Spark plug for internal combustion engine and method for manufacturing the same

Abstract

To provide a spark plug for an internal combustion engine capable of improving welding strength between a ground electrode and a housing, and provide a method of manufacturing the spark plug.SOLUTION: In a spark plug 1, a ground electrode 6 includes: an outer skin layer 61; an intermediate layer 62; and a core layer 63. At least a part of the intermediate layer 62 is disposed inside the outer skin layer 61 and has a higher thermal conductivity than the outer skin layer 61. At least a part of the core layer 63 is disposed inside the intermediate layer 62. At a fixed end part 64 of the ground electrode 6 welded to the housing 2, the outer skin layer 61 has an outer layer bent part 611 bent inward. The outer layer bent part 611 is welded to the housing 2. An aluminum oxide layer is not interposed in a bent welded part 5 which is a welded part between the outer layer bent part 611 and the housing 2.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a spark plug for an internal combustion engine and a method for manufacturing the same. [Background technology]

[0002] For example, as disclosed in Patent Document 1, there is known a spark plug for an internal combustion engine that includes a ground electrode having an outer skin layer, an intermediate layer disposed inside the outer skin layer and having a higher thermal conductivity than the outer skin layer, and a core material layer disposed inside the intermediate layer. This spark plug aims to increase the weld strength between the ground electrode and the housing by preventing the intermediate layer from being included in the weld interface between the ground electrode and the housing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6170526 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the spark plug described in Patent Document 1 does not fully consider the strength of the weld between the outer skin layer and the housing. That is, for example, during the manufacturing process of a spark plug, the ground electrode may be annealed to prevent cracks caused by work hardening when the ground electrode is bent. In the spark plug described in Patent Document 1, the outer skin layer contains aluminum. Therefore, annealing the ground electrode may result in the formation of an aluminum oxide layer on the surface of the outer skin layer. If the ground electrode is resistance-welded to the housing with the aluminum oxide layer formed on the surface of the outer skin layer, the presence of the aluminum oxide layer at the weld between the outer skin layer and the housing may prevent the outer skin layer from being sufficiently joined to the housing. Therefore, it can be said that the spark plug described in Patent Document 1 has room for further improvement in terms of ensuring the strength of the weld between the ground electrode and the housing.

[0005] The present invention has been made in view of the above-mentioned problems, and aims to provide a spark plug for an internal combustion engine that can improve the welding strength between the ground electrode and the housing, and a method for manufacturing the same. [Means for solving the problem]

[0006] One aspect of the present invention is a cylindrical insulator (3), a center electrode (4) held on the inner circumferential side of the insulator and exposed from the insulator to a tip end side (Z1); a cylindrical housing (2) that holds the insulator on its inner periphery; a ground electrode (6) welded to the housing and forming a discharge gap (G) between itself and the center electrode, The ground electrode has an outer skin layer (61), an intermediate layer (62) at least a portion of which is disposed inside the outer skin layer and has a thermal conductivity higher than that of the outer skin layer, and a core layer (63) at least a portion of which is disposed inside the intermediate layer, At the fixed end (64) of the ground electrode welded to the housing, the outer skin layer has an outer layer bent portion (611) bent inward, the outer layer bent portion is welded to the housing, In the spark plug (1) for an internal combustion engine, no aluminum oxide layer is present at a bent welded portion (5) where the outer layer bent portion and the housing are welded to each other.

[0007] Another aspect of the present invention is a method for manufacturing the above spark plug, comprising: a forming step of forming the electrode member (60) to be the ground electrode into an elongated shape; a cutting step of cutting the electrode member using a cutting blade (8) along a direction perpendicular to the longitudinal direction thereof after the forming step; an annealing step of annealing the electrode member after the forming step to reduce the hardness of the electrode member; a removing step of removing an aluminum oxide layer (7) formed on the surface of the outer skin layer by carrying out the annealing step after the annealing step; and a ground welding step of joining the electrode member to the housing by resistance welding in a state where a cut surface (65) of the electrode member formed by cutting with the cutting blade is pressed against the housing after the cutting step and the removing step, In the method for manufacturing a spark plug, the removing step removes the aluminum oxide layer formed on the surface of the outer cover layer at least in a part of the electrode member that becomes the fixed end. [Effects of the Invention]

[0008] In the above spark plug, the bent welded portion does not have an aluminum oxide layer therebetween, which allows the outer cover layer and the housing to be firmly joined together, thereby improving the welding strength between the ground electrode and the housing.

[0009] In the above-described method for manufacturing a spark plug, the ground welding step is performed after the removing step. Therefore, the outer cover layer can be welded to the housing without the aluminum oxide layer being present in the bent welded portion. As a result, the weld strength between the ground electrode and the housing can be improved.

