Spark plug and method for manufacturing the same

Abstract

To provide a spark plug capable of inhibiting peeling occurring between a ground electrode and a housing even if hardness of an outer cover layer in the ground electrode is increased.SOLUTION: A base end (51) of a spark plug has an outer cover layer (511), an intermediate layer (512), and a core material layer (513). A junction interface (51a) of the base end (51) joined to a housing (11) is formed only by the core material layer (513). A first angle (θ1) formed between a tangent line (L1) passing through an intersection point (P1) at which an outer surface (11b) of the housing intersects with the junction interface (51a) and contacting with the junction interface, and an outside line (11bL) along the outer surface, is an acute angle.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to spark plugs and methods of manufacturing spark plugs. [Background technology]

[0002] Patent Document 1 listed below discloses an invention related to a spark plug. In the spark plug described in Patent Document 1 listed below, the base end of the ground electrode has an outer shell portion located on the surface side, an intermediate portion having a higher thermal conductivity than the outer shell portion, and a core portion having a higher hardness than the intermediate portion. The weld interface between the housing and the ground electrode only contains the outer shell portion and the core portion, and does not include the intermediate portion. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-27754 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 describes that the intermediate portion, whose material strength is weaker than the other two portions, is not included in the weld interface with the housing, thereby preventing a decrease in the weld strength of the ground electrode. Patent Document 1 also describes that the outer shell is made of a metal material with the highest hardness and heat resistance among the ground electrodes, such as a Ni-based heat-resistant alloy. The outer shell contains a predetermined percentage of aluminum. The intermediate portion is made of, for example, pure Cu or a Cu alloy. The core portion is made of, for example, pure Ni or a Ni alloy.

[0005] In Patent Document 1, elements such as aluminum are added to the material that makes up the outer cover, which increases the hardness of the outer cover but reduces its ductility. As a result, there is a concern that peeling may occur at the joint interface between the ground electrode and the housing when the ground electrode is bent by applying force in a direction that pulls it away from the housing.

[0006] An object of the present disclosure is to provide a spark plug that can suppress peeling between the ground electrode and the housing even when the hardness of the outer coating layer of the ground electrode is increased. [Means for solving the problem]

[0007] The present disclosure provides a spark plug comprising an insulator (12) that houses a center electrode (13), a housing (11) that houses the insulator, and a ground electrode (14) having a tip end (52) disposed to form a predetermined gap with respect to the center electrode, and a base end (51) that extends along the center electrode and is joined to the housing. The base end has an outer skin layer (511) disposed on a surface side thereof, an intermediate layer (512) having a higher thermal conductivity than the outer skin layer, and a core material layer (513) having a higher hardness than the intermediate layer. In a cross section including the center axis of the spark plug and the base end, a joint interface (51a) of the base end with the housing is formed solely by the core material layer, and a first angle (θ1) formed by an outer line (11bL) along the outer surface of the housing that passes through an intersection (P1) between the joint interface and an outer surface (11b) of the housing and tangent to the joint interface is an acute angle.

[0008] The present disclosure relates to a method for manufacturing a spark plug, comprising: preparing a housing (11) that accommodates an insulator including a center electrode; preparing a ground electrode material (50M) that is joined to the housing to form a ground electrode, the ground electrode material having an outer cover material (511M) disposed on the surface side, an intermediate material (512M) having a higher thermal conductivity than the outer cover material, and a core material (513M) having a higher hardness than the intermediate material; forming a mating surface of the ground electrode material with the housing to have a first surface (50Ma) that is aligned with a front end surface (110) of the housing and a second surface (50Mb) that is disposed from the first surface toward the outer cover material; inclining the second surface toward an outer side surface (11b) of the housing so as to be away from the front end surface; and abutting the mating surface of the ground electrode material against the front end surface of the housing and resistance welding the ground electrode material to join the ground electrode material to the housing to form a ground electrode. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a spark plug that can suppress peeling between the ground electrode and the housing even when the hardness of the outer skin layer of the ground electrode is increased. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a partial cross-sectional view of a spark plug according to this embodiment. [Figure 2] FIG. 2 is a perspective view showing the vicinity of the ground electrode of the spark plug according to this embodiment. [Figure 3] FIG. 3 is a cross-sectional view showing the ground electrode of the spark plug of this embodiment. [Figure 4] FIG. 4 is a cross-sectional view showing the ground electrode of the spark plug of this embodiment. [Figure 5] FIG. 5 is a cross-sectional view showing the ground electrode of the spark plug of this embodiment. [Figure 6] FIG. 6 is a diagram for explaining a method for manufacturing the spark plug according to this embodiment. [Figure 7] FIG. 7 is a diagram for explaining a method for manufacturing the spark plug according to this embodiment. [Figure 8] FIG. 8 is a diagram for explaining a method for manufacturing the spark plug according to this embodiment. [Figure 9] FIG. 9 is a diagram for explaining a method for manufacturing the spark plug according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. To facilitate understanding of the description, the same components in the drawings will be denoted by the same reference numerals as much as possible, and duplicated descriptions will be omitted.

