Aluminum alloy and method for producing aluminum alloy

Abstract

This aluminum alloy (Al alloy) contains Zn: 5.0-6.5 mass%, Mg: 2.0-3.0 mass%, Cu: 1.2-2.0 mass%, and at least one element among the three elements Ni: 2.0-5.0 mass%, Ag: 0.5-3.5 mass%, and Li: 0.1-0.4 mass%, has a Si content of at most 0.25 mass% and a Mn content of at most 0.25 mass%, the remainder being composed of Al and unavoidable impurities, and has a tensile strength of at least 650 MPa. With this Al alloy, it is possible to provide a high-strength Al alloy having excellent surface treatment properties. By improving the strength of the Al alloy, it is possible to reduce the weight of products thereof. Additionally, by improving the surface treatment properties, it is possible to reliably impart anti-corrosion properties and shorten product lead time.

Description

Technical Field

[0001] The present disclosure relates to an aluminum alloy having high strength and high surface treatability.

Background Art

[0002] Light alloy materials meet various needs and are widely used in industry. Considering them as mechanical materials and structural materials, the use of light alloy materials is progressing in applications such as automobiles, high-speed railway vehicles, and aerospace. Among light alloys, aluminum alloys are inexpensive and easy to process, so they are used in major fields in most countries. Hereinafter, aluminum alloys are referred to as Al alloys. The improvement of the mechanical strength of Al alloys is being studied by material manufacturers. However, compared with steel, Ti-based alloys, Ni-based alloys, etc., the strength of Al alloys is low. Even for a high-strength commercially available Al alloy, A7075-T6 (JIS standard), its mechanical strength is considered insufficient depending on the application. For this reason, studies on the optimal metal composition of Al alloys have been continuously conducted.

[0003] For example, in Patent Document 1, in order to obtain a high-strength Al alloy, the composition of the Al alloy is set to Zn: 5-12% by mass, Mg: 2-4% by mass, Cu: 1-2% by mass, and it is necessary to further add Ag in an amount of 0.01-0.1% by mass.

[0004] Also, in Patent Document 2, in order to obtain a high-strength Al alloy, a composition containing Zn: 3.2-8.0% by mass, Mg: 1.2-4.5% by mass, Cu: 0.2-1.5% by mass, Mn: 0.1-1.2% by mass, Cr: 0.1-0.5% by mass, B: 0.005-0.2% by mass, Be: 0.02-1.0% by mass, Ni and / or Co: 0.1-1.2% by mass, Zr and / or Hf: 0.05-1.2% by mass, and Ag: 0.05-2.0% by mass is required.

[0005] In Patent Document 3, in order to obtain a high-strength Al alloy, the composition contains Zn: 5-11% by mass, Mg: 2-4.5% by mass, Cu: 0.5-2.0% by mass, Mn 2-6% by mass, and Ag 0.01-0.5% by mass.

[0006] Among practical metals, the Al alloy is a base metal next to the Mg alloy and is known as a metal that is easily corroded. Therefore, it is necessary to perform surface treatment such as plating or painting on Al alloy products to impart corrosion resistance. In particular, a plastic deformation layer called a Beilby layer is formed on the machined metal surface. For this reason, it is common to perform surface treatment after etching and removing the plastic deformation layer with an alkali or an acid.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

[0008] When the Al alloy is etched with an acid or an alkali, black deposits called smut are generated on the surface. Smut has problems of inhibiting the surface treatment process and the adhesion of plating and painting.

Summary of the Invention

Problems to be Solved by the Invention

[0009] The present disclosure aims to provide an Al alloy in which the generation of smut is suppressed in the surface treatment process and which has high strength.

Means for Solving the Problems

[0010] The aluminum alloy of the present disclosure is Containing 5.0 - 6.5 mass% of Zn, 2.0 - 3.0 mass% of Mg, and 1.2 - 2.0 mass% of Cu, including at least one element out of three elements: 2.0 - 5.0 mass% of Ni, 0.5 - 3.5 mass% of Ag, and 0.1 - 0.4 mass% of Li, with the contained Si being 0.25 mass% or less and the contained Mn being 0.5 mass% or less, the balance being composed of Al and inevitable impurities, and having a tensile strength of 650 MPa or more.

