spark plug

Abstract

Spark plug (10), having: a first electrode (40, 20) with a tip (44, 24) made mainly of a precious metal and a base material (41, 22) made mainly of Ni, wherein the tip (44, 24) is connected to the base material (41, 22) by a fusion section (43, 23); and a second electrode (20, 40) which is provided such that it is directed towards a discharge surface (45, 25) of the tip (44, 24), and wherein the fusion section (43, 23) has an overlap region (48, 62) in which a first interface (46, 60) between the tip (44, 24) and the fusion section (43, 23) and a second interface (47, 61) between the base material (41, 22) and the fusion section (43, 23) overlap each other in a first direction (Z) that is perpendicular to the discharge surface (45, 25), and When considering a cross-section passing through a centroid (49, 63) of the overlap area (48, 62) projected onto a virtual surface parallel to the discharge surface (45, 25) and perpendicular to the discharge surface (45, 25), in one end section (50, 64) of the overlap area (48, 62) in a second direction (Y) extending along the discharge surface (45, 25) the precious metal content is greater than 50 mass-%, and in the other end section (51, 65) of the overlap area (48, 62) in the second direction (Y) the Ni content is greater than 50 mass-%.

Description

GENERAL STATE OF THE ART

[0001] The present invention relates to a spark plug, and more precisely a spark plug formed by connecting a tip consisting mainly of a precious metal with a base material consisting mainly of Ni (nickel).

[0002] For example, international publication WO 2010 / 113404A1 discloses such a spark plug, which is formed by joining a tip consisting mainly of a precious metal to a base material consisting mainly of nickel (Ni) via a fusion section. JP 2013-118082A discloses a spark plug and a spark plug manufacturing process. US 2012 / 0001533A1 discloses a spark plug and its manufacturing process. BRIEF SUMMARY OF THE INVENTION

[0003] However, in the international publication WO 2010 / 113404A1, a thermal stress occurs at the fusion section due to a difference in the coefficient of linear expansion between the base material and the tip. This stress results from temperature changes in an engine in which the spark plug is installed, and there is a possibility that a crack will form in the fusion section and develop around it, eventually causing the tip to detach from the base material. Therefore, a technique is needed to solve this problem, i.e., a technique to suppress the tip detaching from the base material even if a crack develops at the fusion section due to thermal stress.

[0004] The present invention was developed to fulfill the above requirement. An object of the present invention is therefore to provide a spark plug in which the separation of the tip from the base material can be suppressed.

[0005] To achieve the above objective, a spark plug according to one aspect of the present invention comprises a first electrode with a tip consisting or formed mainly of a noble metal and a base material consisting or formed mainly of Ni, wherein the tip is connected to the base material by a fusion section; and a second electrode provided such that it faces a discharge surface of the tip.The fusion section has an overlap region in which a first interface between the tip and the fusion section and a second interface between the base material and the fusion section overlap each other in a first direction perpendicular to the discharge surface, wherein, when considering a cross-section passing through a centroid of the overlap region projected onto a virtual surface parallel to the discharge surface and perpendicular to the discharge surface, in one end section of the overlap region in a second direction extending along the discharge surface, the precious metal content is greater than 50 wt%, and in the other end section of the overlap region in the second direction, the Ni content is greater than 50 wt%.

[0006] According to the spark plug described above, in the cross-section perpendicular to the discharge surface, the precious metal content in one end section of the overlap region, extending along the discharge surface of the tip, is greater than 50 wt%, and in the other end section of the overlap region, the nickel content is greater than 50 wt%. Therefore, in one end section of the overlap region, the thermal stress occurring at the second interface between the base material and the fusion section is greater than the thermal stress occurring at the first interface between the tip and the fusion section. Conversely, in the other end section of the overlap region, the thermal stress occurring at the first interface is greater than the thermal stress occurring at the second interface.Consequently, a crack on one end section tends to occur at the second interface, while a crack on the other end section tends to occur at the first interface. The crack tends to propagate along the interface. However, even if the cracks do develop, the tendency for cracks propagating along the first and second interfaces to connect can be reduced thanks to the above design. Thus, tip separation from the base material can be suppressed.

[0007] In the spark plug described above, the overlap area in the cross-section has such a shape that the distance between the first interface and the second interface along the first direction gradually increases with the progression in the second direction, and that in the overlap area in the cross-section there is a central region in which the precious metal content is 50% by mass and the Ni content is also 50% by mass, with respect to a central position in the second direction of the overlap area on the side of the second direction.