[0012] The schematic configuration of a spark plug 10 according to an embodiment will be described with reference to FIG. 1. The spark plug 10 is attached to, for example, an engine head block. The spark plug 10 ignites an air-fuel mixture in an engine cylinder by generating a spark discharge based on the application of voltage. The spark plug 10 includes a housing 11, an insulator 12, a center electrode 13, and a ground electrode 14.

[0013] The housing 11 is cylindrically formed around a plug central axis m10, which is the central axis of the spark plug 10. The housing 11 is formed of a metal material such as carbon steel. The lower end of the insulator 12 is inserted into the housing 11. A threaded portion 114 is formed on the outer peripheral surface of the lower part of the housing 11. The spark plug 10 can be fastened and fixed to the engine head block by screwing the threaded portion 114 of the housing 11 into a threaded hole formed in the engine head block. Hereinafter, the direction along the plug central axis m10 will also be referred to as the "plug axial direction Da." The circumferential direction about the plug central axis m10 will also be referred to as the "plug circumferential direction Dc." The side facing the ground electrode 14 will also be referred to as the tip side, and the opposite side will also be referred to as the base side.

[0014] The insulator 12 is formed in a cylindrical shape centered on the plug central axis m10. The insulator 12 is made of an insulating material such as alumina. A housing 11 is integrally assembled to the outer periphery of the insulator 12. An axial hole 120 is formed inside the insulator 12. The axial hole 120 is formed so as to penetrate the insulator 12 from the tip end to the base end along the plug central axis m10. A center electrode 13, a first seal body 15, a resistor 16, a second seal body 17, and a terminal fitting 18 are inserted into the axial hole 120 in this order from the tip end. The center electrode 13 is inserted into the insulator 12 so as to be exposed from the tip end of the insulator 12.

[0015] The center electrode 13 has a center electrode base material 30 and a center electrode tip 40. The center electrode base material 30 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode base material 30 is made of a nickel (Ni) alloy or the like, which has excellent heat resistance. The center electrode tip 40 is joined to the front end of the center electrode base material 30. The center electrode tip 40 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode tip 40 is made of an iridium alloy or the like. A first seal body 15, a resistor 16, and a second seal body 17 are sandwiched between the base end side of the center electrode 13 and the front end side of the terminal fitting 18.

[0016] The terminal fitting 18 is formed in a substantially cylindrical shape centered on the plug central axis m10. The terminal fitting 18 is made of steel or the like. A terminal portion 180 is provided on the base end side of the terminal fitting 18. The terminal portion 180 is exposed to the outside from the base end side of the insulator 12.

[0017] The ground electrode 14 has a ground electrode base material 50 and a ground electrode tip 60. The ground electrode base material 50 is made of a nickel alloy or the like. The ground electrode base material 50 is joined to a front end surface 110 of the housing 11. The ground electrode base material 50 is formed so as to extend from the front end surface 110 of the housing 11 to a position facing the center electrode tip 40. The ground electrode tip 60 is joined to the front end of the ground electrode base material 50. The ground electrode tip 60 is made of a precious metal alloy such as an iridium alloy or a platinum alloy. The ground electrode tip 60 is positioned so as to face the center electrode tip 40. The gap formed between the center electrode tip 40 and the ground electrode tip 60 is called a spark gap 19.

