JP2020125524A - Method for producing aluminum alloy member - Google Patents

Abstract

To provide a method for producing an aluminum alloy member having excellent resistance to stress corrosion cracking while having high strength.SOLUTION: The method includes the steps of: subjecting a member composed of a 7000-series aluminum alloy to solid solution treatment at 400-500°C; and performing air cooling at a cooling rate of 0.1-2.0°C/seconds. After that, artificial aging treatment is performed.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a structural member and the like made of an aluminum alloy, which has high strength and excellent resistance to stress corrosion cracking.

Since JIS 7000 series aluminum alloys such as Al-Zn-Mg series and Al-Zn-Mg-Cu series obtain high strength, they are used as vehicle structural members such as automobiles and railway cars, or as transportation of ships and aircraft. It is used as a structural member of machines.
However, these structural members are often used in a state where stress is applied, and countermeasures against stress corrosion cracking are necessary.
For example, in Patent Document 1, heating is performed at a rate of temperature increase of 0.4° C./sec or more, holding in a temperature range of 200 to 550° C. for more than 0 seconds, and then at a cooling rate of 0.5° C./sec or more. A manufacturing method is disclosed in which the residual stress is reduced by performing a restoration process of cooling and then expanding the pipe within 72 hours.
However, in the process disclosed in the publication, alloy components are largely limited, and in order to obtain high strength, it is necessary to add a large amount of transition elements such as Mn, Cr and Zr, so that the influence of the cooling rate becomes large.

Patent Document 2 discloses a manufacturing technique in which after solution treatment, quenching is performed in the range of 150 to 350° C. for 5 minutes to 30 minutes, followed by water cooling and aging.
However, in the process disclosed in the publication, it is necessary to control the microstructure at the crystal grain boundaries, and it is necessary to add a high concentration of Cu component to obtain high strength.

Patent Document 3 discloses a method of improving stress corrosion cracking resistance by setting the average thickness of crystal grains to 25 μm or less and the aspect ratio to 4 or more.
This technique has a problem that it is difficult to control the microstructure.

JP, 2014-141728, A Japanese Patent No. 5343333 Japanese Patent No. 4229307

An object of the present invention is to provide a method for producing an aluminum alloy member having high strength and excellent stress corrosion cracking resistance.

The method for producing an aluminum alloy member according to the present invention comprises the steps of solution treatment of a member made of a 7000 series aluminum alloy at a temperature of 400 to 500° C. and air cooling at a cooling rate of 0.1 to 2.0° C./sec. It is characterized in that it has a step to be carried out, and then an artificial aging treatment is carried out.
In this case, the solution heat treatment may utilize processing heat in extrusion molding, the member is extruded using a 7000 series aluminum alloy, and immediately thereafter, the cooling rate is 0.1 to 2.0° C./sec. It may have a step of performing air cooling, and then perform artificial aging treatment.

In conventional 7000 series aluminum alloys, as a method for obtaining high strength, water quenching using water cooling for quenching after solution treatment is generally adopted as T6 treatment.
However, although water quenching can cool rapidly, a large difference is likely to occur in the cooling rate between the surface layer and the inside of the member, and in the case of a member having a relatively thick wall such as a structural member, the difference is large. There was a problem that large residual stress was likely to occur in the part.
Therefore, the present invention is characterized in that the quenching of the surface layer portion is slightly weakened relative to the inside by controlling the cooling rate within a predetermined range.

According to the investigation by the present inventors, in the member made of the 7000 series aluminum alloy, if it was solution-treated in the range of 400 to 500° C. and was cooled at a predetermined rate in the initial stage, the strength was not significantly reduced by the subsequent cooling. ..
Therefore, it is desirable to perform air cooling at a cooling rate of 0.1 to 2.0° C./sec until the temperature of the member falls to about 300° C. to 200° C. or less.

In the present invention, the surface layer portion of the member is expressed as a surface layer portion within 20% of the total thickness of the member, on the surface side.
Therefore, the portion inside thereof is the inside.
In the present invention, 0.2% proof stress or tensile strength of the surface layer portion after the artificial aging treatment is preferably 4% or more lower than that of the inside, and preferably 6% or more.
In terms of hardness HRB, the hardness of the surface layer portion is preferably 3% or more, more preferably 5% or more lower than the internal hardness.
Further, when compared in terms of electric conductivity, it is preferable that the surface layer portion has an IACS value higher than that of the inside by 2% or more.

The artificial aging treatment is preferably performed under the condition that the maximum strength of the material can be obtained.
For example, the second stage aging treatment condition of the first stage: 90 to 120° C. for 1 to 24 hours and the second stage: 130 to 180° C. for 1 to 24 hours can be mentioned as an example.

