Abstract:
A TIG welding torch  70  is set more adjacent to a first bus bar  30  than a second bus bar  40  to generate an arc. Consequently, the second bus bar  40  serving as a low melting point member is prevented from being over heated, occurrence of blow holes is suppressed, and inadequate melting of the first bus bar  30  serving as a high melting point member is improved, resulting in enhancement of welding strength. The method is applicable to electrical equipment, especially to a motor, provided to a vehicle.

Description:
INCORPORATION BY REFERENCE  
       [0001]     This application is based on and claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2004-043938 filed on Feb. 20, 2004. The contents of that application are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a TIG (Tungsten Inert Gas) welding method for TIG welding a high melting point member and a low melting point member, where melting points of the two members are different from each other, and a welded structure made by the method.  
         [0004]     2. Description of the Related Art  
         [0005]     In a general TIG welding method, two members are overlapped with each other and a welding arc is generated with a TIG welding torch facing and being adjacent to the overlapped portion of two members. The two members melt concurrently and mix with each other, then solidify to form a bead.  
         [0006]     However, in the aforementioned conventional TIG welding method, differences between melting points of the two members to be welded causes non-conformity between melting degrees of the two members. Therefore, sufficient welding strength is not achieved. For example, in case that tough pitch copper whose melting point is relatively high is weld to copper-zinc alloy whose melting point is relatively low by means of the conventional TIG welding method, the copper-zinc alloy melts sufficiently but not the tough pitch copper. On the other hand, at a temperature where the tough pitch copper melts adequately, the tin included in the copper-zinc alloy sublimates to a gas, thus generating a number of blow holes. In either case, sufficient welding strength is not achieved by the conventional TIG welding method.  
         [0007]     Notably, although “JIS (Japanese Industrial Standard) Industrial Term Dictionary” (published by Japanese Standards Association) describes the TIG welding method, there have been no bulletins or documents describing techniques for TIG welding two members whose melting points differ from each other.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been devised in consideration of the aforementioned circumstances, and aims to provide a TIG welding method and welded structure that are capable of enhancing, by comparison to the conventional method and structure, welding strength between members whose melting points differ from each other.  
         [0009]     In order to achieve the above and other objects, the present invention provides a TIG welding method for welding a high melting point member and a low melting point member whose melting points are different from each other. In the method, a TIG welding torch is set more adjacent to the high melting point member than the low melting point member to generate an arc.  
         [0010]     The present invention provides a welded structure by TIG welding a high melting point member and a low melting point member whose melting points are different from each other. In the structure, the high melting point member protrudes from a side surface or an end surface of the low melting point member, and the protrusion is melted to form a bead to cover the bonding portion between the high melting point member and the low melting point member. The bead can exhibit a volume of blow holes which is less than 5% of the bead volume. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawing, in which:  
         [0012]      FIG. 1  is an exploded view of parts composing a motor concerning to a first embodiment of the present invention.  
         [0013]      FIG. 2  is a sectional side view of stator cores and a cylindrical housing of the motor.  
         [0014]      FIG. 3  is a plan view of the cylindrical housing and bus bar holder.  
         [0015]      FIG. 4 (A) is a perspective view showing the state that bus bars are assembled.  
         [0016]      FIG. 4 (B) is a perspective view showing the state that bus bars are welded together.  
         [0017]      FIG. 5  is a lateral view showing a TIG welding torch and bus bars.  
         [0018]      FIG. 6  is a table indicating pictures of test pieces.  
         [0019]      FIG. 7  is a table showing properties of tough pitch copper and copper-zinc alloy.  
         [0020]      FIG. 8  is a table showing conditions of welding.  
         [0021]      FIG. 9 (A) is a perspective view showing an embodiment in which a wire is assembled to a bus bar.  
         [0022]      FIG. 9 (B) is a perspective view showing the state that the wire is welded to the bus bar.  
         [0023]      FIG. 10  is a lateral view showing a TIG welding torch, a bus bar and a wire according to another embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     A preferred embodiment, which is applied to a connecting portion included in a motor, of the present invention will be described on the basis of  FIG. 1  to  FIG. 5 . A motor  10  shown in  FIG. 1  may be a brushless motor for an electric power steering system incorporated in an automobile, and has a stator core  12  in a cylindrical housing  11 .  
         [0025]     The stator core  12  has a structure dividable into a number of core bodies in the circumferential direction. A coil  15  is wounded around each core body  14  in the longitudinal direction (axial direction). Both ends of a wire  16  configuring the coil  15  are arranged at one end (upper end in  FIG. 2 ) of the stator core  12  and penetrate a ring-shaped bus-bar holder  20 . As shown in  FIG. 3 , a number of first bus bars  30  are held in the bus-bar holder  20  and are insulated from each other. Protrusions of each first bus bar  30  are directed toward the inner periphery of the cylindrical housing  11 , and each of the protrusions is welded to one end portion of the associated wire  16 . In addition, the wires  16  configuring the coils  15  are, at the other ends thereof, electrically connected to one another to construct a three-phase motor circuit.  