[0010] As described above, according to the above-described aspect, it is possible to provide a spark plug for an internal combustion engine and a method for manufacturing the same that can improve the welding strength between the ground electrode and the housing. In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view of a spark plug according to a first embodiment, taken along the plug axis direction. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is a view taken along arrow III in FIG. 2. [Figure 4] 4 is a cross-sectional view of the upright portion perpendicular to the plug axial direction in the first embodiment, taken along the line IV-IV in FIG. 2. FIG. [Figure 5] 4 is a cross-sectional view taken along the line VV in FIG. 3, showing the vicinity of the fixed end portion of the plug in the axial direction of the plug in the first embodiment. [Figure 6] FIG. 3 is a perspective view of a first member and the like that constitute the electrode member before a molding step in the first embodiment. [Figure 7] FIG. 3 is a perspective view of an electrode member before a molding step in the first embodiment. [Figure 8] FIG. 3 is a perspective view of the electrode member after a molding step in the first embodiment. [Figure 9] FIG. 10 is a diagram showing a state in which a removal step is being performed in the first embodiment. [Figure 10] 5A to 5C are views showing a state in which a cutting step is performed in the first embodiment. [Figure 11]FIG. 4 is a plan view of the electrode member after a cutting step in the first embodiment. [Figure 12] FIG. 3 is a plan view of the vicinity of a cut surface of the electrode member in the first embodiment. [Figure 13] 4 is a cross-sectional view showing a state in which an electrode member is resistance-welded to a housing in the first embodiment. FIG. [Figure 14] FIG. 4 is a cross-sectional view showing a state in which an outer layer bent portion is formed during a ground welding process in the first embodiment. [Figure 15] 4 is a cross-sectional view showing a current during a ground welding process in the first embodiment. FIG. [Figure 16] FIG. 10 is a cross-sectional view showing a state in which an electrode member is resistance-welded to a housing in a comparative example. [Figure 17] FIG. 10 is a cross-sectional view showing a state in which an outer layer bent portion is formed during a ground welding process in a comparative example. [Figure 18] FIG. 10 is a cross-sectional view showing a current during a ground welding process in a comparative example. [Figure 19] 2 is a graph showing the results of tensile strength tests of Examples and Comparative Examples 1 to 3 in Experimental Examples. [Figure 20] FIG. 10 is a cross-sectional view of the vicinity of the fixed end portion of the sample of Comparative Example 1 before the tensile strength test in the experimental example. [Figure 21] 10 is a cross-sectional view of the vicinity of the fixed end portion of the sample of Comparative Example 1 after a tensile strength test in an experimental example. [Figure 22] 10 is a cross-sectional view of the vicinity of the fixed end portion of the sample of the example after a tensile strength test in an experimental example. [Figure 23] FIG. 10 is a cross-sectional view taken along the axial direction of the plug near the fixed end in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Embodiment 1) An embodiment of a spark plug for an internal combustion engine and a method for manufacturing the same will be described with reference to FIGS. As shown in FIGS. 1 to 3, a spark plug 1 for an internal combustion engine according to this embodiment includes a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, and a ground electrode 6. The center electrode 4 is held on the inner periphery of the insulator 3 and is exposed from the insulator 3 on a tip side Z1. The housing 2 holds the insulator 3 on its inner periphery. The ground electrode 6 is welded to the housing 2 and forms a discharge gap G between itself and the center electrode 4.

[0013] 2, 4, and 5, the ground electrode 6 has an outer skin layer 61, an intermediate layer 62, and a core material layer 63. At least a portion of the intermediate layer 62 is disposed inside the outer skin layer 61 and has a higher thermal conductivity than the outer skin layer 61. At least a portion of the core material layer 63 is disposed inside the intermediate layer 62.

[0014] 5, at the fixed end 64 of the ground electrode 6 welded to the housing 2, the outer layer 61 has an outer layer bent portion 611 that is bent inward. The outer layer bent portion 611 is welded to the housing 2. No aluminum oxide layer is present at the bent welded portion 5, which is the welded portion between the outer layer bent portion 611 and the housing 2.

[0015] The spark plug 1 of this embodiment can be used, for example, as an ignition means in an internal combustion engine of an automobile, etc. The spark plug 1 is attached to an internal combustion engine (not shown) by threading the threaded portion 22 (see FIGS. 1 and 3) of the housing 2 into the female threaded portion of a plug hole in a cylinder head (not shown).

[0016] One end of the spark plug 1 in the axial direction Z is disposed in a combustion chamber (not shown) of an internal combustion engine. In the axial direction Z of the spark plug 1, the side exposed to the combustion chamber is referred to as the tip side Z1, and the opposite side is referred to as the base side Z2. The axial direction Z of the spark plug 1 may also be referred to as the plug axial direction Z as appropriate. The plug radial direction refers to the radial direction of a circle centered on the central axis C of the spark plug 1 on a plane perpendicular to the central axis C of the spark plug 1. The plug circumferential direction refers to the direction along the circumference of a circle centered on the central axis C of the spark plug 1. In this embodiment, the central axis C is also the central axis of the housing 2 and the center electrode 4.

[0017] As shown in FIG. 1 , the center electrode 4 is inserted into the axial bore 31 of the insulator 3. A first glass seal layer 11, a resistor 12, a second glass seal layer 13, and a terminal fitting 14 are arranged on the base end side Z2 of the center electrode 4 within the axial bore 31. The first glass seal layer 11 and the second glass seal layer 13 may be made of glass containing copper powder, for example. The resistor 12 has the function of absorbing radio noise generated when a discharge spark is generated. The resistor 12 may be made of, for example, an aggregate in which a conductive material such as a carbon material is dispersed in a substrate containing a glass material and an aggregate. The end of the terminal fitting 14 on the base end side Z2 is exposed from the insulator 3. The spark plug 1 is electrically connected to an ignition coil (not shown) at the terminal fitting 14. The terminal fitting 14 may be made of, for example, an iron alloy.