Advantages of the Invention

[0011] According to the Al alloy of the present disclosure, a high-strength Al alloy with excellent surface treatability can be provided. The improvement in the high strength of the Al alloy can reduce the weight of the product. Also, the improvement in surface treatability can achieve stable impartation of corrosion resistance and shortening of the lead time of the product.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0013] Embodiment 1. The Al alloy of Embodiment 1 will be described below. In the following description, there are a raw material alloy which is an Al alloy and an Al alloy finally produced from the raw material alloy. For the purpose of distinguishing between the two, these Al alloys may be referred to as the raw material alloy, the Al alloy manufactured from the raw material alloy, and the final alloy. Also, the final alloy may be denoted as "Al alloy (final alloy)". The final alloy is an Al alloy in which the powder is solidified in the solidification process described later. The Al alloy (final alloy) of Embodiment 1 is manufactured by rapid solidification. The tensile strength of the Al alloy (final alloy) of Embodiment 1 is 650 MPa or more. When the Al alloy (final alloy) of Embodiment 1 is immersed in an alkali of 10 mass% or more or an acid of 10 mass% or more, the smart film is 1 μm or less.

[0014] <S10: Melting step> FIG. 1 shows the manufacturing process of the final alloy. Referring to FIG. 1, the manufacturing method of the final alloy will be described. Step S10 is the melting step. In the melting step, the following raw material alloy is put into a melting furnace, melted at a temperature of 800 - 1200 °C, and tapped. Then, the molten raw material alloy is led to the molten metal ejection port of the atomizing nozzle. The raw material alloy is an Al alloy. The aluminum alloy of the raw material alloy has the following composition. Zn: 5.0 - 6.5 mass%, Mg: 2.0 - 3.0 mass%, and contains Cu: 1.2 - 2.0 mass%, and, Ni: 2.0 - 5.0 mass% and, Ag: 0.5 - 3.5 mass% and, Li: 0.1 - 0.4 mass% of the three elements, contains at least one element, and as other inevitable metal elements, Ti is 1.0 mass% or less, Fe is 1.0 mass% or less, Zr is contained in a state of 1.0 mass% or less, and the balance is composed of Al. Note that the elements contained in the raw material alloy will be further described later.

[0015] <S20: Powder Generation Process> Step S20 is a powder generation process. In step S20, an inert gas such as nitrogen, argon, or helium is jetted at high pressure from a nozzle hole to produce rapidly solidified Al alloy powder.

[0016] <Mass % of Zn, Mg, and Cu in the raw material alloy> The relationship between the mass % of Zn, Mg, and Cu in the raw material alloy and the diameter of the crystal particles of the powder will be described below. Powders with a particle diameter exceeding 300 μm are not adopted because they are not rapidly solidified. Zn, Mg, and Cu form coarse intermetallic compounds, Zn: 6.5 mass % or less, Mg: 3.0 mass % or less, Cu: 2.0 mass % or less. Also, Zn: less than 5.0 mass %, Mg: less than 2.0 mass %, When Cu: less than 1.2 mass %, a high tensile strength cannot be obtained. Therefore, Zn, Mg, and Cu are Zn: 5.0 - 6.5 mass %, Mg: 2.0 - 3.0 mass %, Cu: 1.2 - 2.0 mass %, within this concentration range.

[0017] Note that the notation "5.0 - 6.5 mass %" for Zn means "5.0 mass % or more and 6.5 mass % or less". Similarly, notations such as Mg: 2.0 - 3.0 mass % and Cu: 1.2 - 2.0 mass % have the same meaning.

[0018] <Addition concentrations of Ni, Ag, and Li in the raw material alloy> When the addition concentrations of Ni, Ag, and Li are low, a strength of 650 MPa or more in tensile strength cannot be achieved. On the other hand, when the addition concentrations of Ni, Ag, and Li are high, coarse intermetallic compounds are likely to form, and hot cracks are likely to occur during solidification. Therefore, the raw material alloy is Ni: 2.0 - 5.0 mass % and Ag: 0.5 - 3.5 mass% and Li: 0.1 - 0.4 mass% For the three elements, at least one element is included within the range of each mass%. For example, when only Ni is included, Ni: 2.0 - 5.0 mass%. When two elements, Ni and Ag, are included, Ni: 2.0 - 5.0 mass% and Ag: 0.5 - 3.5 mass%. When three elements, Ni, Ag, and Li, are included, each element is within the three ranges shown above. When two or more of Ni, Ag, and Li are included, one of them may be the lower limit of the allowable mass% range.

