A spark plug and a method of making the same

Abstract: A spark plug includes a cylindrical metallic shell having an insulator in which a center electrode is placed, a front end of which somewhat extends beyond the insulator; a noble metal portion is boded to an outer side wall of the front end of the center electrode by means of laser beams so as to form a spark gap between the noble metal portion and a ground electrode which is depended from the metallic shell. The noble metal portion has a molten alloy layer in which the center electrode is melted into the noble metal portion, and a diffused alloy layer in which the noble metal portion is diffused into the center electrode.

Description: This invention relates to a spark plug and a method of making a spark plug in which a spark-erosion resistant noble metal is secured to the front end of its center electrode. In a spark plug for an internal combustion engine, a noble metal tip is welded to the front end of its center electrode or a discharge end of a ground electrode so as to increase a spark-erosion resistance property. The noble metal tip is usually made of Pt, Pd, Ir or alloys with these metals as main components. The tip is usually bonded to the electrode by means of electric resistance welding, and then the tip is extruded together with the front end of the center electrode as shown in Japanese Patent Application Publication No. 62-31797. The electric resistance welding causes a diffused alloy layer in which the noble metal is diffused into the front end of the center electrode. Considering that the center electrode is made of a nickel-based alloy, a stress concentration occurs at an interface between the noble metal tip and the front end of the center electrode due to repeated thermal stress caused by differential thermal expansion therebetween when the center electrode is repeatedly exposed to heat-cool cycles in a combustion chamber. For this reason, there arises a possibility of cracks developing at the interface between the tip and the center electrode so that the tip falls off the center electrode when the cracks are sufficiently large. Therefore, it is one of the objects of the invention to provide a spark plug and a method of making the spark plug in which a noble metal portion may be secured to a center electrode which can prevent the noble metal portion from become detached from the center electrode, and thus contributing to an extended service life. JP-A-1289084, on which the precharacterising portions of appended claims 1 and 3 are based, discloses a spark plug in which a ring-form flange is geared to the tip of a centre electrode and connected by laser welding. WO-A-89/01717 discloses a process for manufacturing a spark plug. A noble metal is deposited on the front surface of the metal electrode by laser radiation so as to form an alloy region which compensates for different coefficients of thermal expansion of the electrode and the coating. EP-A-0,549,368, which is relevant to the present invention only by virtue of Article 54(3) EPC, discloses forming a firing tip on a spark plug electrode by placing a slug of nobel metal concentrically on an end surface of the metallic clad of the electrode. Laser beam welding is applied to the slug to thermally melt the slug such that the end surface of the metallic clad is partly fused into the slug. According to a first aspect of the present invention, there is provided a spark plug comprising: a cylindrical metallic shell having a tubular insulator in which a columnar center electrode is positioned, a front end of the center electrode extending beyond the insulator; a noble metal portion laser beam welded to an outer side wall of the front end of the center electrode so as to form a weld zone of sash-like formation around the outer side wall of the front end of the center electrode and defining a spark gap (Gp) between the noble metal portion and a ground electrode; characterised in that the weld zone comprises a first alloy layer in which, the center electrode has melted into the noble metal portion, and a second alloy layer in which noble metal has diffused into the center electrode. According to a second aspect of the present invention, there is provided a method of making a spark plug comprising positioning a noble metal wire or nugget in the form of a sash-like formation around the outer side wall of the front end of the center electrode so as to define a spark gap (Gp) between the noble metal and a ground electrode; characterised by laser beam welding the noble metal wire or nugget to form a weld zone at an outer side wall of the front end of the center electrode comprising a first alloy layer in which the center electrode has melted