Patent Publication Number: US-6700317-B2

Title: Spark plug for an internal combustion engine and manufacturing method of the same

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship and an iridium (Ir) alloy firing tip provided on at least one of opposing portions of the electrodes. Furthermore, the present invention relates to a method for manufacturing this spark plug. 
     Spark plugs are employed in internal combustion engines of automotive vehicles, cogeneration facilities and gas compressors. For example, to extend lifetime and improve performance of the spark plug, a spark discharge electrode member, made of platinum (Pt) or Pt alloy, is disposed on at least one of opposing portions of the center and ground electrodes disposed in an opposed relationship. 
     U.S. Pat. No. 5,456,624 discloses this type of conventional spark plug which uses a rivet platinum firing tip having a head formed at a front end thereof. The head of the firing tip is fixed to an opposing electrode surface by resistance welding. 
     The spark plugs in future will be subjected to severe engine specifications, i.e., will be used in thermally severe environments. It is predicted that wearability of the firing tip, if it is made of Pt alloy, will be insufficient in such severe conditions. Regarding the wearability, the melting point of iridium (Ir) alloy is higher than that of the Pt alloy. Thus, the iridium alloy is believed to be a prospective material for the future spark discharge electrode member. 
     The inventors of this application have conducted durability tests on some samples prepared based on conventional spark plug arrangement employing Ir alloy firing tips, with a conclusion that fixation of the Ir alloy firing tip is insufficient according to the conventional spark plug. 
     More specifically, according to the above-described conventional spark plug, the firing tip is shallowly welded on the opposing electrode surface in such a manner that only a front end or top of the firing tip head sinks in the opposing electrode. If the firing tip is made of Pt or Pt alloy having a linear expansion coefficient similar to that of the electrode base material (Ni-based alloy or the like), it will be possible to obtain sufficient bonding strength and durability. 
     However, when the firing tip is made of Ir alloy having a linear expansion coefficient larger than that of the electrode base material, the conventional firing tip arrangement cannot assure sufficient bonding strength and durability. In fact, according to an engine test based on practical environments, the firing tip has fallen out of the electrode. Alternatively, it may be possible to use a laser welding for connecting the Ir alloy firing tip to the electrode. However, the laser welding is expensive compared with the resistance welding. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems, the present invention has an object to provide a spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship and an iridium alloy firing tip provided on at least one of opposing portions of the electrodes. More specifically, the present invention provides a low-cost method for surely fixing the iridium alloy firing tip to the electrode. 
     To accomplish the above and other related objects, the present invention provides a first spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on at least one of opposing portions of said electrodes. The iridium alloy firing tip comprises a stem and a head. The firing tip head is integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. A distal end of the firing tip stem is opposed to an opponent electrode. The head is embedded in the opposing portion of the electrode. The head is pointed or tapered in a direction opposite to the stem from a maximum diameter portion in such a manner that a cross-sectional area of the head continuously decreases with increasing distance from the maximum diameter portion. The maximum diameter portion is positioned inside the opposing portion of the electrode, and a base end of the stem extending from the maximum diameter portion is wrapped or surrounded by the opposing portion of the electrode. 
     According to the first spark plug of the present invention, the front end (i.e., top) of the firing tip head is pointed or tapered from the maximum diameter portion in such a manner that the cross-sectional area of the firing tip head continuously decreases with increasing distance from the maximum diameter portion (hereinafter, referred to as tapered configuration of the firing tip head). Thus, when a pressing force is applied during the low-cost resistance welding, the firing tip head can easily sink in the melted opposing portion of the electrode. 
     According to the embedding arrangement of the first spark plug, the opposing portion of the electrode surrounds or wraps the maximum diameter portion of the firing tip head as well as the base end of the stem extending from the maximum diameter portion. Thus, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the electrode. Hence, the first spark plug of the present invention makes it possible to prevent the Ir alloy firing tip from falling out of the electrode based on low-cost resistance welding. 
