EP2141253B1

EP2141253B1 - Process for producing a 7000 aluminum alloy extrudate

Info

Publication number: EP2141253B1
Application number: EP08738747.8A
Authority: EP
Inventors: Tomoo Yoshida; Arata Yoshida; Shinji Makino
Original assignee: Aisin Keikinzoku Co Ltd
Current assignee: Aisin Keikinzoku Co Ltd
Priority date: 2007-03-26
Filing date: 2008-03-24
Publication date: 2015-09-16
Anticipated expiration: 2028-03-24
Also published as: US20110017366A1; JPWO2008123184A1; JP5588170B2; EP2141253A1; US20090053098A1; EP2141253A4; WO2008123184A1

Description

TECHNICAL FIELD

The present invention relates to a method of producing a 7000-series aluminum alloy extruded product that advantageously suppresses a positive effect due to natural aging even when the aluminum alloy is allowed to stand at room temperature after extrusion and then subjected to artificial aging, as compared with the case of subjecting the aluminum alloy to artificial aging immediately after extrusion.

BACKGROUND ART

An extruded product produced using a 7000-series aluminum alloy is generally subjected to artificial aging after extrusion to obtain desired mechanical properties.
When producing an automotive structural member using such an extruded product, the extruded product is generally subjected to secondary processing (e.g., bending) in a state in which the proof stress is low (i.e., before artificial aging), and then subjected to artificial aging.
However, an automotive structural member may be required to have an impact energy absorption within a given range.
For example, when an automotive bumper reinforcement member has high strength, but exhibits a low energy absorption during side impact, the automobile is deformed to a large extent. As a result, the repair cost may increase, or the safety may be impaired.
A related-art 7000-series aluminum alloy extruded product shows an increase in proof stress after artificial aging when the extruded product is allowed to stand at room temperature after extrusion. Therefore, cracks tend to occur during side impact even if the proof stress is high so that the impact resistance (toughness) decreases. In this case, secondary processing (e.g., bending) must be completed immediately after extrusion. This makes process management difficult.
The patent document 1 discloses an automotive bumper reinforcement member made of a 7000-series aluminum alloy. When using the 7000-series aluminum alloy disclosed in the patent document 1, transition elements such as Mn, Cr, and Zr must be added to obtain a fiber internal structure. Moreover, since overaging is required, the hardenability (quench sensitivity) must be taken into consideration. Therefore, the proof stress may not increase depending on the cross section of the extruded product. This complicates the production process so that the production cost increases.
Patent Document 1: Japanese Patent No. 3772962
Further patent documents: JP H09 310 141 A and JP H08 144 031 A each disclose a high strength AL-Zn-Mg alloy extruded member. Another high strength aluminum alloy is described in JP H09 268 342 A .

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The invention provides the method of claim 1 of producing a 7000-series aluminum alloy extruded product of which an increase in proof stress or the like due to natural aging is suppressed by suppressing the positive effect due to storage at room temperature after extrusion.

Means for Solving the Problems

With regard to the method of the invention, the aluminum alloy extruded product comprises a 7000-series aluminum alloy according to the Japanese Industrial Standards (JIS), the 7000-series aluminum alloy having an excess Mg content or an excess Zn content with respect to a stoichiometric composition shown by MgZn₂ of less than 0.5 mass%.
Note that the stoichiometric composition shown by MgZn₂ means that the ratio of components added is MgZn₂, and does not necessarily mean that the precipitate is MgZn₂.
In the aluminum alloy extruded product, the aluminum alloy extruded product according to the invention hasan Mg content of 0.95 to 1.95 mass% and a Zn content of 5.10 to 7.90 mass%.
The alloy is designed so that the Mg content and the Zn content are within the above ranges, and the value A=Zn-5.36xMg (mass%) is -2.64 to 0.50.
In the invention, other components may optionally be added to the aluminum alloy insofar as the aluminum alloy is an Al-Zn-Mg alloy containing aluminum as a base metal.
In the aluminum alloy extruded product, an increase in proof stress due to natural aging is 15 MPa or less when comparing the proof stress of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 50°C or less for one week after extrusion and then subjecting the resulting product to artificial aging with the proof stress of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging immediately after extrusion.
In the aluminum alloy extruded product, an increase in hardness HV due to natural aging is seven or less when comparing the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 50°C or less for one week after extrusion and then subjecting the resulting product to artificial aging with the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging immediately after extrusion.
Note that HV indicates Vickers hardness.
The invention directed to a method of producing an aluminum alloy extruded product comprises homogenizing a billet that is cast using the 7000-series aluminum alloy at 480 to 520°C, preheating the homogenized product at 480 to 540°C, extruding the preheated product, and subjecting the extruded product to press quenching at a cooling rate of 29°C/min or more.
When heating a cylindrical billet to a given temperature and directly or indirectly extruding the billet using an extrusion press, a high-temperature extruded product is extruded from an extrusion die. The term "press quenching" used herein refers to cooling the extruded product using a fan or the like to achieve effects of quench (quenching effects).
In order to achieve sufficient effects of quench, it is preferable that the preheating temperature of the billet be set at 480°C or more and the cooling rate after extrusion be set at 29°C/min or more.
Each component of the aluminum alloy is described below.

