Patent Publication Number: US-9837799-B2

Title: Spark plug

Description:
RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP15/02984 filed Jun. 15, 2015, which claims the benefit of Japanese Patent Application No. 2014-133806, filed Jun. 30, 2014. 
     FIELD OF THE INVENTION 
     The present invention relates to spark plugs. 
     BACKGROUND OF THE INVENTION 
     As a conventional spark plug, a spark plug has been known which includes an electrode (a center electrode or a ground electrode) to which an electrode tip (hereinafter referred to as “precious metal tip”) formed of a precious metal or an alloy containing a precious metal as a main component is joined (see Japanese Patent Application Laid-Open (kokai) No. 2013-178912, for example). Generally, a precious metal tip is joined to an electrode base material by laser welding. Specifically, the precious metal tip is irradiated with a laser beam along its outer periphery, whereby the precious metal tip is joined to the electrode base material. When the precious metal tip is welded to the electrode base material, usually, a melt portion in which the material of the precious metal tip and the material of the electrode base material are melted is formed between the precious metal tip and the electrode base material. 
     As described above, in the spark plug including the precious metal tip, at an interface (hereinafter also referred to as a melt portion interface) between the melt portion and the precious metal tip, an oxide film (hereinafter also referred to as an oxide scale) may be formed on the surface of the melt portion. The oxide scale is formed, at the melt portion interface, so as to gradually grow from an outer peripheral portion near the outside air toward the inside of the melt portion interface. 
     When the spark plug is used, heating and cooling cycles are repeated, whereby a stress is caused by a difference in thermal expansion coefficient between the precious metal tip and the electrode base material, near a joint portion of the precious metal tip and the electrode base material. Generally, an oxide scale is lower in strength (is more fragile) than the melt portion or the precious metal tip. Therefore, when a stress occurs as described above, crack is likely to occur in the oxide scale having the relatively low strength. When crack occurs in the oxide scale and the air enters the crack, oxidation of the melt portion interface progresses, and the oxide scale further grows toward the inside of the melt portion interface. The growth of the oxide scale toward the inside of the melt portion interface causes the crack to extend toward the inside of the melt portion interface, leading to falling off of the precious metal tip, which makes it difficult to secure reliability of the joint between the precious metal tip and the electrode base material. 
     Conventionally proposed measures to improve the reliability of the joint between the precious metal tip and the electrode base material are: forming the melt portion to be thicker; and adjusting the shape of the melt portion to suppress the stress that occurs between the precious metal tip and the electrode base material (refer to Patent Document 1, for example). Since the melt portion has an intermediate composition between the precious metal tip and the electrode base material, a difference in thermal expansion coefficient between the precious metal tip and the melt portion is smaller than the difference in thermal expansion coefficient between the precious metal tip and the electrode base material. Therefore, for example, by increasing the thickness of the melt portion, a stress that occurs near the interface between the precious metal tip and the melt portion can be suppressed, and crack is suppressed from occurring in the oxide scale due to the stress. 
     However, measures to suppress growth of the oxide scale at the interface between the precious metal tip and the electrode base material have not been sufficiently investigated. Therefore, it has been desired to suppress growth of the oxide scale and improve the reliability of the joint between the precious metal tip and the electrode base material. 
     SUMMARY OF THE INVENTION 
     The present invention is made to address, at least partially, the above problem, and can be embodied in the following modes. 
     (1) According to one aspect of the present invention, a spark plug is provided which includes an electrode obtained by welding a cylindrical precious metal tip which contains a precious metal and allows discharge at an end surface on one end side with respect to a center axis thereof, to an electrode base material disposed on the other end side, in a direction of the center axis, with respect to the precious metal tip. The electrode has a melt portion in which the precious metal tip and the electrode base material are melted, between the other end of the precious metal tip and the electrode base material. The melt portion of the spark plug includes a melt sag over an entire circumference on a side surface of the precious metal tip. Further, in this spark plug, in an arbitrary cross section, including the center axis, of the electrode: a length of a line S corresponding to the end surface on the one end side of the precious metal tip is D; two straight lines apart from the center axis by a distance of “9D/20” are virtual straight lines L 1 , L 2 , respectively; an intersection point of each virtual straight line L 1 , L 2  and an interface between the precious metal tip and the melt portion is an intersection point P 1 , P 2 , respectively; a straight line connecting the intersection points P 1  and P 2  is a virtual straight line L 3 ; of both end points of the line S, the end point located on the same side as the virtual straight line L 1  with respect to the center axis is an end point P 3 , and the end point located on the same side as the virtual straight line L 2  with respect to the center axis is an end point P 4 ; a straight line passing each end point P 3 , P 4  and parallel to the center axis is a virtual straight line L 4 , L 5 , respectively; of end points of the melt sag at the one end side on the virtual straight lines L 4  and L 5 , the end point on the virtual straight line L 4  is an end point P 5 , and the end point on the virtual straight line L 5  is an end point P 6 ; an intersection point of each virtual straight line L 4 , L 5  and the virtual straight line L 3  is an intersection point P 7 , P 8 , respectively; and each of a distance X 1  between the intersection point P 7  and the end point P 5  and a distance X 2  between the intersection point P 8  and the end point P 6  is 0.092 mm or more. According to the spark plug of this mode, since the melt sag having a predetermined shape is formed over the entire circumference on the side surface of the precious metal tip, entry of the air into the interface between the precious metal tip and the melt portion can be suppressed, thereby suppressing formation of an oxide scale at the interface between the precious metal tip and the melt portion. As a result, when the heating and cooling cycles are repeated in the spark plug, it is possible to suppress occurrence of crack due to a difference in thermal expansion coefficient between the precious metal tip and the electrode base material at the interface between the precious metal tip and the melt portion, whereby reliability of the joint between the precious metal tip and the electrode base material can be improved. 
