AU2017367371B2

AU2017367371B2 - Aluminum alloy for extruded material, extruded material using the same, and method for producing extruded material

Info

Publication number: AU2017367371B2
Application number: AU2017367371A
Authority: AU
Inventors: Karin Shibata; Tomoo Yoshida
Original assignee: Aisin Keikinzoku Co Ltd
Current assignee: Aisin Keikinzoku Co Ltd
Priority date: 2016-11-30
Filing date: 2017-09-29
Publication date: 2022-09-08
Anticipated expiration: 2037-09-29
Also published as: WO2018100867A1; JP7093611B2; JP2018090839A; AU2017367371A1

Abstract

The purpose of the present invention is to provide: a high strength aluminum alloy extruded material of excellent toughness and stress corrosion cracking resistance as well as high extrudability; an aluminum alloy for an extruded material that is suitable therefor; and an extruded material production method. This aluminum alloy for an extruded material is characterized: in being made, in mass%, of 6.0–7.0% of a Zn component, 1.0–1.6% of a Mg component, 0.15–0.23% of a Zr component, 0.001–0.05% of a Ti component, 0.5% or less of a Cu component, 0.5% or less of an Mn component, 0.02% or less of a Cr component, 0.20% or less of an Fe component, 0.10% or less of an Si component, the balance being made of Al and unavoidable impurities; and in the average crystal grain diameter of the billet cast structure being 250 µm or less.

Description

ALUMINUM ALLOY FOR EXTRUDED MATERIAL, EXTRUDED MATERIAL USING THE SAME, AND METHOD FOR PRODUCING EXTRUDED MATERIAL TECHNICAL FIELD

The present invention relates to a high strength aluminum alloy extruded material having

excellent toughness (impact absorption) and stress corrosion cracking resistance and high

extrusion productivity, and particularly relates to an aluminum alloy suitable therefor and a

method for producing the extruded material.

BACKGROUND ART

An Al-Zn-Mg alloy is known as a high strength aluminum alloy.

For example, a high strength aluminum extruded material for an impact absorbing

member for an automobile disclosed in Patent Document 1 has composition of 5.0 to 7.0 % by

weight of Zn, 1.0 to 1.5 % by weight of Mg, 0.1 to 0.3 % by weight of Cu, 0.05 to 0.2 % by

weight of Zr, 0.001 to 0.05 % by weight of Ti, 0.03 to 0.2 % by weight of Cr, and 0.3 % by

weight or less of Mn, with the balance being Al and unavoidable impurities, wherein the

thickness of the surface recrystallized layer is 7% or less of the wall thickness and average

crystal grain size of surface crystal is 150 pm or less.

The extruded material disclosed in the same publication comprises 0.03 to 0.2 % by

weight of a Cr component as an essential component for the purpose of refining the crystal

grains.

However, the inventors of the invention have revealed that the Cr component causes the

quenching sensitivity in cooling immediately after extrusion processing to be too strong.

The aluminum alloy disclosed in the same publication has an extrusion speed of 3 m/min

to a square pipe of 60x45 mm and t = 2 mm as described in the same publication, indicating poor

extrudability.

Patent Document 1: JP-B-2928445

SUMMARY OF THE INVENTION

In some embodiments, the invention advantageously provides a high strength aluminum

alloy extruded material having not only excellent toughness and stress corrosion cracking

resistance but also good extrudability, an aluminum alloy for an extruded material suitable

therefor, and a method for producing the extruded material.

It is an object of the present invention to overcome or ameliorate at least one of the

disadvantages of the prior art, or to provide a useful alternative.

Unless the context clearly requires otherwise, throughout the description and the claims,

the words "comprise", "comprising", and the like are to be construed in an inclusive sense as

opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not

limited to".

Any discussion of the prior art throughout the specification should in no way be

considered as an admission that such prior art is widely known or forms part of common general

knowledge in the field.

