mass % or more and (0.6002×Cu content (mass %)+0.5606) mass % or less, or (0.6002×Cu content (mass %)+0.5606) mass % or more and 0.80 mass % or less (excluding a composition in which the Cu content is 0.15 mass % and the Si content is 0.80 mass %), Al, and unavoidable impurities.

When the entire raw material of the aluminum alloy is 100 mass %, the Cu content is 0.15 mass % or more, 0.18 mass % or more in one embodiment, 0.20 mass % or more in one embodiment, and 0.22 mass % or more in one embodiment, and is 0.40 mass % or less, 0.38 mass % or less in one embodiment, 0.36 mass % or less in one embodiment, 0.34 mass % or less in one embodiment, 0.32 mass % or less in one embodiment, 0.30 mass % or less in one embodiment, and 0.28 mass % or less in one embodiment.

The Cu content can be adjusted, for example, by mixing two or more kinds of raw material of the aluminum alloy with known compositions at any ratio and optionally adding an additive such as an additive known in the art to adjust the Cu content in the raw material of the aluminum alloy (pure copper or an alloy or compound containing Cu (e.g., an oxide)).

The Cu content can be measured by optical emission spectroscopy.

The Si content may vary depending on the draft in the hot working step (iii).

When the draft in the hot working step (iii) is 40% or more, the Si content is, when the entire raw material of the aluminum alloy is 100 mass %, 0.65 mass % or more, 0.67 mass % or more in one embodiment, 0.69 mass % or more in one embodiment, 0.71 mass % or more in one embodiment, and 0.73 mass % or more in one embodiment, and is 0.80 mass % or less, 0.78 mass % or less in one embodiment, and 0.76 mass % or less in one embodiment. Hereinafter, the Cu content and the Si content suitable for the case where the draft in the hot working step (iii) is 40% or more will be referred to as “CuSi contents A.” The CuSi contents A are in a range surrounded by four points (x, y)=(0.15, 0.65), (0.40, 0.65), (0.40, 0.80), and (0.15, 0.80) when an x-y graph is created by taking the Cu content on the x-axis and the Si content on the y-axis.

When the draft in the hot working step (iii) is less than 40%, the Si content is 0.65 mass % or more when the entire raw material of the aluminum alloy is 100 mass %. When the draft is less than 40%, the upper limit value of the Si content depends on the Cu content and is (0.6002×Cu content (mass %)+0.5606) mass %. Hereinafter, the Cu content and the Si content suitable for the case where the draft in the hot working step (iii) is less than 40% will be referred to as “CuSi contents B.” The CuSi contents B are in a range surrounded by three points (x, y)=(0.15, 0.65), (0.40, 0.65), and (0.40, 0.80) when an x-y graph is created by taking the Cu content on the x-axis and the Si content on the y-axis.

Alternatively, when the draft in the hot working step (iii) is 40% or less, the lower limit value of the Si content depends on the Cu content and is (0.6002×Cu content (mass %)+0.5606) mass %. The Si content is 0.80 mass % or less when the entire raw material of the aluminum alloy is 100 mass %. This range excludes a composition in which the Cu content is 0.15 mass % and the Si content is 0.80 mass %. In one embodiment, this range excludes compositions in which the Cu content is in a range of 0.15 mass % or more and 0.28 mass % or less and the Si content is in a range of (0.5385×Cu content (mass %)+0.6492) mass % or more and 0.80 mass % or less. Hereinafter, the Cu content and the Si content suitable for the case where the draft in the hot working step (iii) is 40% or less will be referred to as “CuSi contents B′.” The CuSi contents B′ are in a range surrounded by three points (x, y)=(0.15, 0.65), (0.15, 0.80), and (0.40, 0.80) when an x-y graph is created by taking the Cu content on the x-axis and the Si content on the y-axis, excluding a composition in which the Cu content is 0.15 mass % and the Si content is 0.80 mass %, and in one embodiment, excluding a composition in a range surrounded by three points (x, y)=(0.15, 0.73), (0.15, 0.80), and (0.28, 0.80).

The Si content can be adjusted, for example, by mixing two or more kinds of raw material of the aluminum alloy with known compositions at any ratio and optionally adding an additive such as an additive known in the art to adjust the Si content in the raw material of the aluminum alloy (pure silicon or an alloy or compound containing Si (e.g., an oxide)).

The Si content can be measured by optical emission spectroscopy.

In the aluminum alloy, the corrosion resistance can be improved by reducing the Cu content, and the strength can be improved by increasing the Si content. As a result, the strength of the obtained aluminum alloy can be increased and the corrosion resistance can also be improved.

In addition to the above elements, the raw material of the aluminum alloy may contain elements such as magnesium (Mg), zinc (Zn), iron (Fe), manganese (Mn), nickel (Ni), tin (Sn), chromium (Cr), titanium (Ti), calcium (Ca), strontium (Sr), and sodium (Na).

