Spark plug

Abstract

To provide a spark plug capable of reducing welding distortion of a metal shell.SOLUTION: A spark plug includes a center electrode, a metal shell that insulates and holds the center electrode on the inside, and a ground electrode that protrudes from the metal shell toward the center electrode, the metal shell includes a cylindrical portion with a male thread on its outer periphery and a hole that penetrates the cylindrical portion in the radial direction, and the ground electrode is connected to the cylindrical portion in the hole via a fusion portion. In a cross section including the center line of the hole and the fusion portion, the penetration depth of the fusion portion into the cylindrical portion is 0 mm or more and 0.4 mm or less.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a spark plug in which a ground electrode is connected to a main fitting via a melting part.

Background Art

[0002] Prior art related to a spark plug including a center electrode, a cylindrical main fitting that insulates and holds the center electrode inside, and a ground electrode that protrudes from the main fitting toward the center electrode is disclosed in Patent Document 1. In the prior art, a part of the ground electrode fits into a hole penetrating the main fitting, and the part in the hole is joined to the main fitting via a melting part where the main fitting and the ground electrode are melted. The melting part is provided from the chamfered part of the outer surface around the hole and from the outer end surface of the ground electrode toward the inner side in the radial direction of the main fitting.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, when a melting part is provided in the radial direction of the main fitting without providing the melting part in the chamfered part around the hole of the main fitting, if the heat input is increased to reduce welding incomplete parts such as poor fusion and cavities, the welding strain of the main fitting may increase.

[0005] The present invention has been made to solve this problem, and an object thereof is to provide a spark plug capable of reducing the welding strain of the main fitting.

Means for Solving the Problems

[0006] To achieve this object, a first aspect of the present invention includes a central electrode, a main fitting that insulates and holds the central electrode inside, and a ground electrode that protrudes from the main fitting toward the central electrode. The main fitting includes a cylindrical portion provided with a male thread on the outer periphery, a hole penetrating the cylindrical portion in the radial direction, and a spark plug in which the ground electrode is connected to the cylindrical portion through a molten portion in the hole. In a cross section including the center line of the hole and the molten portion, the penetration depth of the molten portion into the cylindrical portion is 0 mm or more and 0.4 mm or less.

[0007] A second aspect is that in the first aspect, the penetration depth is 0 mm or more and 0.2 mm or less.

[0008] A third aspect is that in the first or second aspect, the hole is tapered such that the inner size in the radial direction of the cylindrical portion is smaller than the outer size in the radial direction.

[0009] A fourth aspect is that in the first or second aspect, the hole has a constant size in the radial direction of the cylindrical portion.

[0010] A fifth aspect is that in any one of the first to fourth aspects, the first element having the highest content among the elements included in the main fitting is different in type from the second element having the highest content among the elements included in the ground electrode, and the element having the highest content among the elements included in the molten portion is the second element.

[0011] A sixth aspect is that in any one of the first to fifth aspects, a cap for closing the tip side of the main fitting is provided, and a through hole for communicating the inside and the outside of the space closed by the cap is provided in the cap.

Advantages of the Invention

[0012] According to the present invention, in a hole penetrating the cylindrical portion of the main fitting in the radial direction, the ground electrode is connected to the cylindrical portion through a molten portion. Since the penetration depth of the molten portion into the cylindrical portion in a cross section including the center line of the hole and the molten portion is 0 mm or more and 0.4 mm or less, the welding strain of the main fitting can be reduced by ensuring the amount of the ground electrode melted in the molten portion.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view of a spark plug 10 in one embodiment. FIG. 1 shows a cross-section including the axis O of the tip side portion of the spark plug 10. In FIG. 1, the lower side of the paper surface is the tip side of the spark plug 10, and the upper side of the paper surface is the rear end side of the spark plug 10 (the same applies to FIG. 2). As shown in FIG. 1, the spark plug 10 includes an insulator 11, a center electrode 15, a main body fitting 20, and a ground electrode 25.

[0015] The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O, and is formed of ceramics such as alumina that is excellent in mechanical properties and insulation properties at high temperatures. The insulator 11 includes a locking portion 13 and a tip portion 14 adjacent to the tip side of the locking portion 13. The outer diameter of the tip portion 14 is smaller than the outer diameter of the locking portion 13.

[0016] The center electrode 15 is disposed in the axial hole 12 along the axis O at least from the locking portion 13 to the tip portion 14 of the insulator 11. The center electrode 15 includes a rod-shaped base material 16 mainly composed of Ni, a chip 17 mainly composed of one or more of noble metals such as Pt, Rh, Ru, and Ir disposed at the tip of the base material 16, and a melting portion 18 that joins the chip 17 and the base material 16. The chip 17 and the melting portion 18 can be omitted.

