EP1519460B1 - Method for producing a spark plug and spark plug - Google Patents
Method for producing a spark plug and spark plug Download PDFInfo
- Publication number
- EP1519460B1 EP1519460B1 EP04255860.1A EP04255860A EP1519460B1 EP 1519460 B1 EP1519460 B1 EP 1519460B1 EP 04255860 A EP04255860 A EP 04255860A EP 1519460 B1 EP1519460 B1 EP 1519460B1
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- EP
- European Patent Office
- Prior art keywords
- noble metal
- tip
- side face
- spark plug
- molten bond
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 238000004519 manufacturing process Methods 0.000 title description 2
- 229910000510 noble metal Inorganic materials 0.000 claims description 63
- 238000003466 welding Methods 0.000 claims description 28
- 229910052751 metal Inorganic materials 0.000 claims description 20
- 239000002184 metal Substances 0.000 claims description 20
- 238000000034 method Methods 0.000 claims description 20
- 239000012212 insulator Substances 0.000 claims description 10
- 229910045601 alloy Inorganic materials 0.000 claims description 6
- 239000000956 alloy Substances 0.000 claims description 6
- 229910000990 Ni alloy Inorganic materials 0.000 claims description 4
- 230000002093 peripheral effect Effects 0.000 claims description 4
- 229910001260 Pt alloy Inorganic materials 0.000 claims description 3
- 229910002845 Pt–Ni Inorganic materials 0.000 claims 1
- 229910018967 Pt—Rh Inorganic materials 0.000 claims 1
- 229910000629 Rh alloy Inorganic materials 0.000 claims 1
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 6
- 230000005684 electric field Effects 0.000 description 6
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Substances [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- 239000007769 metal material Substances 0.000 description 4
- 229910052759 nickel Inorganic materials 0.000 description 4
- 230000003647 oxidation Effects 0.000 description 4
- 238000007254 oxidation reaction Methods 0.000 description 4
- 229910000575 Ir alloy Inorganic materials 0.000 description 3
- 229910052697 platinum Inorganic materials 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 2
- 229910001055 inconels 600 Inorganic materials 0.000 description 2
- 229910052741 iridium Inorganic materials 0.000 description 2
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 229910052703 rhodium Inorganic materials 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N Alumina Chemical compound [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 238000000641 cold extrusion Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 238000005246 galvanizing Methods 0.000 description 1
- 230000017525 heat dissipation Effects 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 238000004381 surface treatment Methods 0.000 description 1
- 230000002195 synergetic effect Effects 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01T—SPARK GAPS; OVERVOLTAGE ARRESTERS USING SPARK GAPS; SPARKING PLUGS; CORONA DEVICES; GENERATING IONS TO BE INTRODUCED INTO NON-ENCLOSED GASES
- H01T13/00—Sparking plugs
- H01T13/20—Sparking plugs characterised by features of the electrodes or insulation
- H01T13/39—Selection of materials for electrodes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01T—SPARK GAPS; OVERVOLTAGE ARRESTERS USING SPARK GAPS; SPARKING PLUGS; CORONA DEVICES; GENERATING IONS TO BE INTRODUCED INTO NON-ENCLOSED GASES
- H01T21/00—Apparatus or processes specially adapted for the manufacture or maintenance of spark gaps or sparking plugs
- H01T21/02—Apparatus or processes specially adapted for the manufacture or maintenance of spark gaps or sparking plugs of sparking plugs
Definitions
- the present invention relates to a method of producing a spark plug, and also to a spark plug.
- JP-A-2002-237365 discloses in Fig. 26 a ground electrode in which a noble metal tip is laser-welded to an inner side face that is formed into a tapered shape as it advances toward the tip end side, so as to protrude from the inner side face.
- the present invention has been achieved in view of the above-described problem. It is therefore an object of the present invention to provide a method of producing a spark plug in which the reliability of a molten bond can be improved.
- the above object has been achieved by providing a method of producing a spark plug as defined in claim 1 and a spark plug as defined in claim 6.
- the laser welding is conducted after the noble metal tip primarily containing a noble metal is positioned such that the minimum distance between either of the tapered faces and the tip end face of the ground electrode, and the noble metal tip is set to 0.1 mm or more and 0.8 mm or less.
- the laser welding of the noble metal tip when the irradiation angle of a laser beam is about ⁇ 20° with respect to an extension face of the inner side face of the ground electrode to which the discharge portion is to be bonded, the laser welding can be stably conducted.
- the laser beam can be focused within a range of about 0.8 mm or less.