In a first aspect of the invention, there is provided an aluminum alloy for an extruded

material, comprising, in % by weight, 6.0 to 7.0% of a Zn component, 1.0 to 1.6% of a Mg

component, 0.15 to 0.23% of a Zr component, 0.001 to 0.05% of a Ti component, 0.5% or less of

a Cu component, 0.5% or less of a Mn component, 0.02% or less of a Cr component, 0.20% or

less of a Fe component, 0.10% or less of a Si component, with the balance being Al and

unavoidable impurities, wherein the aluminum alloy for the extruded material forms a billet and

-0 the aluminum alloy for the extruded material has an average crystal grain size of a cast structure

of 250 pm or less.

In a second aspect of the invention, there is provided an aluminum alloy extruded material

having high strength and excellent toughness and stress corrosion cracking resistance, using the

aluminum alloy for the extruded material according to the first aspect, wherein a proof stress is

380 N/mm2 or more and a Charpy impact value is 10 J/cm2 or more.

In a third aspect of the invention, there is provided a method for producing an aluminum

alloy extruded material having high strength and excellent toughness and stress corrosion

cracking resistance, the method comprising: homogenizing the billet formed from the aluminum alloy for the extruded material according to the first aspect at 500 to 560°C; and air-cooling the extruded material immediately after extruding the billet at a cooling rate of

50 to 500°C/min. In accordance with one of some embodiments, there is provided an aluminum alloy for an extruded material, comprising, in % by weight, 6.0 to 7.0% of a Zn component, 1.0 to 1.6% of a

Mg component, 0.15 to 0.23% of a Zr component, 0.001 to 0.05% of a Ti component, 0.5% or

less of a Cu component, 0.5% or less of a Mn component, 0.02% or less of a Cr component, 0.20% or less of a Fe component, 0.10% or less of a Si component, with the balance being Al

and unavoidable impurities, wherein the aluminum alloy for the extruded material forms a billet and the aluminum alloy for the extruded material has an average crystal grain size of a cast

structure of 250 pm or less. Using the aluminum alloy for the extruded material according to the invention allows to

obtain an extruded material having a high strength as a proof stress (0.2% proof stress value) of

380 N/mm2 or more as well as excellent toughness with a Charpy impact value of 10 J/cm2 or more by air cooling (die end quenching) immediately after extrusion processing.

In addition, the aluminum alloy for the extruded material according to the invention, for example, enables extrusion processing at an extrusion speed of 6 m/min or more in an "B"

shaped hollow section having a wall thickness of 2 mm. The aluminum alloy for the extruded

-0 material according to the invention secures the extruded material having high strength, high toughness, and high stress corrosion cracking resistance by not only adjusting the amounts of Zn

and Mg components but also adding 0.15 to 0.23% of a Zr component and by air cooling immediately after extrusion processing (referred to as die end quenching); and comprises the cast

structure of the present aluminum alloy for the extruded material forming a billet used for

extrusion, that is a fine structure with an average crystal grain size of 250 pm or less

- 2a -

Casting of billets consisting of an aluminum alloy for an extruded material adopts a float

type casting method, a hot top casting method, or the like in which molten metal is supplied to a

circular mold from above and continuously cast downward in a cylindrical shape.

At this time, the size of crystal grains in the billet after casting changes depending on the

casting speed.

The inventors revealed that the casting speed of 65 mm/min or more in an 8-inch billet

causes the average crystal grain size to be 250 pm or less.

The composition of the aluminum alloy according to the invention will be described.

Zn and Mg components

Zn and Mg are components having artificial age hardenability due to the formation of

these intermetallic compounds, and significantly contributing to strength.

In the invention, the Zn component is selected in the range of 6.0 to 7.0%, and the Mg

component is selected in the range of 1.0 to 1.6%, as expressed in % by weight herein.

In the case of less than 6.0% of Zn and less than 1.0% of Mg, the strength is below the

target, and in the case of more than 7 .0% of Zn and particularly more than 1.6% of Mg, the

extrudability is reduced.

Zr component

The Zr component refines the crystal grains and suppresses the surface recrystallization

depth of the extruded material.

In the invention, the content of the Zr component is in the range of 0.15 to 0.23%, and

particularly the content of the Cr component is limited to 0.02% or less.

Mn component

The content of Mn component is 0.5% or less.

