阅读说明：本技术 火花塞 (Spark plug ) 是由 河合友纪 于 2019-06-11 设计创作，主要内容包括：本发明提供一种火花塞,能够抑制端头从母材脱落。火花塞具备：第一电极,具备以贵金属为主体的端头和以Ni为主体且经由熔融部而与端头连接的母材；及第二电极,与端头的放电面相对,熔融部具有端头与熔融部的第一边界面和母材与熔融部的第二边界面在垂直于放电面的第一方向上重合的重合部。关于重合部,在观察经过投影到与放电面平行的假想面上的重合部的重心且垂直于放电面的截面时,沿着放电面的第二方向上的一端部的贵金属的含有率高于50质量％,第二方向上的另一端部的Ni的含有率高于50质量％。(The invention provides a spark plug capable of preventing an end tip from falling off from a base material. The spark plug is provided with: a first electrode including a tip mainly made of a noble metal and a base material mainly made of Ni and connected to the tip through a melting portion; and a second electrode facing the discharge surface of the tip, the melting portion having an overlapping portion where a first boundary surface between the tip and the melting portion and a second boundary surface between the base material and the melting portion overlap in a first direction perpendicular to the discharge surface. When a cross section perpendicular to the discharge surface and passing through the center of gravity of the overlapping portion projected on a virtual plane parallel to the discharge surface is observed, the content of the noble metal at one end portion in the second direction along the discharge surface is higher than 50 mass%, and the content of Ni at the other end portion in the second direction is higher than 50 mass%.)

1. A spark plug is provided with:

a first electrode including a tip mainly made of a noble metal and a base material mainly made of Ni and connected to the tip through a melting portion; and

a second electrode opposing the discharge surface of the tip,

the melting portion has an overlapping portion where a first boundary surface of the tip and the melting portion and a second boundary surface of the base material and the melting portion overlap in a first direction perpendicular to the discharge surface,

when a cross section perpendicular to the discharge surface and passing through the center of gravity is observed at the overlapping portion,

a noble metal content in one end portion in a second direction along the discharge surface is higher than 50 mass%, a Ni content in the other end portion in the second direction is higher than 50 mass%,

the center of gravity is a center of gravity of the overlapping portion projected onto an imaginary plane parallel to the discharge surface.

2. The spark plug of claim 1,

in the cross-section in question,

the overlapping portion has a shape in which a distance between the first boundary surface and the second boundary surface along the first direction gradually increases toward the second direction,

the intermediate portion of the overlapping portion, in which the noble metal content is 50 mass% and the Ni content is 50 mass%, is located on the second direction side with respect to the center position of the overlapping portion in the second direction.

3. The spark plug of claim 1,

in the cross-section in question,

a shortest portion of the overlapping portion, at which a distance between the first boundary surface and the second boundary surface along the first direction is shortest, is located at a portion other than the first end portion and the other end portion,

the intermediate portion of the overlapping portion, in which the noble metal content is 50 mass% and the Ni content is 50 mass%, is located at a position other than the shortest portion.

4. The spark plug according to any one of claims 1 to 3,

the cross section is a cross section having the longest length in the second direction of the overlapping portion.

Technical Field

The present invention relates to a spark plug, and more particularly to a spark plug in which a tip mainly composed of a noble metal is joined to a base material mainly composed of Ni.

Background

There is known a spark plug in which a tip mainly made of a noble metal is connected to a base material mainly made of Ni through a fusion zone (for example, patent document 1). Since the base material has a linear expansion coefficient different from that of the tip, thermal stress is generated in the melted portion due to a temperature change of an engine to which the spark plug is attached.

Patent document 1: international publication No. 2010/113404

Disclosure of Invention

Technical problem to be solved by the invention

In this technique, there is a demand for a technique for suppressing the tip from falling off from the base material even if a crack generated in the melting portion due to thermal stress develops.

The present invention has been made in view of the above-described demand, and an object thereof is to provide a spark plug capable of suppressing tip separation.

