JP3454755B2 - Shock absorbing member with excellent pressure-resistant cracking resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to Al--Mg--Si.
Impact absorbing member made of extruded aluminum-based aluminum alloy and having an action of absorbing the impact load when it receives a compressive impact load, and is particularly suitable as a side member, a bumper stay and a side frame in a frame structure of an automobile. It relates to an absorbing member.

[0002]

2. Description of the Related Art In a frame structure of an automobile, application of an aluminum alloy hollow extruded material as a shock absorbing member such as a side member or a bumper stay has been studied for weight reduction. These shock-absorbing members that receive a compressive shock load in the axial direction cause Euler buckling of the entire profile when a load is applied in the axial direction of the extrusion (buckling in which the entire profile bends like a dogleg).
It is required that the structure does not occur, and that it contracts and deforms in a bellows shape without causing crush cracking to obtain stable high energy absorption and has the strength (proof strength) required as an automobile frame structural material.

As an aluminum alloy extruded material which can be used as a shock absorbing member, Al--Mg-- which is relatively superior in corrosion resistance among high strength aluminum alloys and superior in recyclability to other types of aluminum alloys.
Many Si-based aluminum alloy extruded materials have been studied (for example, JP-A-6-25783 and JP-A-7-54).
090, JP-A-7-118782, JP-A-9-256096, etc.).

As described in the above publication, Al
When applying a Mg-Si-based aluminum alloy extruded material to a shock absorbing member, generally, after press quenching online or off-line solution heat treatment,
Aging treatment is applied. The aging treatment is performed here to improve the strength of the extruded material and to stabilize the structure and prevent the natural aging during use from degrading the pressure crack resistance.

[0005]

However, Al-Mg-
Extruded Si-based aluminum alloys may be exposed to high temperatures during use as side members, etc., and natural aging may progress, resulting in deterioration of pressure crack resistance. To prevent this, aging treatment is a heat treatment type. This is an indispensable requirement when using the Al-Mg-Si-based aluminum alloy extruded material as a shock absorbing member, but when the strength is increased by performing T5 and T6 treatment, crush cracking occurs when it is compressed and deformed in the axial direction. Is a problem. When crush cracking occurs, the bellows-like contraction deformation is hindered and stable energy absorption cannot be obtained. Recently, from the viewpoint of recyclability, the same Al-Mg-Si-based aluminum alloy extruded material can be used at the same time as other automobile frame structural materials such as side frames subjected to a lateral impact load. It has become required to do. Therefore, in the present invention, Al with high strength (proof stress) is given.
-Mg-Si-based aluminum alloy extruded material is provided with excellent axial crush resistance and excellent energy absorption,
At the same time, the purpose is to give excellent resistance to pressure cracking in the lateral direction.

[0006]

In an Al-Mg-Si based aluminum alloy extruded material, when it is desired to increase the strength after aging treatment, Mg, Si and transition elements (Cu, M) are generally used.
(n, Cr, Zr, etc.) is increased, but in that case, precipitates inevitably increase at the grain boundaries and strain concentrates at the grain boundaries. In addition, if the amount of added elements is large, quenching sensitivity generally becomes sharp, and at low cooling rates (for example, air-cooled press quenching, which is advantageous in terms of dimensional accuracy and cost), the amount of precipitates at grain boundaries also increases and a large impact load is applied. When it is crushed by crushing, cracks are likely to occur. On the other hand, the present inventors have found that Al-Mg-
When the macrostructure of the Si-based aluminum alloy extruded material is mainly composed of a recrystallized structure, crush cracking is likely to occur, but a fibrous structure (a fibrous structure formed by extrusion is also regenerated during the heat treatment process after the extrusion process). It was found that even when the strength is high, the occurrence of crush cracking can be suppressed when the main structure is the structure that remains as it is without crystallization. on the other hand,
It is advantageous to form a surface recrystallized layer of a certain thickness in order to loosen the fibrous structure when performing solution hardening / quenching off-line and to obtain the required properties by online press hardening. I also understood that. The present invention has been made based on this finding.