     According to the first spark plug of the present invention, it is preferable that the opposing portion of the electrode surrounds or wraps the firing tip head by a thickness t1 equal to or larger than 0.3 mm. The thickness t1 satisfying this condition assures a sufficient force for fixing the Ir ally firing tip to the opposing portion of the electrode. 
     Furthermore, according to the first spark plug of the present invention, it is preferable that a pointed or tapered end of the firing tip head is configured into a spherical surface. Alternatively, it is preferable that the pointed or tapered end of the firing tip head is configured into a flattened surface which satisfies a relationship A&lt;D/2, where “A” represents a planar length of the flattened surface and “D” represents a diameter of the maximum diameter portion. If the planar length “A” is equal to or larger than D/2, the firing tip head will not smoothly sink in the opposing portion of the electrode during the resistance welding operation. 
     Furthermore, the present invention provides a manufacturing method for a spark plug employed in an internal combustion engine, the spark plug comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip fixed to at least one of opposing portions of the electrodes by resistance welding, wherein the iridium alloy firing tip comprises a stem and a head, the firing tip head being integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. This manufacturing method comprises a welding operation for fixing the iridium firing tip to the opposing portion of the electrode by resistance welding. The welding operation comprises a step of bringing the head of the iridium alloy firing tip into contact with the opposing portion of the electrode, and a step of applying a pressing force to the head during resistance welding operation for enforcing a maximum diameter portion to sink in a melted portion of the electrode until a base end of the stem extending from the maximum diameter portion is embedded in the melted portion of the electrode. 
     According to the manufacturing method of the present invention, the above-described first spark plug can be manufactured adequately. During resistance welding operation, the firing tip head sinks in the melted portion of the electrode when it thermally deforms due to welding heat. This makes it possible to surround or wrap the base end of the stem extending from the maximum diameter portion. Alternatively, according to the present invention, it is possible to caulk the opposing portion of the electrode after the firing tip head is embedded in the opposing portion of the electrode. 
     Furthermore, the present invention provides a second spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on at least one of opposing portions of said electrodes. The iridium alloy firing tip comprises a stem and a head. The firing tip head is integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. A distal end of the firing tip stem is opposed to an opponent electrode. The firing tip head is placed in a hole formed in the opposing portion of the electrode. The head has a maximum diameter portion positioned in the hole. The opposing portion of the electrode is caulked so that a base end of the stem extending from the maximum diameter portion is wrapped or surrounded by an inside wall of the hole. 
     According to the second spark plug of this present invention, the firing tip head is securely fixed in the hole with the maximum diameter portion placed in the hole and the stem surrounded by the deformed inside wall of the hole. Accordingly, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the opposing portion of the electrode. Thus, according to the second spark plug of the present invention, it becomes possible to effectively fixing the Ir alloy firing tip to the electrode based on low-cost caulking operation. 
     Furthermore, the present invention provides a third spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on an opposing portion of said ground electrode. The iridium alloy firing tip is provided on an opposing portion of the ground electrode. The iridium alloy firing tip comprises a stem and two heads formed at both ends of this stem, with a diameter of each head larger than a diameter of the stem. A through-hole, provided on the opposing portion of the electrode, accommodates the stem. Two heads are fixed to the opposing portion of the electrode so as to close each end of the through-hole. 
     According to the third spark plug of this present invention, the heads are fixed to the opposing portion of the electrode so as to close the both ends of the through-hole. Thus, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the opposing portion of the electrode. This arrangement can be easily realized, for example, by inserting an Ir alloy tip rod into a through-hole and deforming a protruding portion of the tip rod to form the firing tip head. Thus, according to the third spark plug of the present invention, it becomes possible to effectively fixing the Ir alloy firing tip to the opposing portion of the electrode based on low-cost operation. 