     (2) In accordance with a second aspect of the present invention, there is provided a spark plug, as described above, wherein each of the distances X 1  and X 2  may be 0.110 mm or more. According to the spark plug of this mode, growth of the oxide scale at the interface between the precious metal tip and the melt portion is more suppressed, whereby reliability of the joint between the precious metal tip and the electrode base material can be further improved. 
     The present invention can be embodied in various forms other than the spark plug. For example, the present invention can be embodied in forms of an internal combustion engine on which the spark plug is mounted, a vehicle including the internal combustion engine, and the like. Further, the present invention can be embodied in the form of a method for manufacturing the spark plug. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cross-sectional view of a spark plug. 
         FIGS. 2(A) and 2(B)  are enlarged explanatory views showing the structure of a front end portion of a center electrode. 
         FIG. 3  is a cross-sectional view for explaining the specific shape of a melt sag. 
         FIG. 4  is an explanatory view showing specs of electrodes subjected to a thermal test. 
         FIG. 5  is an explanatory view having a horizontal axis indicating the length of the melt sag, and a vertical axis indicating the oxide scale formation ratio. 
         FIG. 6  is an explanatory view having a horizontal axis indicating the length of the melt sag, and a vertical axis indicating the oxide scale formation ratio. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. Schematic Structure of Spark Plug: 
       FIG. 1  is a partially cross-sectional view of a spark plug  100  as an embodiment of the present invention. The spark plug  100  has an elongated shape extending along an axis Ax (a center axis of the spark plug  100 ). In  FIG. 1 , the right side of the axis Ax indicated by a dot-dash line shows an exterior front view, and the left side of the axis Ax shows a sectional view where the spark plug  100  is sectioned by a plane that passes the axis Ax. In the following description, in a direction parallel to the axis Ax, the lower side in  FIG. 1  (indicated by an arrow X in  FIG. 1 ) is referred to as the front side, and the upper side in  FIG. 1  is referred to as the rear side. 
     The spark plug  100  includes a ceramic insulator  10 , a center electrode  20 , a ground electrode  30 , a metal terminal  40 , and a metallic shell  50 . The rod-shaped center electrode  20  projecting from a front end of the ceramic insulator  10  extends through the interior of the ceramic insulator  10  and is electrically connected to the metal terminal  40  provided at a rear end of the ceramic insulator  10 . The outer periphery of the center electrode  20  is held by the ceramic insulator  10 , and the outer periphery of the ceramic insulator  10  is held by the metallic shell  50  at a position apart from the metal terminal  40 . 
     The ground electrode  30  electrically connected to the metallic shell  50  forms a spark gap which is a gap for generating a spark, between the ground electrode  30  and the front end of the center electrode  20 . The spark plug  100  is attached, through the metallic shell  50 , to a threaded attachment hole  201  provided in an engine head  200  of an internal combustion engine. When a high voltage of 20,000 to 30,000 V is applied to the metal terminal  40 , a spark is generated at the spark gap formed between the center electrode  20  and the ground electrode  30 . 
     The ceramic insulator  10  is an insulator formed through firing of a ceramic material such as alumina. The ceramic insulator  10  is a cylindrical member having, in the center thereof, an axial hole  12  in which the center electrode  20  and the metal terminal  40  are accommodated. The ceramic insulator  10  has a central trunk portion  19  formed at the center thereof in the axial direction and having an increased outer diameter. A rear trunk portion  18  for insulation between the metal terminal  40  and the metallic shell  50  is formed on the metal terminal  40  side relative to the central trunk portion  19 . A front trunk portion  17  having an outer diameter smaller than that of the rear trunk portion  18  is formed on the center electrode  20  side relative to the central trunk portion  19 . A leg portion  13  having an outer diameter which is smaller than that of the front trunk portion  17  and decreases toward the front side is formed frontward of the front trunk portion  17 . 
