Patent Publication Number: US-2016221059-A1

Title: Electromagnetic forming coil device and method of making electromagnetically formed product

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electromagnetic forming coil device and a method of making an electromagnetically formed product. To be more specific, the present invention relates to an electromagnetic forming coil device and a method of making an electromagnetically formed product that are used to electromagnetically form a portion to be processed of a workpiece. 
     2. Description of the Related Art 
     Electromagnetic forming technology is used to expand or compress a pipe made of a conductor. Electromagnetic forming is a method of plastically forming a conductor by using an electromagnetic force. In electromagnetic forming, a high-voltage charge is instantaneously discharged to a conductor coil to generate a strong magnetic field around the conductor coil in a short time. A workpiece disposed in the magnetic field is processed by using a repulsive force generated between the workpiece and the conductor coil. 
     For example, Japanese Unexamined Patent Application Publication No. 2007-275909 discloses a technology related to an electromagnetic forming coil that can be used for compression forming. The electromagnetic forming coil includes an electromagnetic coil body made by helically winding a conductive wire and a tubular conductor disposed outside of the electromagnetic coil body so as to surround the electromagnetic coil body. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-520353 discloses a technology, related to a drive shaft, in which a current is induced by using an electric field shaper and thereby two components are compressively joined together. 
     According to the disclosure in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-520353, a coil is disposed at a position at which the coil does no overlap the magnetic field shaper. Therefore, a magnetic field generated from the coil might influence a portion of the workpiece other than a portion to be processed. 
     When electromagnetic forming is performed on a workpiece by using existing technologies, an electromagnetic force may be applied to a portion of the workpiece that is not to be processed, and this portion may become deformed. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide an electromagnetic forming coil device and a method of making an electromagnetically formed product that can reduce occurrence of deformation, due to an electromagnetic force, of a portion of a workpiece other than a portion to be processed when electromagnetic forming is performed. 
     An electromagnetic forming coil device according to the present invention includes a conductor coil that is helically wound; and a magnetic field shaper including a tubular portion that is disposed in the conductor coil along the coil in a longitudinal direction, and an end wall portion that extends from a base end portion thereof, which is at one end of the tubular portion in the longitudinal direction, to an extended end thereof toward an axis of the conductor coil, the end wall portion having a cavity surface at the extended end, the cavity surface surrounding a workpiece and being formed along an outer periphery of the workpiece. The tubular portion and the end wall portion of the magnetic field shaper are each divided into a plurality of sections in the longitudinal direction, and the magnetic field shaper includes insulating layers disposed on the divided sections of each of the tubular portion and the end wall portion and on the cavity surface. The extended end of the end wall portion extending toward the axis is disposed at a position protruding outward from a position of the conductor coil in the longitudinal direction. 
     In the electromagnetic forming coil device, the base end portion of the end wall portion may be disposed so as to protrude outward from the position of the conductor coil in the longitudinal direction. 
     A method of making an electromagnetically formed product according to the present invention uses a magnetic field shaper including a tubular portion, an end wall portion, and insulating layers, the tubular portion being disposed in a conductor coil, which is helically wound, along the coil in a longitudinal direction, the end wall portion extending from a base end portion thereof, which is at one end of the tubular portion in the longitudinal direction, to an extended end thereof toward an axis of the conductor coil, the end wall portion having a cavity surface at the extended end, the cavity surface being formed along an outer periphery of the workpiece, the tubular portion and the end wall portion of the magnetic field shaper each being divided into a plurality of sections in the longitudinal direction, the insulating layers being disposed on the divided sections of each of the tubular portion and the end wall portion and on the cavity surface. The method includes a step of arranging the magnetic field shaper and the workpiece so that the cavity surface surrounds a portion to be processed of the workpiece; a step of disposing the conductor coil around the magnetic field shaper so that the extended end of the end wall portion is located at a position protruding outward from a position of the conductor coil in the longitudinal direction; and a step of passing an electric current through the conductor coil to generate magnetic flux. The portion to be processed of the workpiece is processed by using an electromagnetic force generated by the magnetic flux. 
     In the method of making an electromagnetically formed product, the conductor coil may be disposed around the magnetic field shaper after the portion to be processed of the workpiece has been surrounded by the cavity surface of the magnetic field shaper. 
     With the present invention, it is possible to provide an electromagnetic forming coil device and a method of making an electromagnetically formed product that can reduce occurrence of deformation, due to an electromagnetic force, of a portion of a workpiece other than a portion to be processed when electromagnetic forming is performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetic forming coil device according to an embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view, taken along a line II-II in  FIG. 1 , illustrating an example of the structure of the electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 3  is a schematic longitudinal sectional view illustrating an electromagnetically formed product formed by using the electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 4A  is a schematic longitudinal sectional view illustrating another example of the structure of an electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 4B  is a schematic longitudinal sectional view illustrating another example of the structure of an electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 5  is a schematic longitudinal sectional view illustrating still another example of the structure of an electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 6  is a cross-sectional view corresponding to  FIG. 2  and illustrating an example of the structure of the electromagnetic forming coil device according to the embodiment of the present invention, in which slit portions of the electromagnetic forming coil device are shown; 
         FIG. 7  is a cross-sectional view corresponding to  FIG. 2  and illustrating an example of the structure of the electromagnetic forming coil device according to the embodiment of the present invention, in which insulating layers of the electromagnetic forming coil device according to the embodiment of the present invention are shown; 
         FIG. 8  is a cross-sectional view corresponding to  FIG. 2  and illustrating still another example of the structure of the electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 9  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetic forming coil device to be compared with the electromagnetic forming coil device according to the embodiment of the present invention; 
         FIG. 10  is a schematic longitudinal sectional view illustrating another example of the structure of an electromagnetic forming coil device to be compared with the electromagnetic forming coil device according to the embodiment of the present invention; and 
         FIG. 11  is a schematic longitudinal sectional view illustrating an electromagnetically formed product that can be formed by using the electromagnetic forming coil devices shown in  FIGS. 9 and 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail. In each of the embodiments described below, an example in which two pipe-shaped workpieces are processed by electromagnetic forming will be described. However, the present invention is not limited to the embodiments described below. 
