A method of manufacturing a centre electrode for a spark plug

Abstract: In a method of manufacturing a center electrode for a spark plug, a composite column is provided by enclosing a heat-conductor core into a nickel-alloyed clad by means of plastic working. A front end of the nickel-alloyed clad is severed to define a severing end surface with which an axial bore is provided to reach to a front end of the heat-conductor core. A straight neck portion around the axial bore is provided by diametrically reducing the front end of the nickel-alloyed clad. A firing tip made of precious metal is placed in the axial bore. In this instance, a front end of the firing tip extends beyond the front end of the straight neck portion, while a rear end of the firing tip comes in thermally transferable contact with the front end of the heat-conductor core. An outer surface of the firing tip is bonded to an inner surface of the axial bore by means of laser beam welding or electronic beam welding.

Description: This invention relates to a method of manufacturing a centre electrode for a spark plug in which a corrosion-resistant firing tip is welded to the front end of a centre electrode. In a spark plug for an internal combustion engine, a firing tip is welded to the front end of a centre electrode. It is known that the tip may be made of a noble metal such as platinum-based alloy so as to impart spark erosion resistance to the front end of the centre electrode. It has been contemplated to reduce the diameter of the firing tip in order to maintain good sparking or ignitability condition of the spark plug; the reduced diameter tip, however, is subject to even higher temperatures thus accelerating its spark erosion by oxidation-evaporation and dispersion, although it advantageously concentrates spark discharge to lower the spark discharge voltage. Therefore, the invention is made on the basis of the concept that a heat-conductor core is enclosed in a nickel-alloy so as to provide an heat escape path from the firing tip to the heat-conductor core, thus preventing the temperature of the firing tip from exceedingly rising. It is an object of the invention to provide a method of manufacturing a centre electrode for a spark plug in which a firing tip is provided in improved thermal contact with the heat-conductive core, thus militating against the firing tip being worn, and contributing to an extended service life at reduced cost. US-A-2 955 222 discloses extruding two different metals to form a lower end of a spark plug center electrode, with one of the metals forming a sleeve over the other metal. The document is concerned with avoiding problems with joining the noble metal firing tip onto the center electrode after extrusion. In the Fig. 1-4 embodiment this is achieved by making the firing tip with a frusto-conical head and fitting the tip in a hole formed in the lower block before extrusion - see Fig. 1. The Fig. 5 embodiment is different. In this embodiment the firing tip is welded or soldered into a recess formed in the front surface of the lower billet block. The billet assembly is then extruded "directly into the article shown in Fig. 6". The material forming the recess in the front surface of the lower billet block is, therefore, deformed to cover the enlarged base of the firing tip to retain it in the recess - see Fig. 6. According to one aspect of the present invention, there is provided a method of manufacturing a centre electrode for a spark plug comprising the steps of: providing a composite column by enclosing a heat-conductive core in a nickel-alloy cladding preferably by means of plastic working; severing the front end of the nickel-alloy cladding to define a severed end surface; providing a substantially axial bore at the severed end surface of the nickel-alloy cladding which reaches the heat-conductive core; providing a neck portion around the bore by diametrically reducing the front end of the nickel-alloy cladding; providing a firing tip made of a precious metal in the bore, a front end of the firing tip extending beyond the front end of the neck portion, and a rear end of the firing tip being in thermal contact with the front end of the heat-conductive core; and bonding at each part of the periphery of the firing tip to the inner surface of the bore by means of laser beam welding or electronic beam welding. According to a second aspect of the present invention, there is provided a method of manufacturing a centre electrode for a spark plug the comprising steps of: providing a composite column by enclosing a heat-conductive core in a nickel-alloy cladding by means of plastic working; severing the front end of the nickel-alloy cladding to define a severed end surface; providing a neck portion by diametrically reducing the front end of the nickel-alloy cladding; providing a substantially axial