- Nickel which is the principal component of the ground electrode base member is more easily melted than the noble metal tip primarily containing a noble metal. Because of these reasons, when laser welding is conducted after positioning the discharge portion in accordance with the invention as described above, the laser welding can be stably conducted.
- the distance between the tip end face of the noble metal tip and the molten bond is increased.
- the height (t) of the unmelted portion of the noble metal tip is set to 0.3 mm or more.
- the height (t) of the unmelted portion is defined by the minimum distance between the tip end face of the noble metal tip and the molten bond.
- the spark plug Since the spark plug has a discharge portion in which the diameter is 0.8 mm or less and the height is 0.5 mm or more, the electric field strength is easily concentrated at the tip end of the discharge portion. Moreover, the molten bond is formed so as to extend from the inner side face to the tapered faces, and a corner formed between the inner side face and the tapered faces has a rounded shape. Therefore, concentration of electric field strength hardly occurs in these portions. Because of a synergistic effect due to these two structural features, the electric field strength is concentrated at the tip end of the discharge portion, and hence stable spark discharge is enabled at a low discharge voltage. The spark plug is formed by laser-welding the noble metal tip after the tapered faces are formed. Therefore, the molten bond is not broken by vibrations in the process of forming the tapered faces.
- Preferred examples of a material of the noble metal tip are Pt alloys such as Pt-20 wt% Ni, Pt-20 wt% Rh, and Pt-20 wt% Rh-5 wt% Ni, and Ir alloys such as Ir-5 wt% Pt, Ir-20 wt% Rh, Ir-5 wt% Pt-1 wt% Rh-1 wt% Ni, and Ir-11 wt% Ru-8 wt% Rh-1 wt% Ni.
- the material is not restricted to these examples, and other known noble metal tips can be suitably applied.
- Fig. 1 shows a spark plug which is produced by the production method of the embodiment.
- the spark plug has a cylindrical metal shell 1.
- the metal shell 1 comprises a thread portion 1a for fixing the spark plug to an engine block which is not shown.
- An insulator 2 which is made of alumina ceramic (Al 2 O 3 ) or the like is fixed to the inside of the metal shell 1.
- a center electrode 3 is fixed to an axial hole 2a of the insulator 2.
- a tip end portion 2b of the insulator 2 is exposed from the metal shell 1.
- the center electrode 3 is a columnar member in which a metal material having a high thermal conductivity, such as Cu is placed inside the electrode, and another metal material that has high thermal resistance and corrosion resistance, such as a nickel-base alloy consisting of INCONEL 600 (trademark), covers the outside of the metal material having a high thermal conductivity.
- a tip end portion 51 of the center electrode is exposed from the tip end portion 2b of the insulator 2.
- the tip end portion 51 is formed by a noble metal tip made of an iridium alloy.
- the tip end portion 51 is formed so as to have a circular shape in section. In consideration of the heat dissipation property of the tip end portion 51 and the flame quenching effect of the center electrode 3, for example, the tip end portion 51 has a diameter of 0.6 mm and a length of 0.8 mm.
- the center electrode 3 has a small-diameter portion 3c at the tip end side, and has a straight portion at the tip end of the small-diameter portion 3c.
- a noble metal tip made of 95 wt% of iridium and 5 wt% of platinum is placed on the tip end of the straight portion, and then bonded by laser welding, thereby forming the tip end portion 51.
- the outer diameter of the straight portion is slightly larger than that of the noble metal tip.
- the laser welding is conducted at eight spots at an outer periphery of the noble metal tip which are arranged at intervals of 45° in a circumferential direction.
- a ground electrode 4 is fixed by welding to one end of the metal shell 1.
- the ground electrode 4 is made of a metal material such as a nickel-base alloy consisting of INCONEL 600 (trademark), and has an inner side face (a face opposed to the center electrode) 4a having a width that is smaller as it advances toward the tip end side, in a portion of the inner side face positioned between a pair of tapered faces 4b.
- a noble metal tip 52a primarily containing a noble metal is bonded to the inner side face 4a by laser welding so as to protrude by about 0.8 mm from the inner side face 4a, thereby forming a discharge portion 52.
- a discharge gap 6 is formed by the discharge portion 52 and the tip end portion 51 of the center electrode 3.
- the discharge portion 52 has a circular section shape having a diameter of 0.7 mm, and is formed of an alloy of 80 wt% of platinum and 20 wt% of iridium.
- the ground electrode 4 is formed so as to have a width of about 2.2 to 2.8 mm, and the tip end face positioned between the pair of tapered faces is formed so as to have a width of about 0.6 to 1.2 mm.
- primarily containing a noble metal means that the content of a noble metal(s) is larger than 50 wt%.
