Patent Publication Number: US-9837798-B1

Title: Spark plug

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2016-094220, which was filed on May 10, 2016, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a spark plug, and particularly to a spark plug that allows improvement of durability of a ground electrode. 
     Description of Related Art 
     A spark plug in which a tip containing a noble metal is joined to an electrode base material of a ground electrode, in order to improve spark wear resistance of the ground electrode, is known (Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 1 is Japanese Patent Application Laid-Open (kokai) No. 2003-217792. 
     SUMMARY OF THE INVENTION 
     However, in the technique disclosed in Patent Document 1, the tip is jointed to the electrode base material via a welded portion formed over the entirety of an interface of the tip, so that thermal stress may cause peeling at the tip or falling-off of the tip. Therefore, a problem arises that durability of the ground electrode may be reduced. 
     The present invention is made in order to solve the aforementioned problem, and an object of the present invention is to provide a spark plug that allows improvement of durability of a ground electrode. 
     In order to attain the above object, a spark plug according to a first aspect of the present invention includes a tip that contains a noble metal and forms a spark gap between the tip and a center electrode (i.e., positioned to form a spark gap between the tip and the center electrode). The tip is joined to a joining surface of an electrode base material (or, as used interchangeably herein, an electrode base) of a ground electrode via a welded portion. On cross-sections of the tip and the electrode base material in a longitudinal direction of the joining surface, the welded portion has a void above the joining surface, and a continuous distance of the welded portion on the joining surface is less than or equal to 0.5 mm. In other words, the tip defines one or more voids above the joining surface and between the plurality of welded portions, and the welded portions are each less than or equal to 0.5 mm in the longitudinal direction. Therefore, as compared to a case where the entirety of the interface of the tip is joined to the electrode base material, thermal stress due to a difference in thermal expansion between the tip and the electrode base material can be reduced. A total of continuous distances of the welded portions on the joining surface, are 0.4 times to 0.8 times a length from an end of the tip to another end thereof, whereby joining strength of the welded portion can be assured. As a result, peeling at the tip or falling-off of the tip due to thermal stress, vibration, or the like can be less likely to occur. Therefore, an effect of improving durability of the ground electrode can be obtained. 
     In the spark plug according to a second aspect of the present invention, the tip has a plurality of divisional tips arranged on the joining surface. The welded portions adjoin each of the divisional tips to the joining surface. The size of the divisional tip can be reduced as compared to an integral tip. Therefore, in addition to the effect by the first aspect being obtained, an effect of reducing thermal stress caused by a difference in thermal expansion between the tip and the electrode base material can be enhanced. Further, a maximum spatial distance, on the joining surface, between the divisional tips adjacent to each other is less than or equal to 0.3 mm. Therefore, spark discharge at the electrode base material between the divisional tips can be less likely to occur. As a result, an effect of reducing spark wear of the electrode base material between the divisional tips can be obtained. 
     In the spark plug according to a third aspect of the present invention, on the cross-sections of the tip and the electrode base material in the longitudinal direction of the joining surface, the length from the end of the tip to the other end thereof is greater than or equal to 1.5 mm. The greater the length of the tip is, the greater the dimensional change due to heat is, so that peeling at the tip or falling-off of the tip is likely to occur. However, a plurality of voids are formed above the joining surface at the welded portion, whereby the peeling or falling-off can be prevented. As a result, in addition to the effect by the first aspect or the second aspect being obtained, even when the tip has the length which is greater than or equal to 1.5 mm, peeling at the tip or falling-off of the tip due to thermal stress can be less likely to occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a cross-sectional view of a spark plug according to a first embodiment of the present invention. 
         FIG. 2( a )  is a plan view of a tip.  FIG. 2( b )  is a front view of the tip.  FIG. 2( c )  is a bottom view of the tip.  FIG. 2( d )  is a side view of the tip. 
         FIG. 3( a )  is a plan view of a ground electrode.  FIG. 3( b )  is a cross-sectional view of the ground electrode taken along the line represented by arrows IIIb-IIIb shown in  FIG. 3( a ) . 
         FIG. 4( a )  is a plan view of a tip of a spark plug according to a second embodiment.  FIG. 4( b )  is a front view of the tip.  FIG. 4( c )  is a bottom view of the tip.  FIG. 4( d )  is a side view of the tip. 
         FIG. 5( a )  is a plan view of a ground electrode.  FIG. 5( b )  is a cross-sectional view of the ground electrode taken along the line represented by arrows Vb-Vb shown in  FIG. 5( a ) . 
         FIG. 6( a )  is a plan view of a ground electrode of a spark plug according to a third embodiment.  FIG. 6( b )  is a cross-sectional view of the ground electrode taken along the line represented by arrows VIb-VIb shown in  FIG. 6( a ) . 
         FIG. 7( a )  is a plan view of a ground electrode of a spark plug according to a fourth embodiment.  FIG. 7( b )  is a cross-sectional view of the ground electrode taken along the line represented by arrows VIIb-VIIb shown in  FIG. 7( a ) . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a cross-sectional view of a spark plug  10 , according to a first embodiment of the present invention, taken along a plane including a central axis O. In  FIG. 1 , the lower side on the surface of the drawing sheet is referred to as a front side of the spark plug  10 , and the upper side on the surface of the drawing sheet is referred to as the rear side of the spark plug  10 . As shown in  FIG. 1 , the spark plug  10  includes a metal shell  20 , a ground electrode  30 , an insulator  40 , a center electrode  50 , a metal terminal  60 , and a resistor  70 . 
