JP4311679B2 - Manufacturing method of energy absorbing member for automobile - Google Patents

Description

  The present invention relates to a method for manufacturing an energy absorbing member made of aluminum alloy used as a reinforcing member for automobiles.

As shown in FIG. 2, when a load (P) is applied from the compression side to the central portion while supporting both ends of the aluminum alloy door beam (referred to as a three-point bending test), the central portion of the door beam is pushed in. This causes bending deformation, and tensile force acts on the tension side (occupant side) flange. Further, when the displacement amount (δ) increases and the tensile force exceeds the fracture limit value of the material, a fracture (crack) occurs in the tension side flange.
In order to increase the displacement until fracture (breaking displacement), the following Patent Document 1 discloses that the position of the neutral axis of bending is biased by a necessary amount toward the tension side, and the following Patent Document 2 discloses a neutral axis. It is disclosed that after the maximum bending strength is achieved, local buckling is induced on the compression side and the stress applied to the tension side flange is rapidly reduced.

JP-A-5-246242 JP-A-6-171362

However, with the recent increase in safety measures, it has become necessary to further increase the breaking displacement.
For example, in the above Patent Document 2, the height (H) of the door beam material is set to 30 to 35 mm, and the door beam having the same height is described in the above Patent Document 1, but the initial rigidity is increased without increasing the weight. In order to earn, it is effective to increase the sectional moment by increasing the height around the axis subjected to bending, and therefore, a door beam designed with a beam height larger than 35 mm has come out. However, in that case, the tension side flange breaks with a small displacement compared to the conventional one.
In addition, the application types of door beams tend to spread to small cars, and in this case, the beam length becomes shorter. For example, when this is applied to the rear door of a small four-door car, the beam length may be 700 mm or less (the above Patent Document 2 targets 700 mm or more), and accordingly, a conventional door beam having a long beam length. Breakage occurs with a small displacement compared to.

  As a safety measure, if it is only necessary to prevent breakage of the flange on the tension side during bending deformation, an element that improves the strength, that is, Zn, Mg, Cu, etc., is added, and annealing is performed to increase the elongation. However, in this case, the strength, proof stress, and energy absorption amount that the material can exhibit are greatly sacrificed, and the demand for weight reduction cannot be met.

  In view of the present situation where such further safety measures and weight reduction are required for the energy absorbing member of an automobile including an aluminum alloy door beam, the present invention, when the aluminum alloy energy absorbing member receives a bending load, An object of the present invention is to obtain an energy absorbing member that can obtain a larger breaking displacement than before and that does not excessively sacrifice the potential of the material itself in terms of maximum strength and energy absorption.

  The energy absorbing member according to the present invention is made of a heat-treated aluminum alloy extruded material that has been over-aged. This heat-treatable aluminum alloy is made of Al-- from the viewpoint of strength, maximum load, breaking displacement, energy absorption amount until breaking, and the like. A Zn—Mg-based aluminum alloy is particularly suitable, and its specific composition is: Zn: 4-7%, Mg: 0.8-1.5%, Ti: 0.005-0.3%, Cu: One or two selected from 0.05 to 0.6%, Mn: 0.2 to 0.7%, Cr: 0.05 to 0.3%, Zr: 0.05 to 0.25% It contains the above, and the balance consists of Al and inevitable impurities. And the manufacturing method of the energy-absorbing member according to the present invention is characterized in that after heat-treating the heat-treated aluminum alloy extruded material so as to obtain the maximum strength, an overaging treatment is performed using a paint baking process. .

  According to the present invention, the breaking displacement of an energy absorbing member made of heat treated aluminum alloy extruded material can be greatly improved. For example, a door beam can obtain a large breaking displacement even with a short beam length or a large beam height. it can.

By performing an overaging treatment on the heat-treated aluminum alloy extruded material, the maximum load in bending deformation is somewhat reduced, but the fracture displacement is greatly improved. In addition, looking at the examples to be described later, there is no improvement in elongation due to overaging treatment, and therefore, the improvement in fracture displacement is not due to a decrease in strength and an improvement in elongation, unlike in the case of annealing, and is completely different. I guess it is due to the mechanism.
Here, the overaging treatment is to perform an aging treatment at a higher temperature or longer time than the aging treatment conditions for obtaining the maximum strength. Specifically, for example, if the maximum strength at T ₁ ° C. is obtained at H ₁ min when aging treatment is performed at a processing temperature T ₁ ° C., the processing condition of T ₁ ° C. × (H ₁ + α) min is applied. If the maximum strength at H ₂ min is obtained at T ₂ ° C. when the aging treatment is performed at a treatment time of H ₂ min, the processing condition of (T ₂ + β) ° C. × H ₂ min is obtained. When applied, it becomes an overaging treatment. α and β are positive values.

  The maximum strength here is the maximum value of yield strength obtained by aging treatment of extruded material that has been solution-quenched or press-quenched (quenched immediately after extrusion), and is solution-quenched or press-quenched under the same conditions. If it is an extruded material, its value can be specified. The treatment conditions vary depending on the treatment temperature and the treatment time is not uniquely determined. For example, in the case of an Al—Zn—Mg alloy, it is, for example, 117 to 123 ° C. × 18 to 24 hours or 127 to 133 ° C. × 11. ~ 14 hr. In the case of an Al—Mg—Si based alloy, for example, 177 to 183 ° C. × 7.5 to 8.5 hr or 187 to 193 ° C. × 2.5 to 3.5 hr.