     According to the present invention, the iridium alloy firing tip chiefly contains iridium with at least one additive selected from the group consisting of rhodium (Rh), platinum (Pt), ruthenium (Ru), palladium (Pd) and tungsten (W). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a half cross-sectional view showing overall arrangement of a spark plug in accordance with a first embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view showing a firing tip fixed on the front end of a center electrode of the spark plug shown in FIG. 1; 
     FIGS. 3A and 3B are views showing appearance and configuration of Ir alloy firing tip itself in accordance with the first embodiment of the present invention; 
     FIG. 4A is a view showing appearance of a flat firing tip; 
     FIG. 4B is a cross-sectional view explaining definition of an embedding depth H; 
     FIG. 4C is a graph showing relationship between the welding current and the embedding depth H; 
     FIG. 5A is a view showing appearance and configuration of a tapered Ir alloy firing tip with a flat top surface in accordance with the first embodiment of the present invention; 
     FIG. 5B is a graph showing relationship between a head planar length A and the embedding depth H; 
     FIG. 6 is a cross-sectional view schematically showing a modified embodiment which has an Ir alloy firing tip fixed on the opposing portion of a ground electrode in accordance with the first embodiment of the present invention; 
     FIGS. 7A to  7 H show various configurations of the Ir alloy firing tip in accordance with the first embodiment of the present invention; 
     FIGS. 8A to  8 D are cross-sectional views schematically illustrating caulking steps for fixing the Ir alloy firing tip to the electrode in accordance with a second embodiment of the present invention; 
     FIG. 9 is a cross-sectional view schematically showing a caulking operation using a separate member in accordance with the second embodiment of the present invention; and 
     FIGS. 10A to  10 C are cross-sectional views schematically showing the caulking steps for fixing the Ir alloy firing tip to the electrode in accordance with a third embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be explained with reference to attached drawings. 
     First Embodiment 
     FIG. 1 is a half cross-sectional view showing overall arrangement of a spark plug  100  in accordance with a first embodiment of the present invention. The spark plug  100  is applicable as an ignitor to an internal combustion engine of an automotive vehicle. The spark plug  100  is inserted into and fixedly engaged with a screw hole opened on an engine head (not shown) which forms and defines a combustion chamber of the engine. 
     The spark plug  100  has a cylindrical metal fitting (i.e., mounting bracket)  10  made of electrically conductive steel material (e.g., low-carbon steel). The metal fitting  10  has a thread ridge  11  which is securely engaged with a corresponding thread hole formed on the engine head. An insulator  20 , made of alumina ceramic (Al 2 O 3 ) etc., is securely coupled in an inside hollow space of the metal fitting  10 . The insulator  20  has a front end  21  exposed to the outside from an opening of one axial end of the metal fitting  10 . 
     The insulator  20  has an axially extending inside hole  22  for securely supporting a center electrode  30  therein. Thus, the metal fitting  10  supports the center electrode  30  via the insulator  20 . The center electrode  30  is a columnar member which has an inside metallic member, such as Cu (i.e., copper), having excellent thermal conductivity and an outside metallic member, such as Ni (i.e., nickel)-based alloy, having excellent heat resistivity and corrosive resistivity. As shown in FIG. 1, a front end  31  of the center electrode  30  is exposed to the outside from the front end  21  of the insulator  20 . 
     A ground electrode  40  has a proximal end  42  fixed to the axial end of the metal fitting  10 , an intermediate portion substantially bent into an L shape, and a distal end  41  opposing via a discharging gap  50  to the front end  31  of the center electrode  30 . The ground electrode  40  is a square rod member made of a Ni-based alloy or the like. 
     The front end  31  of the center electrode  30  and the distal end  41  of the ground electrode  40  serve as opposing portions of the electrodes of the present invention. According to the first embodiment, an Ir alloy firing tip  60  is attached on the front end  31  of the center electrode  30  by resistance welding. The Ir alloy firing tip  60  serves as a spark discharge electrode member. 