In the present disclosure, the raw material of the aluminum alloy having the adjusted composition may be homogenized. Examples of the homogenization method include a method of simply mixing the raw material of the aluminum alloy. This method can be adopted when the raw material of the aluminum alloy is in a form that is easily mixed, such as powder or granule.

Examples of the homogenization method also include a method of melting the raw material of the aluminum alloy to prepare a molten aluminum alloy.

In the continuous casting step (ii), the raw material of the aluminum alloy having the composition adjusted in step (i) is cast to produce an ingot.

Casting refers to pouring molten metal (including an alloy) melted at high temperature, generally 680° C. to 700° C. in the case of an aluminum alloy, into a hollow part (cavity) of a mold made of sand, metal, etc., and cooling the molten metal generally to 200° C. to 350° C. until it solidifies, thereby producing an ingot.

Examples of the casting method include ordinary melting casting methods such as continuous casting, continuous casting rolling, semi-continuous casting (direct-chill (DC) casting), and hot top casting, and die casting.

The aluminum alloy obtained by casting may be subjected to homogenization treatment.

The homogenization treatment can be carried out by any method known in the art. For example, the aluminum alloy obtained by casting is subjected to heat treatment generally at 400° C. to 500° C. for 2 hours to 10 hours.

In the hot working step (iii), the ingot produced in step (ii) is subjected to hot working such that the draft is appropriate according to the composition of the aluminum alloy.

The hot working refers to a working method in which the ingot produced in step (ii), such as a continuously cast bar, is subjected to heat treatment while applying pressure.

As shown in FIG. 1, the draft means the degree of rolling reduction calculated as (a−b)/a×100(%) when a continuously cast bar (thickness before forming: a) is subjected to hot working to produce an aluminum alloy (thickness after forming: b).

When the Cu content and the Si content are adjusted to the CuSi contents A in the adjustment step (i), the draft is set to 40% or more in the hot working step (iii). When the Cu content and the Si content are adjusted to the CuSi contents A, the upper limit value of the draft is not limited. The draft is generally less than 100%, 99% or less in one embodiment, 98% or less in one embodiment, 97% or less in one embodiment, 96% or less in one embodiment, 95% or less in one embodiment, 90% or less in one embodiment, 85% or less in one embodiment, and 80% or less in one embodiment.

When the Cu content and the Si content are adjusted to the CuSi contents B in the adjustment step (i), the draft in the hot working step (iii) may be less than 40%, 39% or less in one embodiment, 38% or less in one embodiment, 37% or less in one embodiment, 36% or less in one embodiment, 35% or less in one embodiment, 30% or less in one embodiment, and 25% or less in one embodiment. When the Cu content and the Si content are adjusted to the CuSi contents B, the lower limit value of the draft is not limited. The draft is generally more than 0%, 1% or more in one embodiment, 2% or more in one embodiment, 3% or more in one embodiment, 4% or more in one embodiment, 5% or more in one embodiment, 10% or more in one embodiment, 15% or more in one embodiment, and 20% or more in one embodiment.

In other words, when the Cu content and the Si content adjusted in the adjustment step (i) are in the range surrounded by the three points (x, y)=(0.15, 0.65), (0.15, 0.80), and (0.40, 0.80) in the x-y graph created by taking the Cu content on the x-axis and the Si content on the y-axis, the draft is set to 40% or more. When the Cu content and the Si content adjusted in the adjustment step (i) are the CuSi contents B, the draft may be less than 40%.

Alternatively, when the Cu content and the Si content are adjusted to the CuSi contents B′ in the adjustment step (i), the draft in the hot working step (iii) may be 40% or less, 39% or less in one embodiment, 38% or less in one embodiment, 37% or less in one embodiment, 36% or less in one embodiment, 35% or less in one embodiment, 30% or less in one embodiment, and 25% or less in one embodiment. When the Cu content and the Si content are adjusted to the CuSi contents B′, the lower limit value of the draft is not limited. The draft is generally more than 0%, 1% or more in one embodiment, 2% or more in one embodiment, 3% or more in one embodiment, 4% or more in one embodiment, 5% or more in one embodiment, 10% or more in one embodiment, 15% or more in one embodiment, and 20% or more in one embodiment.

The hot working temperature is not limited, but is generally 450° C. or more, and 500° C. or more in one embodiment, and is generally 600° C. or less, and 550° C. or less in one embodiment.

The adjustment step (i) and the hot working step (iii) impart sufficient HG-SCC resistance to the aluminum alloy. As a result, it is possible to produce an aluminum alloy that can pass the HG-SCC test.

The obtained aluminum alloy may then be subjected to solution treatment and/or aging treatment.

The solution treatment can be carried out by any method known in the art. For example, the aluminum alloy obtained by casting is subjected to heat treatment generally at 500° C. to 600° C. for 2 hours to 4 hours.

By cooling after the solution treatment, a supersaturated solid solution of metal elements that may affect the strength and toughness of the aluminum alloy can be formed.

