JP7507726B2 - Spark plug and method for manufacturing the same - Google Patents

Description

This disclosure relates to spark plugs.

Conventionally, electrodes of spark plugs used for ignition in internal combustion engines have sometimes been provided with precious metal tips to improve durability. In the spark plug described in Patent Document 1, the ground electrode and the precious metal tip are welded by laser welding to join the precious metal tip to the ground electrode, and a fused portion is formed by the welding where the ground electrode and the precious metal tip are fused together.

JP 2019-139977 A

In recent years, there has been a demand to increase the size of the precious metal tip in order to improve the durability of spark plugs. However, when laser welding is performed with the same output, the larger the size of the precious metal tip, the less the thickness and surface area of the molten part formed by laser welding become, and there is a risk that the peeling resistance of the precious metal tip will deteriorate. In addition, the ground electrode is generally made of a material that is more likely to melt and expand thermally than the precious metal tip. For this reason, when the output of laser welding is increased, the proportion of material derived from the ground electrode in the molten part increases, and as a result, the molten part may not be able to absorb the stress caused by the difference in thermal expansion between the ground electrode and the precious metal tip. In this case, there is a risk that the formation of cracks will progress due to the cold-hot cycle in the combustion chamber, making the precious metal tip more likely to peel off. For this reason, there has been a demand for technology that can suppress the deterioration of the peeling resistance of the precious metal tip.

This disclosure can be realized in the following forms:

(1) According to one embodiment of the present disclosure, a spark plug is provided. The spark plug includes a center electrode, a cylindrical insulator that holds the center electrode on its inner periphery, a cylindrical metal shell that holds the insulator on its inner periphery, and a ground electrode, the ground electrode having one end attached to the tip of the metal shell and the other end to which a precious metal tip having a discharge surface that forms a gap for spark discharge between the ground electrode and the tip of the center electrode is joined, and the precious metal tip is joined to the other end via a fusion zone formed by the ground electrode and the precious metal tip being fused together, and is characterized in that the spark plug satisfies the following conditions (a) and (b): (a) ) In a first cross section along the boundary between the ground electrode and the precious metal tip, the fusion portion has a plurality of protrusions each protruding in a first direction from the other end toward the one end and formed side by side in a second direction perpendicular to the first direction; (b) In a second cross section perpendicular to the first direction and passing through the protrusions, each of the protrusions satisfies B<A, where A is a first dimension that is the maximum dimension of the thickness of the precious metal tip in the tip thickness direction that is perpendicular to the first direction and the second direction, and B is a second dimension that is the maximum dimension in the second direction. According to this form of spark plug, in the first cross section along the boundary between the ground electrode and the precious metal tip, the fusion portion has a plurality of protrusions protruding in the first direction, and in the second cross section perpendicular to the first direction and passing through the protrusions, each of the protrusions satisfies B<A, so that the surface area of the fusion portion can be increased. As a result, the ground contact area between the precious metal tip and the fusion portion can be increased, so that the peeling resistance of the precious metal tip can be prevented from deteriorating.

(2) In the spark plug of the above embodiment, in the second cross section, each of the protrusions may satisfy D<C, where C is the maximum dimension in the tip thickness direction of the first region located on the discharge surface side of the boundary, and D is the maximum dimension in the tip thickness direction of the second region located on the opposite side of the boundary from the discharge surface side. According to this spark plug, since the fourth dimension of each of the protrusions in the second cross section is smaller than the third dimension, excessive melting of the ground electrode when forming the fusion portion can be suppressed. As a result, the proportion of the material originating from the ground electrode in the fusion portion can be suppressed from becoming excessively large. This allows the fusion portion to absorb stress caused by the thermal expansion difference between the ground electrode and the precious metal tip, and as a result, the progression of crack formation can be suppressed, and peeling of the precious metal tip can be suppressed.