[0017] The tip of the central electrode 15 protrudes from the insulator 11 toward the tip side. The central electrode 15 is electrically connected to the terminal fitting 19 within the axial hole 12. The terminal fitting 19 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a conductive metal material (such as low-carbon steel, etc.). The terminal fitting 19 is fixed to the rear end of the insulator 11.

[0018] The main body fitting 20 is a substantially cylindrical member formed of a conductive metal material (such as copper, copper alloy, low-carbon steel, etc.). The main body fitting 20 is disposed on the outer periphery of the insulator 11. The main body fitting 20 includes a cylindrical portion 21 located on the outer peripheral side of at least the locking portion 13 and the tip portion 14 of the insulator 11. The cylindrical portion 21 is a cylindrical part provided with an external thread 22 on the outer periphery, and a shelf portion 23 is provided on the inner periphery. The external thread 22 engages with an internal thread provided in the plug hole of an engine (not shown). The shelf portion 23 is located on the tip side of the locking portion 13 of the insulator 11 and locks the locking portion 13.

[0019] A hole 24 that penetrates the cylindrical portion 21 in the radial direction is provided in a portion of the cylindrical portion 21 on the tip side of the shelf portion 23. In the present embodiment, the hole 24 is provided at the position of the external thread 22, and the cross-section of the hole 24 is circular. A ground electrode 25 is disposed in the hole 24 of the cylindrical portion 21. The ground electrode 25 disposed in the hole 24 protrudes from the cylindrical portion 21 toward the central electrode 15.

[0020] A space 29 is provided inside the cap 26 by the cap 26 that closes the tip side of the main body fitting 20. Examples of the material of the cap 26 include metal materials mainly composed of one or more of Fe, Ni, Cu, etc. In the present embodiment, the cap 26 is connected to the tip side of the cylindrical portion 21 via a molten portion 28. A through hole 27 that communicates the inside and the outside of the space 29 is provided in the cap 26.

[0021] FIG. 2 is a cross-sectional view of the spark plug 10 with the portion shown in II of FIG. 1 enlarged. The inner surface 30 of the hole 24 provided in the cylindrical portion 21 is connected to the outer peripheral surface 31 of the cylindrical portion 21, and a ground electrode 25 is provided between the inner peripheral surface 32 of the cylindrical portion 21 and the inner surface 30 of the hole 24. The ground electrode 25 includes, for example, a base material 33 mainly composed of Ni, a chip 34 mainly composed of one or more noble metals such as Pt, Rh, Ru, and Ir, and a molten portion 35 that joins the chip 34 and the base material 33. The chip 34 and the molten portion 35 can be omitted. The end surface 36 of the base material 33 is located inside the hole 24. The hole 24 provided in the cylindrical portion 21 is blocked by the ground electrode 25.

[0022] In this embodiment, the portion of the base material 33 inside the hole 24 is cylindrical, and the portion of the base material 33 inside the hole 24 is thicker than the portion where the molten portion 35 is provided. The chip 34 of the ground electrode 25 faces the side surface of the chip 17 of the center electrode 15, and a spark gap G is provided between the tip of the ground electrode 25 and the side surface of the center electrode 15.

[0023] The base material 33 of the ground electrode 25 is connected to the cylindrical portion 21 through a molten portion 37 inside the hole 24. The molten portion 37 is formed by irradiating a laser beam on the end surface 36 of the base material 33 of the ground electrode 25 arranged inside the hole 24. The molten portion 37 is formed by melting a portion including a part of the end surface 36 of the base material 33 and a part of the inner surface 30 of the hole 24 of the cylindrical portion 21.

[0024] The spark plug 10 attached to an engine (not shown) allows fuel gas to flow from the combustion chamber of the engine through the through-hole 27 into the space 29 by the valve operation of the engine. The spark plug 10 generates a flame kernel by discharge between the center electrode 15 and the ground electrode 25. When the flame kernel grows, it ignites the fuel gas in the space 29 and the fuel gas burns. The expansion pressure generated by the combustion of the fuel gas generates a gas flow containing a flame, and the gas containing the flame is injected from the through-hole 27 into the combustion chamber. The fuel gas in the combustion chamber burns by the jet of the flame. That is, the space 29 inside the cap 26 functions as a sub-combustion chamber provided inside the combustion chamber of the engine.

[0025] Figure 3 is a cross-sectional view of the spark plug 10 taken along line III-III in Figure 2. In Figure 3, a part of the cross-section of the cylindrical portion 21 and the ground electrode 25 including the center line C of the hole 24 is shown. The lower side of the paper is the inner side in the radial direction of the cylindrical portion 21, and the upper side of the paper is the outer side in the radial direction of the cylindrical portion 21 (the same applies in Figure 4). The center line C of the hole 24 is a straight line passing through the geometric centers of a plurality of cross-sections obtained by cutting the hole 24 such that the cutouts on the inner surface 30 of the hole 24 form a ring, regarded as a plurality of planar figures. In the present embodiment, since the melting portions 37 are provided over the entire length around the center line C of the hole 24, in Figure 3 which is a cross-sectional view including the center line C, the melting portions 37 and 38 appear on both sides of the center line C respectively.