     An electromagnetic forming coil device according to an embodiment of the present invention includes a conductor coil that is helically wound and a magnetic field shaper. 
     The magnetic field shaper includes a tubular portion that is disposed in the conductor coil along the coil in the longitudinal direction, and an end wall portion that extends from a base end portion thereof, which is at one end of the tubular portion in the longitudinal direction, to an extended end thereof toward the axis of the conductor coil. The end wall portion has a cavity surface at the extended end, the cavity surface surrounding a workpiece and being formed along an outer periphery of the workpiece. The tubular portion and the end wall portion of the magnetic field shaper are each divided into a plurality of sections in the longitudinal direction. The magnetic field shaper includes insulating layers disposed on the divided sections of each of the tubular portion and the end wall portion and on the cavity surface. 
     In the electromagnetic forming coil device according to the present embodiment, the extended end of the end wall portion is disposed so as to protrude outward from the position of the conductor coil in the longitudinal direction. 
     Referring to  FIGS. 1 to 8 , the electromagnetic forming coil device according to the present embodiment will be described. First, referring to  FIGS. 1 to 3 , the electromagnetic forming coil device according to the present embodiment and a method of making an electromagnetically formed product will be described. Next, referring to  FIGS. 4 to 8 , the structure of the electromagnetic forming coil device according to the present embodiment will be described further. 
       FIG. 1  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetic forming coil device  11  according to the present embodiment.  FIG. 2  is a schematic cross-sectional view taken along a line II-II in  FIG. 1 .  FIG. 3  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetically formed product formed by using the electromagnetic forming coil device according to the present embodiment. 
     As illustrated in  FIG. 1 , the electromagnetic forming coil device  11  according to the present embodiment includes a conductor coil  12  that is helically wound and a magnetic field shaper  13 . 
     For example, the conductor coil  12  may be made by winding a conductive wire around a shaft, such as a bobbin, helically, or preferably, solenoidally. The material of the conductive wire is not particularly limited, and any appropriate material used for existing electromagnetic forming coil devices can be used. Examples of the material of the conductive wire include copper, a copper alloy such as chromium copper, and an aluminum alloy. The conductor coil  12  can be connected to an electric circuit (not shown), including a capacitor, a switch, and the like. 
     The magnetic field shaper  13  as a whole has a substantially tubular shape and includes a tubular portion  132 , which is the tubular peripheral wall. The tubular portion  132 , which has an outer surface  132 A and an inner surface  132 B, is disposed in the conductor coil  12  along the coil in the longitudinal direction (see the direction of an arrow D 2  in  FIG. 1 ). The meaning of “the tubular portion of the magnetic field shaper is disposed in the conductor coil along the coil in the longitudinal” includes the meaning that, in the longitudinal direction of the tubular portion and the conductor coil, the length of the tubular portion is greater than that of the conductor coil or the length of the conductor coil is substantially the same as that of the tubular portion. Thus, in the longitudinal direction of the tubular portion  132  and the conductor coil  12 , the tubular portion  132  is disposed at a position that overlaps the conductor coil  12  and the conductor coil  12  is disposed at a position that does not extend beyond the magnetic field shaper  13 . 
     The magnetic field shaper  13  includes an end wall portion  134  that extends from a base end portion  133 , which is at one end of the tubular portion  132  in the longitudinal direction (also referred to as the axial direction, see the direction of the arrow D 2  in  FIG. 1 ), to an extended end thereof toward the axis of the conductor coil  12  (see an arrow D 1  in  FIG. 1 ). The end wall portion  134  of the magnetic field shaper  13  has a cavity surface  135  at the extended end extending toward the axis, the cavity surface  135  surrounding a first workpiece  10  and being formed along the outer periphery of the first workpiece  10 . The cavity surface, which is the surface of the extended end of the end wall portion of the magnetic field shaper extending toward the axis, serves as an end surface for forming a portion to be processed of a workpiece (an end surface that is closest to the portion to be processed of the workpiece). In the present disclosure, the end surface is called a cavity surface, because a space formed between the end surface (cavity surface) and the outer surface of the portion to be processed of the workpiece serves as a cavity. 
     In the present embodiment, because the magnetic field shaper  13  is divided into a plurality of sections, the cavity surface  135  is a combination of surfaces, on the axis side of the end portion  134 , of the sections into which the magnetic field shaper  13  is divided. 
     In the electromagnetic forming coil device  11  according to the present embodiment, the outer surface  132 A of the tubular portion  132  is disposed in the conductor coil  12  so as to face the conductor coil  12 . The inner surface  132 B of the tubular portion  132 , which is located inside of the outer surface  132 A, is disposed so as to face the first workpiece  10 . 
     Preferably, the outer surface  132 A of the tubular portion  132  of the magnetic field shaper  13  has a cylindrical shape corresponding to the shape of the conductor coil  12  so that magnetic flux generated from the conductor coil  12  can be concentrated onto the magnetic field shaper  13 . The cross-sectional shape of the inner surface  132 B of the tubular portion  132  is not particularly limited. Generally, the cross-sectional shape is the same as that of the outer surface  132 A. The cross-sectional shape of the inner surface  132 B of the tubular portion  132  may be a shape corresponding to the first workpiece  10  disposed in a space  14  of the magnetic field shaper  13 . 
