JP2005105327A - Extruded material of aluminum alloy to be electro-magnetic-formed - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an extruded material of an aluminum alloy superior in electro-magnetic formability, which is free from cracking when being electro-magnetic-formed at a tube-expanding rate even as high as 15% or more. <P>SOLUTION: The extruded material of the aluminum alloy includes 0.2-1.5% Si (mass%, hereafter the same), 0.3-1.5% Mg and 95% or more Al; and has crystal grains with an average aspect ratio of 5.0 or lower in the center of a plate thickness. The extruded material is preferably used for manufacturing a bumper stay through the step of forming a flange 26 in the edge of the hollow extruded material 25 by electro-magnetic forming. The tube expanding rate is expressed by δ=ä(l-l<SB>0</SB>)/l<SB>0</SB>}×100 (%), wherein l<SB>0</SB>represents an outer circumferential length in a not-yet-expanded part of the tube, and l represents an outer circumferential length of an expanded part of the tube (the outer circumferential length of the flange if the flange is formed). The alloy preferably limits each content of Ti, Cu, Cr and Zr to 0.1% or less, Mn to 0.15% or less, the above total content to 0.3% or less and Fe to 0.35% or less, and contains 98% or more Al. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an aluminum alloy extruded material for electromagnetic forming that is excellent in electromagnetic formability and suitable for forming a frame or a joint portion of an automobile, a railway vehicle, or a building member in addition to an automobile bumper stay.

  For example, bumper reinforcement is provided as a reinforcing member inside a bumper installed at the front end (front) and rear end (rear) of an automobile body such as a passenger car or a truck. The bumper reinforcement is a hollow member having a front wall and a rear wall that are generally perpendicular to the load direction, and a lateral wall that connects them, and is supported by a pair of bumper stays from the rear side. It is fixed to the tip of the side member (front or rear).

  In recent years, an aluminum alloy extruded material has been used for bumper reinforcement to reduce the weight, and an aluminum alloy extruded material has also started to be used for a bumper stay. Bumper stays made of an aluminum alloy extruded material are roughly classified into a vertical crushing type and a lateral crushing type, and the vertical crushing type bumper stay 1 is a hollow that forms a shaft portion 2 as shown in FIG. Plate-like mounting flanges 3 and 4 (for mounting the bumper reinforcement 5 and the side member 6) are welded to the front and rear ends of the extruded material. As shown in FIG.1 (b), it consists of the extrusion material in which the front-and-rear walls 11 and 12 (for attachment of the bumper reinforcement 5 and the side member 6) were integrally formed in the front-and-rear end of the vertical walls 8-10, The direction of the extrusion axis is in the vertical direction of the vehicle body.

  The lateral crush type bumper stay is low in manufacturing cost and suitable for mass production. As shown in Fig. 1 (b), it is easy even if the bumper reinforcement end mounting position is inclined or curved with respect to the vehicle width direction. Although there is an advantage that can be coped with, it has been pointed out that the amount of energy absorption by weight is smaller than that of the vertical crush-type bumper stay, and the advantageous lightening effect cannot be obtained. Conversely, the vertical crush-type bumper stay has the advantage that it has a large energy absorption amount when crushing vertically into a bellows shape, and an advantageous lightening effect can be obtained. It is difficult to integrally form a mounting flange, and as shown in FIG. 1A, generally, a flange plate is welded to the end of the extruded profile.

  However, when mounting the flange by welding, (1) the flange surface is distorted by welding distortion, and the adhesion to the bumper reinforcement or the side member mounting surface is reduced. (2) the bumper reinforcement or the side member mounting surface is in close contact In order to improve the performance, it is necessary to remove the weld bead that is raised on the mounting surface side of the flange. (3) There is a problem that the strength of the bumper stay is governed by the strength of the welded part and unexpected weakening occurs. . On the other hand, in order to avoid welding, as seen in Patent Document 1 below, it is considered that a part of the end of the extruded material in the circumferential direction is cut off and the remaining part is bent outward to form a flange. In this case, since the flange cannot be formed on the entire circumference of the extruded material, the rigidity of the flange is lowered.