[0018] In the spark plug 10, an external circuit capable of applying a high voltage is connected to the terminal portion 180 of the terminal fitting 18. When a high voltage is applied to the terminal portion 180 from the external circuit, a spark discharge is formed between the center electrode tip 40 of the center electrode 13 and the ground electrode tip 60 of the ground electrode 14. This spark discharge ignites the air-fuel mixture in the cylinder of the engine, forming a flame, and the air-fuel mixture burns.

[0019] Next, the structures of the housing 11 and the ground electrode 14 will be described in detail. As shown in FIG. 2, the ground electrode base material 50 has a base end 51 and a tip end 52. The base end 51 is joined to the tip end surface 110 of the housing 11. The base end 51 is formed to extend from the tip end surface 110 of the housing 11 in the plug axial direction Da. The tip end portion 52 is formed to extend from the tip of the base end 51 to a position facing the center electrode tip 40 of the center electrode 13. The ground electrode base material 50 is formed into a generally L-shape as a whole by being composed of the base end 51 and the tip end portion 52. In this embodiment, the surface connected to the tip end surface 110 of the housing 11 and facing the plug center axis m10 is referred to as the inner surface 11a, and the outer surface opposite the inner surface 11a is referred to as the outer surface 11b.

[0020] As shown in FIG. 3, the base end portion 51 of the ground electrode base material 50 has an outer skin layer 511 , an intermediate layer 512 , and a core layer 513 .

[0021] The outer skin layer 511 is the layer disposed on the outermost side when viewed from the center of the base end portion 51. The outer skin layer 511 is formed on both the inner surface 11a side and the outer surface 11b side. The outer skin layer 511 is made of, for example, a Ni-based heat-resistant alloy containing nickel (Ni) as its main component. In this specification, the term "main component" refers to the material component with the largest content. It is desirable that the alloy constituting the outer skin layer 511 contains aluminum (Al) at a predetermined ratio.

[0022] The intermediate layer 512 is a layer disposed inside the outer layer 511. The intermediate layer 512 is made of a metal material having a higher thermal conductivity than the outer layer 511. The intermediate layer 512 is desirably made of a metal material having a higher thermal conductivity than the metal material constituting the core layer 513. The intermediate layer 512 is made of, for example, pure Cu or a Cu alloy.

[0023] Core layer 513 is a layer disposed inside skin layer 511 and / or intermediate layer 512. Core layer 513 is made of a metal material that is harder than intermediate layer 512. Core layer 513 is made of, for example, pure Ni or a Ni alloy.

[0024] In the example shown in FIG. 3, the joint interface 51a of the base end portion 51 with the housing 11 is formed only by the core layer 513.

[0025] A first angle θ1 is an acute angle formed between a tangent line L1 that passes through an intersection P1 between the outer side surface 11b of the housing 11 and the joint interface 51a and is tangent to the joint interface 51a, and an outer line 11bL along the outer side surface 11b. A second angle θ2 is an acute angle formed between a tangent line L2 that passes through an intersection P2 between the inner side surface 11a of the housing 11 and the joint interface 51a and is tangent to the joint interface 51a, and an inner line 11aL along the inner side surface 11a. In the example shown in FIG. 3, both the first angle θ1 and the second angle θ2 are acute angles, but only one of them may be an acute angle.

[0026] A spark plug 10 according to this embodiment includes an insulator 12 that houses a center electrode 13, a housing 11 that houses the insulator 12, and a ground electrode 14 that has a tip end 52 disposed so as to form a predetermined gap with respect to the center electrode 13, and a base end 51 that extends along the center electrode 13 and is joined to the housing 11. The base end 51 has an outer cover layer 511 disposed on the surface side, an intermediate layer 512 having a higher thermal conductivity than the outer cover layer 511, and a core layer 513 having a higher hardness than the intermediate layer 512. In a cross section including the plug center axis m10, which is the center axis of the spark plug, and the base end 51, a joint interface 51a of the base end 51 with the housing 11 is formed only by the core layer 513.