(3) According to another aspect of the present disclosure, a method for manufacturing a spark plug is provided. The method for manufacturing a spark plug includes a center electrode, a cylindrical insulator that holds the center electrode on its inner periphery, a cylindrical metal shell that holds the insulator on its inner periphery, and a ground electrode, the ground electrode having one end attached to the tip of the metal shell and the other end to which a precious metal tip having a discharge surface that forms a gap for spark discharge between the tip of the center electrode and the precious metal tip is joined, the precious metal tip being joined to the other end via a fusion zone formed by the melting of the ground electrode and the precious metal tip, the method for manufacturing a spark plug includes irradiating a laser beam from the other end side of the ground electrode to a boundary between the ground electrode and the precious metal tip, and applying a second laser beam across the boundary to the ground electrode. The present invention is characterized in that the method includes a step of forming the molten portion that satisfies the following conditions (a) and (b) by repeating the above-mentioned steps intermittently multiple times in the direction of the boundary: (a) in a first cross section along the boundary, the molten portion has a plurality of protrusions that protrude in a first direction from the other end to the one end and are formed side by side in a second direction perpendicular to the first direction; (b) in a second cross section perpendicular to the first direction and passing through the protrusions, each of the protrusions satisfies B<A, where A is a first dimension that is the maximum dimension in the tip thickness direction that is perpendicular to the first and second directions of the thickness of the precious metal tip, and B is a second dimension that is the maximum dimension in the second direction. According to this form of spark plug, a molten portion that satisfies the above conditions (a) and (b) can be formed, so that a spark plug with improved peeling resistance of the precious metal tip can be manufactured.

(4) In the above-described method for manufacturing a spark plug, the laser light emitted multiple times in the process may have a longer irradiation distance for the earlier laser light irradiated than for the later laser light irradiated. In general, the earlier laser light irradiated tends to have a smaller amount of heat than the later laser light irradiated. According to this method for manufacturing a spark plug, the earlier laser light irradiated has a longer irradiation distance than the later laser light irradiated, so that it is possible to suppress the occurrence of a bias in the amount of heat applied to the ground electrode and the precious metal tip in the second direction.

(5) In the above-described embodiment of the method for manufacturing a spark plug, the laser light irradiated multiple times in the process may be irradiated symmetrically with respect to the center of gravity of the precious metal tip in the second direction, gradually moving away from the center of gravity. According to this embodiment of the method for manufacturing a spark plug, the laser light is irradiated multiple times symmetrically with respect to the center of gravity of the precious metal tip in the second direction, gradually moving away from the center of gravity, thereby further suppressing the occurrence of bias in the amount of heat applied to the ground electrode and the precious metal tip in the second direction.

The present invention can be realized in various forms, such as a method for manufacturing a ground electrode of a spark plug, a tip welding method for a ground electrode of a spark plug, an engine head with a spark plug attached, etc.

1 is a partial cross-sectional view showing a schematic configuration of a spark plug; FIG. 4 is an enlarged view of the other end of the ground electrode as viewed from the rear end side in the axial direction. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2 . FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. FIG. 4 is an explanatory diagram showing an example of a laser light irradiation position. FIG. 4 is an explanatory diagram for explaining a method for determining the formation rate of oxide scale. FIG. 4 is an explanatory diagram showing the evaluation results of oxide scale in the examples and the comparative examples. FIG. 11 is an explanatory diagram illustrating a schematic view of a laser light irradiation position in another embodiment. FIG. 11 is an explanatory diagram illustrating a schematic view of a laser light irradiation position in another embodiment. FIG. 11 is an explanatory diagram illustrating a schematic view of a laser light irradiation position in another embodiment. FIG. 11 is an explanatory diagram illustrating a schematic view of a laser light irradiation position in another embodiment. FIG. 11 is an explanatory diagram illustrating a schematic view of a laser light irradiation position in another embodiment.

A. Embodiments:
FIG. 1 is a partial cross-sectional view showing a schematic configuration of a spark plug 100 according to an embodiment of the present disclosure. In FIG. 1, the outer shape of the spark plug 100 is shown on the right side of the drawing, and the cross-sectional shape of the spark plug 100 is shown on the left side of the drawing, with the axis CA being the axis of the spark plug 100 as the boundary. In the following description, the lower side of FIG. 1 along the axis CA (the side where a ground electrode 40 described later is arranged) is called the leading end side, the upper side of FIG. 1 (the side where a terminal metal fitting 50 described later is arranged) is called the rear end side, and the direction along the axis CA is called the axial direction AD. In FIG. 1, for convenience of description, an engine head 90 to which the spark plug 100 is attached is shown by a broken line. The spark plug 100 is attached to the engine head 90 so that its leading end is exposed in a combustion chamber 95.

