Abstract: An igniter having a two-piece center electrode assembly that includes an upper electrode member and an iridium lower electrode member. The two electrode members are assembled together by insertion of an expanded head section of the iridium electrode into a blind hole within the upper electrode. The upper electrode is then swaged over the expanded head section to provide a positive mechanical interlock of the two electrode members together. Thereafter, the two components are brazed together by heating of a brazing compound that is pre-loaded into the blind hole prior to insertion of the expanded head section. This assembly results in both a mechanical and metallurgical bond between the two components without subjecting the iridium electrode to deformation that could otherwise result in axial cracking of the iridium material.

Abstract: In accordance with the present invention, there is provided a spark plug for producing a plurality of sparks which define a three-dimensional volume in a combustion chamber of an internal combustion engine. The spark plug is provided with an insulative body having a support end. The spark plug is further provided with a central positive electrode which extends through the insulative body. The central positive electrode has a spark ignition end. The spark plug is further provided with a conductive outer shell which is disposed about the insulative body. The spark plug is further provided with at least two neutral electrodes which are attached to the support end of the insulative body. One of the neutral electrodes is adjacent to the spark ignition end of the central positive electrode and spaced apart therefrom to form a spark gap therebetween. Another one of the neutral electrode is adjacent to the conductive outer shell and spaced apart therefrom to form a spark gap therebetween.

Abstract: In a spark plug, whose shell is provided with an eccentrically positioned, full-length, longitudinal bore designed to hold an insulating element, parts of the shell having varying wall thicknesses and a pressure measuring device being located in a part of greater wall thickness. In order to simplify connection with the ignition cable or coil, respectively, and to avoid breakage of the insulator, the spark plug insulator is configured as a multi-piece element and comprise at least an upper piece carrying the high voltage connector, a lower piece adjacent to the combustion chamber, and an insulating intermediate element of flexible material, which is placed between the upper and the lower piece of the insulator.

Abstract: A spark plug for igniting an air/fuel mixture in a combustion chamber of an internal combustion engine includes a spark gap having a variable distance defined between a central electrode and a ground electrode portion of a ground terminal. The ground terminal is formed using either a bimetallic layer arrangement wherein one of the metallic materials has a different coefficient of thermal expansion than the other metallic material, or is formed using a strut or by thickening the metal where the ground terminal is connected to a metal housing of the spark plug. The ground terminal deflects away from the central electrode when subjected to an increased temperature, such as may occur when the engine warms up during operation.

Abstract: A spark plug with a metallic housing (10) is proposed, which has an insulator (11) embedded in the housing (10), in which a rod-shaped internal conductor device is disposed. The internal conductor device has a contact pin (13), a current-limiting erosion resistor (17), and a central electrode (14). A sealing material (16) secures the central electrode (14) in a sealed fashion in the insulator (11). The temperature resistant sealing material (16) essentially contains at least one metallic component and at least one ceramic component with a low thermal expansion, wherein the metallic component has a melting temperature that is higher than the operating temperature.

Abstract: A spark plug comprises a main metal fitting, a center electrode mounted to and held by the main metal fitting in a manner insulated therefrom and a grounding electrode having one end welded to the main metal fitting and another end opposing to the center electrode. A welding burr is further formed to the welded portion of the main metal fitting and the grounding electrode so as to have an area expanding outside the grounding electrode, and the main metal fitting of the main metal fitting is plated, whereas the grounding electrode and the welding burr are not plated.

Abstract: An igniter having a two-piece center electrode assembly that includes an upper electrode member and an iridium lower electrode member. The two electrode members are assembled together by insertion of an expanded head section of the iridium electrode into a blind hole within the upper electrode. The upper electrode is then swaged over the expanded head section to provide a positive mechanical interlock of the two electrode members together. Thereafter, the two components are brazed together by heating of a brazing compound that is pre-loaded into the blind hole prior to insertion of the expanded head section. This assembly results in both a mechanical and metallurgical bond between the two components without subjecting the iridium electrode to deformation that could otherwise result in axial cracking of the iridium material.

Abstract: A spark plug having enhanced heat transfer capabilities is provided. The spark plug comprises a tubular insulator body, a center electrode and a thermally conductive filler material. The tubular insulator body has an inner surface, and the center electrode has an outer surface. The center electrode is positioned within the tubular insulator body such that a gap is formed between at least a portion of the inner surface of the tubular insulator body and at least a portion of the outer surface of the center electrode. The thermally conductive filler material has a high heat transfer coefficient. It is positioned within at least a portion of the gap so as to intimately contact the inner surface of the tubular insulator body and the outer surface of the center electrode. Positioned accordingly in the gap, the thermally conductive filler material enhances the ability of the spark plug to transfer heat away from the firing tip and into the body of the spark plug.