[0027] A first angle θ1 is an acute angle formed between a tangent line L1 that passes through an intersection point P1 between the outer surface 11b of the housing 11 and the bonded interface 51a and is tangent to the bonded interface 51a, and an outer line 11bL along the outer surface 11b. For example, when an external force that bends the base end 51 from the outer surface 11b side of the housing 11 toward the inner surface 11a is applied to the base end 51, tensile stress is generated at the bonded interface 51a on the outer surface 11b side because the first angle θ1 is an acute angle. The tensile stress is stress along a direction that intersects the bonded interface 51a from the housing 11 to the base end 51. Therefore, compared to shear stress that would occur when the first angle θ1 is an obtuse angle, stress is generated in a direction that increases the bond strength between the housing 11 and the base end 51, thereby suppressing peeling.

[0028] In the spark plug 10 according to this embodiment, a second angle θ2 is formed between a tangent line L2 that passes through an intersection point P2 between the inner surface 11a of the housing and the joint interface 51a and is tangent to the joint interface 51a, and an inner line 11aL that runs along the inner surface 11a. For example, when an external force that bends the base end 51 from the inner surface 11a side of the housing 11 toward the outer surface 11b is applied to the base end 51, tensile stress is generated at the joint interface 51a on the inner surface 11a side because the second angle θ2 is an acute angle. The tensile stress is a stress that runs along a direction that intersects the joint interface 51a from the housing 11 to the base end 51. Therefore, compared to a shear stress that would occur when the second angle θ2 is an obtuse angle, the stress is generated in a direction that increases the joint strength between the housing 11 and the base end 51, thereby suppressing the occurrence of separation.

[0029] When the first angle θ1 and the second angle θ2 are both acute angles as shown in FIG. 3, the effects achieved when the first angle θ1 is an acute angle and the effects achieved when the second angle θ2 is an acute angle are both achieved.

[0030] 4, a part of the joint interface 51a of the base end portion 51 with the housing 11 is formed of a core material layer 513 and an outer skin layer 511. More specifically, on the outer surface 11b side of the housing 11, the joint interface 51a is formed of the core material layer 513 and the outer skin layer 511.

[0031] 4, similarly to the example shown in Fig. 3, the base end portion 51 has an outer skin layer 511 disposed on the surface side, an intermediate layer 512 having a higher thermal conductivity than the outer skin layer 511, and a core material layer 513 having a higher hardness than the intermediate layer 512. In the example shown in Fig. 4, in a cross section including the plug central axis m10, which is the central axis of the spark plug, and the base end portion 51, a joint interface 51a of the base end portion 51 with the housing 11 is formed by the core material layer 513 and the outer skin layer 511.

[0032] In the example shown in FIG. 4 , a third angle θ3 is an acute angle formed between a tangent line L3 that passes through an intersection point P3 between the outer surface 11b of the housing 11 and the bonded interface 51a and is tangent to the bonded interface 51a, and an outer line 11bL along the outer surface 11b. For example, when an external force that bends the base end 51 from the outer surface 11b side of the housing 11 toward the inner surface 11a is applied to the base end 51, tensile stress is generated at the bonded interface 51a on the outer surface 11b side because the third angle θ3 is an acute angle. The tensile stress is stress along a direction that intersects the bonded interface 51a from the housing 11 to the base end 51. Therefore, compared to shear stress that would occur when the third angle θ3 is an obtuse angle, stress is generated in a direction that increases the bond strength between the housing 11 and the base end 51, thereby suppressing peeling.

[0033] 5, the joint interface 51a of the base end 51 with the housing 11 is formed of the core material layer 513 and the outer skin layer 511. More specifically, the joint interface 51a is formed of the core material layer 513 and the outer skin layer 511 on both the outer surface 11b side and the inner surface 11a side of the housing 11.

[0034] In the example shown in Fig. 5, in addition to the third angle θ3, the fourth angle θ4 is also an acute angle. The third angle θ3 is the same as that described with reference to Fig. 4. In the example shown in Fig. 5, the fourth angle θ4, which is the angle formed by a tangent line L4 that passes through an intersection P4 between the inner surface 11a of the housing 11 and the joint interface 51a and is tangent to the joint interface 51a, and an inner line 11aL along the inner surface 11a, is an acute angle.

