Abstract:
Disclosed is a novel electric joining method and apparatus that can overcome the defects associated with the prior art while taking advantage of the technology for the pulse excited sintering process such as spark plasma sintering process or the hot-press joining process. According to the present invention, an electric joining apparatus for joining a plurality of members to be joined at the joining surfaces thereof, comprises: a pair of current-currying electrodes  13, 16  capable of contacting with the members so as to apply electricity to the members; a power supply  17  connected to the pair of energizing electrodes for supplying the energizing electrodes with at least either one of a DC current or a pulsated current; and a pressurizing unit  14  for compressing each of the pair of electrodes against the joining surfaces, wherein the pair of members are sandwiched between the energizing electrodes and applied with at least either one of the DC current or the pulsated current from the power supply under a desired pressure, thus to be joined, without using a die made of graphite.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a joining and bonding method and an apparatus for joining two members by using a DC current and/or a pulsated current and also to a unit of members joined by the same method, and in particular, to a joining method and an apparatus and also a unit of members joined by the same method, in which a plurality of members to be joined are placed in such a way that surfaces to be joined abut one another and a pulsated current and/or a DC current, in other words, at least either one of the pulsated current or the DC current is applied thereto so that the surfaces to be joined of these members are first joined temporarily, by using a technology such as pulse-excited sintering, which includes spark plasma sintering process (SPS), or a technology of hot-press joining, and thereafter heat-treating said temporarily joined to provide a permanent joint.  
           [0002]    Various conventional methods have been used practically for joining a plurality of members desired. There are some well known methods including; (1) welding, in which areas to be joined are heated with a burner, an arc discharge or the like, and a weld-assisting material (i.e. a welding rod) having the same or similar material properties to the members to be joined is melted to be deposited on the members, thereby accomplishing joining; (2) brazing, which unlike welding uses a brazing filler metal (an alloy of Cu, Ag or the like, or an amorphous metal) consisting of a material different from members to be joined, which is melted between the joining surfaces of the members in an inactive atmosphere such as a vacuum atmosphere, nitrogen or argon atmosphere or the like, whereby those members are joined by the medium of said brazing filler metal; and (3) spot welding, in which relatively thin metal sheets are sandwiched between a pair of electrodes and electricity is applied therethrough, to cause welding.  
           [0003]    Although the welding operation as described above in (1) includes a process for injecting an inactive gas such as nitrogen or argon to a heated portion to prevent it from being oxidized, since in this method a welding rod is used as the weld-assisting material to be deposited on the member (i.e. to form a padding) along an outer periphery of the joining surfaces, thereby allowing a plurality of members to be joined, such welding is not suitable for use when it is desired to join the entire surfaces of the members. In addition, welding has some defects including, (a) a weld-assisting member must be used; (b) a certain volume of a joining portion of a member must be cut off in advance to enable a welding rod melt to be deposited; (c) a welded portion has to be post-treated or processed, because it is impossible to deposit the melted substance from the welding rod precisely to a desired portion; (d) training is required to learn how to control a heating or welding temperature so that the metal composition of a member to be joined does not change; (e) training is also required to control a cooling temperature and rate so as not to cause a weld crack in the welded portion; (f) a resultant thermal deformation in the areas other than the joined portion and an increase in volume of the joined portion would occur inevitably; (g) the joining of members made of different materials is difficult; and (h) the joining is not applicable to a specific metal such as SUS, due to a change in the properties possibly caused by the welding.  
           [0004]    Although brazing as described in (2) is advantageous in that it uses an intervening brazing filler metal placed over the entire joining surfaces, and joining may be accomplished at a temperature lower than that for the above described welding, it still involves some defects such as (i) a brazing filler metal must be used; (j) a joining strength obtained is no greater than the strength of the brazing filler metal; (k) there is an overflow of brazing filler metal upon brazing and thus finish processing must be carried out on the joined portion; (l) it is impossible to join the members in the desired areas precisely due to the overflow of brazing filler metal; (m) a joint uniform in appearance is difficult to attain; (n) strength of the joint will not be uniform especially in the outer periphery, from which the brazing filler metal is likely to flow out; and (o) joining of some metals such as stainless steel (SUS) is not feasible.  