[0026] The interface 39 between one melting portion 37 and the cylindrical portion 21 connects the intersection 41 between the outer surface 40 of the melting portion 37 on the outer side in the radial direction of the cylindrical portion 21 and the inner surface 30 of the hole 24, and the intersection 42 where the inner peripheral surface 32 of the cylindrical portion 21 intersects the melting portion 37. A part of the interface 39 exists on the outer side in the radial direction of the cylindrical portion 21 than the base material 33 (the end surface 36 in the present embodiment). Compared with the case where the entire interface 39 exists on the inner side in the radial direction of the cylindrical portion 21 than the base material 33, the joint area of the melting portion 37 can be increased.

[0027] The penetration depth D (the thickness of the welding metal) of the melting portion 37 into the cylindrical portion 21 is 0 mm or more and 0.4 mm or less. The penetration depth D into the cylindrical portion 21 is the distance between a point 0.1 mm away from the intersection 41 in the outer side in the radial direction on the inner surface 30 and the tip position of the interface 39 in the radial direction of the hole 24 (the direction from the center line C of the hole 24 to the inner surface 30). When the penetration depth D is within this range, the amount of melting of the cylindrical portion 21 into the melting portion 37 can be reduced. Therefore, by ensuring the amount of melting of the base material 33 (the ground electrode 25) into the melting portion 37, it is possible to reduce the welding distortion of the cylindrical portion 21 (the main fitting 20) while reducing welding incomplete parts such as fusion defects and cavities.

[0028] The reason for determining the penetration depth D based on a point 0.1 mm radially outward from the intersection point 41 on the inner surface 30 is that the measurement accuracy can be improved compared to determining the penetration depth D based on the intersection point 41. The penetration depth D is obtained by acquiring an image of the melted portion 37 appearing in the cross-section of the cylindrical portion 21 including the center line C with an optical microscope and performing image analysis.

[0029] In order to increase the bonding strength of the ground electrode 25, the penetration depth D of the melted portion 37 is preferably 0.05 mm or more and 0.4 mm or less. In order to further reduce the welding strain of the cylindrical portion 21, the penetration depth D of the melted portion 37 is preferably 0 mm or more and 0.2 mm or less. The penetration depth E of the melted portion 37 into the base material 33 (ground electrode 25) (the distance measured in the direction opposite to the direction in which the penetration depth D is measured) is preferably greater than the penetration depth D of the melted portion 37. This is to ensure the bonding strength of the ground electrode 25.

[0030] In this embodiment, the outermost position in the radial direction on the surface 40 of the melted portion 37 is the intersection point 41. Since the build-up of the melted portion 37 can be prevented from becoming excessive, the stress generated at the intersection point 41 can be reduced. As a result, the fatigue strength of the cylindrical portion 21 and the melted portion 37 can be prevented from decreasing.

[0031] The interface 43 between the other melted portion 38 and the cylindrical portion 21 connects the intersection point 45 between the outer surface 44 of the melted portion 38 in the radial direction of the cylindrical portion 21 and the inner surface 30 of the hole 24, and the intersection point 46 where the inner peripheral surface 32 of the cylindrical portion 21 intersects the melted portion 38. A part of the interface 43 is located radially outside the cylindrical portion 21 with respect to the base material 33 (the end surface 36 in this embodiment). Since the intersection points 41 and 45 are located radially inside the intersection point between the inner surface 30 of the hole 24 and the groove of the male screw 22 (see FIG. 1), the male screw 22 can be fitted into the female screw provided in the spark plug hole of the engine.

[0032] Since the interface 43 exists radially outside the cylindrical portion 21 with respect to the base material 33, the interface 43 can be extended radially outside the cylindrical portion 21. Compared with the case where the melted portion 38 does not exist radially outside the ground electrode 25, the interface 43 becomes longer, so that the bonding area related to the strength of the melted portion 38 can be increased.

[0033] The penetration depth D of the molten part 38 into the cylindrical part 21 is 0 mm or more and 0.4 mm or less. The penetration depth D into the cylindrical part 21 is the distance between a point 0.1 mm radially outward from the intersection point 45 on the inner surface 30 and the tip position of the interface 43 in the radial direction of the hole 24. Since the amount of melting of the cylindrical part 21 into the molten part 38 can be reduced, by ensuring the amount of melting of the base material 33 (grounding electrode 25) into the molten part 38, it is possible to reduce the welding strain of the cylindrical part 21 (main fitting 20) while reducing incomplete welding parts such as fusion defects and cavities.