[0018] In this embodiment, the ground electrode 6 is welded to the front end surface 21 of the housing 2. The front end surface 21 is flat and is formed so as to be perpendicular to the plug axial direction Z. The ground electrode 6 protrudes from a fixed end portion 64 fixed to the front end surface 21 toward the front end side Z1.

[0019] 1 and 2, the ground electrode 6 has an upright portion 671, a ground bent portion 672, and an extended portion 673. The upright portion 671 has a fixed end portion 64 and extends from the front end surface 21 of the housing 2 toward the front end side Z1 along the plug axial direction Z. The ground bent portion 672 bends from the front end of the upright portion 671 toward the inside in the plug radial direction. The extended portion 673 extends from the ground bent portion 672 toward the inside in the plug radial direction, and forms a discharge gap G between itself and the center electrode 4.

[0020] As shown in FIG. 4, a cross section of the ground electrode 6 perpendicular to the extension direction is substantially rectangular. The width W1 of the ground electrode 6 is greater than the thickness T1 of the ground electrode 6. In this embodiment, the thickness direction of the upright portion 671 is the direction along the arrangement of the upright portion 671 and the center electrode 4 when viewed from the plug axial direction Z, as shown in FIG. 3. As shown in FIG. 4, the ground electrode 6 has a multilayer structure in which an outer skin layer 61, an intermediate layer 62, and a core layer 63 are stacked in both the thickness direction and the width direction of the ground electrode 6. As shown in FIGS. 1, 2, and 5, the outer skin layer 61, the intermediate layer 62, and the core layer 63 are each formed from the fixed end portion 64 to the extension portion 673.

[0021] Furthermore, the outer skin layer 61 and the core layer 63 have a higher hardness than the intermediate layer 62. The outer skin layer 61 has a higher hardness than the core layer 63. The outer skin layer 61 and the core layer 63 may be made of, for example, nickel or a nickel alloy containing nickel as a main component. In this embodiment, the outer skin layer 61 is made of a nickel-based alloy, and the core layer 63 is made of nickel. Furthermore, the intermediate layer 62 may be made of, for example, copper or an alloy containing copper as a main component. In this embodiment, the intermediate layer 62 is made of copper.

[0022] The outer layer 61 also contains aluminum. The aluminum content in the outer layer 61 is 2 mass % or more. The outer layer 61 can be made of a material such as Inconel 600 (registered trademark) containing aluminum, for example.

[0023] 5, the outer skin layer 61 and the core material layer 63 at the fixed end 64 are directly fixed to the front end surface 21 of the housing 2. The outer layer bent portion 611 of the outer skin layer 61 and the front end surface 21 are joined to each other without an aluminum oxide layer therebetween. In addition, the fixed end 64 and the housing 2 are joined to each other without an aluminum oxide layer therebetween.

[0024] In this embodiment, the outer layer bent portion 611 is in contact with the core material layer 63. Furthermore, the intermediate layer 62 is not in contact with the ground weld 50, which is the weld between the fixed end 64 and the housing 2. The outer layer bent portion 611 covers a part of the base end of the intermediate layer 62 from the base end side Z2. The intermediate layer 62 is sealed by the outer layer 61 and the core material layer 63.

[0025] The outer layer bent portion 611 is formed on one side in the thickness direction of the fixed end portion 64. The outer layer bent portion 611 is also formed on the outer or inner side in the plug radial direction of the fixed end portion 64. In this embodiment, the outer layer bent portion 611 is formed on the inner side in the plug radial direction of the fixed end portion 64.

[0026] When viewed from the plug axial direction Z (not shown), the outer layer bent portion 611 is formed to overlap with a portion of the outer coat layer 61 of the standing portion 671 that is closer to the tip Z1 than the outer layer bent portion 611. At the fixed end 64, the outer layer bent portion 611 protrudes toward the core material layer 63 and also protrudes to one side in the thickness direction of the fixed end 64. In this embodiment, the outer layer bent portion 611 protrudes toward one side in the plug radial direction. Specifically, the outer layer bent portion 611 protrudes outward in the plug radial direction.

[0027] The base end of the core layer 63 has a core protrusion 631 that protrudes toward one side in the plug radial direction. The core protrusion 631 protrudes toward the outside in the plug radial direction. The core protrusion 631 is welded to the front end surface 21 of the housing 2.

[0028] Furthermore, no aluminum oxide layer is formed on the surface of the outer skin layer 61 at the fixed end 64. In this embodiment, no aluminum oxide layer is formed on the surface of the outer skin layer 61 at the standing portion 671. Furthermore, no aluminum oxide layer is formed on the entire surface of the outer skin layer 61 of the ground electrode 6.

[0029] In this embodiment, the ground weld 50 does not have an aluminum oxide layer. Furthermore, when observed with an EPMA (Electron Probe Micro Analyzer), the surface of the outer layer bent portion 611 facing the bent weld 5 has the same concentrations of aluminum atoms and oxygen atoms as the interior of the outer layer bent portion 611. In other words, when observed with an EPMA, the entire surface of the outer layer bent portion 611 facing the bent weld 5 does not exhibit any high concentrations of aluminum atoms or oxygen atoms, which would indicate the presence of an aluminum oxide layer. Furthermore, it is preferable that the bent weld 5 does not also have a metal oxide layer, such as chromium oxide, in addition to the aluminum oxide layer.

