Patent Publication Number: US-10784654-B2

Title: Spark plug

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a spark plug, and more particularly to a spark plug formed by joining a tip principally made of noble metal to a base material principally made of Ni (nickel) together. 
     As such spark plug, for instance, International Publication WO2010113404 discloses a spark plug formed by joining a tip principally made of noble metal to a base material principally made of Ni (nickel) together through a fusion portion. 
     SUMMARY OF THE INVENTION 
     In the International Publication WO2010113404, however, since there is a difference in coefficient of linear expansion between the base material and the tip, a thermal stress occurs at the fusion portion due to temperature change of an engine in which the spark plug is mounted, and there is a possibility that a crack will appear at the fusion portion due to the thermal stress and develop around the fusion portion, then the tip will come off the base material. A technique of solving this problem, i.e. a technique of suppressing the coming-off of the tip from the base material even if the crack appearing at the fusion portion due to the thermal stress develops, has therefore been required. 
     The present invention was made to meet the above requirement. An object of the present invention is therefore to provide a spark plug that is capable of suppressing the coming-off of the tip from the base material. 
     To achieve the above object, according to one aspect of the present invention, a spark plug comprises: a first electrode having a tip principally made of noble metal and a base material principally made of Ni, the tip being joined to the base material through a fusion portion; and a second electrode provided so as to face a discharge surface of the tip. And, the fusion portion has an overlap portion where a first interface between the tip and the fusion portion and a second interface between the base material and the fusion portion overlap each other in a first direction that is perpendicular to the discharge surface, and when viewing a cross section which passes through a center of gravity of the overlap portion projected onto a virtual surface parallel to the discharge surface and which is perpendicular to the discharge surface, a noble metal content is greater than 50 mass % at one end portion of the overlap portion in a second direction that extends along the discharge surface, and a Ni content is greater than 50 mass % at the other end portion of the overlap portion in the second direction. 
     According to the above spark plug, on the cross section perpendicular to the discharge surface, the noble metal content is greater than 50 mass % at the one end portion of the overlap portion in the second direction extending along the discharge surface of the tip, and the Ni content is greater than 50 mass % at the other end portion of the overlap portion in the second direction. Therefore, at the one end portion of the overlap portion, a thermal stress occurring at the second interface between the base material and the fusion portion is greater than a thermal stress occurring at the first interface between the tip and the fusion portion. On the other hand, at the other end portion of the overlap portion, a thermal stress occurring at the first interface is greater than a thermal stress occurring at the second interface. Consequently, at the one end portion side, a crack tends to appear at the second interface, whereas at the other end portion side, a crack tends to appear at the first interface. The crack tends to develop along the interface. However, even if the cracks develop, thanks to the above structure, it is possible to reduce a tendency for the cracks developing along the first and second interfaces to join together. Hence, coming-off of the tip from the base material can be suppressed. 
     According to the above spark plug, the overlap portion has a shape on the cross section such that a distance between the first interface and the second interface along the first direction is gradually longer toward the second direction, and in the overlap portion on the cross section, a middle portion at which the noble metal content is 50 mass % and also the Ni content is 50 mass % exists on the second direction side with respect to a center position in the second direction of the overlap portion. 
     Therefore, a position where the cracks developing along the first interface and the second interface respectively overlap each other in the first direction tends to shift to or get closer to the second direction side with respect to the center position. Thus, even if the cracks develop along the first direction at this position, since a distance between the first interface and the second interface at this position is relatively long, in addition of the above effect, the coming-off of the tip can be further suppressed. 
     According to the above spark plug, in the overlap portion on the cross section, a shortest portion at which a distance between the first interface and the second interface along the first direction is shortest exists at a portion except the one end portion and the other end portion, and a middle portion at which the noble metal content is 50 mass % and also the Ni content is 50 mass % exists at a portion except the shortest portion in the overlap portion on the cross section. 
     Therefore, a position where the cracks developing along the first interface and the second interface respectively overlap each other in the first direction tends to be located at a portion except the shortest portion. Thus, even if the cracks develop along the first direction at this position, since a distance between the first interface and the second interface at this position is relatively long, in addition of the above effect, the coming-off of the tip can be further suppressed. 