     The extended end of the end wall portion  134 , extending from the base end portion  133  of the tubular portion  132  toward the axis of the conductor coil  12 , is disposed so as to protrude from the position of the conductor coil  12  outward in the longitudinal direction of the tubular portion  132 . At this position, the cavity surface  135  of the end wall portion  134  is disposed so as to face a portion R 0  to be processed of the first workpiece  10 . When performing electromagnetic forming, magnetic flux generated from the conductor coil  12  passes through the tubular portion  132  and the end wall portion  134  of the magnetic field shaper  13 , and is concentrated toward the cavity surface  135  at the extended end of the end wall portion  134 . Thus, it is possible to locally concentrate an electromagnetic force onto the portion R 0  to be processed of the first workpiece  10 . 
     In the electromagnetic forming coil device  11  illustrated in  FIG. 1 , a part of the extended end of the end wall portion  134  extending toward the axis of the conductor coil  12  and a part of the cavity surface  135  at the extended end are disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction. As in this case, it is sufficient that at least a part of the extended end of the end wall portion  134  and at least a part of the cavity surface  135  at the extended end may be disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction of the tubular portion  132 . Accordingly, the magnetic field shaper  13  may be structured in such a way that the entirety of the extended end of the end wall portion  134  extending toward the axis of the conductor coil  12  and the entirety of the cavity surface  135  at the extended end may be disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction. 
     In the magnetic field shaper  13  according to the present embodiment, the end wall portion  134 , which extends from the base end portion  133  of the tubular portion  132  toward the axis of the conductor coil  12 , forms a space  14  between the inner surface  132 B of the tubular portion  132  and the first workpiece  10 . The space  14  has a sufficient distance between the inner surface  132 B of the tubular portion  132  and the first workpiece  10  so that magnetic flux is not applied to the first workpiece  10 . Accordingly, it is possible to easily block magnetic flux by using the space  14 . Therefore, magnetic flux toward a portion of the first workpiece  10  other than the portion R 0  to be processed (hereinafter, referred to as a “portion R 1  not to be processed”) can be suppressed, and it is possible to reduce the influence of magnetic flux generated by the conductor coil  12  during electromagnetic forming on the portion R 1  not to be processed of the first workpiece  10 . 
     The size of the space  14  in the magnetic field shaper  13  may vary in accordance with, for example, the sizes and the materials of the first workpiece  10  and a second workpiece  20  (hereinafter referred to as “workpieces  10  and  20 ”). Preferably, the size of the space  14  is such that the space  14  can block magnetic flux toward the first workpiece  10 . 
     For example, when the first workpiece  10  is a cylindrical member made of a 7000 series aluminum alloy and having an outside diameter of 40 mm and a thickness of 2 mm, the size of the space  14  may be as follows. In order to block magnetic flux, in the longitudinal sectional view shown in  FIG. 1 , the distance L 1  from the inner surface  132 B of the tubular portion  132  of the magnetic field shaper  13  to the outer surface of the first workpiece  10  is preferably 3 mm or more, more preferably 5 mm or more, and further preferably 10 mm or more. In order to make magnetic flux be easily concentrated onto the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13 , the distance L 1  is preferably 100 mm or less, more preferably 50 mm or less, and further preferably 30 mm or less. In order to apply an electromagnetic force to the portion R 0  to be processed of the first workpiece  10 , the distance L 2  from the end surface of the cavity surface  135  of the magnetic field shaper  13  to the outer surface of the first workpiece  10  is preferably in the range of, for example, 0.1 to 2 mm. 
     As illustrated in  FIG. 2 , the magnetic field shaper  13  according to the present embodiment has slit portions  131  extending in the longitudinal direction of the tubular portion  132  (see the direction of the arrow D 2  in  FIG. 1 ). The slit portions  131  connect the outer surface  132 A of the tubular portion  132 , the inner surface  132 B of the tubular portion  132 , and the cavity surface  135  of the end wall portion  134  to each other. In the magnetic field shaper  13 , due to the presence of the slit portions  131 , each of the tubular portion  132  and the end wall portion  134  is divided into a plurality of sections in the longitudinal direction. 
     The magnetic field shaper  13  illustrated  FIG. 2  includes three slit portions  131  in the circumferential direction of the magnetic field shaper  13 , so that the magnetic field shaper  13  is divided into three sections in the circumferential direction. 
     The slit portions  131  may extend over the entirety of or a part of the length of the magnetic field shaper  13 , including the tubular portion  132  and the end wall portion  134 , in the axial direction of the magnetic field shaper  13  (see the direction of the arrow D 2  in  FIG. 1 ). 
     Because the magnetic field shaper  13  includes the slit portions  131  extending in the axial direction, when an electric current is applied to the conductor coil  12  to perform electromagnetic forming, it is possible to form a closed circuit of an induced current circulating through the outer surface  132 A and the inner surface  132 B of the magnetic field shaper  13 . 
     Due to the closed circuit of the induced current, flow of an induced current is generated also on the inner surface  132 B side of the magnetic field shaper  13 , and magnetic repulsion between the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13  and the outer surface of the first workpiece  10  occurs. As a result, it is possible to compressively form the portion R 0  to be processed of the first workpiece  10 . 
     The magnetic field shaper  13  includes insulating layers  15   b , which are disposed on the divided sections of each of the tubular portion  132  and the end wall portion  134 , to be specific, facing surfaces of the divided sections that face each other with the slit portions  131  therebetween. The magnetic field shaper  13  includes insulating layers  15   a  on the cavity surface  135  of the end wall portion  134 . 