     The metallic shell  50  is a cylindrical metallic member that surrounds a portion of the ceramic insulator  10  extending from a part of the rear trunk portion  18  to the leg portion  13  to hold the ceramic insulator  10 . In the present embodiment, the metallic shell  50  is formed of low carbon steel, and the entirety of the metallic shell  50  is subjected to plating such as nickel plating or zinc plating. The metallic shell  50  has a tool engagement portion  51 , a threaded attachment portion  52 , and a gasket receiving portion  54 . 
     A tool (not shown) used for fixing the spark plug  100  to an engine head  200  is engaged with the tool engagement portion  51  of the metallic shell  50 . The threaded attachment portion  52  of the metallic shell  50  has a screw thread to be screwed into the threaded attachment hole  201  of the engine head  200 . The gasket receiving portion  54  of the metallic shell  50  projects radially outward relative to the threaded attachment portion  52  and is formed in a flange shape, on the rear side of the threaded attachment portion  52 . 
     In addition, a gasket  5  which is a substantially annular-shaped solid member is fitted to the metallic shell  50  so as to be in contact with a front-side end portion of the gasket receiving portion  54 . The gasket  5  secures sufficient seal between the gasket receiving portion  54  of the spark plug  100  and the engine head  200 . A front end surface  57  of the metallic shell  50  is formed in a circular shape having an opening in a center portion thereof. At the center portion, the center electrode  20  projects from the leg portion  13  of the ceramic insulator  10 . 
     A thin crimp portion  53  is provided on the rear side of the metallic shell  50  with respect to the tool engagement portion  51 . In addition, a compressive deformation portion  58 , as thin as the crimp portion  53 , is provided between the gasket receiving portion  54  and the tool engagement portion  51 . Annular ring members  6  and  7  are interposed between an inner peripheral surface of the metallic shell  50  from the tool engagement portion  51  to the crimp portion  53 , and an outer peripheral surface of the rear trunk portion  18  of the ceramic insulator  10 . Further, powder of talc  9  is charged between the ring members  6  and  7 . 
     When the spark plug  100  is manufactured, crimping is performed in which the crimp portion  53  is bent inward and pressed frontward, whereby the compressive deformation portion  58  is compressed and deformed. As a result of the crimping, the ceramic insulator  10  is pressed frontward in the metallic shell  50  through the ring members  6  and  7  and the talc  9 . As a result of this pressing, the talc  9  is compressed in the direction of the axis Ax, whereby the airtightness of the metallic shell  50  is improved. 
     At the inner periphery of the metallic shell  50 , a ceramic step portion  15  located at a base end of the leg portion  13  of the ceramic insulator  10  is pressed, through a ring-shaped sheet packing  8 , against a metal-shell internal step portion  56  formed at the position of the threaded attachment portion  52 . The sheet packing  8  is a member for maintaining airtightness between the metallic shell  50  and the ceramic insulator  10 , and prevents combustion gas from flowing out. 
     The center electrode  20  includes an electrode base material  25  which is a rod-shaped member extending in the direction of the axis Ax. The electrode base material  25  is formed of a nickel alloy containing nickel as a main component. In the present embodiment, the electrode base material  25  has, therein, a core member formed of a material having higher thermal conductivity than the electrode base material  25 , such as copper or an alloy containing copper as a main component. The center electrode  20  according to the present embodiment further includes, at a front end of the electrode base material  25 , a precious metal tip for improving resistance to spark-induced erosion and resistance to oxidation-induced erosion. The structure of the front end portion of the center electrode  20  will be described later in detail. The center electrode  20  is inserted in the axial hole  12  of the ceramic insulator  10 , with the front end of the electrode base material  25  projecting from the axial hole  12  of the ceramic insulator  10 , and is electrically connected to the metal terminal  40  through a ceramic resistor  3  and a seal body  4 . 
     The ground electrode  30  is a rod-shaped member, and has a base end welded to the front end surface  57  of the metallic shell  50 . The front side of the ground electrode  30  is bent in a direction intersecting the axis Ax, and the front end portion of the ground electrode  30  faces the front end surface of the center electrode  20  on the axis Ax. A precious metal tip similar to the center electrode  20  may be provided at a position opposed to the center electrode  20  at the front end portion of the ground electrode  30 . 
     B. Structure of Precious Metal Tip and its Vicinity: 
       FIGS. 2(A) and 2(B)  are enlarged explanatory views showing the structure of the front end portion of the center electrode  20 .  FIG. 2(A)  is a side view showing the appearance of the front end portion of the center electrode  20 , and  FIG. 2(B)  is a cross-sectional view showing a cross section including a center axis O of a precious metal tip  27  included in the center electrode  20 . In  FIG. 2(A) , the above-described front side of the spark plug  100  is indicated by an arrow X. In the spark plug  100  according to the present embodiment, the center axis O of the precious metal tip  27  coincides with the axis Ax as the center axis of the spark plug  100 . 