The aging treatment can be carried out by any method known in the art. For example, the aluminum alloy subjected to the solution treatment is subjected to heat treatment generally at 150° C. to 200° C. for 2 hours to 10 hours.

The aging treatment can stabilize the metal structure precipitated in the aluminum alloy and improve the strength.

In the present disclosure, the composition does not change when the aluminum alloy is produced from the aluminum alloy raw material. Therefore, the aluminum alloy raw material and the obtained aluminum alloy have the same composition.

The aluminum alloy produced in the present disclosure is an aluminum alloy casting. The term “casting” refers to a molded product obtained by casting. Therefore, castings include molded products obtained by low-pressure casting, gravity casting, die casting, etc.

The aluminum alloy produced in the present disclosure has sufficient HG-SCC resistance. Specifically, the aluminum alloy of the present disclosure can pass the HG-SCC test (HPISE 103: 2018) established by the High Pressure Institute of Japan in 2018 by reducing the case where the crack length exceeds 0.16 mm. By molding the aluminum alloy of the present disclosure by casting, it can be used as a material of a tank for storing high-pressure gas, in particular, hydrogen gas.

As described above, the aluminum alloy produced in the present disclosure has the HG-SCC resistance, but the structure and properties of the aluminum alloy that should be changed due to the production method of the present disclosure have not been clarified yet. This is presumably because the structure and properties of the aluminum alloy need to be analyzed from multiple perspectives by obtaining not only measurement results using commonly used indices such as compositions and photographs of specific portions of the aluminum alloy, but also, for example, measurement results ranging from the local (micro) structure to the overall (macro) structure related to the composition and structure of the aluminum alloy. Such an analysis is not easily accomplished using current analytical technologies and requires time, effort, and cost. Therefore, at the current level of analytical technologies, the aluminum alloy produced in the present disclosure can be expressed only by the production method. The aluminum alloy of the present disclosure has impossible or impractical circumstances that cannot be expressed by any method other than the production method.

Hereinafter, some examples of the present disclosure will be described, but these examples are not intended to limit the present disclosure to the forms shown in the examples.

1. Sample Preparation

An aluminum alloy raw material containing chemical components shown in Table 1 was melted. The molten metal was cast into a continuously cast bar, and the obtained continuously cast bar was homogenized at 470° C. for 7 hours. Subsequently, the homogenized continuously cast bar was subjected to hot working at 520° C. to obtain a draft shown in Table 1. The alloy after the hot working was subjected to solution treatment at 530° C. for 3 hours and then to aging treatment at 180° C. for 6 hours to prepare a sample.

2. HG-SCC Test

The obtained sample was subjected to the HG-SCC test (HPISE 103: 2018) established by the High Pressure Institute of Japan in 2018. Specifically, the HG-SCC test was performed on the prepared sample through the procedure of: (1) preparation of a test piece (prepared based on FIG. 3); (2) pre-fatigue crack introduction; (3) constant load test (adjusted such that a load of a crack stress intensity factor K_IAPP=0.056×σ_0.2 was input to the crack tip of the test piece relative to a 0.2% yield strength (σ_0.2) obtained in a tensile test for each substrate performed in advance; test environment: air atmosphere at 25±5° C., relative humidity of 85% or more; test period: 90 days); (4) post-fatigue crack introduction and fracture; (5) SCC length measurement; and (6) evaluation of material compatibility (an SCC length of 0.16 mm or less was considered to be a pass). The results are shown in Table 1 and FIG. 2.

Table 1 and FIG. 2 demonstrate that, in the HG-SCC test, a disadvantage arises when the Si content is large, the Cu content is small, and the draft is small. Specifically, the following have been found. When the draft is 40% or more, an aluminum alloy that can pass the HG-SCC test can be produced by adjusting the Cu content in the aluminum alloy to the range of 0.15 mass % or more and 0.40 mass % or less and the Si content in the aluminum alloy to the range of 0.65 mass % or more and 0.80 mass % or less. When the draft is less than 40%, an aluminum alloy that can pass the HG-SCC test can be produced by adjusting the Cu content in the aluminum alloy to the range of 0.15 mass % or more and 0.40 mass % or less and the Si content in the aluminum alloy to the range of 0.65 mass % or more and (0.6002×Cu content (mass %)+0.5606) mass % or less. When the draft is 40% or less, an aluminum alloy that can pass the HG-SCC test can be produced by adjusting the Cu content in the aluminum alloy to the range of 0.15 mass % or more and 0.40 mass % or less and the Si content in the aluminum alloy to the range of (0.6002×Cu content (mass %)+0.5606) mass % or more and 0.80 mass % or less, excluding a composition in which the Cu content is 0.15 mass % and the Si content is 0.80 mass %, and preferably excluding compositions in which the Cu content is in the range of 0.15 mass % or more and 0.28 mass % or less and the Si content is in the range of (0.5385×Cu content (mass %)+0.6492) mass % or more and 0.80 mass % or less.