     The materials and the like of the insulating layers  15   a  and  15   b  are not particularly limited. The insulating layers  15   a  and  15   b  may be made of, for example, a resin or a rubber. The insulating layers  15   a  and  15   b  can be formed by covering, with a resin film or a rubber film, the surface of the cavity surface  135  of the end wall portion  134  and the facing surfaces facing each other with the slit portion  131  therebetween. The materials of the insulating layers  15   a  and  15   b  are not particularly limited. Examples of the materials include a fluororesin, an acrylic resin, a urethane resin, a vinyl chloride resin, an epoxy resin, and a silicone resin. 
     Because the insulating layers  15   a  and  15   b  are disposed on the divided sections of each of the tubular portion  132  and the end wall portion  134  of the magnetic field shaper  13  and on the cavity surface  135  of the end wall portion  134 , it is possible to suppress occurrence of a spark in the slit portions  131  or on the cavity surface  135 . Therefore, the electromagnetic forming coil device  11  including the magnetic field shaper  13  can be used more safely. 
     The material of the magnetic field shaper  13  is not particularly limited, as long as the magnetic field shaper  13  can generate an induced current from magnetic flux generated from the conductor coil  12  and can concentrate the magnetic flux. Examples of such a material include, for example, copper, a chromium-copper alloy, a beryllium-copper alloy, a silver-copper alloy, aluminum, and a 6000 series aluminum alloy. From such a conductive material, the magnetic field shaper  13  can be made to have a substantially tubular shape having the slit portion  131 . Because the shape of the magnetic field shaper  13  is substantially tubular, the magnetic field shaper  13  can be disposed in the conductor coil  12  so as to be substantially coaxial with the conductor coil  12 . In this case, the magnetic field shaper  13  is disposed so that the outer surface  132 A of the tubular portion  132  faces the inner periphery of the conductor coil  12  and so that the inner surface  132 B of the tubular portion  132  and the cavity surface  135  of the end wall portion  134  face the outer surface of the first workpiece  10 . 
     As described above, the conductor coil  12  of the electromagnetic forming coil device  11 , including the magnetic field shaper  13 , according to the present embodiment may be connected to an electric circuit (not shown) including a capacitor and a switch. By structuring the electric circuit so that the capacitor is discharged when the switch, which is connected to a power source, is turned on, it is possible to supply a large pulse current from the electric circuit to the conductor coil  12 . 
     When a large pulse current flows through the conductor coil  12 , magnetic flux generated from the conductor coil  12  is concentrated onto the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13 . As a result, an induced current is generated in the first workpiece  10  disposed in the magnetic field shaper  13 . Due to interaction between the induced current and the electromagnetic field, a compressive force (electromagnetic force) is applied to the portion R 0  to be processed of the first workpiece  10 , which is disposed at a position corresponding to the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13 . Thus, as illustrated in  FIG. 3 , the portion R 0  to be processed of the first workpiece  10  is compressed, and an electromagnetically formed product  30 , in which the first workpiece  10  and the second workpiece  20  are clinched to each other, can be obtained. 
     In the electromagnetic forming coil device  11  according to the present embodiment, the conductor coil  12  and the magnetic field shaper  13  are disposed as described above. Therefore, by using the electromagnetic forming coil device  11 , it is possible to obtain an electromagnetically formed product  30  in which the portions R 1  and R 2  not to be processed of the workpieces  10  and  20  have substantially no deformation. 
     The electromagnetically formed product  30  can be made by using a method of making an electromagnetically formed product according to the present embodiment, which uses the magnetic field shaper  13 . 
     In the method of making an electromagnetically formed product, the electromagnetic forming coil device  11  described above can be used. In this method, as described below, the conductor coil  12  can be disposed after the workpieces  10  and  20  have been surrounded by the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13 . Therefore, it can be said that the conductor coil  12  and the magnetic field shaper  13  are used in the method of making an electromagnetically formed product according to the present embodiment. 
     In the method of making an electromagnetically formed product according to the present embodiment, the magnetic field shaper  13  described above, which includes the tubular portion  132 , the end wall portion  134 , and the insulating layers  15   a  and  15   b  is used. 
     The method of making an electromagnetically formed product according to the present embodiment includes a step of arranging the magnetic field shaper  13  and the first workpiece  10  so that the cavity surface  135  of the end wall portion  134  surrounds the portion R 0  to be processed of the first workpiece  10 . When clinching the first workpiece  10  and the second workpiece  20  to each other as in the present embodiment, preferably, the second workpiece  20  is inserted into the first workpiece  10  beforehand. 
     The method of making an electromagnetically formed product according to the present embodiment includes a step of disposing the conductor coil  12  around the magnetic field shaper  13  so that the extended end of the end wall portion  134  is located at a position protruding outward from the position of the conductor coil  12  in the longitudinal direction (axial direction) of the tubular portion  132 . In this step, the cavity surface  135 , which is at the extended end of the end wall portion  134  of the magnetic field shaper  13 , is disposed so that at least a part thereof is located at a position protruding outward from an end of the conductor coil  12  in the longitudinal direction. 
     The method includes a step of passing an electric current through the conductor coil  12  to generate magnetic flux. By using an electromagnetic force generated by the magnetic flux, the portion R 0  to be processed of the first workpiece  10  can be processed. 
     In the method of making an electromagnetically formed product according to the present embodiment, preferably, the conductor coil  12  is disposed around the magnetic field shaper  13  after the workpieces  10  and  20  have been surrounded by the cavity surface  135  of the end wall portion  134  of the magnetic field shaper  13 . By disposing the conductor coil  12  after the workpieces  10  and  20  have been disposed, the position of the cavity surface  135  relative to the position of the conductor coil  12  in the longitudinal direction of the tubular portion  132  (a distance by which the cavity surface  135  protrudes outward in the longitudinal direction) can be easily adjusted. 