[0019] <Content of Si and Mn in the raw material alloy> Also, in the raw material alloy, the contained Si is 0.25 mass% or less, and the contained Mn is 0.5 mass% or less. The contents of Si and Mn will be described later in relation to Smart.

[0020] <S30: Solidification process> Step S30 is the solidification process. The rapidly solidified powder of the raw material alloy produced in the powder production process was hot extruded and densified and solidified. Then, the solidified rapidly solidified powder was solution treated and aged under the tempering conditions equivalent to T6 of A7075 in the JIS standard. As a result, a high-strength Al alloy was obtained as the final alloy.

[0021] <Intermetallic compound> Figure 2 shows the result of binarizing the SEM image obtained by cross-sectional SEM observation of the Al alloy (final alloy) obtained by the manufacturing method of the flowchart described in Figure 1. The black color in the slow-cooling and rapid-cooling SEM images in Figure 2 represents the precipitation of intermetallic compounds. Figure 2 shows a length of 100 μm. In the Al alloy using the slowly cooled raw material alloy powder in Figure 2, coarsened intermetallic compounds can be confirmed. The appearance rate of intermetallic compounds in slow cooling was 10% or more in a 300 μm × 300 μm field of view as a result of cross-sectional observation by SEM. On the one hand, as shown in FIG. 2, in the Al alloy using rapidly solidified raw material alloy powder, intermetallic compounds hardly appear. In rapid solidification, the appearance rate of intermetallic compounds was 1% or less in a field of view of 300 μm × 300 μm. When the intermetallic compounds coarsen, a high tensile strength of 650 MPa or more cannot be obtained. In FIG. 2, the appearance rate of intermetallic compounds was 1% or less in a field of view of 300 μm × 300 μm. To obtain a tensile strength of 650 MPa or more, the total appearance area of the intermetallic compounds appearing in the Al alloy (final alloy), when observed in a scanning electron microscope field of view of 300 μm × 300 μm, is preferably 2% or less, and more preferably 1% or less.

[0022] <S40: Cleaning process> Step S40 is a cleaning process. The produced Al alloy (final alloy) removes organic substances such as oil by degreasing and cleaning. Then, alkali cleaning is performed.

[0023] <Alkali cleaning> In alkali cleaning, an aqueous solution of sodium hydroxide or potassium hydroxide with a concentration of 10% by mass or more and 20% by mass or less is used, and the surface is etched by immersion at a temperature of 50 °C or higher for 10 - 30 seconds to expose a clean surface. In addition to sodium hydroxide, alkali cleaning may be an aqueous solution of a strong alkali such as potassium hydroxide.

[0024] <Smut> The thickness of the smut formed during alkali cleaning was measured by spectroscopic ellipsometry. When the smut is thin and the reflected light from the base metal can be measured, the smut thickness can be measured. In the case where it can be measured, the measurement result of the smut thickness was 1 μm or less. On the other hand, when the smut was thick and showed black, it was difficult to measure. In the case where it was difficult to measure, the smut thickness was 1 μm or more. The poor surface treatability can be visually judged by the blackening due to the thick smut.

[0025] <Pickling> In pickling, the degreased and cleaned final alloy was immersed in an aqueous solution containing 10% to 20% by mass of nitric acid. By immersing at a temperature of 50°C or higher for 10 - 30 seconds, the surface smoothness was examined in the same manner as in alkaline cleaning. The surface treatment property was judged by the surface smoothness film thickness and blackening. Note that for pickling, strong acids such as sulfuric acid and hydrochloric acid may be used in addition to nitric acid, or a mixed acid may be used. Also, for the mixed acid, a mixed acid of a weak acid such as phosphoric acid or carbonic acid and a strong acid may be used. In particular, a mixed acid of phosphoric acid and nitric acid, and a mixed acid of phosphoric acid and sulfuric acid are known as chemical polishing liquids. These are used to improve the surface smoothness.