     According to the above spark plug, at least one relationship described above is established on the cross section on which a length of the overlap portion in the second direction becomes longest. Therefore, in addition of the above effect, lengths of the first interface and the second interface on which the cracks tend to develop can be longest. 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a spark plug according to an embodiment of the present invention. 
         FIG. 2A  is a plan view of a ground electrode.  FIG. 2B  is a sectional view of the ground electrode, taken along a line IIb-IIb of  FIG. 2A . 
         FIG. 3A  is a schematic view when joining a tip to a base material.  FIG. 3B  is a schematic view when joining the tip to the base material according to a modified example. 
         FIG. 4A  is a bottom view of a center electrode.  FIG. 4B  is a sectional view of the center electrode, taken along a line IVb-IVb of  FIG. 4A . 
         FIG. 5A  is a schematic view when joining a tip to a base material.  FIG. 5B  is a schematic view when joining the tip to the base material according to a modified example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be explained below with reference to the drawings.  FIG. 1  is a sectional view of a spark plug  10  according to an embodiment of the present invention with axis O being a boundary. In  FIG. 1 , a lower side of the drawing is called a front end side (or a top end side) of the spark plug  10 , and an upper side of the drawing is called a rear end side of the spark plug  10 . As shown in  FIG. 1 , the spark plug  10  has a center electrode  20  and a ground electrode  40 . 
     An insulator  11  is a substantially tubular member provided with an axial hole  12  that extends along the axis O. The insulator  11  is made of ceramic such as alumina which is superior in mechanical characteristics and insulation performance under high temperature. The insulator  11  has, at a front side on an inner peripheral surface of the axial hole  12  thereof, a rear-end-facing surface  13  that is an annular surface facing the rear end side. A diameter of the rear-end-facing surface  13  is reduced toward the front end side. 
     The center electrode  20  is a rod-shaped member engaged with and supported on the rear-end-facing surface  13 . A top end of the center electrode  20  protrudes from a top end of the insulator  11  toward the front end side. The center electrode  20  is formed by covering a core  21  principally made of copper with a closed-bottomed tubular base material  22 . The base material  22  has a chemical composition containing 50 wt % or more of Ni. Here, the core  21  could be omitted. A tip  24  is joined to a top end of the base material  22  through a fusion portion (or a melting portion)  23 . The tip  24  has a chemical composition containing 50 wt % or more of at least one noble metal selected from Pt, Rh, Ir, Ru etc. A discharge surface  25  of the tip  24  faces the ground electrode  40 . The center electrode  20  is electrically connected to a metal terminal  26  in the axial hole  12 . 
     The metal terminal  26  is a rod-shaped member to which a high-tension cable (not shown) is connected. The metal terminal  26  is made of metal material (e.g. low-carbon steel) having conductivity. The metal terminal  26  is fixed at a rear end side of the insulator  11  with a top end of the metal terminal  26  inserted into the axial hole  12  of the insulator  11 . 
     A metal shell  30  is secured to an outer periphery at the top end side of the insulator  11  by caulking. The metal shell  30  is a substantially tubular member made of metal material (e.g. low-carbon steel) having conductivity. The metal shell  30  has a brim-shaped seat portion  31  extending or bulging in a radially outward direction and a thread portion  32  formed on an outer peripheral surface at a top end side of the metal shell  30  with respect to the seat portion  31 . By screwing the thread portion  32  into a screw hole (not shown) of an engine (a cylinder head), the metal shell  30  is fixed to the engine (the cylinder head). The ground electrode  40  is connected to a top end portion of the metal shell  30 . 
     The ground electrode  40  is a rod-shaped member made of metal material having conductivity. The ground electrode  40  has a base material  41  connected to the metal shell  30  and a tip  44  located on an inner surface  42 , which faces the center electrode  20 , of the base material  41  and joined to the base material  41  through a fusion portion (or a melting portion)  43 . The base material  41  has a chemical composition containing 50 wt % or more of Ni. The tip  44  has a chemical composition containing 50 wt % or more of at least one noble metal selected from Pt, Rh, Ir, Ru etc. A discharge surface  45  of the tip  44  faces the center electrode  20 . A spark gap G is formed between the discharge surface  45  of the tip  44  and the center electrode  20 . 