[0008] Therefore, a position where the cracks developing along the first and second interfaces overlap tends to shift or approach the central position in the second direction. Thus, even if the cracks develop along the first direction at this position, in addition to the effect described above, tip release can be suppressed even more strongly because the distance between the first and second interfaces is relatively long at this position.

[0009] In the spark plug described above, the cross-section in the overlap area contains a shortest section in which the distance between the first interface and the second interface along the first direction is shortest, located in a different area than one end section and the other end section, and the cross-section in the overlap area contains a middle area in which the precious metal content is 50% by mass and the Ni content is also 50% by mass, located in a different area than the shortest section.

[0010] Therefore, there is a tendency for the position where the cracks, each developing along the first and second interfaces, overlap in the first direction, to be located in a different section than the shortest section. Thus, even if the cracks at this position develop along the first direction, in addition to the effect mentioned above, tip separation can be suppressed even more strongly because the distance between the first and second interfaces at this position is relatively long.

[0011] In the spark plug described above, at least one of the relationships described above occurs in the cross-section where the length of the overlap area in the second direction is longest. Therefore, in addition to the effect described above, the lengths of the first and second interfaces, where cracks tend to develop, can be designed to be the longest.

[0012] The further tasks and features of this invention will be understood from the following description with reference to the accompanying drawings, without being limited thereto. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a sectional view of a spark plug according to an embodiment of the present invention. Fig. 2A is a top view of a ground electrode. Fig. 2B is a sectional view of the ground electrode along a line IIb-IIb in Fig. 2A. Fig. 3A is a schematic view of a tip being connected to a base material. Fig. 3B is a schematic view when the tip is connected to the base material according to a modified example. Fig. 4A is a bottom view of a center electrode. Fig. 4B is a sectional view of the center electrode along a line IVb-IVb in Fig. 4A. Fig. 5A is a schematic view of a tip being connected to a base material. Fig. 5B is a schematic view when the tip is connected to the base material according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be explained below with reference to the drawings. Fig. Figure 1 is a sectional view of a spark plug 10 according to an embodiment of the present invention with an axis O as its boundary. Fig. 1. A lower side of the drawing is referred to as the front end (or upper end) of the spark plug, and 10. A top side of the drawing is referred to as the rear end of the spark plug. As in Fig. As shown in Figure 1, the spark plug 10 has a center electrode 20 and a ground electrode 40.

[0014] An insulator 11 is essentially a tubular element provided with an axial hole 12 extending along axis O. The insulator 11 is made of a ceramic such as aluminum oxide, which possesses excellent mechanical properties and excellent insulating capacity at high temperatures. The insulator 11 has a surface 13 facing the rear end on an inner circumferential surface of its axial hole 12. This surface 13 is an annular shape facing the rear end. The diameter of the rear-facing surface 13 decreases towards the front end.

[0015] The central electrode 20 is a rod-shaped element that engages with and is held against the surface 13 facing the rear end. An upper end of the central electrode 20 projects from an upper end of the insulator 11 to the front end. The central electrode 20 is formed by covering a core 21, consisting mainly of copper, with a tubular base material 22 having a base. The base material 22 has a chemical composition containing 50 wt.% or more Ni. The core 21 could be omitted here. A tip 24 is connected to an upper end of the base material 22 by a fusion section (or a melting section) 23. The tip 24 has a chemical composition containing 50 wt.% or more of at least one noble metal selected from Pt, Rh, Ir, Ru, etc. A discharge surface 25 of the tip 24 faces the ground electrode 40.The central electrode 20 is electrically connected to a metal terminal 26 in the axial hole 12.

[0016] The metal connector 26 is a rod-shaped element to which a high-voltage cable (not shown) is connected. The metal connector 26 is made of a conductive metal material (e.g., low-carbon steel). The metal connector 26 is fixed to a rear end of the insulator 11, with one upper end of the metal connector 26 inserted into the axial hole 12 of the insulator 11.

[0017] A metal housing 30 is fixed to an outer circumference of the upper (front) end face of the insulator 11 by crimping. The metal housing 30 is essentially a tubular element made of a conductive metallic material (e.g., low-carbon steel). The metal housing 30 has a rim-shaped seat section 31 extending or bulging in a radially outward direction and a threaded section 32 formed on an outer circumferential surface of an upper (front) end face of the metal housing 30 with respect to the seat section 31. The metal housing 30 is fixed to the engine (cylinder head) by screwing the threaded section 32 into a screw hole (not shown). The ground electrode 40 is connected to an upper end section of the metal housing 30.