[0035] For example, when an external force is applied to the base end 51 so as to bend the housing 11 from the outer surface 11b toward the inner surface 11a, the effect is the same as that described with reference to FIG. 4 . On the other hand, when an external force is applied to the base end 51 so as to bend the housing 11 from the inner surface 11a toward the outer surface 11b, tensile stress is generated at the bonding interface 51a on the inner surface 11a side because the fourth angle θ4 is an acute angle. The tensile stress is a stress along a direction intersecting the bonding interface 51a from the housing 11 to the base end 51. Therefore, compared to the shear stress generated when the fourth angle θ4 is an obtuse angle, for example, stress is generated in a direction that increases the bonding strength between the housing 11 and the base end 51, thereby suppressing the occurrence of peeling. In the example shown in FIG. 5 , both the third angle θ3 and the fourth angle θ4 are acute angles, but only one of them may be an acute angle.

[0036] A method for manufacturing the spark plug 10 will be described with reference to Figures 6, 7, and 8. As shown in Figure 6, a presser J1, a stopper J2, an upper electrode E1, and a lower electrode E2 are prepared. The housing 11 is placed on the presser J1. The presser J1 is configured to hold the housing 11 and apply pressure in the direction of the arrow in the figure. A ground electrode material 50M, which will become the ground electrode base material 50 after processing, is prepared and held between the upper electrode E1 and the lower electrode E2. One end of the ground electrode material 50M is abutted against the housing 11, and the other end is held by the stopper J2. After being arranged in this manner, current is passed through the upper electrode E1 and the lower electrode E2, performing resistance welding.

[0037] The joining state between the ground electrode material 50M and the housing 11 during resistance welding will be described with reference to Fig. 7. Fig. 7 is an enlarged view of part VII in Fig. 6 rotated 90° clockwise.

[0038] 7(A), the ground electrode material 50M has an outer cover material 511M, an intermediate material 512M, and a core material 513M. The outer cover material 511M is a portion that will become the outer cover layer 511 after processing, and is made of the same material as the outer cover layer 511. The intermediate material 512M is a portion that will become the intermediate layer 512 after processing, and is made of the same material as the intermediate layer 512. The outer cover material 511M, the intermediate material 512M, and the core material 513M are uniformly configured to abut against the housing 11.

[0039] The outer cover material 511M is inclined so as to be away from the front end surface 110 of the housing 11. On the other hand, the intermediate material 512M and the core material 513M abut against the front end surface 110 of the housing 11. The abutting surface of the ground electrode material 50M with the housing 11 has a first surface 50Ma that is along the front end surface 110 of the housing 11, and a second surface 50Mb that is disposed on the outer cover material 511M side from the first surface 50Ma.

[0040] As shown in FIGS. 7(B) and 7(C), current is passed through the intermediate material 512M and the core material 513M, so that the joining progresses so as to form a gentle concave surface near the center of the housing 11 in the thickness direction.

[0041] As shown in Fig. 7(D), as the joining process progresses further, the core material 513M is deformed so as to be pushed outward. The deformation of the core material 513M ejects the intermediate material 512M, and the outer cover material 511M is also pushed outward. When the core material 513M is cut along the dashed line in Fig. 7(D), the ground electrode base material 50 joined to the housing 11 is formed, as shown in Fig. 7(E).

[0042] Fig. 8 shows a contact state between the upper electrode E1 and the lower electrode E2 and the ground electrode material 50M at a stage corresponding to Fig. 7(D). As shown in Fig. 8, a rectangular melted portion is formed between a corner of the lower electrode E2 and the housing 11. By tilting the lower electrode E2 in a direction away from the housing 11, as in the case of a lower electrode E2A shown in Fig. 9, the melted portion can be directed in a direction away from the housing 11.