[0026] <Relationship between surface smoothness and Si, Mn> The thickening and blackening of the surface smoothness are greatly affected by the amounts of Si and Mn. As a result of the investigation, Si: 0.25% by mass or less, Mn: 0.25% by mass or less, It was found that this is desirable. Si lowers the melting point of the Al alloy and increases the solid solubility of other metal elements. Reducing the amount of Si added makes it difficult for other metal elements to dissolve in the Al alloy, so intermetallic compounds are likely to precipitate. Therefore, the solidification of the Al alloy needs to be carried out by rapid cooling. The condition for rapid solidification is desirably a cooling rate of 100°C / sec or more.

[0027] Figure 3 shows the results of the surface treatment property and the tensile strength of the Al alloy (final alloy) produced by the method of Figure 1. In Figure 3, Examples 1 to 11, Comparative Examples 1 to 4, and the Al alloy of A7075 are shown. Examples 1 to 11 are the final alloys, and Comparative Examples 1 to 4 and A7075 correspond to the final alloys. In Examples 1 to 11, for Zn, Mg, and Cu, Zn: 5.5% by mass, Mg: 2.5% by mass, Cu: 1.5% by mass. For Si and Mn, Si: 0.2% by mass, Mn: 0.1 - 0.2% by mass. For Ni, Ag, and Li, Ni: 2.0 - 5.0% by mass, Ag: 0.5 - 3.5% by mass, Li: 0.1 - 0.4% by mass. On the left side of Figure 3, the tensile strength (MPa), the presence or absence of blackening after alkaline cleaning, and the surface smoothness thickness (μm) of the Al alloy (final alloy) are shown.

[0028] ***Description of the Effects of Embodiment 1*** Using the raw material alloy having the composition described above, a high-strength Al alloy with excellent surface treatability can be manufactured by the manufacturing method described in FIG. 1. Therefore, by improving the strength of the Al alloy, the weight of the product can be reduced. In addition, by improving the surface treatability, it is possible to achieve stable imparting of corrosion resistance and shortening of the lead time of the product.

[0029] Supplement Embodiment 1. In order to suppress the generation of smut by alkali or acid and improve the surface treatability, it is necessary to make the smut thickness 1 μm or less. For this purpose, it was necessary to make the Si content in the Al alloy 0.25 mass% or less and the Mn content 0.5 mass% or less. Patent Document 1 and Patent Document 2 allow the Si content to be 0.25 mass% or more, and Patent Document 3 allows the Mn content to be 2-6 mass%. Therefore, in Patent Document 1 and Patent Document 2, it is not possible to make the smut thickness 1 μm or less. On the other hand, by restricting the addition amounts of Si and Mn, it was necessary to determine the optimal composition ratio of the additive elements to the Al alloy. In order to obtain a high-strength Al alloy with a tensile strength of 650 MPa or more, Zn: 5.0-6.5 mass%, Mg: 2.0-3.0 mass%, Cu: 1.2-2.0 mass%, and it is necessary to contain at least one element or more among Ni: 2.0-5.0 mass%, Ag: 0.5-3.5 mass%, and Li: 0.1-0.4 mass%. It is desirable that the additive elements are dissolved in the Al matrix phase as much as possible. In addition, since a higher tensile strength can be obtained without generating precipitates that are coarse intermetallic compounds, it was produced by rapid solidification.

[0030] <Modification: Application to 3D Printer> As a modification of Embodiment 1, the application to a 3D printer 40 will be described. With reference to FIGS. 4 and 5, a method of directly manufacturing a product by a layer manufacturing method using a 3D printer 40 with the powder produced in step S20 will be described. FIG. 4 shows a flowchart of a manufacturing method of an Al alloy (final alloy) when using a layer manufacturing method. In FIG. 4, steps S30 and S40 in FIG. 1 are different in that they are steps S30-1 and S40-1. FIG. 5 is a diagram for explaining a layer manufacturing method. In FIG. 5, a conceptual perspective of the 3D printer 40 in XYZ coordinates is shown at the upper left, the XY plane at the upper right, the YZ plane at the lower left, and the ZX plane at the lower right. In the 3D printer 40 in XYZ coordinates, the powder generated in step S20 is arranged on the base plate 32. This powder will hereinafter be referred to as powder 31. The XY plane and the ZX plane show the diameter φd of the crystal grains 20. The diameter of the crystal grains 20 may also be called the particle diameter.

