Method Of Producing High-Intensity Aluminum Alloy Extruded Material Excellent In Resistance To Stress Corrosion Cracking And Aluminum Alloy Used Therefor

Abstract

A method of producing a high-intensity aluminum alloy extruded material excellent in resistance to stress corrosion cracking, the method comprising: preparing an Al—Zn—Mg-based aluminum alloy, subjecting the aluminum alloy to extrusion so that a temperature of an extruded material obtained immediately after the extrusion is 440° C. or more; allowing the extruded material to cool in an air at a cooling rate of less than 100° C./min until the temperature range of the extruded material reaches 300 to 400° C. from immediately after the extrusion for one minute to 3 minutes and 24 seconds; and then forced cooling the extruded material at a cooling rate of 100 to 2000° C./min until the temperature of the extruded material reaches 150° C. or less, followed by an aging treatment.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority to Japanese Patent Application No. 2023-049527 filed on Mar. 27, 2023, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present disclosure relates to a method of producing an extruded material including an Al—Zn—Mg-based aluminum alloy having a high intensity and an excellent resistance to stress corrosion cracking (resistance to SCC) and an aluminum alloy used therefor.

In the field of vehicles and the like, an aluminum alloy extruded material having a high intensity has been required for the purpose of weight reduction.

Among aluminum alloys, the Al—Zn—Mg-based alloy has been known to have a high intensity. However, there is the following technical problem: when the Al—Zn—Mg-based alloy has a higher intensity, the resistance to SCC decreases.

For example, JP-B-2928445 discloses that when the thickness of surface recrystallization is set to 7% or less of the wall thickness and an average particle diameter of the surface recrystallization is 150 μm or less, the resistance to SCC is improved, but the tensile strength is about 450 MPa that is a low level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a composition of an aluminum alloy used for evaluation, and the balance is Al and inevitable impurities;

FIG. 2 illustrates the casting conditions and the homogenization treatment (HOMO treatment) conditions of a billet for extrusion;

FIG. 3 illustrates the extrusion conditions and the cooling conditions immediately after extrusion of the extruded material; and

FIG. 4 illustrates the evaluation results of the extruded material.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being “connected” or “coupled” to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected or coupled to each other with one or more other intervening elements in between.

An object of the present disclosure is to provide a method of producing an extruded material including an aluminum alloy having a high intensity and an excellent resistance to SCC using an Al—Zn—Mg-based aluminum alloy having a given component composition under given extrusion conditions and an aluminum alloy used therefor.

In accordance with one of some embodiments, an aluminum alloy used for extrusion includes, by mass: Zn: 6.0 to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.30 or less, Cr: 0.25% or less, a total of [Mn+Cr+Zr]: 0.10 to 0.50%, and the balance of Al and inevitable impurities, wherein an average crystalline particle diameter of the aluminum alloy after casting is 250 μm or less.

In order to obtain a casting texture having an average crystalline particle diameter of 250 μm or less, for example, a molten metal of the aluminum alloy having the above-referenced alloy composition may be used to be cast at a casting rate of 50 mm/min or more and may be subjected to a homogenization treatment (HOMO treatment) at 480 to 520° C. for 1 to 12 hours, followed by cooling at a cooling rate of 50° C.//min or more.

In accordance with one of some embodiments, a method of producing a high-intensity aluminum alloy extruded material excellent in resistance to SCC includes: subjecting the above-referenced aluminum alloy to extrusion so that a temperature of an extruded material obtained immediately after the extrusion is 440° C. or more; allowing the extruded material to cool in an air to a temperature range of 300 to 400° C. from immediately after the extrusion for one minute to 3 minutes and 24 seconds from immediately after the extrusion until a temperature of the extruded material reaches a range of 300 to 400° C.; and then cooling the extruded material at a cooling rate of 100 to 2000° C./min until the temperature of the extruded material reaches 150° C. or less, followed by an aging treatment.

As a result, in some embodiments, the tensile strength of 500 MPa or more and 0.2% proof stress of 470 MPa or more can be achieved.

Here, the term “allowing the extruded material to cool” means that the extruded material is extruded so that the temperature of the extruded material obtained immediately after the extrusion is 440° C. or more and is then allowed to cool in an air to a temperature range of 300 to 400° C. for one minute to 3 minutes and 24 seconds without forced cooling at, for example, a cooling rate of preferably less than 100° C./min, more preferably 59 to 90° C./min. If the cooling time exceeds 3.4 min (3 minutes 24 seconds), the intensity of the extruded material will be poor.

In addition, the cooling at a cooling rate of 100 to 2000° C./min, more preferably more than 200° C./min is performed by forced cooling, for example, forced air cooling using a fan or water cooling. Water cooling ensures a cooling rate of 1000° C./min or more.