The spark plug 100 includes an insulator 10, a center electrode 20, a metal shell 30, a ground electrode 40, a precious metal tip 45, and a terminal metal fitting 50. The axis CA of the spark plug 100 coincides with the axis of each of the insulator 10, the center electrode 20, the metal shell 30, and the terminal metal fitting 50.

The insulator 10 has a generally cylindrical external shape with a through hole 11 formed along the axial direction AD. The through hole 11 accommodates a part of the center electrode 20 at the front end and a part of the terminal metal fitting 50 at the rear end. Thus, the insulator 10 holds the center electrode 20 on its inner periphery. The front end portion of the insulator 10 is accommodated in an axial hole 38 of the metal shell 30 described below, and the rear end portion is exposed from the axial hole 38. The insulator 10 is made of an insulating porcelain formed by sintering a ceramic material such as alumina.

The center electrode 20 is a rod-shaped electrode extending along the axial direction AD. The tip 21 of the center electrode 20 protrudes from the tip of the through hole 11. A precious metal tip made of, for example, platinum or an iridium alloy may be joined to the tip 21. The center electrode 20 of this embodiment is made of a nickel alloy containing nickel as the main component.

A part of the center electrode 20 is inserted into the leading end side of the through hole 11 of the insulator 10, and a part of the terminal metal fitting 50 is inserted into the trailing end side of the through hole 11 of the insulator 10. In the through hole 11 of the insulator 10, a leading end seal material 61, a resistor 62, and a trailing end seal material 63 are arranged between the center electrode 20 and the terminal metal fitting 50 in this order from the leading end side to the trailing end side. Therefore, the center electrode 20 is electrically connected to the terminal metal fitting 50 at the trailing end side via the leading end seal material 61, the resistor 62, and the trailing end seal material 63.

The resistor 62 is made of ceramic powder, conductive material, glass, and adhesive. The resistor 62 functions as an electrical resistor between the terminal fitting 50 and the center electrode 20, thereby suppressing the generation of noise when spark discharge occurs. The leading end sealing material 61 and the trailing end sealing material 63 each contain conductive glass powder as a material. In this embodiment, the leading end sealing material 61 and the trailing end sealing material 63 contain a powder mixture of copper powder and calcium borosilicate glass powder as a material.

The metal shell 30 has a generally cylindrical exterior shape with an axial hole 38 formed along the axial direction AD, and holds the insulator 10 within the axial hole 38. In other words, the metal shell 30 holds the insulator 10 on its inner periphery. The metal shell 30 is made of, for example, low carbon steel, and is entirely plated with nickel, zinc, or the like.

A tool engagement portion 31 and a male thread portion 32 are formed on the outer periphery of the metal shell 30. The tool engagement portion 31 engages with a tool (not shown) when attaching the spark plug 100 to the engine head 90. The male thread portion 32 has a thread formed on the outer periphery at the tip of the metal shell 30, and is screwed into the female thread portion 93 of the engine head 90.

The terminal metal fitting 50 is provided at the rear end of the spark plug 100. The tip side of the terminal metal fitting 50 is accommodated in the through hole 11 of the insulator 10, and the rear end side of the terminal metal fitting 50 is exposed from the through hole 11. A high voltage cable (not shown) is connected to the terminal metal fitting 50, and high voltage is applied. This application generates a spark discharge in the gap G, which will be described later. The spark generated in the gap G ignites the mixture in the combustion chamber 95.

The ground electrode 40 has one end 41 attached to the tip 37 of the metal shell 30 and the other end 42 facing the tip 21 of the center electrode 20. The ground electrode 40 of this embodiment is formed by bending a rod-shaped metal member having a generally rectangular cross section. More specifically, the ground electrode 40 is bent so that one end 41 is connected to the tip 37 of the metal shell 30 and the other end 42 faces the tip 21 of the center electrode 20. The ground electrode 40 of this embodiment is formed from a nickel alloy containing nickel as the main component. In this specification, the term "main component" means the component that is contained in the largest amount.

A precious metal tip 45 is joined to the other end 42 of the ground electrode 40. The precious metal tip 45 has a discharge surface 46. The discharge surface 46 forms a gap G for spark discharge between the tip 21 of the center electrode 20.