[0034] In order to increase the bonding strength of the grounding electrode 25, the penetration depth D of the molten part 38 is preferably 0.05 mm or more and 0.4 mm or less. In order to further reduce the welding strain of the cylindrical part 21, the penetration depth D of the molten part 38 is preferably 0 mm or more and 0.2 mm or less. The penetration depth E of the molten part 38 into the base material 33 (grounding electrode 25) (the distance measured in the direction opposite to the direction in which the penetration depth D is measured) is preferably larger than the penetration depth D of the molten part 38. This is to ensure the bonding strength of the grounding electrode 25.

[0035] In this embodiment, the outermost position in the radial direction of the surface 44 of the molten part 38 is the intersection point 45. Since the excess of the molten part 38 can be prevented from becoming excessive, the stress generated at the intersection point 45 can be reduced. As a result, the fatigue strength of the cylindrical part 21 and the molten part 38 can be prevented from decreasing.

[0036] Both the interfaces 39 and 43 are present outside the cylindrical part 21 in the radial direction with respect to the base material 33. Thereby, the strength of the molten parts 37 and 38 can be increased as compared with the case where one of the interfaces 39 and 43 is present outside the cylindrical part 21 in the radial direction with respect to the base material 33.

[0037] When a portion including the end face 36 of the base material 33 melts and the molten portions 37, 38 extend toward the inner side in the radial direction of the cylindrical portion 21, the entire interfaces 39, 43 come to exist on the outer side in the radial direction of the cylindrical portion 21 than the base material 33. At this time, compared with the case where a part of the interfaces 39, 43 exists on the outer side in the radial direction of the cylindrical portion 21 than the base material 33, a large amount of thermal energy for melting the base material 33 and the cylindrical portion 21 is applied to the base material 33 and the cylindrical portion 21, so there is a risk that the base material 33 and the cylindrical portion 21 may be deformed. By having a part of the interfaces 39, 43 exist on the outer side in the radial direction of the cylindrical portion 21 than the base material 33 as in the present embodiment, the strength of the molten portions 37, 38 can be ensured, and the deformation of the base material 33 and the cylindrical portion 21, particularly the deformation of the male screw 22, can be reduced.

[0038] In the present embodiment, the hole 24 has a constant size in the radial direction of the cylindrical portion 21. This makes it easier to control the conditions for laser welding to form the molten portions 37, 38. The fact that the size of the hole 24 is constant in the radial direction of the cylindrical portion 21 means that the difference between the maximum value and the minimum value when the dimension (size of the hole 24) of the inner surface 30 of the hole 24 is measured at a plurality of positions over the entire length in the radial direction of the cylindrical portion 21 is 0.20 mm or less. The size of the hole 24 is measured up to the third decimal place to obtain the maximum value and the minimum value, and the third decimal place of the difference between the maximum value and the minimum value is rounded off. When the angle of the inner surface 30 of the hole 24 with respect to the center line C is 5.7° or less, it can also be said that the hole 24 has a constant size in the radial direction of the cylindrical portion 21.

[0039] The size of the hole 24 is preferably larger than 1 mm and 3 mm or less. This is because if the size of the hole 24 is 1 mm or less, it becomes difficult to irradiate the laser beam into the hole 24 to form the molten portions 37, 38. If the size of the hole 24 exceeds 3 mm, it becomes difficult to form the molten portions 37, 38 on the inner surface 30 of the hole 24 excluding the portion of the male screw 22.

[0040] Among the elements included in the cylindrical portion 21 (main fitting 20), the first element with the highest content (Cu or Fe in this embodiment) and the second element with the highest content among the elements included in the base material 33 (ground electrode 25) (Ni in this embodiment) are of different types. Although the first element and the second element are included in the molten portions 37 and 38, it is preferable that the element with the highest content among the elements included in the molten portions 37 and 38 is the second element. This is because by reducing the ratio of the melting amount of the cylindrical portion 21 in the molten portions 37 and 38, the welding strain of the cylindrical portion 21 (main fitting 20) can be reduced.

[0041] Since the means for elemental analysis of the molten portion 37 is the same as that of the molten portion 38, the elemental analysis of the molten portion 38 will be described, and the description of the elemental analysis of the molten portion 37 will be omitted. For elemental analysis, a cross-section of the molten portion 38 is analyzed by wavelength-dispersive X-ray spectroscopy (WDS) using an electron probe microanalyzer (EPMA). The analysis positions are the centroids of the ranges between the straight line 47 and the straight line 49, between the straight line 49 and the straight line 50, and between the straight line 50 and the straight line 48 when straight lines 47, 48, 49, and 50 that are perpendicular to the center line C and parallel to each other are drawn on the molten portion 38. The centroid is the geometric center when each range is regarded as a planar figure. The results of the three-point analysis are averaged to obtain the element with the highest content among the elements included in the molten portion 38. The straight line 47 is a straight line in contact with the surface 44 of the molten portion 38, and the straight line 48 is a straight line passing through the intersection point 46. The straight lines 49 and 50 are straight lines that divide the distance between the straight line 47 and the straight line 48 into three equal parts.