bore at the neck portion of the nickel-alloy cladding which reaches the heat-conductor core; providing a firing tip made of a precious metal in the bore, a front end of the firing tip extending beyond the front end of the neck portion, and a rear end of the firing tip being in thermal contact with the front end of the heat-conductive core; and bonding at least part of the periphery of the firing tip to the inner surface of the bore by means of laser beam welding or electronic beam welding. With the provision of the neck portion, which is preferably substantially straight, the volume of the front end of the nickel-alloy cladding is reduced thus preventing the firing tip from melting due to welding heat when the firing tip is welded to the neck portion. In this structure, the firing tip is in good thermal contact with the heat-conductive core, and provides a path for heat from the firing tip to the heat-conductor core, thus preventing excessive temperature rises in the firing tip thus imparting spark erosion resistance to the firing tip when the firing tip is diametrically reduced. By means of laser beam welding or electronic beam welding, the firing tip may be securely welded to the neck portion and thus be prevented from inadvertently falling off the straight neck portion due to thermal cycling during operation. The step of providing the straight neck portion may precede the step of providing the axial bore. With the provision of the thermal treating step, residual stresses can be removed so as to prevent the centre electrode from unfavourably deforming, and avoiding breaking the tubular insulator of a spark plug during operation. The thermal treating step is preferably be carried out after the welding step. It is preferable that a thermal treating step precedes the step of providing the axial bore if the axial bore is formed by means of blanking. The laser beams are preferably directed at an angle of substantially 45 degrees against the composite column when bonding the outer surface of the firing tip to the inner surface of the axial bore by means of laser beam welding. Preferably the laser beam welding hermetically welds the firing tip to the neck portion without involving the heat-conductor core in the welding portion, and prevents entry of combustion gas into the axial bore. With the relationship among D, d and L as 0.2 mm ≦ (D-d)/2 ≦ 0.5 mm and 0.2 mm ≦ L ≦ 1.0 mm, the firing tip is readily welded to the straight neck portion with stabilized welding strength. If the dimension of (D-d) is less than 0.2 mm, the nickel-alloy cladding can be short of strength so that cracks occur at the rear end of the straight neck portion due to thermal stress. A dimension of (D-d) exceeding 0.5 mm requires an increased output from the welding laser beam so as to melt the firing tip. If the dimension L exceeds 1.0 mm, cracks can occur on a rear end of the straight neck portion due to thermal stress during the heat-cool cycle operation. When the dimension L is less than 0.2 mm, the nickel-alloy cladding tends to absorb a considerable amount of heat at the time of welding the firing tip to the neck portion, thus requiring an increased output from the welding laser beam to melt the firing tip. Unless the clearance between the outer diameter of the firing tip and the inner diameter of the neck portion is less than 0.05 mm, an increased number of voids appear in the welding portion between the firing tip and the neck portion. A length of the firing tip exceeding 1.5 mm reduces the heat-dissipating properties of the tip thus increasing its operating temperature resulting in an increased amount of spark-erosion. By providing a flange at the front end of the firing tip, spark-erosion resistance is imparted to the firing tip, while reduced temperature of the centre electrode is maintained. The laser beams are preferably directed substantially along the interface between the flange and the front end of the neck portion when bonding the firing tip to the straight neck portion by means of the laser beam welding. Thus the welding portion makes it possible to hermetically seal the interface between the flange and the front end of the straight neck portion. By providing a recess at the front end surface of the flange, the flange is divided into plural areas, thus increasing the intensity of the electrical field between the centre electrode and an outer electrode and lowering the spark discharge voltage therebetween. The recess is provided prior to inserting the firing tip to the axial bore, so that shape of the recess can be precisely maintained resulting in reduced variation of the spark discharge voltage. Furthermore, pressure can be applied in the direction in which the firing tip is brought into engagement with the heat-conductive core after positioning the firing tip in the axial bore. This makes it possible to