     The metal shell  20  is an almost cylindrical member that is fixed in a screw hole (not shown) of an internal combustion engine, and has a through hole  21  that penetrates therethrough along the central axis O. The metal shell  20  is formed of a metal material (for example, low-carbon steel or the like) having conductivity. The metal shell  20  includes: a seat portion  22  that protrudes outward in the radial direction so as to be flange-shaped; and a screw portion  23  formed on the outer circumferential surface forward of the seat portion  22 . An annular gasket  24  is fitted between the seat portion  22  and the screw portion  23 . When the screw portion  23  is fitted into the screw hole of the internal combustion engine, the gasket  24  seals a gap between the metal shell  20  and the internal combustion engine (engine head). 
     The ground electrode  30  includes: an electrode base material  31  which is made of a metal (for example, a nickel-based alloy) and is joined to the front end of the metal shell  20 ; and a tip  32  joined to the end of the electrode base material  31 . The electrode base material  31  is a rod-shaped member that is bent toward the central axis O so as to intersect the central axis O. The tip  32  is a member formed of a noble metal such as platinum, iridium, ruthenium, or rhodium, or an alloy containing such a noble metal as a main component. The tip  32  is joined by laser beam welding, resistance welding, or the like at such a position that the tip  32  intersects the central axis O. The melting point of the tip  32  is higher than the melting point of the electrode base material  31 , and the thermal expansion coefficient of the tip  32  is less than the thermal expansion coefficient of the electrode base material  31 . 
     The insulator  40  is an almost cylindrical member formed of alumina or the like which is excellent in mechanical property and insulation property at a high temperature. The insulator  40  has an axial hole  41  that penetrates therethrough along the central axis O. The insulator  40  is inserted into the through hole  21  of the metal shell  20  and the metal shell  20  is fixed on the outer circumference of the insulator  40 . The front end and the rear end of the insulator  40  are exposed from the through hole  21  of the metal shell  20 . 
     The axial hole  41  has: a first hole portion  42  disposed on the front side of the insulator  40 ; a step portion  43  that connects to the rear end of the first hole portion  42  and has the diameter enlarged toward the rear side; and a second hole portion  44  disposed on the side rearward of the step portion  43 . The inner diameter of the second hole portion  44  is set to be larger than the inner diameter of the first hole portion  42 . 
     The center electrode  50  is a rod-shaped electrode in which, in a tubular electrode base material having the bottom, a core material  53  having a thermal conductivity that is more excellent than the electrode base material is embedded. The core material  53  is formed of copper or an alloy containing copper as a main component. The center electrode  50  includes: a head portion  51  disposed at the step portion  43  of the axial hole  41 ; and a leg portion  52  that extends along the central axis O on the first hole portion  42  side. 
     The front end of the leg portion  52  is exposed from the first hole portion  42 , and a tip  54  is joined thereto by laser beam welding. The tip  54  is a columnar member formed of a noble metal such as platinum, iridium, ruthenium, or rhodium, or an alloy containing such a noble metal as a main component, and opposes the tip  32  of the ground electrode  30  via a spark gap. 
     The metal terminal  60  is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a metal material (for example, low-carbon steel or the like) having conductivity. The front side portion of the metal terminal  60  is disposed in the axial hole  41  of the insulator  40 . 
     The resistor  70  is a member for reducing electric wave noise generated when spark occurs, and is disposed in the second hole portion  44  between the metal terminal  60  and the center electrode  50 . A conductive glass seal  71  is disposed between the resistor  70  and the center electrode  50 , and a conductive glass seal  72  is disposed between the resistor  70  and the metal terminal  60 . The glass seal  71  contacts with the resistor  70  and the center electrode  50 , and the glass seal  72  contacts with the resistor  70  and the metal terminal  60 . As a result, the center electrode  50  and the metal terminal  60  are electrically connected with each other via the resistor  70  and the glass seals  71 ,  72 . 
     The spark plug  10  is manufactured in, for example, a method described below. Firstly, the center electrode  50  is inserted through the second hole portion  44  of the insulator  40 . The center electrode  50  has the tip  54  welded to the front end of the leg portion  52 . The center electrode  50  is disposed such that the head portion  51  thereof is supported by the step portion  43 , and the front end portion thereof is exposed to the outside from the front end of the axial hole  41 . 
     Next, raw material powder of the glass seal  71  is charged through the second hole portion  44  into a portion around the head portion  51  and a rear end side portion thereof. A compression bar member (not shown) is used to preform preliminary compression on the raw material powder, of the glass seal  71 , having been charged through the second hole portion  44 . Onto a formed body of the formed raw material powder of the glass seal  71 , the raw material powder of the resistor  70  is charged. The compression bar member (not shown) is used to perform preliminary compression on the raw material powder, of the resistor  70 , having been charged through the second hole portion  44 . Next, onto the raw material powder of the resistor  70 , the raw material powder of the glass seal  72  is charged. The compression bar member (not shown) is used to perform preliminary compression on the raw material powder, of the glass seal  72 , having been charged through the second hole portion  44 . 
     Thereafter, a front end portion  61  of the metal terminal  60  is inserted through the rear end side of the axial hole  41 , and the metal terminal  60  is positioned such that the front end portion  61  contacts with the raw material powder of the glass seal  72 . Next, for example, while heating up to a temperature higher than a softening point of a glass component contained in each raw material powder is performed, the metal terminal  60  is pressed in until the front end surface of a protrusion portion  62  provided on the rear end side of the metal terminal  60  contacts with the rear end surface of the insulator  40 , and load is applied, in the axial direction, to the raw material powder of each of glass seal  71 , the resistor  70 , and glass seal  72  by the front end portion  61 . As a result, each raw material powder is compressed and sintered, and the glass seal  71 , the resistor  70 , and the glass seal  72  are formed in the insulator  40 . 