Abstract: A spark plug fabricating method and a spark plug fabricated by the same are provided. The spark plug is equipped with a piezoelectric sensor working to measure the pressure of combustion of fuel in an engine and required to apply a given preload to the piezoelectric sensor at all the time. The piezoelectric sensor is disposed within a holder and retained between the holder and a porcelain insulator. The fabrication method includes the steps of pressing the holder to apply a required preload to the piezoelectric sensor in a longitudinal direction of the porcelain insulator and joining the holder to a metal shell while maintaining the preload applied to the piezoelectric sensor as it is.

Abstract: In a creeping discharge type spark plug, the firing surface forming portions of a center electrode and a ground electrode are made of a metallic material containing a corrosion impeding component consisting of one or more of Fe, Cr and Cu. The spark plug is attached to an internal combustion engine. When it is operated under conditions of a predetermined high speed and high load, a corrosion impeding layer containing a corrosion impeding component is formed on the surface of the top end of an insulating member, with progression of spark discharge in spark discharge gaps. The corrosion impeding layer thus formed protects the center electrode from the attack by the creeping discharge spark. Therefore, the channeling is considerably effectively prevented or restricted.

Abstract: A ceramic sheathed element glow plug has a tubular metal casing holding a ceramic U-shaped heating device cantilevered at its end facing the combustion chamber, and on the end remote from the combustion chamber, it has a terminal pin for applying a voltage to the ceramic heating device. In the metal casing, a ceramic bushing and a ring follow the ceramic heating device at the end remote from the combustion chamber.

Abstract: A contact glass composition for use in resistor-type spark plugs is described. The contact glass composition is a mixture of: (a) at least one metal; (b) graphite; and (c) silicon. Additionally, borosilicate glass, barium borate glass, magnesium aluminum silicate, ball clay, and aluminum may also be included in the contact glass composition.

Abstract: A spark plug according to this invention includes an insulator having a center through-hole formed therein, a center electrode held in the center through-hole, a metallic shell holding the insulator by swaging, and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion formed on the outer circumferential surface of a front end portion of the metallic shell, and a tightening portion formed on the outer circumferential surface of the metallic shell, located at a rear side with respect to the male-threaded portion. The tightening portion has a width-across-flats W of not greater than 14 mm. A cushion material is filled into a space defined by the metallic shell and an insulator to form a cushion-material charged portion. The cushion-material charged portion has an axial length L of from 0.5 mm to 10.0 mm inclusive and a thickness M of from 0.5 mm to 1.3 mm inclusive.

Abstract: In a spark plug having a straight pillar type ground electrode whose protruding length into the combustion chamber is relatively short, an intermediate member is provided at a bridging portion between the end of the housing and the leading end of the ground electrode so as to easily adjust the spark discharge gap.

Abstract: In a spark plug, a chip or a plurality of chips made of Ir alloy are bonded by resistance welding through a stress releasing layer on the respective chip mounting portions of the center and ground electrodes made of Ni base alloy. At the temperature of 900° C., the value of Young's modulus of the stress releasing layer is less than those of the Ir alloy and the Ni base alloy and, further, the value of linear expansion co-efficient of the stress releasing layer is intermediate between those of the Ir alloy and the Ni base alloy. The bonded junction of the chip and the stress releasing layer is shaped as a curved surface.

Abstract: Process for producing a middle electrode with precious metal reinforcement, a middle electrode being used with an ignition tipwhich is tapered relative to the middle electrode body, such that a precious metal wire with a cross section which corresponds to the tapered ignition tip is welded flat onto the tapered ignition tipand such that this precious metal wire is cut off at the desired height, with a sharp edge.

Abstract: In a spark plug, a chip made of Ir alloy is partly buried into and bonded by laser beam welding on the ground electrode made of Ni base alloy. A molten portion is extended from the inside of the ground electrode through a junction of the chip and the electrode into the inside of the chip, but not formed on the spark discharge surface of the chip. Where A is a maximum length from the end of the junction on the side of the center electrode to the end of the molten portion on the junction on the side opposite to the center electrode, within the length A, B is a length of both ends of the molten portion, t is a length of the chip perpendicularly extending from the junction and d is a sum of the length t and a length of the molten portion protruding from the junction into the ground electrode, a ratio of A/B is not less than 0.5 and a ratio of d/t is not less than 2, but more than 4.