strengthen the direct engagement between the firing tip and the heat-conductive core, improving thermal contact. In addition, if the pressure is applied when the firing tip is bonded to the neck portion by means of laser beam welding, the contact is further improved. The invention will be further understood from the following description, when taken together with the attached drawings, which are given by way of example only, and in which: Fig. 1 is an enlarged longitudinal cross sectional view of a main part of a centre electrode according to the invention; Figs, 2a through 2f are sequential process views showing how the centre electrode is manufactured; Fig. 3 is an explanatory view showing how inconvenience appears when electrical resistance welding is carried out; Fig. 4 is graph showing the relationship between thermal expansion of the centre electrode (mm) and heat-treatment temperature (°C) in a third embodiment of the invention; Fig. 5 is an explanatory cross sectional view showing how the centre electrode may be manufactured according in a fourth embodiment of the invention; Fig. 6 is an explanatory cross sectional view showing how the centre electrode may be manufactured according in a fifth embodiment of the invention; Fig. 7 is an explanatory cross sectional view showing how inconvenience appears when (D-d) is less than 0.2 mm; Fig. 8 is an explanatory cross sectional view showing how inconvenience appears when (D-d) exceeds 0.5 mm; Fig. 9 is an explanatory cross sectional view showing how the centre electrode may be manufactured according to a sixth embodiment of the invention; Fig. 10 is graph showing a relationship between occurrence of voids (%) and clearance (R) in the sixth embodiment of the invention; Fig. 11 is an explanatory cross sectional view showing how a center electrode is manufactured according in a seventh embodiment of the invention; Fig. 12a is an enlarged longitudinal cross sectional view of a main part of a center electrode according to an eighth embodiment of the invention; Fig. 12b is a plan view of a flange of the firing tip in Fig. 12a; Fig. 13 is a plan view of a flange of a firing tip in a ninth embodiment of the invention; Fig. 14 is a plan view of a flange of a firing tip in a tenth embodiment of the invention; Fig. 15 is an explanatory cross sectional view showing how the center electrode may be manufactured in an eleventh embodiment of the invention; and Fig. 16 is an explanatory cross sectional view showing how the center electrode may be manufactured in a twelfth embodiment of the invention. Referring to Fig. 1, a center electrode (A) for a spark plug has a clad sheath 1 whose front end has a diameter-reduced straight neck 11. Within the clad sheath 1, is a core 2 extruded. To the straight neck 11, is a firing tip 3 inserted. A rear end of the core 2 may be exposed to outside from a rear end of the clad sheath although not shown. The center electrode (A) thus assembled is manufactured as follows: (1) A composite column 401 is made by plastic working such as extruding a heat-conductor core 201 into a nickel-alloyed clad 101 as shown in Fig. 2a. The heat-conductor core 201 is made of copper, while the nickel-alloyed clad 101 made of Inconel by way of illustration. (2) A front end 402 of the nickel-alloyed clad 101 is severed to provide a front end surface 404 of the clad 101 as shown in Fig. 2b. The front end surface 404 of the clad101 is milled to expose a front end 201a of the heat-conductor core 201 from the front end surface 404 of the clad 101. (3) A drilling tool is applied on a center of the front end surface 404 of the clad 101 to provide an axial bore 11a concentrically with the nickel-alloyed clad 101 and the heat-conductor core 201 as shown in Fig. 2c. The axial bore 11a is provided by means of blanking. The axial bore 11a is circular in section, and equi-diameter all through its depth. Such is the depth of the axial bore 11a as to be lengthwisely equivalent to a rear end portion of the firing tip 3 which is to be inserted into the axial bore 11a as described below. The firing tip 3 is made of precious metal such as an iridium-alloy including Y 2 O 3 (2.5 wt%) for example. (4) A front end 407 of the nickel-alloyed clad around axial bore 11a is diametrically reduced to form a straight neck tube 11 which is concentrical with the axial bore 11a as shown in Fig. 2d. (5) The columnar firing tip 3 is inserted to the axial bore 11a. A rear end 30 of the tip 3 comes in thermally transferable contact with the front end of the heat-conductor core 201, while a front end of the tip 3 slightly extends beyond the straight neck tube 11 as shown in Fig. 2e. (6) A laser beam welding makes it possible to securely bond an interface between an outer surface 11x of the firing tip 3 and an inner surface 11y of the axial bore 11a so as to form a welding portion (g) as shown in Fig. 2f. It is appreciated that the step of providing the straight neck tube 11 precedes the step of providing the axial bore 11a so as to serve as a second embodiment of the invention. The following advantages are obtained. (i) The rear end 30 of the firing tip 3 comes in thermally transferable contact with the front end of the heat-conductor core 201, thus enabling to provide a heat escape path with the firing tip 3 by way of the heat-conductor core 201. This makes it possible to avoid an excessive temperature rise of the firing tip 3 to significantly reduce its spark erosion when the tip 3 is diametrically thin. (ii) The straight neck tube 11 enables to favorably ignite combustion gas consecutively, and sparking the center electrode (A) with a reduced discharge voltage. (iii) With the use of the laser beam welding, the beams is readily focused so that the laser beam welding is, as well as an electronic beam welding, suited for welding the firing tip 3 to the straight neck tube 11. With an employment of an electrical resistant welding, an electrical current (I) flows from a noble metal tip to a copper core, thus failing to strongly bond the tip to a front end of a nickel-alloyed metal as shown in Fig. 3. An employment of an argon welding (TIG) makes it difficult to control an amount of output heat so as to melt the firing tip, thus rendering it difficult to keep the tip in original good shape. After carrying out the step (5) of Fig. 2e and previously before the step (6) of Fig. 2f, is a thermal treatment provided with the firing tip 3 and the composite column 401 in a vacuum atmosphere over recrystalization temperature for more than 30 minutes so as to serve as a third embodiment of the invention. By way of example, when the thermal treatment is carried out in a vacuum atmosphere under the conditions of 900 °C × 1 Hr., it is found that the thermal expansion of the center electrode (A) falls on 0.01 mm as shown in Fig. 4 after conducting a burner experiment in which the center electrode (A) is subjected alternately to heat-cool cycle 1000 times from 900 °C × 1 min. to 100 °C × 1 min. On the contrary, the thermal expansion of the center electrode reaches 0.06 mm when non-thermal treatment as evidenced by Fig. 4. Fig. 5 shows a fourth embodiment of the invention in which laser beams 5a, 5b are directed to form an angle of 45 degrees against an axial direction (j) of the center electrode (A). The laser beams 5a, 5b impinges on a front end surface 11b of the straight neck tube 11 to cover between an innermost edge 52 and outermost edge 51 of the straight neck tube 11. This way of welding enables to hermetically seal the interface between the firing tip 3 and the straight neck tube 11 without involving the heat-conductor core 201 in the welding portion (g), thus preventing the tip 3 from bulging out of the straight neck tube 11, and at the same time, effectively avoiding an inflow of the combustion gas into the straight neck tube 11. Figs. 6 through 8 show a fifth embodiment of the invention in which the firing tip 3 is made of an iridium alloy including Y 2 O 3 (2.5 wt%), and is 0.5 mm in diameter, while the nickel-alloyed clad 101 is made of Inconel 600. A thickness (D-d) of the straight neck tube 11 is 0.3 mm, while a length (L) of the straight neck tube 11 is 0.6 mm as shown in Fig. 6. The thickness (D-d) is equivalent to a difference between an outer diameter (D) of the straight neck tube 11 and an inner diameter (d) of the axial bore 11a. In order to examine an optimum range of the thickness (D-d) and the length (L), an experiment is carried out with the center electrode mounted on a 2000 cc, six-cylinder engine which is alternately operated in accordance with heat-cool cycle from full throttle 5000 rpm × 1 min. to an idling rpm × 1 min. As a result, it is found that it is favorable when the thickness (D-d) falls within a range from 0.2 mm to 0.5 mm (more preferably 0.25 mm ∼ 0.35 mm), while the length (L) within a range from 0.2 mm to 1.0 mm (more preferably 0.5 mm ∼ 0.8 mm). If the thickness (D-d) is less than 0.2 mm, the nickel-alloyed clad 101 becomes short of sufficient strength, and cracks (k) appear on a rear end 11c of the straight neck tube 11 due to thermal stress as shown in Fig. 7. In the meanwhile, the thickness exceeding 0.5 mm results in an increased output of the laser beam welding which melts the firing tip 3 as shown in Fig. 8. The length (L) exceeding 1.0 mm causes cracks appeared on the straight neck tube 11 due to thermal stress, while the length (L) less than 0.2 mm contributes for the straight neck tube 11 to absorb a large amount of heat at the time of welding the firing tip 3, thus requiring an increased output for the laser beam welding to compensate an amount of heat absorbed by the nickel-alloyed clad 101, thus causing to melt the firing