     The firing tip  60  chiefly contains Ir (iridium) with at least one additive selected from the group consisting of rhodium (Rh), platinum (Pt), ruthenium (Ru), palladium (Pd) and tungsten (W). For example, the firing tip  60  is an Ir-10Rh alloy with 90 weight % Ir and 10 weight % Rh. The discharge gap  50  is a clearance between the firing tip  60  and the distal end  41  of the ground electrode  40 . For example, the discharge gap  50  is approximately 1 mm. 
     FIG. 2 is an enlarged cross-sectional view showing the Ir alloy firing tip  60  welded to the front end  31  of the center electrode  30 . 
     FIGS. 3A and 3B show appearance and configuration of the Ir alloy firing tip  60  itself. FIG. 3B shows the Ir alloy firing tip  60  seen from the direction of an arrow Y shown in FIG.  3 A. The Ir alloy firing tip  60  has a circular stem  61  with a diameter “d” and a length “t0” which is integrally formed from a circular head  62 . The firing tip head  62  has a diameter larger than that (i.e., diameter “d”) of the circular stem  61 . In other words, the Ir alloy firing tip  60  is configured into a rivet shape. For example, the Ir alloy firing tip  60  can be formed by cold forging or hot forging the Ir alloy. 
     According to this embodiment, the circular firing tip head  62  has a maximum diameter (i.e., D as shown in FIG. 3A) at a maximum diameter portion  63 . The maximum diameter portion  63  is a bottom flat portion closest to the circular stem  61  of the firing tip  60 . The opposite side (i.e., top) of the circular head  62  is configured into a spherical shape. The diameter of the circular head  62  gradually decreases with increasing distance from the maximum diameter portion  63  (due to tapered configuration of the firing tip head). Accordingly, when taken along a plane normal to the axis of the stem  61  (refer to FIG.  2 ), the cross-sectional area of the circular head  62  continuously decreases with increasing distance from the maximum diameter portion  63 . 
     As shown in FIG. 2, the maximum diameter portion  63  of the circular head  62  is embedded in the front end (i.e. one opposing portion)  31  of the center electrode  30 . A free end of the circular stem  61  is directed toward the distal end (i.e., the other opposing portion)  41  of the ground electrode  40 . The circular head  62  is deeply positioned so as to be completely embedded in the front end  31  of the center electrode  30 . The base end of the circular stem  61  extending from the maximum diameter portion  63  is also embedded in the front end  31  of the center electrode  30 . 
     Next, a manufacturing method for the above-described spark plug  100  will be explained chiefly for a step of fixing the Ir alloy firing tip  60  to the front end  31  of the center electrode  30  in the following manner. Regarding manufacturing steps for other portions of the spark plug  100  are conventionally known and therefore not explained. 
     This embodiment employs the resistance welding for fixing the Ir alloy firing tip  60  to the front end  31  of the center electrode  30 . The Ir alloy firing tip  60  is held up side down relative to the center electrode  30 . More specifically, the front end (i.e., top) of the circular head  62  is brought into contact with the front end  31  of the center electrode  30  while a pressing force is applied to the circular stem  61  in the axial direction. Holding this state, the resistance welding is performed. 
     During the resistance welding operation, the front end  31  of the center electrode  30  melts due to welding heat and the circular head  62  sinks into melted portion of the center electrode  30  due to the pressing force applied thereon. Deformation of the front end  31  of the center electrode  30  results in a condition that the base end of the circular stem  61  extending from the maximum diameter portion  63  is embedded together with the circular head  62  in the melted front end  31  of the center electrode  30  as shown in FIG.  2 . In other words, deformation of the center electrode  30  advances in such a manner that the maximum diameter portion  63  of the circular head  62  is gradually surrounded or wrapped by the melted front end  31  of the center electrode  30 . Thus, as a result of resistance welding operation, the circular head  62  of the Ir alloy firing tip  60  is completely embedded in the center electrode  30  as shown in FIG.  2 . 
