US20180347017A1

US20180347017A1 - Aluminum alloy material and production method therefor

Info

Publication number: US20180347017A1
Application number: US15/777,798
Authority: US
Inventors: Taichi Suzuki; Hidenori HATTA; Shuhei Shakudo
Original assignee: UACJ Corp
Current assignee: UACJ Corp
Priority date: 2015-11-20
Filing date: 2016-11-18
Publication date: 2018-12-06
Also published as: CN108291279A; JP6954722B2; WO2017086470A1; JP2017095754A; CN108291279B; KR20180081603A

Abstract

An aluminum alloy material as one aspect of the present disclosure has a chemical composition including: Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% or less; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: less than 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less; and Ti: 0.001% or more and 0.05% or less, a balance including Al and inevitable impurities. In the aluminum alloy material, a mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less, and a metallographic structure includes an equigranular recrystallized structure.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This international application claims the benefit of Japanese Patent Application No. 2015-227926 filed on Nov. 20, 2015 with the Japan Patent Office, and the entire disclosure of Japanese Patent Application No. 2015-227926 is incorporated herein.

TECHNICAL FIELD

The present disclosure relates to an aluminum alloy material and a production method therefor.

BACKGROUND ART

Conventional 7000-series aluminum alloys with Zn and Mg added to Al have been known as aluminum alloys exhibiting a high strength. Such 7000-series aluminum alloys exhibit a high strength due to age precipitation of Al—Mg—Zn-based fine precipitates. 7000-series aluminum alloys to which Cu has been added in addition to Zn and Mg exhibit the highest strength among aluminum alloys.
7000-series aluminum alloys are produced by, for example, hot extrusion or other process, and are used in applications requiring a high strength, including transportation equipment, such as aircraft and vehicles, and machine parts, as well as sporting goods and so on. Properties that 7000-series aluminum alloys are required to have when used in such applications include impact absorbability (toughness), resistance to stress corrosion cracking (hereinafter referred to as resistance to SCC, which is an abbreviation of Stress Corrosion Cracking), and so on, in addition to strength. Proposed as an example of 7000-series aluminum alloys is, for example, an aluminum alloy extruded material disclosed in Patent Document 1.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-119904

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In 7000-series aluminum alloys, when an amount of Zn and Mg added to achieve a high strength is increased, a strength improving effect is obtained, whereas a problem of decrease in workability, such as extrusion processability, arises.
Further, in the above-described applications, good appearance properties are required in addition to the above-described various properties; thus, surface quality such as surface texture and visual appearance is regarded as important. In general 7000-series aluminum alloys, when a surface treatment such as anodization is performed for the purpose of preventing surface scratches, compounds precipitated on a grain boundary are preferentially etched at pretreatment, whereby streak patterns or the like are generated on the surface-treated surface, resulting in a problem in surface quality. Especially in a case where the metallographic structure is made to be fibrous in order to obtain a higher strength, such streak patterns are conspicuous because the compounds precipitated on the grain boundary are arranged along the fibrous metallographic structure. As a result, it is difficult to obtain a good surface quality.
Means for solving the above-described problems in surface quality, such as generation of the streak patterns, include to make the metallographic structure to be a recrystallized structure, which is not fibrous but equigranular. With such a recrystallized structure, a situation can be inhibited in which the compounds precipitated on the grain boundary are arranged linearly, whereby generation of streak patterns can be reduced. However, it is known that, in the case where a 7000-series aluminum alloy has the recrystallized structure, its strength is lowered and its toughness and resistance to SCC are also decreased in some cases, as compared with the case of having the fibrous structure. In addition, with the recrystallized structure, scale-like patterns are conspicuous although generation of the streak patterns can be reduced. In this way, conventional 7000-series aluminum alloys have been difficult to use in the applications requiring properties such as resistance to SCC and surface quality as well, in addition to a high strength and a high toughness.
In one aspect of the present disclosure, it is desirable to provide a high-strength aluminum alloy material that is excellent in surface quality, toughness, and resistance to SCC; and a production method therefor.

Means for Solving the Problems

An aluminum alloy material as one aspect of the present disclosure has a chemical composition comprising: Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% or less; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: less than 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less; and Ti: 0.001% or more and 0.05% or less, a balance comprising Al and inevitable impurities. In the aluminum alloy material, a mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less, and a metallographic structure comprises an equigranular recrystallized structure.
The above-described aluminum alloy material has the above-specified chemical composition, and its metallographic structure comprises the equigranular recrystallized structure. This makes it possible to inhibit poor surface quality after surface treatment such as anodization, as compared with a case in which its metallographic structure is a fibrous structure. In particular, regulation of the upper limit of the Mg content makes it possible to inhibit precipitation of the compounds on the grain boundary while ensuring a high strength, thereby inhibiting generation of scale-like patterns on the surface caused by the recrystallized structure after surface treatment such as anodization. Moreover, regulation of the upper limit of the Cu content makes it possible to inhibit the surface from becoming yellowish in color tone by surface treatment. As a result, a good surface quality can be obtained. Furthermore, by setting the mass ratio of Zn to Mg (Zn/Mg) to the above-specified range, toughness and resistance to SCC can be improved while ensuring a high strength.
A production method for an aluminum alloy material as another aspect of the present disclosure is a method for producing an aluminum alloy material, a metallographic structure of which comprises an equigranular recrystallized structure. The method comprises: preparing an ingot having a chemical composition comprising: Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less; Mg: 0.5% or more and 1.5% or less; Cu: 0.10% or less; Fe: 0.30% or less; Si: 0.30% or less; Mn: less than 0.05%; Cr: less than 0.05%; Zr: 0.05% or more and 0.10% or less; and Ti: 0.001% or more and 0.05% or less, a balance comprising Al and inevitable impurities, wherein a mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less; and performing a homogenizing treatment in which the ingot is heated at a temperature higher than 540° C. and 580° C. or lower for 1 hour or longer and 24 hours or shorter.
In the above-described production method for the aluminum alloy material, the ingot having the above-specified chemical component and having the mass ratio of Zn to Mg (Zn/Mg) set to the above-specified range is prepared in the production process. Then, the ingot is subjected to the homogenizing treatment under the above-specified conditions. In particular, by setting the heating temperature in the homogenizing treatment to a high temperature, which is higher than 540° C. and 580° C. or lower, it becomes possible to easily obtain the above-described aluminum alloy material, that is, a high-strength aluminum alloy material, a metallographic structure of which comprises an equigranular recrystallized structure and which is excellent in surface quality, toughness, and resistance to SCC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a bending test method.