Abstract: A spark plug is provided having a copper core side wire including a nickel alloy sheath rolled over the end of the copper core. When the side wire is welded to the steel shell of the spark plug, the rolled over portion of the sheath provides enhanced surface area for bonding to the steel shell. Further, a central portion of the copper core is in direct contact with the steel shell for enhanced thermal conductivity.

Abstract: A spark plug wherein a tip comprised of an iridium alloy essentially consisting of iridium is resistance welded through a relieving layer to a front end of a center electrode comprised of an iron-based alloy and a facing portion of a ground electrode and wherein the material of the relieving layer has a coefficient of linear expansion between the iridium alloy and the iron-based alloy and has a Young's modulus of elasticity smaller than that of the two alloys.

Abstract: A spark plug has at least two electrodes having ends facing toward one another so as to produce an electrical discharge when an electrical potential is applied to said electrodes, and a layer of an erosion-resistant material applied on each of said electrodes by electro-spark deposition so the lifetime of the spark plug comparing to existing is substantially increased.

Abstract: A spark plug including an insulation porcelain having a throughhole; a center electrode held at one end of the throughhole; a housing holding the insulation porcelain; a ground electrode disposed at a front end of the housing and facing the center electrode; a spark gap defined by the center electrode and the ground electrode; and at least one of the center and ground electrodes having a nobel metal member bonded to a discharging spot of its front end, wherein the nobel metal member consists of 90 wt % or more of an Ir—Rh alloy containing 1 to 60 wt % Rh and the balance substantially consisting of at least one of Pt, Pb, Ru, Au and Ni.

Abstract: A spark plug for an internal combustion engine or a sensor element for an ignition or combustion process is provided in which at least one of the electrodes consists of two materials. The electrodes are designed such that the distance from a first area of one electrode to the other electrode is less than the distance from the one additional area of one electrode to the other electrode. The two areas consist of different materials, and at least one of the electrodes has a slight radius of curvature in the first area at the point at which the distance from the other electrode is minimal. Regardless of or in connection with this design, the shape of the surface of the electrode can be constant or continuous, at least at the transition between the two areas. One of the electrodes may have a material projection at a point that is opposite a point on the other electrode at which deposits can develop on the other electrode during operation.

Abstract: A spark plug defines a spark gap G in a range of 0.2 mm≦G<0.4 mm, between a noble metal chip disposed on a central electrode and an earth electrode. The noble metal chip has a groove portion facing the earth electrode so that a space, which is larger than a sphere having a diameter equal to or larger than 0.4 mm, is provided in the spark gap G. As a result, a flame core having a diameter equal to or larger than 0.4 mm can be held in the space not to be quenched, resulting in high ignition property of the spark plug.

Abstract: A spark plug for an internal combustion engine comprises a noble metallic firing tip laser welded on the opposing surface of either a central electrode or a grounded electrode. A fused junction layer, formed between the noble metallic firing tip and the base electrode member, contains 40˜70 wt % noble metallic firing tip component. The noble metallic firing tip has a non-fused portion axially extending by a length “L”, where 0.2 mm ≦L≦0.7 mm. The fused junction layer axially extends by a length “M”, where 0.2 mm ≦M≦0.7 mm. And, the relationship B≧1.3A is established when “A” represents the diameter of the noble metallic firing tip and “B” represents the diameter of a contact area at the boundary between the fused junction layer and the base electrode member, where 0.3 mm ≦A≦0.6 mm.

Abstract: In a spark plug, a resistor is placed between a terminal and a center electrode within a through hole of an insulator. The through hole of the insulator has a first portion which allows the center electrode to be inserted therethrough, and a second portion which is formed on a rear side of the first portion so as to be larger in diameter than the first portion and which accommodates the resistor therein, where the second portion is connected to the first portion via a connecting portion including a two- or more-stepped reduced-diameter portion. Then, an electrically conductive glass seal layer is placed at a position corresponding to the connecting portion between the resistor and the center electrode.

Abstract: A resistor spark plug includes an insulator having an axially extending through-hole. A center electrode is disposed within the through-hole of the insulator such that the center electrode projects from a tip end of the insulator. A terminal metal piece is disposed within the through-hole of the insulator such that the terminal metal piece projects from a tail end of the insulator. A conductive coupling layer is disposed within the through-hole and located between the center electrode and the terminal metal piece. The conductive coupling layer comprises a conductive glass seal layer formed on at least an end facing the terminal metal piece. A surface layer region of the terminal metal piece that comes into contact with the conductive glass seal layer is formed mainly of one or more metals selected from the group consisting of Zn, Sn, Pb, Rh, Pd, Pt, Cu, Au, Sb, and Ag.