[0030] Next, a method for manufacturing the spark plug 1 of this embodiment will be described. The manufacturing method of the spark plug 1 of this embodiment includes a forming step, a cutting step, an annealing step, a removing step, and a ground welding step. The forming step is a step of forming an electrode member 60 that will become the ground electrode 6 into a long shape as shown in FIG. 8. The cutting step is a step of cutting the electrode member 60 using a cutting blade 8 in a direction perpendicular to its longitudinal direction as shown in FIG. 10 after the forming step. The annealing step is a step of annealing the electrode member 60 after the forming step to reduce the hardness of the electrode member 60. The removing step is a step of removing an aluminum oxide layer 7 that was unintentionally formed on the surface of the outer cover layer 61 as a result of the annealing step as shown in FIG. 9 after the annealing step. The ground welding step is a step of joining the electrode member 60 to the housing 2 by resistance welding a cut surface 65 of the electrode member 60, formed by cutting with the cutting blade 8, in a state in which the cut surface 65 is pressed against the housing 2 as shown in FIGS. 13 to 15 after the cutting and removing steps. In the removing step, at least the aluminum oxide layer 7 formed on the surface of the outer skin layer 61 in the part of the electrode member 60 that will become the fixed end 64 is removed.

[0031] Next, each step will be described in detail. In this embodiment, the electrode member 60 has a substantially rectangular prism shape before being welded to the housing 2. To manufacture the electrode member 60, first, as shown in FIG. 6 , a bottomed, cylindrical first member 601 that will become the outer skin layer 61, a cylindrical second member 602 that will become the intermediate layer 62, and a third member 603 that will become the core layer 63 are prepared. The third member 603 has a cylindrical shaft portion 604 and a flange portion 605 that is provided at one end of the shaft portion 604 and protrudes radially outward from the shaft portion 604. Then, as shown by arrow M1 in FIG. 6 , the shaft portion 604 of the third member 603 is inserted into a through hole formed in the second member 602. Next, as shown by arrow M2 in FIG. 6 , the third member 603 to which the second member 602 is attached is placed inside the first member 601, thereby manufacturing the electrode member 60 before the molding process shown in FIG. 7 .

[0032] Next, in the forming step, the electrode member 60 is subjected to a stretching process by cold forging. In this embodiment, a part of the electrode member 60 shown in Fig. 7 is passed through a die (not shown) with a rectangular opening and formed into a substantially square prism shape as shown in Fig. 8. In the part of the electrode member 60 formed into a substantially square prism shape, a first member 601, a second member 602, and a third member 603 are respectively stacked in layers as an outer skin layer 61, an intermediate layer 62, and a core layer 63.

[0033] Next, an annealing process is performed. The annealing process is a process for reducing the hardness of the electrode member 60, which has become too hard due to the forming process. In other words, the annealing process is a process for preventing cracks from occurring in the outer skin layer 61 when the electrode member 60 is bent to form the ground bent portion 672 after the ground welding process, as described below. In the annealing process, for example, the electrode member 60 is heated in a vacuum high-temperature environment at approximately 850°C for several hours. This reduces the hardness of the electrode member 60, which has been work-hardened by the forming process. In other words, the annealing process recrystallizes the electrode member 60. The hardness of the electrode member 60 is then reduced to a hardness suitable for the bending process to form the ground bent portion 672, which is performed after the ground welding process. The annealing process is performed in a vacuum environment in a vacuum furnace to prevent an aluminum oxide layer from forming on the surface of the outer skin layer 61. However, even if the annealing process is performed in a vacuum environment, oxygen molecules that adhere to the inner wall surface of the furnace at room temperature will separate at high temperatures and react with aluminum on the surface of the outer skin layer 61, resulting in the unintended formation of an aluminum oxide layer 7. The aluminum oxide layer 7 is a layer whose main component is Al2O3 (i.e., aluminum oxide).

[0034] Next, a removal step is performed to remove the unintentionally formed aluminum oxide layer 7. In the removal step of the manufacturing method of this embodiment, as shown in Fig. 9, at least a part of the electrode member 60 that will become the fixed end portion 64 is immersed in an acidic solution 10 to remove the aluminum oxide layer 7 formed on the outer layer 61.

[0035] Specifically, in the removal step, as shown in FIG. 9 , the electrode member 60 after the annealing step is immersed in an acidic solution 10 contained in a solution tank 100. In this embodiment, the electrode member 60 is supported by a guide portion 101, and the portion of the electrode member 60 that will become the ground electrode 6 is immersed in the solution 10 for a predetermined time, thereby removing the aluminum oxide layer 7 formed on the surface of the outer layer 61. In this embodiment, in the removal step, the entire portion of the electrode member 60 that will become the ground electrode 6 is immersed in the acidic solution 10. The acidic solution 10 used in the removal step may contain, for example, nitric acid, hydrogen chloride, or phosphoric acid.

[0036] Next, a cutting step is performed. In this embodiment, the cutting step is performed using a fixed blade 81 and a cutting blade 8, which is a movable blade that is movable relative to the fixed blade 81, as shown in FIG. 10. Specifically, the electrode member 60 is clamped between the fixed blade 81 and a pressing member 82 in the thickness direction of the electrode member 60, and the cutting blade 8 is moved along the thickness direction of the electrode member 60 as shown by arrow S in FIG. 10, thereby cutting the electrode member 60. As a result, a cut surface 65 is formed in the electrode member 60, as shown in FIGS. 11 and 12. Furthermore, in the cutting step, cutting is performed on the portion of the electrode member 60 where the outer skin layer 61, the intermediate layer 62, and the core layer 63 overlap each other in a layered manner. In this way, the electrode member 60 having a substantially rectangular prism shape is manufactured before the ground welding step.