2 is an enlarged view of the other end 42 of the ground electrode 40 as viewed from the rear end side in the axial direction AD. The precious metal tip 45 is made of a precious metal or an alloy mainly composed of a precious metal. The precious metal tip 45 of this embodiment is made of platinum, but is not limited to platinum and may be made of a precious metal such as iridium, rhodium, or ruthenium, or an alloy mainly composed of these precious metals. The precious metal tip 45 of this embodiment has an external shape of a thin rectangular parallelepiped. The precious metal tip 45 is joined to the other end 42 of the ground electrode 40 via a fusion zone 70 formed by the fusion of the ground electrode 40 and the precious metal tip 45. A detailed description of the fusion zone 70 will be given later.

Fig. 3 is a cross-sectional view taken along line III-III in Fig. 2. Fig. 4 is a cross-sectional view taken along line IV-IV in Fig. 3. The molten portion 70 shown in Figs. 2 to 4 is formed by fusing the ground electrode 40 and the noble metal tip 45 together by laser welding for joining the noble metal tip 45 to the ground electrode 40. The joining of the noble metal tip 45 to the ground electrode 40 is performed in the following order of steps S110 to S130.
(Step S110) A recess 43 is formed in the other end 42.
(Step S120) The noble metal tip 45 is placed in the recess 43.
(Step S130) The boundary 44 between the ground electrode 40 and the noble metal tip 45 is laser welded.

4, in step S110, the recess 43 is formed at a position where the precious metal tip 45 is to be placed. The recess 43 is formed, for example, by pressing a pressing member (not shown) having a shape corresponding to the recess 43 against the other end 42 using a press machine or the like. In step S120, the precious metal tip 45 is placed in the recess 43 formed in the other end 42 of the ground electrode 40. By step S120, the surface of the tip side of the precious metal tip 45 in the axial direction AD and the bottom surface of the recess 43 come into contact with each other. In the following description, as shown in FIG. 3, the surface where the precious metal tip 45 and the recess 43 come into contact with each other is also referred to as the "boundary 44 between the ground electrode 40 and the precious metal tip 45". In this embodiment, the boundary 44 is formed parallel to the discharge surface 46. Note that the formation of the recess 43 may be omitted, and the precious metal tip 45 may be directly placed on the rear end side surface of the other end 42.

In step S130, laser light is intermittently applied to the boundary 44 from the other end 42 side of the ground electrode 40. In FIG. 4, the direction of laser light application is shown diagrammatically by a hollow arrow. A detailed explanation of the method of applying laser light will be given later. The ground electrode 40 and the precious metal tip 45 are melted by the laser irradiation in step S130, and then solidified to form a molten portion 70. Therefore, the molten portion 70 contains material components of the ground electrode 40 and the precious metal tip 45.

In FIG. 4, a cross section along the boundary 44 between the ground electrode 40 and the precious metal tip 45 is shown. In the following description, the cross section along the boundary 44 between the ground electrode 40 and the precious metal tip 45 shown in FIG. 4 is also referred to as the first cross section CS1. The position of the boundary 44 can be confirmed, for example, by scraping the other end 42 of the ground electrode 40 from the end. In this embodiment, the first cross section CS1 is a cross section parallel to the discharge surface 46 and perpendicular to the lamination direction of the ground electrode 40 and the precious metal tip 45. As shown in FIG. 3, the lamination direction of the ground electrode 40 and the precious metal tip 45 coincides with the axial direction AD of the spark plug 100 and the tip thickness direction TD of the precious metal tip 45. In this embodiment, the fusion portion 70 satisfies the following conditions (a) to (c). Note that the following condition (c) is an optional condition and does not have to be satisfied.

(a) In a first cross section CS1 along the boundary 44 between the ground electrode 40 and the precious metal tip 45, the molten portion 70 has a plurality of protrusions 72 each protruding in a first direction D1 from the other end 42 toward the one end 41 and formed side by side in a second direction D2 perpendicular to the first direction D1.
(b) In a second cross section CS2 perpendicular to the first direction D1 and passing through the convex portion 72, each convex portion 72 satisfies B<A when a first dimension A is the maximum dimension in the chip thickness direction TD, which is the direction perpendicular to the first direction D1 and the second direction D2, of the thickness of the precious metal chip 45, and a second dimension B is the maximum dimension in the second direction D2.
(c) In the second cross section CS2, when the third dimension, which is the maximum dimension in the chip thickness direction TD of the first region 75 located on the discharge surface 46 side of the boundary 44, is C, and the fourth dimension, which is the maximum dimension in the chip thickness direction TD of the second region 76 located on the opposite side of the boundary 44 from the discharge surface 46 side, is D, each convex portion 72 satisfies D<C.