Abstract: In a spark plug in which a space between a center electrode at the top end side and a terminal electrode at a rear end side which are arranged to be opposite to each other in an axial hole of an insulator, the glass seal is effected at a temperature within the range of 500 to 1000.degree. C. and under a condition where the concentration of oxygen is 12 vol % or less.

Abstract: A spark plug has a central electrode, an insulator, a metal shell and a ground electrode. The insulator is provided exterior to the central electrode. The metal shell is provided exterior to the insulator. The ground electrode opposes the central electrode. At least one of the central electrode and the ground electrode has a multi-layered structure comprising a core and a high heat conducting layer that covers at least part of the outermost surface of the core. An inner layer of the core is more heat conductive than the outermost layer of the core that is in contact with high heat conducting layer.

Abstract: A spark plug having a center electrode with a cylindrical body with a tip at one end and a terminal near the other end with an insulator radially surrounding the center electrode. A ground shield surrounds the insulator and includes a ground electrode near one end, having a portion thereof with an annular opening disposed above and radially separated from the center electrode tip to form a radial spark gap, with a sealing portion disposed between the ground electrode and the other end of the ground shield for sealing the engine combustion chamber from the outside environment when the spark plug is installed. The ground shield can be formed from two parts which are deformed during manufacturing to be in mechanical engagement and securely holding the insulator in place. The spark plug can also have also have an integral or separate retainer for securing the spark plug to the engine.

Abstract: To improve the consumption resistance and lifetime, a spark plug for an internal combustion engine comprises: an insulation porcelain having a throughhole; a center electrode held at one end of the throughhole; a housing holding the insulation porcelain; a ground electrode disposed at an front end of the housing and facing the center electrode; a spark gap defined by the center electrode and the ground electrode; and at least one of the center electrode and the ground electrode having a noble metal chip bonded to a discharging spot of an front end thereof, wherein the noble metal chip consisting of an Ir-Rh alloy contains Rh in an amount of from 1 wt % to 60 wt %.

Abstract: In a spark plug for an internal combustion engine, a noble metal chip such as an iridium alloy chip is bonded on the tip of a center electrode made of a material such as nickel by laser beam welding. The noble metal chip contains another noble metal such as rhodium having a melting point lower than that of the noble metal chip. By laser welding, a molten bond containing the noble metal melted thereinto from the noble metal chip is formed at the junction of the noble metal chip and the center electrode. Alternatively, the noble metal to be melted into the molten bond may be supplied by a separate noble metal plate. The molten bond thus made has a high bonding strength and a small thermal stress, and thereby durability of the spark plug is improved.

Abstract: A spark electrode assembly and method of assembling same is disclosed. A ceramic housing having a cylindrical passage provides clearance for a conductive metal electrode. The electrode has a tubular body with rod shaped wires inserted at each end, the wires extending outwardly from the tubular body at each of its ends colinearly. During assembly, the wires are crimped into the tubular body in such a manner that the tubular body is deformed outwardly forming bosses. The bosses are of such a size as to cause an interference fit between the tubular body and the housing passage so that the electrode is captured within the housing. An impact force on the electrode forces the tubular body to expand radially outwardly, thereby further engaging the ceramic housing for holding the electrode in the housing. That part of the electrode body that is not in contact with the housing still has ample room to expand within the housing.

Abstract: A hole is formed on a central electrode. A chip has a leg portion. A distal end of the leg portion is entirely united with a bottom surface of the hole by resistance welding. A plurality of fused portions are formed at a boundary between the chip and the central electrode. The fused portions integrate the chip and the central electrode by fusing them together. Each fused portion extends in a radial direction of the chip. The entire periphery of a pointed end of each fused portion penetrates radially inside the outer cylindrical surface of the leg portion of the chip.

Abstract: A spark plug electrode wire composed of an iridium-rhodium alloy exhibits improved erosion resistance and enhanced durability in leaded and unleaded fuel environments. The alloy is formed into a wire having high thermal conductivity, low electrical resistivity, good oxidation resistance, good corrosion resistance, good wear resistance. It exhibits desirable work function in pure metals and alloys such as iridium and iridium-rhodium. The wire is inserted into the center electrode of the spark plug and brazed to an Inconel base metal and copper core thereof using a silver-based brazing alloy. After electrical engine dynamometer testing for 200 hours, an improvement in erosion resistance as high as eight fold was observed with an 60% iridium-40% rhodium alloy, when compared to a conventional 90% platinum-10% nickel alloy. The iridium-rhodium spark plug electrode wire can be used to minimize erosion and wear resistance in leaded and unleaded fuel environments.