Technical solution for solving technical problem

In order to achieve the object, a spark plug according to the present invention includes: a first electrode including a tip mainly made of a noble metal and a base material mainly made of Ni and connected to the tip through a melting portion; and a second electrode facing the discharge surface of the tip, the melting portion having an overlapping portion where a first boundary surface between the tip and the melting portion and a second boundary surface between the base material and the melting portion overlap in a first direction perpendicular to the discharge surface. When a cross section perpendicular to the discharge surface is viewed through the center of gravity of the overlapping portion projected on a virtual plane parallel to the discharge surface, the content of the noble metal at one end portion in the second direction along the discharge surface is higher than 50 mass%, and the content of Ni at the other end portion in the second direction is higher than 50 mass%.

Effects of the invention

According to the spark plug of claim 1, in a cross section perpendicular to the discharge surface, the overlapping portion has a noble metal content of more than 50 mass% at one end portion in the second direction along the discharge surface of the tip, and a Ni content of more than 50 mass% at the other end portion in the second direction. Thus, the thermal stress generated at the second boundary surface between the melting portion and the base material at the one end portion of the overlapping portion is larger than the thermal stress generated at the first boundary surface between the tip and the melting portion. On the other hand, at the other end portion of the overlapping portion, the thermal stress generated at the first boundary surface is larger than the thermal stress generated at the second boundary surface. As a result, cracks are likely to occur in the second boundary surface near the one end portion, and cracks are likely to occur in the first boundary surface near the other end portion. Since the crack tends to progress along the boundary surface, even if the crack progresses, the cracks respectively progressing along the first boundary surface and the second boundary surface can be made difficult to connect to each other. This can prevent the tip from falling off the base material.

In the spark plug according to claim 2, in the cross section, the overlapping portion has a shape in which a distance between the first boundary surface and the second boundary surface along the first direction perpendicular to the discharge surface gradually increases toward the second direction. The intermediate portion of the overlapping portion, in which the noble metal content is 50 mass% and the Ni content is 50 mass%, is located on the second direction side with respect to the center position of the overlapping portion in the second direction. This makes it possible to easily bring the position where the cracks that have progressed along the first boundary surface and the second boundary surface overlap in the first direction to the second direction side of the center position. Even if the crack progresses in the first direction at this position, the distance between the first boundary surface and the second boundary surface is long, and therefore, in addition to the effect of claim 1, the tip slip-off can be further suppressed.

In the spark plug according to claim 3, in the cross section, a shortest portion, where a distance along the first direction between the first boundary surface and the second boundary surface of the overlapping portion is shortest, is located at a portion other than the first end portion and the other end portion. The intermediate portion of the overlapping portion having a noble metal content of 50 mass% and an Ni content of 50 mass% is located at a position other than the shortest portion. This makes it possible to easily set the position where the cracks respectively propagating along the first boundary surface and the second boundary surface overlap in the first direction to a position other than the shortest portion. Even if the crack progresses in the first direction at this position, the distance between the first boundary surface and the second boundary surface is long, and therefore, in addition to the effect of claim 1, the tip slip-off can be further suppressed.

According to the spark plug of claim 4, the above-described arbitrary relationship is established in the cross section where the length of the overlapping portion in the second direction is the longest. As a result, the lengths of the first boundary surface and the second boundary surface where cracks are likely to develop can be made longest, and therefore, in addition to the effects of any of claims 1 to 3, the end drop can be further suppressed.

Drawings

FIG. 1 is a cross-sectional side view of one embodiment of a spark plug.

Fig. 2(a) is a plan view of the ground electrode, and fig. 2(b) is a sectional view of the ground electrode taken along line IIb-IIb in fig. 2 (a).

Fig. 3(a) is a schematic view when an end is joined to a base material, and fig. 3(b) is a schematic view when an end is joined to another base material.

Fig. 4(a) is a bottom view of the center electrode, and fig. 4(b) is a cross-sectional view of the center electrode at the line IVb-IVb of fig. 4 (a).

Fig. 5(a) is a schematic view when an end is joined to a base material, and fig. 5(b) is a schematic view when an end is joined to another base material.