tip and the nickel-alloyed clad. Figs. 9 and 10 show a sixth embodiment of the invention in which the length of the firing tip 3 is less than 1.5 mm inclusive, while an outer diameter of the firing tip 3 is smaller than the diameter (d) of the axial bore 11a by at most 0.05 mm as shown in Fig. 9. When a clearance (R) between the diameter (d) of the axial bore 11a and the outer diameter of the firing tip 3 exceeds 0.05 mm, it is found that voids (v) appear on the welding portion (g). It is also found that the length of the firing tip 3 exceeding 1.5 mm causes to be short of its heat-dissipating effect, and bring an excessive temperature rise so as to accelerate the spark erosion. Fig. 10 shows a relationship between the occurrence of voids (v) and the clearance (R) when the firing tip 3 is 1.3 mm in length and 0.5 mm in diameter, while the straight neck tube 11 is 0.6 mm in length and 1.1 mm in diameter. Fig. 11 shows a seventh embodiment of the invention in which a front end of a firing tip 3a integrally has a circular flange 31 whose diameter (1.2 mm) is equivalent to that of the straight neck tube 11. The firing tip 3a is made of platinum alloy into which zirconia (0.06 ∼ 0.3 wt%) is dispersed to enhance its mechanical strength. An employment of the laser beam welding makes it possible to bond an interface (Int) between an upper surface 31a of the flange 31 and the front end surface of the straight neck tube 11. In this instance, the laser beams are directed to the interface (Int) to form an angular range from 70 to 110 degrees against the axial direction (j) of the center electrode (A). In this embodiment, the flange 31 acts as a spark-corrosive surface, while a diameter-reduced portion 32 of the firing tip 3a serves as a provider of the heat escape path toward the heat-conductor core 201 so as to improve the spark-erosion property and avoiding the excessive temperature rise. Figs. 12a and 12b show an eighth embodiment of the invention in which a front end of a firing tip 3b integrally has a circular flange 31b. The firing tip 3b is made of platinum alloy including zirconia (0.06 wt%). By means of header process, a criss cross groove 33 is provided with an lower surface of the flange 31b prior to inserting the firing tip 3b to the axial bore 11a. Fig. 13 shows a nineth embodiment of the invention in which a front end of a firing tip 3c integrally has a circular flange 31c. The firing tip 3b is made of platinum alloy including zirconia (0.06 wt%). By means of header process, a criss cross groove 33a is provided with a lower surface of the flange 31c prior to insering the firing tip 3a to the axial bore 11a. A width of the groove 33a is somewhat larger than that of the groove 33 of Fig. 12b. Fig. 14 shows a tenth embodiment of the invention in which a front end of a firing tip 3d integrally has a circular flange 31d. The firing tip 3d is made of platinum alloy including zirconia (0.06 wt%). By means of header process, a criss cross recess 34 is provided with a lower surface of the flange 31d prior to inserting the firing tip 3d to the axial bore 11a. In Figs. 12a, 12b, 13 and 14, various tangible dimensions are depicted concerning to the grooves 33, 33a and the recess 34. With the grooves 33, 33a and the recess 34, the flanges 31b, 31c and 31d are respectively divided into plural areas to increase an intensity of an electrical field between electrodes so as to discharge therebetween with a reduced voltage. With the grooves 33, 33a and the recess 34 each provided with the respective flanges prior to inserting the corresponding firing tips to the axial bore 11a, the divided shape of the grooves and the recess are rigidly kept with least variation of the spark discharge voltage. Fig. 15 shows an eleventh embodiment of the invention in which the firing tip 3 is 1.5 mm in length, and the diameter of the tip 3 is smaller than that of the axial bore 11a by at most 0.05 mm. After inserting the firing tip 3 to the axial bore 11a, the firing tip 3 is pressed by a mandrel (P) in the direction in which the tip 3 is brought into engagement with the front end of the heat-conductor core 201 concurrently when the firing tip 3 is bonded to the straight neck tube 11 by means of laser beam welding. Fig. 16 shows a twelfth embodiment of the invention in which the firing tip 3a described at the seventh embodiment in Fig. 11 is employed. After inserting the firing tip 3a to the axial bore 11a, the firing tip 3a is pressed by the mandrel (P) in the direction in which the tip 3 is brought into engagement with the front end of the heat-conductor core 201 concurrently when the firing tip 3a is bonded to the straight neck portion 11 by means of the laser beam welding. In each of the embodiments, the firing tip is brought in tight contact with the heat-conductor core 201 so as to achieve the heat-dissipating effect.