           [0005]    Further, although the spot welding described above in (3) can generates thermal energy between the pair of electrodes in a short time and allows narrow areas to be joined in a short time, it still has defects such that (p) only a member of a thin plate-like material can be joined; (q) post-welding distortion is great; (r) an extremely small area can be welded due to the occurrence of partial welding; and (s) post-welding deformation is considerable due to the occurrence of partial welding.  
           [0006]    Still further, the above mentioned joining methods are applicable only to the joining process between metal members, and are not suitable for joining a metal member and a non-metal member, or non-metal members.  
           [0007]    On the other hand, there has been another joining method suggested that takes advantage of the principle of hot-press joining using a DC current applied to the members to be joined under a predetermined pressure thus accomplishing joining; and there is known a spark plasma sintering process (SPS) which uses a DC pulsated current applied to members to thereby accomplish joining, but a joining method employing SPS technology is disadvantageous in that members to be joined are required to be placed within a die made of graphite and to be joined within said die, which makes the process too complicated. Further there are many restrictions on the shape of the members that can be joined, and, in addition, a sufficiently strong joint can not be obtained. There is known another joining method taking advantage of the principle of the spark plasma sintering process as disclosed in Japanese Patent Application Laid-open Publication No. Hei 11-158514. However, this method also suffers from some problems in that it is applicable only to members having rough surfaces, which entails pre-roughening surfaces for joining, which restrict its use, making it inapplicable, for example, to mirror-surfaces, and the like, and making it difficult to attain a sufficiently strong joint. Still further, even if the conventional hot-press joining process is applied to the joining of bulk members without any modifications, it is difficult to attain a sufficiently strong joint.  
         SUMMERY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a novel electric joining method and apparatus which can overcome the above-mentioned defects associated with the prior art while taking advantage of the technology of the pulse excited sintering process such as a spark plasma sintering process or a hot-press joining process, and also to provide a unit of members joined by the same method and apparatus.  
           [0009]    Another object of the present invention is to provide an electric joining method and apparatus which allows a rapid and rigid joining to be accomplished in joining areas by applying at least either one of a DC current or a pulsated current to a plurality of members to be joined to one another, and also to provide a unit of members joined by the same method and apparatus.  
           [0010]    Still another object of the present invention is to provide an electric joining method and apparatus which allows an easy and rigid joining to be performed even between the members each made of different materials, and also to provide a unit of members joined by the same method and apparatus.  
           [0011]    Still another object of the present invention is to provide an electric joining method and apparatus which makes possible not only joining of metal members but also joining between a metal member and a non-metal member or between non-metal members, and also to provide a unit of members joined by the same method and apparatus.  
           [0012]    Still another object of the present invention is to provide an electric joining method and apparatus which allows the joining process to be applied relatively easily and reliably even to the joining between joining surfaces having the shapes different from each other, and also to provide a unit of members joined by the same method and apparatus.  
           [0013]    Still another object of the present invention is to provide an electric joining method and apparatus which allows joining to be accomplished with minimal deformation after joining, and also to provide a unit of members joined by the same method and apparatus.  
           [0014]    An electric joining method according to the present invention, comprises the steps of; placing the joining surfaces of a plurality of members to be joined so as to butt against one another, without using a die made of graphite; applying at least one desired current selected from a DC current and a pulsated current to said members while applying a desired level of pressure to them so as to joint them temporarily; and applying a heat treatment to said temporarily joined members under a desired temperature condition.  