The condition (a) will be described below. In the first cross section CS1, the first direction D1 from the other end 42 to the one end 41 coincides with the direction from the axis CA of the spark plug 100 to the radially outward direction along the ground electrode 40. In addition, the second direction D2 perpendicular to the first direction D1 in the first cross section CS1 coincides with the width direction of the ground electrode 40. In the first cross section CS1, the multiple protrusions 72 each protrude in the first direction D1 and are formed side by side in the second direction D2. The number of protrusions 72 that the fusion zone 70 has in the first cross section CS1 is not particularly limited as long as it is multiple. For example, in FIG. 4, six protrusions 72 are shown as an example of the fusion zone 70, but the number is not limited to six and may be any number.

Condition (b) will be described below. In FIG. 3, a second cross section CS2 perpendicular to the first direction D1 and passing through the convex portion 72 is shown. In the second cross section CS2, a first dimension A is the maximum dimension of each convex portion 72 in the chip thickness direction TD, and a second dimension B is the maximum dimension of each convex portion 72 in the second direction D2. In this case, in each convex portion 72, the first dimension A is greater than the second dimension B. That is, each convex portion 72 satisfies B<A. Note that, as a preferred ratio between the second dimension B and the first dimension A, when the length of the second dimension B is 1, the length of the first dimension A may be, for example, 2 or more and 3 or less.

The condition (c) will be described below. In the second cross section CS2, the region of the convex portion 72 located on the discharge surface 46 side of the boundary 44 is defined as the first region 75, and the region of the convex portion 72 located on the opposite side of the boundary 44 from the discharge surface 46 side is defined as the second region 76. Then, the third dimension, which is the maximum dimension of each convex portion 72 in the chip thickness direction TD of the first region 75, is defined as C, and the fourth dimension, which is the maximum dimension of each convex portion 72 in the chip thickness direction TD of the second region 76, is defined as D. In this case, the fourth dimension D is smaller than the third dimension C in each convex portion 72. That is, each convex portion 72 satisfies D<C. Note that, as a preferable ratio between the fourth dimension D and the third dimension C, when the length of the fourth dimension D is 1, the length of the third dimension C may be, for example, 2 or more and 3 or less.

Here, in a configuration in which the size of the precious metal tip is relatively large in order to improve the durability of the spark plug, etc., when laser welding is performed with the same output, the thickness of the molten part formed by the laser welding may be insufficient, and as a result, the surface area of the molten part may be insufficient. If the surface area of the molten part is insufficient, the contact area between the precious metal tip and the molten part cannot be secured, and as a result, the precious metal tip may be prone to peeling off.

In contrast, in the spark plug 100 of this embodiment, the fusion zone 70 satisfies the above conditions (a) and (b), so the configuration of the fusion zone 70 can be made more complex to increase the surface area of the fusion zone 70. As a result, the contact area between the fusion zone 70 and the precious metal tip 45 can be increased, so that the peeling resistance of the precious metal tip 45 can be prevented from deteriorating.

In addition, since the thermal expansion coefficient of the ground electrode and the thermal expansion coefficient of the precious metal tip are generally different, stress is generated in the ground electrode and the precious metal tip due to the thermal cycle in the combustion chamber. For this reason, the fusion part is required to have the function of absorbing the stress caused by the thermal expansion difference between the ground electrode and the precious metal tip. Here, the ground electrode is generally made of a material that is more easily melted and thermally expanded than the precious metal tip. For this reason, if the output of the laser welding between the ground electrode and the precious metal tip is increased in order to increase the thickness of the fusion part and increase the surface area, the amount of melting of the ground electrode increases, and the proportion of the material originating from the ground electrode in the fusion part increases. As a result, the fusion part may not be able to absorb the stress caused by the thermal expansion difference between the ground electrode and the precious metal tip. In this case, the thermal cycle in the combustion chamber generates stress at the interface between the fusion part and the precious metal tip and at the interface between the fusion part and the ground electrode, which may lead to the formation of cracks and the precious metal tip becoming more likely to peel off.