[0031] The powder particle size distribution of the atomized powder produced in the powder generation step of step S20 is adjusted as follows. D10: The particle diameter is 10 μm or more and 50 μm or less. D50: The particle diameter is 20 μm or more and 70 μm or less. D90: The particle diameter is 40 μm or more and 100 μm or less. By adjusting the particle diameter, in the solidification step of step S30-1, a product of an Al alloy (final alloy) can be directly manufactured by layer manufacturing (Additive Manufacturing) using a 3D printer 40.

[0032] Step S30-1 will be described with reference to FIG. 5. In FIG. 5, the Z direction is the height direction. It is preferable to perform layer manufacturing at a pitch P of 100 μm or less in the Z direction by irradiation of the laser beam 11 by the laser device 10, and it is more preferable to perform layer manufacturing at a pitch P of 30 μm or less. The YZ plane shows layers #1 to #4. The YZ plane shows the pitch P between the layers. By performing layer manufacturing such as the YZ plane, a product can be manufactured by rapid solidification at 100°C / sec or more.

[0033] <Concentration of oxygen and nitrogen> By irradiating the laser beam 11, the temperature can be raised and lowered in a short time. Therefore, the powder 31 generated in step S20 is easily oxidized and nitrided. Therefore, the concentration control of oxygen and nitrogen is important. In particular, since oxidation is a cause of brittle fracture, product shaping by the 3D printer 40 in a non-oxygen atmosphere is good, and product shaping in an Ar gas atmosphere is desirable. The oxygen contained in the shaped product by the 3D printer 40 produced in a non-oxygen atmosphere is preferably 0.1% by mass or less, and more preferably 0.05% by mass or less. Also, the nitrogen contained in the shaped product by the 3D printer 40 is preferably 0.01% by mass or less, and more preferably 0.001% by mass or less.

[0034] <Laser wavelength> When the laser wavelength of the laser beam 11 used for melting the powder 31 produced in step S20 is 700 nm or less, the light absorption rate by Al decreases. For this reason, the range of the laser wavelength is preferably 700 nm or more and 1100 nm or less. In the powder 31 produced in step S20, since the Si content is small, the melting temperature becomes high. Therefore, it is more preferable to irradiate with a laser wavelength of 800 nm or more and 1100 nm or less, which is a wavelength region where light is easily absorbed.

[0035] <Diameter φd of the crystal grains 20 after irradiation with the laser beam 11> The smaller the diameter φd of the crystal grains 20 of the rapidly solidified powder 31, the higher the tensile strength. When a product is produced by the 3D printer 40, due to the nature of the 3D printer 40, the produced product has anisotropy in the diameter φd of the crystal grains 20. When the laser beam 11 as a heat source is irradiated from the Z direction, the diameter φd of the crystal grains 20 becomes long in the Z direction. Let the Z direction be the height direction. After the irradiation of the laser beam 11, as shown in the XY plane of FIG. 5, in the XY direction which is the planar direction with respect to the laser beam 11 as the heat source, the diameter φd of the crystal particles 20 is preferably 100 μm or less, and more preferably 30 μm or less. This particle size is the particle size of the product after being directly fabricated by the 3D printer 40. The XY direction means all directions of 360 degrees on the XY plane with respect to the direction of the double arrow shown as φd in the XY plane. After the irradiation of the laser beam 11, as shown in the ZX plane of FIG. 5, in the Z direction of the diameter φd of the crystal particles 20, it is preferably 400 μm or less, and still more preferably 200 μm or less. This particle size is the particle size of the product after being directly fabricated by the 3D printer 40.

[0036] As shown in the perspective view of the XYZ coordinates in FIG. 5, the shaped object of the 3D printer 40 is shaped on the aluminum base plate 32. The temperature of the base plate 32 is preferably implemented at 200°C or less. After shaping by the 3D printer 40, without performing the solution treatment of the product, by implementing either one or both of the aging treatment and the stress relaxation heat treatment on the product, a high-strength Al alloy product can be obtained.