     Preferably, the shapes of the first workpiece  10  and the second workpiece  20 , which are to be processed by using the electromagnetic forming method using the electromagnetic forming coil device  11  according to the present embodiment, are substantially cylindrical shapes, substantially rectangular tubular shapes, or substantially polygonal tubular shapes. Each of the workpieces may have a non-tubular shape, such as a plate-like shape or a bar-like shape. 
     The workpiece may be a bracket member including a tubular body and a rib portion, which protrudes from the outer surface of the tubular body outward (toward the magnetic field shaper or toward the conductor coil). Preferably, the rib portion of the bracket member is formed so as to protrude from a base end portion thereof on a part of the outer surface of the tubular body in the circumferential direction. Preferably, the rib portion is formed on the outer surface of the tubular body along the axial direction. In such a bracket member, due to the presence of the rib portion, the rigidity of a part of the tubular body near the base end portion of the rib portion is higher than those of the other parts of the tubular body. When such a bracket member is used as the first workpiece, the part of the tubular body near the base portion of the rib portion, which has a high rigidity, is compressed by a smaller amount than the other parts of the tubular portion, and therefore nonuniform compression occurs in the circumferential direction of the tubular body. As a result, it is possible to make the bracket member (first workpiece) not to be easily extracted from the second workpiece. 
     Regarding the materials of the first workpiece  10  and the second workpiece  20 , it is sufficient that the material of at least the first workpiece  10 , which is disposed outside, is a material that can be plastically formed by electromagnetic forming. Preferably, the material of the first workpiece  10  is a metal having high conductivity, such as copper, aluminum, or an aluminum alloy; and more preferably a 2000 series, a 6000 series, or a 7000 series aluminum alloy. The material of the second workpiece  20  may be any of the aforementioned metals having high conductivity or a steel, so that the second workpiece  20  can be plastically formed easily by electromagnetic forming. It is also possible to process and deform the second workpiece  20  by deforming the first workpiece  10  by using an electromagnetic force. Therefore, the material of the second workpiece  20  may be a ceramic, a plastic, a rubber, or the like. 
     As described above in detail, in the electromagnetic forming coil device  11  according to the present embodiment, the magnetic field shaper  13  includes the tubular portion  132  disposed in the conductor coil  12  along the conductor coil  12  in the longitudinal direction; and the end wall portion  134  extending from the base end portion  133 , which is at one end of the tubular portion  132 , to the extended end toward the axis of the conductor coil  12 . The extended end of the end wall portion  134  extending toward the axis of the conductor coil  12  is disposed at a position protruding outward from the position of the conductor coil  12  in the longitudinal direction of the conductor coil  12 . With such structures, while concentrating magnetic flux generated by the conductor coil  12  onto the cavity surface  135  at the extended end of the end wall portion  134 , it is possible to suppress magnetic flux toward the portion R 1  not to be processed of the first workpiece  10  and magnetic flux toward the portion R 2  not to be processed of the second workpiece  20 . As a result, it is possible to obtain the electromagnetically formed product  30 , in which the portion R 0  to be processed of the first workpiece  10  is compressed, the first workpiece  10  and the second workpiece  20  are clinched to each other, and the portion R 1  not to be processed of the first workpiece  10  and the portion R 2  not to be processed of the second workpiece  20  have substantially no deformation. 
     In the electromagnetic forming coil device  11  according to the present embodiment, the tubular portion  132  and the end wall portion  134  are each divided into a plurality of sections in the longitudinal direction of the conductor coil  12  (the longitudinal direction of the tubular portion  132 ), and the insulating layers  15   b  are disposed on the divided sections. Moreover, in the electromagnetic forming coil device  11 , the insulating layer  15   a  is disposed also on the cavity surface  135  of the end wall portion  134 . With such structures, even when a large pulse current flows through the conductor coil  12  during electromagnetic forming, it is possible to prevent occurrence of a spark on the divided sections of the tubular portion  132  and the end wall portion  134  (the slit portions  131 ) or on the cavity surface  135  of the end wall portion  134 . Therefore, the electromagnetic forming coil device  11  can be more safely used. 
     In contrast, when electromagnetic forming is performed by using a device having a structure in which the portion R 1  not to be processed of the first workpiece  10  is influenced by a magnetic flux, the portion R 1  not to be processed of the first workpiece  10  may become deformed undesirably.  FIGS. 9 and 10  illustrate examples of the structures of such a device.  FIG. 9  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetic forming coil device  1 A to be compared with the electromagnetic forming coil device  11  according to the present embodiment.  FIG. 10  is a schematic longitudinal sectional view illustrating an example of the structure of another electromagnetic forming coil device  1 B to be compared with the electromagnetic forming coil device  11  according to the present embodiment.  FIG. 11  is a schematic longitudinal sectional view illustrating an electromagnetically formed product  300  that can be formed by using the electromagnetic forming coil devices shown in  FIGS. 9 and 10 . 
     The electromagnetic forming coil device  1 A illustrated in  FIG. 9  includes a magnetic field shaper  3  that has a substantially trapezoidal shape in a longitudinal sectional view, and the magnetic field shaper  3  forms a space  4  between the magnetic field shaper  3  and the first workpiece  10 . However, the electromagnetic forming coil device  1 A does not have a structure corresponding to the end wall portion of the magnetic field shaper  3  described above. Therefore, the space  4  is small, and magnetic flux toward the portion R 1  not to be processed of the first workpiece  10  cannot be easily suppressed (see blank arrow in  FIG. 9 ). 