           [0015]    In said electric joining method, said joining surfaces may be mirror-surfaces or rough surfaces. Further, said plurality of members may be made of substantially the same material or different materials. Still further, in said electric joining method, three or more pieces of said members to be joined may be disposed in series, so that they may be joined at a plurality of pairs of joining surfaces. Yet further, in said electric joining method, said pressure applied to said members may be not more than 50 Mega-pascal (MPa), said heat treatment may be conducted under inactive atmosphere, and said heat treatment may be conducted with the temperature controlled so as to equal to or lower than the temperature of 85% of the lowest melting point of the members to be joined. Besides a locating pin may be used in order to improve a locating accuracy of the joining surfaces.  
           [0016]    Another invention disclosed in the present application provides an electric joining apparatus for joining a plurality of members to be joined to one another at respective joining surfaces thereof, said apparatus comprising: an electric joining device including a pair of energizing electrodes capable of contacting with said members so as to apply electricity thereto, a power supply connected to said pair of current-supply electrodes for supplying said energizing electrodes with at least either one of a DC current or a pulsated current, and a pressurizing unit for compressing each of said air of electrodes against said joining surfaces, wherein said pair of members are sandwiched between said energizing electrodes and applied with at least either one of the DC current or the pulsated current from said power supply under a desired pressure, thus to be joined, without using a die made of graphite; and a heat treatment device for heat-treating under a desired atmosphere the members temporarily joined by applying current, and may still further comprises an inactive gas supplying source coupled to said heat treatment equipment for supplying inactive gas into said heat treatment equipment.  
           [0017]    Still another invention disclosed in the present application provides a unit of joined members formed by joining a plurality of members using an electric joining method, wherein a heat treatment is applied to said plurality of members having been temporarily joined by applying electricity to said members.  
           [0018]    Said unit of joined members may be composed of a plurality of members having their joining surfaces processed to be mirror-surfaces. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a perspective view of two members to be joined by an electric joining method according to the present invention;  
         [0020]    [0020]FIG. 2 is a side elevation view illustrating a conceptual configuration of an electric joining apparatus according to the present invention;  
         [0021]    FIGS.  3  (A) and (B) are perspective views illustrating round bars to be joined in the embodiments 1 and 2 respectively according to the present invention;  
         [0022]    FIGS.  4  (A), (B), (C) and (D) are perspective views of exemplary applications of an electric joining method according to the present invention;  
         [0023]    [0023]FIG. 5 is a perspective view of a plastic gear molding die formed by an electric joining method according to the present invention;  
         [0024]    [0024]FIG. 6 is a perspective views illustrates respective members of the molding die of FIG. 5 being disassembled, showing relative positioning of grooves, through-holes and communicating channels formed in respective members;  
         [0025]    [0025]FIG. 7 is a side elevation view of an alternative electric joining apparatus according to the present invention; and  
         [0026]    [0026]FIG. 8 is a cross-sectional view taken along line A-A of FIG. 7, illustrating an arrangement of members on a energizing electrode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Embodiments of the present invention will now be described with reference to the attached drawings.  
         [0028]    First of all, there are prepared, in this embodiment two members  1  and  3  to be joined (hereinafter referred to as a member for simplicity), as shown in FIG. 1. These two members may be made of same material or of different materials. The faces which are to be joined of respective members  1  and  3 , i.e. joining surfaces  2  and  4  are preferably flat, but can be, for example, curved surfaces having the same curvature, so long as no gap is created between the surfaces. These joining surfaces  2  and  4  may be rough (with surface-roughness in the order of ∇ grade as defined in JIS), but mirror-surfaces are more preferable because the joining strength for two members will be greater, and deformation resulting from such joining will be less when applied to mirror-surfaces.  
         [0029]    Then, the members  1  and  3  are placed one on the other such that those joining surfaces  2  and  4  are butted against each other, and the members  1  and  3  are positioned between a pair of energizing electrodes of an electric joining apparatus  10 . The electric joining apparatus  10 , in this embodiment, comprises an electric joining section  11  and a heat treatment section  21 . The electric joining section  11  comprises; a lower energizing electrode  13  secured on a base  12  via an insulating member by a know manner so as to be electrically insulated from said base  12 ; a fluid-pressure cylinder  14  arranged above the base  12  and secured in a known manner with respect to the base  12 ; and an upper energizing electrode  16  secured to the end (the lower end portion in the illustrated drawing) of a piston rod  15  of the fluid-pressure cylinder  14  via an insulating member in a know manner so as to be electrically insulated from the piston rod  15 . Preferably, the electrodes  13  and  16  are made of graphite.  