     The precious metal tip  27  is a cylindrical member formed of a precious metal (e.g., platinum, iridium, ruthenium, rhodium, or the like) or an alloy containing not less than 50 wt % of a precious metal as a main component, and is joined to the front end surface of the electrode base material  25  by laser welding. Therefore, a melt portion  26  in which the electrode base material  25  and the precious metal tip  27  are melted is formed between the front end surface of the electrode base material  25  and the precious metal tip  27 . In the present embodiment, the melt portion  26  is formed so as to cover the entire front end surface of the electrode base material  25 . 
     In the precious metal tip  27  welded to the electrode base material  25 , the diameter of the cross section perpendicular to the center axis O (the diameter of the end surface on the front side) can be, for example, 0.3 mm or more, and preferably 0.4 mm or more. In addition, the diameter of the cross section of the precious metal tip  27  can be, for example, 1.5 mm or less, and preferably 1.2 mm or less. In addition, in the electrode base material  25 , the front end surface to which the precious metal tip  27  is joined may have a size enough to be in contact with the entire rear end surface of the precious metal tip  27 . For example, when the electrode base material  25  is a cylindrical member, the diameter of the front end surface of the electrode base material  25  may be about 0.2 to 0.4 mm larger than the diameter of the rear end surface of the precious metal tip  27 , in terms of facilitating welding. 
     Further, in the present embodiment, a melt sag  28  is formed in the melt portion  26 . When the electrode base material  25  and the precious metal tip  27  are melted and thereby the melt portion  26  is formed, the melt sag  28  is formed as a portion of a melt forming the melt portion  26 . That is, the melt sag  28  is formed such that a portion of the melt extends frontward along the side surface of the precious metal tip  27 , from near the interface between the electrode base material  25  and the precious metal tip  27 , when the electrode base material  25  and the precious metal tip  27  are welded (refer to  FIG. 2(B) ). In the present embodiment, the melt sag  28  is formed over the entire circumference on the side surface of the precious metal tip  27 . 
     Welding of the electrode base material  25  and the precious metal tip  27  may be performed by bringing the front end surface of the electrode base material  25  into contact with the rear end surface of the precious metal tip  27 , and irradiating an area including the contact portions thereof, with a laser beam, from the outer peripheral side of the precious metal tip  27  toward the inside thereof. In the present embodiment, the irradiation with the laser beam is performed from the outer peripheral side of the precious metal tip  27  toward the center axis O of the precious metal tip  27 . It is desirable that the irradiation with the laser beam is uniformly performed over the entire circumference of the precious metal tip  27 . 
     Welding of the electrode base material  25  and the precious metal tip  27  may employ various devices capable of emitting laser beams, such as a YAG laser, a carbon dioxide gas laser, a semiconductor laser, and a fiber laser. The employed laser may be a pulse wave (PW) oscillation laser or a continuous wave (CW) oscillation laser. In welding, in order to form the melt sag  28  having a desired shape described later, for example, it is desirable that the amount of energy per irradiation width is increased in a profile of the laser beam. In order to increase the amount of energy per irradiation width in the profile of the laser beam, for example, a lens or an oscillator in the laser irradiation device may be optimized, and a condition selected from the laser output and the laser irradiation time may be adjusted. In particular, it is desirable to use a fiber laser in terms of increasing the amount of energy per irradiation width. 
       FIG. 3  is a cross-sectional view for explaining a specific shape of the melt sag  28 .  FIG. 3  shows a cross section, including the center axis O, of the front end portion of the center electrode  20 . In  FIG. 3 , a line corresponding to a front-side end surface of the precious metal tip  27  is indicated as a line S, and the line S has a length D. The melt sag  28  formed in the center electrode  20  according to the present embodiment has a shape as follows. 
     In the cross section shown in  FIG. 3 , two straight lines apart from the center axis O by a distance of “9D/20” are referred to as virtual straight lines L 1  and L 2 , respectively. An intersection of the virtual straight line L 1 , L 2  and the interface between the precious metal tip  27  and the melt portion  26  is referred to as an intersection point P 1 , P 2 , respectively. A straight line connecting the intersection points P 1  and P 2  is referred to as a straight line L 3 . 