     According to the spark plug  100  of this embodiment, the front end (i.e., top) of the circular head  62  is pointed or tapered in a direction opposite to the stem  61  from its maximum diameter portion  63  in such a manner that the cross-sectional area of the circular head  62  continuously decreases with increasing distance from the maximum diameter portion  63 . Thus, when a pressing force is applied during the resistance welding operation as described above, the circular head  62  can easily sink in the melted portion of the center electrode  30 . 
     According to the embedding arrangement for the Ir alloy firing tip  60 , the front end  31  of the center electrode  30  surrounds or wraps the entire body of the circular head  62  as well as the base end of the circular stem  61  extending from the maximum diameter portion  63 . Thus, it becomes possible to securely fix the Ir alloy firing tip  60  to the front end (i.e., opposing portion)  31  of the center electrode  30  in a hooked condition, thereby effectively preventing the Ir alloy firing tip  60  from being mechanically pulled out of the center electrode  30 . Hence, the spark plug  100  of this embodiment makes it possible to realize reliable fixation of Ir alloy firing tip  60  to the opposing portion  31  based on low-cost resistance welding, i.e., without relying on expensive laser welding. 
     However, the above-described fixing method based on the resistance welding can be modified in the following manner. After embedding the circular head  62  into the opposing portion  31  of the center electrode  30 , the opposing portion  31  can be caulked so as to surround or wrap the entire body of the circular head  62  as well as the base end of the circular stem  61  extending from the maximum diameter portion  63 . This makes it possible to ensure accurate surrounding or wrapping shape of the opposing portion of the electrode. 
     Next, shape of Ir alloy firing tip  60  according to this embodiment and its embedded arrangement will be explained in more detail. 
     In the Ir alloy firing tip  60  shown in FIGS. 3A and 3B, a preferable range of the diameter “d” (hereinafter referred to as thin diameter “d”) of the circular stem (i.e., discharge side smaller diameter portion)  61  is in a range of 0.3 mm to 0.8 mm. When the circular stem  61  satisfies this condition, the electric field can be strengthened while the mechanical strength can be assured. 
     Regarding the relationship between the thin diameter “d” and the maximum diameter D of the circular head  62 , it is desirable that the maximum diameter D is within a range of (d+0.2) mm to (d+0.8) mm. When the difference between the maximum diameter D and the thin diameter “d” is less than 0.2 mm, a hooking depth of the center electrode  30  relative to the circular head  62  is so shallow that an insufficient force will be obtained for securely holding the circular head  62 . When the difference between the maximum diameter D and the thin diameter “d” is larger than 0.8 mm, the firing tip  60  will no sink in the melted portion of the center electrode  30 . 
     The thickness T0 (in the axial direction of firing tip  60 ) of the circular head  62  should be determined considering the following points. When the thickness T0 is too thick, the circular head  62  cannot completely sink in the front end (i.e., opposing portion)  31  of the center electrode  30 . When the thickness T0 is too thin, the circular head  62  will deform and cannot assure a sufficient fixing force for preventing the Ir alloy firing tip  60  from falling out of the opposing portion  31 . For example, an appropriate value of the thickness T0 of the circular head  62  is 0.3 mm. 
     Furthermore, it is desirable that a thickness (i.e., surrounding or wrapping depth) t1 of the opposing portion  31  relative to the circular head  62  is equal to or larger than 0.3 mm. This value is based on practical-level durability test (e.g., durability test equivalent to traveling distance 100,000 km based on actual vehicle) conducted on the spark plug  100  for checking the effect of preventing the Ir alloy firing tip  60  from falling out of the opposing portion  31 . When the thickness t1 is equal to or larger than 0.3 mm, a sufficient force can be obtained for fixing the Ir alloy firing tip  60  to the opposing portion  31 , thereby assuring the above-described anti-falling effect. 
     Furthermore, it is desirable that a length (i.e., stem protruding length) t2 of the stem  61  protruding relative to the opposing portion  31  is equal to or larger than 0.3 mm. When the stem protruding length t2 is too short, spark discharge may occur at an unpredictable point on the opposing portion (i.e., front end of center electrode)  31  other than the stem  61 . The opposing portion  31  may be so exhausted that the Ir alloy firing tip  60  falls out of the opposing portion  31 . 