         [0030]    Said fluid-pressure cylinder  14  functions as a pressurizing device for compressing members to be joined. In order to fulfill the function for the pressurizing device, the upper energizing electrode may be moved up and down by way of an electric motor in conjunction with a screw mechanism as a substitute for the fluid-pressure cylinder  14 . The upper and lower energizing electrodes are electrically connected to a power supply  17 .  
         [0031]    The power supply is adapted to supply a DC pulsated current. The power supply  17  includes an internal switch mechanism (not shown) for connecting and disconnecting the electric circuit between the power supply and the energizing electrodes. The power supply will generally be such as to afford a voltage equal to or lower than 100V and a current as high as, for example, 2000 to 5000 A. Although in the above embodiment, the lower electrode is fixedly secured, while the upper electrode is allowed to move, those electrodes may be arranged in reverse orientation, or both may be adapted to move. Preferably, the heat treatment section  21  may a vacuum heat treatment furnace having a known configuration. Example of such a vacuum heat treatment furnace may be the furnace having Model No. VTTC-324T or NVFC-180PT purchasable from Ishikawajima-Harima Heavy Industries Co. Ltd.(IHI). Alternatively, the electric joining section  11  and the heat treatment section  21  may be integrated to have a structure allowing for a movement, or may be located separately.  
         [0032]    When the members  1  and  3  are to be joined by said apparatus, first the members  1  and  3  are placed between the electrodes  12  and  16  with their joining surfaces  2  and  4  butted against each other, and then the fluid-pressure cylinder  14  is activated to compress the energizing electrode  16  in the direction toward the members  1  and  3  with the piston rod  15 . Thereby, the pair of electrodes  12  and  16  sandwiched between the base  12  and the piston rod  15  is forced to compress against the members, so that the pair of members  1  and  3  is pushed in the direction toward the respective joining surfaces. This compressive force is controlled appropriately depending on the material of the member but may be equal to or less than 50 Mega-pascal. When the DC pulsated current of desired value from the power supply  17  is applied via the energizing electrodes  12  and  16  to the members  1  and  3  while holding them under the compressive force, the both members are joined to each other at the joining surfaces  2  and  4 .  
         [0033]    Although the detailed mechanism for this joining process is not necessarily apparent, it is considered that the joining takes place by the plasmas generated between the joining surfaces, the thermal diffusion effect by the Joule heat, the electro-chemical diffusion effect by electric field and the like. On the other hand, even when only the DC current of a desired value is applied to the members  1  and  3 , or a DC current and a pulsated current are concurrently applied to the members  1  and  3 , the both members  1  and  3  may be joined to each other at the joining surfaces  2  and  4 . The joining between both members in this condition is not complete from the viewpoint of joining strength. Consequently, this joining state is referred to as temporary joining, and the members temporarily joined are referred to collectively as a temporarily joined unit.  
         [0034]    The temporarily joined unit consisting of the temporarily joined members  1  and  3  is heat-treated in the heat treatment furnace  22  of the heat treatment section  21 . The inside of the furnace  22  constituting the heat treatment section  21  may be controlled to house an inactive atmosphere by means of an atmosphere control system  23  having a known structure and function. The temperature and time required to carry out this heat treatment depends on the material and size of the members. Through this heat treatment, joining between the joining surfaces having been temporarily joined becomes complete, thus becoming a completely joined unit, with a joint strength equivalent to the strength of the material of the members. As described above, the DC current, the pulsated current or the combination of the DC current and the pulsated current are available for the current to be used with the joining method and apparatus according to the present invention, and in the case where the pulsated current or the combination of the DC current and the pulsated current are used, the above method and apparatus could be referred to as a pulse excited joining method and apparatus, because there is necessarily applied a pulsated current.  