     Of both end points of the line S, the end point positioned on the same side as the virtual straight line L 1  with respect to the center axis O is referred to as an end point P 3 , and the end point positioned on the same side as the virtual straight line L 2  with respect to the center axis O is referred to as an end point P 4 . A straight line which passes the end point P 3 , P 4  and is parallel to the center axis O is referred to as a virtual straight line L 4 , L 5 , respectively. Of end points at the front side of the melt sag  28  on the virtual straight lines L 4  and L 5 , the end point on the virtual straight line L 4  is referred to as an end point P 5 , and the end point on the virtual straight line L 5  is referred to as an end point P 6 . An intersection point of the virtual straight line L 4 , L 5  and the virtual straight line L 3  is referred to as an intersection point P 7 , P 8 , respectively. At this time, a distance X 1  between the intersection point P 7  and the end point P 5  and a distance X 2  between the intersection point P 8  and the end point P 6  are each 0.092 mm or more. 
     Further, in the cross section shown in  FIG. 3 , at the interface between the precious metal tip  27  and the melt portion  26 , a rear end of a portion overlapping the virtual straight line L 4  is referred to as a point P 9 , and a rear end of a portion overlapping the virtual straight line L 5  is referred to as a point P 10 . Regarding an area where the precious metal tip  27  and the melt portion  26  are in contact with each other, an area between the points P 5  and P 9  and an area between the points P 6  and P 10  each are an area where the surface of the precious metal tip  27  is not substantially melted. In contrast, in the area where the precious metal tip  27  and the melt portion  26  are in contact with each other, an area between the points P 9  and P 10  is an area where the surface of the precious metal tip  27  is melted. Therefore, in the specification of the present application, in the area where the precious metal tip  27  and the melt portion  26  are in contact with each other, the area between the points P 9  and P 10  is also referred to as an “interface between the precious metal tip  27  and the melt portion  26 ”. The interface indicated by P 9 -P 10  where the surface of the precious metal tip  27  is melted is an area which significantly contributes to the joint strength between the precious metal tip  27  and the electrode base material  25 . 
     In the spark plug  100  according to the present embodiment, the shape of the melt sag  28  described with reference to  FIG. 3  is obtained at any cross section, including the center axis O, of the front end portion of the center electrode  20 . 
     According to the spark plug  100  of the present embodiment configured as described above, the melt sag  28 , which is a part of the melt portion  26  extending frontward, is formed on the side surface of the precious metal tip  27  disposed at the front end portion of the center electrode  20 . Therefore, entry of the air into the interface between the precious metal tip  27  and the melt portion  26  can be suppressed, thereby suppressing formation of the oxide scale that is lower in strength than the precious metal tip  27  and the melt portion  26 , at the interface between the precious metal tip  27  and the melt portion  26 . As a result, when the heating and cooling cycles are repeated in the spark plug  100 , it is possible to suppress occurrence of crack due to a difference in thermal expansion coefficient between the precious metal tip  27  and the electrode base material  25  at the interface between the precious metal tip  27  and the melt portion  26 . By suppressing occurrence of crack, entry of the air into the interface between the precious metal tip  27  and the melt portion  26  is suppressed, whereby further growth of the oxide scale can be suppressed. By suppressing extension of crack in this way, falling off of the precious metal tip  27  is suppressed, whereby reliability of the joint between the precious metal tip  27  and the electrode base material  25  can be improved. 
     That is, it is conceivable that the melt sag  28  has a function as a seal portion which suppresses entry of the air into the interface between the precious metal tip  27  and the melt portion  26 . Therefore, by forming the melt sag  28  to be long along the center axis O, the effect of suppressing growth of the oxide scale and extension of crack at the interface between the precious metal tip  27  and the melt portion  26  can be improved. 
     In particular, in the present embodiment, the condition that the length (corresponding to the distance X 1 , X 2  in  FIG. 3 ) of the melt sag  28  from a predetermined reference position corresponding to the virtual straight line L 3  shown in  FIG. 3  along the direction of the center axis O is 0.092 mm or more, is satisfied over the entire circumference on the side surface of the precious metal tip  27 . Therefore, growth of the oxide scale can be effectively suppressed over the entire interface between the precious metal tip  27  and the melt portion  26 . 
     As described above, according to the present embodiment, the melt sag, which has been considered to be undesirable in terms of appearance, is intentionally formed to a predetermined length or more, whereby reliability of the joint of the precious metal tip  27  is improved, resulting in increase in durability of the spark plug  100 . 
     The larger the length of the melt sag  28  in the direction of the center axis O is, the more the effect of suppressing growth of the oxide scale at the interface between the precious metal tip  27  and the melt portion  26  can be improved. It is particularly desirable that the length of the melt sag  28  (the distance X 1 , X 2  in  FIG. 3 ) is 0.110 mm or more. With such a configuration, even when the temperature at which the precious metal tip  27  is exposed is high in the heating and cooling cycles, reliability of the joint between the precious metal tip and the electrode base material can be secured. However, the upper limit of the length of the melt sag  28  (the distance X 1 , X 2  in  FIG. 3 ) is preferably equal to the distance between the virtual straight line L 3  and the point P 3 , P 4  which is the end point of the line S corresponding to the front-side end surface of the precious metal tip  27 . In other words, it is desirable that the melt sag  28  is not present on the front-side end surface of the precious metal tip  27 . The cause of this is to suppress the melt sag  28  from adversely affecting ignitability in the spark plug  100 . 