       FIG. 2A  is a plan view of the ground electrode  40  (a first electrode), viewed from a direction of the axis O.  FIG. 2B  is a sectional view of the ground electrode  40 , taken along a line IIb-IIb of  FIG. 2A . An arrow Z indicates a first direction that is perpendicular to the discharge surface  45  of the tip  44 . If the ground electrode  40  is defined as the first electrode, the center electrode  20  is a second electrode. In the present embodiment, the base material  41  has a rod-shape having a substantially rectangular cross section, and the tip  44  has a rectangular parallelepiped. A part of the tip  44  is placed in a recessed groove that is formed by being set back into the inner surface  42  located at a top end portion of the base material  41  along a side surface  41   b  of the base material  41 . A position of the tip  44  is limited by a wall surface  42   a  of the groove. The tip  44  is joined to the base material  41  through the fusion portion  43 . The fusion portion  43  is a portion where the tip  44  and the base material  41  are fused together. 
     The fusion portion  43  has an overlap portion  48  where a first interface (or a first boundary)  46  between the tip  44  and the fusion portion  43  and a second interface (or a second boundary)  47  between the base material  41  and the fusion portion  43  overlap each other in the first direction (the arrow Z direction).  FIG. 2B  is also a sectional view of the ground electrode  40 , cut by a cutting-plane line (the line IIb-IIb) passing through a center of gravity  49  of a projected planform of the overlap portion  48  onto a virtual surface (a surface parallel to the drawing of  FIG. 2A ) parallel to the discharge surface  45  of the tip  44 . An arrow Y indicates a second direction that is a direction parallel to the discharge surface  45  and extends on the cutting-plane line (the line IIb-IIb). Although the cutting-plane line passing through the center of gravity  49  can be drawn innumerably, in the present embodiment, the cutting-plane line is drawn on a diagonal line of the discharge surface  45  of the tip  44  such that a length of the overlap portion  48  in the second direction becomes a maximum (becomes longest). Analysis on its cross section is then carried out. 
     An example of a method of producing the ground electrode  40  will be explained with reference to  FIG. 3A .  FIG. 3A  is a schematic view when joining the tip  44  to the base material  41 , and shows a state before the fusion portion  43  (indicated by a two-dot chain line) is formed.  FIG. 3A  is a cross section cut by a cutting-plane line that is perpendicular to a top end surface  41   a  of the base material  41  and parallel to the side surface  41   b  of the base material  41 .  FIG. 3B  is similar to the above-explained  FIG. 3A , namely that  FIG. 3B  is a cross section cut by the above cutting-plane line and shows a state before the fusion portion  43  (indicated by a two-dot chain line) is formed. 
     A groove bottom  42   b , which is a bottom of the groove on the inner surface  42  of the base material  41 , inclines or slopes from the wall surface  42   a  toward the top end surface  41   a  such that a depth of the groove is deeper from the wall surface  42   a  toward the top end surface  41   a . A bottom surface  45   a  of the tip  44  also inclines or slopes such that a portion, located close to the wall surface  42   a  of the base material  41 , of the tip  44  is thinner than a portion, located close to the top end surface  41   a  of the base material  41 , of the tip  44 . 
     After placing the tip  44  on the groove of the base material  41 , high-energy beam such as laser beam and electron beam is radiated from a beam-machining head  54  provided so as to face to the top end surface  41   a  of the base material  41 . By moving the beam-machining head  54  along the groove bottom  42   b  while radiating the beam, the fusion portion  43  is formed, then the tip  44  is joined to the base material  41 . Since the beam is radiated to the top end surface  41   a  of the base material  41 , a melting amount at the top end surface  41   a  side is large as compared with that at the wall surface  42   a  side. Further, as mentioned above, since the bottom surface  45   a  of the tip  44  and the groove bottom  42   b  of the inner surface  42  of the base material  41  slope, at the top end surface  41   a  side in the fusion portion  43 , a melting amount of the tip  44  is larger than a melting amount of the base material  41 , whereas at the wall surface  42   a  side in the fusion portion  43 , a melting amount of the base material  41  is larger than a melting amount of the tip  44 . 