As the aluminum alloy that can be used in the present invention, general JIS-7000 series alloys of Al-Zn-Mg type and Al-Zn-Mg-Cu type can be used.
For example, in the case of JIS 7075 material, Zn: 6.40 to 6.90%, Mg: 2.1 to 2.9%, Cu: 1.20 to 2.20%, Mn: 0 in all mass% below. 0.3% or less, Cr: 0.18 to 0.28%, Fe: 0.5% or less, Si: 0.4% or less, Ti: 0.005 to 0.05%, the balance being Al and impurities. ing.
As a particularly preferable high-strength material, the following aluminum alloy (A) can be given as an example.
Alloy (A): Zn: 6.40 to 6.90%, Mg: 1.60 to 1.80%, Cu: 0.20 to 0.30, Mn: 0.20 to 0.30%, Zr: 0.17 to 0.23%, Cr: 0.20% or less, Ti: 0.005 to 0.05%, Fe: 0.20% or less, Si: 0.10% or less, and the balance being Al and impurities. is there.

In the method for manufacturing an aluminum alloy member according to the present invention, after the solution treatment, the cooling rate until the temperature of the member becomes about 300°C to 200°C or less is in the range of 0.1 to 2.0°C/sec. By controlling so that the physical property values can be inclined between the surface layer portion and the inside, a high strength member excellent in stress corrosion cracking resistance can be obtained.

The production method according to the present invention is effectively applied to a thick member, and the thickness may be 3 mm or more, for example, 3 to 20 mm or 5 to 15 mm.
As a result, structural members made of aluminum alloy for various vehicles and transport aircraft can be obtained.

The evaluation results of the aluminum alloy members are shown in the table.

The manufacturing method according to the present invention and the conventional T6 treatment were compared and evaluated, and will be described below.
In the table of FIG. 1, Examples 1 to 5 are obtained by quenching the alloy (A) described above at the cooling rate in the table after solution treatment.
In Comparative Example 1, the alloy (A) was used for conventional water cooling (T6 treatment).
In Comparative Example 2, 7075 alloy was used for conventional water cooling.
The details will be described below.

In each of Examples 1 to 5 and Comparative Examples 1 and 2, an extruded material (member) having a thickness of 10 mm was used.
In Examples 1 to 5, after solution treatment at about 450° C., cooling was performed at average cooling rates of 0.16, 0.47, 0.77, 1.30, and 2.02° C./sec, respectively.
Thereafter, a two-stage artificial aging treatment of 90°C to 120°C + 130 to 180°C was performed.
Comparative Examples 1 and 2 are water-cooled directly after solution heat treatment.
The mechanical properties were evaluated based on JIS-Z2241 after cutting out the evaluation site.
The electrical conductivity was comparatively evaluated based on ASTM E1004 with the value of International Annealed Copper Standard (IACS) being 100%.
The SCC resistance was evaluated based on ASTM G47, and the corrosion test was evaluated based on ASTM G34.

From the evaluation results shown in the table, when quenching by water cooling is performed as in Comparative Examples 1 and 2, both the yield strength and the tensile strength of the surface (surface layer portion) and the inside are equal to or less than 2% of the surface layer portion. It was estimated that the residual stress was large, and the SCC resistance and corrosion resistance were poor.
On the other hand, in Examples 1 to 5, the cooling rate in the initial stage of quenching was sequentially increased.
At the cooling rate of 2.02° C./sec in Example 5, the surface layer portion had a yield strength of 4% lower than that of the inside, but the tensile strength had a difference of only 2%, and the SCC resistance was barely the target. It has been clarified that the cooling rate is preferably 2.0° C./second or less.
When the cooling rate was set to 0.16° C./sec as in Example 1, the surface layer portion had a yield strength of 9% lower and a tensile strength of 10% lower than the interior, and was excellent in SCC resistance and corrosion resistance.
It should be noted that the slower the cooling rate in the initial stage of quenching within the cooling rate range of 0.1 to 2.0° C./second, the SCC resistance and the corrosion resistance are improved, but the mechanical properties tend to be slightly lowered. To be
When an aluminum alloy (A) is used, in order to obtain excellent SCC resistance and corrosion resistance while securing a proof stress of 420 MPa and a tensile strength of 470 MPa or more for the entire member, a cooling rate of 0.5 to 2.0° C./ It has also become clear that a range of seconds is preferred.
In addition, the present invention suppresses the residual stress in the surface layer portion by controlling the cooling rate in the initial stage of quenching within a predetermined range. When the temperature of the member becomes 300°C or lower, high-speed cooling of 20°C/sec or more is performed. It may be two-stage cooling using.
In that case, quenching quality is more stable.

Claims (4)

A step of subjecting a member made of a 7000 series aluminum alloy to a solution treatment at a temperature of 400 to 500° C.;
A step of performing air cooling at a cooling rate of 0.1 to 2.0° C./second,
A method for manufacturing an aluminum alloy member, characterized in that the artificial aging treatment is performed thereafter. Extruding the member using 7000 series aluminum alloy,
Immediately thereafter, there is a step of performing air cooling at a cooling rate of 0.1 to 2.0° C./second,
A method for manufacturing an aluminum alloy member, characterized in that the artificial aging treatment is performed thereafter. The method for producing an aluminum alloy member according to claim 1 or 2, wherein the surface layer portion of the member after the artificial aging treatment has a lower strength than the inside thereof. The method for producing an aluminum alloy member according to claim 1, wherein the surface layer portion of the member after the artificial aging treatment has higher electrical conductivity than the inside thereof.

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