JP7366553B2 - Method for manufacturing aluminum alloy parts - Google Patents

Description

TECHNICAL FIELD The present invention relates to structural members etc. made of aluminum alloy that have high strength and excellent stress corrosion cracking resistance.

JIS 7000 series aluminum alloys such as Al-Zn-Mg series and Al-Zn-Mg-Cu series have high strength and are therefore used as structural components of vehicles such as automobiles and railway vehicles, or for transportation such as ships and aircraft. It is used in structural components of aircraft.
However, these structural members are often used under stress, and therefore, countermeasures against stress corrosion cracking are required.
For example, Patent Document 1 discloses that heating is performed at a temperature increase rate of 0.4°C/sec or more, maintained in a temperature range of 200 to 550°C for more than 0 seconds, and then cooled at a cooling rate of 0.5°C/sec or more. A manufacturing method is disclosed in which residual stress is reduced by performing a cooling restoration process and further performing tube expansion within 72 hours.
However, the process disclosed in the publication has major restrictions on alloy components, and in order to obtain high strength, it is necessary to add a large amount of transition elements such as Mn, Cr, Zr, etc., and the cooling rate has a large effect.

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

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 in that it is difficult to control the microstructure.

JP2014-141728A Patent No. 5343333 Patent No. 4229307

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

The method for manufacturing an aluminum alloy member according to the present invention includes the steps of solution-treating a member made of 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. The method is characterized in that it has a step of performing an artificial aging treatment after that.
In this case, the solution treatment may be performed using the processing heat during extrusion molding, in which the member is extruded using 7000 series aluminum alloy, and immediately thereafter, the cooling rate is 0.1 to 2.0°C/sec. The method may include a step of performing air cooling, followed by an 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 employed as T6 treatment.
However, although water quenching allows for rapid cooling, there tends to be a large difference in the cooling rate between the surface layer and the interior of the component, and in relatively thick components such as structural components, the difference is large and the surface layer There was a problem that large residual stress was likely to occur in the parts.
Therefore, the present invention is characterized in that by controlling the cooling rate within a predetermined range, the hardening of the surface layer is made slightly weaker than that of the inside.

According to the inventors' research, in a member made of 7000 series aluminum alloy, if it is solution-treated in the range of 400 to 500°C and cooled at a predetermined rate in the initial stage, there is no significant decrease in strength due to 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 below about 300° C. to 200° C.

In the present invention, the surface layer portion of a member is expressed as the depth on the surface side within 20% of the overall wall thickness of the member.
Therefore, the part inside is considered to be internal.
In the present invention, after artificial aging treatment, the 0.2% yield strength or tensile strength of the surface layer portion is preferably lower than that of the inner portion by 4% or more, preferably 6% or more.
In terms of hardness HRB, the hardness of the surface layer is preferably 3% or more, more preferably 5% or more lower than the internal hardness.
Furthermore, when comparing electrical conductivity, it is preferable that the IACS value of the surface layer is 2% or more higher than that of the inside.

Artificial aging treatment is preferably performed under conditions that allow the maximum strength of the material to be obtained.
For example, a two-stage aging treatment condition may be given, in which 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.

As the aluminum alloy that can be used in the present invention, general JIS 7000 series alloys such as Al-Zn-Mg series and Al-Zn-Mg-Cu series can be used.
For example, in the case of JIS7075 material, the following are all mass%: Zn: 6.40 to 6.90%, Mg: 2.1 to 2.9%, Cu: 1.20 to 2.20%, Mn: 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 remainder being Al and impurities. ing.
As a particularly preferable high-strength material, the following aluminum alloy (A) is exemplified.
Alloy (A): Zn: 6.40-6.90%, Mg: 1.60-1.80%, Cu: 0.20-0.30, Mn: 0.20-0.30%, 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, the remainder being Al and impurities. be.

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 set in a range of 0.1 to 2.0°C/sec. By controlling this, it is possible to create a gradient in the physical property values between the surface layer and the inside, and a high-strength member with excellent stress corrosion cracking resistance can be obtained.