FIG. 2 is an explanatory diagram showing a method for observing metallographic structures.

EXPLANATION OF REFERENCE NUMERALS

10 . . . test piece, 20 . . . specimen

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present disclosure will be described below. It is needless to say that the present disclosure is not limited to the below-described embodiments, and that the present disclosure can be practiced in various forms within the scope not departing from the gist of the present disclosure.
Detailed explanation will be given of a composition of respective components of aluminum alloy materials in the embodiments of the present disclosure.
Zn:
Zn coexists with Mg to precipitate a η′ phase, and provides an effect of improving strength. The range of Zn content is more than 6.5% and 8.5% or less. If the Zn content is 6.5% or less, a precipitation amount of the η′ phase is reduced, thus decreasing the strength improving effect. In contrast, if the Zn content is more than 8.5%, hot workability is reduced to thereby decrease productivity. A preferred range of the Zn content is 7.0% or more and 8.0% or less.
Mg:
Mg coexists with Zn to precipitate a η′ phase, and provides the effect of improving strength. The range of Mg content is 0.5% or more and 1.5% or less. In particular, by regulating the upper limit of the Mg content to 1.5% or less, it is possible to inhibit precipitation of compounds on a grain boundary (a crystal grain boundary, a sub-grain boundary, or the like), while obtaining the strength improving effect. This makes it possible to reduce, at the time of surface treatment such as anodization, an amount of the compounds that have precipitated on the grain boundary to be etched at pretreatment, to thereby inhibit generation of scale-like patterns on the surface-treated surface.
If the Mg content is less than 0.5%, a precipitation amount of the η′ phase is reduced, thus decreasing the strength improving effect. In contrast, if the Mg content is more than 1.5%, coarse compounds are likely to be generated on the grain boundary, thus increasing an amount of the compounds to be etched at pretreatment of surface treatment such as anodization. Therefore, scale-like patterns are generated on the surface-treated surface, resulting in poor surface quality. To obtain a good surface quality and a higher strength, the Mg content is preferably 1.0% or more and 1.3% or less.
Cu:
Cu may get mixed in when a recycled material is used as a raw material for an aluminum alloy material. In a 7000-series aluminum alloy, inclusion of Cu contributes to improvement in strength, whereas change in color tone or the like occurs, such as yellowing of the color tone of the surface caused by surface treatment such as anodization. Such change in color tone may cause poor surface quality. Thus, when an emphasis is particularly placed on the color tone of the surface-treated surface, the upper limit of Cu content needs to be regulated. Regulation of the upper limit of the Cu content to 0.10% or less makes it possible to reduce the above-described poor surface quality. The Cu content is preferably 0.08% or less.
Fe, Si, Mn, and Cr:
Fe and Si may get mixed in as impurities of aluminum metal. Mn and Cr may get mixed in when a recycled material is used as a raw material for an aluminum alloy material. Of the above-described four components, Fe, Si, and Mn have an effect of inhibiting recrystallization by forming Al—Mn-based, Al—Mn—Fe-based, and/or Al—Mn—Fe—Si-based intermetallic compounds in combination with Al. Cr has an effect of inhibiting recrystallization by forming Al—Cr-based intermetallic compounds in combination with Al. Thus, inclusion of the above-described four components results in inhibiting formation of a recrystallized structure, and instead results in formation of a fibrous structure.
That is, excessive inclusion of the above-described four components results in formation of the fibrous structure and, in combination with the size and distribution of the compounds, streak patterns are generated on the surface subjected to surface treatment such as anodization, leading to poor surface quality. Thus, by regulating Fe content to 0.30% or less, Si content to 0.30% or less, Mn content to less than 0.05%, and Cr content to less than 0.05%, formation of the fibrous structure is inhibited, and the above-described poor surface quality, specifically generation of the streak patterns, can thereby be inhibited.
Zr:
Zr is added to obtain a fine and uniform recrystallized structure. The range of Zr content is 0.05% or more and 0.10% or less. Zr forms fine Al—Zr-based compounds in combination with Al. In the process of producing the aluminum alloy material, the crystal structure of the Al—Zr-based compounds changes depending on the temperature at which the ingot is subjected to homogenizing treatment. If the temperature in the homogenizing treatment is 540° C. or lower, a metastable phase is formed which has an L1₂structure commensurate with the matrix, thus inhibiting recrystallization in the structure subjected to hot working and readily leading to formation of a fibrous structure. In contrast, if the homogenizing treatment is performed at a temperature higher than 540° C. and 580° C. or lower, the Al—Zr-based compounds change into an equilibrium phase having a D0₂₃structure. This results in formation of an equigranular recrystallized structure, not a fibrous structure, after hot working, and also inhibits recrystallized grains from coarsening by blocking movement of the crystal grain boundary.
If the Zr content is less than 0.05%, the effect of inhibiting the recrystallized grains from coarsening is less likely to be obtained, resulting in formation of a nonuniform metallographic structure in which the recrystallized grains have partially coarsened. This causes a problem that mottled patterns are visually confirmed on the surface subjected to surface treatment such as anodization, or other problem, and results in poor surface quality. On the other hand, if the Zr content is more than 0.10%, the Al—Zr-based compounds are distributed more densely; thus, recrystallization is inhibited to form a fibrous structure. This causes generation of streak patterns on the surface-treated surface, and results in poor surface quality.
Ti:
Ti is added to seek micronization of crystal grains in the ingot. The range of Ti content is 0.001% or more and 0.05% or less. If the Ti content is less than 0.001%, an effect of micronizing the crystal grains is reduced. Thus, mottled patterns are likely to be generated on the surface subjected to surface treatment such as anodization, resulting in poor surface quality. On the other hand, if the Ti content is more than 0.05%, a point defect is likely to occur on the surface-treated surface due to Al—Ti-based intermetallic compounds formed in combination with Al or other cause, resulting in poor surface quality.
Other Elements:
Contained other than the above-listed elements may be basically Al and inevitable impurities. Elements to be generally added to the aluminum alloy other than the above-listed elements are allowed to be present as inevitable impurities, within a range not greatly affecting the properties of the aluminum alloy.
In the above-described aluminum alloy material, the mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less. As described above, 7000-series aluminum alloys can generally obtain higher strength by addition of Zn and Mg. However, addition of a large amount of Zn reduces hot workability, and addition of a large amount of Mg facilitates generation of coarse compounds to thereby reduce surface treatmentability and toughness. Further, general 7000-series alloys are known as having a decreased resistance to SCC when the metallographic structure thereof is a recrystallized structure. In the present disclosure, upper limits of the Zn content and the Mg content are regulated and, further, the mass ratio (Zn/Mg) is set to be within the above-specified range. As a result, the following properties can be obtained.