The Mn component also has the effect of refining the crystal grains and the invention

aimed at a proof stress of 380 N/mm2 or more, but for example, in order to obtain a proof stress

of 480 N/mm2 level, the Mn component is added in the range of 0.15 to 0.5%.

In this case, Zr + Mn = 0.30 to 0. 7 3 %.

Ti component

The Ti component has the effect of refining the structure at billet casting, and a small

amount of the Ti component is added in the range of 0.001 to 0.05%.

Cu component

In the invention, regardless of the Cu component that is not essential, the addition of a

small amount of the Cu component alleviates the potential difference in the grain boundaries and

the grains and improves the stress corrosion cracking resistance, and hence the Cu component

may be added in the range of 0.5% or less and preferably in the range of 0.1 to 0.4%.

Fe and Si components

The Fe and Si components have a high possibility of being mixed as impurities in the

process of producing the aluminum alloy billet, and they cause deterioration of the toughness.

Therefore, Fe and Si are limited to 0.20% or less and 0.10% or less, respectively.

In order to fully use the properties of the aluminum alloy according to the invention, the

method for producing the aluminum alloy extruded material preferably has homogenizing the

billet formed from the aluminum alloy for the extruded material according to claim 1 at 500 to

560°C; and air-cooling the extruded material immediately after extruding the billet at a cooling

rate of 50 to 500°C/min.

In JIS 7000 series alloy (Al-Zn-Mg alloy), the homogenization treatment temperature

(HOMO temperature) after casting of the billet is preferably less than 500°C in consideration of

the melting point of the Zn component in Japanese Industrial Standards.

On the other hand, in the invention, in order to provide high strength and high toughness

of the extruded material by die end quenching with air cooling, the homogenization treatment

temperature of the billet is preferably in the range of 500 to 560C.

Further, conditions are set so that the temperature of the extruded material immediately

after extrusion processing is 500 to 585°C. That is, the billet is preheated at 400 to 470°C.

Immediately after extrusion processing, the extruded material is air-cooled (die end

quenching) at a cooling rate of 50 to 500°C/min.

Here, the cooling rate is an average cooling rate when the extruded material is cooled

until the extruded material temperature becomes 100°C or less.

Mechanical properties and quality properties targeted by the invention can be obtained by

performing a two-stage artificial aging treatment at 85 to 110C for 2 to 6 hours and 110 to

160°C for 2 to 12 hours.

ADVANTAGEOUS EFFECTS OF INVENTION

The aluminum alloy extruded material obtained in the invention has high strength and

excellent toughness and stress corrosion cracking resistance, and also good extrudability, thus

providing not only a solid cross section but also a hollow cross section.

Applications of the aluminum alloy extruded material obtained by the invention include

an impact absorbing member for a vehicle, and specific examples include bumper reinforcements,

crash boxes, and door beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the composition (% by weight) of the aluminum alloy used for the evaluation.

FIG. 2 is the casting conditions and extrusion conditions of the billet used for the

evaluation.

FIG. 3 is the evaluation results of the extruded material.

FIG. 4 is the examples of structure photographs of a billet, and A illustrates the present

Example No. 8 and B illustrates Comparative Example No. 10.

DESCRIPTION OF EMBODIMENTS

A molten metal having the component composition illustrated in the table of FIG. 1 was

prepared, and an 8-inch billet was cast at the casting speed illustrated in the table of FIG. 2.

The casting speed of the billet was adjusted by the water-cooling amount of the casting

mold.

The structure photographs of the billet of Example No. 8 and Comparative Example No.

10 are illustrated in FIG. 4.

The invention was aimed at an average crystal grain size of 250 pm or less.

Example No. 8 had an average crystal grain size of 180 pm, and Comparative Example

No. 10 had an average crystal grain size of 475 pm.

The average crystal grain size was measured with an optical microscope after mirror

polishing the surface of the test piece and etching with a 0.5% hydrogen fluoride reagent.

"HOMO holding temperature" in the table of FIG. 2 indicates the billet homogenization

treatment conditions.

The cross-sectional shape of an "B" shape (100 mm x 50 mm, middle rib and surrounding

wall thickness of 2 mm) was extruded at an extrusion speed of 6 to 7 m/min under the extrusion

conditions of Table 2.