The shock-absorbing member excellent in crush resistance according to the present invention contains 0.40 to 0.80 wt% of Mg and 0.50 to 1.0 wt% of Si, and is aged after press hardening. It is made of an Al-Mg-Si-based aluminum alloy hollow extruded material, has a proof stress of 200 MPa or more, a surface recrystallization layer thickness of 1 to 50% of the wall thickness, and the surface recrystallization layer has a grain size in the thickness direction of 200 μm. It is characterized by the following. The inside of the surface recrystallized layer of the extruded material is a fibrous texture layer. This shock absorbing member is particularly suitable for side members and bumper stays that receive a compressive impact load in the extrusion axis direction, and also as other automobile frame structural materials such as a side frame that receives a compressive impact load in the lateral direction. Available.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The Al-Mg-Si based aluminum alloy described above contains C as an additional element other than the above, if necessary.
u, Mn, Cr, Zr, Ti, F as unavoidable impurities
e, other elements may be included. Hereinafter, the composition and the like of the extruded material forming the impact absorbing member of the present invention will be described.

Mg, Si Mg and Si combine to form Mg ₂ Si and improve alloy strength. In order to obtain the strength required as an automobile frame structural material, it is necessary to add 0.40% or more of Mg. However, if it exceeds 0.80%, the quenching sensitivity becomes sharp, and if the quenching rate becomes low, quenching does not occur and the required strength cannot be obtained. Therefore, the Mg content is 0.4
0 to 0.80%. A more desirable range is 0.40
It is 0.60%. On the other hand, if the Si content is less than 0.50%, the required strength cannot be obtained, and if it exceeds 1.0%, the quenching sensitivity becomes sharp, and if the quenching rate is also low, the quenching does not occur and the required strength cannot be obtained. Therefore, the Si content is 0.50 to 1.0%. Within this Mg and Si amount range, the Si amount is particularly preferably 0.50 to 0.70% as a range in which high strength is obtained and quenching sensitivity is not so sharp.

Mn, Cr, and Zr Mn, Cr, and Zr have the action of forming a fibrous structure in the extruded material and improving the resistance to pressure cracking, and one or more of these are Mn: 0. .05 to 0.40%, Cr:
It is added in the range of 0.05 to 0.20% and Zr: 0.05 to 0.20%. If the addition amount of these transition elements is less than the lower limit, a fibrous structure will not be formed, or the surface recrystallized layer will be thick and crush cracking will occur, and if it exceeds the upper limit, quenching sensitivity will become sharp, and quenching will occur when the quenching speed is low. As a result, the strength required as a structural material for automobile frames cannot be obtained. At this time, the desirable range of each element is Mn: 0.05 to 0.25.
%, Cr: 0.05 to 0.15%, Zr: 0.05 to
It is 0.15%. The total addition amount of these transition elements is 0.05 to 0.60%, and 0.10 to 0.40%
Is more desirable, and 0.2 to 0.3% is desirable.

Cu Cu has the effect of increasing the strength of the Al--Mg--Si system aluminum alloy and improving the resistance to stress corrosion cracking, and is added as necessary. However, if it is less than 0.05%, the action is insufficient, and if it exceeds 0.7%, the extrudability and general corrosion resistance are deteriorated, so the content is preferably 0.05 to 0.7%. A more desirable range is 0.10 to 0.35%. Ti Ti has the effect of refining the ingot structure and is added as appropriate. However, if it is less than 0.005%, the effect of miniaturization is not sufficient, and if it is more than 0.2%, it is saturated and a huge compound is generated. Therefore, the Ti content is 0.005
~ 0.2%. A more desirable range is 0.01-0.
10%, more desirable range is 0.015-0.05%
Is.