     Returning to  FIG. 2B , this will be explained in detail. In the present embodiment, at one side end portion  50  (one end portion) of the overlap portion  48  in the second direction (the arrow Y direction) along the discharge surface  45  of the tip  44 , since the melting amount of the tip  44  is larger than the melting amount of the base material  41 , a noble metal content is greater than 50 mass %. On the other hand, at the other side end portion  51  (the other end portion) of the overlap portion  48  in the second direction, since the melting amount of the base material  41  is larger than the melting amount of the tip  44 , a Ni content is greater than 50 mass %. Here, each of the end portions  50  and  51  is a line segment whose both ends are defined by the first and second interfaces  46  and  47 . Each of the end portions  50  and  51  is perpendicular to the discharge surface  45 . 
     As mentioned above, since the differences in the noble metal content and the Ni content exist between the end portions  50  and  51 , at the end portion  50 , a thermal stress occurring at the second interface  47  is greater than a thermal stress occurring at the first interface  46 . On the other hand, at the end portion  51 , a thermal stress occurring at the first interface  46  is greater than a thermal stress occurring at the second interface  47 . Consequently, at the end portion  50  side, a crack tends to appear at the second interface  47 , whereas at the end portion  51  side, a crack tends to appear at the first interface  46 . Further, the crack appearing at the first interface  46  tends to develop along the first interface  46 , and the crack appearing at the second interface  47  tends to develop along the second interface  47 . However, even if the cracks develop in this way, thanks to the above structure, it is possible to reduce a tendency for the cracks developing along the first and second interfaces  46  and  47  to join together. Therefore, as compared with a case where cracks appearing at both ends of one interface develop toward the middle of the interface along the interface, coming-off of the tip  44  from the base material  41  due to rupture of the fusion portion  43  can be suppressed. 
     Here, quantitative analysis to measure the noble metal content and the Ni content at the end portions  50  and  51  of the overlap portion  48  can be carried out by WDS (Wavelength Dispersive Spectrometry) analysis using EPMA (Electron Probe Micro Analyzer). A width of each of the end portions  50  and  51  (a thickness of each line segment) in the second direction is a width required for the quantitative analysis (in the present embodiment, it is at least 20 μm). Each of the noble metal content and the Ni content at the end portions  50  and  51  can be measured by taking an average of measurement values of a plurality of measurement points which are set at the same regular intervals on both line segments of the end portions  50  and  51 . Instead of this, a measurement value of a midpoint of each line segment of the end portions  50  and  51  could be a central value. 
     As mentioned above, in the fusion portion  43 , since the melting amount at the top end surface  41   a  side is large as compared with that at the wall surface  42   a  side, the overlap portion  48  is shaped so that a distance between the first interface  46  and the second interface  47  along the first direction (the arrow Z direction) is gradually longer toward the second direction (the arrow Y direction). In the overlap portion  48 , a middle portion  53  at which the noble metal content is 50 mass % and also the Ni content is 50 mass % exists on the second direction side (the arrow Y direction side) with respect to a center position  52  in the second direction of the overlap portion  48 . The center position  52  is a position including a middle point that is located at the same L distance from the end portion  50  and from the end portion  51 . 