[0037] Furthermore, by cutting the electrode member 60 in the cutting process, a sagging portion 66 formed in a convex curved shape and a burr portion 67 protruding to one side in the thickness direction of the electrode member 60 are formed at the cut portion of the electrode member 60. The sagging portion 66 is formed on the side of the electrode member 60 where cutting by the cutting blade 8 starts, and the burr portion 67 is formed on the side of the electrode member 60 opposite the cutting start side in the thickness direction. In the plan view of the electrode member 60 in FIG. 12 as viewed from the width direction, the angle θ between an extension line 65L of the cut surface 65 and a tangent line 66L to the end of the outer surface of the sagging portion 66 on the cut surface 65 side is an acute angle. Furthermore, part or all of the outer skin layer 61 at the sagging portion 66 becomes an outer layer bent portion 611 formed after the ground welding process.

[0038] 13 to 15, in a ground welding process, a substantially rectangular prism-shaped electrode member 60 is welded to the front end surface 21 of the housing 2. When joining the electrode member 60, the longitudinal direction of the electrode member 60 is aligned with the longitudinal direction of the housing 2, and current is passed through the electrode member 60 while the cut surface 65 of the electrode member 60 is pressed against the front end surface 21 of the housing 2, thereby performing resistance welding. Note that the housing 2 to which the electrode member 60 is welded in the ground welding process is the housing 2 before the center electrode 4 and the insulator 3 are assembled.

[0039] Specifically, first, as shown by arrow P in Fig. 13 , current is applied while the electrode member 60 is pressed against the housing 2 in the plug axial direction Z. As a result, as shown in Fig. 14 , the heat generated by the current welds the base end of the electrode member 60 to the front end surface 21 of the housing 2 and deforms the base end of the electrode member 60, forming an outer layer bent portion 611. Furthermore, by applying current while pressing the electrode member 60 toward the housing 2, as shown in Fig. 15 , the outer layer 61, which was the hanging portion 66, is further deformed, and the portion of the outer layer 61, which was the surface of the hanging portion 66, is welded to the front end surface 21. In other words, the welded portion between the surface of the outer layer 61, which was the hanging portion 66 after the aluminum oxide layer 7 was removed, and the front end surface 21 forms the bent welded portion 5. Furthermore, by performing the ground welding process, the core layer 63 and the front end surface 21 are also welded to each other.

[0040] Next, after the electrode member 60 is welded to the housing 2, a part of the outer covering layer 61 protruding in the plug radial direction is removed as necessary.

[0041] Thereafter, the insulator 3 holding the center electrode 4 and other components on its inner periphery is assembled into the housing 2. Next, a bending process is performed in which the electrode member 60, which has a substantially rectangular prism shape, is bent radially inward to form the ground electrode 6 having an upright portion 671, a ground bent portion 672, and an extended portion 673, as shown in FIG. 2. Thereafter, the ground electrode 6 is slightly deformed as necessary to finely adjust the discharge gap G to an appropriate distance. This allows the spark plug 1 of this embodiment to be manufactured, as shown in FIGS. 1 to 3.

[0042] Next, the effects of this embodiment will be described. In the above spark plug 1, no aluminum oxide layer is present at the bent welded portion 5. This allows the outer skin layer 61 and the housing 2 to be firmly joined together, thereby improving the welding strength between the ground electrode 6 and the housing 2.

[0043] In this embodiment, the electrode member 60 is formed by cold forging as described above, and therefore undergoes work hardening during the manufacturing process. If the electrode member 60 is bent to form a discharge gap while still in a work-hardened state, cracks or the like may occur in the electrode member. Therefore, as described above, the electrode member 60 after the forming process is subjected to the annealing process. Furthermore, the annealing process forms an aluminum oxide layer 7 on the electrode member 60. This aluminum oxide layer 7 is stable and does not easily decompose during resistance welding. Therefore, if the electrode member 60 is welded to the housing 2 without removing the aluminum oxide layer 7 in the removal process, as in the comparative spark plug 9 shown in FIGS. 16 to 18 , the aluminum oxide layer 7 is likely to remain in the bent weld portion 5. Specifically, as indicated by arrow P in FIG. 16 , current is applied to the electrode member 60 while it is pressed against the housing 2 along the plug axial direction Z. In this case, as shown in FIG. 17 , heat generated by energization welds the base end of the electrode member 60 to the front end surface 21 of the housing 2, deforming the base end of the electrode member 60 and forming an outer layer bent portion 611. Furthermore, by energizing the electrode member 60 while pressing it toward the housing 2, as shown in FIG. 18 , the outer layer 61 of the hanging portion 66 becomes the outer layer bent portion 611, and the aluminum oxide layer 7 formed on the surface of the outer layer 61 of the hanging portion 66 becomes interposed in the bent weld portion 5. Here, the aluminum oxide layer 7 has insulating properties. Therefore, as shown in FIGS. 17 and 18 , if resistance welding is performed in a state where the aluminum oxide layer 7 is interposed in the region R between the outer layer bent portion 611 and the housing 2, current E is unlikely to flow from the electrode member 60 to the housing 2 in the region R. As a result, sufficient heat is not generated in the region R that will become the bent weld portion 5, which may result in insufficient weld strength between the outer layer bent portion 611 and the housing 2. Therefore, when the spark plug 9 of the comparative example is mounted on an internal combustion engine, there is a risk that peeling or the like will occur at the bent welded portion 5, where the aluminum oxide layer 7 is present, due to, for example, the difference in the linear expansion coefficient between the housing 2 and the ground electrode 6 and thermal stress caused by temperature changes accompanying operation of the internal combustion engine.If the entire ground electrode 6 tilts due to peeling at the bent welded portion 5, the distance of the discharge gap G increases, which may result in an excessively high voltage being required to generate a discharge in the discharge gap G. This may result in an inability to generate a discharge in the discharge gap G, resulting in a misfire, or in a failure to ignite the fuel. In the worst case scenario, the ground electrode may even fall off. Therefore, in the spark plug 1 of this embodiment, an aluminum oxide layer is prevented from being present at the bent welded portion 5. That is, a removal process is performed before the ground welding process. As a result, as shown in FIG. 15 , a sufficiently large current E can flow from the electrode member 60 to the housing 2 via the outer layer bent portion 611 during resistance welding, thereby firmly joining the outer layer bent portion 611 and the housing 2. This effectively prevents the outer layer bent portion 611 from peeling off from the housing 2. As a result, the life of the spark plug 1 can be extended.