     The above-described length of the melt sag  28  is the length for securing the function thereof as the seal portion that suppresses entry of the air into the interface between the precious metal tip  27  and the melt portion  26 . Therefore, the effect achieved by setting the length of the melt sag  28  in the direction of the center axis O to the above-described value is the effect achieved regardless of the size of the precious metal tip  27  and the material of the precious metal tip  27 . 
     In the present embodiment, the virtual straight line L 3  is a position to be a reference for specifying the length, in the direction of the center axis O, of the melt sag  28  formed on the side surface of the precious metal tip  27 . The virtual straight line L 3  is specified as follows. 
     As already described, the effect obtained by providing the melt sag  28  is achieved when the melt sag  28  covers the side surface of the precious metal tip  27  to suppress entry of the air into the interface between the precious metal tip  27  and the melt portion  26 . Therefore, it is considered that the reference that defines the length of the melt sag  28  should be determined on the basis of the position of the interface between the precious metal tip  27  and the melt portion  26  at the rear-side end portion of the precious metal tip  27 . However, the shape of the interface between the precious metal tip  27  and the melt portion  26  can vary depending on the welding condition. In particular, in the case where the melt sag  28  is provided on the side surface of the precious metal tip  27  as in the present embodiment, when the high-temperature melt formed of the precious metal tip  27  and the electrode base material  25  being melted extends frontward on the side surface of the precious metal tip  27 , the side surface of the precious metal tip  27  is melted to some degree by being in contact with the melt. The degree of the melting of the side surface of the precious metal tip  27  is greater in the position closer to the rear side where the high-temperature melt is supplied. The distance of “9D/20” from the center axis O, which defines the virtual straight line L 1 , L 2  used for obtaining the virtual straight line L 3  in the present embodiment is a value experientially obtained by the inventors of the present application, as a position at which influence of the melting of the side surface of the precious metal tip  27  due to the high-temperature melt is sufficiently reduced. In the present embodiment, the virtual straight line L 3  to be the reference is obtained by connecting the intersection points P 1  and P 2  which are the intersection points of the virtual straight lines L 1  and L 2  each apart from the center axis O by the distance of “9D/20” and the interface between the precious metal tip  27  and the melt portion  26 , respectively. Thus, the length, in the direction of the center axis O, of the melt sag  28  formed on the side surface of the precious metal tip  27  is specified by determining the position of the rear-side end portion on the side surface of the precious metal tip  27  while suppressing influence of the melted and deformed side surface of the precious metal tip  27  due to the high-temperature melt. 
     C. Modifications: 
     Modification 1 (Modification of Shape of Melt  26 ): 
     In the above embodiment, when the precious metal tip  27  is welded, the entire circumference of the precious metal tip  27  is uniformly irradiated with the laser beam, whereby the length of the melt sag  28  in the direction of the center axis O is almost uniform over the entire circumference of the side surface of the precious metal tip  27 . However, another configuration may be adopted. The length of the melt sag  28  in the direction of the center axis O may be non-uniform. For example, in the cross section shown in  FIG. 3 , the distance X 1  and the distance X 2  may be different from each other. In all the cross sections of the precious metal tip  27  including the center axis O, the length (the distance X 1 , X 2  in  FIG. 3 ) of the melt sag  28  described with reference to  FIG. 3  may be 0.092 mm or more. In addition, the center electrode  20  may have a portion in which the precious metal tip  27  and the electrode base material  25  are in direct contact with each other without the melt portion  26  intervening therebetween. Even in such a configuration, the same effect as in the above embodiment can be achieved as long as the melt sag  28  of the melt portion  26  has the same length, in the direction of the center axis O, as the length in the above embodiment. 
     Modification 2 (Modification of Welding Method): 
     Welding of the precious metal tip  27  and the electrode base material  25  may be performed by other welding methods than the above-described laser welding, such as electron beam welding. In this case, as long as the electron beam welding is capable of melting the precious metal tip  27  and the electrode base material  25  by irradiating the precious metal tip  27  with an energy beam, from the outer peripheral side thereof toward the inside thereof, and welding them together to form the melt portion  26  having the melt sag  28 , the present invention can be applied to the electron beam welding as in the above embodiment. 