In contrast, in the spark plug 100 of this embodiment, the molten portion 70 satisfies the above condition (c), so that excessive melting of the ground electrode 40 during laser welding when forming the molten portion 70 can be suppressed. As a result, the proportion of material originating from the ground electrode 40 in the molten portion 70 can be suppressed from increasing. This allows the molten portion 70 to absorb the stress caused by the thermal expansion difference between the ground electrode 40 and the precious metal tip 45, and as a result, the progression of crack formation can be suppressed, so that peeling of the precious metal tip 45 can be suppressed. Therefore, in the spark plug 100 of this embodiment, the molten portion 70 satisfies the above conditions (c) in addition to the above conditions (a) and (b), so that the surface area of the molten portion 70 can be increased and the increase in the amount of melting of the ground electrode 40 can be suppressed, so that the peeling resistance of the precious metal tip 45 can be effectively suppressed from deteriorating.

The molten portion 70 that satisfies the above conditions can be formed in the above step S130 by, for example, repeating the operation of irradiating the boundary 44 with laser light from the other end 42 side of the ground electrode 40 intermittently in the second direction D2 across the boundary 44 multiple times as follows:

Figure 5 is an explanatory diagram showing an example of the irradiation position of the laser light. In Figure 5, the boundary 44 between the ground electrode 40 and the precious metal tip 45 is shown as viewed from the other end 42 side of the ground electrode 40. In Figure 5, the area shown by cross hatching indicates the irradiation position LB of the laser light, and the numbers surrounded by circles conveniently indicate the order in which the laser light is irradiated. The dashed lines connecting the irradiation positions LB of the laser light indicate that the laser is turned off. In the example shown in Figure 5, the operation of irradiating the laser light so as to straddle the boundary 44 along the chip thickness direction TD is repeated 14 times intermittently in the second direction D2. More specifically, as shown by the white arrows, the operation of irradiating the laser light from the rear end side to the tip side of the axial direction AD is performed with the positions of the second direction D2 being different from each other. By such intermittent irradiation, in the example shown in Figure 5, a molten part 70 having 14 protrusions 72 is formed. In this way, by repeating the operation of irradiating the laser light intermittently multiple times in the second direction D2 across the boundary 44, it is possible to easily form a molten part 70 having multiple protrusions 72 on the first cross section CS. In addition, with this irradiation method, the irradiation time of each laser can be shortened, so that excessive melting of the ground electrode 40 can be suppressed.

In the example shown in FIG. 5, the earlier the laser light is irradiated, the longer the irradiation distance is than the later laser light. In other words, the earlier the laser light is irradiated, the longer the distance the laser is turned on is set to than the later laser light. Generally, the earlier the laser light is irradiated, the smaller the amount of heat tends to be than the later laser light. However, according to the irradiation method of this embodiment, the earlier the laser light is irradiated, the longer the irradiation distance is than the later laser light, so that it is possible to suppress the occurrence of bias in the amount of heat applied to the ground electrode 40 and the precious metal tip 45 in the second direction D2.

In the example shown in FIG. 5, the laser light is irradiated multiple times symmetrically about the center of gravity of the precious metal tip 45 in the second direction D2, gradually moving away from the center of gravity. According to this irradiation method, the laser light is irradiated intermittently multiple times in the second direction D2, symmetrically about the center of gravity of the precious metal tip 45 in the second direction D2, so that it is possible to further suppress the occurrence of bias in the amount of heat applied to the ground electrode 40 and the precious metal tip 45 in the second direction D2.

B. Examples:
The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

<Sample>
As the sample of the embodiment, a spark plug 100 was used in which a fusion portion 70 that satisfied the above conditions (a) to (c) was formed in the first cross section CS1. As the sample of the comparative example, a spark plug was used in which a fusion portion that did not satisfy the above conditions (a) to (c) was formed. The sample of the embodiment and the sample of the comparative example had the same configuration except for the fusion portion 70, and the number of samples was three for each.