     Therefore, as compared with a section of the first interface  46  from the end portion  50  up to the middle portion  53 , at a section of the first interface  46  from the end portion  51  up to the middle portion  53 , the crack appearing at the end portion  51  side tends to develop along the first interface  46 . On the other hand, as compared with a section of the second interface  47  from the end portion  51  up to the middle portion  53 , at a section of the second interface  47  from the end portion  50  up to the middle portion  53 , the crack appearing at the end portion  50  side tends to develop along the second interface  47 . Consequently, a position where the cracks developing along the first interface  46  and the second interface  47  respectively overlap each other in the first direction (the arrow Z direction) tends to shift to or get closer to the second direction (the arrow Y direction) side with respect to the center position  52 . Therefore, even if the cracks develop along the first direction (the arrow Z direction) at this position in the fusion portion  43 , since a distance between the first interface  46  and the second interface  47  at this position is longer than that at the end portion  51  side with respect to the center position  52  of the overlap portion  48 , rupture of the fusion portion  43  is suppressed, then the coming-off of the tip  44  from the base material  41  can be further suppressed. 
     It is noted that a relationship showing that the noble metal content is greater than 50 mass % at the one side end portion  50  and the Ni content is greater than 50 mass % at the other side end portion  51  is established on the cross section on which the length of the overlap portion  48  in the second direction (the arrow Y direction) becomes a maximum (becomes longest). Since lengths of the first interface  46  and the second interface  47  on which the cracks tend to develop are longest at this cross section position, the coming-off of the tip  44  from the base material  41  can be further suppressed. 
     A modified example of the ground electrode  40  will be explained with reference to  FIG. 3B .  FIG. 3B  is a schematic view when joining the tip  44  to the base material  41 . Unlike the case of  FIG. 3A , a groove bottom  42   c , which is a bottom of the groove on the inner surface  42  of the base material  41 , inclines or slopes from the wall surface  42   a  toward the top end surface  41   a  such that a depth of the groove is shallower from the wall surface  42   a  toward the top end surface  41   a . A bottom surface  45   b  of the tip  44  also inclines or slopes such that a portion, located close to the wall surface  42   a  of the base material  41 , of the tip  44  is thicker than a portion, located close to the top end surface  41   a  of the base material  41 , of the tip  44 . 
     After placing the tip  44  on the groove of the base material  41 , by radiating high-energy beam from the beam-machining head  54  provided so as to face to the top end surface  41   a  of the base material  41 , the fusion portion  43  is formed, then the tip  44  is joined to the base material  41 . Because of the slopes of the bottom surface  45   b  of the tip  44  and the groove bottom  42   c  of the inner surface  42  of the base material  41 , at the top end surface  41   a  side in the fusion portion  43 , a melting amount of the base material  41  is larger than a melting amount of the tip  44 , whereas at the wall surface  42   a  side in the fusion portion  43 , a melting amount of the tip  44  is larger than a melting amount of the base material  41 . 
     Returning to  FIG. 2B , this will be explained in detail. In this example, at one side end portion  50  (one end portion) of the overlap portion  48  in the second direction (the arrow Y direction) along the discharge surface  45  of the tip  44 , since the melting amount of the base material  41  is larger than the melting amount of the tip  44 , a Ni content is greater than 50 mass %. On the other hand, at the other side end portion  51  (the other end portion) of the overlap portion  48  in the second direction, since the melting amount of the tip  44  is larger than the melting amount of the base material  41 , a noble metal content is greater than 50 mass %. 
     Therefore, at the end portion  50 , a thermal stress occurring at the first interface  46  is greater than a thermal stress occurring at the second interface  47 . On the other hand, at the end portion  51 , a thermal stress occurring at the second interface  47  is greater than a thermal stress occurring at the first interface  46 . Consequently, at the end portion  50  side, a crack tends to appear at the first interface  46 , whereas at the end portion  51  side, a crack tends to appear at the second interface  47 . Further, the crack appearing at the first interface  46  tends to develop along the first interface  46 , and the crack appearing at the second interface  47  tends to develop along the second interface  47 . However, even if the cracks develop in this way, thanks to the above structure, it is possible to reduce a tendency for the cracks developing along the first and second interfaces  46  and  47  to join together. Therefore, as compared with a case where cracks appearing at both ends of one interface develop toward the middle of the interface along the interface, coming-off of the tip  44  from the base material  41  due to rupture of the fusion portion  43  can be suppressed. 