     Next, the metal shell  20  to which the ground electrode  30  is previously joined, is mounted to the outer circumference of the insulator  40 . Thereafter, the tip  32  is welded to the electrode base material  31  of the ground electrode  30 , and the electrode base material  31  is bent such that the tip  32  of the ground electrode  30  opposes the tip  54  of the center electrode  50  in the axial direction, to obtain the spark plug  10 . 
     The tip  32  will be described with reference to  FIG. 2 .  FIG. 2( a )  is a plan view of the tip  32 ,  FIG. 2( b )  is a front view of the tip  32 ,  FIG. 2( c )  is a bottom view of the tip  32 , and  FIG. 2( d )  is a side view of the tip  32 . 
     The tip  32  is a member that is formed, in a rectangular parallelepiped, of a noble metal or an alloy containing a noble metal as a main component. The tip  32  has: a rectangular top surface  33  that opposes the center electrode  50  (see  FIG. 1 ); a rectangular bottom surface  36  disposed opposite to the top surface  33 ; and side surfaces  34 ,  35  that connect between the top surface  33  and the bottom surface  36 . The side surfaces  35  have long sides (edges that connect between the side surfaces  35  and the top surface  33 ) which are longer than long sides (edges that connect between the side surfaces  34  and the top surface  33 ) of the side surfaces  34 . 
     The bottom surface  36  has a plurality of protrusions  37  that protrude from the bottom surface  36 . In the present embodiment, three protrusions  37  are disposed almost parallel to the long sides of the side surfaces  34  so as to be spaced from each other. The protrusions  37  are formed on the bottom surface  36  of the tip  32  by a base material of the tip  32  being, for example, rolled or cut. 
     The ground electrode  30  will be described with reference to  FIG. 3 .  FIG. 3( a )  is a plan view of the ground electrode  30 , and  FIG. 3( b )  is a cross-sectional view of the ground electrode  30  taken along the line represented by arrows IIIb-IIIb shown in  FIG. 3( a ) . In  FIG. 3( a ) , a portion of the electrode base material  31  in the longitudinal direction (the left-right direction in  FIG. 1 ) is not shown. In  FIG. 3( b ) , a portion of the electrode base material  31  in the thickness direction is not shown. An arrow L in  FIG. 3( a )  represents the longitudinal direction of the electrode base material  31 . 
     As shown in  FIG. 3( a ) , the tip  32  is disposed on a joining surface  38  (see  FIG. 3( b ) ) of the electrode base material  31  such that the longitudinal direction of the tip  32  is along the longitudinal direction (the direction represented by the arrow L) of the electrode base material  31 . As shown in  FIG. 3( b ) , the joining surface  38  opposes the bottom surface  36  of the tip  32 , and the tip  32  is joined to the joining surface  38 . In other words, the joining surface  38  is a projection surface (surface representing the outer shape of the tip  32 ) obtained by the outer shape of the tip  32  being projected on the surface of the electrode base material  31 . In the present embodiment, the tip  32  is joined to the electrode base material  31  by resistance welding. 
       FIG. 3( b )  is a cross-sectional view of the tip  32  and the electrode base material  31  taken along the longitudinal direction (direction in which the line represented by arrows IIIb-IIIb extends) of the joining surface  38 . The tip  32  is joined to the electrode base material  31  by welded portions  80 . The welded portions  80  are formed by the tip  32  and the electrode base material  31  being melted, and are formed at positions at which the protrusions  37  contact with the electrode base material  31 . The protrusions  37  protrude from the bottom surface  36  of the tip  32 . Therefore, when pressure is appropriately applied to the tip  32  and the electrode base material  31  and electricity is applied thereto, the welded portions  80  are formed between the protrusions  37  and the electrode base material  31  due to Joule heat generated in contact resistance between the protrusions  37  and the electrode base material  31 . Simultaneously when the welded portions  80  are formed, voids  81  are formed above the joining surface  38  between the protrusions  37 ,  37  adjacent to each other. The voids  81  are portions at which the electrode base material  31  and the bottom surface  36  of the tip  32  do not connect with each other. 
     The welded portion is divided into a plurality of welded portions  80  (three welded portions in the present embodiment) on the joining surface  38  by the voids  81  (two voids in the present embodiment) being formed above the joining surface  38 . A continuous distance L 1 , a continuous distance L 2 , and a continuous distance L 3  of the welded portions  80  on the joining surface  38  are each set to be less than or equal to 0.5 mm (excluding 0). As a result, as compared to a case where the entirety of the bottom surface  36  of the tip  32  is joined to the electrode base material  31 , thermal stress caused by a difference in thermal expansion between the tip  32  and the electrode base material  31  can be reduced. In the present embodiment, L 1 =L 2 =L 3  is satisfied. However, L 1 , L 2 , and L 3  are not limited thereto. The distances L 1 , L 2 , and L 3  can be set as appropriate in such a range that the distances L 1 , L 2 , and L 3  are each less than or equal to 0.5 mm. 
     If the distances L 1 , L 2 , and L 3  of the welded portions  80  are each greater than 0.5 mm, as each distance becomes greater, dimensional change, due to heat, of the electrode base material  31  and the tip  32  becomes too great to be ignored due to difference between a thermal expansion coefficient of the tip  32  and a thermal expansion coefficient of the electrode base material  31 . Thus, ends of the welded portions  80  tend to be more likely to be peeled due to thermal stress. In the present embodiment, the distances L 1 , L 2 , and L 3  are each set to be less than or equal to 0.5 mm, whereby peeling at the welded portions  80  due to thermal stress can be inhibited. 