The manufacturing method according to the present invention is effectively applied to thick-walled members, and the wall thickness may be 3 mm or more, for example, in the range of 3 to 20 mm, 5 to 15 mm.
As a result, structural members made of aluminum alloy for various vehicles and transportation machines can be obtained.

The evaluation results for 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 explained below.
In the table of FIG. 1, Examples 1 to 5 were obtained by using the alloy (A) described above and quenching at the cooling rate shown in the table after solution treatment.
Comparative Example 1 is an example in which Alloy (A) was subjected to conventional water cooling (T6 treatment).
Comparative Example 2 uses 7075 alloy and performs conventional water cooling.
This will be explained in detail below.

In Examples 1 to 5 and Comparative Examples 1 and 2, extruded materials (members) with a wall thickness of 10 mm were used.
Examples 1 to 5 were cooled at average cooling rates of 0.16, 0.47, 0.77, 1.30, and 2.02°C/sec after solutionization at about 450°C, respectively.
Thereafter, a two-stage artificial aging treatment was performed at 90°C to 120°C + 130 to 180°C.
Comparative Examples 1 and 2 were water-cooled immediately after solution treatment.
Mechanical properties were evaluated based on JIS-Z2241 by cutting out the evaluation site.
The electrical conductivity was comparatively evaluated based on ASTM E1004, with the International Annealed Copper Standard (IACS) value set as 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 is performed by water cooling as in Comparative Examples 1 and 2, the yield strength and tensile strength of the surface layer are only 2% or less different between the surface (surface layer) and the inside. 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 at the initial stage of quenching was gradually increased.
At a cooling rate of 2.02°C/sec in Example 5, although the yield strength of the surface layer was 4% lower than that of the inside, the difference in tensile strength was only 2%, and the SCC resistance was just below the target. It has become clear that the cooling rate is preferably 2.0° C./second or less.
When the cooling rate is set to 0.16° C./sec as in Example 1, the yield strength of the surface layer portion is 9% lower and the tensile strength is 10% lower than that of the inner portion, resulting in excellent SCC resistance and corrosion resistance.
It should be noted that when the cooling rate is in the range of 0.1 to 2.0°C/sec, the SCC resistance and corrosion resistance are improved when the cooling rate is slow in the initial stage of quenching, but there is a tendency for the mechanical properties to deteriorate slightly. It will be done.
When using aluminum alloy (A), in order to obtain excellent SCC resistance and corrosion resistance while ensuring proof stress of 420 MPa and tensile strength of 470 MPa or more for the entire member, a cooling rate of 0.5 to 2.0°C/ It has also been found that a range of seconds is preferred.
In addition, the present invention suppresses residual stress in the surface layer by controlling the cooling rate at the initial stage of quenching within a predetermined range, and when the temperature of the member falls below 300°C, high-speed cooling of 20°C/second or more is possible. Two-stage cooling using .
That way, the quenching quality will be more stable.

Claims (3)

a step of solution-treating a member made of 7000 series aluminum alloy and having a wall thickness of 10 to 20 mm at a temperature of 400 to 500°C;
Two-stage cooling in which air cooling is performed at an average cooling rate of 0.5 to 2.0°C/sec until the temperature of the member reaches 300°C after the solution treatment, and then rapid cooling is performed at a rate of 20°C/sec or more. the step of
After that, there is a step of artificial aging treatment,
A method for manufacturing an aluminum alloy member, characterized in that the yield strength of a surface layer portion at a depth of 20% from the surface with respect to the overall thickness of the member is 4% or more lower than the yield strength of the inner part. 2. The method of manufacturing an aluminum alloy member according to claim 1, wherein the solution treatment is performed by extrusion processing of the member. The aluminum alloy contains Zn: 6.40 to 6.90%, Mg: 1.60 to 1.80%, Cu: 0.20 to 0.30, Mn: 0.20 to 0.30 in mass %. %, Zr: 0.17-0.23%, Cr: 0.20% or less, Ti: 0.005-0.05%, Fe: 0.20% or less, Si: 0.10% or less, the balance is The method for manufacturing an aluminum alloy member according to claim 1 or 2, characterized in that the aluminum and impurities are used.

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