         [0026]     A tip end portion of each first bus bar  30  protrudes upward from the bus-bar holder  20  in  FIG. 2 , for forming tongue pieces  39 . Further, as shown in  FIG. 3 , a connector  51  is attached to a peripheral surface of the cylindrical housing  11  and accommodates three second bus bars  40  by way of insert forming. A tip end portion of each second bus bar  40  protrudes inward from an inner peripheral surface of the cylindrical housing  11  and is welded to the tip end portion (tongue piece  39 ) of a first bus bar  30  protruding from the bus-bar holder  20  (with reference to  FIG. 4 (B)).  
         [0027]     In the present embodiment, the first bus bars  30  are made of tough pitch copper as the “high melting point member” of the present invention. On the other hand, the second bus bars  40  are made of copper-zinc alloy as the “low melting point member” of the present invention. Notably, “high melting point” and “low melting point” indicate a relative relationship between the melting points of the two members. When there are two members whose melting points differ from each other, the member having a relatively high melting point is designated as “high melting point,” and member having a relatively low melting point is designated as “low melting point.” 
         [0028]     The present embodiment embodies the invention in a welding portion between the first bus bars  30  and the second bus bars  40 . Specifically,  FIG. 4 (A) shows the state before the first bus bar  30  and the second bus bar  40  are welded together. As shown in this Figure, the second bus bar  40  is formed with a slit  46  breaching the tip end portion thereof. The width of the slit  46  is approximately the same as the thickness of the first bus bar  30 . The depth of the slit  46  is approximately the same as the width of the tongue piece  39  of the first bus bar  30 .  
         [0029]     As shown in  FIG. 4 (A), protrusions  43  formed at both sides of the slit  46  of the second bus bar  40  are linear-symmetric about the slit  46 . At an outer edge of each protrusion  43  is formed a tapered surface  45  which tapers toward the slit  46  as it approaches the tip end thereof. The tongue piece  39  of the first bus bar  30  is inserted into the bottom of the slit  46  and protrudes from the upper surface of the second bus bar  40 . The length of the protrusion is approximately one to three times of the thickness of the second bus bar  40 .  
         [0030]     The first bus bar  30  and the second bus bar  40 , configured as described above, are welded together by way of TIG welding. Specifically, as shown in  FIG. 5 , a tip end portion  70 A of a TIG welding torch  70  is positioned adjacent to the tongue piece  39  of the first bus bar  30 , then an arc is generated. Therefore, the first bus bar  30  (the high melting potion member) which is adjacent to the TIG welding torch  70  receives much more heat in comparison with the second bus bar  40  (the low melting point member). Accordingly, the first bus bar  30  and the second bus bar  40  melt properly. In addition, since protrusions  43  of the second bus bar  40  are formed into a tapered shape, the tip end sides of the protrusions  43  are melted surely. Then, the material of the first bus bar  30  and the second bus bar  40  are mixed in the melted state. When the melted metal solidifies, a bead  50  is formed at a crossing portion between the first bus bar  30  and the second bus bar  40  as shown in  FIG. 4 (B), thus integrally connecting the first bus bar  30  and the second bus bar  40 .  
         [0031]     Since the wire  16  is made of tough pitch copper which may be the same as the first bus bar  30 , the wire  16  and the first bus bar  30  are welded together by way of conventional welding, i.e. welding between members having the same melting point, to connect them integrally.  
         [0032]     According to the TIG welding method and welded structure of the embodiment described above, the TIG welding torch  70  is positioned closer to the first bus bar  30  than the second bus bar  40 , then an arc is generated. Thus, the second bus bar  40  as the low melting point member is prevented from being overheated, generation of blow holes is suppressed, insufficient melting of the first bus bar  30  as the high melting point member is diminished, and welding strength is improved in comparison with the conventional method and structure. According to the TIG welding method and the welded structure of this embodiment, it is possible to weld two metal members whose melting points differ each other, in the electrical equipment such as aforementioned motor  10 , which is applied to a vehicle and suffers from vibrations and changes of temperature.  
       EXAMPLES  
       [0033]     The following experiment was executed in order to confirm the effects of the embodiment of the invention.  
         [0034]     1) Rectangular-rod-shape high melting point members  71 ,  72  and  73  made of tough pitch copper whose properties are shown in  FIG. 7  were provided (referring to  FIG. 6 ).  
         [0035]     2) Rectangular-rod-shape low melting point members  81 ,  82  and  83  made of copper-zinc alloy whose properties are shown in  FIG. 7  were provided (referring to  FIG. 6 ).  