Description of the reference numerals

10 spark plug

20 heart electrode (first electrode, second electrode)

22. 41 base material

23. 43 melting part

24. 44 end

25. 45 discharge surface

40 ground electrode (first electrode, second electrode)

46. 60 first side interface

47. 61 second boundary surface

48. 62 overlapping part

49. 63 center of gravity

50. 51 end (one end, the other end)

One end part of 64

65 the other end portion

52 center position

53. 67 intermediate part

66 shortest part

Detailed Description

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a cross-sectional side view of an embodiment of a spark plug 10, bounded by an axis O. In fig. 1, the lower side of the paper surface is referred to as the front end side of the spark plug 10, and the upper side of the paper surface is referred to as the rear end side of the spark plug 10. As shown in fig. 1, the spark plug 10 includes a center electrode 20 and a ground electrode 40.

The insulator 11 is a substantially cylindrical member having a shaft hole 12 formed along the axis O, and is formed of a ceramic such as alumina having excellent mechanical properties and high-temperature insulation properties. The insulator 11 has a rear end facing surface 13 formed on the front end side of the inner peripheral surface formed by the shaft hole 12, and the rear end facing surface 13 is an annular surface facing the rear end side. The diameter of the rear end facing surface 13 is reduced toward the front end side.

The center electrode 20 is a rod-shaped member locked to the surface 13 facing the rear end. The front end of the center electrode 20 protrudes from the front end of the insulator 11 toward the front end side. The core member 21 of the center electrode 20, which mainly contains copper, is covered with a bottomed cylindrical base member 22. The base material 22 has a chemical composition containing 50 wt% or more of Ni. The core material 21 can be omitted. A tip 24 is connected to the tip of the base material 22 through a fusion zone 23. The tip 24 has a chemical composition including 50 wt% or more of one or two or more of noble metals such as Pt, Rh, Ir, Ru, and the like. The discharge surface 25 of the tip 24 is opposite the ground electrode 40. The center electrode 20 is electrically connected to the terminal fitting 26 in the axial hole 12.

The terminal fitting 26 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a metal material having electrical conductivity (for example, mild steel). The terminal fitting 26 is fixed to the rear end side of the insulator 11 with the front end side inserted into the axial hole 12.

A metal shell 30 is fixed by caulking to the outer periphery of the front end side of the insulator 11. The metal shell 30 is a substantially cylindrical member formed of a metal material having electrical conductivity (for example, mild steel). The main metal fitting 30 includes: a seat portion 31 extending outward in the radial direction in a flange shape; and a screw portion 32 formed on the outer peripheral surface of the front end side of the seat portion 31. The body metal fitting 30 is fixed by fastening the screw portion 32 to a screw hole (not shown) of an engine (cylinder head). A ground electrode 40 is connected to the tip end of the metal shell 30.

The ground electrode 40 is a rod-shaped member formed of a metal material having conductivity. The ground electrode 40 includes: a base material 41 joined to the main body metal fitting 30; and a tip 44 disposed on the inner surface 42 of the base material 41 facing the center electrode 20 and connected to the inner surface 42 via the melting portion 43. The base material 41 has a chemical composition containing 50 wt% or more of Ni. The tip 44 has a chemical composition containing 50 wt% or more of one or two or more of noble metals such as Pt, Rh, Ir, and Ru. The discharge surface 45 of the tip 44 is opposite the center electrode 20. A spark gap G is formed between the discharge surface 45 of the tip 44 and the center electrode 20.

Fig. 2(a) is a plan view of the ground electrode 40 (first electrode) as viewed from the direction of the axis O, and fig. 2(b) is a sectional view of the ground electrode 40 taken along line IIb-IIb in fig. 2 (a). Arrow Z indicates a first direction perpendicular to the discharge surface 45 of the tip 44. If the ground electrode 40 is set as the first electrode, the center electrode 20 is the second electrode. In the present embodiment, the base material 41 has a rod shape having a substantially rectangular cross section, and the tip 44 has a rectangular parallelepiped shape. A part of the tip 44 is disposed in a groove formed by recessing the inner surface 42 of the tip portion of the base material 41 along the side surface 41b of the base material 41. The position of the tip 44 is limited by the wall surface 42a of the groove. The tip 44 is connected to the base material 41 through the fusion zone 43. The fusion zone 43 fuses the tip 44 and the base material 41.