[0042] The tip side of the main fitting 20 of the spark plug 10 is closed by a cap 26 (see FIG. 1), fuel gas burns in the space 29 inside the cap 26, and the expansion pressure is applied to the cap 26, the cylindrical portion 21, and the molten portions 37 and 38. The molten portions 37 and 38 require mechanical strength to withstand the pressure. Since the penetration depth D of the molten portions 37 and 38 into the cylindrical portion 21 of the spark plug 10 is 0 mm or more and 0.4 mm or less, by ensuring the amount of the base material 33 (ground electrode 25) melted in the molten portion 37, the welding strain of the cylindrical portion 21 (main fitting 20) can be reduced while reducing the incomplete welding portion. Therefore, it is suitable for the spark plug 10 provided with the cap 26.

[0043] Referring to FIG. 4, the second embodiment will be described. In the first embodiment, the case where the hole 24 provided in the cylindrical portion 21 has a constant size in the radial direction of the cylindrical portion 21 was described. In contrast, in the second embodiment, the case where the hole 51 provided in the cylindrical portion 21 has a tapered shape in which the size on the inner side in the radial direction of the cylindrical portion 21 is smaller than the size on the outer side in the radial direction will be described. In the second embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the following description thereof will be omitted.

[0044] FIG. 4 is a cross-sectional view including the center line C of the hole 51 provided in the cylindrical portion 21 of the spark plug 10 in the second embodiment. The hole 51 is provided at the position of the male thread 22 (see FIG. 1) of the cylindrical portion 21. The hole 51 has a tapered shape in which the inner surface 52 becomes narrower toward the inner side in the radial direction of the cylindrical portion 21. In the present embodiment, a molten portion formed by melting the base material 33 and the cylindrical portion 21 is provided between the base material 33 and the cylindrical portion 21 over the entire length around the center line C of the hole 51. Therefore, in FIG. 4 which is a cross-sectional view including the center line C, the molten portions 53 and 54 appear on both sides of the center line C, respectively.

[0045] The interface 55 between one molten portion 53 and the cylindrical portion 21 connects the intersection 57 between the outer surface 56 in the radial direction of the cylindrical portion 21 and the inner surface 52 of the hole 51 in the molten portion 53 and the intersection 59 between the inner surface 58 in the radial direction of the molten portion 53 and the inner peripheral surface 32 of the cylindrical portion 21. The penetration depth D of the molten portion 53 into the cylindrical portion 21 is 0 mm or more and 0.4 mm or less. The penetration depth D into the cylindrical portion 21 is the distance between a point 0.1 mm away from the intersection 57 toward the outer side in the radial direction on the inner surface 52 and the tip position of the interface 55 in the radial direction of the hole 51 (the direction from the center line C of the hole 51 toward the inner surface 52). Since the amount of melting of the cylindrical portion 21 into the molten portion 53 can be reduced, by ensuring the amount of melting of the base material 33 (ground electrode 25) into the molten portion 53, it is possible to reduce the welding distortion of the cylindrical portion 21 (main body fitting 20) while reducing welding incomplete portions such as fusion defects and cavities.

[0046] In order to increase the bonding strength of the ground electrode 25, the penetration depth D of the molten part 53 is preferably 0.05 mm or more and 0.4 mm or less. In order to further reduce the welding strain of the cylindrical part 21, the penetration depth D of the molten part 53 is preferably 0 mm or more and 0.2 mm or less. The penetration depth E of the molten part 53 into the base material 33 (ground electrode 25) (the distance measured in the direction opposite to the direction in which the penetration depth D is measured) is preferably larger than the penetration depth D of the molten part 53. This is to ensure the bonding strength of the ground electrode 25.

[0047] The interface 60 between the other molten part 54 and the cylindrical part 21 connects the intersection 62 between the outer surface 61 in the radial direction of the cylindrical part 21 and the inner surface 52 of the hole 51 in the molten part 54, and the intersection 64 between the inner surface 63 in the radial direction of the molten part 54 and the inner circumferential surface 32 of the cylindrical part 21. The penetration depth D of the molten part 54 into the cylindrical part 21 is 0 mm or more and 0.4 mm or less. The penetration depth D into the cylindrical part 21 is the distance between a point 0.1 mm away from the intersection 62 in the radial direction outside on the inner surface 52 and the tip position of the interface 60 in the radial direction of the hole 51 (the intersection 64 in this embodiment). Since the amount of melting of the cylindrical part 21 into the molten part 54 can be reduced, by ensuring the amount of melting of the base material 33 (ground electrode 25) into the molten part 54, it is possible to reduce the welding strain of the cylindrical part 21 (main fitting 20) while reducing welding incomplete parts such as fusion defects and cavities.