     Further, the total L 1 +L 2 +L 3  of the continuous distance L 1 , the continuous distance L 2 , and the continuous distance L 3  of the welded portions  80  on the joining surface  38  is set to be 0.4 times to 0.8 times a length L (the length of the long side of the top surface  33 ), in the longitudinal direction, from the end of the tip  32  to the other end thereof. Thus, joining strength resistant to, for example, vibration of the internal combustion engine (not shown) to which the spark plug  10  is mounted, can be assured. As a result, peeling at the tip  32  or falling-off of the tip  32  can be inhibited against an external force such as thermal stress or vibration, thereby improving durability of the ground electrode  30 . 
     If the total L 1 +L 2 +L 3  of the distances L 1 , L 2 , and L 3  is less than 0.4 times the length L of the tip  32 , the less the total L 1 +L 2 +L 3  is, the lower the joining strength of the welded portions  80  tends to be. Meanwhile, if the total L 1 +L 2 +L 3  of the distances L 1 , L 2 , and L 3  is greater than 0.8 times the length L of the tip  32 , the greater the total L 1 +L 2 +L 3  is, the more easily peeling at the welded portions  80  due to thermal stress tends to occur. In the present embodiment, the total L 1 +L 2 +L 3  of the distances L 1 , L 2 , and L 3  is set to be 0.4 times to 0.8 times the length L of the tip  32 . Therefore, while peeling at the welded portions  80  due to thermal stress is inhibited, joining strength can be assured. 
     Next, a second embodiment will be described with reference to  FIG. 4  and  FIG. 5 . In the first embodiment, the ground electrode  30  having the tip  32  in which the protrusions  37  are arranged almost parallel to each other, is described. Meanwhile, in the second embodiment, a tip  90  having twilled protrusions  91  is used. The same components as described for the first embodiment will be denoted by the same reference numerals, and the description thereof is not given. 
       FIG. 4( a )  is a plan view of the tip  90  of a spark plug according to the second embodiment.  FIG. 4( b )  is a front view of the tip  90 .  FIG. 4( c )  is a bottom view of the tip  90 .  FIG. 4( d )  is a side view of the tip  90 . 
     The tip  90  is a member that is formed, in a rectangular parallelepiped, of a noble metal or an alloy containing a noble metal as a main component. The tip  90  has a plurality of protrusions  91  on the rectangular bottom surface  36  disposed opposite to the top surface  33 . In the present embodiment, the protrusions  91  are formed by twilled knurls which are obtained by grooves  92  being formed by rolling, cutting, or the like. The protrusions  91  can be easily formed by knurling. 
     A ground electrode  93  will be described with reference to  FIG. 5 .  FIG. 5( a )  is a plan view of the ground electrode  93 .  FIG. 5( b )  is a cross-sectional view of the ground electrode  93  taken along the line represented by arrows Vb-Vb shown in  FIG. 5( a )  (the longitudinal direction of the joining surface  38 ). In  FIG. 5( a )  and  FIG. 5( b ) , a portion of the electrode base material  31  in the longitudinal direction (the direction represented by the arrow L) is not shown. In  FIG. 5( b ) , a portion of the electrode base material  31  in the thickness direction is not shown. Instead of the tip  32  of the spark plug  10  described for the first embodiment, the tip  90  is joined to the electrode base material  31 . 
     As shown in  FIG. 5( a ) , the tip  90  is joined to the joining surface  38  (see  FIG. 5( b ) ) of the electrode base material  31  by resistance welding such that the longitudinal direction of the tip  90  is along the longitudinal direction (the direction represented by the arrow L) of the electrode base material  31 . As shown in  FIG. 5( b ) , the tip  90  is joined to the electrode base material  31  by welded portions  94 . The welded portions  94  are formed by the tip  90  and the electrode base material  31  being melted. The protrusions  91  protrude relative to the grooves  92 . Therefore, in a case where the welded portions  94  are formed by resistance welding, voids  95  are formed by the grooves  92  above the joining surface  38  between the protrusions  91  adjacent to each other. 
     In a case where the voids  95  are formed above the joining surface  38 , the welded portion is divided into n (n is an integer greater than or equal to 2) welded portions  94  on the joining surface  38 . In the present embodiment, the welded portion is divided into three welded portions  94 . A continuous distance L 1  to a continuous distance Ln of the welded portions  94  on the joining surface  38  are each set to be less than or equal to 0.5 mm, and the total of the distances L 1  to Ln is set to be 0.4 times to 0.8 times the length L from the end of the tip  90  to the other end thereof. Thus, the same action and effect as in the first embodiment can be obtained. 
     Further, the protrusions  91  (knurls) are uniformly arranged in the surface direction on the bottom surface  36  of the tip  90 . Therefore, the welded portions  94  can be uniformly arranged on the joining surface  38 . As a result, thermal stress generated in the welded portions  94  can be uniformly dispersed. Therefore, an effect of inhibiting, for example, peeling at the tip  90  can be enhanced. 
     Next, a third embodiment will be described with reference to  FIG. 6 . In the first embodiment and the second embodiment, the integral tips  32  and  90  are arranged on the electrode base materials  31  of the ground electrodes  30  and  93 , respectively. Meanwhile, in the third embodiment, a tip  101  is formed by a plurality of divisional tips  102 . The same components as described for the first embodiment will be denoted by the same reference numerals, and the description thereof is not given. 
       FIG. 6( a )  is a plan view of a ground electrode  100  of a spark plug according to the third embodiment.  FIG. 6( b )  is a cross-sectional view of the ground electrode  100  taken along the line represented by arrows VIb-VIb shown in  FIG. 6( a ) . In  FIG. 6( a ) , a portion of the electrode base material  31  in the longitudinal direction (the direction represented by the arrow L) is not shown. In  FIG. 6( b ) , portions of the electrode base material  31  in the longitudinal direction and the thickness direction are not shown. 