     Furthermore, it is desirable that a length t0 of the stem  61  is equal to or larger than 0.6 mm in view of preferable values of the surrounding or wrapping depth t1 (≧0.3 mm) and the stem protruding length t2 (≧0.3 mm). When the length t0 of the stem  61  is too long, the stem  61  may cause buckling when a pressing force is applied during the resistance welding. The length t0 of the stem  61  should be determined based on such considerations, and is preferably equal to or larger than 0.6 mm. 
     FIGS. 4A,  4 B and  4 C show evaluation result on the embedding arrangement of the Ir alloy firing tip  60  relative to the opposing portion  31  in relation to the welding current of the above-described resistance welding. In view of embedding of the head  62  as well as anti-buckling of the stem  61 , a preferable pressing force applied to the Ir alloy firing tip  60  during the resistance welding is within a range of 200N to 400N. The test shown in FIGS. 4A to  4 C was conducted under the pressing force of 250N and the cycle number of 10 with the parameter of welding current. 
     Two Ir alloy firing tips were tested. One of the tested firing tips is a spherical firing tip  60  having the spherical head  62  shown in FIGS. 3A and 3B. The other one of the tested firing tips is a flat firing tip  70  having a flattened heat  62  as shown in FIG.  4 A. The tested firing tips  60  and  70  have the same thin diameter (d=0.7 mm), the same maximum diameter (D=1.2 mm), the same stem length (t0=0.6 mm), and the same head thickness (T0=0.3 mm). The spherical firing tip  60  has a curvature radius R equivalent to D/2 (=0.6 mm). 
     FIG. 4C shows the test result, with an abscissa representing welding current (kA) and an ordinate representing embedding depth H (mm). FIG. 4B shows the embedding depth H (mm). In FIG. 4C, a black plotting curve represents the spherical firing tip  60  while a while plotting curve represents the flat firing tip  70 . The point×represents a buckling point of the stem  61  in each of the firing tips  60  and  70 . 
     As understood from FIG. 4C, in the case of flat firing tip  70 , the flattened head  62  did not sink in the opposing portion  31  and the stem  61  has buckled when the welding current is increased up to 1.2 kA. Like the flat firing tip  70 , the stem  61  of the spherical firing tip  60  has buckled when the welding current is increased up to 1.2 kA. And, at 0.6 kA level of the welding current, the spherical head  62  of the spherical firing tip  60  did not completely sink in the opposing portion  31 . However, when the welding current is within the range of 0.8 kA to 1.0 kA, the spherical head  62  has completely sunk in the opposing portion of the electrode without causing buckling of the stem  61 . 
     As understood from the test result, the spherical configuration of this embodiment is advantageous in that the head  62  can easily sink in the opposing portion  31  under application of an appropriate pressing force during the resistance welding operation. FIG. 4C merely shows one test result. As far as the firing tip  60  has the above-described appropriate size, adequate embedding performance can be assured. 
     The configuration of the firing tip head  62  is not limited to the above-described spherical shape and therefore can be modified into other one. For example, the firing tip head  62  may have a flat surface at its top when the cross-sectional area of the head  62  continuously decreases with increasing distance from the maximum diameter portion  63  as shown in FIG.  5 A. FIG. 5A shows appearance and configuration of a tapered Ir alloy firing tip  60  whose diameter monotonously decreases from the maximum diameter portion  63  to the top (i.e., front end). 
     When the firing tip head  62  has a flattened top, it is preferable that the radial length (i.e., head planar length) “A” of the top surface is smaller than a half of the maximum D, i.e., A&lt;D/2, as shown in FIG.  5 A. FIG. 5B is a graph showing a test result conducted for the relationship between the head planar length A and the embedding depth H. 