     Next, the center electrode  20  will be explained.  FIG. 4A  is a bottom view of the center electrode  20  (a first electrode), viewed from a direction of the axis O.  FIG. 4B  is a sectional view of the center electrode  20 , taken along a line IVb-IVb of  FIG. 4A . An arrow Z indicates a first direction that is perpendicular to the discharge surface  25  of the tip  24 . If the center electrode  20  is defined as the first electrode, the ground electrode  40  is a second electrode. In the present embodiment, the base material  22  has, as an outside shape, a cylindrical-columned shape extending along the axis O, and the tip  24  has a disc shape. The tip  24  is placed at a top end in the axis direction of the base material  22 , and joined to the base material  22  through the fusion portion  23 . The fusion portion  23  is a portion where the tip  24  and the base material  22  are fused together. 
     The fusion portion  23  has an overlap portion  62  where a first interface (or a first boundary)  60  between the tip  24  and the fusion portion  23  and a second interface (or a second boundary)  61  between the base material  22  and the fusion portion  23  overlap each other in the first direction (which is identical with the axis O direction, the arrow Z direction).  FIG. 4B  is also a sectional view of the center electrode  20 , cut by a cutting-plane line (the line IVb-IVb) passing through a center of gravity  63  of a projected planform of the overlap portion  62  onto a virtual surface (a surface parallel to the drawing of  FIG. 4A ) parallel to the discharge surface  25  of the tip  24 . A position of the center of gravity  63  is substantially identical with a position of the axis O. An arrow Y indicates a second direction that is a direction parallel to the discharge surface  25  and extends on the cutting-plane line (the line IVb-IVb). 
     An example of a method of producing the center electrode  20  will be explained with reference to  FIG. 5A .  FIG. 5A  is a schematic view when joining the tip  24  to the base material  22 , and shows a state before the fusion portion  23  (indicated by a two-dot chain line) is formed.  FIG. 5B  is similar to the above  FIG. 5A . 
     A top end surface  22   a  of the base material  22  and an end surface  24   a , located at an opposite side to the discharge surface  25 , of the tip  24  are flat surfaces that obliquely cross the axis O. With these shapes, regarding both side portions  24   b  and  24   c  of the tip  24  which are located at opposite sides of the axis O, a length of the portion  24   b  between the discharge surface  25  and the end surface  24   a  of the tip  24  is longer than that of the portion  24   c . In other words, a length of the portion  24   c  between the discharge surface  25  and the end surface  24   a  is shorter than that of the portion  24   b . The tip  24  is placed on the base material  22  with its end surface  24   a  contacting the top end surface  22   a  of the base material  22  so that the discharge surface  25  of the tip  24  is orthogonal to the axis O. 
     After placing the tip  24  on the base material  22 , by radiating high-energy beam such as laser beam and electron beam from a beam-machining head  54  provided so as to face to side surfaces of the base material  22  and the tip  24  while turning the base material  22  and the tip  24  on the axis O, the fusion portion  23  is formed, then the tip  24  is joined to the base material  22 . Since the beam is radiated to the side surface of the base material  22 , a melting amount at an outer side in a radial direction of the base material  22  is large as compared with that at a middle in the radial direction of the base material  22 . Further, since the top end surface  22   a  of the base material  22  and the end surface  24   a  of the tip  24  slope, at the portion  24   b  of the tip  24  in the fusion portion  23 , a melting amount of the tip  24  is larger than a melting amount of the base material  22 , whereas at the portion  24   c  opposite to the portion  24   b  with respect to the axis O, a melting amount of the base material  22  is larger than a melting amount of the tip  24 . 
     Returning to  FIG. 4B , this will be explained in detail. In the present embodiment, at one end portion  64  of the overlap portion  62  in the second direction (the arrow Y direction) along the discharge surface  25  of the tip  24 , since the melting amount of the tip  24  is larger than the melting amount of the base material  22 , a noble metal content is greater than 50 mass %. On the other hand, at the other end portion  65  of the overlap portion  62  in the second direction, since the melting amount of the base material  22  is larger than the melting amount of the tip  24 , a Ni content is greater than 50 mass %. 