- the tip end portion 51 is made of an iridium alloy having a higher wear resistance against spark discharge
- the discharge portion 52 is made of a platinum alloy in which oxidation and volatilization can be avoided even at a high temperature.
- a substantially cylindrical metal shell 1' which has not yet been subjected to a threading process is formed by a process such as a cold extrusion process and a cutting process.
- a tool engagement portion 1d having a hexagonal section shape is formed on on one end side with respect to an axial middle portion 1b, and a thread forming portion 1a' which is substantially cylindrical, and in which the diameter is smaller than that of the center portion 1 b, is formed on the other end side (see Fig. 2A ).
- the ground electrode 4 having the tapered faces 4b formed at the tip end is resistance-welded to a tip end face 1e of the thread forming portion 1a' (see Fig. 2B ). Then, a rolling process is applied to the thread forming portion 1 a' of the metal shell 1' to form the thread portion 1 a ( Fig. 2C ). Next, a surface treatment such as galvanizing is applied to the metal shell 1', and the insulator 2 holding the center electrode 3 to which the noble metal tip is welded to form the tip end portion 51 is attached to the metal shell 1' ( Fig. 2D ).
- the noble metal tip 52a is placed in a portion of the inner side face 4a which is positioned between the pair of tapered faces 4b, and in which the width is smaller than the original width of the ground electrode.
- the interface between the inner side face 4a and the noble metal tip 52a is irradiated with a laser beam in a substantially horizontal direction, thereby forming the discharge portion 52 in the ground electrode 4 ( Fig. 2E ).
- the ground electrode 4 in which the tapered faces are previously formed is resistance-welded to thread forming portion 1a'.
- the tapered faces may be formed after the resistance welding is conducted.
- the discharge portion 52 may be formed by provisionally welding the noble metal tip 52a to the inner side face 4a of the ground electrode 4 by resistance welding or the like, forming the tapered faces, and thereafter conducting laser welding.
- the tapered faces may be formed in the ground electrode 4 at any step so long as laser welding has not yet been conducted.
- Fig. 3 is a diagram showing positioning of the discharge portion 52 with respect to the inner side face 4a of the ground electrode 4, i.e., the minimum distance L between the tapered faces 4b and the tip end face 4c of the ground electrode 4, and the noble metal tip 52a.
- Fig. 4 shows test results obtained in evaluating weldability in the case where the minimum distance L is set to have a value of 0 to 1.0 mm. The weldability was evaluated in the following manner.
- a spark plug was repeatedly subjected to 1,000 cycles in each of which the tip end of the spark plug on the side of the spark discharge gap was heated by a gas burner for two minutes to 1,000°C in the vicinity of the molten bond between the ground electrode 4 and the noble metal tip 52a, and then air cooled for one minute (corresponding to a travel distance of about 100,000 km in a durability test on an actual engine under usual traveling conditions). Then, the spark plug which had undergone the test was cut and polished in a plane passing through the center axis of the discharge portion 52, and the section was magnified and observed under a microscope. The length of an oxidized portion (oxidation length) at the interface between the noble metal tip 52a and the ground electrode 4 was measured in the observation field.
- the measured length of the oxidized portion was divided by the total length of the interface, and the division result was set as an oxidation rate.
- the oxidation rate was larger than 50% the weldability was judged not good (X), that in which the rate was 30 to 50% was judged to have good peel resistance ( ⁇ ), and that in which the rate was smaller than 30% was judged as being excellent ( ⁇ ).
- Fig. 5 is a diagram showing a state after laser welding in the case where the discharge portion 52 is formed by placing the noble metal tip 52a on the inner side face 4a of the ground electrode 4 so as to attain the value of L at which the best result is obtained.
- Fig. 5B is a partial sectional view taken along the line A-A' in Fig. 5A .
- a molten bond 53 is formed so as to extend from the inner side face 4a to the tapered faces 4b.
- the molten bond 53 has a curved shape which protrudes outward in a convex shape, and has a radius of curvature R, at a corner between the inner side face 4a and the tapered faces 4b.
- the radius of curvature R is in the range of 0.3 mm to 1.0 mm (in the examples, about 0.4 mm).
- a portion (unmelted portion) which is not melted by the laser welding has a height t of 0.45 mm.
- the minimum distance (the height t of the unmelted portion) between the tip end face of the noble metal tip 52a and the molten bond 53 is set to 0.3 mm or more. In the resulting structure, therefore, discharge at the molten bond hardly occurs.
- the noble metal tip 52a is laser-welded so as to satisfy a relationship of t ⁇ 0.78 ⁇ S between the height (t) of the unmelted portion of the noble metal tip 52a and a horizontal sectional area S of the noble metal tip 52a.