<Cold-heat cycle test>
The sample of the embodiment and the sample of the comparative example were subjected to 1000 cycles of heating for 2 minutes and slow cooling for 1 minute by high frequency testing. Heating was performed so that the maximum temperature of the other end 42 of the ground electrode 40 was 1100°C ± 20°C. After the thermal cycle test, the ground electrode 40 of the embodiment sample was cut at a cross section passing through the center of gravity of the precious metal tip 45 along the first direction D1. The ground electrode of the comparative example sample was also cut at a similar cross section. After that, the cut cross section of each sample was observed with a metal microscope, and the formation rate of the oxide scale was obtained by the method shown below. Note that "oxide scale" means the degree of crack formation.

Fig. 6 is an explanatory diagram for explaining a method for determining the formation rate of oxide scale. Fig. 6 shows a schematic cross section passing through the center of gravity of the precious metal tip 45 and along the first direction D1. In Fig. 6, the location where a crack occurs in the boundary between the precious metal tip 45 and the ground electrode 40 is shown by a thick line. The rate (%) of oxide scale is indicated by the rate of the portion where the fusion zone 70 is peeled off in the cross section shown in Fig. 6, and can be determined by the following formula (1).
Oxide scale (%)=(a+b)/L×100 Formula (1)

As shown in FIG. 6, in the above formula (1), a indicates the dimension of the crack 78 along the first direction, b indicates the dimension of the crack 79 along the first direction, and L indicates the dimension of the molten portion 70 along the first direction. Note that 100% oxide scale indicates a state in which the molten portion 70 has completely peeled off in the cross section shown in FIG. 6, and the smaller the percentage of oxide scale (%), the higher the peeling resistance of the precious metal tip 45.

Figure 7 is an explanatory diagram showing the evaluation results of oxide scale for the Example and Comparative Example. In Figure 7, the vertical axis shows the percentage of oxide scale measured by the above method. Linear markers show the average value for each sample. From the results shown in Figure 7, it was found that the percentage of oxide scale was significantly smaller in the Example sample than in the Comparative Example sample. Therefore, it was found that the peeling resistance of the precious metal tip 45 was improved in the Example sample compared to the Comparative Example sample.

C. Other embodiments:
The configuration of the fusion portion 70 in the above embodiment is merely an example and can be modified in various ways. For example, the fusion portion 70 does not have to satisfy the above condition (c). That is, in the second cross section CS2, at least one of the multiple convex portions 72 may satisfy C≦D. Also, for example, the first dimension A of the multiple convex portions 72 may be the same as each other or different from each other. Also, for example, the second dimension B of the multiple convex portions 72 may be the same as each other or different from each other. Even with such a configuration, the fusion portion 70 satisfies the above conditions (a) and (b), so that the surface area of the fusion portion can be increased, and the ground contact area between the precious metal tip and the fusion portion can be increased, thereby suppressing deterioration of the peeling resistance of the precious metal tip.

8 to 12 are explanatory diagrams showing the irradiation position of the laser light in other embodiments. In Figs. 8 to 12, the same cross section as Fig. 5 is shown, and the area shown by cross hatching indicates the irradiation position LB of the laser light. The method of laser welding the ground electrode 40 and the precious metal tip 45 in the above embodiment, i.e., the method of forming the molten part 70, is merely an example and can be modified in various ways. For example, as shown in Figs. 8 and 9, the number of times the laser is turned on and off, i.e., the number of times the laser is intermittently irradiated, may be decreased or increased. Also, for example, the operation of irradiating the laser light from the tip side to the rear end side in the axial direction AD may be repeated intermittently multiple times in the second direction D2. Also, for example, the laser light may be irradiated multiple times symmetrically with respect to the center of gravity of the precious metal tip 45 in the second direction D2 so as to gradually approach the center of gravity. Also, as shown in Figures 10, 11 and 12, the laser light is not limited to being irradiated along the chip thickness direction TD, but may be irradiated across the interface of the boundary 44 along a direction intersecting the chip thickness direction TD and the second direction. Also, as shown in Figure 11, irradiation may be performed multiple times at the same position in the second direction D2 in multiple irradiations. That is, in general, the operation of irradiating the boundary 44 with the laser light from the other end 42 side of the ground electrode 40 may be repeated multiple times intermittently in the second direction D2 across the boundary 44. This method can also form a fusion zone 70 that satisfies the above conditions.

The present invention is not limited to the above-described embodiments, and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments corresponding to the technical features in each form described in the Summary of the Invention column can be replaced or combined as appropriate to solve some or all of the above-described problems or to achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate.