The melting portion 43 has an overlapping portion 48 where a first boundary surface 46 of the tip 44 and the melting portion 43 and a second boundary surface 47 of the base material 41 and the melting portion 43 overlap in the first direction (the direction of the arrow Z). Fig. 2b is also a cross-sectional view of the ground electrode 40 cut along a cutting line (line IIb-IIb) passing through the center of gravity 49, and the center of gravity 49 is the center of gravity of a planar view of the overlapping portion 48 projected onto a virtual plane (plane parallel to the paper plane of fig. 2 a) parallel to the discharge surface 45. Arrow Y indicates a second direction parallel to discharge surface 45 and on the cutting line (line IIb-IIb). Although a few cutting lines can be drawn through the center of gravity 49, in the present embodiment, the cutting lines are drawn on the diagonal lines of the discharge surface 45 of the tip 44 so that the length of the overlapping portion 48 in the second direction is longest, and the cross sections are analyzed.

An example of the method for manufacturing the ground electrode 40 will be described with reference to fig. 3 (a). Fig. 3(a) is a schematic view of the tip 44 joined to the base material 41, and shows a state before the melted portion 43 (indicated by a two-dot chain line) is formed. Fig. 3 a shows a cross section at a cutting line perpendicular to the front end surface 41a of the base material 41 and parallel to the side surface 41b (the same applies to fig. 3 b).

The groove bottom 42b formed in the base material 41 is inclined so that the groove becomes deeper from the wall surface 42a toward the distal end surface 41 a. The bottom surface 45a of the tip 44 is inclined so that the thickness of the portion of the tip 44 disposed near the wall surface 42a of the base material 41 is thinner than the thickness of the portion disposed near the distal end surface 41 a.

After the tip 44 is disposed on the base material 41, a high-energy beam such as a laser beam or an electron beam is irradiated from the processing head 54 facing the front end surface 41a of the base material 41. The machining head 54 is moved along the groove bottom 42b while irradiating the beam to form the molten portion 23, and the tip 44 is joined to the base material 41. Since the distal end surface 41a of the base material 41 is irradiated with the beam, the amount of melting in the vicinity of the distal end surface 41a of the base material 41 is larger than the amount of melting in the vicinity of the wall surface 42a of the base material 41. Further, since the bottom surface 45a and the groove bottom 42b of the tip 44 are inclined, the melting portion 43 melts more of the tip 44 than the base material 41 near the front end surface 41a, and melts more of the base material 41 than the tip 44 near the wall surface 42 a.

The description will be made with reference to fig. 2 (b). In the present embodiment, at one end 50 (one end) of the overlapping portion 48 in the second direction (arrow Y direction) along the discharge surface 45 of the tip 44, the melting amount of the tip 44 is larger than the melting amount of the base material 41, and therefore the content of the noble metal is higher than 50 mass%. On the other hand, at the other end 51 (the other end) of the overlapping portion 48 in the second direction, the melting amount of the base material 41 is larger than that of the tip 44, and therefore the Ni content is higher than 50 mass%. The end portions 50 and 51 are line segments having the first boundary surface 46 and the second boundary surface 47 as both ends, and each line segment (end portions 50 and 51) is perpendicular to the discharge surface 45.

Since the end portions 50 and 51 have different contents of the noble metal and Ni, the thermal stress generated at the second boundary surface 47 is larger at the end portion 50 than at the first boundary surface 46. On the other hand, at the end portion 51, the thermal stress generated at the first boundary surface 46 is larger than the thermal stress generated at the second boundary surface 47. As a result, cracks tend to occur in the second boundary surface 47 near the end 50, and cracks tend to occur in the first boundary surface 46 near the end 51. The cracks generated at the first boundary surface 46 tend to progress along the first boundary surface 46, and the cracks generated at the second boundary surface 47 tend to progress along the second boundary surface 47, so that even if the cracks progress, the cracks respectively progressing along the first boundary surface 46 and the second boundary surface 47 can be made difficult to connect with each other. Therefore, compared to the case where the cracks generated at both ends of one boundary surface progress toward the center of the boundary surface along the boundary surface, it is possible to suppress the tip 44 from falling off from the base material 41 due to the breakage of the melting portion 43.