     Modification 3 (Modification of Electrode): 
     In the above embodiment, the length, in the direction of the center axis O, of the melt sag  28  formed by welding the precious metal tip  27  to the electrode base material  25  of the center electrode  20  is defined. However, another configuration may be adopted. The present invention may be applied to the ground electrode  30  instead of or in addition to the center electrode  20 . 
     EXAMPLES 
     Various precious metal tips  27  having different components and sizes were welded to the electrode base material  25 , thereby manufacturing a plurality of electrodes having different lengths of the melt sag  28  in the direction of the center axis O. A thermal test was performed to expose these electrodes to heating and cooling cycles, and the degree of the oxide scale formed at the interface between each precious metal tip  27  and the melt portion  26  was examined. Regarding the thermal test, two types of tests (a first thermal test and a second thermal test) having different heating conditions were performed. The thermal test was performed as a desk test in which the precious metal tips were welded onto a welding base material imitating the electrode base material  25  and heated by using a burner, instead of actually fabricating a spark plug and actually performing ignition operation using the spark plug. 
     [Electrodes for Test] 
       FIG. 4  is an explanatory view showing the specs of the electrodes subjected to the thermal test. In the electrodes according to Spec  1  and Spec  4 , the precious metal tip  27  made of iridium-platinum (Ir—Pt) alloy with the content ratio of platinum being 10 wt % was used. In the electrodes according to Spec  2  and Spec  5 , the precious metal tip  27  made of iridium-rhodium (Ir—Rh) alloy with the content ratio of rhodium being 10 wt % was used. In the electrodes according to Spec  3  and Spec  6 , the precious metal tip  27  made of iridium-ruthenium (Ir—Ru) alloy with the content ratio of ruthenium being 8 wt % was used. In addition, in the electrodes according to Spec  1  to Spec  3 , the cylindrical precious metal tip  27  with the diameter of the end surface thereof being 0.6 mm and the height being 0.75 mm was used. In the electrodes according to Spec  4  to Spec  6 , the cylindrical precious metal tip  27  with the diameter of the end surface thereof being 0.8 mm and the height being 0.5 mm was used. 
     In each electrode subjected to the thermal test, as the welding base material imitating the electrode base material  25 , a cylindrical member made of INCONEL 600 (INCONEL is a registered trademark) as a nickel-base alloy was used. In manufacturing each electrode, the welding base material was used in which the diameter of the end surface thereof to which the precious metal tip is to be welded was 0.3 mm larger than the diameter of the end surface of the precious metal tip to be welded. 
     The welding conditions in manufacturing each electrode to be subjected to the thermal test are as follows. Welding was performed by using a pulse wave (PW) oscillation fiber laser. In advance of laser welding, each precious metal tip  27  was disposed on the end surface of the welding base material, and pressed and fixed by means of a pin. Then, laser welding was performed while rotating the welding base material on which the precious metal tip  27  was fixed, around the center axis O at a rotation speed of 60 rpm. For each spec, various electrodes were manufactured with different average laser outputs ranging from 30 to 45 W. In addition, for each spec, various electrodes were manufactured with different numbers of laser shots ranging from 11 to 14 shots. In each electrode, the irradiation time of laser per shot was 5 msec. The interval of laser irradiation was uniformly adjusted such that laser irradiation was finished within one rotation of the welding base material to which the precious metal tip was fixed. The laser irradiation was adjusted such that, in each electrode, a region irradiated with laser first time was about one-half overlapped with a region irradiated with laser last time. 
     Thus, for each spec, the various electrodes were manufactured with different average laser outputs and different number of laser shots, X-ray CT observation was performed on these electrodes, and the length of the melt sag  28  in each electrode in the direction of the center axis O was measured by nondestructive internal observation. While the melt sag  28  was substantially uniformly formed over the entire circumference on the side surface of the precious metal tip  27 , the length of the melt sag  28  was measured at a position where the melt sag  28  has the shortest length in the direction of the center axis O. Then, for each of the first thermal test and the second thermal test, six electrodes having different welding conditions were selected for each spec in such a manner that the length of the melt sag  28  in the direction of the center axis O varied as uniformly as possible among the six electrodes within a range from about 0.01 mm to about 0.18 mm (in  FIG. 4 , for each spec, the number of electrodes is  6 ). 
     [Conditions of First Thermal Test] 
     All the electrodes, which were selected by six for each of Spec  1  to Spec  6  as described above, were subjected to the first thermal test. The first thermal test includes, as one cycle, an operation to heat the precious metal tip  27  with a burner and an operation to stop the heating, and was performed by 1,000 cycles. In the heating operation, heating was performed so that the temperature of the precious metal tip  27  reached 950° C. while measuring the temperature of the precious metal tip  27  with a radiation thermometer. The heating time per cycle was 2 minutes. The operation to stop the heating was one minute per cycle. 