When the temperature of the extruded material is 150° C. or less, the intensity or the resistance to SCC of the extruded material is not affected.

In accordance with one of some embodiments, reasons for selecting the composition of the aluminum alloy used for extrusion are as follows.

Zn and Mg Components

Zn does not decrease extrudability even at a relatively high concentration and contributes to improvement of the intensity of the extruded material. Addition of Mg precipitates MgZn₂ in a texture and increases the intensity of the extruded material.

However, when a large amount of Mg is added, the extrudability decreases, an amount of MgZn₂ precipitations is too large, and there is a possibility that toughness of the extruded material decreases.

Therefore, the combination of the ranges of Zn: 6.0 to 10.0% and Mg: 1.5 to 3.5% is desirable. The combination of the ranges of Zn: 6.5 to 9.5% and Mg: 1.6 to 3.45% is preferable, and the combination of the ranges of Zn: 6.7 to 8.9% and Mg: 1.69 to 3.42% is more preferable.

Cu Component

Addition of a Cu component is effective to improve the intensity of the extruded material because of the solution effect. However, when an amount of the Cu component added is large, typical corrosion resistance decreases. Therefore, the range of Cu: 0.20 to 2.50% is desirable.

Zr, Mn, and Cr Components

All these components, which are transition elements, reduce the recrystallization depth on the surface of the extruded material at the extrusion, and have an effect of micronizing crystal grains.

This improves the resistance to SCC of the extruded material.

Among them, the Cr component has the strongest quench sensitivity. The Mn component has less quench sensitivity than the Cr component but has a larger influence on the quench sensitivity than Zr.

In accordance with one of some embodiments, when Zr: 0.10 to 0.25% is satisfied and Mn is added, Mn: 0.30% or less is satisfied, and when Cr is added, Cr: 0.25% or less is satisfied. Cr may be not necessarily added, but when Mn is added, Mn: 0.30% or less (excluding 0) is satisfied. In addition, the total of [Mn+Cr+Zr] may fall within the range of 0.10 to 0.50%. The total of [Mn+Cr+Zr] or [Mn+Zr] may preferably fall within the range of 0.10 to 0.47%, and more preferably fall within the range of 0.10 to 0.18% or 0.46 to 0.47%.

Ti Component

A Ti component has an effect of micronizing crystal grains when a billet used for extrusion is cast, and a very slight amount of a B component is generally added.

A slight amount of Ti: 0.005 to 0.05% may be added.

Other Components

In the casting process or the like of 7000 series aluminum alloy, Fe component and Si component are often contained as impurities. However, a large amount of the above-referenced components affects, for example, extrudability and resistance to SCC. Therefore, Fe: 0.2% or less, preferably 0.18% or less, and Si: 0.1% or less, preferably 0.07% or less are preferably satisfied.

An aluminum alloy having a given composition is subjected to extrusion, and cooling conditions (die end quenching) immediately after the extrusion are controlled. Then, an extruded material having a high intensity while ensuring excellent resistance to SCC can be obtained.

Exemplary embodiments are described below. Note that the following exemplary embodiments do not in any way limit the scope of the content defined by the claims laid out herein. Note also that all of the elements described in the present embodiment should not necessarily be taken as essential elements.

A molten metal having the composition of the aluminum alloy illustrated in FIG. 1 was adjusted, and a billet (BLT) was cast under the conditions illustrated in FIG. 2, followed by a homogenization treatment (HOMO treatment).

Next, as illustrated in FIG. 3, the billet (BLT) was preheated to 440° C. or more, to produce a hollow extruded shape or a solid extruded shape having a wall thickness of 2 to 3 mm, a height of 50 to 60 mm, and a width of 110 to 120 mm at an extrusion rate of 5 m/min or more.

All the temperatures of the extruded materials obtained immediately after extrusion were 440° C. or more, as illustrated in the table of FIG. 3.

FIG. 3 illustrates the conditions of Examples 1 to 14 and Comparative Examples 1 to 16, and the extruded materials were evaluated under various cooling conditions immediately after extrusion.

In FIG. 3, the target ranges of the respective conditions are illustrated. When the target range was satisfied, “circle mark” was illustrated. When the target range was not satisfied, “x mark” was illustrated.

FIG. 4 illustrates the evaluation results, and the evaluation conditions are as follows.

Mechanical Properties

According to JIS-Z2241, No. JIS-5 test piece was prepared, and a tensile tester according to the JIS standard was used to determine the tensile strength, the σ_0.2 proof stress, and the elongation.