[0048] In order to increase the bonding strength of the ground electrode 25, the penetration depth D of the molten part 54 is preferably 0.05 mm or more and 0.4 mm or less. In order to further reduce the welding strain of the cylindrical part 21, the penetration depth D of the molten part 54 is preferably 0 mm or more and 0.2 mm or less. The penetration depth E of the molten part 54 into the base material 33 (ground electrode 25) is preferably larger than the penetration depth D of the molten part 54. This is to ensure the bonding strength of the ground electrode 25.

[0049] The intersection points 57 and 62 are located radially inside the valleys of the male screw 22 (see FIG. 1) on the inner surface 52 of the hole 51. Since the hole 51 is tapered such that the outer size in the radial direction of the cylindrical portion 21 is larger than the inner size in the radial direction, the male screw 22 is less likely to be affected by the heat of laser welding that irradiates the end face 36 of the base material 33 disposed in the hole 51 to provide the molten portions 53 and 54. Therefore, deformation of the male screw 22 can be reduced.

[0050] It is preferable that the element with the highest content among the elements included in the molten portions 53 and 54 is the second element (Ni in this embodiment) with the highest content among the elements included in the base material 33 (ground electrode 25). This is because the welding strain of the cylindrical portion 21 (main fitting 20) can be reduced by decreasing the ratio of the melting amount of the cylindrical portion 21 in the molten portions 53 and 54.

[0051] Since the means for elemental analysis of the molten portion 53 is the same as that of the molten portion 54, the elemental analysis of the molten portion 54 will be described, and the description of the elemental analysis of the molten portion 53 will be omitted. For elemental analysis, the cross-section of the molten portion 54 is analyzed by WDS using EPMA. The analysis positions are the centroids of the ranges between the straight line 65 and the straight line 67, between the straight line 67 and the straight line 68, and between the straight line 68 and the straight line 66 when straight lines 65, 66, 67, and 68 that are perpendicular to the center line C and parallel to each other are drawn on the molten portion 54. The analysis results of the three points are averaged to obtain the element with the highest content among the elements included in the molten portion 54. The straight line 65 is a straight line passing through the intersection point 62 of the molten portion 54, and the straight line 66 is a straight line passing through the intersection point 64. The straight lines 67 and 68 are straight lines that divide the distance between the straight line 65 and the straight line 66 into three equal parts.

Example

[0052] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0053] (Preparation of Samples) The tester prepared a main metal fitting 20 made of low-carbon steel, which had an external thread 22 on the outer periphery of a cylindrical portion 21 with a radial thickness of 1.5 mm according to the spark plug 10 in the first embodiment, and a hole 24 with a diameter of 2 mm penetrated through the portion of the external thread 22. The nominal diameter of the external thread 22 was 10 mm, and the external thread 22 conformed to JIS B8031:2006. The hole 24 was circular with a constant size in the radial direction of the cylindrical portion 21, and the center line C of the hole 24 intersected perpendicularly with the axis O of the cylindrical portion 21.

[0054] After inserting a cylinder into the hole such that the end face of the cylinder was located at a position 0.3 mm radially inward from the bottom of the external thread 22, a laser beam was irradiated on the end face of the cylinder to laser-weld the entire circumference of the cylinder to the cylindrical portion 21. The material of the cylinder was Ni-based alloy (NCF601), the total length of the cylinder was 3.0 mm, and the diameter of the cylinder was 0.1 mm smaller than the diameter of the hole 24. The tester obtained main metal fittings 20 with various melted portions by varying the angle of the laser beam with respect to the center line C of the hole 24. The main metal fitting 20 was assembled on the outer periphery of the insulator 11 on which the center electrode 15 and the terminal fitting 19 were arranged, and samples of the spark plugs in No.1 - 9 shown in Table 1 were produced.

[0055]

Table 1

[0056] (Air tightness test) For the samples whose screw inspection results were A, in accordance with the airtightness test specified in JIS B8031:2006, the samples were kept in an atmosphere of 150 °C for 30 minutes, and then an air pressure of 1.5 MPa was applied to the inside of the cylindrical portion 21 in that state, and the air leakage amount from the hole 24 of each sample was measured. Samples with an air leakage amount of 0.5 mL / min or less were judged as A, samples exceeding 0.5 mL / min and 1 mL / min or less were judged as B, samples exceeding 0.5 mL / min and 1 mL / min or less were judged as B, and samples exceeding 1 mL / min were judged as C. The results are shown in Table 1.

[0057] (Elemental analysis of the molten part) Elemental analysis of three points in the molten part that appears in a cross-section including the center line C of the hole 24 of each sample and perpendicular to the axis O of the cylindrical portion 21 was performed by WDS, and the first decimal place of the average value of the analysis results of the three points in the molten part was rounded off. The results are shown in Table 1.

