JP2012246555A - 7000-based aluminum alloy extruded material for case-type - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a 7000-based aluminum alloy extruded material with which after working the 7000-based aluminum alloy material into a case-type with a cutting-work (shaving), when an anodic oxidation treatment is applied on the surface, the generation of the color-irregularity on a film is prevented to generate the silver-color film without showing yellowish.SOLUTION: The 7000-based aluminum alloy extruded material is composed by mass of 5.5-7.5% Zn, 1.2-2.2% Mg, 0.01-0.1% Ti, and regulated to ≤0.05% the total of Mn, Cr and Zr and ≤0.10% Cu and the balance Al with inevitable impurities, and has an equiaxial granular structure in the cross-sectional surface. The extruded material is worked into the case-type with cutting work and on the surface, the anodic oxidation treatment is applied. The case-type is suitable for a laptop personal computer, cellular phone, digital camera, etc.

Description

  The present invention relates to a 7000 series aluminum alloy extruded material used for casings of notebook personal computers, mobile phones, digital cameras and the like.

In general, the materials used in the case of notebook computers, mobile phones, digital cameras, etc. have good appearance and high strength to reduce weight or prevent deformation of the case when dropped. Is needed. From such a point of view, aluminum alloys are frequently used in these cases, and usually the surface is often used after anodizing.
The following Patent Documents 1 to 9 describe that an aluminum alloy sheet material or extruded material belonging to various alloy systems is subjected to anodizing treatment on the surface and used for various applications.

JP 2009-209426 A JP 2006-52436 A JP 2006-26938 A JP 2004-137517 A JP 2001-115227 A JP-A-10-226857 Japanese Patent Laid-Open No. 9-184095 Japanese Patent Laid-Open No. 9-143602 JP-A 63-297543

Since the 7000 series aluminum alloy has high strength among various alloy series, it is suitable for reducing the weight and strength of the member. For example, Patent Document 9 describes that a 7000 series aluminum alloy extruded material having a recrystallized surface layer and a fibrous structure inside has high strength and excellent alumite (anodizing) color tone. Yes.
On the other hand, for cases such as laptop computers, mobile phones, digital cameras, etc., especially when good appearance and precision and robustness of the product are required, the thick plate-like aluminum alloy is cut and processed. Molded by surface cutting. However, when such a molding method is applied to the above-mentioned 7000 series aluminum alloy extruded material and the surface of the molded casing is anodized, a streak that is visually observed on part or all of the surface. There is a problem that the appearance is deteriorated such as uneven color (glossy shade) or yellowing of the anodized film.

  Accordingly, the present invention prevents the occurrence of uneven color in the coating when the surface is subjected to anodization after the 7000 series aluminum alloy extruded material is processed into a casing by cutting (machining), and The object is to obtain a silver-colored film with reduced yellowness.

  A 7000 series aluminum alloy extruded material for a casing according to the present invention is formed into a casing by cutting, and then an anodized film is formed on the surface. Zn: 5.5 to 7.5 Mass%, Mg: 1.2-2.2 mass%, Ti: 0.01-0.1% by mass, Mn, Cr, Zr total 0.05 mass% or less, Cu 0.10 mass %, And the remainder is made of equiaxed crystals having a recrystallized structure.

Using the aluminum alloy extrudate, after processing into a housing by cutting (cutting out), when anodizing the surface, it can prevent color unevenness from occurring in the anodized film, and A silver-colored anodized film that is not yellowish or suppresses yellowishness is obtained.
The aluminum alloy extruded material, by performing T5 or T6 treatment, the tensile strength of 400~600N / mm ^2, it is possible to yield strength to obtain a high strength of 300~500N / mm ^2.

It is an optical micrograph of the microstructure of the surface layer part, 1 / 4t part, and 1 / 2t part of the cross section perpendicular to the sheet thickness direction parallel to the extrusion direction of the specimen of Example A2.

The alloy composition and structure of the 7000 series aluminum alloy extruded material according to the present invention will be described below.
(Zn)
Zn needs to be added in an amount of 5.5% by mass or more in order to ensure the strength after the heat treatment. However, if it exceeds 7.5%, the extrudability decreases. Therefore, Zn content shall be 5.5-7.5 mass%.
(Mg)
Mg needs to be added in an amount of 1.2% by mass or more in order to ensure the strength after heat treatment together with Zn. However, if it exceeds 2.2% by mass, the extrudability decreases. Therefore, Mg content shall be 1.2-2.2 mass%.
(Ti)
Ti is added to refine the ingot structure. If the Ti content is less than 0.01% by mass, the effect of miniaturization is not sufficient. If the Ti content is more than 0.1% by mass, the compound may be saturated and a giant compound may be crystallized. Therefore, the Ti content is set to 0.01% to 0.1% by mass.