into the noble metal portion, and a second, diffused alloy layer in which the noble metal has diffused into the center electrode. The provision of the molten alloy layer makes it possible to decrease the thermal stress between the noble metal portion and the center electrode, and can prevent the growth of cracks thus protecting the noble metal portion against peeling of the center electrode when the center electrode is repeatedly exposed to heat-cool cycles in a combustion chamber. The present invention will be further understood from the following description, when taken together with the accompanying drawings, which are given by way of example only, and in which: Fig. 1 is a side view of a spark plug; Fig. 2 is an enlarged cross sectional view of the front portion of the spark plug; Figs. 3a and 3b are perspective views of a front end of a center electrode showing how a noble metal portion is welded to the center electrode in a first embodiment of the invention; Fig. 4 is an enlarged longitudinal cross-sectional view of the front end of the center electrode; Fig. 5 is a perspective view of a front end of a center electrode showing a noble metal portion welded to the center electrode in the second embodiment of the invention; Fig. 6 is an enlarged longitudinal cross-sectional view of a firing end of the spark plug of a third embodiment of the invention; Fig. 7a is an enlarged perspective view of a firing end of the spark plug of a fourth embodiment of the invention; Fig. 7b is an enlarged longitudinal cross-sectional view of a firing end of the spark plug in a fourth embodiment of the invention; Fig. 8 is a graph showing how an endurance time period changes depending on how much the content of the metal of the center electrode in the noble metal portion; and Fig. 9 is a graph showing how the spark gap increment changes with service life according to the content of the metal of the center electrode in the noble metal portion. Fig. 1 shows a spark plug 100 according to a first embodiment of the invention; the spark plug 100 has a cylindrical metallic shell 2, to a front end of which a ground electrode 1 is secured by means of welding. Within the metallic shell 2, a tubular insulator 3 is concentrically supported through a packing 22 by resting a stepped portion 31 of the insulator 3 on a shoulder portion 21 which is provided with an inner wall of the metallic shell 2 as shown in Fig. 2. The metallic shell 2 has a rear head 23 inturned to engage on the insulator 3 by means of caulking so as to secure the insulator 3 against the removal. In the meanwhile, an inner space of the insulator 3 serves as an axial bore 32 in which a center electrode 4 is placed. A front end of the center electrode 4 extends somewhat beyond the insulator 3 to be in flush with the front end of the ground electrode 1, and at the same time, forming a spark gap (Gp) with the ground electrode 1 through a noble metal portion 6 described hereinafter in detail. The center electrode 4 and a Terminal electrode 5 are heat sealed in the insulator 3 by an electrically conductive glass sealant 52. The spark plug 100, thus structured, is secured to a cylinder head of the internal combustion engine by way of a gasket 25 and a threaded portion 24 provided at an outer surface of the metallic shell 2. The center electrode 4 has a composite column 40 including a clad metal 42 and a heat-conductive core 43 embedded in the clad metal 42. The clad metal 42 is made of a nickel-based alloy including 8.0 % iron (Fe) and 15.0 % chromium (Cr), while the heat-conductive core 43 made of an alloyed metal with a copper (Cu) or silver (Ag) as a main component. To an outer wall 41 of the front end of the composite column 40, the noble metal portion 6 is provided to form the spark gap (Gp) with a discharge end 11 of the ground electrode 1. The ground electrode 1 is made of a nickel-based alloy including chromium (Cr) and iron (Fe). In this instance, the ground electrode 1 may be made in integral with the metallic shell 2. The noble metal portion 6 is made of a noble metal material 60 such as platinum (Pt), iridium (Ir), Pt-Ir alloy, Pt-Ni alloy or Ir-alloy including oxides of rare earth metals. The noble metal material 60 is welded to the composite column 40 of the center electrode 4 as follows: (i) The noble metal material 60 is prepared into a ring-shape configuration, an inner diameter of which is substantially the same as an outer diameter of the noble metal ring 60. Then, the ring 60 is fitted to the outer side wall 41 the composite column 40 of the center electrode 4, and provisionally held in place by an appropriate means as shown in Fig. 3a. (ii) As shown in Fig. 3b, the laser beam welding is carried