[0018] The ground electrode 40 is a rod-shaped element made of a conductive metal material. The ground electrode 40 has a base material 41 connected to the metal housing 30 and a tip 44 located on an inner circumferential surface 42 of the base material 41 facing the central electrode 20 and connected to the base material 41 by a fusion section 43. The base material 41 has a chemical composition containing 50 wt.% or more Ni. The tip has a chemical composition containing 50 wt.% or more of at least one noble metal selected from Pt, Rh, Ir, Ru, etc. A discharge surface 45 of the tip 44 faces the central electrode 20. A spark gap G is formed between the discharge surface 45 of the tip 44 and the central electrode 20.

[0019] Fig. 2A is a top view of the ground electrode 40 (a first electrode) seen from one direction of the axis O. Fig. 2B is a sectional view of the ground electrode 40 along a line IIb-IIb in Fig. 2A. An arrow Z indicates a first direction, which is perpendicular to the discharge surface 45 of the tip 44. If the ground electrode 40 is defined as the first electrode, the center electrode 20 is a second electrode. In the present embodiment, the base material 41 has a rod shape with a substantially rectangular cross-section, and the tip 44 is a rectangular parallelepiped. A portion of the tip 44 is arranged in a recessed groove formed by recessing it into the inner surface 42 located at an upper end section of the base material 41 along a side surface 41b. The position of the tip 44 is limited by a wall surface 42a of the groove. The tip 44 is connected to the base material 41 by the fusion section 43. The fusion section 43 is a section in which the tip 44 and the base material 41 are fused together.

[0020] The fusion section 43 has an overlap area 48 in which a first interface (or a first boundary) 46 between the tip 44 and the fusion section 43 and a second interface (or a second boundary) 47 between the base material 41 and the fusion section 43 overlap each other in the first direction (the direction of the arrow Z). Fig. 2B is also a sectioned sectional view of the ground electrode 40, which is projected along a section plane line (the line IIb-IIb) through a centroid 49 of a plan shape of the overlap section 48 onto a virtual surface parallel to the discharge surface 45 of the tip 44 (one corresponding to the drawing of Fig. The cross-section is cut along the discharge surface 45 (2A parallel surface). An arrow Y indicates a second direction, which is parallel to the discharge surface 45 and runs along the cutting plane line (line IIb-IIb). Although countless cutting plane lines passing through the centroid 49 could be drawn, in the present embodiment the cutting plane line is drawn along a diagonal line of the discharge surface 45 of the tip 44 such that the length of the overlap area 48 is maximized (longest) in the second direction. An analysis is then performed on its cross-section.

[0021] With reference to Fig. Section 3A will explain an example of a method for manufacturing the mass electrode 40. Fig. 3A is a schematic view when the tip 44 is joined to the base material 41, and shows a state before the fusion section 43 (indicated by a double dotted dashed line) is formed. Fig. 3A is a sectional view cut along a section plane line which is perpendicular to an upper end surface 41a of the base material 41 and parallel to the side surface 41b of the base material 41. Fig. 3B is the one explained above. Fig. 3A equals, that means that Fig. 3B is a cross-section cut along the above section plane line and shows a state prior to the formation of the fusion section 43 (indicated by a double dotted dashed line).

[0022] A groove bottom 42b, which is the bottom of the groove on the inner surface 42 of the base material 41, slopes from the wall surface 42a to the upper end surface 41a, or is chamfered, such that the depth of the groove increases from the wall surface 42a to the upper end surface 41a. A bottom surface 45a of the tip 44 also slopes or is chamfered such that a section of the tip 44 located near the wall surface 42a of the base material 41 is thinner than a section of the tip 44 located near the upper end surface 41a of the base material 41.