Specifically, by regulating the upper limits of the Zn content and the Mg content, the absolute value of the generation amount of MgZn₂compounds is made smaller. Further, by setting the mass ratio (Zn/Mg) to 16 or less, that is, by decreasing the Mg content relatively and also by regulating the mass ratio (Zn/Mg) to 16 or less, the MgZn₂compounds are inhibited from growing coarse. As a result, fine compounds are obtained and toughness can be improved.
The resistance to SCC will be discussed below. In general 7000-series aluminum alloys, an electric potential of the matrix in the vicinity of the grain boundary is nobler than that of the MgZn₂compounds precipitated on the grain boundary. Such an electric potential difference causes a local anodic dissolution under a stress corrosion environment, thus generating a crack in the vicinity of the grain boundary. This is considered to cause stress concentration and, thus, generation and progress of cracking. In the present disclosure, the mass ratio (Zn/Mg) is set to 5 or more, that is, an amount of Zn that is solid-solved in the matrix is made to be relatively large and also the mass ratio (Zn/Mg) is regulated to 5 or more. This makes it possible to alleviate the electric potential difference from the MgZn₂compounds present on the grain boundary, thus improving the resistance to SCC even in the recrystallized structure.
As described above, in the present disclosure, a high-strength aluminum alloy material, which has a good surface quality and is excellent in toughness and resistance to SCC can be obtained by regulating the upper limits of the Zn content and the Mg content and also by setting the mass ratio (Zn/Mg) to 5 or more and 16 or less.
In the above-described ranges of the Zn content and the Mg content, if the mass ratio (Zn/Mg) is less than 5, the effect of reducing and micronizing the compounds composed of Zn and Mg is decreased, and the effect of improving toughness cannot be sufficiently obtained. On the other hand, if the mass ratio (Zn/Mg) is more than 16, the Zn content becomes larger to thereby cause anodic dissolution in the vicinity of the grain boundary more likely, resulting in decrease in resistance to SCC. A preferable range of the mass ratio (Zn/Mg) is 7 or more and 16 or less.
The metallographic structure of the above-described aluminum alloy material comprises an equigranular recrystallized structure. The recrystallized structure means a metallographic structure comprising equigranular recrystallized grains. The metallographic structure can be confirmed by, for example, observing a surface or a cross-section of the aluminum alloy material with a polarizing microscope.
In the above-described aluminum alloy material, it is preferable that the recrystallized structure be such that: an average grain diameter of the crystal grains in a cross-section parallel to a direction orthogonal to a working direction of the aluminum alloy material (e.g., a direction of extrusion in the case of an extruded material) is 500 μm or less; and also such that a difference between the maximum value and the minimum value of the grain diameters of the crystal grains is less than 300 μm. In this case, the grain diameters of the crystal grains in the recrystallized structure are more uniform, and a good surface quality is thereby obtained. “Working” as in the “working direction” means extruding, rolling, or other processing. The “cross-section parallel to a direction orthogonal to a working direction” means, for example, a cross-section parallel to a width direction (a cross-section orthogonal to a thickness direction) when the working direction is assumed to be a length direction.
If the average grain diameter of the crystal grains in the recrystallized structure is more than 500 μm, the crystal grains are excessively coarse, resulting in a risk that mottled patterns caused by the coarse crystal grains may be generated on the surface subjected to surface treatment such as anodization. If the difference between the maximum value and the minimum value of the grain diameters of the crystal grains is 300 μm or more, the metallographic structure is nonuniform, resulting in a risk that a light reflection state may be nonuniform on the surface subjected to surface treatment.
The yield strength, as defined in JIS Z2241 (ISO 6892-1), of the above-described aluminum alloy material is preferably 300 MPa or more, and more preferably 350 MPa or more. This makes it possible to relatively easily obtain strength properties applicable to a lesser wall thickness for weight reduction.
Next, in a production method for the above-described aluminum alloy material, an ingot is prepared which comprises the above-described chemical components and in which the mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less, and then a homogenizing treatment is performed in which the ingot is heated at a temperature of higher than 540° C. and 580° C. or lower for 1 hour or longer and 24 hours or shorter.
If the heating temperature in the above-described homogenizing treatment is 540° C. or lower, the Al—Zr-based compounds present in the ingot form a metastable phase having an L1₂structure commensurate with the matrix, thus inhibiting recrystallization in the structure subjected to hot working and readily leading to formation of a fibrous structure. This causes generation of streak patterns on the surface subjected to surface treatment such as anodization, and results in poor surface quality. Further, a segregated layer in the ingot is not homogenized, and the structure subjected to hot working becomes a nonuniform recrystallized structure. As a result, a final surface quality becomes similarly poor. On the other hand, if the heating temperature in the above-described homogenizing treatment is higher than 580° C., the ingot may be melt locally, resulting in difficulty in practical production.
Accordingly, the heating temperature in the above-described homogenizing treatment is set to be higher than 540° C. and 580° C. or lower, whereby the Al—Zr-based compounds present in the ingot change to an equilibrium phase having a D0₂₃structure. This results in formation of an equigranular recrystallized structure, not a fibrous structure, after hot working, and also inhibits the recrystallized grains from coarsening by blocking movement of the crystal grain boundary.
If the heating time for the above-described homogenizing treatment is shorter than 1 hour, the segregated layer in the ingot is not homogenized, and the structure subjected to hot working becomes a nonuniform recrystallized structure. As a result, a final surface quality becomes poor similarly to the above. On the other hand, if the heating time for the above-described homogenizing treatment exceeds 24 hours, the segregated layer in the ingot is sufficiently homogenized; thus, no further effect can be expected. Accordingly, the heating time for the above-described homogenizing treatment is set to 1 hour or longer and 24 hours or shorter.
The above-described aluminum alloy material includes, for example, an extruded material, a plate material, and so on made of aluminum alloy. The present disclosure can be applied to various aluminum alloy materials and production methods therefor.