[0037] In the surface treatment step S40-1, it is washed using an alkali and an acid under the same conditions as in FIG. 1. By these washings, it can be confirmed that the matte thickness is 1 μm or less and there is no blackening in the appearance.

[0038] As described above, the Al alloy (final alloy) manufactured by solidification has been irradiated with a heat source from the direction along the Z axis in the solidification process, and the diameter φd of the crystal particles 20 has anisotropy with respect to the laser beam 11 as the heat source. The diameter φd is 30 μm or less in a cross-section parallel to the XY plane and 200 μm or less in a cross-section parallel to the ZX plane. Further, in the Al alloy (final alloy) manufactured by solidification, the contained oxygen is 0.1 mass% or less and the contained nitrogen is 0.01 mass% or less.

[0039] ***Explanation of the effects of Embodiment 1*** According to the production of products by the 3D printer 40 described with reference to FIGS. 4 and 5, products with high precision, high strength, and excellent surface treatability can be produced.

[0040] The Al alloy (final alloy) described in the above Embodiment 1 is as follows. The Al alloy (final alloy) contains 5.0 - 6.5 mass% of Zn, 2.0 - 3.0 mass% of Mg, and 1.2 - 2.0 mass% of Cu, contains at least one of the three elements: 2.0 - 5.0 mass% of Ni, 0.5 - 3.5 mass% of Ag, and 0.1 - 0.4 mass% of Li, the contained Si is 0.25 mass% or less, and the contained Mn is 0.25 mass% or less, the balance consists of Al and unavoidable impurities, and the tensile strength is 650 MPa or more. Also, the appearance area showing the total area of the intermetallic compounds appearing in the Al alloy (final alloy) is 2% or less when observed in a scanning electron microscope field of 300 μm × 300 μm.

Explanation of Reference Numerals

[0041] 10 Laser device, 11 Laser light, 20 Particles, 31 Powder, 32 Base plate, 40 3D printer.

Claims

1. In an aluminum alloy, containing Zn: 5.0 - 6.5% by mass, Mg: 2.0 - 3.0% by mass, Cu: 1.2 - 2.0% by mass, containing at least one of the three elements of Ni: 2.0 - 5.0% by mass, Ag: 0.5 - 3.5% by mass, and Li: 0.1 - 0.4% by mass, the contained Si is 0.25% by mass or less, and the contained Mn is 0.25% by mass or less, the balance consisting of Al and unavoidable impurities, an aluminum alloy having a tensile strength of 650 MPa or more.

2. The appearance area showing the total area of the intermetallic compounds appearing in the aluminum alloy is 2% or less when observed in a scanning electron microscope field of 300 μm × 300 μm. The aluminum alloy according to Claim 1.

3. containing Zn: 5.0 - 6.5% by mass, Mg: 2.0 - 3.0% by mass, Cu: 1.2 - 2.0% by mass, containing at least one of the elements of Ni: 2.0 - 5.0% by mass, Ag: 0.5 - 3.5% by mass, and Li: 0.1 - 0.4% by mass, the contained Si is 0.25% by mass or less, and the contained Mn is 0.25% by mass or less, a melting step of melting a raw material alloy consisting of Al and unavoidable impurities, a powder generation step of rapidly solidifying the melted raw material alloy at a cooling rate of 100 °C / sec or more to generate powder, a solidification step of manufacturing an aluminum alloy by solidifying the powder, comprising, the tensile strength of the aluminum alloy produced by solidification is A method for manufacturing an aluminum alloy having a tensile strength of 650 MPa or more.

4. The aluminum alloy produced by solidification is In the solidification process, heat is irradiated from a direction along the Z-axis, the crystal grain size has anisotropy with respect to the heat source, the cross-section parallel to the XY plane is 30 μm or less, the cross-section parallel to the ZX plane is 200 μm or less, the contained oxygen is 0.1% by mass or less, and the contained nitrogen is 0.01% by mass or less. The method for producing an aluminum alloy according to claim 3.

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