     FIG. 5B has an abscissa representing the head planar length A (mm) and an ordinate representing the embedding depth H (mm). The tested firing tip  60  has the same size (d=0.7 mm, D=1.2 mm, t0=0.6 mm, and T0=0.3 mm) as that shown in FIGS. 4A to  4 C. As understood from this graph, the firing tip head  62  can easily sink when the head planar length A is small and does not sink when the head planar length A exceeds 0.6 mm. 
     From the foregoing, in the case of the Ir alloy firing tip  60  shown in FIG. 5A, sufficient embedding performance is not assured when the head planar length A is equal to or larger than D/2. Namely, the firing tip head cannot surely sink in the opposing portion of the electrode during the resistance welding operation. Hence, it is desirable that the relationship A&lt;D/2 is satisfied. In the graph of FIG. 5B, when the head planar length A is 0, configuration of the firing tip head  62  is conical or the one shown in FIGS. 3A and 3B. 
     As described above, the present invention makes it possible to realize the spark plug  100  using the If alloy firing tip  60  which can assure long life (e.g., equivalent to traveling distance 100,000 km based on actual vehicle). Furthermore, by optimizing the size, configuration and embedding performance of firing tip  60  relative to the electrode, it becomes possible to obtain the reliable and excellent spark plug  100  capable of surely fixing the firing tip  60  based on low-cost resistance welding and preventing the firing tip  60  from falling out of the electrode. 
     The above-described spark plug  100  has two opposing portions  31  and  41  on the center electrode  30  and the ground electrode  40  which are disposed in an opposed relationship, with the Ir alloy firing tip  60  fixed on the opposing portion  31  of the center electrode  30  by resistance welding. Alternatively, it is possible to fix the Ir alloy firing tip  60  on the opposing portion (i.e., distal end)  41  of the ground electrode  40  by resistance welding. Furthermore, it is possible to fix the Ir alloy firing tip  60  on each of the opposing portions  31  and  41  of the center and ground electrodes by resistance welding. 
     FIG. 6 schematically shows a modified embodiment of the first embodiment which has the Ir alloy firing tip  60  fixed on the opposing portion  41  of the ground electrode  40  by resistance welding. According to this modified embodiment, the Ir alloy firing tip  60  has the same arrangement and embedding structure as those of the firing tip  60  of the above-described spark plug  100 . The resistance welding can be performed in the same manner. Thus, the modified embodiment can operate in the same manner with the same effects. 
     Besides the firing tips shown in FIGS. 2 to  5 , the configuration of Ir alloy firing tip  60  applicable to the present invention are shown in FIGS. 7A to  7 H. FIGS. 7B,  7 D, and  7 F are views seen from the direction Y shown in FIGS. 7A,  7 C and  7 E, respectively. Each embodiment shows unique rivet configuration or unique pointed or tapered head configuration. 
     Second Embodiment 
     According to the above-described first embodiment, the head  62  of the rivet Ir alloy firing tip  60  is fixed by resistance welding to at least one of the opposing portions  31  and  41  of the center electrode  30  and the ground electrode  40  which are disposed in an opposed relationship. The second embodiment is characterized in that fixing of Ir alloy firing tip  60  is performed based on caulking. Hereinafter, characteristic features of the second embodiment different from the first embodiment will be explained. 
     FIGS. 8A to  8 D are cross-sectional views illustrating the caulking steps for fixing the firing tip  60  to the opposing portion  31  or  41 . First, an axially extending hole  35  is opened on the opposing portion  31  or  41  by drilling operation (refer to FIG. 8A) so that the head  62  of the Ir alloy firing tip  60  can be inserted in this hole  35 . Next, the Ir alloy firing tip  60  having the same arrangement as that disclosed in the first embodiment is inserted into the hole  35 , with the head  62  as a leading side and the stem  61  following the head  62  and placed partly in the hole  35  (refer to FIG.  8 B). 