     The electromagnetic forming coil device  1 B illustrated in  FIG. 10  includes a conductor coil  2  at a position protruding beyond the position of the magnetic field shaper  13  (and the tubular portion  132  of the magnetic field shaper  13 ). With the electromagnetic forming coil device  1 B, because the tubular portion  132  is not disposed along the conductor coils  2  and  12  in the longitudinal direction (the conductor coil  2  is not disposed at a position overlapping the tubular portion  132  of the magnetic field shaper  13 ), the portion R 1  not to be processed of the first workpiece  10  may be influenced by magnetic flux from the conductor coil  2  (see wave-shaped arrows in  FIG. 10 ). 
     When electromagnetic forming of the workpieces  10  and  20  is performed by using the electromagnetic forming coil devices  1 A and  1 B shown in  FIGS. 9 and 10 , an electromagnetic force is likely to be applied also to the portion R 1  not to be processed of the first workpiece  10 . As a result, as in the electromagnetically formed product  300  illustrated in  FIG. 11 , the portion R 1  not to be processed of the first workpiece  10  might become deformed undesirably. In contrast, with the electromagnetic forming coil device  11  according to the present embodiment described above, it is possible to avoid problems that might occur due to the electromagnetic forming coil devices  1 A and  1 B. 
     Next, referring to  FIGS. 4 to 8 , examples of structures that the magnetic field shaper according to the present invention can have, besides the aforementioned structure of the magnetic field shaper  13 , will be described. 
     Regarding the magnetic field shapers and the electromagnetic forming coil devices described below, elements that are the same as those of the magnetic field shaper  13  and the electromagnetic forming coil device  11  described above will be denoted by the same numerals and redundant descriptions of such elements will be omitted. Description of a method of making an electromagnetically formed product using the magnetic field shapers will be omitted, because it is the same the method of forming the electromagnetically formed product  30  described above. 
       FIGS. 4A and 4B  are schematic longitudinal sectional views illustrating the structures of electromagnetic forming coil devices  21   a  and  21   b  according to the embodiment of the present invention. As illustrated in  FIGS. 4A and 4B , the electromagnetic forming coil devices  21   a  and  21   b  include conductor coils  12  and magnetic field shapers  23   a  and  23   b  disposed in the conductor coils  12 . As with the magnetic field shaper  13  described above, the magnetic field shapers  23   a  and  23   b  include tubular portions  232   a  and  232   b  disposed along the conductor coils  12  in the longitudinal direction; and end wall portions  234   a  and  234   b  extending from base end portions  233   a  and  233   b , which are at ends of the tubular portions  232   a  and  232   b , to extended ends thereof toward the axes of the conductor coils  12 . Cavity surfaces  235   a  and  235   b  of the end wall portions  234   a  and  234   b  are formed along the outer peripheries of the first workpieces  10  so as to face the portions R 0  to be processed of the first workpieces  10 . 
     The magnetic field shapers  23   a  and  23   b  differ from the magnetic field shaper  13  described above in that the magnetic field shapers  23   a  and  23   b  include flange portions  236   a  and  236   b . The flange portions  236   a  and  236   b  are formed so as to protrude outward in the longitudinal directions of the tubular portions  232   a  and  232   b  (axial directions) from the extended ends of the end wall portions  234   a  and  234   b  extending toward the axes of the conductor coils  12 . The flange portions  236   a  and  236   b  of the magnetic field shapers  23   a  and  23   b  can be formed so as to be continuous with the end wall portions  234   a  and  234   b.    
     In the electromagnetic forming coil device  21   a  illustrated in  FIG. 4A , the cavity surface  235   a  at the extended end of the end wall portion  234   a  of the magnetic field shaper  23   a  is disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction by the length of the flange portion  236   a.    
     In the electromagnetic forming coil device  21   b  illustrated in  FIG. 4B , the base end portion  233   b  of the end wall portion  234   b  of the magnetic field shaper  23   b  is disposed so as to protrude outward from the position of an end of the conductor coil  12  in the longitudinal direction of the conductor coil  12  (the longitudinal direction of the tubular portion  232   b ). Thus, the entirety of the end wall portion  234   b  is located outward from the position of the conductor coil  12  in the longitudinal direction of the conductor coil  12  (the longitudinal direction of the tubular portion  232   b ). Accordingly, the cavity surface  235   b  at the extended end of the end wall portion  234   b  is located outward from the position of an end of the conductor coil  12  in the longitudinal direction. The end wall portion  234   b  of the magnetic field shaper  23   b  may be partially disposed outward from the position of the end of the conductor coil  12  in the longitudinal direction. 
     With the electromagnetic forming coil devices  21   a  and  21   b  according to the present embodiment, which are illustrated in  FIGS. 4A and 4B , the magnetic field shapers  23   a  and  23   b  including the flange portions  236   a  and  236   b  can easily concentrate magnetic flux generated by the conductor coils  12  toward the flange portions  236   a  and  236   b  of the magnetic field shapers  23   a  and  23   b . Therefore, by disposing the portions R 0  to be processed of the first workpieces  10  so as to face the cavity surfaces  235   a  and  235   b  of the end wall portions  234   a  and  234   b , it is possible to locally concentrate electromagnetic forces onto the portions R 0  to be processed. Influence of magnetic flux on the portions R 1  not to be processed of the first workpieces  10  can be suppressed. Thus, with each of the electromagnetic forming coil devices  21   a  and  21   b , in addition to the effects and the advantages that can be obtained by using the electromagnetic forming coil device  11  described above, it is possible to more easily process the portion R 0  to be processed of the first workpiece  10  and to further suppress deformation of the portion R 1  not to be processed. Furthermore, with the electromagnetic forming coil device  21   b  illustrated in  FIG. 4B , because the base end portion  233   b  of the end wall portion  234   b  of the magnetic field shaper  23   b  is disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction, it is possible to further suppress deformation of the portion R 1  not to be processed. 