(Mn, Cr, Zr)
In general, in order to improve the SCC (stress corrosion cracking) resistance and strength in a 7000 series aluminum alloy extruded material, transition elements such as Mn, Cr, and Zr are added in a total amount of about 0.15 to 0.5% by mass, and the extrusion direction. An elongated fibrous structure is formed. However, when the surface of the extruded material whose entire cross section is a fibrous structure is anodized, there is a problem that streaky color unevenness occurs in the anodized film. On the other hand, in Patent Document 9, a predetermined amount of Cr and Zr is added to the alloy, and a special heat treatment is applied before and after extrusion, so that only the surface of the extruded material is recrystallized. Unevenness is prevented. However, when the casing is formed by cutting (cutting out) such an extruded material, the fibrous structure is exposed on a part or all of the surface of the casing, and thus the casing is anodized. In some cases, as described above, color unevenness also occurs in the anodized film.

Therefore, in the present invention, the total content of Mn, Cr, and Zr contained as inevitable impurities is regulated to 0.05% by mass or less, and the entire cross section of the extruded material is made of equiaxed crystals that are recrystallized structures. . If the entire cross section has an equiaxed crystal structure, even when the casing is formed by cutting the extruded material, the entire surface of the casing remains the equiaxed crystal structure. When the anodizing treatment is performed, it is possible to prevent uneven color from occurring in the anodized film.
In the present invention, the equiaxed crystal structure is a structure composed of crystal grains having an average aspect ratio of 4.0 or less. The average aspect ratio is measured in the extrusion direction in a cross section parallel to the extrusion direction and perpendicular to the plate thickness direction. When the average crystal grain size measured is a and the average crystal grain size measured in the thickness direction is b, it is expressed as a / b or b / a. For the measurement of the average aspect ratio, for example, a plurality of regions representing a cross section such as a surface layer portion, a 1/4 plate thickness portion, and a 1/2 plate thickness portion are selected, and the average aspect ratio is 4.0 or less in all the regions. In this case, it is defined that the entire cross section has an equiaxed crystal structure. The average aspect ratio is desirably 3.0 or less in all regions. The average crystal grain size may be measured according to the cutting method of JISH0501.

(Cu)
Cu is contained in the aluminum alloy as an inevitable impurity, or is added as necessary to increase the strength. In any case, Cu is restricted to 0.10% by mass or less. As the Cu content increases, the anodized film becomes yellowish. Therefore, when a silver-colored anodized film with reduced yellowness is desired as in the present invention, the Cu content is 0.10% by mass. It is necessary to regulate the following. In order to obtain a silver-colored anodized film that is not yellowish and nearly white, the Cu content is preferably 0.05% by mass or less.
(Other)
Other unavoidable impurity elements are not affected by the color tone and color unevenness of the anodized film as long as they are contained in a commonly used 7000 series aluminum alloy.

(Anodized film)
The color tone of the anodized film formed on the housing according to the present invention is a brightness index L * (L * value) and a chromaticness index (a * value, b * value) according to the Hunter color difference formula described in JISZ8730. (For example, see Patent Documents 3 and 4). The L * value has an upper limit of 100, and the higher the value, the brighter the tone. The a * value becomes reddish as the numerical value is higher on the positive side with respect to 0, and becomes greener as the numerical value is higher on the negative side. The b * value becomes yellowish as the numerical value is higher on the positive side with respect to 0, and becomes bluer as the numerical value is higher on the negative side.
In the casing according to the present invention, the color tone of the anodized film is targeted to be a silver color that is bright and close to white. When expressed as L *, a *, and b *, 65 <L * <95, − 1.5 <a * <1.5 and −1.5 <b * <1.5. This color tone can be obtained with a 7000 series aluminum alloy extruded material having the composition and crystal structure of the present invention. The b * value related to yellowness is more preferably b * <1.0.

  The concentration of the silver color of the anodized film varies depending on the film thickness, and the film thickness is preferably 3 to 30 μm in the casing according to the present invention. The thinner the anodized film, the lower the cost. However, a sufficient silver color may not be obtained, and if it is less than 3 μm, it becomes difficult to stably form the anodized film. On the other hand, if the thickness of the anodized film exceeds 30 μm, the cost increases, cracks are likely to occur in the deformed film, and the silver color of the anodized film becomes too dark, resulting in the bright white color described above. A silver color close to is not obtained.

Next, examples of the present invention will be described.
First, an aluminum alloy ingot having the composition shown in Table 1 below was melted by an ordinary method. Cu was added to A4, A5, B2, and B3, and Mn, Cr, and Zr were added to B4. Even in other compositions, Cu, Mn, Cr, and Zr are contained in trace amounts as inevitable impurities, and Fe and Si are also inevitable impurities. Each aluminum alloy ingot was homogenized at 470 ° C x 4h, and extrusion was performed under conditions of an extrusion temperature of 470 ° C and an extrusion speed of 10m / min. It was cooled at about 100 ° C./min) to obtain an extruded material (extruded plate) having a thickness of 10 mm × width of 90 mm. Thereafter, high temperature aging treatment was performed under the conditions of 90 ° C. × 3 h → 140 ° C. × 8 h.