Furthermore, the content of noble metal and Ni in the end portions 50 and 51 of the overlapping portion 48 can be quantitatively analyzed by WDS analysis using EPMA. The width of the end portions 50, 51 in the second direction (the thickness of the line segment) is a width (at least 20 μm in the present embodiment) necessary for quantitative analysis. The content of the noble metal and Ni in the end portions 50 and 51 can be obtained by averaging the analysis values of a plurality of measurement points set at equal intervals on the end portions 50 and 51. Further, the representative value may be an analysis value of the midpoint position of each of the end portions 50 and 51 (line segment).

Since the amount of melting near the distal end surface 41a of the melting portion 43 is larger than the amount of melting near the wall surface 42a of the base material 41, the overlapping portion 48 has a shape in which the distance along the first direction (the direction of the arrow Z) between the first boundary surface 46 and the second boundary surface 47 gradually increases toward the second direction (the direction of the arrow Y). The intermediate portion 53 of the overlapping portion 48, which has a noble metal content of 50 mass% and an Ni content of 50 mass%, is located on the second direction (arrow Y direction) side with respect to the center position 52 of the overlapping portion 48 in the second direction. Further, the center position 52 is a position including an intermediate point located at a distance L equal to the distance from the end portions 50, 51.

Thus, the crack generated in the vicinity of the end portion 51 tends to progress along the first boundary surface 46 at the portion from the end portion 51 to the intermediate portion 53 in the first boundary surface 46, as compared with the portion from the end portion 50 to the intermediate portion 53 in the first boundary surface 46. On the other hand, a crack generated in the vicinity of the end portion 50 tends to progress along the second boundary surface 47 at a portion from the end portion 50 to the intermediate portion 53 in the second boundary surface 47, as compared with a portion from the end portion 51 to the intermediate portion 53 in the second boundary surface 47. As a result, the position where the cracks respectively developing along the first boundary surface 46 and the second boundary surface 47 overlap in the first direction (the direction of arrow Z) can be easily brought closer to the second direction (the direction of arrow Y) side of the center position 52. Even if the crack progresses in the first direction (arrow Z direction) in the melted portion 43 at this position, the distance between the first boundary surface 46 and the second boundary surface 47 is longer than the distance between the overlapping portion 48 and the end 51 of the central portion 52, so that breakage of the melted portion 43 can be suppressed, and the tip 44 can be further suppressed from dropping off from the base material 41.

In the cross section of the overlapping portion 48 having the longest length in the second direction (the direction of the arrow Y), the relationship is established that the content of the noble metal is higher than 50 mass% at one end portion 50 and the content of Ni is higher than 50 mass% at the other end portion 51. The position of the cross section can make the length of the first boundary surface 46 and the second boundary surface 47 where cracks are likely to progress longest, and therefore, the tip 44 can be further suppressed from coming off.

Another embodiment of the ground electrode 40 will be described with reference to fig. 3 (b). FIG. 3(b) is a schematic view showing the joining of the tip 44 to the other base material 41. Unlike fig. 3(a), the groove bottom 42c formed in the base material 41 is inclined so that the groove becomes shallower from the wall surface 42a toward the distal end surface 41 a. The bottom surface 45b of the tip 44 is inclined so that the thickness of the portion of the tip 44 disposed near the wall surface 42a of the base material 41 is greater than the thickness of the portion disposed near the distal end surface 41 a.

After the tip 44 is disposed on the base material 41, a high-energy beam is irradiated from a processing head 54 facing the front end surface 41a of the base material 41 to form a melted portion 43, and the tip 44 is joined to the base material 41. Since the bottom surface 45b and the groove bottom 42c of the tip 44 are inclined, the melting portion 43 has a larger melting amount of the base material 41 than the tip 44 near the front end surface 41a, and has a larger melting amount of the tip 44 than the base material 41 near the wall surface 42 a.