Zn and Mg

Zn and Mg are bonded to improve the proof stress due to precipitation hardening.
Therefore, the Zn content and the Mg content are designed corresponding to the required proof stress. The invention is characterized in that the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ is less than 0.5 mass%.
When designing the Mg content and the Zn content as described above, the Mg content is set at 0.95 to 1.95 mass% and set the Zn content at 5.10 to 7.90 mass%.
In this case, the value A=Zn-5.36×Mg is -2.64 to 0.50, taking the atomic weights of Mg and Zn into consideration.

Cu

Cu reduces the potential difference between the grain boundary and the inside of the grain with a small amount of addition to improve the stress corrosion cracking resistance. Cu also improves the proof stress.
If the Cu content exceeds 0.4 mass%, the extrudability and the corrosion resistance deteriorate.
The Cu content is preferably 0.3 mass% or less from the viewpoint of corrosion resistance.

Mn, Cr, and Zr

Mn, Cr, and Zr are bonded to Al to form minute compounds to suppress recrystallization so that a fiber structure can be obtained.
Although each of Mn, Cr, and Zr serves as a fiber structure-forming element, it is effective to add these elements in combination. In particular, it is preferable to add Zr in an amount greater than those of Mn and Cr from the viewpoint of suppressing recrystallization. It is necessary to control the content of each of these elements to less than 0.25 mass%. If the total content of these elements exceeds 0.25 mass%, the hardenability increases so that a sufficient strength cannot be obtained by air cooling.
Moreover, the size of compounds increases so that the toughness deteriorates.

Fe

Fe is an unavoidable impurity. Fe is bonded to Al and Si to form an Al-Fe-Si compound, or is bonded to Al to form an Al-Fe compound.
Such a compound tends to serve as a breakage starting point to decrease the toughness. Therefore, the Fe content is 0.35 mass% or less, and preferably 0.20 mass% or less.

Si

Si is an unavoidable impurity. Si is bonded to Al and Fe to form an Al-Fe-Si compound.
Such a compound tends to serve as a breakage starting point to decrease the toughness. Therefore, the Si content is 0.1 mass% or less, and preferably 0.05 mass% or less.

Homogenization of billet

A billet is homogenized to eliminate segregation of the main components (e.g., Mg, Zn, and Cu) in the billet and to divide and reduce the size of coarse Mn, Cr, Zr, Fe, and Si compounds that are crystallized during casting to decrease the toughness. The homogenization temperature differs depending on the aluminum alloy components (alloy series). The solution treatment temperature suitable for a 7000-series Al-Zn-Mg alloy is 450 to 550°C.
The homogenization temperature of the billet is high, 480°C or more, and ideally 520°C, while controlling the total content of elements (e.g., Mn, Cr, and Zr) that tend to undergo segregation at 0.25 mass% or less.
In general, the upper limit of the homogenization temperature could be set at 550°C because local melting may occur if the billet is held at a temperature of more than 550°C for a specific period of time.
If the homogenization temperature were less than 450°C, crystallized products produced when casting the billet would not be sufficiently divided and reduced in size. As a result, the toughness decreases.

Extrusion conditions

An Al-Zn-Mg high-strength aluminum alloy exhibits poor extrudability as compared with a 6000-series alloy. Therefore, the extrusion conditions are also important factors.
The heating temperature of the billet is 480 to 540°C. If the heating temperature is less than 480°C, the billet may not be extruded due to high extrusion resistance. If the heating temperature exceeds 540°C, the proof stress tends to decrease.
The temperature of the extrusion die is 440 to 500°C. If the temperature of the extrusion die is less than 440°C, the billet may not be extruded due to a decrease in material temperature. If the temperature of the extrusion die exceeds 500°C, the die tends to break during annealing.
The temperature of the extruded product immediately after extrusion is preferably 580°C or less. If the temperature of the extruded product exceeds 580°C, a pickup occurs on the surface of the extruded product, whereby the appearance may deteriorate.

Hollow cross-sectional shape of extruded product

FIGS. 3, 4A and 4B show cross section examples used for evaluation tests.
A double hollow cross section shown in FIG. 3 has a dimension a of 70 to 150 mm, a dimension b of 50 to 100 mm, and a thickness t of 1 to 6 mm.
A triple hollow cross section shown in FIG. 4A has a dimension a of 40 mm<a≤75 mm, a dimension b of b≤120 mm, and rib thicknesses of 3≤t₁≤8, 1≤t₂≤6, 1≤t₃₁≤6, and 1≤t₃₂≤6.
A cross section shown in FIG. 4B has a dimension a of a≤40 mm, a dimension b of b≤140 mm, and rib thicknesses of 3≤t₁≤8, 1≤t₂≤6, 1≤t₃₁≤6, and 1≤t₃₂≤6.
Note that FIGS. 4A and 4B show schematic cross sections. An upright rib may be provided outside the peripheral rib.
The cross sections shown in FIGS. 3, 4A and 4B are examples of the cross section of a bumper reinforcement member provided on the front side and the rear side of an automobile.
The side impact energy absorption during collision is increased by forming a bumper reinforcement member having a double hollow cross section or a triple hollow cross section.
Moreover, cracks rarely occur during side impact so that the toughness increases.

Effects of the Invention

In the invention, the content of Mg and Zn as the main components of the 7000-series aluminum alloy are set so that the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ is less than 0.5 mass%. Therefore, a positive effect due to storage at room temperature can be suppressed so that a decrease in side impact energy absorption can be suppressed. Moreover, the time management from extrusion to secondary processing is facilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an aluminum alloy composition.
FIG. 2 shows evaluation results for an aluminum alloy extruded product.
FIG. 3 shows an example of a double hollow cross section of an aluminum alloy extruded product according to one aspect of the invention.
FIGS. 4A and 4B show an example of a triple hollow cross section of an aluminum alloy extruded product according to one aspect of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Molten metal having the composition shown in FIG. 1 (table) was prepared, and was cast into a cylindrical billet with a diameter of 204 mm. The billet was homogenized at 480 to 520°C for about 12 hours or more.
The value of each component shown in FIG. 1 indicates an analytical value or a significant value calculated from the analytical value.
Extruded products having a double hollow cross section shown in FIG. 3 and extruded products having a triple hollow cross section shown in FIGS. 4A and 4B were air-cooled using a fan immediately after extrusion, subjected to press quenching, subjected to two-stage artificial aging (90°C×4 hours and 140°C×14 hours), which does not belong to the present invention, or subjected to artificial aging (90°C×4 hours and 140°C×14 hours) after natural aging at 40 (i.e., 50°C or less) for one week (seven days) to obtain specimens. FIG. 2 (table) shows the 0.2% proof stress (significant value) and the Vickers hardness HV (significant value) (load: 5 kg) of each specimen.
A specimen for measuring the 0.2% proof stress was prepared based on a JIS Z 2201 metal material tensile test specimen, and the 0.2% proof stress was evaluated in accordance with JIS Z 2241 "Metal Material Tensile Test Method".
The Vickers hardness HV was evaluated in accordance with JIS Z 2244 "Vickers Hardness Test Method".
Examples 1 to 7 indicate aluminum alloy extruded products according to the examples of the invention. Comparative Examples 1 to 11 are provided to clarify the characteristics of the aluminum alloy extruded products according to Examples 1 to 7 of the invention.
In the table, an Mg content of 0.95 to 1.95 is indicated as "Good", and a Zn content of 5.10 to 7.90 is indicated as "Good".
A value A=Zn-5.36×Mg of -2.64≤A≤0.50 is indicated as "Good", an increase in 0.2% proof stress of 15 MPa or less is indicated as "Good", and an increase in hardness HV (load: 5 kg) of 7 or less is indicated as "Good".
The amount of MgZn₂ added was 6.38% in Example 1, 7.95% in Example 2, and 8.90% in Example 3. The proof stress increased along with an increase in the amount of MgZn₂ added.
This tendency was also observed for the comparative examples. However, when comparing Example 1 with Comparative Examples 1, 2, 3, 4, and 8, an increase in proof stress due to natural aging was 9 MPa (i.e., 15 MPa or less) in Example 1 in which the excess Zn content (+exZn) was 0.02%. On the other hand, an increase in proof stress due to natural aging was more than 15 MPa in Comparative Examples 1, 2, 3, 4, and 8.
An increase in hardness HV due to natural aging was four (i.e., seven or less) in Example 1. On the other hand, an increase in hardness HV due to natural aging was 10 or more in Comparative Examples 1, 2, 3, 4, and 8.
Example 2 indicates a composition in which Zn and Mg were balanced. In Example 3 in which the excess Mg content (+exMg) was 0.41%, an increase in proof stress due to natural aging was 15 MPa or less, and an increase in hardness HV due to natural aging was seven or less.
In Comparative Examples 5, 6, and 7 in which the Zn content was increased to 5.40% (i.e., the Mg content was decreased), an increase in proof stress due to natural aging was more than 15 MPa.
In Examples 4 to 7, the Mg content was set at 0.95 to 1.95 and the Zn content was set at 5.10 to 7.90, and the relationship between the value A=Zn-5.36×Mg and the positive effect due to natural aging was investigated while setting the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ at less than 0.5 mass%.
When the value A was -2.64 to 0.50, an increase in proof stress due to natural aging (40°C×7 days) was 15 MPa or less, and an increase in hardness HV due to natural aging was seven or less.
In Comparative Example 8 in which the Mg content and the Zn content were within the design ranges, but the excess Mg content was 0.72 mass% (i.e., 0.5 mass% or more) and the value A was -3.86 (i.e., -2.64 or less), an increase in proof stress was 16 MPa and an increase in hardness HV was 11 (i.e., the target values of the examples of the invention were exceeded).
In Comparative Examples 9, 10, and 11, when the excess Mg content or the excess Zn content was less than 0.5 mass%, but the Mg content was 5.10% or less or the Zn content was 0.95% or less, an increase in proof stress and an increase in hardness HV exceeded the target values of the examples of the invention. Therefore, it was found that it is preferable to set the Mg content and the Zn content within the above-mentioned ranges, and set the amount of MgZn₂ at 5.4% or more, and preferably 6.0% or more.
In the examples of the invention, the difference due to natural aging at 40°C for one week was evaluated by the proof stress value and the hardness. Since it was confirmed that the positive effect due to natural aging is suppressed, it is considered that the toughness is stabilized due to artificial aging so that the impact resistance increases.

INDUSTRIAL APPLICABILITY

Since the aluminum alloy extruded products according the examples of the invention can suppress the positive effect due to artificial aging after extrusion, the artificial aging effect after secondary processing is stabilized even if the extruded product is allowed to stand at room temperature for a long period of time. Therefore, the aluminum alloy extruded products can be widely used as 7000-series aluminum alloy extruded products utilized in the field in which the required quality is strictly limited to a narrow range, such as automotive bumper reinforcement members.

Claims

A method of producing an aluminum alloy extruded product, the method comprising
- homogenizing a billet that has been cast using a 7000-series aluminum alloy at 480 to 520°C for 12 hours or more,

- preheating the homogenized product at 480 to 540°C,

- extruding the preheated product using an extrusion die that has a temperature of 440 to 500°C,

- subjecting the extruded product to press quenching at a cooling rate of 29°C/min or more, and

- subjecting the resulting product to artificial aging at 90°C for 4 hours and at 140°C for 14 hours after subjecting the product to natural aging at 50°C or less,
the 7000-series aluminum alloy
- having an Mg content of 0.95 to 1.95 mass% and a Zn content of 5.10 to 7.90 mass%,

- having an excess Mg content or an excess Zn content with respect to a stoichiometric composition shown by MgZn₂ of less than 0.5 mass%,

- having a value A indicated by a relational expression A=Zn-5.36Mg (mass%) of -2.64 to 0.50 and

- an increase in proof stress due to natural aging being 15 MPa or less and an increase in hardness HV due to natural aging being seven or less when comparing the

- proof stress and the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 40°C or less for 7 days after extrusion and then subjecting the resulting product to artificial aging at 90°C for 4 hours and at 140°C for 14 hours
with the
- proof stress and the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging at 90°C for 4 hours and at 140°C for 14 hours immediately after extrusion.