       EXAMPLE 1  
       [0035]    Two round bars  1   a  and  3   a  made of SUS304 (stainless steel), each having a circular shape in cross-sectional view with a diameter of 20 mm and a length of 90 mm, as shown in  3 [A], were prepared and after each of their joining surfaces  2   a  and  4   a  are processed to have a mirror-surface, they are set in the electric joining apparatus  10  as described above. Then, a DC pulsated current with a voltage within the range of 3 to 5V, for example 3.2V in this Example 1, and a current within the range of 1000 to 2000 A, for example 1850 A in this Example 1, was applied to those round bars from the power supply via the energizing electrodes for a period within the range of 10 to 30 minutes, for example 12 minutes in this Example 1, to join them temporarily. Then, the temporarily joined round bars were subjected to a mutual diffusion heat treatment in the inactive atmosphere at a temperature within the range of 1000 to 1100° C., for example 1080° C., for a period within the range of 30 to 90 minutes, for example 60 minutes in this Example 1. The unit of joined round bars, after the heat treatment had been completed, was ground so that the central portion thereof might have the average diameter of 15.05 mm, and was subjected to the tensile strength test with a gauge length of 40 mm. The result of the tensile strength test shows the resultant length of 52.0 mm, the elongation of 30%, and the tensile strength of 551 N/mm 2 . These values for the tensile strength are comparatively almost equivalent to those obtained from a single piece of member of the same material and size.  
       EXAMPLE 2  
       [0036]    Two round bars  1   b  and  3   b  made of SKD61 (alloy tool steels), each having a circular shape in cross-sectional view with a diameter of 20 mm and a length of 90 mm, as shown in  3 [B], were prepared and after each of their joining surfaces  2   b  and  4   b  was processed to have a mirror-surface, they were set in the pulse excited joining apparatus  10  as described above. Then, a DC pulsated current with a voltage within the range of 3 to 5V, for example 3.6V in this Example 2, and a current within the range of 1000 to 2000 A, for example 1680 A in this Example 2, was applied to those round bars from the power supply via the energizing electrodes for a period within the range of 10 to 30 minutes, for example 15 minutes in this Example 2, to join them temporarily. Then, the temporarily joined round bars were subjected to the mutual diffusion heat treatment in the inactive atmosphere at a temperature within the range of 1000 to 1100° C., for example 1025° C., for a period within the range of 30 to 90 minutes, for example 60 minutes in this Example 2, and thereafter were annealed completely. The unit of joined round bars, after the heat treatment had been completed, was ground so that the central portion thereof might have the average diameter of 15.05 mm, and was subjected to the tensile strength test with a gauge length of 40 mm. The result of the tensile strength test shows the resultant length of 46.7 mm, the elongation of 16.8%, and the tensile strength of 695 N/mm 2 . These values for the tensile strength are comparatively almost equivalent to those obtained from a single piece of member of the same material and size.  
       EXAMPLE 3  
       [0037]    As similar to the example 2, two round bars  1   b  and  3   b  made of SKD61 (alloy tool steels), each having a circular shape in cross-sectional view with a diameter of 20 mm and a length of 90 mm, as shown in  3  [B], were prepared and after each of their joining surfaces  2   b  and  4   b  was processed to have a mirror-surface, they were set in the electric joining apparatus  10  as described above. Then, a DC current with a voltage within the range of 5 to 10V, for example 5.4V in this Example 3, and a current within the range of 800 to 2000 A, for example 1450 A in this Example 3, was applied to those round bars from the power supply via the energizing electrodes for a period within the range of 60 to 120 minutes, for example 60 minutes in this Example 3, to join them temporarily. Then, the temporarily joined round bars were subjected to the mutual diffusion heat treatment in the inactive atmosphere at a temperature within the range of 1000 to 1100° C., for example 1025° C. for a period within the range of 30 to 90 minutes, for example 60 minutes in this Example 3, and thereafter were annealed completely. The unit of joined round bars, after the heat treatment had been completed, was ground so that the central portion thereof might have the average diameter of 15.05 mm, and was subjected to the tensile strength with a gauge length of 40 mm. The result of the tensile strength test shows the resultant length of 44.6 mm, the elongation of 11.5%, and the tensile strength of 685 N/mm 2 . These values for the tensile strength are comparatively almost equivalent to those obtained from a single piece of member of the same material and size.  