EXAMPLES

Example 1

Examples of the aluminum alloy material of the present disclosure will be described through comparison with comparative examples, with reference to Table 1 and Table 2. The below-described examples show one embodiment of the present disclosure, and the present disclosure is not limited to these.
As shown in Table 1 and Table 2, a plurality of specimens of the aluminum alloy material (examples: Specimen 1 to Specimen 23, comparative examples: Specimen 24 to Specimen 38) containing different chemical components were prepared under the same production conditions, and various evaluations were conducted on each specimen. A preparation method and various evaluation methods for the specimens will be described below.
<Method for Preparing Specimen>
A cylindrical ingot (billet) having a diameter of 90 mm containing chemical components shown in Table 1 is forged by semicontinuous casting. Then, a homogenizing treatment is performed in which the ingot is heated at 560° C. for 12 hours. The heating temperature in the homogenizing treatment may be higher than 540° C. and 580° C. or lower. Subsequently, the ingot is subjected to hot extrusion with the temperature of the ingot maintained at 520° C. In this way, an extruded material having a width of 150 mm and a thickness of 10 mm is obtained.
Next, a quenching treatment is performed in which the extruded material subjected to hot extrusion is cooled to 100° C. at a cooling rate of 1500° C./min. Then, after the quenched extruded material is cooled to room temperature, an artificial aging treatment is performed in which the extruded material is heated at 140° C. for 12 hours. In this way, a specimen of the aluminum alloy material (extruded material) is obtained.
<Method for Evaluating Mechanical Properties>
A test piece is prepared from the specimen by a method based on JIS Z2241 (ISO 6892-1), and a tensile strength, a yield strength, and an elongation of the test piece are measured. The test piece having a yield strength of 300 MPa or more is determined to be acceptable. The criterion for determining the yield strength is just an example.
As for a bending test, as shown in FIG. 1, a test piece 10 having a thickness of 10 mm, a width of 10 mm, and a length of 120 mm is prepared from a width-direction central portion of the specimen, and an amount Δ of bending deformation of the test piece 10 is measured by a three-point bending test. Specifically, a jig comprising a base 11 and two supporting portions 12 is prepared, and the test piece 10 is left at rest on the two supporting portions 12. At this time, the two supporting portions 12 each support the test piece 10 at a position 10 mm from the corresponding end of the test piece 10, so that a distance between supporting points becomes 100 mm. Then, a downward load in a direction orthogonal to the width direction of the specimen is applied to the specimen by an indenter 13, the dimension of which at a leading end surface is 10 mm×10 mm. Here, if the amount Δ of bending deformation after application of the load of 4000 kgf for 10 seconds is more than 4 mm, the test piece 10 is determined to be unacceptable “X”; if more than 2 mm and 4 mm or less, the test piece 10 is determined to be acceptable “◯”; and if 2 mm or less, the test piece 10 is determined to be desirable “⊚”.
<Method for Evaluating Toughness>
A Charpy impact test is performed by a method based on JIS Z2242. Specifically, a test piece having a thickness of 7.5 mm, a width of 10 mm, and a length of 55 mm is prepared. A longitudinal direction of the test piece is parallel to a direction of extrusion, and the test piece has a U-shaped notch having a depth of 2 mm, formed so as to be orthogonal to the direction of extrusion. The Charpy impact test is performed on the test piece, and an impact value is measured. If the impact value is 15 J/cm²or more, the test piece is determined to be acceptable, and if less than 15 J/cm², the test piece is determined to be unacceptable. The criteria for determining the impact value is just an example.
<Method for Evaluating Resistance to SCC>
An SCC test is performed by a method based on JIS Z8711. Specifically, a test piece having a C-ring shape (outside diameter: 19 mm, inside diameter: 16 mm, thickness: 8 mm) is prepared. Then, a stress of 90% of the yield strength is applied to the test piece such that a direction of application of a tensile stress at a stress-concentrated part corresponds to a direction of extrusion of the test piece. In such a state and under a temperature environment of 25° C., the test piece is immersed in salt water with the concentration of 3.5% for 10 minutes and then dried for 50 minutes. Such steps as one cycle are repeatedly performed. Thirty days later, whether a cracking is generated in the test piece is visually confirmed. If no cracking is generated, the test piece is determined to be acceptable, and if a cracking is generated, the test piece is determined to be unacceptable.
<Method for Observing Metallographic Structure>
A texture observation of the specimen is performed at a cross-section parallel to a width direction when the working direction (the direction of extrusion here) is assumed to be a length direction. In particular, a portion in the vicinity of a width-direction center of the cross-section is observed. As shown in FIG. 2, an extruded material 20 as the specimen is cut, and three cross-sections in total, that is, a cross-section at a thickness-direction central position of the extruded material 20 and cross-sections at ¼ positions from the top and the bottom in the thickness directions of the extruded material 20, are electrolytically polished. Then, a microscopic image (e.g., a photograph shown in a lower part of FIG. 2) of each cross-section at 50 to 100-fold magnification is obtained using a polarizing microscope. Subsequently, whether the metallographic structure is an equigranular recrystallized structure is confirmed from the obtained microscopic image. If the metallographic structure is fibrous, the specimen is determined to be acceptable. If the metallographic structure is nonuniform, the specimen is determined to be unacceptable. As shown in FIG. 2, a direction of observation is the thickness direction of the specimen.
Furthermore, as for the specimen whose metallographic structure is an equigranular recrystallized structure, the obtained microscopic image thereof is subjected to image analysis. Equivalent circle diameters of the crystal grains on the respective cross-sections are found, and an average grain diameter of the crystal grains on each cross-section is calculated. In addition, the greatest diameters and the smallest diameters of the crystal grains on the respective cross-sections are found, and the greatest one of the greatest diameters and the smallest one of the smallest diameters are respectively referred to as a maximum value and a minimum value. Then, a difference between the maximum value and the minimum value of the grain diameters of the crystal grains (a grain diameter difference) is calculated. If the average grain diameter of the crystal grains on each cross-section is 500 μm or less and the difference between the maximum value and the minimum value of the grain diameters of the crystal grains on all the cross-sections observed (the grain diameter difference) is less than 300 μm, the specimen is determined to be desirable.
<Method for Evaluating Surface Quality>
After a surface of the specimen is mechanically polished (buffed), the specimen is etched with an aqueous sodium hydroxide and is further desmutted. Then, the desmutted specimen is chemically polished by a phosphoric acid-nitric acid method for 1 minute at a temperature of 90° C.
Next, the chemically polished specimen is anodized at a current concentration of 150 A/m²in a 15% sulfuric acid bath to form an anodized coating having a thickness of 10 μm. Then, the anodized specimen is immersed in boiling water to perform a sealing treatment on the anodized coating. In this way, the specimen is subjected to a surface treatment (anodization).
Subsequently, the surface-treated (anodized) surface of the specimen is visually observed. First, the specimen is observed from a viewpoint vertical to a surface thereof, and the specimen having no surface defect, such as a scale-like pattern, a streak pattern, a mottled pattern, or a point defect, generated on its surface is determined to be acceptable. Further, the specimen is observed from a viewpoint at an angle of 30° with respect to its surface, and the specimen whose light reflection state on its surface is uniform is determined to be desirable.
Among the above-described surface defects, the scale-like pattern is a pattern looking like scales along a grain boundary (a pattern in which crystal grains are seen more conspicuously) generated as a result of etching the compounds precipitated on the grain boundary at pretreatment of the surface treatment, in a case where the metallographic structure is an equigranular recrystallized structure. The streak pattern is a pattern looking like a streak along a grain boundary generated as a result of etching the compounds precipitated on the grain boundary at pretreatment of the surface treatment, in a case where the metallographic structure is a fibrous structure. The mottled pattern is a pattern generated because differences in the crystal grain size make the crystal grains partially coarse or fine and such larger and smaller crystal grains look like mottles after the surface treatment. The point defect is caused when, for example, coarse compounds come off by being etched. Concave pits are formed in a position where the compounds were present, and such concave pits look like points after the surface treatment.

	TABLE 1

	Chemical Composition (mass %)	Mass Ratio

Specimen	Zn	Mg	Cu	Zr	Si	Fe	Mn	Cr	Ti	Al	(Zn/Mg)