out by using YAG (yttrium, aluminum and garnet) laser beams (L) emitted at 10 mm underfocus with one shot energy and pulse duration of 6.5 Joules and 2.0 milliseconds respectively. The laser beams (L) are intermittently applied several times perpendicular to the noble metal ring 60 while rotating the composite column 40 of the center electrode 4 in a direction shown at arrow (Aw) in Fig. 3b. The laser beams (L) make it possible to melt the noble metal ring 60 and the outer side wall 41 of the composite column 40 simultaneously to form sash-like noble metal portion 6. The noble metal portion 6 includes a molten alloy layer 61 in which a component of the clad metal 42 of the composite column 40 is thermally fused into the noble metal ring 60, and a diffused alloy layer 63 in which the noble metal ring 60 is diffused into the outer side wall 41 of the clad metal 42 of the composite column 40 between the molten alloy layer 61 and the clad metal 42 of the composite column 40 as shown in Fig. 4. The molten alloy layer 61 contains a component of the clad metal 42 in the range of 0.5 ∼ 80.0 % by weight. The diffused alloy layer 63 has a width extending from several µm to several hundred µm. In the diffused alloy layer 63, the diffused degree of the noble metal progressively decreases in the layer 63 away from a base end 62 of the molten alloy layer 61. The component of the clad metal 42 is melted into the base end 62 of the molten alloy layer 61 so that the thermal expansional coefficient of the base end 62 aproaches that of the clad metal 42. With the formation of the diffused alloy layer 63 and the base end 62 of the molten alloy layer 61, it is possible to prevent the thermal stress from locally affecting the welded portion when the center electrode is exposed to the repeated heat-cool cycle. It also decreases the thermal stress itself by reducing the differing degree of the thermal expansional coefficients in the direction from the welded portion to the clad metal 42. This makes it possible to prevent the growth of cracks at the welded portion or in the proximity of the welded portion so as to prevent the molten alloy layer 61 from peeling off the outer side wall 41 of the clad metal 42. Figs. 5a and 5b in turn show second and third embodiments of the invention. As shown in Fig. 5a, the noble metal material 60 is initially in the form of wire in the second embodiment of the invention. A leading end 64 of the noble metal wire 60 is placed around the outer side wall 41 of the front end of the composite column 40 while applying the laser beams (L) to the leading end 64 so as to form the annular noble metal portion 6 all around the outer side wall 41 in the same manner as described in the first embodiment of the invention. In this instance, the rotation of the composite column 40 accompanies with the laser beam welding operation. The use of the noble metal wire 60 eliminates the necessity of forming the noble metal material into the ring-shape configuration, and obviating the provisional holding of the noble metal wire 60 in place at the time of welding it to the outer wall 41 of the composite column 40, thus making it possible to advantageously reduce the number of assembly processes. Fig. 6 shows a third embodiment of the invention in which the diametrically opposed ground electrodes 1 project into a combustion chamber of the internal combustion engine. The noble metal material 60 is laser-welded to the outer side wall 41 of the composite column 40 to be in registration with a discharge end 11 of the ground electrode 1 so as to form the noble metal portion 6. Figs. 7a and 7b show a fourth embodiment of the invention in which a surface discharge gap (Ga) and an air gap (Gb) are provided in a semi-surface-discharge type spark plug. The noble metal material 60 is laser-welded to the outer side wall 41 of the composite column 40 to provide the noble metal portion 6. The surface-discharge gap (Ga) is a distance measured along a discharge-surface 33 between a noble metal portion 6 and an outer surface of the insulator 3. The air gap (Gb) is a distance between an outer surface 34 of the insulator 3 and the discharge end 11 of the ground electrode 1 as shown in Fig. 7b. Fig. 8 is a graph showing how many hours are required for the noble metal portion 6 to peel off the clad metal 42 depending on how much the molten layer 61 contains the component of the clad metal 42. The graph is obtained after carrying out an endurance heat-cool cycle alternately between a full throttle (5000 rpm) for 1 min. and an idle operation for 1 min. with a spark plug (A) and a prior art counterpart mounted on an internal combustion engine (six-cylinder, 2000 cc) respectively. In the prior