[0044] The outer skin layer 61 and the core material layer 63 have higher hardness than the intermediate layer 62. The outer layer bent portion 611 is in contact with the core material layer 63. Furthermore, the intermediate layer 62 is not in contact with the ground weld portion 50. This further improves the welding strength between the ground electrode 6 and the housing 2. In other words, the intermediate layer 62, which has a relatively low hardness, is not welded to the housing 2, while the outer skin layer 61 and the core material layer 63, which have a relatively high hardness, are welded to the housing 2. This further improves the welding strength between the ground electrode 6 and the housing 2.

[0045] The outer skin layer 61 contains aluminum. Therefore, when the spark plug 1 is installed in an internal combustion engine, the outer skin layer 61 is heated by combustion in the combustion chamber. As a result, even if the aluminum oxide layer is removed in the removal process, a dense aluminum oxide passivation layer is newly formed on the surface of the outer skin layer 61. This makes it easy to suppress further oxidation of the outer skin layer 61 and improve the durability of the ground electrode 6. This is the original purpose of including aluminum in the ground electrode 6, and as a result, the life of the ground electrode 6 can be extended.

[0046] The aluminum content in the outer layer 61 is 2% by mass or more. Therefore, when the spark plug 1 is mounted in an internal combustion engine, a dense aluminum oxide passivation layer is more likely to be formed on the surface of the outer layer 61. As a result, the durability of the ground electrode 6 is more likely to be improved.

[0047] No aluminum oxide layer is formed on the surface of the outer layer 61 at the fixed end 64. Therefore, even if the length or degree of bending of the outer layer bent portion 611 varies among ground electrodes 6, the ground electrode 6 and the housing 2 can be welded to each other while reliably preventing an aluminum oxide layer from being present at the bent welded portion 5. In other words, before the ground welding process, the aluminum oxide layer is removed from a wider area of the outer layer 61 of the electrode member 60 than the area expected to become the outer layer bent portion 611. Therefore, even if the area welded to the housing 2 varies among electrode members 60, the aluminum oxide layer can be reliably prevented from being present at the bent welded portion 5. As a result, the ground electrode 6 and the housing 2 can be joined sufficiently firmly while improving productivity.

[0048] In the method for manufacturing the spark plug 1, the ground welding step is performed after the removing step. Therefore, the outer cover layer 61 can be welded to the housing 2 so that no aluminum oxide layer is present at the bent welded portion 5. As a result, the welding strength between the ground electrode 6 and the housing 2 can be improved.

[0049] In the removal step, at least a portion of the electrode member 60 that will become the fixed end 64 is immersed in the acidic solution 10 to remove the aluminum oxide layer 7 formed on the outer skin layer 61. This allows the aluminum oxide layer 7 formed on the outer skin layer 61 to be removed efficiently and sufficiently. As a result, the welding strength between the ground electrode 6 and the housing 2 can be further improved, and productivity can be improved.

[0050] The intermediate layer 62 is primarily composed of copper. This makes it easier to dissipate heat from the ground electrode 6 to the outside. In other words, the intermediate layer 62, which is primarily composed of copper, has high thermal conductivity, making it easier to dissipate heat from the ground electrode 6 to the outside via the housing 2. This makes it possible to further suppress overheating of the ground electrode 6. As a result, it is possible to further suppress the occurrence of pre-ignition.

[0051] As described above, according to the present embodiment, it is possible to provide the spark plug 1 for an internal combustion engine and the manufacturing method thereof, which are capable of improving the welding strength between the ground electrode 6 and the housing 2.

[0052] In the first embodiment, the removal step is performed by immersing the electrode member 60 in an acidic solution 10. However, the removal step can also be performed by, for example, shot blasting, barrel polishing, laser cleaning, cutting the surface of the electrode member, or the like.

[0053] In the first embodiment, the surface of the outer layer 61 of the ground electrode 6 does not have an aluminum oxide layer formed thereon.