In order to solve this problem, in the specification and drawings attached to Japanese Patent Application No. 2002-357820, it is proposed to integrally form a flange at the end of a hollow extruded material by electromagnetic forming.
Electromagnetic forming means that a strong current of, for example, a level of 10 kA or more is instantaneously passed through a coil to create a strong magnetic field, and the interaction between the eddy current and magnetic field generated in a molded object (conductor) placed in the coil. For example, as described in Patent Documents 2 to 9 below, it is a known technique.

JP 2002-67840 A Japanese Patent Laid-Open No. 58-4601 JP-A-6-31226 JP-A-7-116751 JP-A-9-166111 Japanese Patent Laid-Open No. 10-314869 Japanese Patent Laid-Open No. 11-20434 JP 2000-264246 A JP 2002-86228 A

As shown in FIG. 2, the method described in the specification and drawings attached to the Japanese Patent Application No. 2002-357820 includes a mold 16 for electromagnetic molding (a plurality of divided molds) around a tubular extruded material 15. The end of the extruded material 15 protrudes from the end surfaces 17 and 18 (molded surfaces) of the mold 16 and is inserted into the electromagnetic forming coil 19 inserted into the extruded material 15. Thus, the end peripheral wall of the extruded material 15 is expanded in the outer diameter direction (radial direction) and pressed against the molding surfaces 17 and 18 of the molds, so that the flanges 21 and 22 (one side as required) The bumper stay 20 having the same structure). The width of the flange 21 is indicated by W.
According to this method, by forming the molding surface of the mold into an appropriate shape, not only a flange having a surface perpendicular to the axial direction, but also a flange inclined to the surface perpendicular to the axial direction, or a curved surface A flange having a shape corresponding to the shape of the mounting surface of the bumper reinforcement 5 (see FIG. 1), such as a flange made of, can be formed by one electromagnetic forming.

However, when an aluminum alloy having a practical level of strength as a bumper stay is formed by expanding the end of the extruded material by electromagnetic forming to form a flange, for example, a flange having a width (W) that can form a bolt hole As shown in FIG. 3 (a), a fan-shaped crack 23 (fissure) directed in the radial direction occurs in the flange, or necking (local thinning) occurs even if the crack does not occur. It was found that it occurred. Even in the case of so-called tube expansion, if the tube expansion rate is large, as shown in FIG. 3B, cracks 24 (fissures) along the extrusion axis direction occur.
Such cracking and necking problems at the time of electromagnetic forming have not been particularly apparent until now, but this may be difficult to obtain a power source with high power and a coil for electromagnetic forming. This is thought to be due to the fact that the tube expansion rate during electromagnetic forming could not be increased.
The present invention has been made to solve this problem, and an object of the present invention is to obtain an aluminum alloy extruded material excellent in electromagnetic formability without occurrence of cracking even when electromagnetic forming is performed with a large tube expansion ratio.

The aluminum alloy extruded material for electromagnetic forming according to the present invention is particularly used for electromagnetic forming with a tube expansion ratio of 15% or more. Si: 0.2 to 1.5% (mass%, the same applies hereinafter), Mg: 0 It is made of an aluminum alloy extruded material containing 3 to 1.5% and Al: 95% or more, and the average aspect ratio of the crystal grains in the center portion of the plate thickness is 5.0 or less.
This aluminum alloy for electromagnetic forming is suitable for manufacturing a member having a mounting flange electromagnetically formed at an end portion with a tube expansion rate of 15% or more, for example, a bumper stay for automobiles.

In the present invention, the tube expansion rate δ is defined by the following equation (1), where l ₀ is the outer peripheral length of the hollow material before tube expansion (or unexpanded portion) by electromagnetic forming, and l is the outer peripheral length after tube expansion. Is done. To explain a specific example, in the case of flange molding, this is also regarded as a kind of expanded pipe, and as shown in FIG. 4A, the outer peripheral length of the non-expanded pipe portion 25 is l ₀ and the outer peripheral length of the flange 26 is l. In the case of so-called pipe expansion, as shown in FIG. 4B, the outer peripheral length of the non-expanded pipe portion 27 is l ₀ , and the outer peripheral length (location of the maximum diameter) of the pipe expanded portion 28 is l.
δ = {(l−l ₀ ) / l ₀ } × 100 (%) (1)

  According to the present invention, an aluminum alloy extruded material excellent in electromagnetic formability without cracking can be obtained even when electromagnetic forming is performed with a large tube expansion rate of 15% or more. Thereby, the member in which the wide flange was formed by electromagnetic forming, for example, a bumper stay, can be manufactured using the aluminum alloy extrusion material.