       EXAMPLE 4  
       [0038]    As similar to the example 2, two round bars  1   b  and  3   b  made of SKD61 (alloy tool steels), each having a circular shape in cross-sectional view with a diameter of 20 mm and a length of 90 mm, as shown in  3  [B], were prepared and after each of their joining surfaces  2   b  and  4   b  was processed to have a mirror-surface, they were set in the electric joining apparatus  10  as described above. Then, a DC current with a voltage within the range of 5 to 10V, for example 5.4V and a current within the range of 800 to 2000 A, for example 1450 A in this Example 4, and a pulsated current with a current within the range of 500 to 1000 A, for example 800 A in this Example 4, were applied to those round bars from the power supply via the energizing electrodes for a period within the range of 10 to 25 minutes, for example 20 minutes in this Example 4, to join them temporarily. Then, the temporarily joined round bars were subjected to the mutual diffusion heat treatment in the inactive atmosphere at a temperature within the range of 1000 to 1100° C., for example 1025° C., for a period within the range of 30 to 90 minutes, for example 60 minutes in this Example 4, and thereafter were annealed completely. The unit of joined round bars, after the heat treatment had been completed, was ground so that the central portion thereof might have the average diameter of 15.05 mm, and was subjected to the tensile strength test with a gauge length of 40 mm. The result of the tensile strength test shows the resultant length of 45.8 mm, the elongation of 14.5%, and the tensile strength of 690 N/mm 2 . These values for the tensile strength are comparatively almost equivalent to those obtained from a single piece of member of the same material and size.  
         [0039]    According to the above electric joining method in which only the pulsated current or both of the pulsated current and the DC current are applied for performing the joining process, joining is feasible not only between the bar members of the same sectional area and the same material as described with reference to the above examples, but also between metal members of different materials, for example, between a member made of SKD61 and a member made of aluminum alloy, between a member made of SUS304 and a member made of SUS 420J2, between a member made of copper alloy and a member made of SUS420J2, and between a member made of SKH51 (high speed tool steels) and a member made of SKD61, and further between a metal member and a non-metal member or between non-metal members, without using any weld-assisting materials or brazing filler metals.  
         [0040]    When members made of different materials, for example metal and ceramic or plastic, are joined by the present joining method, a member having gradient functionality (i.e., gradual variation in properties of the member from one side of the member to the other) is positioned between the members.  
         [0041]    Further, the joining has been made possible between two members  1   c  and  3   c  having the same shape in plan view when they are staggered as shown in FIG. 4[A], and also between two members  1   d  and  3   d  having different shapes in plan view as shown in FIG. 4[B]. Still further, the joining has been made possible between three members  1   e,    3   e  and  5   e  each having the same shape in plan view when they are staggered, that is, the joining can take place for a plurality of pairs of joining surfaces, between joining surfaces  2   e  and  6   e  and between the joining surfaces  7   e  and  4   e,  at one time, as shown in FIG. 4[C]. Yet further, as shown in FIG. 4[D], a plurality of members  3   f  and  5   f  of any arbitrary shapes (quarters of a cylindrical member  1   f,  in this example) is allowed to be joined onto the joining surface  2   f  of the member  1   f.  In this case, if the all the members are made of the same material, the above method allows the fabrication of parts having complicated shapes without any difficult mechanical processing. In particular, when the members  1   f,    3   f  and  5   f  are joined in the above method, there is no rounding produced in the surrounding of the joining areas, but if the shapes as shown in FIG. 4[D] is created by using the mechanical processing, the rounding is inevitably produced in the processed area due to the inherent properties of the tooling. It is needless to say that the above members can be joined in the same manner when only the DC current is applied.  