[0023] After positioning the tip 44 in the groove of the base material 41, a high-energy beam, such as a laser beam and / or an electron beam, is emitted from a beam processing head 54, which is positioned so that it is directed towards the upper end face 41a of the base material 41. By moving the beam processing head 54 along the groove bottom 42b while the beam is emitted, the fusion section 43 is formed and the tip 44 is joined to the base material 41. Since the beam is directed towards the upper end face 41a of the base material 41, the extent of the fusion on the upper end face 41a is greater than that on the wall face 42a.Furthermore, since, as mentioned above, the bottom surface 45a of the tip 44 and the groove bottom 42b of the inner surface 42 of the base material 41 are chamfered, in the fusion section 43 on the side of the upper (front) end surface 41a the melting extent of the tip 44 is greater than the melting extent of the base material 41, while in the fusion section 43 on the side of the wall surface 42a the melting extent of the base material 41 is greater than the melting extent of the tip 44.

[0024] This is called Return to Fig. 2B is explained in detail. In the present embodiment, in a lateral end section 50 (an end section) of the overlap region 48 in the second direction (the direction of arrow Y) along the discharge surface 45 of the tip 44, the precious metal content is greater than 50 wt%, since the melt volume of the tip 44 is greater than the melt volume of the base material 41. On the other hand, in the other lateral end section 51 (the other end section) of the overlap region 48 in the second direction, the Ni content is greater than 50 wt%, since the melt volume of the base material 41 is greater than the melt volume of the tip 44. Here, each of the end sections 50 and 51 is a line segment whose two ends are defined by the first and second interfaces 46 and 47. Each of the end sections 50 and 51 runs perpendicular to the discharge surface 45.

[0025] Since, as mentioned above, there are differences in precious metal and nickel content between end sections 50 and 51, a thermal stress occurring at the second interface 47 in end section 50 is greater than a thermal stress occurring at the first interface 46. Conversely, a thermal stress occurring at the first interface 46 in end section 51 is greater than a thermal stress occurring at the second interface 47. Consequently, a crack on the side of end section 50 tends to occur at the second interface 47, while a crack on the side of end section 51 tends to occur at the first interface 46. Furthermore, the crack occurring at the first interface 46 tends to propagate along the first interface 46, and the crack occurring at the second interface 47 tends to propagate along the second interface 47.However, even if the cracks develop in this way, the tendency for the cracks developing along the first and second interfaces 46 and 47 to connect to each other can be reduced thanks to the above structure. Therefore, separation of the tip 44 from the base material 41 due to a fracture of the fusion section 43 can be suppressed compared to a case in which cracks occurring at both ends of an interface develop along the interface towards the center of the interface.

[0026] Here, a quantitative analysis to measure the precious metal content and the nickel content in the end sections 50 and 51 of the overlap region 48 can be performed by WDS (wavelength dispersive spectroscopy) analysis using an EPMA (electron probe microanalyzer). The width of each of the end sections 50 and 51 (the thickness of each line segment) in the second direction is a thickness required for the quantitative analysis (in the present embodiment, it is at least 20 µm). Each of the precious metal content and the nickel content in the end sections 50 and 51 can be measured by averaging measurements from several data points established at equal regular intervals on both line segments of the end sections 50 and 51.Instead, a measurement of the midpoint of each line segment of end sections 50 and 51 could be a mean value.

[0027] Since, as mentioned above, the melting extent in the fusion section 43 is large on the side of the upper end surface 41a compared to that on the side of the wall surface 42a, the overlap area 48 is shaped such that the distance between the first interface 46 and the second interface 47 gradually increases along the first direction (the direction of arrow Z) with the second direction (the direction of arrow Y). Within the overlap area 48, there is a central region 53, in which the precious metal content is 50 wt% and the Ni content is also 50 wt%, with respect to a central position 52 in the second direction of the overlap area 48 on the side of the second direction (the direction of arrow Y). The central position 52 is a position containing a center point equidistant L from the end section 50 and the end section 51.

[0028] Therefore, the crack that occurs on the side of the end section 51 tends to develop along the first interface 46 in a region of the first interface 46 from the end section 51 to the central region 53, compared to a region of the first interface 46 from the end section 50 to the central region 53. On the other hand, the crack that occurs on the side of the end section 50 tends to develop along the second interface 47 in a region of the second interface 47 from the end section 50 to the central region 53, compared to a section of the second interface 47 from the end section 51 to the central region 53.Consequently, a position where the cracks developing along the first interface 46 and the second interface 47 overlap each other in the first direction (the direction of arrow Z) tends to shift towards or approach the side of the second direction (the direction of arrow Y) with respect to the central position 52. Therefore, even if the cracks at this position in the fusion section 43 develop along the first direction (the direction of arrow Z), a fracture of the fusion section 43 is suppressed because the distance between the first interface 46 and the second interface 47 at this position is greater than that of the overlap area 48, which is located further than the central position 52 on the side of the end section 51. Furthermore, the separation of the tip 44 from the base material 41 can be suppressed even more effectively.