     As shown in  FIG. 6( a ) , in the ground electrode  100 , the tip  101  that includes the plurality of divisional tips  102  is disposed on the electrode base material  31 . In the present embodiment, the divisional tips  102  are each a spherical body that is formed of a noble metal or an alloy containing a noble metal as a main component and that has a radius of about 0.1 mm to about 0.3 mm. The plurality of divisional tips  102  are substantially tightly arranged on the joining surface  38  (see  FIG. 6( b ) ) of the electrode base material  31  such that the shape of the tip  101  is almost rectangular as a whole in the planar view, and the divisional tips  102  are not stacked and layered on each other. The divisional tips  102  are spherical bodies having no directivity. Therefore, in a case where a region of the tip  101  formed by the divisional tips  102  is regulated, the divisional tips  102  can be easily arranged on the electrode base material  31 . 
     The joining surface  38  is a projection surface (surface representing the outer shape of the tip  101 ) obtained by the tip  101  formed by arrangement of the divisional tips  102  being projected on the surface of the electrode base material  31 . In the present embodiment, the divisional tips  102  are joined to the electrode base material  31  by resistance welding. In a case where the divisional tips  102  oppose the joining surface  38 , a maximum spatial distance L 4  (maximum distance between the divisional tips  102  on the projection surface formed on the joining surface  38  in the case of the divisional tips  102  being projected on the joining surface  38 ) between the divisional tips  102  adjacent to each other, is set to be less than or equal to 0.3 mm. 
       FIG. 6( b )  is a cross-sectional view of the tip  101  and the electrode base material  31  taken along the longitudinal direction (direction in which the line represented by arrows VIb-VIb extends) of the joining surface  38 . The divisional tips  102  are joined to the electrode base material  31  (joining surface  38 ) by welded portions  103 . The welded portions  103  are formed by the divisional tips  102  and the electrode base material  31  being melted, and the welded portion  103  is formed for each divisional tip  102 . Voids  104  are formed between the divisional tips  102  adjacent to each other so as to divide a welded portion into the welded portions  103  on the joining surface  38 . The voids  104  are regions, formed by the divisional tips  102  contacting with each other, where the welded portions  103  cannot be formed. 
     The welded portion is divided into n (n is an integer greater than or equal to 2) welded portions  103  on the joining surface  38  by the voids  104  being formed above the joining surface  38 . A continuous distance L 1  to a continuous distance Ln (indicated as L 1 , L 2 , L 3  in  FIG. 6( b ) ) of the welded portions  103  on the joining surface  38  are each set to be less than or equal to 0.5 mm, and the total of the distances L 1  to Ln is set to be 0.4 times to 0.8 times the length L from the end of the tip  101  to the other end thereof. Thus, the same action and effect as in the first embodiment can be obtained. 
     The maximum spatial distance L 4  between the divisional tips  102  is set to be less than or equal to 0.3 mm. Therefore, spark discharge can be less likely to occur between the center electrode  50  and regions of the electrode base material  31  which are located between the divisional tips  102 . As a result, while spark wear of the electrode base material  31  is reduced, peeling at the tip or falling-off of the tip due to thermal stress, vibration, or the like can be less likely to occur, whereby durability of the ground electrode can be improved. 
     Next, a fourth embodiment will be described with reference to  FIG. 7 . In the first embodiment to the third embodiment, the tips  32 ,  90 , and  101  are joined to the electrode base materials  31  by resistance welding. Meanwhile, in the fourth embodiment, a tip  111  (divisional tips  112 ) is joined to the electrode base material  31  by laser beam welding. The same components as described for the first embodiment will be denoted by the same reference numerals, and the description thereof is not given. 
       FIG. 7( a )  is a plan view of a ground electrode  110  of a spark plug according to the fourth embodiment.  FIG. 7( b )  is a cross-sectional view of the ground electrode  110  taken along the line represented by arrows VIIb-VIIb shown in  FIG. 7( a ) . In  FIG. 7( a ) , a portion of the electrode base material  31  in the longitudinal direction (the direction represented by the arrow L) is not shown. In  FIG. 7( b ) , a portion of the electrode base material  31  in the thickness direction is not shown. Instead of the ground electrode  30  of the spark plug  10  described for the first embodiment, the ground electrode  110  is joined to the metal shell  20 . 
     As shown in  FIG. 7( a ) , in the ground electrode  110 , the tip  111  that includes a plurality of divisional tips  112  is disposed on the electrode base material  31 . In the present embodiment, the divisional tips  112  are each a member which is formed, into almost a quadrangular prism, of a noble metal or an alloy containing a noble metal as a main component. The plurality of divisional tips  112  are arranged on the joining surface  38  (see  FIG. 7( b ) ) of the electrode base material  31  such that the shape of the tip  111  is almost square as a whole in the planar view. 
     The joining surface  38  is a projection surface (surface representing the outer shape of the tip  111 ) obtained by the tip  111  formed by arrangement of the divisional tips  112  being projected on the surface of the electrode base material  31 . In the present embodiment, the divisional tips  112  are joined to the electrode base material  31  by laser beam welding. In a case where the divisional tips  112  oppose the joining surface  38 , a maximum spatial distance L 4  (maximum distance between the divisional tips  112  on the projection surface formed on the joining surface  38  in the case of the divisional tips  112  being projected on the joining surface  38 ) between the divisional tips  112  adjacent to each other, is set to be less than or equal to 0.3 mm. Thus, as in the third embodiment, spark wear can be reduced in the electrode base material  31  (region of L 4 ). 
       FIG. 7( b )  is a cross-sectional view of the tip  111  and the electrode base material  31  taken along the longitudinal direction (direction in which the line represented by arrows VIIb-VIIb extends) of the joining surface  38 . The joining surface  38  has an almost square shape. Therefore, the longitudinal direction of the joining surface  38  may be set to be the same as the longitudinal direction (the direction represented by the arrow L in  FIG. 7( a ) ) of the electrode base material  31 , or may be set to be the same as the transverse direction (the direction orthogonal to the direction represented by the arrow L) of the electrode base material  31 . In the present embodiment, the longitudinal direction of the joining surface  38  is set so as to be the same as the transverse direction of the electrode base material  31 . 