[0054] In addition, in the first embodiment, the outer layer bent portion 611 is formed on one side in the plug radial direction of the fixed end portion 64. However, the outer layer bent portion may also be formed, for example, on one side in the plug circumferential direction of the fixed end portion.

[0055] In the first embodiment, the cutting step is performed after the removing step. However, for example, the removing step can also be performed after the cutting step.

[0056] (Experimental example) In this example, as shown in Fig. 19, a plurality of spark plug samples were used that had the same basic structure as the spark plug of embodiment 1 but were manufactured by different methods, and a tensile strength test was conducted to examine the welding strength between the ground electrode and the housing. Furthermore, in this example, five samples each of the example and comparative examples 1 to 3 were used to conduct the tensile strength test. Furthermore, all of the samples used in this example had outer layer bent portions.

[0057] In this example, the annealing process was performed at approximately 850° C. The tensile strength of the ground electrode was evaluated by pulling the ground electrode toward the tip end using a tension jig (not shown) while the housing was fixed to a fixture (not shown) until the ground electrode broke.

[0058] The samples of the example had the same configuration as that of the first embodiment, but no aluminum oxide layer was present in the bent weld. The samples of the example were annealed using a vacuum furnace, and a removal process was performed using an acidic aqueous solution containing 7.5% nitric acid, 7.5% hydrogen chloride, 12.5% phosphoric acid, and 12.5% nitrogen compounds. The removal process was performed by immersing the electrode member in the acidic aqueous solution maintained at 25°C for 60 minutes. This condition was determined by using an EPMA to identify the immersion time at which high concentrations of aluminum and oxygen atoms, indicating the presence of an aluminum oxide layer, were no longer observed on the surface of the outer layer. Furthermore, elemental analysis of the ground weld of the example sample was performed using an EPMA, and no high concentrations of aluminum and oxygen atoms were observed, confirming the absence of an aluminum oxide layer in the ground weld, including the bent weld. The samples were otherwise manufactured under the same conditions as those of the first embodiment.

[0059] The sample of Comparative Example 1 is a sample having the same configuration as the spark plug 9 of the comparative embodiment shown in FIG. 18. That is, the sample of Comparative Example 1 is a sample in which an aluminum oxide layer 7 is interposed in the bent welded portion 5 as shown in FIG. 20. The sample of Comparative Example 1 was subjected to the annealing process without using a vacuum furnace, and the ground welding process without performing the removal process. Other than that, it was produced by the same production method as the samples of the example.

[0060] The sample of Comparative Example 2 was subjected to the annealing process in a vacuum furnace and the ground welding process without the removal process. It was confirmed that an aluminum oxide layer was present in the bent welded portion of the sample of Comparative Example 2. The outer skin layer contains aluminum, which is easily oxidized. Therefore, in Comparative Example 2, although the electrode member was annealed in a vacuum furnace, oxygen adsorbed on the furnace wall and the like was liberated during vacuum heating and bonded with the aluminum on the surface of the outer skin layer to form an aluminum oxide layer. The rest of the sample was manufactured using the same manufacturing method as Comparative Example 1.

[0061] The sample of Comparative Example 3 is a sample that does not undergo the removal process, similar to Comparative Example 1. Furthermore, in the ground welding process for the sample of Comparative Example 3, the current value was increased by 20% compared to the current value during resistance welding in the example, and the electrode member was joined to the housing. In other words, the sample of Comparative Example 3 is a sample that underwent the ground welding process by increasing the current energy during resistance welding to forcibly cause dielectric breakdown in the aluminum oxide layer between the outer layer bent portion and the housing, thereby allowing current to pass through. Other than that, the sample was manufactured using the same manufacturing method as Comparative Example 1.

[0062] Next, the results of the tensile strength test will be described. As shown in Fig. 19, Comparative Example 1 exhibited a tensile strength of 1500 to 1600 N, while the Examples exhibited a tensile strength of 2050 to 2150 N. In other words, it was confirmed that the Examples exhibited a tensile strength that was improved by nearly 40% on average compared to the tensile strength of Comparative Example 1.

[0063] In Comparative Example 1, as shown in FIG. 21 , after the tensile strength test, peeling was confirmed at the bent weld 5 where the aluminum oxide layer 7 was present. On the other hand, in all of the samples of the Examples, as shown in FIG. 22 , no peeling was confirmed at the ground weld 50, including the bent weld 5, and fracture of the ground electrode 6 was confirmed at the portion on the tip side Z1 of the fixed end 64. From this result, it is thought that the tensile strength of Comparative Example 1 was lower than that of the Examples due to the peeling at the bent weld 5 where the aluminum oxide layer 7 was present. In other words, when the aluminum oxide layer 7 is present between the outer layer bent portion 611 and the housing 2, it is thought that the weld strength between the outer layer bent portion 611 and the housing 2 is weakened.

[0064] Furthermore, Comparative Example 2 also exhibited a tensile strength similar to that of Comparative Example 1, as shown in Fig. 19. It is believed that the sample of Comparative Example 2 also had an aluminum oxide layer present in the bent welded portion, as in Comparative Example 1, which is why the tensile strength was lower than that of the samples of the example. Furthermore, it was confirmed from these results that vacuum annealing alone was not sufficient to suppress the formation of an aluminum oxide layer on the surface of the outer layer bent portion, and that a separate process for removing the aluminum oxide layer was necessary.