Abstract: By heating a glass sealing material, a center electrode is sealed and fixed within an axial bore of an insulator. Formation of a groove in a head portion of the center electrode makes it possible to protect the head portion of the center electrode from deformation under a pressure which is applied upon filling the glass sealing material in a powder form in the axial bore. The glass sealing material is therefore assured to flow to a peripheral side wall of a flange portion of the center electrode, thereby enhancing mechanical engagement between the center electrode and the insulator, that is, uniting strength for them. A spark plug is hence obtained with improved impact resistance and accordingly, with sufficient durability.

Abstract: The present process for the production of sparkplugs provides coated sparkplugs that are resistant to carbonization or lead fouling of the central ceramic insulator. A liquid metal carboxylate composition is applied to the insulator of the sparkplug and subjected to heat treatment at a temperature preferably above 400.degree. C. to form a durable metal oxide layer on the insulator.

Abstract: A spark plug (80) having a center electrode (82) with a cylindrical body and having a tip (84) at one end and a terminal (123) near the other end with an insulator (86) extending around the center electrode (82). A ground shield and retainer unit extends around the insulator (86) and includes a threaded portion (88) near one end and a ground electrode (85) near the other end, having a portion thereof aligned with the center electrode tip (84) to form a spark gap, with a sealing portion (92) disposed therebetween for sealing the engine combustion chamber from the outside environment when the spark plug (80) is installed. The ground shield and retainer unit can be formed from two parts (88,90) which are deformed during manufacturing to be in mechanical engagement and securely holding the insulator (86) in place. The spark plug (80) can also have two or more center electrodes (82,84) with two or more corresponding ground electrodes (83,85) projecting from the ground shield (90).

Abstract: A spark plug is provided having a center electrode disposed in a metallic shell through an insulator so that the front end of the center electrode forms a spark gap with an outer electrode. A platinum tip is secured to the outer electrode positioned to oppose the front end of the center electrode. The outer electrode is provided with a heat-conductive core metal cladded with a matrix metal which is made of a nickel-based alloy containing 10-20 wt. % chromium. The platinum tip is welded to the outer electrode to form a welded portion between the platinum tip and the outer electrode and the platinum tip is positioned to terminate slightly short of the core metal so that the distance between a circumferential edge of the welded portion and a front end of the core metal is in the range 0.0 mm-2.0 mm measured lengthwise along direction the outer electrode.

Abstract: In a spark plug having a noble metal layer disposed on a firing end of an electrode, the noble metal layer being laser welded to the firing end of the electrode, and subsequently heat treated to increase a crystallized granulation particle size of the noble metal layer.

Abstract: A spark plug (11) having a center conductive path (21) with a cylindrical body having a tip (33, 51, 77) at one end and a terminal (23) near the other end has an insulator (41, 45) radially surrounding the center electrode and a substantially cylindrical body with at least first and second diameter sections separated by a shoulder (29). A ground shield (37) surrounds the insulator first diameter section and including near one end a frustoconical section (31) juxtaposed with the insulator shoulder. There is a ground electrode (55, 57, 73) near the other end having a portion thereof (53, 75) aligned with the center electrode tip to form a spark gap. An annular retainer (59) surrounds the insulator second diameter section and including a threaded portion (61) threadedly engaging the threaded portion (17) of a generally cylindrical opening in the head of an internal combustion engine.

Abstract: An electrode material for a spark plug 10 for use in an internal combustion engine. The electrode material includes a core 20 having a high thermal conductivity for reducing the temperature of the spark plug 10 by thermally conducting heat away from the spark plug gap to a cooler part of the engine. The core 20 extends continuously from an arcing surface at an end of a spark plug electrode to the cooler part of the engine. Surrounding the core 20 is a cladding layer 22 for resisting corrosion when exposed to high temperatures caused by arcing.

Abstract: In a spark plug which has an electrode metal made from a heat- and erosion-resistant nickel alloy whose front end has a noble metal tip made of iridium or ruthenium, the electrode metal has a thermal conductivity of at least 30 W/m.multidot.K so as to avoid rapid temperature rise in the noble metal tip to thereby minimize oxidation-evaporation and attendant wear thereof.