[0029] It is noted that a relationship showing that in one lateral end section 50 the precious metal content is greater than 50 wt% and in the other lateral end section 51 the Ni content is greater than 50 wt% occurs in the cross-section where the length of the overlap region 48 is maximal (longest) in the second direction (the direction of arrow Y). Since the lengths of the first interface 46 and the second interface 47, where the cracks tend to develop, are longest at this cross-sectional position, the separation of the tip 44 from the base material 41 can be suppressed even more.

[0030] With reference to Fig. 3 will explain a modified example for the ground electrode 40. Fig. Figure 3B is a schematic view showing the tip 44 connected to the base material 41. Unlike in the case of Fig. 3A A groove bottom 42c, which is a bottom of the groove on the inner surface 42 of the base material 41, slopes from the wall surface 42a towards the upper end surface 41a, or is chamfered such that the depth of the groove becomes shallower from the wall surface 42a to the upper end surface 41a. A bottom surface 45b of the tip 44 also slopes or is chamfered such that a section of the tip 44 located near the wall surface 42a of the base material 41 is thicker than a section of the tip 44 located near the upper end surface 41a of the base material 41.

[0031] After the tip 44 is positioned in the groove of the base material 41, the fusion section 43 is formed by emitting a high-energy beam from the beam processing head 54, which is positioned so that it faces the upper end surface 41a of the base material 41, and the tip 44 is joined to the base material 41. Due to the chamfers of the bottom surface 45b of the tip 44 and the groove bottom 42c of the inner surface 42 of the base material 41, in the fusion section 43 on the side of the upper (front) end surface 41a, the melt volume of the base material 41 is greater than the melt volume of the tip 44, while in the fusion section 43 on the side of the wall surface 42a, the melt volume of the tip 44 is greater than the melt volume of the base material 41.

[0032] This is called Return to Fig. 2B is explained in detail. In this example, in a lateral end section 50 (an end section) of the overlap region 48 in the second direction (the direction of arrow Y) along the discharge surface 45 of the tip 44, the Ni content is greater than 50 wt%, since the melt volume of the base material 41 is greater than the melt volume of the tip 44. On the other hand, in the other lateral end section 51 (the other end section) of the overlap region 48 in the second direction, the precious metal content is greater than 50 wt%, since the melt volume of the tip 44 is greater than the melt volume of the base material 41.

[0033] Therefore, in end section 50, a thermal stress occurring at the first interface 46 is greater than a thermal stress occurring at the second interface 47. Conversely, in end section 51, a thermal stress occurring at the second interface 47 is greater than a thermal stress occurring at the first interface 46. Consequently, a crack on the side of the first end section 50 tends to occur at the first interface 46, while a crack on the side of the second end section 51 tends to occur at the second interface 47. Furthermore, the crack occurring at the first interface 46 tends to propagate along the first interface 46, and the crack occurring at the second interface 47 tends to propagate along the second interface 47.However, even if the cracks develop in this way, the tendency for the cracks developing along the first and second interfaces 46 and 47 to connect to each other can be reduced thanks to the above structure. Therefore, separation of the tip 44 from the base material 41 due to a fracture of the fusion section 43 can be suppressed compared to a case in which cracks occurring at both ends of an interface develop along the interface towards the center of the interface.

[0034] Next, the central electrode 20 will be explained. Fig. 4A is a bottom view of the center electrode 20 (a first electrode) seen from one direction of the axis O. Fig. 4B is a sectional view of the center electrode 20 along a line IVb-IVb in Fig. 4A. An arrow Z indicates a first direction, perpendicular to the discharge surface 25 of the tip 24. If the center electrode 20 is defined as the first electrode, the ground electrode 40 is a second electrode. In the present embodiment, the base material 22 has a cylindrical-columnar shape extending along the axis O, and the tip 24 has a disk shape. The tip 24 is arranged at an upper end in the axial direction of the base material 22 and is connected to the base material 22 by the fusion section 23. The fusion section 23 is a section in which the tip 24 and the base material 22 are fused together.