         [0042]    [0042]FIGS. 5 and 6 show an embodiment for the case where one specific product, i.e. a plastic gear molding die, is fabricated according to the joining method of the present invention. In this embodiment, a plurality of members (four in this embodiment), which have been divided in advance to enable coolant passages to be formed therein for supplying coolant into the molding die, are prepared and joined after formation of the coolant passages, by the joining method according to the present invention. In this way, an integrated base material is formed for the molding die, and this base material is then mechanically processed to form a molding die.  
         [0043]    The base material for the plastic molding die  100  comprises four members  110 ,  120 ,  130  and  140 , which are made of SKD61 and cylindrical or disk-like in shape. One face  111  (the top face in FIG. 6) of the lowest member (in FIG. 5)  110  has been processed to be flat (preferably, to be a mirror-surface) and has an annular groove  113  formed therein centering around the axial center of the member. Also, the faces  121  and  131  in one side (the top faces in FIG. 6) of the intermediate members  120  and  130  have been respectively processed to be flat (preferably, to be mirror-surfaces) and have annular grooves  123  and  133  formed therein respectively centering around the axial centers of the members. Further, the lower face (in FIG. 6)  142  of the topmost member (in FIG. 5)  140  and the faces in the other side (the lower faces in FIG. 6) of the intermediate members  120  and  130  have also been processed to be flat (preferably, to be mirror-surfaces). These grooves  113 ,  123  and  133  have been respectively formed on a circle having the same size and a center at the central axis line of the members.  
         [0044]    Each of the members  110 ,  120 ,  130  and  140  has a pair of locating holes  116 ,  126 ,  136  and  146  which are diametrically spaced apart from one another. These locating holes have been arranged so as to be aligned with one another, and each of the paired locating holes  126  and  136  is formed to provide a through-holes. Each of the locating holes  116  of the member  110  extends downwardly from the upper face  111  to a predetermined depth. Still further, each of the locating holes  146  of the member  140  extends upwardly from the lower face  142  to a predetermined depth. The locating pins (not shown) are to be inserted to bring the four members in position.  
         [0045]    The upper two members (in FIG. 5)  130  and  140  have respectively four through-holes  137  or  147  equally spaced in the circumferential direction penetrating therethrough. In the upper face  131  of the member  130 , communicating channels  138  have been formed extending in the radial direction from the annular groove  133  for making a communication between the annular groove  133  and the two through holes  137  among the four through holes  137 , which two holes  137  have been diametrically spaced apart (in the horizontal direction in FIG. 6). The member  120  has a pair of through holes  127  which are diametrically spaced away (located in the horizontal direction in FIG. 6) penetrating therethrough. In the upper face  121  of the member  120 , communicating channels  28  have been formed extending in the radial direction from the annular groove  123  for providing communication between the annular groove  123  formed in the upper face  121  and the opening ends of the remaining two through-holes  137  of the four through-holes  137  in the side of the lower face  132 , which remaining two holes  137  have been diametrically spaced apart (in the vertical direction in FIG. 6). In the upper face  111  of the member  110 , communicating channels  118  have been formed extending in the radial direction from the annular groove  113  for communicating with the two through holes  127  opened on the lower face  122  of the member  120  (said two holes spaced away in the horizontal direction in FIG. 6).  