[0065] Furthermore, Comparative Example 3 exhibited a tensile strength of 1900 to 2180 N. Although the maximum tensile strength of Comparative Example 3 was comparable to that of the Examples, Comparative Example 3 resulted in a large variation in tensile strength between samples. In Comparative Example 3, even when the current value during resistance welding was increased, the dielectric breakdown removal of the aluminum oxide layer was insufficient, so stable welding could not be performed, which is thought to have resulted in a large variation in tensile strength between samples. On the other hand, in the Examples, the variation in tensile strength between samples was smaller than in Comparative Example 3. In other words, it can be said that the Examples have higher joining stability than Comparative Example 3.

[0066] As described above, from the results of this example, it can be said that the spark plug 1 of embodiment 1, in which no aluminum oxide layer is present in the bent welded portion 5, can improve the welding strength between the ground electrode 6 and the housing 2 and can also improve the joining stability.

[0067] (Embodiment 2) In this embodiment, as shown in FIG. 23, an intermediate layer 62 is welded to the housing 2.

[0068] 23, the intermediate layer 62 is directly welded to the housing 2. The outer layer bent portion 611 is not in contact with the core layer 63.

[0069] Furthermore, before the ground welding process, the electrode member 60 has no bond between the intermediate layer 62 and the outer layer 61. However, after the ground welding process, an alloy layer is formed at the boundary between the intermediate layer 62 and the outer layer bent portion 611. This further strengthens the bond strength between the ground electrode 6 and the housing 2. Therefore, although not shown, when a test similar to the tensile strength test described in the experimental example is performed using the spark plug 1 of this embodiment, the standing portion 671 breaks partway through, similar to the result shown in FIG. 22 of the example. Other aspects are the same as those of embodiment 1. Note that, among the symbols used in embodiment 2 and onwards, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0070] In this embodiment, the intermediate layer 62, which has high thermal conductivity, is directly welded to the housing 2. This allows heat to be efficiently transferred from the ground electrode 6 to the housing 2. This further reduces overheating of the ground electrode 6. As a result, the occurrence of pre-ignition can be further reduced. In addition, the same effects as those of the first embodiment are achieved.

[0071] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments within the scope of the present invention. [Explanation of symbols]

[0072] 1...spark plug, 2...housing, 3...insulator, 4...center electrode, 5...bent welded portion, 6...ground electrode, 61...outer layer, 62...intermediate layer, 63...core material layer, 64...fixed end, 611...outer layer bent portion, G...discharge gap, Z1...tip side

Claims

1. A cylindrical insulator (3), a center electrode (4) held on the inner circumferential side of the insulator and exposed from the insulator to a tip end side (Z1); a cylindrical housing (2) that holds the insulator on its inner periphery; a ground electrode (6) welded to the housing and forming a discharge gap (G) between itself and the center electrode, The ground electrode has an outer skin layer (61), an intermediate layer (62) at least a portion of which is disposed inside the outer skin layer and has a thermal conductivity higher than that of the outer skin layer, and a core layer (63) at least a portion of which is disposed inside the intermediate layer, At the fixed end (64) of the ground electrode welded to the housing, the outer skin layer has an outer layer bent portion (611) bent inward, the outer layer bent portion is welded to the housing, A spark plug (1) for an internal combustion engine, wherein an aluminum oxide layer is not present at a bent welded portion (5) where the outer layer bent portion and the housing are welded to each other.

2. 2. The spark plug for an internal combustion engine according to claim 1, wherein the outer skin layer and the core material layer have a hardness higher than that of the intermediate layer, the outer layer bent portion is in contact with the core material layer, and the intermediate layer is not in contact with a ground weld portion (50) that is a weld portion between the fixed end portion and the housing.

3. 2. The spark plug for an internal combustion engine according to claim 1, wherein said intermediate layer is directly welded to said housing.

4. 4. The spark plug for an internal combustion engine according to claim 1, wherein the outer cover layer contains aluminum.

5. 5. The spark plug for an internal combustion engine according to claim 4, wherein the aluminum content in the outer cover layer is 2% by mass or more.

6. 4. The spark plug for an internal combustion engine according to claim 1, wherein an aluminum oxide layer is not formed on the surface of the outer cover layer at the fixed end portion.

7. A method for manufacturing a spark plug according to any one of claims 1 to 3, a forming step of forming the electrode member (60) to be the ground electrode into an elongated shape; a cutting step of cutting the electrode member using a cutting blade (8) along a direction perpendicular to the longitudinal direction thereof after the forming step; an annealing step of annealing the electrode member after the forming step to reduce the hardness of the electrode member; a removing step of removing an aluminum oxide layer (7) formed on the surface of the outer skin layer by carrying out the annealing step after the annealing step; and a ground welding step of joining the electrode member to the housing by resistance welding in a state where a cut surface (65) of the electrode member formed by cutting with the cutting blade is pressed against the housing after the cutting step and the removing step, In the removing step, the aluminum oxide layer formed on the surface of the outer cover layer in at least a portion of the electrode member that becomes the fixed end portion is removed.

8. 8. The method for manufacturing a spark plug according to claim 7, wherein in the removing step, the aluminum oxide layer formed on the outer coating layer is removed by immersing at least a portion of the electrode member that will become the fixed end in an acidic solution.

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