     Therefore, at the one end portion  64 , a thermal stress occurring at the second interface  61  is greater than a thermal stress occurring at the first interface  60 . On the other hand, at the other end portion  65 , a thermal stress occurring at the first interface  60  is greater than a thermal stress occurring at the second interface  61 . Consequently, at the one end portion  64  side, a crack tends to appear at the second interface  61 , whereas at the other end portion  65  side, a crack tends to appear at the first interface  60 . Further, the crack appearing at the first interface  60  tends to develop along the first interface  60 , and the crack appearing at the second interface  61  tends to develop along the second interface  61 . However, even if the cracks develop in this way, thanks to the above structure, it is possible to reduce a tendency for the cracks developing along the first and second interfaces  60  and  61  to join together. Therefore, as compared with a case where cracks appearing at both ends of one interface develop toward the middle of the interface along the interface, coming-off of the tip  24  from the base material  22  due to rupture of the fusion portion  23  can be suppressed. 
     As mentioned above, in the fusion portion  23 , since the melting amount at the outer side in the radial direction of the base material  22  is large as compared with that at the middle in the radial direction of the base material  22 , the overlap portion  62  is shaped so that a distance between the first interface  60  and the second interface  61  along the first direction (the arrow Z direction) is gradually shorter from the outer side toward the middle. Thus, in the overlap portion  62 , between the one end portion  64  and the other end portion  65 , a shortest portion  66  at which the distance between the first interface  60  and the second interface  61  along the first direction is shortest exists at a portion except the one end portion  64  and the other end portion  65 . Further, in the overlap portion  62 , a middle portion  67  at which the noble metal content is 50 mass % and also the Ni content is 50 mass % exists at a portion except the shortest portion  66 . 
     Therefore, as compared with a section of the first interface  60  from the one end portion  64  up to the middle portion  67 , at a section of the first interface  60  from the other end portion  65  up to the middle portion  67 , the crack appearing at the other end portion  65  side tends to develop along the first interface  60 . On the other hand, as compared with a section of the second interface  61  from the other end portion  65  up to the middle portion  67 , at a section of the second interface  61  from the one end portion  64  up to the middle portion  67 , the crack appearing at the one end portion  64  side tends to develop along the second interface  61 . Since the middle portion  67  is positioned at a different position from the shortest portion  66  in the second direction (the arrow Y direction), a position where the cracks developing along the first interface  60  and the second interface  61  respectively overlap each other in the first direction (the arrow Z direction) tends to be located at a portion except the shortest portion  66 . Therefore, even if the cracks develop along the first direction at this position, since a distance between the first interface  60  and the second interface  61  at this position is longer than that at the shortest portion  66 , rupture of the fusion portion  23  is suppressed, then the coming-off of the tip  24  from the base material  22  can be further suppressed. 
     A modified example of the center electrode  20  will be explained with reference to  FIG. 5B .  FIG. 5B  is a schematic view when joining the tip  24  to the base material  22 . Unlike the case of  FIG. 5A , an end surface  24   d  of the tip  24  is parallel to the discharge surface  25 , and a top end surface  22   b  of the base material  22  is a surface that is perpendicular to the axis O. After placing the tip  24  on the base material  22  with its end surface  24   d  contacting the top end surface  22   b  of the base material  22 , by radiating high-energy beam from the beam-machining head  54  provided so as to face to side surfaces of the base material  22  and the tip  24  while turning the base material  22  and the tip  24  on the axis O and while moving the beam-machining head  54  backwards and forwards along the axis O, a path or a trail of the beam scanning surfaces of the base material  22  and the tip  24  becomes an oval shape. 
     Also in this case, a relationship, showing that in a sectional view of the center electrode  20  cut by a cutting-plane line passing through a center of gravity  63  of a projected planform of the overlap portion  62  onto a virtual surface parallel to the discharge surface  25  of the tip  24 , the noble metal content is greater than 50 mass % (the melting amount of the tip  24  is larger than the melting amount of the base material  22 ) at the end portion of the overlap portion  62  on the portion  24   b  side and the Ni content is greater than 50 mass % (the melting amount of the base material  22  is larger than the melting amount of the tip  24 ) at the other end portion of the overlap portion  62  on the other portion  24   c  side, is established. Hence, the same mechanism and effect can be obtained. 