- Fig. 6 shows that when tapered faces are formed after a noble metal tip satisfying t ⁇ 0.78 ⁇ S is laser-welded, the crack occurrence rate is high.
- the noble metal tip is welded to the ground electrode in which the tapered faces are previously formed. Even in the case of a noble metal tip satisfying t ⁇ 0.78 ⁇ S, therefore, it is possible to prevent cracks from occurring.
- the edges formed at a corner between the tapered faces 4b of the ground electrode and the tip end face 4c are melted by a laser beam to have a curved shape which protrudes outward in a convex shape, and which has a radius of curvature r.
- the electric field strength can be further suppressed from concentrating at such edges as compared with the case where the edges formed by the tapered faces 4b of the ground electrode and the tip end face 4c are angular as shown in Fig. 3 . Therefore, this configuration prevents the molten bond 53 from becoming damaged.
- a middle portion of the molten bond 53 on the side of the metal shell (the rear end side) has an inward recessed shape.
- this portion has a structure in which the width (the minimum distance between the outer peripheral edge of the molten bond 53 and the outer peripheral face of the noble metal tip 52a) is smaller than the width of another portion.
- the reason therefor is as follows.
- the front face of the noble metal tip on the side of the metal shell is hardly irradiated with a laser beam, because of obstruction by the metal shell. Therefore, the laser irradiation is conducted in an oblique direction.
- the width of the molten bond is smaller than that of another portion. Even in such a shape, in order to obtain sufficient bonding strength between the noble metal tip 52a and the ground electrode 4, preferably, a minimum width of 0.25 mm or more is ensured in the recessed portion.
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- Manufacturing & Machinery (AREA)
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Description
- The present invention relates to a method of producing a spark plug, and also to a spark plug.
-
JP-A-2002-237365 US-A1-2002/0105254 ) discloses in Fig. 26 a ground electrode in which a noble metal tip is laser-welded to an inner side face that is formed into a tapered shape as it advances toward the tip end side, so as to protrude from the inner side face. - When the ground electrode is formed into a tapered shape after the noble metal tip is laser welded, impact occurring during formation of the tapered shape may cause a crack or the like in a molten bond formed by the welding. Particularly, this phenomenon has a high tendency to occur when laser welding is conducted in a state where, in order to improve ignitability, a noble metal tip having a small diameter of 0.8 mm or less protrudes by 0.5 mm or more from the inner side face of the ground electrode.
- The present invention has been achieved in view of the above-described problem. It is therefore an object of the present invention to provide a method of producing a spark plug in which the reliability of a molten bond can be improved.
- The above object has been achieved by providing a method of producing a spark plug as defined in
claim 1 and a spark plug as defined inclaim 6. - In the case of a diameter of 0.8 mm or less in which a molten bond has a small sectional area, the weld strength is easily reduced. In the case where the protrusion distance of the noble metal tip is 0.5 mm or more, stress due to vibration in the process of forming the tapered faces tends to be easily concentrated in the molten bond. By contrast, when a spark plug is produced by the method of the invention, it is possible to avoid this problem.
- Preferably, the laser welding is conducted after the noble metal tip primarily containing a noble metal is positioned such that the minimum distance between either of the tapered faces and the tip end face of the ground electrode, and the noble metal tip is set to 0.1 mm or more and 0.8 mm or less.
- In the laser welding of the noble metal tip, when the irradiation angle of a laser beam is about ±20° with respect to an extension face of the inner side face of the ground electrode to which the discharge portion is to be bonded, the laser welding can be stably conducted. During a laser welding process, although both the discharge portion and the ground electrode must be simultaneously melted, the laser beam can be focused within a range of about 0.8 mm or less. Nickel which is the principal component of the ground electrode base member is more easily melted than the noble metal tip primarily containing a noble metal. Because of these reasons, when laser welding is conducted after positioning the discharge portion in accordance with the invention as described above, the laser welding can be stably conducted.
- In order to prevent spark discharge at the molten bond, preferably, the distance between the tip end face of the noble metal tip and the molten bond is increased. Specifically, the height (t) of the unmelted portion of the noble metal tip is set to 0.3 mm or more. In this manner, a spark plug in which the height of the unmelted portion protruding from the molten bond is large tends to be easily broken. This is because of stress concentration due to vibrations in the process of forming the tapered faces. When the laser welding is conducted after the tapered faces are formed, it is possible to avoid stress concentration on the molten bond occurring during formation of the tapered faces.