10...insulator, 11...through hole, 20...center electrode, 21...tip portion, 30...metal shell, 31...tool engagement portion, 32...male thread portion, 37...tip portion, 38...shaft hole, 40...ground electrode, 41...one end portion, 42...other end portion, 43...recess, 44...boundary, 45...precious metal tip, 46...discharge surface, 50...terminal metal, 61...tip side seal material, 62...resistor, 63...rear end side seal material, 70...Melted part, 72...Convex part, 75...First region, 76...Second region, 78, 79...Crack, 90...Engine head, 93...Female thread, 95...Combustion chamber, 100...Spark plug, AD...Axial direction, CA...Axial line, CS1...First cross section, CS2...Second cross section, D1...First direction, D2...Second direction, G...Gap, LB...Irradiation position, TD...Chip thickness direction

Claims (4)

a center electrode, a cylindrical insulator that holds the center electrode on an inner periphery side, a cylindrical metal shell that holds the insulator on an inner periphery side, and a ground electrode,
the ground electrode has one end attached to a tip of the metallic shell, and the other end to which a precious metal tip having a discharge surface that forms a gap for spark discharge between the ground electrode and the tip of the center electrode is joined,
The noble metal tip is joined to the other end portion via a fusion zone formed by melting the ground electrode and the noble metal tip.
A spark plug characterized by satisfying the following conditions (a), (b) and (c) :
(a) in a first cross section along a boundary between the ground electrode and the precious metal tip, the fusion zone has a plurality of protrusions each protruding in a first direction from the other end portion toward the one end portion and formed side by side in a second direction perpendicular to the first direction;
(b) in a second cross section perpendicular to the first direction and passing through the protrusion, each of the protrusions satisfies B<A, where A is a first dimension that is the maximum dimension in the tip thickness direction, which is a direction perpendicular to the first direction and the second direction, of the thickness of the precious metal tip, and B is a second dimension that is the maximum dimension in the second direction ;
(c) In the second cross section, each of the convex portions satisfies D<C, where C is the third dimension, which is the maximum dimension in the chip thickness direction of a first region located on the discharge surface side of the boundary, and D is the fourth dimension, which is the maximum dimension in the chip thickness direction of a second region located on the opposite side of the boundary from the discharge surface side. a center electrode, a cylindrical insulator that holds the center electrode on an inner periphery side, a cylindrical metal shell that holds the insulator on an inner periphery side, and a ground electrode,
the ground electrode has one end attached to a tip of the metallic shell, and the other end to which a precious metal tip having a discharge surface that forms a gap for spark discharge between the ground electrode and the tip of the center electrode is joined,
a fusion zone formed by fusing the ground electrode and the precious metal tip, the precious metal tip being joined to the other end portion of the spark plug,
a step of irradiating a boundary between the ground electrode and the precious metal tip with laser light from the other end side of the ground electrode in the second direction across the boundary, the step of intermittently repeating the irradiation a plurality of times to form the fusion zone that satisfies the following conditions (a), (b) , and (c) :
(a) in a first cross section along the boundary, the fusion portion has a plurality of protrusions each protruding in a first direction from the other end portion toward the one end portion and formed side by side in a second direction perpendicular to the first direction;
(b) in a second cross section perpendicular to the first direction and passing through the protrusion, each of the protrusions satisfies B<A, where A is a first dimension that is the maximum dimension in the tip thickness direction, which is a direction perpendicular to the first direction and the second direction, of the thickness of the precious metal tip, and B is a second dimension that is the maximum dimension in the second direction ;
(c) In the second cross section, each of the convex portions satisfies D<C, where C is the third dimension, which is the maximum dimension in the chip thickness direction of a first region located on the discharge surface side of the boundary, and D is the fourth dimension, which is the maximum dimension in the chip thickness direction of a second region located on the opposite side of the boundary from the discharge surface side. 3. The method for producing a spark plug according to claim 2 ,
In the above step, the laser light irradiated multiple times has a longer irradiation distance than the laser light irradiated later.
A method for manufacturing a spark plug. 4. The method for producing a spark plug according to claim 3 ,
The laser light irradiated multiple times in the step is irradiated symmetrically with respect to the center of gravity of the precious metal tip in the second direction so as to gradually move away from the center of gravity.
A method for manufacturing a spark plug.

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