     Although the present invention is explained on the basis of the above embodiment, the present invention is not limited to the above embodiment. The present invention includes all design modifications and equivalents belonging to the technical scope of the present invention. 
     The above embodiment shows the example in which the groove is formed on the base material  41  of the ground electrode  40 , and the tip  44 , a part of which is accommodated in the groove, is joined to the base material  41 . However, structures of the base material  41  and the tip  44  are not limited to this example. The base material  41  is not necessarily provided with the groove. And, the tip  44  could be joined to the base material  41  without forming the groove on the base material  41 . 
     The above embodiment shows the example in which a top end surface of the tip  44  is positioned at a slightly inner side with respect to the top end surface  41   a  of the base material  41 . However, the position of the tip  44  is not limited to this example. For instance, the tip  44  could be set so that its top end surface is positioned at an outer side with respect to the top end surface  41   a  of the base material  41 , namely that the top end surface of the tip  44  protrudes from the top end surface  41   a  of the base material  41 . 
     The above embodiment shows the example in which the tip  44  is joined to the inner surface  42  of the base material  41  of the ground electrode  40 . However, the joining of the tip  44  is not limited to this example. The tip  44  could be joined to other portions such as the top end surface  41   a  of the base material  41 , except the inner surface  42 . 
     The above embodiment shows the example in which the tip  44  of the ground electrode  40  has the rectangular parallelepiped (a square column). However, a shape of the tip  44  is not limited to this example. As the shape of the tip  44 , a cylindrical column and a polygonal column except the square column could be employed as necessary. 
     The above embodiment shows the example in which the tip  44  is directly joined to the base material  41  of the ground electrode  40  through the fusion portion  43 . However, the joining of the tip  44  is not limited to this example. It could be possible to interpose an intermediate member principally made of Ni between the base material and the tip, and join the tip to the intermediate member joined to the base material through the fusion portion. 
     The above embodiment shows the example in which the relationship, showing that the noble metal content is greater than 50 mass % at the one end portions of the overlap portions  48  and  62  and the Ni content is greater than 50 mass % at the other end portions of the overlap portions  48  and  62 , is established in both of the center electrode  20  and the ground electrode  40 . However, the present invention is not limited to this example. As long as this relationship is established in either one of the center electrode  20  and the ground electrode  40 , the present invention can be realized, and the tip of the electrode having this relationship can be prevented from coming off the base material. 
     In the above embodiment, as the example of the production of (the fusion portion  23  of) the center electrode  20 , the tip  24  is set on the base material  22 , and the high-energy beam is radiated while turning this set of the base material  22  and the tip  24  on the axis O. However, the production of (the fusion portion  23  of) the center electrode  20  is not limited to this example. For instance, the fusion portion  23  could be formed by setting the tip  24  on the base material  22  and performing the high-energy beam scan around the base material  22  and the tip  24  using one or more mirrors with this set of the base material  22  and the tip  24  remaining at rest. 
     EXPLANATION OF REFERENCE 
     
         
           10  . . . spark plug 
           20  . . . center electrode (first electrode, second electrode) 
           22 ,  41  . . . base material 
           23 ,  43  . . . fusion portion 
           24 ,  44  . . . tip 
           25 ,  45  . . . discharge surface 
           40  . . . ground electrode (first electrode, second electrode) 
           46 ,  60  . . . first interface (first boundary) 
           47 ,  61  . . . second interface (second boundary) 
           48 ,  62  . . . overlap portion 
           49 ,  63  . . . center of gravity 
           50 ,  51  . . . end portion (one end portion, the other end portion) 
           64  . . . one end portion 
           65  . . . the other end portion 
           52  . . . center position 
           53 ,  67  . . . middle portion 
           66  . . . shortest portion 
       
    
     The entire contents of Japanese Patent Applications No. 2018-112958 filed on Jun. 13, 2018 is incorporated herein by reference. 
     Although the invention has been described above by reference to certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.