Claims: A method of manufacturing a centre electrode for a spark plug comprising the steps of: providing a composite column by enclosing a heat-conductive core in a nickel-alloy cladding preferably by means of plastic working; severing the front end of the nickel-alloy cladding to define a severed end surface; providing a substantially axial bore at the severed end surface of the nickel-alloy cladding which reaches the heat-conductive core; providing a neck portion around the bore by diametrically reducing the front end of the nickel-alloy cladding; providing a firing tip made of a precious metal in the bore, a front end of the firing tip extending beyond the front end of the neck portion, and a rear end of the firing tip being in thermal contact with the front end of the heat-conductive core; and bonding at each part of the periphery of the firing tip to the inner surface of the bore by means of laser beam welding or electronic beam welding. A method of manufacturing a centre electrode for a spark plug the comprising steps of: providing a composite column by enclosing a heat-conductive core in a nickel-alloy cladding by means of plastic working; severing the front end of the nickel-alloy cladding to define a severed end surface; providing a neck portion by diametrically reducing the front end of the nickel-alloy cladding; providing a substantially axial bore at the neck portion of the nickel-alloy cladding which reaches the heat-conductor core; providing a firing tip made of a precious metal in the bore, a front end of the firing tip extending beyond the front end of the neck portion, and a rear end of the firing tip being in thermal contact with the front end of the heat-conductive core; and bonding at least part of the periphery of the firing tip to the inner surface of the bore by means of laser beam welding or electronic beam welding. A method according to claim 1 or 2, further comprising the step of thermally treating the composite column and the firing tip to remove residual stress therefrom. A method according to claim 1, 2 or 3, further comprising the step of directing laser beams, preferably at an angle of 45 degrees, against the composite column at the time of bonding the periphery of the firing tip to the inner surface of the bore by means of the laser beam welding. A method according to any one of claims 1 to 4, wherein a relationship among D, d and L is as follows: 0.2 mm ≤ (D-d)/2 ≤ 0.5 mm, 0.2 mm ≤ L ≤ 1.0 mm where D is the outer diameter of the neck portion, d is the inner diameter of the bore, L is the length of the neck portion. A method according to any one of claims 1 to 5, wherein the length of the firing tip is 1.5 mm, and the diameter of the firing tip is less than the diameter of the bore by at most 0.05 mm. A method according to any one of claims 1 to 3, wherein a flange is provided at the front end of the firing tip, the diameter of the flange being substantially equal to the outer diameter of the straight neck portion. A method according to claim 7, wherein laser beams are directed at the interface between the flange and the front end of the neck portion when bonding the firing tip to the neck portion by means of the laser beam welding. A method according to claim 7 or 8, wherein a recess is provided at the front surface of the flange prior to inserting the firing tip in the bore. A method according to any preceding claim further comprising the step of applying pressure in the direction in which the firing tip is brought into engagement with the heat-conductive core after positioning the firing tip in the bore. A method according to claim 10, wherein pressure is applied as the firing tip is bonded to the neck portion by means of the laser beam welding. A spark plug comprising a centre electrode made according to the method of any preceding claim.

Inventors: OKAYAMA TSUTOMU, OSHIMA TAKAFUMI, OKAYAMA, TSUTOMU, OSHIMA, TAKAFUMI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1992
Main classification: H01T21
IPC codes: B23K26, H01T21, H01T13, B23K15