[0035] The fusion section 23 has an overlap area in which a first interface (or a first boundary) 60 between the tip 24 and the fusion section 23 and a second interface (or a second boundary) 61 between the base material 22 and the fusion section 23 overlap each other in the first direction (which is identical to the direction of axis O, the direction of arrow Z). Fig. 4B is also a line along a section plane (the line IVb-IVb) that passes through a centroid 63 of the virtual surface parallel to the discharge surface 25 of the tip 24 (a virtual surface corresponding to the drawing of Fig. 4A parallel surface) projected plan shape of the overlap section 62 runs, sectioned sectional view of the center electrode 20. A position of the center of gravity 63 is essentially identical to a position of axis O. An arrow Y indicates a second direction, which is a direction parallel to the discharge surface 25 and runs along the section plane line (the line IVb-IVb).

[0036] With reference to Fig. Section 5A will explain an example of a method for manufacturing the central electrode 20. Fig. 5A is a schematic view when the tip 24 is joined to the base material 22, and shows a state before the fusion section 23 (indicated by a double dotted dashed line) is formed. Fig. 5A is the one above Fig. 5B similar.

[0037] An upper (front) end surface 22a of the base material 22 and an end surface 24a of the tip, located on the side opposite the discharge surface 25, are flat surfaces that obliquely intersect the axis O. Due to these shapes, with respect to the two side sections 24b and 24c of the tip, which are located on opposite sides of the axis O, the length of section 24b between the discharge surface 25 and the end surface 24a of the tip 24 is longer than that of section 24c. In other words, the length of section 24c between the discharge surface 25 and the end surface 24a is shorter than that of section 24b. The tip 24 is arranged on the base material 22, with its end surface 24a being brought into contact with the upper end surface 22a of the base material 22, so that the discharge surface 25 of the tip 24 is perpendicular to the axis O.

[0038] After the tip 24 is positioned on the base material 22, the fusion section 23 is formed and the tip 24 is joined to the base material 22 by emitting a high-energy beam, such as a laser beam and / or an electron beam, from a beam processing head 54. The beam is positioned so that it is directed towards the side faces of the base material 22 and the tip 24, while the base material 22 and the tip 24 are rotated on axis O. Because the beam is emitted towards the side face of the base material 22, the extent of the fusion on an outer surface in a radial direction of the base material 22 is large compared to that in a center in the radial direction of the base material 22.And since the upper end surface 22a of the base material 22 and the end surface 24a of the tip 24 are inclined, in the fusion section 23 in section 24b of the tip 24 the melting extent of the tip 24 is greater than the melting extent of the base material 22, while in section 24c, which is opposite to section 24b with respect to axis O, the melting extent of the base material 22 is greater than the melting extent of the tip 24.

[0039] This is called Return to Fig. 4B explained in detail. In the present embodiment, in one end section 64 of the overlap region 62 in the second direction (the direction of arrow Y) along the discharge surface 25 of the tip, the precious metal content is greater than 50 wt%, since the melt volume of the tip 24 is greater than the melt volume of the base material 22. On the other hand, in the other end section 65 of the overlap region 62 in the second direction, the Ni content is greater than 50 wt%, since the melt volume of the base material 22 is greater than the melt volume of the seats 24.

[0040] Therefore, in one end section 64, a thermal stress occurring at the second interface 61 is greater than a thermal stress occurring at the first interface 60. Conversely, in the other end section 65, a thermal stress occurring at the first interface 60 is greater than a thermal stress occurring at the second interface 61. Consequently, a crack on the side of one end section 64 tends to occur at the second interface 61, while a crack on the side of the other end section 65 tends to occur at the first interface 60. Furthermore, the crack occurring at the first interface 60 tends to propagate along the first interface 60, and the crack occurring at the second interface 61 tends to propagate along the second interface 61.However, even if the cracks develop in this way, the tendency for the cracks developing along the first and second interfaces 60 and 61 to connect to each other can be reduced thanks to the above structure. Therefore, separation of the tip 24 from the base material 22 due to a fracture of the fusion section 23 can be suppressed compared to a case in which cracks occurring at both ends of an interface develop along the interface towards the center of the interface.