         [0046]    The members  110 ,  120 ,  130  and  140  that have been mechanically processed in advance as described above, are placed with one on the anther as shown in FIG. 5 and located in position by the locating pins (not shown) and the locating holes  116 ,  126 ,  136  and  146 , and then those members are joined with one another to be integrated into one unit by the above described electric joining method in which the pulsated current or the combination of pulsated current and DC current is applied thereto, so as to form the base material  100  used for the molding die. Herein, in the base material for a molding die, the coolant passages are formed from the plurality of annular grooves  113 ,  123  and  133 , the through-holes  127 ,  137  and  147 , and the communicative channels  118 ,  128  and  138 . A through-hole is formed in the central portion (in the inner side of the annular grooves in the radial direction so as not to interfere with said grove) of the base material  100  for the molding die, and a core member for forming a cavity necessary for molding of plastic (the cavity into which the plastic material is injected) is fitted within said through-hole to be formed into a molding die. It should be noted that the explanation to the processes for making up said base material for the molding die into the actual molding die is omitted, because those processes have no direct relevance to the subject invention. When said unit is used as a molding die, the coolant flows from the outside into the coolant passages in the molding die as is indicated by the arrows, to thereby cool the plastic gear molding die. The coolant flow as indicated by the arrows → passes through the annular groove  123  to cool the vicinity thereof, while the coolant flow as indicated by the arrows          passes through the annular grooves  113  and  133  to cool the vicinities thereof. It should be noted that although in the above embodiment, the annular groove and the communicative channel have been formed in only one surface of the two surfaces to be joined with each other, they may be formed in each joining surface plane-symmetrically so that these separate two parts are joined to form one passage. Further, the shape of the groove in the plan view is not limited to the circular shape shown in the above embodiment, but a wide variety of shapes may be possible.  
         [0047]    Although in the electric joining apparatus shown in FIGS. 1 and 2, the electric joining section  11  has applied at least one current selected from the pulsated current and the DC current to a plurality of members to form them into a temporarily joined unit, and then said single piece of temporarily joined unit has been heat-treated by the heat treatment section  21  so as to be made into a completely joined unit, a plurality of temporarily joined units in a group may be heat-treated in the batch processing manner. Further, in the case where the members to be joined are relatively small and are in mass production, the energizing electrodes  13   g  and  16   g  may be modified to have plate-like member (circular or polygonal member) having a larger sectional surface as shown in FIGS. 7 and 8, and a plurality of paired members  1  and  3  may be placed between those electrodes, so that the plurality of paired members can be joined at one time.  
         [0048]    As is obvious from the above description, the present invention can be applied to various kinds of joining, including a joining of an IC substrate and a cooling metal member, a joining of an ultra-hard metal and a normal metal, a joining of a member made of aluminum or the like and a corrosion resistant or wear-resistant member, a joining of cams in multi-rows, a joining of a gear and a pinion, and a joining of a shaft and a gear.  
         [0049]    According to the present invention, the flowing effects are expected to be obtainable.  
         [0050]    (a) Joining is possible without using any weld assisting materials.  
         [0051]    (b) Joining takes effect over entire joining surface and uniformly over the entire surface as well.  
         [0052]    (c) Joining takes place by simply processing a joining surface to be flat.  
         [0053]    (d) Joining strength can be increased by improving the flatness of a joining surface.  
         [0054]    (e) Joining strength is improved to be made equal to that of the material of the metal member to be joined.  
         [0055]    (f) Joining can be accomplished with only a small deformation.  
         [0056]    (g) Such post-treatment as grinding out the weld assisting material or the brazing filler metal is not required in an area proximate to a joined portion.  
         [0057]    (h) Joining in the precise location is possible.  
         [0058]    (i) Joining of fine parts can be performed easily.  
         [0059]    (j) Since a member to be joined can to be joined after it has been formed into a complete part, a product having a complicated shape can be assembled by the joining process.  
         [0060]    (k) Joining is possible without damaging the properties of a member to be joined.  
         [0061]    (l) Members made of different materials can be joined easily.  
         [0062]    (m) Joining is possible with an appropriate thermal control of the portions other than the joining portions.  
         [0063]    (n) A plurality of members of different shapes can be joined in a single operation.