The description will be made with reference to fig. 2 (b). In the present embodiment, the melting amount of the base material 41 is larger than the melting amount of the tip 44 at one end portion 50 (the other end portion) of the overlapping portion 48 in the second direction (the arrow Y direction) along the discharge surface 45 of the tip 44, and therefore the content of Ni is higher than 50 mass%. On the other hand, at the other end 51 (one end) of the overlapping portion 48 in the second direction, the amount of melting of the tip 44 is larger than that of the base material 41, and therefore the content of the noble metal is higher than 50 mass%.

Thus, at the end portion 50, the thermal stress generated at the first boundary surface 46 is larger than the thermal stress generated at the second boundary surface 47. On the other hand, at the end portion 51, the thermal stress generated at the second boundary surface 47 is larger than the thermal stress generated at the first boundary surface 46. As a result, cracks are likely to occur in the first boundary surface 46 near the end 50, and cracks are likely to occur in the second boundary surface 47 near the end 51. The cracks generated at the first boundary surface 46 tend to progress along the first boundary surface 46, and the cracks generated at the second boundary surface 47 tend to progress along the second boundary surface 47, so that even if the cracks progress, the cracks respectively progressing along the first boundary surface 46 and the second boundary surface 47 can be made difficult to connect with each other. Therefore, compared to the case where the cracks generated at both ends of one boundary surface progress toward the center of the boundary surface along the boundary surface, the tip 44 can be prevented from falling off from the base material 41 due to the breakage of the melting portion 43.

The center electrode 20 will be described next. Fig. 4(a) is a bottom view of the center electrode 20 (first electrode) as viewed from the direction of the axis O, and fig. 4(b) is a cross-sectional view of the center electrode 20 at the line IVb-IVb of fig. 4 (a). The arrow Z indicates a first direction perpendicular to the discharge surface 25 of the tip 24. If the center electrode 20 is set as the first electrode, the ground electrode 40 is the second electrode. In the present embodiment, the outer shape of the base material 22 is a cylindrical shape extending along the axis O, and the tip 24 is a disk shape. The tip 24 is disposed at the axial tip of the base material 22 and connected to the base material 22 through the fusion zone 23. The fusion zone 23 fuses the tip 24 and the base material 22.

The melting portion 23 has an overlapping portion 62 where a first boundary surface 60 between the tip 24 and the melting portion 23 and a second boundary surface 61 between the base material 22 and the melting portion 23 overlap each other in a first direction (a direction of an arrow Z which is the same as the direction of the axis O). Fig. 4(b) is also a cross-sectional view of the center electrode 20 cut by a cutting line (line IVb-IVb) passing through the center of gravity 63, and the center of gravity 63 is the center of a planar view of the overlapping portion 62 projected onto a virtual plane (a plane parallel to the paper plane in fig. 4 (a)) parallel to the discharge surface 25. The position of the center of gravity 63 is substantially the same as the position of the axis O. Arrow Y indicates a second direction parallel to the discharge surface 25 and on the cutting line (IVb-IVb line).

An example of the method for manufacturing the center electrode 20 is described with reference to fig. 5 (a). Fig. 5a is a schematic view of the tip 24 being joined to the base material 22, and shows a state before the melted portion 23 (indicated by a two-dot chain line) is formed (the same applies to fig. 5 b).

The distal end surface 22a of the base material 22 and the end surface 24a of the tip 24 opposite to the discharge surface 25 are planes obliquely intersecting the axis O. Thus, the distance between the discharge surface 25 and the end surface 24a of the tip 24 is longer at the portion 24b of the portions 24b and 24c on both sides of the tip 24 across the axis O, and the distance between the discharge surface 25 and the end surface 24a is shorter at the portion 24c than at the portion 24 b. The tip 24 is disposed on the base material 22 by bringing an end surface 24a of the tip 24 into contact with the front end surface 22a of the base material 22 so that the discharge surface 25 of the tip 24 is orthogonal to the axis O.