     In each divisional tip  112 , a tilt surface  113  by which an area of the bottom surface (surface that contacts with the joining surface  38 ) is reduced as compared to a cross-sectional area of the divisional tip  112 , is formed between the bottom surface and the side surfaces. The divisional tips  112  are joined to the electrode base material  31  by welded portions  114 . The welded portions  114  are formed by the electrode base material  31  and the divisional tips  112  being melted, and the welded portion  114  is formed for each divisional tip  112 . The welded portions  114  connect with a melt portion  115  formed by the electrode base material  31  being melted. The melt portion  115  is a portion, of the electrode base material  31 , which is melted by laser light applied from the rear surface side of the electrode base material  31 , and is formed on the rear surface side of the joining surface  38  of the electrode base material  31 . The tilt surfaces  113  of the divisional tips  112  are not joined to the electrode base material  31 . Therefore, the welded portions  114  are formed such that voids  116  are each formed between the divisional tips  112  adjacent to each other so as to connect with the joining surface  38 . 
     In  FIG. 7( b ) , for easy understanding, the welded portions  114  and the melt portion  115  are indicated so as to be distinguished from each other (hatching is different). However, in practice, the welded portions  114  and the melt portion  115  are continuous with each other. In the welded portions  114 , the concentration of the noble metal into which the divisional tips  112  are melted is higher than in the melt portion  115 . However, a boundary between the welded portions  114  and the melt portion  115  is not clearly defined. 
     The welded portion is divided into the three welded portions  114  on the joining surface  38  by the voids  116  being formed above the joining surface  38 . A continuous distance L 1  to a continuous distance L 3  of the welded portions  114  on the joining surface  38  are each set to be less than or equal to 0.5 mm, and the total of the distances L 1  to L 3  is set to be 0.4 times to 0.8 times the length L from the end of the tip  111  to the other end thereof. Thus, the same function effect as in the third embodiment can be obtained. 
     EXAMPLES 
     The present invention will be more specifically described according to examples. However, the present invention is not limited to the examples. 
     Experimental Examples 1 to 20 
     Samples according to experimental examples 1 to 20 were produced in a manner similar to that for the spark plug  10  described in the first embodiment. The samples were each a spark plug which had a screw portion of which the nominal diameter was M12. In the center electrode, a tip formed of iridium in a columnar shape having the diameter of 0.6 mm was joined to the end of the leg portion by resistance welding. 
     The tip of the ground electrode was formed of platinum in a rectangular parallelepiped. The tip had the width of 1 mm, the length of 1.5 mm, and the thickness of 0.4 mm. 19 kinds of tips were prepared such that grooves were formed at one to three portions on the bottom surface of the tip so as to extend in the tip width direction, and protrusions having the same length were formed parallel with each other so as to be separated by various grooves in the tips. 
     Each protrusion of the tip was pressed onto the electrode base material formed of INCONEL (registered trademark) 600, and the tip was joined to the electrode base material by resistance welding, thereby obtaining samples, of experimental examples 1 to 19, having various ground electrodes. In each sample, a 0.2 mm gap was formed between the joining surface of the electrode base material and the groove bottom of the tip. In addition to these tips, a tip having no grooves and no protrusions was prepared, and the bottom surface (the width of 1 mm, the length of 1.5 mm) of the tip was pressed onto the electrode base material to perform resistance welding, thereby obtaining a sample of experimental example 20. 
     The samples of experimental examples 1 to 20 were each mounted to a turbocharged engine (displacement of 1.5L). A test in which the engine revolution was set as the engine revolution for idling for 90 seconds and the engine revolution of 6000 rpm (full throttle) for 90 seconds, was regarded as one cycle of test, and the test was repeatedly performed for 1000 cycles. 
     After the tests, each sample was removed from the engine, and the longitudinal cross-sections of the tip and the electrode base material were observed, and the proportion (length of oxide scale/continuous distance of the welded portion on the joining surface) of the length of a peeled tip portion was measured and evaluated. As the length of the oxide scale, the length of the longest oxide scale among oxide scales in the observed cross-section was adopted. The evaluation is “excellent” when the proportion of the length of the peeled tip portion was less than 30%, is “good” when the proportion thereof was greater than or equal to 30% and less than 50%, is “slightly poor” when the proportion thereof was greater than or equal to 50% and less than 70%, and is “poor” when the proportion thereof was greater than or equal to 70%. 
     Table 1 indicates a list of: the continuous distance (mm) of the welded portion on the joining surface; the number (pieces) of the welded portions; the total length (mm) of voids above the joining surface; the number (pieces) of the voids; a ratio of the total of distances of the welded portions to the length of the tip (represented as “proportion of welded portion”); the length of oxide scale/continuous distance of the welded portion on the joining surface (represented as “proportion of scale (%)”); and evaluation. The continuous distance (each of L 1  to Ln) of the welded portion on the joining surface is determined according to the length of the protrusion, and the number and the total length of the voids are determined according to the number and the total length of grooves. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Welded portion 
                 Void 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 The 
                 Total 
                 The 
                 Proportion 
                 Proportion 
                   