  In the present invention, the use of electromagnetic forming at a tube expansion ratio of 15% or more is limited to a tube expansion ratio smaller than that, and it is practical to develop an aluminum alloy extruded material excellent in electromagnetic formability. When there is a material having a sufficient strength and capable of preventing the occurrence of cracks, and when, for example, a mounting flange is formed at the end of a hollow extruded material to form a bumper stay for an automobile, in practice, 15 If the flange cannot be formed with a tube expansion ratio of at least%, preferably at least 20%, it is difficult to secure a punching space for the bolt holes, and there is no point in using electromagnetic forming.

  In the aluminum alloy hollow extruded material having the above composition, the average aspect ratio of crystal grains (average axis ratio between the major axis and the minor axis) at the center of the plate thickness is regulated to 5.0 or less. This is because An aspect ratio of 5.0 or less means that the crystal grains at the center of the plate thickness are in the form of equiaxed crystals or close to equiaxed crystals. In order to prevent necking during tube expansion, the aspect ratio is more preferably 3 or less. If the aspect ratio is 5.0 or less or 3.0 or less at the center of the thickness of the extruded material, 5.0 or less or 3.0 or less is also obtained from the surface layer.

In a fiber structure often found in an aluminum alloy extruded material, electromagnetic formability is lowered and cracking is likely to occur. This is because in the case of a fiber structure, most of the grain boundaries are parallel to the extrusion direction, and the forming force of the expanded pipe that is instantaneously introduced by electromagnetic forming acts in the direction of breaking (tearing) the grain boundaries. In addition, the fiber structure generally has a small elongation in the direction perpendicular to the extrusion direction. On the other hand, in the case of equiaxed crystals, the elongation does not change significantly in the extrusion direction and in the direction perpendicular to the extrusion direction.
On the other hand, in order to suppress the rough surface of the surface portion after electromagnetic forming, it is desirable that the average grain size of recrystallized grains at least on the surface portion (portion from the outer surface to 500 μm) is 500 μm or less. Further, if the average particle size is larger than that of the equiaxed crystal, the crushing performance is lowered when used as a bumper stay. This average particle size is preferably 300 μm or less, more preferably 100 μm or less.

  In addition to Si and Mg, the Al—Mg-based aluminum alloy may contain, for example, Ti, Cu, Mn, Cr, Zr, V, Fe, Zn, and other elements as additive elements or inevitable impurities. However, the total content of these elements needs to be suppressed to 5% or less (Al: 95% or more). Desirably, Ti: 0.2% or less, Cu: 0.8% or less, Mn: 0.7% or less, Cr: 0.2% or less, Zr: 0.2% or less, V: 0.2% or less Fe: 1.0% or less, Zn: 1.0% or less, and the total of Mn, Cr, Zr and V is 0.8% or less. Further, the Al content is desirably 97.5%, and more desirably 98% or more. This is because the alloy elements or compounds that precipitate at the grain boundaries increase due to the increase in alloy elements, making the grain boundaries brittle and easy to break during electromagnetic forming. This is because the generated current due to the force is consumed as Joule heat (heat generation of the material), and as a result, the generated electromagnetic force is reduced and the tube forming force is reduced. The conductivity is desirably 46% IACS or more.

Hereafter, the effect | action is demonstrated about the main component in the aluminum alloy hollow extrusion material which concerns on this invention.
Si, Mg
Si and Mg are elements that impart strength to the alloy. When the Si content is less than 0.2% or the Mg content is less than 0.3%, the effect of the aging treatment cannot be obtained, and the strength required for a structural member such as a bumper stay of an automobile (a proof stress value σ0.2) ≧ 130 MPa) cannot be obtained, and the required amount of energy absorption cannot be obtained at the same time. On the contrary, when Si exceeds 1.5% or Mg exceeds 1.5%, the formability deteriorates and cracks occur during electromagnetic forming. Desirable ranges of Si and Mg are Si: 0.2 to 1.0% and Mg 0.4 to 0.9%. More desirable ranges are Si: 0.3-0.6% and Mg 0.5-0.7%.