1	6.52	1.11	0.01	0.07	0.11	0.08	0.02	0.02	0.03	bal.	5.87
2	8.47	0.98	0.04	0.06	0.08	0.15	0.01	0.03	0.01	bal.	8.64
3	7.05	1.11	0.08	0.07	0.18	0.23	0.03	0.02	0.008	bal.	6.35
4	7.99	1.21	0.01	0.06	0.14	0.21	0.02	0.03	0.03	bal.	6.60
5	7.13	0.52	0.02	0.08	0.23	0.09	0.02	0.03	0.02	bal.	13.71
6	8.11	1.48	0.05	0.07	0.15	0.22	0.02	0.02	0.01	bal.	5.48
7	7.88	1.05	0.02	0.08	0.12	0.20	0.01	0.03	0.03	bal.	7.50
8	8.12	1.29	0.06	0.06	0.13	0.08	0.03	0.03	0.01	bal.	6.29
9	6.94	0.99	0.09	0.07	0.09	0.09	0.02	0.02	0.03	bal.	7.01
10	7.31	1.19	0.03	0.05	0.11	0.18	0.02	0.03	0.008	bal.	6.14
11	7.22	1.35	0.04	0.09	0.17	0.23	0.03	0.03	0.02	bal.	5.35
12	6.99	1.25	0.03	0.08	0.26	0.11	0.02	0.03	0.01	bal.	5.59
13	7.34	0.82	0.07	0.06	0.13	0.25	0.02	0.03	0.009	bal.	8.95
14	6.98	0.94	0.02	0.06	0.22	0.09	0.04	0.02	0.009	bal.	7.43
15	6.81	1.11	0.07	0.08	0.18	0.18	0.03	0.04	0.03	bal.	6.14
16	7.99	1.31	0.07	0.07	0.21	0.20	0.01	0.02	0.001	bal.	6.10
17	8.10	0.98	0.01	0.06	0.16	0.08	0.02	0.01	0.04	bal.	8.27
18	7.29	1.44	0.05	0.08	0.14	0.23	0.03	0.02	0.02	bal.	5.06
19	8.42	0.53	0.08	0.07	0.20	0.15	0.02	0.03	0.01	bal.	15.89
20	7.62	1.07	0.03	0.06	0.12	0.21	0.02	0.02	0.009	bal.	7.12
21	8.21	0.58	0.08	0.07	0.09	0.18	0.01	0.03	0.01	bal.	14.16
22	7.88	1.32	0.05	0.06	0.15	0.09	0.03	0.02	0.03	bal.	5.97
23	7.05	1.15	0.08	0.06	0.11	0.17	0.02	0.01	0.009	bal.	6.13
24	6.45	1.09	0.03	0.08	0.08	0.14	0.03	0.01	0.03	bal.	5.92
25	8.56	1.23	0.06	0.08	0.23	0.18	0.01	0.02	0.008	bal.	6.96
26	6.77	0.47	0.07	0.06	0.09	0.16	0.02	0.01	0.02	bal.	14.40
27	7.87	1.54	0.05	0.07	0.17	0.18	0.01	0.02	0.01	bal.	5.11
28	8.11	1.02	0.11	0.08	0.18	0.13	0.03	0.03	0.03	bal.	7.95
29	7.88	1.11	0.02	0.04	0.21	0.08	0.03	0.02	0.03	bal.	7.10
30	6.89	1.21	0.01	0.12	0.16	0.15	0.03	0.01	0.02	bal.	5.69
31	7.11	1.19	0.04	0.07	0.32	0.21	0.01	0.01	0.01	bal.	5.97
32	7.96	0.88	0.07	0.08	0.22	0.33	0.02	0.03	0.03	bal.	9.05
33	6.96	1.12	0.07	0.06	0.18	0.18	0.05	0.02	0.01	bal.	6.21
34	7.33	0.91	0.05	0.07	0.14	0.14	0.02	0.05	0.009	bal.	8.05
35	8.01	0.92	0.03	0.06	0.20	0.08	0.03	0.03	0.0008	bal.	8.71
36	7.77	1.22	0.05	0.08	0.13	0.21	0.02	0.01	0.07	bal.	6.37
37	6.97	1.41	0.03	0.08	0.08	0.12	0.03	0.02	0.01	bal.	4.94
38	8.44	0.52	0.06	0.07	0.23	0.08	0.02	0.01	0.02	bal.	16.23

TABLE 2

Resistance	Metallographic Structure Observation	Surface Quality

Mechanical Properties

Toughness

to SCC

Average

Grain

Defect

	Tensil	Yield	Elon-		Impact	Stress	Metallo-	grain	diameter	after	Light
	strength	strength	gation	Bending	value	corrosion	graphic	diameter	difference	surface	reflection