After the tip 24 is disposed on the base material 22, the tip 24 is joined to the base material 22 by irradiating a machining head 54 facing the side surfaces of the base material 22 and the tip 24 with an energy beam such as a laser beam or an electron beam while rotating the base material 22 and the tip 24 about the axis O to form a fusion zone 23. Since the side surface of the base material 22 is irradiated with the beam, the amount of melting on the outer side in the radial direction of the base material 22 is larger than the amount of melting on the center in the radial direction of the base material 22. Further, since the tip end surface 22a of the base material 22 and the end surface 24a of the tip 24 are inclined, the melting portion 23 has a portion 24b of the tip 24 in which the amount of melting of the tip 24 is larger than the amount of melting of the base material 22, and a portion 24c on the opposite side of the axis O in which the amount of melting of the base material 22 is larger than the amount of melting of the tip 24.

The description will be made with reference to fig. 4 (b). In the present embodiment, the melting amount of the tip 24 is larger than the melting amount of the base material 22 at the one end 64 of the overlapping portion 62 in the second direction (arrow Y direction) along the discharge surface 25 of the tip 24, and therefore the content of the noble metal is higher than 50 mass%. On the other hand, at the other end portion 65 of the overlapping portion 62 in the second direction, the melting amount of the base material 22 is larger than that of the tip 24, so the Ni content is higher than 50 mass%.

Thus, at the one end portion 64, the thermal stress generated at the second boundary surface 61 is larger than the thermal stress generated at the first boundary surface 60. On the other hand, at the other end portion 65, the thermal stress generated at the first boundary surface 60 is larger than the thermal stress generated at the second boundary surface 61. As a result, cracks tend to occur in the second boundary surface 61 near the one end 64, and cracks tend to occur in the first boundary surface 60 near the other end 65. The cracks generated at the first boundary surface 60 tend to progress along the first boundary surface 60, and the cracks generated at the second boundary surface 61 tend to progress along the second boundary surface 61, so that even if the cracks progress, the cracks respectively progressing along the first boundary surface 60 and the second boundary surface 61 can be made difficult to connect with each other. Therefore, compared to the case where the cracks generated at both ends of one boundary surface progress toward the center of the boundary surface along the boundary surface, it is possible to suppress the tip 24 from falling off from the base material 22 due to the breakage of the melting portion 23.

In addition, since the amount of melting of the molten portion 23 on the outer side in the radial direction of the base material 22 is larger than the amount of melting of the center in the radial direction of the base material 22, the overlapping portion 62 has a shape in which the distance along the first direction (the direction of arrow Z) between the first boundary surface 60 and the second boundary surface 61 gradually becomes shorter from the outer side toward the center. Thus, in the overlapping portion 62, the shortest portion 66, in which the distance along the first direction between the first boundary surface 60 and the second boundary surface 61 is shortest, is located between the one end portion 64 and the other end portion 65, at a position other than the one end portion 64 and the other end portion 65. The intermediate portion 67 having a noble metal content of 50 mass% and an Ni content of 50 mass% in the overlapping portion 62 is located at a position other than the shortest portion 66.

Thus, the crack generated in the vicinity of the other end 65 tends to progress along the first boundary surface 60 at the portion from the other end 65 to the intermediate portion 67 in the first boundary surface 60, as compared with the portion from the one end 64 to the intermediate portion 67 in the first boundary surface 60. On the other hand, the crack generated in the vicinity of the one end portion 64 tends to progress along the second boundary surface 61 at a portion from the vicinity of the one end portion 64 to the intermediate portion 67 in the second boundary surface 61, as compared with a portion from the vicinity of the other end portion 65 to the intermediate portion 67 in the second boundary surface 61. Since the shortest portion 66 and the intermediate portion 67 are located at different positions in the second direction (arrow Y direction), the position where the cracks that have progressed along the first boundary surface 60 and the second boundary surface 61 respectively overlap in the first direction (arrow Z direction) is likely to be a location other than the shortest portion 66. Even if the crack progresses in the first direction at this position, the distance between the first boundary surface 60 and the second boundary surface 61 is longer than the distance of the shortest portion 66, so that breakage of the melted portion 23 is suppressed, and the tip 24 can be further suppressed from coming off.