               
               
                   
                 Distance 
                 number 
                 length 
                 number 
                 of welded 
                 of scale 
               
               
                   
                 (mm) 
                 (pieces) 
                 (mm) 
                 (pieces) 
                 portion 
                 (%) 
                 Evaluation 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 0.2 
                 2 
                 1.1 
                 1 
                 0.27 
                 60 
                 Slightly 
               
               
                 example 1 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.3 
                 2 
                 0.9 
                 1 
                 0.40 
                 45 
                 Good 
               
               
                 example 2 
               
               
                 Experimental 
                 0.4 
                 2 
                 0.7 
                 1 
                 0.53 
                 40 
                 Good 
               
               
                 example 3 
               
               
                 Experimental 
                 0.5 
                 2 
                 0.5 
                 1 
                 0.67 
                 38 
                 Good 
               
               
                 example 4 
               
               
                 Experimental 
                 0.6 
                 2 
                 0.3 
                 1 
                 0.80 
                 65 
                 Slightly 
               
               
                 example 5 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.7 
                 2 
                 0.1 
                 1 
                 0.93 
                 68 
                 Slightly 
               
               
                 example 6 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.1 
                 3 
                 1.2 
                 1 
                 0.20 
                 65 
                 Slightly 
               
               
                 example 7 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.15 
                 3 
                 1.05 
                 2 
                 0.30 
                 62 
                 Slightly 
               
               
                 example 8 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.2 
                 3 
                 0.9 
                 2 
                 0.40 
                 38 
                 Good 
               
               
                 example 9 
               
               
                 Experimental 
                 0.3 
                 3 
                 0.6 
                 2 
                 0.60 
                 35 
                 Good 
               
               
                 example 10 
               
               
                 Experimental 
                 0.4 
                 3 
                 0.3 
                 2 
                 0.80 
                 38 
                 Good 
               
               
                 example 11 
               
               
                 Experimental 
                 0.45 
                 3 
                 0.15 
                 2 
                 0.90 
                 40 
                 Good 
               
               
                 example 12 
               
               
                 Experimental 
                 0.05 
                 4 
                 1.3 
                 3 
                 0.13 
                 60 
                 Slightly 
               
               
                 example 13 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.1 
                 4 
                 1.1 
                 3 
                 0.27 
                 65 
                 Slightly 
               
               
                 example 14 
                   
                   
                   
                   
                   
                   
                 poor 
               
               
                 Experimental 
                 0.15 
                 4 
                 0.9 
                 3 
                 0.40 
                 35 
                 Good 
               
               
                 example 15 
               
               
                 Experimental 
                 0.2 
                 4 
                 0.7 
                 3 
                 0.53 
                 32 
                 Good 
               
               
                 example 16 
               
               
                 Experimental 
                 0.25 
                 4 
                 0.5 
                 3 
                 0.67 
                 33 
                 Good 
               
               
                 example 17 
               
               
                 Experimental 
                 0.3 
                 4 
                 0.3 
                 3 
                 0.80 
                 40 
                 Good 
               
               
                 example 18 
               
               
                 Experimental 
                 0.35 
                 4 
                 0.1 
                 3 
                 0.93 
                 42 
                 Good 
               
               
                 example 19 
               
               
                 Experimental 
                 1.5 
                 1 
                 0 
                 0 
                 1 
                 70 
                 Poor 
               
               
                 example 20 
               
               
                   
               
            
           
         
       
     
     According to Table 1, it has been confirmed that, in experimental examples 1 to 19 in which the voids were formed at the welded portions, peeling at the tip was less likely to occur as compared to experimental example 20 in which the entirety of the bottom surface of the tip was welded (no voids were formed at the welded portions). In particular, it has been confirmed that, in any of experimental examples 2 to 4, 9 to 11, and 15 to 18 in which the distance of the welded portion was less than or equal to 0.5 mm and the total of the distances of the welded portions was 0.4 times to 0.8 times the length of the tip, the evaluation is “good”. In these experimental examples, the peeling at the tip was able to be inhibited, and it is thus clear that durability of the ground electrode can be improved. 
     Experimental Examples 21 to 26 
     The tip according to the tip of experimental example 16 was cut at positions of the grooves in the width direction, and divided into four divisional tips having the same size such that each divisional tip had the width of 1 mm, the length of 0.35 mm, and the thickness of 0.4 mm. Each divisional tip had a protrusion having the width of 1 mm and the length of 0.2 mm. 
     The four divisional tips were arranged on the electrode base material formed of INCONEL (registered trademark) 600 such that the protrusions were parallel to each other, and the protrusions were pressed onto the electrode base material, and the divisional tips were joined to the electrode base material by resistance welding. When the divisional tips were arranged on the electrode base material, the maximum spatial distance (gap) between the divisional tips adjacent to each other was made different, to obtain samples according to experimental examples 21 to 26. In each sample, a 0.2 mm gap was formed between the joining surface of the electrode base material and the bottom surface of the divisional tip. 
     To the engine used in the test for experimental examples 1 to 20, the samples of experimental examples 21 to 26 were mounted, and the same test was performed for 1000 cycles. After the tests, each sample was removed from the engine, and the electrode base material (joining surface) at the gap between the divisional tips was observed, and the electrode base material was checked for a discharge mark caused by spark discharge. Subsequently, the cross-sections of the four divisional tips and the electrode base material were observed, and the proportion (length of oxide scale/continuous distance of the welded portion on the joining surface) of the length of the peeled divisional-tip portion was measured. 
     As the length of the oxide scale, the length of the longest oxide scale among oxide scales in the observed cross-section was adopted. The evaluation is “poor” when the discharge mark was found even if the oxide scale satisfied the standard. Table 2 indicates a list of: the continuous distance (mm) of the welded portion on the joining surface; the number (pieces) of the welded portions; the maximum spatial distance between the divisional tips (represented as “spatial distance (mm)”); the length of oxide scale/continuous distance of the welded portion on the joining surface (represented as “proportion of scale (%)”); presence or absence of a discharge mark at the electrode base material; and evaluation. For comparison, the result of experimental example 16 is also indicated in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Presence or 
                   