       FIG. 5  is a schematic longitudinal sectional view illustrating an example of the structure of an electromagnetic forming coil device  31  according to the embodiment of the present invention. As illustrated in  FIG. 5 , the electromagnetic forming coil device  31  according to the present embodiment includes a conductor coil  12 , and a magnetic field shaper  33  disposed in the conductor coil  12 . As with the magnetic field shaper  13  described above, the magnetic field shaper  33  includes a tubular portion  332  disposed along the conductor coil  12  in the longitudinal direction, and an end wall portion  334  extending from a base end portion  333 , which is at one end of the tubular portion  332  in the longitudinal direction (axial direction), to an extended end thereof toward the axis of the conductor coil  12 . The magnetic field shaper  33  has a cavity surface  335  at the extended end extending toward the axis of the end wall portion  334 . The cavity surface  335  of the end wall portion  334  surrounds the first workpiece  10 , is formed along the outer periphery of the first workpiece  10 , and is disposed so as to face the portion R 0  to be processed of the first workpiece  10 . 
     The electromagnetic forming coil device  31  differs from the magnetic field shaper  13  described above in that the end wall portion  334  of the magnetic field shaper  33  is formed so as to protrude at an angle outward in the longitudinal direction of the conductor coil  12  (the longitudinal direction of the tubular portion  332 ) and toward the axis of the conductor coil  12 . 
     In the electromagnetic forming coil device  31  according to the present embodiment, the end wall portion  334  of the magnetic field shaper  33  protrudes at an angle outward in the longitudinal direction of the conductor coil  12  (the tubular portion  332 ) and toward the axis of the conductor coil  12  (the tubular portion  332 ). Therefore, it is possible to concentrate magnetic flux generated by the conductor coil  12  onto the cavity surface  335  of the end wall portion  334 . By disposing the portion R 0  to be processed of the first workpiece  10  so as to face the cavity surface  335  of the end wall portion  334  of the magnetic field shaper  33 , it is possible to locally concentrate an electromagnetic force onto the portion R 0  to be processed. Influence of magnetic flux on the portion R 1  not to be processed of the first workpiece  10  can be suppressed. Thus, with the electromagnetic forming coil device  31  according to the present embodiment, in addition to the effects and the advantages that can be obtained by using the electromagnetic forming coil device  11  described above, it is possible to more easily process the portion R 0  to be processed of the first workpiece  10  and to further suppress deformation of the portion R 1  not to be processed. 
     In the electromagnetic forming coil device  31  illustrated in  FIG. 5 , as with the electromagnetic forming coil device  21   b  illustrated in  FIG. 4B , the base end portion  333  of the end wall portion  334  of the magnetic field shaper  33  and the entirety of the cavity surface  335  of the end wall portion  334  are disposed so as to protrude outward from the position of the conductor coil  12  in the longitudinal direction of the conductor coil  12  (a tubular portion  223 ). However, this is not a limitation. For example, a part of the cavity surface  335  of the end wall portion  334  may be disposed so as to protrude outward from the position of an end of the conductor coil  12  in the longitudinal direction. As long as at least a part of the cavity surface  335  of the end wall portion  334  is disposed so as to protrude outward from the position of the end of the conductor coil  12  in the longitudinal direction, the base end portion  333  of the end wall portion  334  may be disposed inside of the conductor coil  12  in the longitudinal direction. 
     In  FIGS. 1, 4A, 4B, and 5 , which are used to describe the electromagnetic forming coil devices  11 ,  21   a ,  21   b , and  31  according to the present embodiment, the sectional shapes of the magnetic field shapers  13 ,  23   a ,  23   b , and  33  have corners. Each of the corners may be rounded or chamfered. For example, in the magnetic field shaper  13  illustrated in  FIG. 1 , the outer corner of the outer surface  132 A of the magnetic field shaper  13  at the boundary between the tubular portion  132  and the end wall portion  134  is preferably a curved surface. Likewise, the inner corner of the inner surface  132 B of the magnetic field shaper  13  at the boundary between the tubular portion  132  and the end wall portion  134  is preferably a curved surface. 
       FIG. 6  is a cross-sectional view corresponding to  FIG. 2  and illustrating an example of the structure of an electromagnetic forming coil device  41  according to the present embodiment. The electromagnetic forming coil device  11  described above includes three slit portions  131  and the magnetic field shaper  13  is divided into three sections in the circumferential direction. However, the number of slit portions of the magnetic field shaper and the number of sections into which the magnetic field shaper is divided are not particularly limited. 
     For example, as illustrated in  FIG. 6 , a magnetic field shaper  43  may include two slit portion  131  in the circumferential direction and may be divided into two sections. The number of the slit portions  131  may be larger than that of the magnetic field shaper  13  described above, and the magnetic field shaper may be divided into four, five, or a larger number of sections in the circumferential direction. Although not shown in the figures, the conductor coil  12  may be divided into a plurality of sections in the circumferential direction. 
     For example, when a workpiece has flange portions or the like at both ends thereof, after performing electromagnetic forming, it may be difficult to remove an electromagnetically formed product from the electromagnetic forming coil device. In this case, because the magnetic field shaper is divided by the slit portions  131 , some of the divided sections of the magnetic field shaper can be removed, and thereby it is possible to easily remove the electromagnetically formed product from the electromagnetic forming coil device. For this purpose, preferably, the conductor coil  12  is also divided into a plurality of sections in the circumferential direction. When the magnetic field shaper and the conductor coil are divided, it is possible to process workpieces having various shapes and to perform electromagnetic forming on wider varieties of workpieces. 