     Next, the opposing portion  31  or  41  is caulked at its front end so that the diameter of the hole  35  becomes smaller than the maximum diameter of the head  62 . For example, the caulking operation can be performed by using a knife member or a roller member. Through this caulking operation, as shown in FIG. 8C or  8 D, the firing tip head  62  is securely fixed in the hole  35  with the maximum diameter portion  63  placed deeply in the hole  35  and the stem  61  surrounded by the deformed inside wall of the hole  35 . 
     Accordingly, it becomes possible to securely fix the Ir alloy firing tip  60  to the opposing portion  31  or  41  in a hooked condition, thereby preventing the Ir alloy firing tip  60  from being mechanically pulled out of the opposing portion  31  or  41 . Thus, this embodiment makes it possible to effectively fix the Ir alloy firing tip  60  to the electrode based on low-cost caulking operation. 
     According to this embodiment, it is preferable to perform the resistance welding in the condition shown in FIGS. 8C or  8 D so as to fix the Ir alloy firing tip  60  to the opposing portion  31  or  41 . Furthermore, in the above-described caulking operation, it is possible to interpose a separate member (i.e., spacer)  80  between the stem  61  and the inside wall of the hole  35  as shown in FIG.  9 . The separate member  80  is made of the same material (Ni-based alloy or the like) as that of the opposing portion  31  or  41 . In the caulking operation, the separate member  80  is deformed integrally with the opposing portion  31  or  41  to securely fix the stem  61 . 
     Furthermore, instead of performing the caulking operation, it is possible to integrate all of the firing tip  60 , the separate member  80  and the opposing portion  31  or  41  by performing resistance welding operation after disposing the separate member  80  in the hole  35 . In this case, it becomes possible to realize substantially the same embedding structure as that of the first embodiment, although the drilling operating is required. 
     Third Embodiment 
     FIGS. 10A to  10 C are cross-sectional views schematically showing the caulking steps for fixing the Ir alloy firing tip to the opposing portion of the electrode in accordance with the third embodiment. Third embodiment is based on a spark plug applicable to an internal combustion engine. The spark plug has an Ir alloy firing tip  60  disposed on the opposing portion (i.e., distal end) of the ground electrode  40 . The Ir alloy firing tip  60  is a twin-headed type having two heads  62  at both ends of the stem  61  as shown in FIG. 10C. A through-hole  36 , opened at the opposing portion  41  of the ground electrode  40 , accommodates the stem  61 . Two heads  62  are fixed to the opposing portion  41  so as to close each end of the through-hole  36 . 
     First, as shown in FIG. 10A, the through-hole  36  is opened on the distal end (i.e., opposing portion)  41  of the ground electrode  40  through drilling operation. An inner diameter of the through-hole  36  is large enough to insert the stem  61  of the Ir alloy firing tip  60  and is smaller than the maximum diameter D of the head  62 . Next, like the first embodiment, the stem  61  of Ir allow firing tip  60  is inserted into the through-hole  36  with its free end far from the head  62  as a leading side (refer to FIG.  10 B). 
     Next, as shown in FIG. 10C, an electrode K 1  of a resistance welding apparatus is depressed on the free end of the stem  61  protruding from the through-hole  36  to thermally deform the free end of the stem  61  with heat available from the electrode K 1  of the resistance welding apparatus. As a result, two heads  62  are formed at both ends of the stem  61 . Each head  62  has a diameter larger than the inner diameter of the through-hole  36 . Thus, the heads  62  are firmly fixed to the opposing portion  41  at both ends of the through-hole  36  so as to prevent the Ir alloy firing tip  60  from being pulled out of the through-hole  36 . 
     As described above, the third embodiment makes it possible to realize an excellent firing tip fixing method based on the resistance welding, i.e., without using expensive laser welding. As a result, the third embodiment provides a low-cost method for preventing the Ir alloy firing tip  60  from being pulling out of the opposing portion  41 . According to this embodiment, it is possible to form two heads by deforming both ends of a simple rodlike firing tip inserted in the through-hole  36 . 
     The present embodiments as described are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them. All changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.