Inevitable Impurities Of the unavoidable impurities, Fe is the most contained impurity in the aluminum base metal, and if more than 0.35% is present in the alloy, coarse intermetallic compounds crystallize during casting and the alloy mechanically acts. Spoil the nature. Therefore, the Fe content is 0.35
% Or less. It is preferably 0.30% or less,
Furthermore, 0.25% or less is desirable. Further, when casting an aluminum alloy, impurities are mixed in through various routes such as a base metal and an intermediate alloy of additive elements. Although various elements are mixed in, if the impurities other than Fe alone are 0.05% or less and the total amount is 0.15% or less, the characteristics of the alloy are hardly affected. Therefore, these impurities are 0.05
% Or less, and the total amount is 0.15% or less. It should be noted that, of the impurities, B is mixed in the alloy in an amount of about 1/5 of the Ti content as Ti is added, but a more desirable range is 0.02% or less, and further 0.01% or less.

Strength (Proof Strength) In consideration of application as an automobile frame structural material (shock absorbing member), if the strength is less than 200 MPa, the amount of energy absorbed decreases, so the strength must be 200 MPa or more. However, if the composition is out of the above range, the strength will not be obtained (the amount of energy absorbed will also be greatly reduced), or a fibrous structure will not be formed, resulting in inferior resistance to pressure cracking.
A desirable range of proof stress is 220 MPa or more.

Surface Recrystallization Layer In the above Al—Mg—Si type aluminum alloy extruded material, it is desirable that the fibrous structure of the extruded material is formed in almost the entire cross section of the extruded material. If the extruded material has a thickness of 1 to 5 mm, the surface recrystallized layer needs to be 50% or less of the total thickness. It is preferably 30% or less. This is because the recrystallized grains have a larger crystal grain size than the fibrous structure, and when the cooling rate is low, many precipitates precipitate at the crystal grain boundaries during the cooling process, and the grain boundaries of the surface recrystallized grains become larger. This is because strain is concentrated and cracks are likely to occur. On the other hand, Al-Mg-S having the above composition and thickness
With the i-type aluminum alloy extruded material, almost all of the cross section of the extruded material (the surface recrystallization layer thickness is less than 1% of the wall thickness) can have a fibrous structure at an extrusion speed of 5 m / min or less.
At such a low extrusion speed, it takes time to enter the quenching zone in press quenching (online), so that quenching hardly occurs and it becomes difficult to obtain required strength, energy absorption amount and maximum load. Therefore, the surface recrystallized layer thickness needs to be 1% or more of the wall thickness. It is preferably at least 5%. The crystal grain size of the surface recrystallized grains is 200 μm.
Hereafter, it is desirable that the thickness be 100 μm or less. This is because the larger the grain size of the surface recrystallized grains, the more strain is collected and cracks are more likely to occur.

[0015]

EXAMPLES Examples of the present invention will be described below. D
Al-Mg-Si having the composition shown in Table 1 by C casting
Molten aluminum alloy billet, 550 ℃ × 4h
Soaking was performed for r. Subsequently, extrusion processing was performed at an extrusion temperature of 500 ° C. and an extrusion rate shown in Table 1, and immediately after extrusion, online press-quenching by air cooling or water cooling was performed.
Extruded material with a hollow rectangular cross section as shown in Fig.
mm, the short side is 54 to 60 mm, and the wall thickness is 2 to 5 mm). The air cooling is fan cooling, and the cooling rate is approximately 190 ° C /
min, water cooling was at a cooling rate of about 10,000 ° C./min. Next, this extruded material was subjected to an aging treatment at 190 ° C. for 3 hours to obtain a test material.

The surface recrystallized layer (GG
Layer) thickness, crystal grain size of the surface recrystallized layer, mechanical properties, and longitudinal and lateral crushing properties were examined in the following manner. The results are shown in Table 1.
And shown in Table 2. Surface recrystallized layer thickness: In the cross section parallel to the extrusion direction, the front surface and the back surface are taken at four points, the recrystallized layer thicknesses are measured, the front and back sides are averaged, and the average value of the front and back sides is averaged. The sum of the values was taken as the GG layer thickness. Surface recrystallized layer grain size: in a cross section parallel to the extrusion direction, from the surface to the center of the wall thickness by the cutting method, the front side 4
The number of spots and four spots on the back side were measured, and the average thereof was taken as the GG layer grain size. Here, the particle size is measured from the surface toward the central part of the wall thickness because the particle size measured in this direction is particularly related to the occurrence of cracks. (In the present invention, when the surface recrystallized layer thickness is not sufficiently larger than the surface recrystallized layer particle size and the particle size measurement is difficult, the surface recrystallized layer particle size is defined as the surface recrystallized layer thickness.) Mechanical properties: JIS No. 5 test pieces were taken from the test material, and J
A tensile test was conducted according to IS Z 2241.