[0043] As described above, the method for manufacturing the spark plug 10 begins with preparing the housing 11 that houses the insulator 12 including the center electrode 13. Next, a ground electrode material 50M is prepared, which is to be joined to the housing 11 to form the ground electrode 14. The ground electrode material 50M includes an outer cover material 511M disposed on the front surface side, an intermediate material 512M having a higher thermal conductivity than the outer cover material 511M, and a core material 513M having a higher hardness than the intermediate material 512M. Next, the surface of the ground electrode material 50M that abuts against the housing 11 is formed to have a first surface 50Ma that is aligned with the front end surface 110 of the housing and a second surface 50Mb that is disposed on the outer cover material 511M side from the first surface 50Ma, and the second surface 50Mb is inclined toward the outer surface 11b of the housing 11 so as to be separated from the front end surface 110. Next, the butt surface of the ground electrode material 50M is brought into contact with the tip surface 110 of the housing 11 and resistance-welded to join the ground electrode material 50M to the housing 11, thereby forming the ground electrode 14.

[0044] In the manufacturing method of the spark plug 10 of this embodiment, the second surface 50Mb formed on the ground electrode material 50M is inclined so as to be away from the tip end surface 110 on the outer surface 11b side of the housing 11, so that it is possible to realize the joining state in which the first angle θ1 described with reference to FIG. 3 is an acute angle and the joining state in which the third angle θ3 described with reference to FIGS. 4 and 5 is an acute angle.

[0045] In the method for manufacturing the spark plug 10 of this embodiment, the second surface 50Mb is inclined so as to be separated from the tip end surface 110 on the inner surface 11a side of the housing 11. By inclining the second surface 50Mb in this manner, it is possible to achieve a joining state in which the second angle θ2 is an acute angle as described with reference to Fig. 3 and a joining state in which the fourth angle θ4 is an acute angle as described with reference to Fig. 5.

[0046] The inclination of the second surface 50Mb may be provided only on the inner surface 11a side of the housing 11, or may be provided only on the outer surface 11b side of the housing 11.

[0047] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Design modifications to these specific examples made by a person skilled in the art as appropriate are also included within the scope of the present disclosure as long as they comprise the features of the present disclosure. The elements of the above-described specific examples, as well as their arrangement, conditions, shape, etc., are not limited to those exemplified and can be modified as appropriate. The elements of the above-described specific examples can be combined in various ways as appropriate, as long as no technical contradictions arise. [Explanation of symbols]

[0048] 11: Housing 11a:Inner surface 11b:Outer surface 12: Insulator 13: Center electrode 14: Ground electrode 51: Proximal end 51a: Bonding interface 511: Outer skin layer 512: Middle class 513: Core layer 52:Tip

Claims

1. A spark plug, an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) having a tip end (52) disposed so as to form a predetermined gap with respect to the center electrode, and a base end (51) extending along the center electrode and joined to the housing, The base end portion is An outer skin layer (511) disposed on the surface side; an intermediate layer (512) having a higher thermal conductivity than the outer skin layer; a core layer (513) having a higher hardness than the intermediate layer; and In a cross section including a center axis and the base end portion of the spark plug, a bonding interface (51a) of the base end portion with respect to the housing is formed only by the core material layer, a first angle (θ1) formed by a tangent (L1) that passes through an intersection (P1) between the outer surface (11b) of the housing and the joint interface and is in contact with the joint interface, and an outer line (11bL) that follows the outer surface, is an acute angle.

2. A spark plug, an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) having a tip end (52) disposed so as to form a predetermined gap with respect to the center electrode, and a base end (51) extending along the center electrode and joined to the housing, The base end portion is An outer skin layer (511) disposed on the surface side; an intermediate layer (512) having a higher thermal conductivity than the outer skin layer; a core layer (513) having a higher hardness than the intermediate layer; and In a cross section including a center axis and the base end portion of the spark plug, a bonding interface (51a) of the base end portion with respect to the housing is formed only by the core material layer, a second angle (θ2) formed by a tangent (L2) that passes through an intersection (P2) between the inner surface (11a) of the housing and the joint interface and is in contact with the joint interface, and an inner line (11aL) that runs along the inner surface, is an acute angle.