- The height (t) of the unmelted portion is defined by the minimum distance between the tip end face of the noble metal tip and the molten bond.
- When the edges formed at a corner between the inner side face and the tapered faces are angular, the electric field is easily concentrated in these portions. As a result, in such a structure, spark discharge at the molten bond easily occurs, and hence the molten bond is susceptible to damage. By contrast, when the edges formed by the inner side face of the ground electrode and the tapered faces are melted during the laser welding and the molten bond is formed into a curved shape at a corner formed between the inner side face and the tapered faces, damage of the molten bond due to concentration of an electric field can be effectively prevented.
- Since the spark plug has a discharge portion in which the diameter is 0.8 mm or less and the height is 0.5 mm or more, the electric field strength is easily concentrated at the tip end of the discharge portion. Moreover, the molten bond is formed so as to extend from the inner side face to the tapered faces, and a corner formed between the inner side face and the tapered faces has a rounded shape. Therefore, concentration of electric field strength hardly occurs in these portions. Because of a synergistic effect due to these two structural features, the electric field strength is concentrated at the tip end of the discharge portion, and hence stable spark discharge is enabled at a low discharge voltage. The spark plug is formed by laser-welding the noble metal tip after the tapered faces are formed. Therefore, the molten bond is not broken by vibrations in the process of forming the tapered faces.
- Preferred examples of a material of the noble metal tip are Pt alloys such as Pt-20 wt% Ni, Pt-20 wt% Rh, and Pt-20 wt% Rh-5 wt% Ni, and Ir alloys such as Ir-5 wt% Pt, Ir-20 wt% Rh, Ir-5 wt% Pt-1 wt% Rh-1 wt% Ni, and Ir-11 wt% Ru-8 wt% Rh-1 wt% Ni. The material is not restricted to these examples, and other known noble metal tips can be suitably applied.
- Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
-
Fig. 1 is a front view showing an embodiment of a spark plug which is produced by the method of the present invention. -
Figs. 2A to 2E are views diagrammatically showing steps of producing the spark plug of the invention. -
Fig. 3 is a diagram showing the minimum distance L between tapered faces and a tip end portion of a ground electrode, and a discharge portion. -
Fig. 4 shows test results obtained in evaluating weldability in the case where the minimum distance L is set to have a value of 0 to 1.0 mm. -
Figs. 5A and 5B are diagrams showing a state after laser welding in the case where the discharge portion is positioned so as to attain the value of L at which a desirable result is obtained. -
Fig. 6 shows the relationship between a ratio of the length (t) of an unmelted portion (straight portion) of a noble metal tip to a horizontal sectional area (S), and a crack occurrence rate after the tapered faces are processed. - Description of Reference Numerals and Symbols:
- 1
- metal shell
- 1a
- thread portion
- 2
- insulator
- 3
- center electrode
- 4
- ground electrode
- 4a
- inner side face
- 4b
- tapered face
- 4c
- tip end face
- 51
- tip end portion
- 52
- discharge portion
- 52a
- noble metal tip
- 53
- molten bond
- 6
- discharge gap
- Hereinafter, a method of producing a spark plug which is a preferred embodiment of the invention will be described. However, the present invention should not be construed as being limited thereto.
-
Fig. 1 shows a spark plug which is produced by the production method of the embodiment. As shown inFig. 1 , the spark plug has acylindrical metal shell 1. Themetal shell 1 comprises athread portion 1a for fixing the spark plug to an engine block which is not shown. Aninsulator 2 which is made of alumina ceramic (Al2O3) or the like is fixed to the inside of themetal shell 1. Acenter electrode 3 is fixed to anaxial hole 2a of theinsulator 2. Atip end portion 2b of theinsulator 2 is exposed from themetal shell 1. - The
center electrode 3 is a columnar member in which a metal material having a high thermal conductivity, such as Cu is placed inside the electrode, and another metal material that has high thermal resistance and corrosion resistance, such as a nickel-base alloy consisting of INCONEL 600 (trademark), covers the outside of the metal material having a high thermal conductivity. Atip end portion 51 of the center electrode is exposed from thetip end portion 2b of theinsulator 2. Thetip end portion 51 is formed by a noble metal tip made of an iridium alloy. Thetip end portion 51 is formed so as to have a circular shape in section. In consideration of the heat dissipation property of thetip end portion 51 and the flame quenching effect of thecenter electrode 3, for example, thetip end portion 51 has a diameter of 0.6 mm and a length of 0.8 mm. - The