               
               
                   
                 Welded portion 
                 The number 
                 Spatial 
                 Proportion 
                 absence of 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Distance 
                 The number 
                 of voids 
                 distance 
                 of scale 
                 discharge 
                   
               
               
                   
                 (mm) 
                 (pieces) 
                 (pieces) 
                 (mm) 
                 (%) 
                 mark 
                 Evaluation 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 &lt;0.1 
                 32 
                 absent 
                 Good 
               
               
                 example 21 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 0.1 
                 25 
                 absent 
                 Excellent 
               
               
                 example 22 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 0.2 
                 25 
                 absent 
                 Excellent 
               
               
                 example 23 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 0.3 
                 24 
                 absent 
                 Excellent 
               
               
                 example 24 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 0.4 
                 21 
                 present 
                 Poor 
               
               
                 example 25 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 0.5 
                 20 
                 present 
                 Poor 
               
               
                 example 26 
               
               
                 Experimental 
                 0.2 
                 4 
                 3 
                 — 
                 32 
                 — 
                 Good 
               
               
                 example 16 
               
               
                   
               
            
           
         
       
     
     According to Table 2, in experimental examples 22 to 24 in which the maximum spatial distance between the divisional tips was 0.1 mm to 0.3 mm, no discharge mark was found in the electrode base material. Further, it has been confirmed that, in experimental examples 22 to 24, peeling at the tip was less likely to occur as compared to experimental example 16. It is assumed that, by the tip being divided, thermal stress can be further reduced. It is assumed that, in experimental examples 22 to 24, peeling at the tip can be reduced, and spark wear of the electrode base material can be also reduced. Therefore, it is clear that durability of the ground electrode can be improved. 
     According to these examples, it has been confirmed that, even if the tip (including the tip in which the total of the lengths of a plurality of arranged divisional tips is greater than or equal to 1.5 mm) has the length which is greater than or equal to 1.5 mm, when a plurality of welded portions each having a continuous distance which is less than or equal to 0.5 mm are provided, peeling at the tip can be less likely to occur. 
     As described above, although the present invention has been described based on the embodiments, the present invention is not limited to the above embodiments at all. It can be easily understood that various modifications can be devised without departing from the gist of the present invention. 
     For easy understanding, in the above embodiments, the voids  81 ,  95 ,  104 ,  116  at the joining surface  38  are formed without bringing the tips  32 ,  90 ,  101 ,  111  and the electrode base material  31  into contact with each other. However, the present invention is not necessarily limited thereto. The voids function to reduce thermal stress unless the tip and the electrode base material are joined to each other. As a matter of course, the voids may be formed also by the tip and the electrode base material contacting with each other (for example, a distance between: the joining surface  38 ; and the bottom surface  36  or the groove  92  is almost zero). 
     In the first embodiment and the second embodiment, the tips  32 ,  90  have the protrusions  37 ,  91  to form the voids  81 ,  95  at the joining surface  38 . However, the present invention is not necessarily limited thereto. As a matter of course, in a case where the tip is joined to the electrode base material  31  by laser beam welding, for example, the joining surface  38  is scanned with laser light from the rear surface side of the electrode base material  31  while energy density is varied, whereby the welded portions and the voids can be formed on the bottom surface of the tip without forming the protrusions  37 ,  91 . 
     In the second embodiment, the tip  90  having twilled knurls is described. However, the present invention is not necessarily limited thereto. As a matter of course, straight knurls or diagonal knurls may be formed on the tip. Further, protrusions may not be regularly formed by knurling. As a matter of course, protrusions may be irregularly formed by cutting. 
     In the third embodiment and the fourth embodiment, the divisional tips  102  having the same size and the divisional tips  112  having the same size are used. However, the present invention is not limited thereto. The sizes of the divisional tips can be determined as appropriate. 
     In each of the above embodiments, the tips  32 ,  90 ,  101 ,  111  are arranged so as to form almost a rectangular shape or almost a square shape in the planar view (when opposing the joining surface  38 ). However, the present invention is not necessarily limited thereto. The shape of the tip can be set, as appropriate, so as to be circular, ellipsoidal, oblong, or the like in the planar view. In a case where the shape of the tip is, for example, ellipsoidal or oblong in the planar view, the cross-section, in the longitudinal direction, of the joining surface  38  means the cross-section, in the major axis direction, of the ellipsoidal or oblong shape. In a case where the shape of the tip is circular in the planar view, the cross-section, in the longitudinal direction, of the joining surface  38  means the cross-section that passes through the center of the circle. 
     In each of the above embodiments, the embodiment may be modified by a part or plural parts of the structure of another embodiment being added to the embodiment or a part or plural parts of the structure being exchanged between the embodiment and another embodiment. 
     For example, in the first embodiment to the third embodiment, the welded portions  80 ,  94 ,  103  are formed by resistance welding. However, as a matter of course, as in the fourth embodiment, the welded portions may be formed by laser beam welding being performed from the rear surface side of the electrode base material  31  toward the tips  32 ,  90 , or the divisional tips  112 . Further, the welded portions may be formed by laser beam welding being performed from the tips  32 ,  90  side or the divisional tips  112  side toward the electrode base material  31 . Similarly, as a matter of course, the divisional tips  112  of the fourth embodiment may be joined to the electrode base material  31  by resistance welding. 
     DESCRIPTION OF REFERENCE NUMERALS
       10 : spark plug     30 ,  93 ,  100 ,  110 : ground electrode     31 : electrode base material     32 ,  90 ,  101 ,  111 : tips     38 : joining surface     50 : center electrode     80 ,  94 ,  103 ,  114 : welded portions     81 ,  95 ,  104 ,  116 : voids     102 ,  112 : divisional tips