[0041] Since, as mentioned above, the extent of fusion in the fusion section 23 is large on the outside in the radial direction of the base material 22 compared to that in the center in the radial direction of the base material 22, the overlap region 62 is shaped such that the distance between the first interface 60 and the second interface 61 gradually decreases along the first direction (the direction of arrow Z) from the outside to the center. Therefore, in the overlap region 62 between one end section 64 and the other end section 65, a shortest section 66, in which the distance between the first interface 60 and the second interface 61 is shortest along the first direction, is located in a different region than between one end section 64 and the other end section 65.Furthermore, in the overlap area 62, a central section 67, in which the precious metal content is 50 mass-% and the Ni content is also 50 mass-%, is present in a different area than the shortest section 66.

[0042] Therefore, the crack that occurs on the side of the other end section 65 tends to develop along the first interface 60 in a region of the first interface 60 from the other end section 65 to the central section 67, compared to a region of the first interface 60 from one end section 64 to the central section 67. Conversely, the crack that occurs on the side of one end section 64 tends to develop along the second interface 61 in a region of the second interface 61 from one end section 64 to the central section 67, compared to a region of the second interface 61 from the other end section 65 to the central section 67.Since the central section 67 is positioned at a location different from the shortest section 66 in the second direction (the direction of arrow Y), there is a tendency for a position where the cracks developing along the first interface 60 and the second interface 61 overlap in the first direction (the direction of arrow Z) to be located in a different region than the shortest section 66. Therefore, even if the cracks develop along the first direction at this position, a fracture of the fusion section 23 is suppressed because the distance between the first interface 60 and the second interface 61 at this position is greater than that at the shortest section 66, and the separation of the tip 44 from the base material 22 can be suppressed even further.

[0043] With reference to Fig. 5B will explain a modified example for the center electrode 20. Fig. 5B is a schematic view when the tip 24 is connected to the base material 22. Unlike in the case of Fig. 5A, an end face 24d of the tip 24 runs parallel to the discharge surface 25, and an upper end face 22b of the base material 22 is a surface perpendicular to the axis O. After positioning the tip 24 on the base material 22, with its end face 24d in contact with the upper end face 22b of the base material 22, an oval shape is achieved for the path of the beam scanning surfaces of the base material 22 and the tip 24 by emitting a high-energy beam from the beam processing head 54, which is positioned so that it is directed towards the side faces of the base material 22 and the tip 24, while the base material 22 and the tip 24 are rotated about the axis O and while the beam processing head 54 is moved back and forth along the axis O.

[0044] In this case, too, a relationship is established which shows that in a cross-sectional view of the central electrode 20, intersected by a section plane line passing through a centroid 63 of the plan view of the overlap section 62 projected onto a virtual surface parallel to the discharge surface 25 of the tip 24, the precious metal content is greater than 50 wt% in the end section of the overlap area 62 on the side of section 24b (the melt volume of the tip 24 is greater than the melt volume of the base material 22), and in the other end section of the overlap area 622 on the side of the other section 24c, the Ni content is greater than 50 wt% (the melt volume of the base material 22 is greater than the melt volume of the tip 24). Therefore, the same mechanism and the same effect can be obtained.

[0045] Although the present invention has been explained based on the embodiment described above, the present invention is not limited to the embodiment described above. The present invention includes all variations and equivalents that fall within the technical scope of the present invention.

[0046] The embodiment above shows an example in which the groove is formed in the base material 41 of the ground electrode 40, and the tip 44, part of which is housed in the groove, is connected to the base material 41. However, the configurations of the base material 41 and the tip 44 are not limited to this example. The base material 41 is not necessarily provided with the groove. And the tip 44 could be connected to the base material 41 without forming the groove in the base material 41.

[0047] The embodiment above shows an example in which an upper end face of the tip 44 is positioned slightly further inward relative to the upper end face 41a of the base material 41. However, the position of the tip 44 is not limited to this example. For instance, the tip 44 could be arranged such that its upper end face is positioned slightly further outward relative to the upper end face 41a of the base material 41, that is, that the upper end face of the tip 44 projects beyond the upper end face 41a of the base material 41.

[0048] The embodiment above shows an example in which the tip 44 is connected to the inner surface 42 of the base material 41 of the ground electrode 40. However, the connection of the tip 44 is not limited to this example. The tip 44 could be connected to other sections besides the inner surface 42, such as the upper end surface 41a of the base material 41.

[0049] The embodiment above shows an example in which the tip 44 of the ground electrode 40 has the shape of a rectangular parallelepiped (a square column). However, the shape of the tip 44 is not limited to this example. Besides the square column, a cylindrical column and a polygonal column could be used as the tip shape, as required.