center electrode 3 has a small-diameter portion 3c at the tip end side, and has a straight portion at the tip end of the small-diameter portion 3c. A noble metal tip made of 95 wt% of iridium and 5 wt% of platinum is placed on the tip end of the straight portion, and then bonded by laser welding, thereby forming thetip end portion 51. The outer diameter of the straight portion is slightly larger than that of the noble metal tip. The laser welding is conducted at eight spots at an outer periphery of the noble metal tip which are arranged at intervals of 45° in a circumferential direction. - A
ground electrode 4 is fixed by welding to one end of themetal shell 1. Theground electrode 4 is made of a metal material such as a nickel-base alloy consisting of INCONEL 600 (trademark), and has an inner side face (a face opposed to the center electrode) 4a having a width that is smaller as it advances toward the tip end side, in a portion of the inner side face positioned between a pair oftapered faces 4b. Anoble metal tip 52a primarily containing a noble metal is bonded to theinner side face 4a by laser welding so as to protrude by about 0.8 mm from theinner side face 4a, thereby forming adischarge portion 52. Adischarge gap 6 is formed by thedischarge portion 52 and thetip end portion 51 of thecenter electrode 3. Thedischarge portion 52 has a circular section shape having a diameter of 0.7 mm, and is formed of an alloy of 80 wt% of platinum and 20 wt% of iridium. Usually, theground electrode 4 is formed so as to have a width of about 2.2 to 2.8 mm, and the tip end face positioned between the pair of tapered faces is formed so as to have a width of about 0.6 to 1.2 mm. As used herein "primarily containing a noble metal" means that the content of a noble metal(s) is larger than 50 wt%. - The wear amount due to spark discharge tends to be larger at the
tip end portion 51 of thecenter electrode 3 than at thedischarge portion 52 of theground electrode 4. The temperature of theground electrode 4 tends to increase more rapidly than the temperature of thecenter electrode 3. In this embodiment, therefore, thetip end portion 51 is made of an iridium alloy having a higher wear resistance against spark discharge, and thedischarge portion 52 is made of a platinum alloy in which oxidation and volatilization can be avoided even at a high temperature. - Next, a method of producing the spark plug will be specifically described with reference to
Fig. 2 . A substantially cylindrical metal shell 1' which has not yet been subjected to a threading process is formed by a process such as a cold extrusion process and a cutting process. In the metal shell 1', atool engagement portion 1d having a hexagonal section shape is formed on on one end side with respect to an axialmiddle portion 1b, and athread forming portion 1a' which is substantially cylindrical, and in which the diameter is smaller than that of thecenter portion 1 b, is formed on the other end side (seeFig. 2A ). - The
ground electrode 4 having the tapered faces 4b formed at the tip end is resistance-welded to atip end face 1e of thethread forming portion 1a' (seeFig. 2B ). Then, a rolling process is applied to thethread forming portion 1 a' of the metal shell 1' to form thethread portion 1 a (Fig. 2C ). Next, a surface treatment such as galvanizing is applied to the metal shell 1', and theinsulator 2 holding thecenter electrode 3 to which the noble metal tip is welded to form thetip end portion 51 is attached to the metal shell 1' (Fig. 2D ). Thenoble metal tip 52a is placed in a portion of theinner side face 4a which is positioned between the pair oftapered faces 4b, and in which the width is smaller than the original width of the ground electrode. The interface between theinner side face 4a and thenoble metal tip 52a is irradiated with a laser beam in a substantially horizontal direction, thereby forming thedischarge portion 52 in the ground electrode 4 (Fig. 2E ). - In this embodiment, the
ground electrode 4 in which the tapered faces are previously formed is resistance-welded tothread forming portion 1a'. Alternatively, the tapered faces may be formed after the resistance welding is conducted. Alternatively, thedischarge portion 52 may be formed by provisionally welding thenoble metal tip 52a to theinner side face 4a of theground electrode 4 by resistance welding or the like, forming the tapered faces, and thereafter conducting laser welding. In other words, the tapered faces may be formed in theground electrode 4 at any step so long as laser welding has not yet been conducted. - A preferred arrangement of the
discharge portion 52 in the above-described embodiment of the invention will now be described with reference toFigs. 3 to 5 . -