Abstract: A spark electrode assembly and method of assembling same is disclosed. A ceramic housing having a cylindrical passage provides clearance for a conductive metal electrode of rod construction. The electrode has a body portion having a relatively large diameter, an emitter portion of relatively smaller diameter extending concentrically from one side of the body portion, and an umbrella-shaped portion extending concentrically from the other side of the body portion. During assembly, the electrode is inserted into the passage of the ceramic housing until the umbrella portion of the electrode makes contact with an annular side wall of the passage. An impact force on the electrode forces the umbrella portion to expand radially outwardly, thereby engaging an annular groove portion of the ceramic housing for holding the electrode in the housing.

Abstract: An electrically conducting sealing compound for a sparkplug contact core with the sealing compound being disposed between a terminal-side section and a spark-side section of a center electrode, while the sealing compound essentially contains a fusible glass component and a powdered, electrically conducting component. The electrically conducting component of the sealing compound is exclusively composed of graphite in an amount of 10 to 30% by volume, based on the powder components of the sealing compound.

Abstract: Method of producing a spark plug having a center electrode 1 provided with precious metal chips 31 and 32 on the tip 10. Production of the center electrode 1 is composed of a flat area formation process wherein a cylindrical center electrode 1 is formed by extrusion of metal material and wherein flat areas 111 and 113 are formed on the sides of the tip 10 of the same, a tip bonding process wherein precious metal chips 31 and 32 are bonded to the flat areas, and a plastic machining process wherein the tip 10 whereupon the precious metal chips are bonded is formed into an abbreviated cylinder by means of plastic machining. By applying an identification mark to the tip surface of the tip 10, the precious metal chips 31 and 32 can accurately and easily be arranged opposite the ground electrodes when mounting the center electrode in a metal housing.

Abstract: A spark plug which realizes reliability against thermal stress even in a case where dissimilar noble metal materials are disposed on the tip portion of the center electrode opposing the ground electrode and which realizes improved productivity while assuring durable reliability. The spark plug includes a center electrode electrically insulated from a housing and having a small-diameter portion at a tip portion which has a diameter smaller than that of a main body of the center electrode and composed of similar kinds of material as the center electrode, and a round-shape material, which is formed into a cylindrical or a ring shape, composed of dissimilar metal material.

Abstract: A spark plug includes a cylindrical metallic shell having a shoulder portion, and a tubular insulator having a stepped portion which rests on the shoulder portion within the metallic shell. The insulator has a rear half made of aluminum oxide and a front half made of aluminum nitride, the rear half having a protrusion, while the front half having a housing, and tightly joining the protrusion within the housing. An overlapping portion of the protrusion and the housing is at least partly located in a plane which is perpendicular to a longitudinal axis of the center electrode, the plane passing through the shoulder portion of the metallic shell.

Abstract: A composite chip formed by joining a discharging layer and a heat stress relieving layer at a joint interface therebetween beforehand is provided on at least one of a central electrode and a ground electrode in its discharge portion made of an electrode material. The discharging layer is made of a precious metal or a precious metal alloy having superior spark- and wear-resistance, and the heat stress relieving layer is made of a metal or an alloy having a linear expansion coefficient between those of the discharging layer and the electrode material. Formed at the joint interface between both the discharging layer and the heat stress relieving layer through mutual diffusion of those materials developed when the two layers are joined to each other is a diffusion layer, in which concentrations of materials of both the layers are continuously changed. A thickness of the diffusion layer is not less than 3 .mu.m in a state that the composite chip is welded to the discharge portion.

Abstract: In a spark plug having a nickel-based electrode whose front end has a firing tip made from a ruthenium- or iridium-based metal in which an oxide of a rare earth metal group is dispersed, the firing tip is welded to the electrode by a solidified alloy layer having a component of the electrode and a component of the firing tip. The firing tip contains the oxide of the rare earth metal group in a range of 5.about.15% by volume (V), and an average grain size (D) of the oxide is in a range of 0.05.about.3.0 .mu.m with a quantitative relationship as D.ltoreq.-0.34V+5.1.

Abstract: In a spark plug, a nickel-alloyed center electrode has a front end which has a constricted end forming a spark gap with a ground electrode by way of a noble metal firing tip. The noble metal firing tip is secured to a front end surface of the constricted end by applying a laser beam welding to an interface between the constricted end and the firing tip all through their circumferential length so as to form a wedge-shaped welding solidification portion.