art counterpart, a noble metal portion is provided by means of electric resistance welding. It is found from Fig. 8 that it takes much longer for the noble metal portion 6 to peel off the outer side wall 41 compared to prior art counterpart when the alloy layer 61 contains the component of the clad metal 42 more than 0.5 % by weight. Fig. 9 is a graph showing how the spark gap increment changes depending on how much the molten layer 61 contains the component of the clad metal 42. The graph is obtained after carrying out an endurance test at full throttle (5500 rpm) with spark plugs (B) ∼ (D) mounted on an internal combustion engine (four-cylinder, 1600 cc) respectively. In the spark plugs (B) ∼ (D), the molten alloy layer 61 in turn contains the component of the clad metal 42 by 90 %, 80 %, 20 % and 10 % by weight. It is found from the endurance test that the spark gap increment augments to accelerate the spark erosion of the clad metal 42 when the molten alloy layer 61 contains the component of the clad metal 42 excessively. Although a relatively small amount of the spark erosion is maintained in the prior art counterpart in which the noble metal tip is provided by means of electric resistance welding, it is possible to control the spark erosion by selecting the kind of the noble metal material 60 and the shooting condition of the laser beams (L) as shown at the spark plug (E) in Fig. 9. With the use of the noble metal portion 6, its flake-resistant property is significantly improved with relatively low cost as evidenced by Fig. 8, it is sufficiently enough to put the spark plug into practical use as long as the molten alloy layer 61 contains the component of the clad metal 42 by 80 % or less. As apparent from the foregoing description, the noble metal portion 6 has a molten alloy layer 61 which contains the component of the clad metal 42, thus making it possible to effectively prevent the development and growth of the cracks at the welding portion or in the neighborhood of the welding portion so as to lead to a long service life. It is noted that the insulator 3 may be made by ceramic material with magnesia as a main component. Further, it is also appreciated that the ground electrode 1 may be made of a composite column in which a copper core is embedded in a clad metal in the same manner as the center electrode 4 is assembled in the embodiment of the invention.

Claims: A spark plug comprising: a cylindrical metallic shell (2) having a tubular insulator (3) in which a columnar center electrode (4) is positioned, a front end of the center electrode extending beyond the insulator (3); a noble metal portion (6) laser beam welded to an outer side wall of the front end of the center electrode (4) so as to form a weld zone of sash-like formation (60) around the outer side wall of the front end of the center electrode (4) and defining a spark gap (Gp) between the noble metal portion (6) and a ground electrode (1); characterised in that the weld zone comprises a first alloy layer (61), in which the center electrode (4) has melted into the noble metal portion (60), and a second alloy layer (63) in which noble metal has diffused into the center electrode (4). A spark plug according to claim 1, wherein the first, molten alloy layer (61) of the noble metal portion contains a concentration of the metal of the center electrode (4) in the range of about 0.5 - 80% by weight. A method of making a spark plug comprising positioning a noble metal wire or nugget (6) in the form of a sash-like formation (60) around the outer side wall of the front end of the center electrode (4) so as to define a spark gap (Gp) between the noble metal (6) and a ground electrode (1); characterised by laser beam welding the noble metal wire or nugget (6) to form a weld zone at an outer side wall of the front end of the center electrode (4) comprising a first alloy layer (61) in which the center electrode (4) has melted into the noble metal portion (6), and a second, diffused alloy layer (63) in which the noble metal (6) has diffused into the center electrode (4). A method according to claim 3, wherein the laser beam welding is effected with a one-shot energy of 6.5 Joules. A method according to claim 4, wherein the first molten alloy layer (61) of the noble metal portion contains a concentration of the metal of the center electrode (4) in the range of about 0.5 - 80% by weight. An internal combustion engine comprising a spark plug according to or made according to the method of any preceding claim.

Inventors: KAGAWA JUNICHI, MATSUTANI WATARU, KAGAWA, JUNICHI, MATSUTANI, WATARU
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1993
Main classification: H01T13
IPC codes: H01T21, H01T13