Ti
Ti has the effect of refining crystal grains during casting, and is added as appropriate in order to improve tube expansion by electromagnetic forming. A desirable addition amount is 0.005% or more. On the other hand, if it exceeds 0.2%, the above-described effect is saturated, and a coarse intermetallic compound is crystallized. When Ti is added, the amount of Ti added is set to 0.005 to 0.2%, more preferably 0.005 to 0.15%, further 0.01 to 0.1%, and still more preferably 0.01 to 0.1%. 05%.

Cu
Cu has the effect of strengthening the matrix and is added as appropriate in order to improve the ductility of the material. However, the addition amount is preferably 0.8% or less, more preferably 0.5% or less, and further preferably 0.3% or less.
Mn, Cr, Zr, V
These elements have the effect of crystallizing and precipitating as intermetallic compounds to refine the crystal grains, and are added as necessary. However, there is an action of suppressing recrystallization, and since the fiber structure stretched in the extrusion direction tends to remain in the extruded material, the viewpoint of obtaining an equiaxed crystal or a structure close to equiaxed crystal having an aspect ratio of 5.0 or less Therefore, it is better that the amount of these elements added is small. If added beyond the above range, the stretched structure cannot be resolved or other adverse effects (coarse recrystallized grains) appear even if the manufacturing conditions described below are devised. Desirably, Mn: 0.2% or less, Cr: 0.1% or less, Zr: 0.1% or less, V: 0.1% or less, and more desirably, Ti and Cu are added to these elements. The total is 0.3% or less.

Other elements Fe is the most abundant impurity in aluminum ingots. If it exceeds 1.0% in the alloy, coarse intermetallic compounds are crystallized during casting, and the mechanical properties of the alloy and expansion by electromagnetic forming Impairs sex. Therefore, the Fe content is regulated to 1.0% or less, desirably 0.35% or less, and more desirably 0.2% or less. Further, when casting an aluminum alloy, impurities are mixed from various routes such as a base metal, an intermediate alloy of an additive element, and a compound. Elements to be mixed vary, but among impurities other than Fe, Zn is 1.0% or less, preferably 0.3% or less, more preferably 0.2% or less, and other impurities are 0.05% by itself. Hereinafter, 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 the Ti content with the addition of Ti, but a more desirable range is 0.02% or less, and further preferably 0.01% or less.

In order to make the structure of the central part of the thickness of the extruded material an aspect ratio of 5.0 or less, it is effective in terms of composition to suppress the addition amount of transition elements such as Mn. In terms of manufacturing conditions, conditions are set such that the extruded material is likely to recrystallize. For example, recrystallization tends to occur when the homogenization condition of the billet is set to a high temperature and long time, or / and the extrusion temperature is increased to grow intermetallic compound grains of these elements to reduce the pinning action. Further, even if the extrusion ratio and the extrusion speed are increased so that the extruded material being extruded has a high temperature, recrystallization easily occurs. Furthermore, in the case of the O material, after extruding the extruded material as it is extruded, it may be possible to perform annealing at a high temperature for a long time so that recrystallization occurs completely to the center of the plate thickness.
On the other hand, if only the electromagnetic formability is considered, it is sufficient to control the aspect ratio. However, considering the rough surface of the surface after electromagnetic forming, it is necessary to prevent the recrystallized grains on the surface from becoming coarse at the same time. is there. For that purpose, when the addition amount of the transition element is so small that the fibrous structure is not formed, the coarsening of the crystal grains is suppressed without excessively increasing the extrusion temperature and the extrusion speed, and when the addition amount of the transition element is larger than that, The homogenization treatment conditions are set to the high temperature and long time side, the extrusion temperature and the extrusion speed are relatively high, and recrystallization is promoted.
Thus, in order to make the aspect ratio 5.0 or less and to prevent the recrystallized grains from becoming too coarse, it is necessary to balance the composition and manufacturing conditions.

The aluminum alloy extruded material according to the present invention can be produced by various extrusion methods, but indirect extrusion rather than direct extrusion is intended to prevent the formation of coarse recrystallized grains on the surface of the extruded material. In addition, the mandrel method is more preferable than the porthole method in terms of ensuring the uniformity of the structure in the cross section (there is no welded portion). The quality of the extruded material can be any of T1, T5, and O. T1 material has good moldability but relatively low conductivity, T5 material has relatively poor moldability but good conductivity, and O material has good moldability and conductivity but low strength and high cost. is there.
In addition, the extrusion material which concerns on this invention is not restricted to the thing of a circular cross section, For example, the thing of an odd-shaped cross section, such as an ellipse and a polygon, is included. Moreover, the case where the thing of circular cross section is expanded to irregular shapes, such as an ellipse and a polygon, is included, or vice versa.