       FIG. 7  is a cross-sectional view corresponding to  FIG. 2  and illustrating another example of the structure of insulating layers formed in the slit portions of the magnetic field shaper. In the magnetic field shaper  13  described above, the insulating layers  15   b  are disposed on facing surfaces that face each other with the slit portions  131  therebetween. As illustrated in  FIG. 7 , insulating layers  25  (insulating plates), which are resin plates, rubber plates, or the like, may be disposed in the slit portions  131 . The material of the insulating layers  25  is not particularly limited. Examples of the material include a phenol resin, a polypropylene resin, a polyethylene terephthalate resin, a polycarbonate resin, an acrylic resin, a butyl rubber, a silicone rubber, and the like. Each of insulating layers  15   a , which are disposed on the cavity surface  135  of the end wall portion of the magnetic field shaper, may be a resin plate, a rubber plate, or the like. 
     With the structure in which the insulating layers  25  are disposed in the slit portions  131 , it is also possible to suppress occurrence a spark in the slit portions  131  and to increase the safety of using the electromagnetic forming coil device  11 . Moreover, with this structure, because it is sufficient to dispose the insulating layers  25  in the slit portions  131 , the insulating layers  25  can be easily provided in the slit portions  131 . 
     In the embodiment described above, the two workpieces  10  and  20 , which are substantially cylindrical, are clinched to each other as workpieces. However, the electromagnetic forming coil device described in each of the embodiments can be used to process a single workpiece, or can be used to process a workpiece having a substantially polygonal tubular shape, a plate-like shape, or a bar-like shape. 
       FIG. 8  is a cross-sectional view corresponding to  FIG. 2  and illustrating an example of the structure of an electromagnetic forming coil device  51  that is used to electromagnetically form a first workpiece  40  having a substantially rectangular tubular shape. As illustrated in  FIG. 8 , the electromagnetic forming coil device  51  can be used to electromagnetically form the first workpiece  40  having a substantially rectangular tubular shape. The first workpiece  40 , which is an example of the aforementioned bracket member, includes a rectangular tubular portion  401  and rib portions  402 , which are formed at four corners (corner portions) of the rectangular tubular portion  401  in a cross-sectional view. The rib portions  402  protrude from base end portions, which are parts of the outer surface of the rectangular tubular portion  401  in the circumferential direction, outward (toward the magnetic field shaper, or toward the conductor coil). Preferably, the rib portions  402  of the first workpiece  40  are disposed on the outer surface of the rectangular tubular portion  401  along the axial direction. 
     The cross-sectional shape of the rectangular tubular portion  401  of the first workpiece  40  may be a rectangle or a rectangle with rounded corners. Although not shown in  FIG. 8 , the cross-sectional shape of the rectangular tubular portion  401  may be a polygon (including a polygon that is not axially symmetrical). Preferably, the cross-sectional shape of a second workpiece  50  is the same as that of the rectangular tubular portion  401 . 
     In the first workpiece  40  illustrated in  FIG. 8 , in a cross-sectional view, the rib portions  402  are formed at the four corners of the rectangular tubular portion  401 . Some of the rib portions  402  may be omitted. The rib portions  402  may be formed at two opposite corners or at only one corner. Although not illustrated in  FIG. 8 , the first workpiece  40  may have a frame-like shape in a plan view. The material of the first workpiece  40  may be the same as that of the first workpiece  10  described above. 
     Also in the electromagnetic forming coil device  51 , each of the tubular portion and the end wall portion of a magnetic field shaper  53  is divided into a plurality of sections in the longitudinal direction of the conductor coil  12  (the axial direction of the tubular portion), and the insulating layers  15   a  and  15   b  are disposed on the divided sections and on a cavity surface  535  of the end wall portion. 
     To be specific, as illustrated in  FIG. 8 , the magnetic field shaper  53  of the electromagnetic forming coil device  51  includes two slit portions  531  at positions corresponding to substantially the centers of two of the four sides of the rectangular tubular portion  401  of the first workpiece  40 . The magnetic field shaper  53  includes the insulating layers  15   b  disposed in the slit portions  531 . The slit portions  531  may have the same structure as the slit portions  131  described above. The insulating layers  15   b  may be resin plates or rubber plates disposed in the slit portions  531 . 
     As with the magnetic field shaper  13  described above, an outer surface  532 A of the magnetic field shaper  53  has a cylindrical shape corresponding to the shape of the conductor coil  12 , but the cavity surface  535  of the end wall portion has a shape corresponding to the shape of the outer periphery of the first workpiece  40 . Because the cavity surface  535  of the magnetic field shaper  53  has a shape corresponding to the outer periphery of the first workpiece  40 , it is possible to locally apply an electromagnetic force to a portion to be processed of the first workpiece  40  and to compress the portion to be processed. 
     As described above, the electromagnetic forming coil device according to the present embodiment can be effectively used to clinch two pipe-shaped workpieces to each other. When clinching two pipe-shaped workpieces to each other, because the two workpieces overlap, in general, it is necessary to apply a stronger electromagnetic force to the portion to be processed than when processing one workpiece. Therefore, a conductor coil that can generate a stronger magnetic flux may be used. In this case, however, deformation of the portion not to be processed of the workpiece might occur. With the magnetic field shaper described above and the electromagnetic forming coil device including the magnetic field shaper, an electromagnetic force can be locally concentrated and deformation of the portion not to be processed can be suppressed. Therefore, the magnetic field shaper and the electromagnetic forming coil device can be more effectively used to clinch two pipe-shaped workpieces to each other. 
     The structures of the embodiments described above can be used in combination within the spirit and scope of the present invention.