[0058] (Evaluation) As shown in Table 1, among the elements contained in the molten part, the element with the highest content was Ni for Sample No. 1-8 and Fe for Sample No. 9. Ni is the element with the highest content among the elements contained in the cylinder, and Fe is the element with the highest content among the elements contained in the main fitting. Since the molten part is formed by melting the cylinder and the main fitting, it means that more of the main fitting has melted in the molten part of Sample No. 9 compared to Sample No. 1-8.

[0059] Considering the screw inspection results, Samples No. 1-7 with a penetration depth of 0.40 mm or less were judged as A, and Samples No. 8 and 9 with a penetration depth of 0.45 mm were judged as B. Although the proportion of the main fitting melted in the molten part of Samples No. 8 and 9 was different, both were judged as B. Therefore, according to Samples No. 1-9, it was clarified that when the penetration depth is 0.40 mm or less, the entire length of the ring gauge can pass through the male screw, that is, the welding strain of the main fitting can be reduced.

[0060] When examining the results of the airtightness test, it was found that for samples No. 1-5 with a penetration depth of 0.2 mm or less, the judgment was A, and for samples No. 6 and 7 with a penetration depth of 0.3 - 0.4 mm, the judgment was B. To increase the penetration depth to 0.3 - 0.4 mm, it is necessary to increase the angle of the laser beam with respect to the center line C of hole 24 compared to the case where the penetration depth is 0.2 mm or less. When this is done, the thickness of the melted portion in the radial direction of the cylindrical portion 21 becomes thinner, so if a cavity occurs in the melted portion, air is likely to leak from the vicinity. Therefore, according to samples No. 1-7, it was clarified that when the penetration depth is 0.2 mm or less, the incomplete welding portion can be reduced.

[0061] In the embodiment, the case where the nominal diameter of the male screw 22 is 10 mm was described, but the same tendency was observed when the nominal diameter of the male screw 22 was 12 mm or 14 mm. Also, in the embodiment, the case of a hole 24 with a constant size in the radial direction of the cylindrical portion 21 was described, but the same tendency was observed in the case of a tapered hole that becomes smaller toward the inside in the radial direction of the cylindrical portion 21.

[0062] As described above, the present invention has been described based on the embodiments, but it is easily inferred that the present invention is not limited to the above embodiments at all, and various improvements and modifications are possible without departing from the spirit of the present invention.

[0063] In the embodiment, the case where holes 24 and 51 are circular was described, but it is not necessarily limited to this. Other shapes of holes 24 and 51 include ellipses, semi-circles, triangles, squares, polygons such as hexagons, and polygons with rounded corners. The cross-sectional shape of the ground electrode 25 disposed in holes 24 and 51 is appropriately set according to the shape of holes 24 and 51.

[0064] In the embodiment, the case where the melting portions are continuously provided around the center line C of the holes 24 and 51 has been described, but it is not necessarily limited to this. It is of course possible to intermittently provide the melting portions around the center line C of the holes 24 and 51. When the melting portions are intermittently provided in the holes 24 and 51, in the cross section including the center line C of the holes 24 and 51 and the melting portions, the melting portions do not necessarily appear on both sides of the center line C as in the embodiment, and the melting portions may appear only on one side of the center line C.

[0065] In the embodiment, the case where a part of the end face 36 of the base material 33 of the ground electrode 25 remains unmelted has been described, but it is not necessarily limited to this. It is possible that the entire end face 36 of the base material 33 has melted into the melting portions 37, 38, 53, and 54 and the end face 36 has disappeared. When the end face 36 of the base material 33 has disappeared, it can be said that the interfaces 39, 43, 55, and 60 between the cylindrical portion 21 and the melting portions 37, 38, 53, and 54 exist on the outside in the radial direction from the ground electrode 25, provided that at least a part of the interfaces 39, 43, 55, and 60 is located on the outside in the radial direction of the cylindrical portion 21 compared to the portion of the interface between the melting portions 37, 38, 53, and 54 and the base material 33 that is located most on the outside in the radial direction.

[0066] In the embodiment, the case where the holes 24 and 51 are provided in the threaded portion 22 of the cylindrical portion 21 has been described, but it is not necessarily limited to this. For example, it is of course possible to provide a cylindrical portion without the thread 22 at the tip of the cylindrical portion 21 and make a hole in the tip (cylindrical portion) of the cylindrical portion 21 to provide the ground electrode 25.