Aluminum alloy ingots having chemical components shown in Table 1 were melted by a semi-continuous casting method and subjected to a homogenization treatment of 520 ° C. × 4 hours, followed by extrusion at an extrusion temperature of 520 ° C. and an extrusion speed of 7 m / min. Processing was performed, and cooling with fan air cooling (cooling rate of about 100 ° C./min) was performed immediately after extrusion to obtain an extruded material having a circular cross section with an outer diameter of 100 mm and a wall thickness of 2.5 mm.
Using this as a test material, various tests were conducted as follows. The results are shown in Tables 1 and 2.

Tensile test: A JIS No. 12 tensile test piece was collected from the test material, and a tensile test was performed in accordance with JISZ2241.
Aspect ratio: Specimens were taken from each specimen, and a cross section perpendicular to the plate surface and parallel to the extrusion direction was observed. The lengths of the major axis and minor axis of 10 crystal grains located at the center of the plate thickness The average value was taken as the aspect ratio.
Electromagnetic forming test: The test material was cut to a length of 150 mm to obtain a test material, and as described above with reference to FIG. 2, the periphery was surrounded by a mold for electromagnetic forming and one end of the test material was used. Project from the end face of the mold (a plane perpendicular to the axial direction), insert a coil for electromagnetic forming inside the test material, input electric energy, and axially the end of the test material A vertical flange was electromagnetically formed and the state of the flange (presence of cracks and necking) was observed. In addition, the protrusion length from a metal mold | die end surface was set so that a pipe expansion rate might be set to 15% in all.
Crushing test: The test material was cut into a length of 100 mm to obtain a test material. As shown in FIG. 5, a static compression load was applied at a constant speed in the axial direction of the test material with an Amsler tester, and the axial direction was 40 mm. After compression, the load-displacement graph shown in FIG. 6 was obtained. Each test material was deformed into a bellows shape. An average crushing load was calculated from the graph, and a case where an average crushing load of 20 kN or more was obtained was evaluated as ◯, and a case where the average crushing load was less than 20 kN was evaluated as ×.

As shown in Table 2, the composition of the alloy is within the specified range of the present invention, and the aspect ratio is low. No. 1 had high strength and crushing load, and neither cracking nor necking occurred during electromagnetic forming. A No. with a slightly higher aspect ratio including transition elements such as Mn. In No. 2, no cracking occurred, but light necking occurred.
On the other hand, no. Nos. 3 and 5 had no cracking or necking during electromagnetic forming, but the strength and crushing load were low, and the Si and Mg contents were higher than specified. In Nos. 4 and 6, cracks occurred during electromagnetic forming and the aspect ratio was higher than that of the present invention. 7 and No. 7 in which the fiber structure was formed. No. 8 cracked during electromagnetic forming.

It is a top view explaining two types of bumper stays. It is the top view (a) explaining the electromagnetic forming method, and its II sectional drawing (b). It is a figure explaining the crack which generate | occur | produces at the time of the pipe expansion by electromagnetic forming. It is a figure explaining the definition of the pipe expansion rate of this invention. It is a figure explaining the crushing test of an Example. It is a figure explaining how to obtain | require the average crushing load of an Example.

Explanation of symbols

1,7 Bumper stay
5 Bumper reinforcement 6 Side member 15 Extruded material 16 Mold 17, 18 End face (molded face)
19 Electromagnetic forming coil 21, 22 Flange 23, 24 Crack (Rear)

Claims (3)

Si: 0.2 to 1.5% (mass%, the same shall apply hereinafter), Mg: 0.3 to 1.5%, and Al: 95% or more of an aluminum alloy extruded material, An aluminum alloy extruded material for electromagnetic forming that is electromagnetically formed at a tube expansion ratio of 15% or more, wherein an average aspect ratio of crystal grains is 5.0 or less. A member comprising the extruded aluminum alloy material for electromagnetic forming according to claim 1 and having a mounting flange that is hollow in cross section and electromagnetically formed at an end portion with a tube expansion ratio of 15% or more. The member according to claim 2, wherein the member is an automobile bumper stay.

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