1. Field of the Invention
The present invention relates to axial members with flange having an axial section made of an aluminum alloy material and a mounting flange arranged at the end of the axial section, to connection members and to production methods of these members.
2. Description of the Related Art
Bumpers arranged at the front end (front) and the rear end (rear) of bodies of automobiles such as passenger cars and trucks have bumper reinforcements as reinforcing members inside thereof. Such a bumper reinforcement is generally a member which is hollow in cross section and includes a front wall and a rear wall pointing substantially perpendicular to a loading direction, and a horizontal wall connecting these walls. The bumper reinforcement is supported by a pair of bumper stays from the rear, and the respective bumper stays are fixed at their rear end to the front end of the side member (front or rear).
Aluminum bumper stays are roughly classified as bumper stays to be vertically crushed, and those to be horizontally crushed. With reference to FIG. 79(a), such a bumper stay to be vertically crushed includes an axial section 1 and sheet mounting flanges 2 and 3. The sheet mounting flanges 2 and 3 serve to mount a bumper reinforcement 4 and a side member 5 and are weld to the front and rear ends of a hollow extrudate constituting the axial section 1. This type of bumper stay has an extrusion axial direction pointing to a cross direction of a car body in a direction substantially perpendicular to the longitudinal direction of the bumper reinforcement 4. With reference to FIG. 79(b), a bumper stay to be horizontally crushed includes an extrudate 6 and has an extrusion axial direction pointing to the vertical direction of a car body in a direction perpendicular to the longitudinal direction of the bumper reinforcement 4. The extrudate 6 integrally has mounting flanges 7 and 8 at the front and rear ends thereof. Examples of bumper stays to be horizontally crushed can be seen typically in Japanese Patent Application Laid-Open (JP-A) No. 08-91154, JP-A No. 2000-318552 and JP-A No. 2001-294106.
Bumper stays to be vertically crushed generally have a high production cost, since the constitutional three parts are integrated by welding. In addition, if a mounting section at the end of such bumper stays is inclined rearward with respect to a width direction of a car as shown in FIG. 79(a), the axial extrudate must be cut obliquely. This invites a decreased yield, an increased cutting cost and an increased welding cost. Bumper stays to be horizontally crushed are low in their production cost and can be easily produced even if the mounting section at the end of the bumper reinforcement is inclined or curved with respect to a width direction of a car. However, these bumper stays have a lower energy absorption per unit weight and lower effects in weight reduction than the bumper stays to be vertically crushed.
Japanese Patent Application Laid-Open (JP-A) No. 2004-42066 describes a technique for forming mounting flanges 12 and 13 at both ends of an axial section 11 by electromagnetic forming, which axial section 11 is made of an aluminum alloy extrudate (tube), as shown in FIG. 79(c).
This article is produced in the following manner as shown in FIG. 80. Initially, an aluminum extrudate is cut to a predetermined length to form an untreated pipe 14, a mould 15 including plural mould parts is allowed to surround the untreated pipe 14 while the ends of the untreated pipe 14 are protruded from end faces (molding faces) 16 and 17 of the mould 15. An electromagnetic forming coil 18 is inserted into the untreated pipe 14, and an electric energy (charge) accumulated at a high voltage in the coil is instantaneously discharged to thereby produce the article. In the electromagnetic forming process, the electromagnetic forming coil 18 generates a strong magnetic field in a very short time as a result of application of electric energy, a work (article to be processed) placed in the magnetic field receives strong expansive force and/or contractive force by the action of a repulsive force of the magnetic field (the Lorentz force in accordance with the Fleming's left-hand rule) and is plastically deformed at high speed to thereby mold the work to a predetermined shape. In the illustrated example, the untreated pipe 14 in a region inside the end faces 16 and 17 expands outward in a radial direction by the action of the strong expansive force, is pressed to the inner face of a through hole 19. The untreated pipe 14 in a region outside the end faces 16 and 17 spreads and strikes against the end faces 16 and 17.
The electromagnetic forming can be applied even to the case where the work must be processed into a complicated shape, since the work is deformed at high speed. In addition, this technique enables a shape with good precision, since the work is pressed to the molding face of the mould to form a predetermined shape. Accordingly, flanges having various shapes corresponding to the shapes of the mounting faces can be obtained by allowing end faces (molding faces) 16 and 17 of the mould 15 to have appropriate corresponding shapes. Examples of possible flanges are a flange having a plane perpendicular to the axial direction (flange 13), as well as a flange having a plane oblique to the vertical plane to the axial direction (flange 12), and a flange having a curved face.
The electromagnetic forming itself has been known, as described typically in JP-A No. 58-4601, JP-A No. 06-312226, JP-A No. 07-116751, JP-A No. 2002-86228, and Search Report of Mechanical Engineering Laboratory, No. 150, “Plastic Working Using Magnetoelectric Force” (March 1990, published by the Mechanical Engineering Laboratory, MITI, Japan)
In the conventional bumper stays to be vertically crushed, the three constitutional parts are weld at an intersection of the axial section and the flanges at which a load upon collision is most applied. Thus, they exhibit decreased properties of the materials, which may invite unexpectable decrease in energy absorption, in addition to the above-mentioned disadvantages.
This problem is solved by allowing both ends of a tubular aluminum alloy extrudate to expand typically by electromagnetic forming and thereby forming flanges integrally with the axial section. In this case, however, deformation of the flanges due to tensile force to a circumferential direction increases with approaching to the outside, which invites reduction in thickness and, in turn, invites cracking. This problem becomes more serious when the flanges have larger diameters as compared with the diameter of the axial section made of an aluminum alloy extrudate.
The cracking problem is specifically serious in the case of an aluminum alloy extrudate having a fiber structure. Such a fiber structure mainly includes grain boundaries in parallel with the extrusion direction, and molding force for expansion applied typically by electromagnetic forming acts in such a direction to break or tear the grain boundaries. In addition, an article having a fiber structure less elongates in a direction perpendicular to the extrusion direction. As is well known, an aluminum alloy extrudate having a fiber structure exhibits excellent crush properties in an axial direction and is highly useful as a bumper stay. However, such an aluminum alloy extrudate exhibits lower moldability in an expansion direction than an article including an equiaxial crystal, invites cracking specifically in the case where flanges are formed at ends thereof and thereby the flanges cannot significantly have sufficient sizes to be connected with a bumper reinforcement and a side member.
FIG. 81 illustrates the state where a flange 12 (and also a flange 13) formed by electromagnetic forming has a decreasing thickness toward the outside. As is illustrated in FIG. 81, the flange 12 has a varying thickness in a radial direction and a front 12a of the flange 12 is not flat even if an end face (molding face) 16 of a mould 15 is a flat plane. When the resulting flange 12 is fixed, for example, to a rear wall 4a of a bumper reinforcement 4, there is a gap G between the flange 12 and the rear wall 4a of the bumper reinforcement 4, as illustrated in FIG. 82. The gap G invites distortion of the flange 12 upon fixation with bolt and nut or by riveting, and invites a space between the fixed flange 12 and the rear wall 4a of the bumper reinforcement 7. This is also true for the case where the flange 13 is fixed to the tip of a side member. This problem becomes more serious when the flanges have larger outer diameters as compared with the diameter of the axial section made of an aluminum alloy extrudate.