Specimen	(MPa)	(MPa)	(%)	test	(J/cm²)	cracking	structure	(μm)	(μm)	treatment	state

1	344	319	19	◯	19.2	None	Equigranular	356	269	None	Uniform
2	383	355	16	⊚	17.6	None	Equigranular	401	245	None	Uniform
3	388	362	16	⊚	17.1	None	Equigranular	365	223	None	Uniform
4	403	378	15	⊚	16.9	None	Equigranular	297	262	None	Uniform
5	338	311	20	◯	19.5	None	Equigranular	321	278	None	Uniform
6	412	381	14	⊚	16.9	None	Equigranular	332	265	None	Uniform
7	396	367	16	⊚	17.0	None	Equigranular	342	281	None	Uniform
8	404	376	15	⊚	16.8	None	Equigranular	358	276	None	Uniform
9	371	345	17	◯	17.9	None	Equigranular	367	254	None	Uniform
10	379	354	17	⊚	17.5	None	Equigranular	376	243	None	Uniform
11	386	359	16	⊚	17.3	None	Equigranular	234	259	None	Uniform
12	391	362	16	⊚	17.0	None	Equigranular	298	228	None	Uniform
13	370	339	18	◯	18.4	None	Equigranular	432	252	None	Uniform
14	353	324	19	◯	18.7	None	Equigranular	339	261	None	Uniform
15	376	349	16	◯	18.1	None	Equigranular	382	281	None	Uniform
16	407	379	15	⊚	16.7	None	Equigranular	412	237	None	Uniform
17	377	348	17	◯	18.0	None	Equigranular	399	231	None	Uniform
18	397	365	16	⊚	16.6	None	Equigranular	288	256	None	Uniform
19	339	314	21	◯	22.8	None	Equigranular	340	267	None	Uniform
20	368	343	17	◯	17.3	None	Equigranular	383	283	None	Uniform
21	338	310	22	◯	24.4	None	Equigranular	299	255	None	Uniform
22	401	372	15	⊚	17.1	None	Equigranular	305	271	None	Uniform
23	363	336	20	◯	18.6	None	Equigranular	389	312	None	Partially
											nonuniform
24	303	272	24	X	20.9	None	Equigranular	410	234	None	Uniform
25	—	—	—	—	—	—	—	—	—	—	—
26	292	264	25	X	22.1	None	Equigranular	399	242	None	Uniform
27	402	381	15	⊚	16.8	None	Equigranular	421	269	Scale-	Nonuniform
										like
										patterns
28	377	352	17	⊚	17.8	None	Equigranular	449	255	Yellowish	Uniform
29	401	372	16	⊚	17.0	None	Coarse and	—	—	Mottled	Nonuniform
							nonuniform			patterns
30	379	353	17	⊚	17.9	None	Fibrous	—	—	Streak	Nonuniform
										patterns
31	365	339	18	◯	18.5	None	Fibrous	—	—	Streak	Nonuniform
										patterns
32	375	348	17	◯	18.2	None	Fibrous	—	—	Streak	Nonuniform
										patterns
33	386	357	16	⊚	17.7	None	Fibrous	—	—	Streak	Nonuniform
										patterns
34	371	343	18	◯	18.1	None	Fibrous	—	—	Streak	Nonuniform
										patterns
35	376	345	18	◯	18.4	None	Equigranular	—	—	Mottled	Nonuniform
										patterns
36	379	351	17	⊚	17.6	None	Equigranular	290	267	Point	Nonuniform
										defect
37	365	339	19	◯	14.1	None	Equigranular	391	246	None	Uniform
38	336	311	21	◯	25.3	Cracking	Equigranular	412	268	None	Uniform
						generated