3. 2. The spark plug according to claim 1, In a cross section including a center axis and the base end portion of the spark plug, a bonding interface between the base end portion and the housing is formed only by the core material layer, a second angle formed by a tangent line passing through an intersection of the inner surface of the housing and the joint interface and contacting the joint interface, and an inner line along the inner surface, is an acute angle.

4. A spark plug, an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) having a tip end (52) disposed so as to form a predetermined gap with respect to the center electrode, and a base end (51) extending along the center electrode and joined to the housing, The base end portion is An outer skin layer (511) disposed on the surface side; an intermediate layer (512) having a higher thermal conductivity than the outer skin layer; a core layer (513) having a higher hardness than the intermediate layer; and In a cross section including a center axis and the base end portion of the spark plug, a joint interface (51a) of the base end portion with respect to the housing is formed by the core material layer and the outer skin layer, a third angle (θ3) formed on the inside of the housing (11), which is an acute angle, is formed between a tangent (L3) that passes through an intersection (P3) between the outer surface (11b) of the housing and the joint interface and is tangent to the joint interface, and an outer line (11bL) along the outer surface.

5. A spark plug, an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) having a tip end (52) disposed so as to form a predetermined gap with respect to the center electrode, and a base end (51) extending along the center electrode and joined to the housing, The base end portion is An outer skin layer (511) disposed on the surface side; an intermediate layer (512) having a higher thermal conductivity than the outer skin layer; a core layer (513) having a higher hardness than the intermediate layer; and In a cross section including a center axis and the base end portion of the spark plug, a joint interface (51a) of the base end portion with respect to the housing is formed by the core material layer and the outer skin layer, a fourth angle (θ4) formed inside the housing (11), which is an acute angle, is formed between a tangent (L4) that passes through an intersection (P4) between the inner surface (11a) of the housing and the joint interface and is in contact with the joint interface, and an inner line (11aL) that runs along the inner surface.

6. 5. The spark plug according to claim 4, In a cross section including a center axis and the base end portion of the spark plug, a joint interface between the base end portion and the housing is formed by the core material layer and the outer skin layer, a fourth angle formed on the inside of the housing (11), which is an acute angle, formed between a tangent line that passes through an intersection of the inner surface of the housing and the joint interface and is in contact with the joint interface, and an inner line along the inner surface.

7. A method for manufacturing a spark plug, comprising: A housing (11) is provided to accommodate an insulator including a center electrode; a ground electrode material (50M) to be joined to the housing and serving as a ground electrode, the ground electrode material (50M) having an outer cover material (511M) disposed on a surface side, an intermediate material (512M) having a higher thermal conductivity than the outer cover material, and a core material (513M) having a higher hardness than the intermediate material; The abutting surface of the ground electrode material with the housing is formed to have a first surface (50Ma) along the tip surface (110) of the housing and a second surface (50Mb) disposed on the outer cover material side from the first surface, The second surface is inclined so as to be spaced apart from the tip surface on the outer surface (11b) side of the housing, abutting a mating surface of the ground electrode material against a front end surface of the housing and resistance welding the ground electrode material to form the ground electrode by joining the ground electrode material to the housing.

8. A method for manufacturing a spark plug, comprising: A housing (11) is provided to accommodate an insulator including a center electrode; a ground electrode material (50M) to be joined to the housing and serving as a ground electrode, the ground electrode material (50M) having an outer cover material (511M) disposed on a surface side, an intermediate material (512M) having a higher thermal conductivity than the outer cover material, and a core material (513M) having a higher hardness than the intermediate material; The abutting surface of the ground electrode material with the housing is formed to have a first surface (50Ma) along the tip surface (110) of the housing and a second surface (50Mb) disposed on the outer cover material side from the first surface, The second surface is inclined so as to be spaced apart from the tip surface on the inner surface (11a) side of the housing, abutting a mating surface of the ground electrode against a front end surface of the housing and resistance welding the abutting surface to form the ground electrode by joining the ground electrode material to the housing.

9. 8. A method for manufacturing a spark plug according to claim 7, comprising: the second surface is inclined so as to be spaced apart from the tip surface on the inner surface side of the housing.

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