     [Conditions of Second Thermal Test] 
     All the electrodes, which were selected by six for each of Spec  1  to Spec  6  as described above, were subjected to the second thermal test. The second thermal test is different from the first thermal test only in that the temperature of heating was 1,000° C. That is, the heating condition is stricter in the second thermal test than in the first thermal test. 
     [Measurement of Oxide Scale] 
     After the first and second thermal tests were executed each by 1,000 cycles, the cross section, including the center axis, of the precious metal tip  27  of each electrode was exposed. Then, in the exposed cross section, the length X 1 , X 2  of the melt sag  28  in the direction of the center axis O was actually measured as shown in  FIG. 3 . The actually measured value coincided well with the numerical value measured by the X-ray CT nondestructive internal observation. The exposed cross section was enlarged by  70  times and observed, and the length of the oxide scale formed at the interface between the precious metal tip  27  and the melt portion  26  was measured. Then, the ratio of the total length of the oxide scale formed between the points P 9  to P 10  to the length of the interface (between the points P 9  and P 10  in  FIG. 3 ) between the precious metal tip  27  and the melt portion  26  was calculated as an oxide scale formation ratio. At the interface between the precious metal tip  27  and the melt portion  26  exposed at the cross section of the electrode, the color of the oxide scale is different in color from other portions and therefore can be easily distinguished. 
       FIG. 5  is an explanatory view in which, regarding each electrode subjected to the first thermal test, the actually measured length (the minimum value of the distance X 1 , X 2  in  FIG. 3 ) of the melt sag  28  of the electrode in the direction of the center axis O is indicated on the horizontal axis, and the calculated oxide scale formation ratio is indicated on the vertical axis. In the first thermal test, the electrodes with the oxide scale formation ratios being 30% or less were evaluated as “passed”. As shown in  FIG. 5 , in each spec, the electrodes with the length of the melt sag  28  in the direction of the center axis O being 0.092 mm or more were evaluated as “passed”. Therefore, it was confirmed that the effect of suppressing growth of the oxide scale was improved by setting the length of the melt sag  28  to be 0.092 mm or more. No difference was observed in the above tendency among the specs of the precious metal tip  27 , i.e., among the different components and sizes of the precious metal tip  27 . 
       FIG. 6  is an explanatory view in which, regarding each electrode subjected to the second thermal test, the actually measured length (the minimum value of the distance X 1 , X 2  in  FIG. 3 ) of the melt sag  28  of the electrode in the direction of the center axis O is indicated on the horizontal axis, and the calculated oxide scale formation ratio is indicated on the vertical axis. In the second thermal test, the electrodes with the oxide scale formation ratios being 50% or less were evaluated as “passed”. As shown in  FIG. 6 , in each spec, the electrodes with the length of the melt sag  28  in the direction of the center axis O being 0.110 mm or more were evaluated as “passed”. Therefore, it was confirmed that, even when the electrodes were subjected to severe heating and cooling cycles with the heating temperature reaching 1000° C., the effect of suppressing growth of the oxide scale was improved by setting the length of the melt sag  28  to be 0.110 mm or more. No difference was observed in the above tendency among the specs of the precious metal tip  27 , i.e., among the different components and sizes of the precious metal tip  27 . 
     The present invention is not limited to the above embodiments, modes, and modifications/variations and can be embodied in various forms without departing from the scope of the present invention. For example, it is feasible to appropriately replace or combine any of the technical features of the aspects of the present invention described in “Summary of the Invention” and the technical features of the embodiments, modes, and modifications/variations of the present invention in order to solve part or all of the above-mentioned problems or achieve part or all of the above-mentioned effects. Any of these technical features, if not explained as essential in the present specification, may be deleted as appropriate. 
     DESCRIPTION OF REFERENCE NUMERALS 
       3  . . . ceramic resistor 
       4  . . . seal body 
       5  . . . gasket 
       6  . . . ring member 
       8  . . . sheet packing 
       9  . . . talc 
       10  . . . ceramic insulator 
       12  . . . axial hole 
       13  . . . leg portion 
       15  . . . ceramic step portion 
       17  . . . front trunk portion 
       18  . . . rear trunk portion 
       19  . . . central trunk portion 
       20  . . . center electrode 
       25  . . . electrode base material 
       26  . . . melt portion 
       27  . . . precious metal tip 
       28  . . . melt sag 
       30  . . . ground electrode 
       40  . . . metal terminal 
       50  . . . metallic shell 
       51  . . . tool engagement portion 
       52  . . . threaded attachment portion 
       53  . . . crimp portion 
       54  . . . gasket receiving portion 
       56  . . . metal-shell internal step portion 
       57  . . . front end surface 
       58  . . . compressive deformation portion 
       100  . . . spark plug 
       200  . . . engine head 
       201  . . . threaded attachment hole 
       600  . . . INCONEL