Evaluation results of the respective specimen are shown in Table 2. As for the specimens that were not determined to be acceptable (that were determined to be unacceptable), evaluation results or the like thereof are indicated with underlines applied thereto in Table 2.
As can be seen from Table 2, Specimens 1 to 23, whose metallographic structures were equigranular recrystallized structures, were determined to be acceptable or to be acceptable and also desirable in all evaluation items, that is, in terms of the mechanical properties (the yield strength and the bending test), the toughness (the impact value), the resistance to SCC (the stress corrosion cracking), the metallographic structure observation (the metallographic structure, the average grain diameter, and the grain diameter difference), and the surface quality (the defect after surface treatment, and the light reflection state). In sum, Specimens 1 to 23 exhibited excellent properties in terms of the strength, the toughness, and the surface quality, and also exhibited excellent properties in terms of the resistance to SCC.
As for Specimen 23, although no defect after surface treatment was observed, the light reflection state was partially nonuniform because the grain diameter difference among the crystal grains (the difference between the maximum value and the minimum value) was slightly large. However, such partial nonuniformity was not bad enough to be a problem in the surface quality. Specimen 23 was determined to be acceptable or to be acceptable and also desirable in all of the evaluation items other than the light reflection state. In sum, Specimen 23 exhibited excellent properties in terms of the strength, the toughness, and the surface quality, and also exhibited excellent properties in terms of the resistance to SCC.
Specimen 24, whose Zn content was too low, was determined to be unacceptable in terms of the yield strength because the strength improving effect was not sufficiently obtained. On the other hand, Specimen 25, whose Zn content was too high, was poor in the hot workability, resulting in difficulty in performing hot extrusion with actually used facilities.
Specimen 26, whose Mg content was too low, was determined to be unacceptable in terms of the yield strength because the strength improving effect was not sufficiently obtained. On the other hand, Specimen 27, whose Mg content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because coarse compounds were present on the grain boundary to generate scale-like patterns on the anodized surface.
Specimen 28, whose Cu content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because its anodized surface was yellowish in color tone.
Specimen 29, whose Zr content was too low, was determined to be unacceptable due to appearance of the defect after surface treatment because a coarse and nonuniform recrystallized structure was formed to generate mottled patterns on the anodized surface. On the other hand, Specimen 30, whose Zr content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because a fibrous structure was formed to generate streak patterns on the anodized surface.
Specimen 31, whose Si content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because a fibrous structure was formed to generate streak patterns on the anodized surface.
Specimen 32, whose Fe content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because a fibrous structure was formed to generate streak patterns on the anodized surface.
Specimen 33, whose Mn content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because a fibrous structure was formed to generate streak patterns on the anodized surface.
Specimen 34, whose Cr content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because a fibrous structure was formed to generate streak patterns on the anodized surface.
Specimen 35, whose Ti content was too low, was determined to be unacceptable due to appearance of the defect after surface treatment because the structure of the ingot was coarse and the metallographic structure subjected to hot extrusion was nonuniform to generate mottled patterns on the anodized surface. On the other hand, Specimen 36, whose Ti content was too high, was determined to be unacceptable due to appearance of the defect after surface treatment because coarse intermetallic compounds were generated to cause a point defect on the anodized surface.
Specimens 27 and 29 to 36, which were determined to be unacceptable in terms of the defect after surface treatment, were nonuniform in the light reflection state.
Specimen 37, whose mass ratio (Zn/Mg) was too low, was determined to be unacceptable in terms of the impact value (toughness) because the impact value was less than 15. On the other hand, Specimen 38, whose mass ratio (Zn/Mg) was too high, was determined to be unacceptable in terms of the stress corrosion cracking (resistance to SCC) because a stress corrosion cracking was generated in the test of resistance to SCC.