In addition, for example, when the aging treatment is stopped once the maximum strength is obtained, and the aging treatment is performed by heating again, the overaging treatment referred to in the present invention is performed.
The conditions of the overaging treatment in this case differ depending on the alloy system and the required fracture displacement and cannot be uniquely determined. However, in the Al—Zn—Mg alloy, the aging treatment in the previous step (the highest strength was obtained). Holding for 15 minutes to 1 hour at a temperature 40 to 60 ° C. higher than the aging temperature of the aging treatment) is mentioned as a standard. Moreover, what is necessary is just to make the intensity | strength (proof stress or tensile strength) into the 5%-10% vicinity from the maximum value.
In an Al—Zn—Mg alloy, this overaging treatment can be performed by using an automobile paint baking process. (In the case of Al-Mg-Si alloys for automobiles and the like, it is well known, for example, in Japanese Patent Application Laid-Open No. 5-44000, to use so-called bake hardware that is age-hardened using a paint baking process to improve strength. However, there is no example of using this process for overaging treatment.)

  The Al—Zn—Mg based aluminum alloy suitable for the present invention is a precipitation hardening type alloy mainly composed of Zn and Mg, and has the following composition. Zn: 4-7%, Mg: 0.8-1.5%, Ti: 0.005-0.3%, Cu: 0.05-0.6%, Mn: 0.2-0.7 %, Cr: 0.05-0.3%, Zr: One or more selected from 0.05-0.25%, the balance being Al and inevitable impurities. The reasons for limiting each component are as follows.

Zn, Mg
Zn and Mg are elements necessary for maintaining the strength of the aluminum alloy. If Zn is less than 4% and Mg is less than 0.8%, the desired strength cannot be obtained. On the other hand, if Zn exceeds 7% and Mg exceeds 1.5%, the extrudability of the aluminum alloy is lowered and the elongation is also lowered, making it impossible to obtain the required characteristic values. Therefore, Zn: 4-7%, Mg: 0.8-1.5%.

Ti
Ti is added to refine the ingot structure. When Ti is less than 0.005%, the effect of miniaturization is not sufficient, and when it is more than 0.3%, saturation occurs and a huge compound is generated. Therefore, the Ti content is set to 0.005 to 0.3%.
Cu, Mn, Cr, Zr
These elements have the effect | action which raises the intensity | strength of an aluminum alloy, and 1 type (s) or 2 or more types are added suitably from these. In addition, Cu improves the stress corrosion cracking resistance of the aluminum alloy. Preferred ranges are: Cu: 0.05-0.6%, Mn: 0.2-0.7%, Cr: 0.05-0.3%, Zr: 0.05-0.25% . If the amount is less than the lower limit, the above action is insufficient, and if the upper limit is exceeded, the extrudability deteriorates, and in the case of Cu, the general corrosion resistance deteriorates.

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.

  In addition, by using the heat-treated aluminum alloy extruded material that has been over-aged, it is possible to obtain an energy-absorbing member that is excellent in fracture resistance during bending deformation, including not only the door beam but also a bumper and the like. Moreover, since the heat-treated aluminum alloy extruded material that has been over-aged is excellent in fracture resistance at the time of bending deformation, it can be widely used as a bending member (a material subjected to bending) for various applications.

  Aluminum alloys having the components shown in Table 1 were melted by a conventional method and cast into an ingot having a diameter of 200 mm. This ingot was soaked at 470 ° C. for 8 hours, extruded at an extrusion temperature of 470 ° C. and an extrusion speed of 4 m / min, and cooled by blowing nitrogen gas at the position immediately after extrusion onto the surface of the extruded material. The cross-sectional shape of the extruded material is as shown in FIG. The extruded material was subjected to an aging treatment of 130 ° C. × 12 hr, and the heat treatment equivalent to baking (170 ° C. × 60 min) was further performed for the comparative examples as they were for the comparative examples to obtain test materials. In FIG. 1, the upper flange is the compression side, and the lower flange is the tension side (occupant side).

  JIS No. 13 B tensile test specimens were collected from the specimens and examined for mechanical properties. Further, a test material is cut out from this test material, a span L is set to 600 mm, a three-point bending test is conducted, and the displacement (δ) is pushed in until 12 inches (305 mm), and the tensile side (occupant side) flange is broken. The amount of displacement that occurred (becomes separated by cracks) was measured. The test results are shown in Table 2.

The extruded material used in this test has a large beam height (40 mm) and a beam length, ie, a short span L (600 mm), but as shown in Table 2, an overaging treatment (heat treatment equivalent to baking). In the test materials of the examples, the maximum load was slightly reduced as compared with the comparative example, but no fracture occurred even with a displacement of 12 inches. It also excels in energy absorption.
In addition, the elongation of the test material of an Example was a little smaller than the comparative example. That is, in the over-aged example, although the elongation was smaller than that of the comparative example, the fracture did not occur with a large displacement.

It is the cross-sectional shape of the door beam used for the Example. It is explanatory drawing which shows the cross-sectional shape example (a) of a door beam, the state (c) of a fracture | rupture by a three-point bending test (b) using the same, and a three-point bending test.

Claims (2)

Zn: 4-7% (mass%, the same applies hereinafter), Mg: 0.8-1.5%, Ti: 0.005-0.3%, Cu: 0.05-0.6%, Mn: Contains one or more selected from 0.2 to 0.7%, Cr: 0.05 to 0.3%, Zr: 0.05 to 0.25%, the balance being Al and inevitable impurities A method for producing an energy absorbing member for an automobile, comprising subjecting an Al—Zn—Mg-based aluminum alloy extruded material made of aging to an aging treatment so as to obtain the highest strength and then performing an overaging treatment using a paint baking process. The method for manufacturing an energy absorbing member for an automobile according to claim 1, wherein the energy absorbing member is a bumper.

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