The cooling conditions depend on fan cooling immediately after extrusion processing.

Two-stage artificial aging treatment was performed at 95°C x 4 hours and 150°C x 7

hours.

The evaluation results of the obtained extruded material are illustrated in the table of FIG.

3.

Evaluation items and target values according to the invention are illustrated in the table.

The evaluation method of each evaluation item is as follows.

For toughness, the test piece was produced based on Japanese Industrial Standards JIS Z

2242, and the Charpy impact value was measured with the Charpy tester based on JIS.

For mechanical properties, tensile test measurement was performed according to Japanese

Industrial Standards JIS Z 2241. The "proof stress" in the table indicates a 0.2% proof stress

value.

For stress corrosion cracking resistance, under the conditions that a stress corresponding

to a proof stress of 80% was applied to a test piece, the following conditions were regarded as

one cycle and the number of cycles (cyc) until occurrence of cracking was measured.

The target is 720 cyc or more.

One cycle

The test piece is immersed in a 3.5% NaCl aqueous solution at 25°C for 10 minutes, left

in 25°C and 40% humidity for 50 minutes, and naturally dried.

Examples No. 1 to No. 8 satisfied the target in all the items.

Among them, Examples No. 7 and 8 had ranges of Zr: 0.15 to 0.23% and Mn: 0.15 to

0.5%, and specifically, Zr: 0.19% and Mn: 0.25%.

For these Examples, the average crystal grain size of the billet structure was set to 250

pm or less, allowing to obtain a proof stress of 480 N/mm2 level, the proof stress values of No. 8: 483 N/mm 2 and No. 9: 482 N/mm2

. In Comparative Example No. 9, the toughness was bad probably because the average

crystal grain size of the billet structure was coarsened, and In No. 10, generation of tear on the

surface of the extruded material caused the extrudability not to be good.

Comparative Examples No. 11 and No. 12 had high proof stress, but toughness was bad.

This is probably because of the large amount of Mg component.

Comparative Examples No. 13 to No. 20 could not satisfy the target of toughness,

probably because the value of one or both of Mg and Mn exceeded the upper limit.

Comparative Example No. 22 had poor cooling after extrusion and could not satisfy the

target of the proof stress, and Comparative Example No. 23, in which die end quenching by

water cooling was performed, had poor stress corrosion cracking resistance.

In Comparative Examples No. 24 and No. 25, the temperature became too high after

extrusion and surface defects occurred.

Comparative Example No. 26 was poor in toughness and stress corrosion cracking

resistance.

This is probably because Mg and Zn exceeded the upper limit.

In Comparative Example No. 27, the stress corrosion cracking resistance deteriorated

because the amount of the Zr component was small.

INDUSTRIAL APPLICABILITY

The invention can be widely used for parts and products that require high toughness and

stress corrosion cracking resistance.

Claims

What is claimed is:

1. An aluminum alloy for an extruded material, comprising, in % by weight, 6.0 to 7.0% of a

Zn component, 1.0 to 1.6% of a Mg component, 0.15 to 0.23% of a Zr component, 0.001 to

0.05% of a Ti component, 0. 5 % or less of a Cu component, 0. 5 % or less of a Mn component,

0.02% or less of a Cr component, 0.20% or less of a Fe component, 0.10% or less of a Si

component, with the balance being Al and unavoidable impurities, wherein the aluminum alloy

for the extruded material forms a billet and the aluminum alloy for the extruded material has an

average crystal grain size of a cast structure of 250 pm or less.

2. An aluminum alloy extruded material having high strength and excellent toughness and

stress corrosion cracking resistance, using the aluminum alloy for the extruded material

according to claim 1, wherein a proof stress is 380 N/mm2 or more and a Charpy impact value is

10 J/cm2or more.

3. A method for producing an aluminum alloy extruded material having high strength and

excellent toughness and stress corrosion cracking resistance, the method comprising:

homogenizing the billet formed from the aluminum alloy for the extruded material

according to claim 1 at 500 to 560°C; and

air-cooling the extruded material immediately after extruding the billet at a cooling rate of

50 to 500°C/min.