[0050] The embodiment above shows an example in which the tip 44 is directly connected to the base material 41 of the ground electrode 40 via the fusion section 43. However, the connection of the tip 44 is not limited to this example. It could be possible to insert an intermediate element, consisting mainly of nickel, between the base material and the tip and to connect the tip to the intermediate element, which is connected to the base material, via the fusion section.

[0051] The embodiment above shows an example where the relationship indicating that the precious metal content is greater than 50 wt% in one end section of the overlapping regions 48 and 62, and the nickel content is greater than 50 wt% in the other end section of the overlapping regions 48 and 62, occurs for both the center electrode 20 and the ground electrode 40. However, the present invention is not limited to this example. As long as this relationship occurs for both the center electrode 20 and the ground electrode 40, the present invention can be achieved and prevent the tip of the electrode from detaching from the base material.

[0052] In the embodiment described above, as an example of the fabrication (of the fusion section 23) of the central electrode 20, the tip 24 is joined to the base material 22 and the high-energy beam is emitted while this assembly of the base material 22 and the tip 24 is rotated on axis O. However, the fabrication (of the fusion section 23) of the central electrode 20 is not limited to this example. For instance, the fusion section 23 could be formed by joining the tip 24 to the base material 22 and performing the high-energy beam scanning around the base material 22 and the tip 24 using one or more mirrors while the assembly of the base material 22 and the tip 24 remains stationary.

[0053] Although the invention has been described above with reference to a specific embodiment, the invention is not limited to the embodiment described above. Those skilled in the art will think of variations and modifications in light of the above teachings. The scope of the invention is defined by reference to the following claims. EXPLANATION OF REFERENCE SYMBOLS 10 Spark plug 20 Middle electrode (first electrode, second electrode) 22, 41 Basic material 23, 43 Merger section 24, 44 peak 25, 45 discharge area 40 Ground electrode (first electrode, second electrode) 46, 60 first boundary surface (first border) 47, 61 second boundary surface (second border) 48, 62 Overlap area 49, 63 Focus 50, 51 End section (one end section, the other end section) 64 a final section 65 the other end section 52 Middle position 53, 67 Middle range 66 shortest section

Claims

[1] Spark plug (10), comprising: a first electrode (40, 20) having a tip (44, 24) made mainly of a noble metal and a base material (41, 22) made mainly of Ni, the tip (44, 24) being connected to the base material (41, 22) by a fusion portion (43, 23); and a second electrode (20, 40) provided to face a discharge surface (45, 25) of the tip (44, 24), and wherein the fusion section (43, 23) has an overlap region (48, 62) in which a first interface (46, 60) between the tip (44, 24) and the fusion section (43, 23) and a second interface (47, 61) between the base material (41, 22) and the fusion section (43, 23) overlap each other in a first direction (Z) perpendicular to the discharge surface (45, 25), and when viewing a cross section passing through a center of gravity (49, 63) of the overlap region (48, 62) projected onto a virtual surface parallel to the discharge surface (45, 25) and perpendicular to the discharge surface (45, 25), in one end section (50, 64) of the overlap region (48, 62) in a second direction (Y) extending along the discharge surface (45, 25), the precious metal content is greater than 50 mass%, and in the other end section (51, 65) of the overlap region (48, 62) in the second direction (Y), the Ni content is greater than 50 mass%. [2] The spark plug (10) according to claim 1, wherein the overlap portion (48) in the cross section has a shape such that a distance between the first interface (46) and the second interface (47) along the first direction (Z) gradually becomes longer as it extends in the second direction (Y), and in the overlap portion (48) in the cross section, a center portion (53) in which the noble metal content is 50 mass% and the Ni content is also 50 mass% is present with respect to a center position (52) in the second direction (Y) of the overlap portion (48) on the second direction (Y) side. [3] Spark plug (10) according to claim 1, wherein in the cross section in the overlap region (62), a shortest section (66) in which a distance between the first interface (60) and the second interface (61) along the first direction (Z) is the shortest is present in a region other than the one end section (64) and the other end section (65), and a central region (67) in which the precious metal content is 50 mass% and the Ni content is also 50 mass% is present in a region other than the shortest section (66) in the overlap region (62) in the cross-section. [4] The spark plug (10) according to any one of the preceding claims 1 to 3, wherein the cross section is a cross section in which a length of the overlap portion (48) in the second direction (Y) becomes the longest.

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