         [0036]     3) As shown in the left column of  FIG. 6 , the high melting point member  71  and the low melting point member  81  were in contact with each other at the respective side walls and were held such that the high melting point member  71  protruded upward from the top surface of the low melting point member  81 . An arc was generated at only the area of clearance between the TIG welding torch and the high melting point member  71 . Thus, the melted high melting point member  71  adhered to the low melting point member  81  located therebelow to form a first test piece  85 .  
         [0037]     4) As shown in the center column of  FIG. 6 , the high melting point member  72  and the low melting point member  82  were in contact with each other at the respective side walls and were held such that the low melting point member  82  protruded upward from the top surface of the high melting point member  72 . An arc was generated at only the area of clearance between the TIG welding torch and the low melting point member  82 . Thus, the melted low melting point member  82  adhered to the high melting point member  72  located therebelow to form a second test piece  86 .  
         [0038]     5) As shown in the right column of  FIG. 6 , the high melting point member  73  and the low melting point member  83  were in contact with each other at the respective side walls and were held such that both top surfaces of the high. melting point member  73  and the low melting point member  83  were approximately aligned. The TIG welding torch was positioned adjacent the contact surface between the top surfaces of the high melting point member  73  and the low melting point member  83 , and then an arc was generated to form a third test piece  87 .  
         [0039]     6) The test pieces  85 - 87  were cut and respective cut surfaces of the test pieces were subjected to etching treatment. Each welded portion of the test piece was examined using a metaloscope to weigh blow holes and the degree of mixing of the high melting point member and the low melting point member. Pictures, which were photographed by the metaloscope, of the cut surfaces of the test pieces  85 - 87  are shown at the bottom of  FIG. 6 . At steps 3), 4) and 5), each TIG welding was performed under the same conditions, as shown in  FIG. 8 .  
         [0040]     In comparing the test pieces  85 - 87 , it was found that blow holes occupied less volume in the first test piece  85  subjected to the TIG welding method of the embodiment than that in the second or third test pieces  86  or  87  subjected to the another TIG welding method. Specifically, the first test piece  85  indicated less than an approximately 1% volume presence of blow holes, the second test piece  86  indicated an approximately 5% volume presence of blow holes and the third test piece  87  indicated an approximately 5% volume presence of blow holes.  
         [0041]     Concerning the second test piece  86 , a boundary R 2  between a bead B and the high melting point member  72  was clearly shown. On the other hand, concerning the first test piece  85  employing the TIG welding method of the embodiment, a boundary R 1  between a bead B and the low melting point member  81  was diffused. Further, when pictures were colored, it was found that a boundary R 3  between a bead B and the high melting point member  73  was clear with respect to the third test piece  87 . This indicates that the high melting point member  71  and low melting point member  81  were mixed to a greater degree in the first test piece  85  subjected to the method of the embodiment in comparison to the second and the third test pieces  86  and  87 .  
         [0000]     Other Embodiments  
         [0042]     The present invention is not limited to the aforementioned embodiment. For example, modifications described below are included in the scope to be protected by the invention.  
         [0043]     (1) In the aforementioned embodiment, the present invention is exemplified by application to the connecting portion within a motor. However, the invention can be applicable to a connecting portion between members that are provided in various electrical equipment other than motors.  
         [0044]     (2) In the embodiment, the present invention is exemplified by applying the welding to tough pitch copper and copper-zinc alloy. However, the invention can be applicable to welding between members of other kinds of copper alloys, and is also applicable to the welding between members of iron alloys or between members of aluminum alloys. Further, the invention can be applicable to the welding, for example, between a member of copper alloy and a member of aluminum alloy, i.e. welding between different kinds of alloys whose base metals are different from each other.  
         [0045]     (3) In the embodiment, the invention is embodied in the welding between the bus bars  30  and  40 . However, as shown in  FIG. 9 (A) and  FIG. 9 (B), a present invention can be applied to a portion of welding between the bus bar  41  and a wire  52 . The wire  52  is disposed in a slit  46 ′ whose tip end structure is approximately the same as that of the second bus bar  40  of the aforementioned embodiment of the bus bar  41 , with the wire protruding from the top surface of the bus bar  41 . In such a state, the TIG welding torch is set adjacent to the tip end of the wire  52  to generate an arc (referring to  FIG. 9 (B)).  
         [0046]     (4) The present invention can be also embodied in welding as shown in  FIG. 10 , for example. Specifically, a tip end of a bus bar  42  extending in the horizontal direction is upwardly bent by a right angle. A wire  53  is brought into contact with the bus bar  42  at the bent tip end thereof to upwardly protrude from the tip end. In such a state, the TIG welding torch is set adjacent to a tip end of the wire  53  to generate an arc.  
         [0047]     The embodiments described herein are to be regarded as illustrative rather than restrictive. Plural objectives are achieved by the present invention, and yet there is usefulness in the present invention as far as one of the objectives is achieved. Variations and changes may be made by others, and equivalents employed, without departing from spirit of the present invention. Accordingly, it is expressly intended that all variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.