[0067] In the first embodiment, the case where the size of the hole 24 is constant in the radial direction of the cylindrical portion 21 has been described, but it is not necessarily limited to this. It is of course possible to provide a countersink connected to the outer peripheral surface 31 of the cylindrical portion 21 around the hole 24. Since the hole 24 expands at the countersink portion, the heat influence of the laser welding for providing the melting portions 37 and 38 by irradiating a laser beam into the hole 24 is less likely to be received by the thread 22. In this case, the intersections 41 and 45 between the melting portions 37 and 38 and the inner surface 30 of the hole 24 are provided on the inside in the radial direction of the cylindrical portion 21 compared to the countersink.

[0068] In the first embodiment, the case where the portion of the ground electrode 25 inside the hole 24 is thicker than the portion closer to the axis O has been described, but it is not necessarily limited to this. It is of course possible to make the thickness of the portion of the ground electrode 25 inside the hole 24 the same as the thickness of the portion closer to the axis O.

[0069] In the embodiment, the molten portions 37, 38, 53, 54 that appear in a cross-section including the center line C of the holes 24, 51 and perpendicular to the axis O of the cylindrical portion 21 have been described, but the cross-section is not necessarily limited to this. Other cross-sections include a cross-section including the center line C of the holes 24, 51 and parallel to the axis O of the cylindrical portion 21, and a cross-section including the center line C of the holes 24, 51 and the axis O of the cylindrical portion 21.

[0070] In the embodiment, the case where a spark gap G is provided between the side surface of the center electrode 15 and the tip of the ground electrode 25 has been described, but it is not necessarily limited to this. While shifting the positions of the holes 24, 51 provided in the cylindrical portion 21 toward the tip side and making the ground electrode 25 slightly longer, a spark gap G may be provided between the tip of the center electrode 15 and the side surface of the ground electrode 25.

[0071] In the embodiment, the case where the cap 26 is arranged on the tip side of the main fitting 20 has been described, but it is not necessarily limited to this. It is of course possible to omit the cap 26.

[0072] In the embodiment, the case where the hemispherical cap 26 is arranged on the main fitting 20 has been described, but it is not necessarily limited to this. The shape of the cap 26 can be set as appropriate. Other shapes of the cap 26 include a bottomed cylindrical shape and a disc shape.

[0073] In the embodiment, the case where the cap 26 is welded to the main fitting 20 has been described, but it is not necessarily limited to this. It is of course possible to prepare a cylindrical member provided with a cap at its tip and connect this to the main fitting 20 to form the space 29. The cylindrical member is a cylindrical member whose tip is closed by a cap, and a female screw for coupling to the male screw 22 of the main fitting 20 is provided on the inner peripheral surface. A male screw for coupling to the female screw of the spark plug hole of the engine is provided on the outer peripheral surface of the cylindrical member. By coupling the female screw of the cylindrical member to the male screw 22 of the main fitting 20, a cap is disposed on the tip side of the main fitting 20. A through hole 27 is provided in this cap.

[0074] The means for connecting the cylindrical member to the main fitting 20 and disposing a cap on the tip side of the main fitting 20 is not limited to coupling the female screw on the inner peripheral surface of the cylindrical member to the male screw 22 of the main fitting 20. It is of course possible to connect the cylindrical member to the main fitting by other means. Examples of other means include joining the cylindrical member and the main fitting by welding or the like. Examples of the material of the cylindrical member include metal materials such as Ni-based alloys and stainless steels, and ceramics such as silicon nitride.

Explanation of Reference Numerals

[0075] 10 Spark plug 15 Center electrode 20 Main fitting 21 Cylindrical portion 22 Male screw 24, 51 Hole 25 Ground electrode 26 Cap 27 Through hole 29 Space 37, 38, 53, 54 Molten portion C Center line of hole

Claims

1. A central electrode, a main fitting that insulatively holds the central electrode inside, and a ground electrode that protrudes from the main fitting toward the central electrode, wherein the main fitting includes a cylindrical portion provided with an external thread on the outer periphery, and a hole penetrating the cylindrical portion in the radial direction, and the spark plug in which the ground electrode is connected to the cylindrical portion through a melted portion in the hole, in a cross section including the center line of the hole and the melted portion, the spark plug in which a penetration depth of the melted portion into the cylindrical portion is 0 mm or more and 0.4 mm or less.

2. The spark plug according to claim 1, wherein the penetration depth is 0 mm or more and 0.2 mm or less.

3. The spark plug according to claim 1 or 2, wherein the hole is tapered such that a size inside in the radial direction of the cylindrical portion is smaller than a size outside in the radial direction.

4. The spark plug according to claim 1 or 2, wherein the hole has a constant size in the radial direction of the cylindrical portion.

5. a first element having the largest content among the elements included in the main fitting and a second element having the largest content among the elements included in the ground electrode are different in type, and the element having the largest content among the elements included in the melted portion is the second element. The spark plug according to claim 1 or 2.

6. a cap that closes the tip side of the main fitting, and a through hole that communicates the inside and the outside of the space closed by the cap is provided in the cap. The spark plug according to claim 1 or 2.

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