The microstructure was observed using this as a test material, and the average aspect ratio of the crystal grains was measured as follows. The results are shown in Table 1.
(Average aspect ratio)
As the area for measuring the average aspect ratio, the surface layer part, 1 / 4t part and 1 / 2t part of the cross section parallel to the extrusion direction and perpendicular to the sheet thickness direction are selected, and the extrusion is performed in accordance with the cutting method of JISH0501 in each area. When the average crystal grain size in the direction and the plate thickness direction is measured, the average crystal grain size in the extrusion direction is a, and the average crystal grain size in the plate thickness direction is b, the average aspect ratio is a / b or b / a The larger value was used. The measurement range of the average aspect ratio is 600 μm in the plate thickness direction from the outermost surface in the surface layer portion, 900 μm in the extrusion direction, and the line thickness of 1/4 t and 1/2 t in the 1/4 t portion and 1/2 t portion, respectively. A range of 350 μm inside and outside the plate thickness direction × 450 μm in the extrusion direction was selected. The microstructure of the specimen A2 is shown in FIG. Note that the same microstructure as that of the cross-section is obtained on the surface of the test material, which is parallel to the plate surface in the surface layer portion, 1 / 4t portion, and 1 / 2t portion.

Next, the obtained specimens were subjected to a tensile test and an anodizing treatment in the following manner, and the color tone of the anodized film was evaluated and the color unevenness was evaluated. The results are shown in Table 2.
(Tensile test)
Tensile test specimens were taken from a JIS No. 5 specimen parallel to the extrusion direction and subjected to a tensile test, and the tensile strength (σB) and proof stress (σ0.2) were measured. A tensile strength (σB) of 400 to 600 N / mm ² and a proof stress (σ0.2) of 300 to 500 N / mm ² were determined to be acceptable.
(Anodizing treatment)
The surface of the extruded material was milled by 3 mm, and as a pretreatment, it was immersed in an aqueous sodium hydroxide solution having a concentration of 5% and a liquid temperature of 60 ° C. for 1 minute, and then immersed in an aqueous nitric acid solution having a concentration of 30% and a liquid temperature of 20 ° C. Then, the surface was dipped in a sulfuric acid aqueous solution having a concentration of 15% and a liquid temperature of 20 ° C., and an anodizing treatment was performed on the milled surface by applying an additional current of 2.0 A / dm 2 over 10 minutes. Then, as a sealing treatment, it was immersed in an aqueous solution of Ni acetate having a concentration of 5% and a liquid temperature of 90 ° C. for 20 minutes. The thickness of the anodized film was 8 μm.

(Measurement of color tone)
As described above, the color tone of the anodized extruded material conforms to the provisions of JISZ8730, and uses a color difference meter (CN-600d, manufactured by Konica Minolta Co., Ltd.) based on the L * a * b * color system. L * value, a * value, and b * value were measured by a method (including regular reflection light).
(Evaluation of uneven color)
The evaluation of color unevenness was visually evaluated in three stages of △△ ×. ○ indicates no color unevenness, Δ indicates slight color unevenness, × indicates color unevenness, and ○ Δ indicates pass.

  As shown in Tables 1 and 2, A1 to A4 in which the alloy composition satisfies the provisions of the present invention and the entire cross section has an equiaxed crystal structure is high in strength, and the color tone (L *, a *) of the surface oxidized film B *) is bright and close to white in the target range described above, and no color unevenness was observed. Similarly, A5 had high strength and no color unevenness was observed, but the b * value of the surface oxidation-treated film was relatively high within the target range, and the color tone was slightly yellowish. This is presumably because the Cu content was relatively high within the specified range. A6 was high in strength and the color of the surface oxidation-treated film was bright and close to white, but slight color unevenness was observed in the surface-oxidized film. This is presumably because the average aspect ratio of the equiaxed structure was relatively large within the allowable range.

On the other hand, B1, which contains less Mg and Zn than in the present invention, has low strength.
B2 and B3 having a Cu content higher than that defined in the present invention were high in strength, but the b * value of the anodized film was high and yellowish.
B4 in which the total amount of addition of Mn, Cr, and Zr is larger than that of the present invention has a noticeable fibrous structure in the cross section, a large average aspect (measurement is difficult), and occurrence of uneven color.

Claims (2)

Zn: 5.5 to 7.5% by mass, Mg: 1.2 to 2.2% by mass, Ti: 0.01 to 0.1% by mass, Mn, Cr, Zr in total 0.05 mass %, Cu is controlled to 0.10% by mass or less, the remainder is made of Al and inevitable impurities, the entire cross section is made of an equiaxed crystal structure, and the surface is anodized after being processed into a casing by cutting. A 7000 series aluminum alloy extruded material for a casing on which a film is formed. The tensile strength is 400 to 600 N / mm ² and the proof stress is 300 to 500 N / mm ² .