Fig. 3 is a diagram showing positioning of thedischarge portion 52 with respect to theinner side face 4a of theground electrode 4, i.e., the minimum distance L between the tapered faces 4b and the tip end face 4c of theground electrode 4, and thenoble metal tip 52a.Fig. 4 shows test results obtained in evaluating weldability in the case where the minimum distance L is set to have a value of 0 to 1.0 mm. The weldability was evaluated in the following manner. A spark plug was repeatedly subjected to 1,000 cycles in each of which the tip end of the spark plug on the side of the spark discharge gap was heated by a gas burner for two minutes to 1,000°C in the vicinity of the molten bond between theground electrode 4 and thenoble metal tip 52a, and then air cooled for one minute (corresponding to a travel distance of about 100,000 km in a durability test on an actual engine under usual traveling conditions). Then, the spark plug which had undergone the test was cut and polished in a plane passing through the center axis of thedischarge portion 52, and the section was magnified and observed under a microscope. The length of an oxidized portion (oxidation length) at the interface between thenoble metal tip 52a and theground electrode 4 was measured in the observation field. The measured length of the oxidized portion was divided by the total length of the interface, and the division result was set as an oxidation rate. In a spark plug in which the oxidation rate was larger than 50% the weldability was judged not good (X), that in which the rate was 30 to 50% was judged to have good peel resistance (○), and that in which the rate was smaller than 30% was judged as being excellent (⊚). - As shown in
Fig. 4 , satisfactory results were obtained when L was 0.1 to 0.8 mm, and the best results were obtained when L was 0.3 to 0.5 mm. -
Fig. 5 is a diagram showing a state after laser welding in the case where thedischarge portion 52 is formed by placing thenoble metal tip 52a on theinner side face 4a of theground electrode 4 so as to attain the value of L at which the best result is obtained.Fig. 5B is a partial sectional view taken along the line A-A' inFig. 5A . As shown inFig. 5B , amolten bond 53 is formed so as to extend from theinner side face 4a to the tapered faces 4b. Themolten bond 53 has a curved shape which protrudes outward in a convex shape, and has a radius of curvature R, at a corner between theinner side face 4a and the tapered faces 4b. Preferably, the radius of curvature R is in the range of 0.3 mm to 1.0 mm (in the examples, about 0.4 mm). - In the
noble metal tip 52a, a portion (unmelted portion) which is not melted by the laser welding has a height t of 0.45 mm. The minimum distance (the height t of the unmelted portion) between the tip end face of thenoble metal tip 52a and themolten bond 53 is set to 0.3 mm or more. In the resulting structure, therefore, discharge at the molten bond hardly occurs. - Moreover, the
noble metal tip 52a is laser-welded so as to satisfy a relationship of t ≥ 0.78 × S between the height (t) of the unmelted portion of thenoble metal tip 52a and a horizontal sectional area S of thenoble metal tip 52a. - With respect to a noble metal tip having a height of 0.8 mm and a tip diameter φ of 0.7 mm (the horizontal sectional area = about 0.385 mm2), while changing the height (t) of an unmelted portion in the range of 0.1 mm to 0.55 mm, the relationship between t/S and the crack occurrence rate was evaluated in the case where tapered faces were formed after the noble metal tip was laser-welded. The results are shown in
Fig. 6 . -
Fig. 6 shows that when tapered faces are formed after a noble metal tip satisfying t ≥ 0.78 × S is laser-welded, the crack occurrence rate is high. By contrast, in the Examples, the noble metal tip is welded to the ground electrode in which the tapered faces are previously formed. Even in the case of a noble metal tip satisfying t ≥ 0.78 × S, therefore, it is possible to prevent cracks from occurring. - In the Examples, as shown in
Fig. 5A , the edges formed at a corner between the tapered faces 4b of the ground electrode and the tip end face 4c are melted by a laser beam to have a curved shape which protrudes outward in a convex shape, and which has a radius of curvature r. According to this configuration, the electric field strength can be further suppressed from concentrating at such edges as compared with the case where the edges formed by the tapered faces 4b of the ground electrode and the tip end face 4c are angular as shown inFig. 3 . Therefore, this configuration prevents themolten bond 53 from becoming damaged. - In
Fig. 5A , as viewed from the tip end face of thenoble metal tip 52a, a middle portion of themolten bond 53 on the side of the metal shell (the rear end side) has an inward recessed shape. Namely, this portion has a structure in which the width (the minimum distance between the outer peripheral edge of themolten bond 53 and the outer peripheral face of thenoble metal tip 52a) is smaller than the width of another portion. The reason therefor is as follows. The front face of the noble metal tip on the side of the metal shell is hardly irradiated with a laser beam, because of obstruction by the metal shell. Therefore, the laser irradiation is conducted in an oblique direction. In the front face of the noble metal tip on the side of the metal shell, as a result, the width of the molten bond is smaller than that of another portion. Even in such a shape, in order to obtain sufficient bonding strength between thenoble metal tip 52a and theground electrode 4, preferably, a minimum width of 0.25 mm or more is ensured in the recessed portion. - It should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the scope of the claims appended hereto.