Example 2

Examples in the production method for the above-described aluminum alloy material will be described through comparison with comparative examples, with reference to Table 3 and Table 4. The below-described examples show one embodiment of the present disclosure, and the present disclosure is not limited to these.
In this example, as shown in Table 3, a plurality of specimens (examples: Specimens A to H, comparative examples: Specimens I to N) of the aluminum alloy material were prepared under different production conditions, and various evaluations were conducted on each specimen. The chemical components of the aluminum alloy material were similar to those of Specimen 10 or Specimen 11 (see Table 1) of Example 1 described above. A preparation method for the specimens will be described below. Various evaluation methods were similar to those in the above-described Example 1.
<Method for Preparing Specimen>
A cylindrical ingot (billet) having a diameter of 90 mm containing chemical components similar to those of Specimen 10 or Specimen 11 (see Table 1) of the above-described Example 1 is forged by semicontinuous casting. Then, a homogenizing treatment is performed in which the ingot is heated at a temperature and for a period of time shown in Table 3. Subsequently, the ingot is subjected to hot extrusion with the temperature of the ingot maintained at 520° C. In this way, an extruded material having a width of 150 mm and a thickness of 10 mm is obtained.
Next, a quenching treatment is performed in which the extruded material subjected to hot extrusion is cooled to 100° C. at a cooling rate of 1500° C./min. Then, the quenched extruded material is cooled to room temperature, and an artificial aging treatment is performed in which the extruded material is heated at a temperature of 140° C. for 12 hours. In this way, the specimen of the aluminum alloy material (extruded material) is obtained.

	TABLE 3

	Homogenizing
	treatment

	Alloy	Temperature	Time
Specimen	(Specimen No.)	(° C.)	(h)

A	10	542	10
B	11
C	10	576	8
D	11
E	10	559	1
F	11
G	10	565	24
H	11
I	10	535	10
J	11
K	10	584	8
L	11
M	10	560	0.5
N	11

TABLE 4

Resistance	Metallographic Structure Observation	Surface Quality

Mechanical Properties

Toughness

to SCC

Average

Grain

Defect

	Tensil	Yield	Elon-		Impact	Stress	Metallo-	grain	diameter	after	Light
	strength	strength	gation	Bending	value	corrosion	graphic	diameter	difference	surface	reflection
Specimen	(MPa)	(MPa)	(%)	test	(J/cm²)	cracking	structure	(μm)	(μm)	treatment	state

A	381	356	17	⊚	17.5	None	Equigranular	379	244	None	Uniform
B	389	362	16	⊚	17.3	None	Equigranular	343	261	None	Uniform
C	379	355	16	⊚	17.4	None	Equigranular	401	233	None	Uniform
D	385	360	17	⊚	17.2	None	Equigranular	299	259	None	Uniform
E	382	356	17	⊚	17.6	None	Equigranular	334	229	None	Uniform
F	388	361	17	⊚	17.1	None	Equigranular	420	282	None	Uniform
G	381	353	16	⊚	17.3	None	Equigranular	405	245	None	Uniform
H	387	364	16	⊚	17.5	None	Equigranular	329	229	None	Uniform
I	380	353	17	⊚	17.7	None	Fibrous	—	—	Streak	Nonuniform
										patterns
J	385	358	16	⊚	17.6	None	Fibrous	—	—	Streak	Nonuniform
										patterns
K	—	—	—	—	—	—	—	—	—	—	—
L	—	—	—	—	—	—	—	—	—	—	—
M	379	354	17	⊚	17.4	None	Coarse and	—	—	Mottled	Nonuniform
							nonuniform			patterns
N	386	359	16	⊚	17.2	None	Coarse and	—	—	Mottled	Nonuniform
							nonuniform			patterns

As can be seen from Table 4, Specimens A to H, whose metallographic structures were equigranular recrystallized structures, were determined to be acceptable or to be desirable in all evaluation items, that is, in terms of the mechanical properties (the yield strength and the bending test), the toughness (the impact value), the resistance to SCC (the stress corrosion cracking), the metallographic structure observation (the metallographic structure, the average grain diameter, and the grain diameter difference), and the surface quality (the defect after surface treatment, and the light reflection state). In sum, Specimens A to H exhibited excellent properties in terms of the strength, the toughness, and the surface quality, and also exhibited excellent properties in terms of the resistance to SCC.
In Specimens I and J, which were each homogenized at too low a temperature, Al—Zr-based compounds having an L1₂structure were present and fibrous structures were formed. Thus, Specimens I and J were determined to be unacceptable due to appearance of the defect after surface treatment because streak patterns were generated on the anodized surface.
In Specimens K and L, which were each homogenized at too high a temperature, local melting occurred to make it difficult to perform hot extrusion in the actually used facilities.
Specimens M and N, which were each homogenized for too short a time, were determined to be unacceptable due to appearance of the defect after surface treatment because their metallographic structures after hot extrusion were nonuniform to generate mottled patterns on the anodized surface.
In the above-described Examples 1 and 2, the extruded materials were evaluated as one embodiment of the aluminum alloy material of the present disclosure. However, results similar to those of the above-described Examples 1 and 2 are obtained also in a case of other embodiments, such as plate materials, for example.

Claims

1. An aluminum alloy material having a chemical composition comprising:

Zn: more than 6.5% (mass %, same applies hereafter) and 8.5% or less;

Mg: 0.5% or more and 1.5% or less;

Cu: 0.10% or less;

Fe: 0.30% or less;

Si: 0.30% or less;

Mn: less than 0.05%;

Cr: less than 0.05%;

Zr: 0.05% or more and 0.10% or less; and

Ti: 0.001% or more and 0.05% or less,

a balance comprising Al and inevitable impurities,

wherein a mass ratio of Zn to Mg (Zn/Mg) is 5 or more and 16 or less, and

wherein a metallographic structure comprises a recrystallized structure, which is equigranular.

2. The aluminum alloy material according to claim 1,

wherein the recrystallized structure comprises crystal grains having an average grain diameter of 500 μm or less in a cross-section parallel to a direction orthogonal to a working direction, and a difference between a maximum value and a minimum value of grain diameters of the crystal grains is less than 300 μm.

3. A production method for an aluminum alloy material, a metallographic structure of which comprises an equigranular recrystallized structure, the method comprising:

preparing an ingot having a chemical composition comprising: