JP2008274441A - Aluminum alloy extruded material excellent in crushing characteristics - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an aluminum alloy extruded material excellent in crushing characteristics, suitable to a bumper reinforcement, etc. <P>SOLUTION: This aluminum alloy extruded material is composed by mass% of 0.5-1.8% Mg, 4.0-8.0% Zn, 0.005-0.3% Ti, 0.05-0.6% Cu and further, one or more kinds of 0.1-0.7% Mn, 0.03-0.3 Cr and 0.05-0.25% Zr and the balance Al with inevitable impurities and has a fibrous structure and has ≥20% thickness reduction ratio in the broken surface when the JIS No.5 test piece is subjected to a tensile strength. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an aluminum alloy extruded material having excellent crushing characteristics suitable as an energy absorbing member for automobiles, for example.

Energy absorbing members are used to improve the collision safety of automobiles. For example, as an energy absorbing member for mitigating damage to a vehicle body at the time of a light collision, there is a bumper reinforcing material for automobiles, and the use of an aluminum alloy extruded material has been studied from the viewpoint of weight reduction (Patent Document 1). ~ 4 etc.). This bumper reinforcing material is formed, for example, as a hollow extruded material having a rectangular cross section, and is a so-called crushable material. When energy is applied from the outside due to a collision or the like, the collision energy is deformed (collapsed) into the hollow portion. So that other members are not damaged as much as possible.
FIG. 1 shows a deformation process of a bumper reinforcing member 1 having a hollow rectangular cross section (flange portions 1a, 1b and web portions 1c, 1d). A compressive load is perpendicular to the outer flange portion 1a side of the bumper reinforcing member 1. When added, the web portions 1c and 1d facing each other are deformed to deform the hollow rectangular cross section (see the phantom line), and energy due to the load is absorbed during the deformation process.

  The amount of energy (minimum value) that should be absorbed by such a bumper reinforcement is determined by legal standards, etc. On the other hand, if the amount of energy that can be absorbed by the bumper reinforcement is too large, the design becomes overdesigned. The weight becomes excessive. Accordingly, the bumper reinforcing material is designed so as to absorb necessary energy and not to be excessively designed and excessively weighted.

Japanese Patent Laid-Open No. 7-70688 JP-A-8-170139 Japanese Patent Laid-Open No. 8-144031 JP-A-9-268342

As described above, the bumper reinforcing material representing the energy absorbing member is required to have a large energy absorption amount and be lightweight. In order to cope with this demand, recently, attempts have been made to increase the material strength of the aluminum alloy extruded material constituting the bumper reinforcement. However, when a high strength material such as the 7000 series (Al-Mg-Zn series) aluminum alloy described in the above publication is used, cracks are likely to occur in the web portion. The energy absorption capacity is reduced.
That is, the energy absorption capacity of the aluminum alloy extruded material and the increase in strength for weight reduction are contradictory problems, and it has been difficult to solve these metallurgically in terms of alloy components, structures, and the like.

  The present invention has been made in view of such a situation, and the strength is increased by using a high-strength material of an Al—Mg—Zn-based aluminum alloy, and at the same time, cracking during crushing is prevented to ensure a high energy absorption amount. The purpose is to do.

  In the course of conducting various experimental studies to develop such an aluminum alloy extruded material with excellent crushing characteristics, the present inventors have made a fracture surface of the test piece when a JIS No. 5 tensile test piece taken from the extruded material is subjected to a tensile test. It was found that the thickness reduction rate of the steel was closely related to the crushing characteristics of the extruded material, and based on this, the present invention could be obtained.

That is, the aluminum alloy extruded material according to the present invention has Mg: 0.5-1.8%, Zn: 4.0-8.0%, Ti: 0.005-0.3%, Cu: 0.05 -0.6%, and Mn: 0.1-0.7%, Cr: 0.03-0.3%, Zr: 0.05-0.25%, one or more Is an aluminum alloy extruded material composed of the balance Al and inevitable impurities, and has a fibrous structure, and when the tensile test is carried out with a JIS No. 5 tensile test piece, the thickness reduction rate of the fracture surface is 20 % Or more. Here, the thickness reduction rate of the fracture surface (hereinafter referred to as “squeezing”) is the thickness at the center of the fracture surface when the fracture surface of the tensile specimen is viewed from the front (perpendicular to the plate surface of the specimen). The measured thickness is represented by (1-a / a ₀ ) × 100, where a (see FIG. 1) is a and the original thickness of the tensile test piece is a ₀ .
This aluminum alloy extruded material has excellent crushing characteristics, and is particularly suitable as a member for automobiles such as a bumper reinforcement, a frame, a door beam, etc., which receives a lateral compressive load.

According to the present invention, it is possible to obtain an aluminum alloy extruded material that has high strength by using a high-strength aluminum alloy and at the same time has excellent crushing characteristics when subjected to a compressive load of collision, particularly excellent crack resistance against lateral crushing. .
The aluminum alloy extruded material according to the present invention includes, for example, a bumper reinforcing material, a door beam (an energy absorbing member disposed in the front-rear direction in the door and receiving an impact load from the side), an impact beam (arranged in the front-rear direction of the door) Energy absorbing member that receives the impact load in the axial direction), steering member (reinforcement material for the instrument panel arranged in the vehicle body width direction), automobile frame (side member, cross member, bumper stay, side frame, etc.), railway It can be used for applications such as frames of vehicles and ships.

Subsequently, the reason for adding each component and the reason for limiting the composition of the aluminum alloy extruded material according to the present invention will be described.
Zn
Zn coexists with Mg and imparts aging properties to the alloy, and has the effect of improving strength by artificial aging. If the Zn content is less than 4.0%, the strength is not sufficiently improved, and the amount of energy absorption decreases. On the other hand, if it exceeds 8.0%, the extrudability is lowered and the elongation and bending workability are also lowered. Furthermore, SCC resistance and general corrosion resistance are reduced. Therefore, the Zn content is 4.0 to 8.0%, and more preferably 6.0 to 7.0%.

Mg
Mg is a main element that increases the strength of the aluminum alloy. However, if the Mg content is less than 0.5%, the effect of improving the strength cannot be sufficiently obtained, and the energy absorption amount decreases. On the other hand, if it exceeds 1.8%, the extrudability is lowered and the elongation is also lowered. Furthermore, SCC resistance and bending workability are reduced. Therefore, the content of Mg is 0.5 to 1.8%, more preferably 0.6 to 1.0%, and still more preferably 0.7 to 0.9%.

Ti
Ti has the effect of refining crystal grains in the aluminum alloy ingot. However, if the content is less than 0.005%, the effect cannot be sufficiently obtained, and if it exceeds 0.3%, the crystal grain refining effect is saturated and a giant compound is generated. Therefore, the Ti content is set to 0.005 to 0.3%.
Cu
Cu has an effect of increasing the strength of the aluminum alloy, and is added to obtain a target high strength. Moreover, Cu has the effect | action which improves SCC resistance. However, if the Cu content is less than 0.05%, the effect is insufficient, the energy absorption amount is small, and the SCC resistance is poor. On the other hand, if it exceeds 0.6%, the extrudability deteriorates, and further, the quenching sensitivity is increased and the strength is lowered, and the bending workability and the general corrosion resistance are deteriorated. Moreover, weldability also deteriorates. Therefore, the Cu content is 0.05 to 0.6%, preferably 0.1 to 0.2%.

Mn, Cr, Zr
Since these elements have a function of forming a fibrous structure in the aluminum alloy extruded material and strengthening the alloy, one or more of them are added. However, if less than 0.1%, 0.03%, and 0.05%, respectively, the effect is insufficient, while if exceeding 0.7%, 0.3%, and 0.25%, respectively, the extrudability Worsens, further increases the quenching sensitivity and causes a decrease in strength. Therefore, it is set as the range of Mn: 0.1-0.7%, Cr: 0.03-0.3%, Zr: 0.05-0.25%. Zr is preferably 0.1 to 0.2%. In order to form a fibrous structure, it is desirable to contain a total of 0.1% or more of one or more of them. In particular, when press quenching by air cooling is performed, a total of 0.4 is used to prevent a decrease in quenching sensitivity. % Or less is desirable.

Inevitable Impurities Among the inevitable impurities, Fe is the most abundant impurity in aluminum ingots. If it exceeds 0.35% in the alloy, coarse intermetallic compounds are crystallized during casting, which impairs the mechanical properties of the alloy. . Therefore, the Fe content is restricted to 0.35% or less. Further, when casting an aluminum alloy, impurities are mixed from various paths such as a metal base and an intermediate alloy of an additive element. The elements to be mixed are various, but impurities other than Fe alone are 0.05% or less, and if the total amount is 0.15% or less, the characteristics of the alloy are hardly affected. Accordingly, these impurities are 0.05% or less as a single substance, and the total amount is 0.15% or less. Of the impurities, B is mixed in the alloy in an amount of about 1/5 of Ti with the addition of Ti, but a more desirable range is 0.02% or less, and further preferably 0.01% or less.

Drawing In the Al-Mg-Si-based aluminum alloy extruded material, when the drawing value defined in the present invention is 20% or more, the value of drawing shows the local deformability of the material. On the other hand, when the extruded material undergoes crushing deformation, the elongation generated on the very surface of the material reaches about 30%, and in such a region, the material is locally deformed like a squeeze. Therefore, it is presumed that the occurrence of cracking was suppressed when the aperture value was greater than or equal to a predetermined value.
In order to set the drawing value to 20% or more within the above composition range, it is effective to increase the cooling rate during quenching, for example, in the case of press quenching by air cooling. As shown in the following examples, it was possible to obtain an aperture of 20% or more with a high strength by aging treatment.

  In the energy absorbing member according to the present invention, the crystal structure of the aluminum alloy extruded material has a fibrous structure. Here, the fibrous structure is a hot-worked structure found in the extruded material, which is a grain structure that has been elongated in the direction of extrusion, and has higher strength and higher drawing than the equiaxed recrystallized structure. To improve the crushability of the extruded material. Even when the surface recrystallized structure is formed, the surface recrystallized structure needs to be formed with a thickness of about 1/2 or more of the cross-sectional thickness of the extruded material. Further, the cross-sectional shape of the aluminum alloy extruded material is not particularly limited, but an appropriate closed cross-sectional shape, for example, the front and rear flange portions that are substantially perpendicular to the load direction, and connecting them to be substantially parallel to the load direction. A substantially rectangular cross section consisting of a pair of web portions.

  Al-Mg-Zn alloys having chemical components shown in Table 1 were melted by a conventional method and cast into an ingot having a diameter of 200 mm by a semi-continuous casting method. After this ingot was homogenized, it was extruded into the cross-sectional shape shown in FIG. 2 and quenched (press-quenched) from the high temperature state (460 ° C.) during extrusion by air cooling. Air cooling was performed by directly jetting high-speed cooling air jetted from the nozzle onto the surface of the extruded material, and the cooling rate was about 300 ° C./min. This is a cooling rate much higher than the normal fan air cooling used for press quenching. However, no. For Nos. 11 and 12, press quenching was performed by ordinary fan air cooling (cooling rate: about 200 ° C./min). Subsequently, this extruded material was cut into short pieces, and similarly subjected to the aging treatment shown in Table 1 to obtain test materials.

A JIS No. 5 test piece was taken in the length direction from the web portion (width 40 mm side) of the test material, and a tensile test was performed to measure the mechanical properties of each test material. Further, using a 30 ton universal testing machine, as shown in FIG. 3, the rigid body 4 is fixed from the upper surface of the test material 3 fixed to the exclusive stay 2 (test material mounting surface: length direction 80 mm × width direction 50 mm) with double-sided tape. And a lateral crush test was performed (displacement amount 35 mm = effective stroke). Table 2 shows the mechanical properties of each test material, the maximum load of each test material by the lateral crush test, the absorbed energy within the effective stroke, and its crushing cracking property. The crushing cracking properties shown in Table 2 are the evaluations of the cracking properties of the web part in three stages: ◎: No cracking, ○: Microcracking, no splitting cracks penetrating the thickness, ×: With splitting cracks, is there.
In addition, No. 1 (Nos. 3 and 11 are the same), the extrusion speed was 7 m / min. The limit extrusion speed at which a surface quality equivalent to 1 was obtained was measured. Equivalent to 1 (in the case of 90% or more of No. 1) was evaluated as ○, 70 to 89% as Δ, and 69% or less as ×. The results are also shown in Table 2.

Further, an Al—Mg—Zn alloy ingot having chemical components shown in Table 1 was subjected to extrusion molding-press quenching under the same production conditions as the above extruded material to obtain a 2 mm × 150 mm extruded material (flat bar). Were subjected to the same heat treatment as test materials. About each test material, the SCC resistance was measured in the following way.
A test using a chromic acid aqueous solution (36 g of chromic anhydride, 30 g of potassium dichromate, 3 g of sodium chloride in 1 liter of pure water) as a test solution so that stress is applied in the direction perpendicular to the extrusion (LT direction) from each test material. Pieces were collected and tested under the conditions of applying a load of 100% and 75% of the material yield strength in the manner of a test temperature of 95 ° C. and three-point bending. Evaluation criteria were after immersion for 360 minutes, observed with a magnifying glass at 25 times, and evaluated by the presence or absence of cracks on the material surface. The case where there was no crack was evaluated as ◯, when only the 100% load test piece was cracked, Δ, and when the 75% load test piece was cracked, it was evaluated as x.

As shown in Table 2, the composition was within the scope of the present invention and the aperture was 20% or more. Nos. 1 and 2 are excellent in lateral crushing properties such as maximum load, absorbed energy, and crack resistance, and are also excellent in extrudability and SCC resistance.
On the other hand, although the composition is within the scope of the present invention, the aperture is less than 20%. 3, 4, 11, 12 are inferior in crack resistance. No. For No. 3, the aging time was insufficient. No. 4 is a recrystallized structure. 11 and 12 may have been affected by the low cooling rate of press quenching by air cooling.
In addition, No. in which the main component deviates from the definition of the present invention. Nos. 4 to 10 have a drawing of 20% or more and relatively good crack resistance, but extrudability (No. 5, 9), SCC resistance (No. 5, 7, 10), and strength (No. 6, 8). 9) and any of the characteristics of maximum load and absorbed energy (No. 6, 8, 9) are inferior.

It is a figure which shows one form (imaginary line) before a deformation | transformation of a bumper reinforcement (solid line) and a deformation process. It is a figure which shows the cross-sectional shape of the energy absorption member used for the Example. It is a figure which shows the lateral crush test method of an Example.

Explanation of symbols

2 Stay 3 Test material 4 Rigid body

Claims (1)

Mg: 0.5-1.8% (mass%, the same shall apply hereinafter), Zn: 4.0-8.0%, Ti: 0.005-0.3%, Cu: 0.05-0.6% In addition, Mn: 0.1 to 0.7%, Cr: 0.03 to 0.3%, Zr: 0.05% to 0.25% of one or two or more types, This is an aluminum alloy extruded material composed of the balance Al and inevitable impurities, which has a fibrous structure and has a thickness reduction rate of 20% or more at the fracture surface when a tensile test is carried out with a JIS No. 5 tensile test piece. An aluminum alloy extruded material with excellent crushing characteristics.

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