Claims (11)
- A method of producing a spark plug, said spark plug including:a center electrode (3);
an insulator (2) which holds said center electrode (3) therein in a state where a tip end portion (51) of said center electrode (3) protrudes therefrom;a metal shell (1) which holds said insulator (2) therein;a ground electrode (4) which is fixed to said metal shell (1), said ground electrode (4) having an inner side face (4a) having a width that is smaller as it advances toward a tip end side, in a portion of the inner side face (4a) positioned between a pair of tapered faces (4b); anda discharge portion (52) which is bonded to said inner side face (4a) of said ground electrode (4) by laser welding so as to attain a diameter of 0.8 mm or less and a height of 0.5 mm or more, a discharge gap (6) being formed between said discharge portion (52) and said tip end portion (51) of said center electrode (3), characterized in that said method comprises:forming said tapered faces (4b) before said discharge portion (52) is laser-welded to said inner side face (4a), forming said molten bond (53) by laser welding so as to extend from said inner side face (4a) to said tapered faces (4b), said molten bond (53) having a curved shape (R) at a corner formed between said inner side face (4a) and said tapered faces (4b). - The method as claimed in claim 1, which comprises forming said discharge portion (52) by bonding a noble metal tip (52a) to said inner side face (4a) after said noble metal tip (52a) primarily containing a noble metal is positioned so as to set a minimum distance between said tip (52a), and either of said tapered faces (4b) and a tip end face (4c) of said ground electrode (4) of from 0.1 to 0.8 mm.
- The method as claimed in claim 1 or 2, which comprises forming a molten bond (53) by laser welding, leaving said noble metal tip (52a) with an unmelted portion having a height of 0.3 mm or more.
- The method as claimed in claim 1, 2 or 3, which comprises forming a molten bond (53) by laser welding, satisfying a relationship of t ≥ 0.78 x S, between a height in mm, t of an unmelted portion of said noble metal tip (52a) and a horizontal sectional area in mm2, S of said noble metal tip (52a).
- The method as claimed in claim 1, wherein said molten bond (53) has a curved shape which has a radius of curvature of from 0.3 to 1.0 mm.
- A spark plug comprising:a center electrode (3);an insulator (2) which holds said center electrode (3) therein in a state where a tip end portion (51) of said center electrode (3) protrudes therefrom;a metal shell (1) which holds said insulator (2) therein;
a ground electrode (4) which is fixed to said metal shell (1), said ground electrode (4) having an inner side face (4a) having a width that is smaller as it advances toward a tip end side, in a portion of the inner side face (4a) positioned between a pair of tapered faces (4b);a discharge portion (52) in which a noble metal tip (52a) is bonded to said inner side face (4a) of said ground electrode (4) by laser welding so as to attain a diameter of 0.8 mm or less and a height of 0.5 mm or more, a discharge gap (6) being formed between said discharge portion (52) and said tip end portion (51) of said center electrode (3); anda molten bond (53), in which said noble metal tip (52a) and said ground electrode (4) are melted together, characterized in that said molten bond (53) extends from said inner side face (4a) to said tapered faces (4b), said molten bond (53) having a curved shape (R) at a corner formed between said inner side face (4a) and said tapered faces (4b). - The spark plug as claimed in claim 6, wherein said molten bond (53) has a curved shape which has a radius of curvature of from 0.3 to 1.0 mm.
- The spark plug as claimed in claim 6 or 7, wherein said noble metal tip (52a) is welded such that an unmelted portion of said noble metal tip (52a) protrudes by 0.3 mm or more from said molten bond (53).
- The spark plug as claimed in claim 6, 7 or 8, wherein said noble metal tip (52a) satisfies a relationship of t ≥ 0.78 × S, between a height in mm, t of said unmelted portion and a horizontal sectional area in mm2, S.
- The spark plug as claimed in any one of claims 6 to 9, wherein said noble metal tip (52a) comprises an alloy selected from the group consisting of a Pt-Ni alloy, a Pt-Rh alloy, a Pt-Rh-Ni alloy, an Ir-Pt alloy, an Ir-Rh alloy, an Ir-Pt-Rh-Ni alloy, and an Ir-Ru-Rh-Ni alloy.
- The spark plug as claimed in any one of claims 6 to 10, wherein said molten bond (53) has a shape in which a middle portion on a side of said metal shell (1) is inward recessed as viewed from a tip end face of said noble metal tip (52a), and a minimum distance between an outer peripheral edge of said molten bond (53) in said recessed portion and an outer peripheral face of said noble metal tip (52a) is 0.25 mm or more.
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EP1519460B1 true EP1519460B1 (en) | 2016-04-27 |
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