Another manufacturing method of the center electrode 20 is described with reference to fig. 5 (b). Fig. 5(b) is a schematic view when the tip 24 is joined to another base material 22. Unlike fig. 5(a), the end surface 24d of the tip 24 is parallel to the discharge surface 25, and the tip end surface 22b of the base material 22 is a surface perpendicular to the axis O. After the end surface 24d of the tip 24 is brought into contact with the front end surface 22b of the base material 22 and the tip 24 is disposed on the base material 22, the machining head 54 facing the side surfaces of the base material 22 and the tip 24 is irradiated with a high-energy beam while reciprocating along the axis O while rotating the base material 22 and the tip 24 about the axis O. Thereby, the trajectory of the beam scanning the surfaces of the base material 22 and the tip 24 becomes elliptical.

In this case, in a cross-sectional view of the center electrode 20 cut by a cutting line passing through the center of gravity 63, the content of noble metal in the end portion of the overlapping portion 62 on the side of the portion 24b is higher than 50 mass% (the amount of melting of the tip 24 is larger than the amount of melting of the base material 22) and the content of Ni in the end portion on the side of the other portion 24c is higher than 50 mass% (the amount of melting of the base material 22 is larger than the amount of melting of the tip 24), where the center of gravity 63 is the center of a plan view pattern of the overlapping portion 62 projected on a virtual plane parallel to the discharge surface 25. This can achieve the same operational effects as those of the above embodiment.

The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments at all, and various modifications can be easily assumed without departing from the spirit of the present invention.

In the embodiment, the case where the groove is formed in the base material 41 of the ground electrode 40 and the tip 44 partially accommodated in the groove is joined to the base material 41 has been described, but the present invention is not necessarily limited thereto. It is not necessary to form a groove on the base material 41. Of course, the tip 44 may be joined to the base material 41 without forming a groove.

In the embodiment, the case where the tip surface of the tip 44 is slightly inward of the tip surface 41a of the ground electrode 40 has been described, but the present invention is not necessarily limited to this. Of course, the tip 44 may be projected from the front end surface 41a of the ground electrode 40 by projecting the tip 44 beyond the front end surface 41a of the ground electrode 40.

In the embodiment, the case where the tip 44 is joined to the inner surface 42 of the base material 41 of the ground electrode 40 has been described, but the present invention is not necessarily limited thereto. Of course, the tip 44 may be joined to the distal end surface 41a of the base material 41 other than the inner surface 42.

In the embodiment, the case where the tip 44 of the ground electrode 40 has a rectangular parallelepiped (quadrangular prism) shape has been described, but the present invention is not necessarily limited thereto. The shape of the end 44 may be appropriately set to a cylindrical shape, a polygonal column other than a quadrangular column, or the like.

In the embodiment, the case where the tip 44 is directly connected to the base material 41 of the ground electrode 40 via the fusion zone 43 has been described, but the present invention is not necessarily limited thereto. Of course, an intermediate member mainly made of Ni may be disposed between the base material and the tip, and the tip may be connected to the intermediate member joined to the base material via the fusion zone.

In the embodiment, the case where the relationship that the content of the noble metal is higher than 50 mass% at one end portion of the overlapping portions 48 and 62 and the content of Ni is higher than 50 mass% at the other end portion is satisfied at both the center electrode 20 and the ground electrode 40 has been described, but the present invention is not necessarily limited thereto. The above relationship may be established in either the center electrode or the ground electrode. In the electrode in which the above-described relationship holds, the tip can be suppressed from coming off.

In the embodiment, the case where the base material 22 and the tip 24 are integrally formed and the base material 22 and the tip 24 are irradiated with the high-energy beam while rotating about the axis O has been described as an example of manufacturing the center electrode 20, but the present invention is not necessarily limited thereto. Of course, the structure in which the base material 22 and the tip 24 are integrated may be stopped, and the fusion zone 23 may be formed by scanning a high-energy beam around the base material 22 and the tip 24 using one or more mirrors.