Billet Crystal Particle Diameter and Microtexture of Extruded Material

A sample surface was subjected to mirror polishing finishing and was etched using a Keller's reagent.

The metal texture of the sample surface obtained after etching was observed by an optical microscope, and an image (×100) was subjected to image processing, to determine an average crystalline particle diameter.

Resistance to Stress Corrosion Cracking (SCC)

With the stress having a proof stress of 80% being applied to the test piece, the test piece that caused no cracking in 720 cycles was considered as achievement of the goal. Here, the following condition was considered as one cycle.

Note that, when cracking occurred during the above-referenced test, the number of cycles at that time was illustrated.

One Cycle

A test piece was immersed in a 3.5% NaCl aqueous solution at 25° C. for 10 minutes and was then allowed to stand at 25° C. and humidity of 40% for 50 minutes, followed by air-drying.

As illustrated in FIG. 4, all Examples 1 to 14 were excellent in resistance to SCC by controlling the cooling conditions immediately after extrusion to the target range, while Comparative Examples 1 to 14 had a tensile strength of 500 MPa or more, a 0.2% proof stress of 470 MPa or more, and an elongation of 8% or more, but did not achieve the goal of the resistance to SCC. Comparative Examples 15-16 achieved the goal of SCC resistance and an elongation of 8% or more, but did not have a tensile strength of 500 MPa or more and a 0.2% proof stress of 470 MPa or more.

In Examples 1 to 14, as shown in FIG. 3, the temperature of the extruded material after extrusion (440° C. or higher) is allowed to cool in an air for at least 1 minute to the extruded material temperature at the start of cooling (300 to 400° C.). On the other hand, Comparative Examples 1 to 16 have a wide range of the cooling time from less than 1 minute to more than 5 minutes. The cooling rate immediately after extrusion in Examples 1 to 14 is determined from [(extruded material temperature after extrusion)−(extruded material temperature at start of cooling)]/(time period to reach start of cooling) in FIG. 3. The cooling rate immediately after extrusion in Examples 1 to 14 are less than 100° C./min, especially 59 to 90° C./min, and are greatly different from Comparative Examples 1 to 14. In Comparative Examples 15-16, the cooling rate during cooling in a air is less than 100° C./min, but the cooling time exceeds 3.4 min (3 minutes 24 seconds), so the intensity of the extruded material is poor.

In particular, Examples 7 to 10 have a tensile strength of 600 MPa or more and a 0.2% yield strength of 600 MPa or more, as shown in FIG. 4. In Examples 7 to 10, unlike Examples 1 to 6 and 11 to 14, the combinations are in the range of Zn: 8.0 to 10.0% and Mg: 1.5 to 3.5% (see FIG. 1), and the high concentration of Zn contributes to improving the strength of the extruded material.

Although only some embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications are intended to be included within scope of this disclosure.

Claims

1. A method of producing a high-intensity aluminum alloy extruded material excellent in resistance to stress corrosion cracking, the method comprising:

preparing an aluminum alloy consisting of, by mass: Zn: 6.0 to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.30 or less, Cr: 0.25% or less, a total of [Mn+Cr+Zr]: 0.10 to 0.50%, and a balance of Al and inevitable impurities, wherein an average crystalline particle diameter of the aluminum alloy after casting is 250 μm or less,

subjecting the aluminum alloy to extrusion so that a temperature of an extruded material obtained immediately after the extrusion is 440° C. or more;

allowing the extruded material to cool in an air at a cooling rate of less than 100° C./min until the temperature of the extruded material reaches 300 to 400° C. from immediately after the extrusion for one minute to 3 minutes and 24 seconds; and

then forced cooling the extruded material at a cooling rate of 100 to 2000° C./min until the temperature of the extruded material reaches 150° C. or less, followed by an aging treatment.

2. The method of producing a high-intensity aluminum alloy extruded material excellent in resistance to stress corrosion cracking according to claim 1, wherein the aluminum alloy extruded material has a tensile strength of 500 MPa or more and a 0.2% proof stress of 470 MPa or more.

3. The method of producing a high-intensity aluminum alloy extruded material excellent in resistance to stress corrosion cracking according to claim 1, wherein the aluminum alloy includes Zn: 8.0 to 10.0%, and the extruded material has a tensile strength of 600 MPa or more and a 0.2% proof stress of 600 MPa or more.

4. An aluminum alloy comprising:

by mass: Zn: 8.0 to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.30 or less, Cr: 0.25% or less, a total of [Mn+Cr+Zr]: 0.10 to 0.50%, and a balance of Al and inevitable impurities,

wherein an average crystalline particle diameter of the aluminum alloy after casting is 250 μm or less.