Longitudinal compression characteristics: Using a test material having a length of 200 mm, an Amsler tester applies a static compressive load in the axial direction as shown in FIG. 2, and compresses this to 100 mm to obtain a load-displacement curve. Then, the maximum load and the absorbed energy up to 100 mm were obtained. The cracking property was evaluated by visual inspection, and those with no cracks were rated as ⊚, those with minute cracks as ◯, and those with opening cracks as x. Lateral compression characteristics: Using a test material having a length of 200 mm, as shown in Fig. 3, it is placed sideways so that the long sides are up and down and a static compressive load is applied, and this is compressed to 20 mm, and the crackability is evaluated. Was visually observed. The case where no crack was generated was evaluated as ⊚, the case where the minute crack was generated was evaluated as ○, and the case where the opening crack was generated was evaluated as ×.

[0018]

[Table 1]

[0019]

[Table 2]

As shown in Table 2, those having a GG layer thickness within the specified range of the present invention (Nos. 1 to 8) do not cause crush cracking, and have high yield strength and excellent energy absorption characteristics (crush crack in both length and width). None, high absorbed energy and maximum load). On the other hand, in No. 1 in which the surface GG layer is not formed. No. 9 has a proof stress of less than 200 MPa, a low absorbed energy and a maximum load, and a GG layer thickness exceeding the specified range of the present invention (No. 10 to 15) has poor crush cracking resistance and a GG layer thickness of this. Even within the stipulated range of the invention, the proof stress does not reach 200 MPa. No. 16 has a low absorbed energy and a maximum load, and has a GG layer grain size of more than 200 μm. No. 17 is inferior in crush cracking resistance.

[0021]

EFFECTS OF THE INVENTION According to the present invention, an Al-Mg-Si-based aluminum alloy extruded material has high strength and excellent energy absorption characteristics by defining the yield strength, the ratio of the surface recrystallized layer, and the grain size. The shock absorbing member shown can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a cross-sectional shape of an extruded material used in Examples.

FIG. 2 is a diagram illustrating a vertical crush test of an example (before crush,
After crushing).

FIG. 3 is a diagram illustrating a lateral crush test of an example (before crush,
After crushing).

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-5-171328 (JP, A) JP-A-6-25783 (JP, A) JP-A-9-256096 (JP, A) JP-A-5- 247575 (JP, A) JP 2000-54049 (JP, A) JP 2000-54051 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 21 / 00-21 / 18 B62D 21/15

Claims (2)

(57) [Claims] 1. Mg of 0.40 to 0.80 wt%, Si
The content is 0.50 to 1.0 wt% and is made of an Al-Mg-Si-based aluminum alloy hollow extruded material which has been aged after press hardening, and has a yield strength of 200 MPa or more and a surface recrystallization layer thickness of 5 to 5 mm. 50% (less than 100 μm
Except that the grain size in the thickness direction of the surface recrystallized layer is 2
Compressed in the axial direction of extrusion , characterized by being less than 00 μm
Shock absorbing member with excellent resistance to crushing and cracking under the impact load of . 2. Mg of 0.40 to 0.80 wt% and Si
The content is 0.50 to 1.0 wt% and is made of an Al-Mg-Si-based aluminum alloy hollow extruded material which has been aged after press hardening, and has a yield strength of 200 MPa or more and a surface recrystallization layer thickness of 5 to 5 mm. 50% (less than 100 μm
Except that the grain size in the thickness direction of the surface recrystallized layer is 2
A side member having excellent resistance to pressure cracking and cracking, which is characterized by having a thickness of 00 μm or less.