Methods for manufacturing spark plugs

Abstract

Method for manufacturing a spark plug (10), wherein the spark plug (10) has a ground electrode (17) to which a connecting surface (31) of a plate (30) is welded, the method comprising: a machining process of machining the ground electrode (17) such that an area (26) located within a rim (23) of a part (22) intended for welding, where the plate (30) is welded onto the ground electrode (17), and which includes a center (24) of the part (22) intended for welding, is rougher than a section (29) that does not include the area (26) of the part (22) intended for welding; and a joining process of connecting the plate (30) with the part (22) of the ground electrode (17) intended for welding, which was subjected to the machining process, by resistance welding.

Description

TECHNICAL AREA

[0001] The present invention relates to a method for manufacturing a spark plug, in particular to a method for manufacturing a spark plug in which a plate is joined to a ground electrode by resistance welding. GENERAL STATE OF THE ART

[0002] To improve spark wear resistance, a spark plug is known in which a precious metal-containing chip is connected to a ground electrode. One method for connecting the chip to the ground electrode is resistance welding. Patent document 1 discloses a technique in which, after roughening a surface of the ground electrode to which the chip is to be connected by grinding, the chip is placed on the surface of the ground electrode and inserted between the electrodes. Current flowing through the electrodes then causes resistance heating, forming a weld nugget through the melting and solidification of the chip and the ground electrode.In patent document 1, by increasing the roughness of an entire contact surface of the ground electrode with the plate, a contact resistance between the plate and the ground electrode is increased, thus increasing a heating degree, then the welding lens is formed. CITATION LIST PATENT DOCUMENTS

[0003] Patent document 1: Publication of the Japanese unexamined patent application No. JP 2003-123 937 A.

[0004] DE 11 2017 006 811 T5 shows a pretreatment of a section of a ground electrode before joining a tip by resistance welding.

[0005] US 2011 / 0148275A1 discloses a spark plug and a method for manufacturing it using resistance welding after partial laser treatment of a joining surface on a ground electrode.

[0006] DE 10 2015 108 551 A1 discloses a spark plug and its manufacture by means of resistance welding with advantageously suppressed scattering. SUMMARY OF THE INVENTIONAL PROBLEM

[0007] Because the current flowing through the electrodes expands or spreads due to a scattering phenomenon as soon as it flows through the connecting elements (plate and ground electrode), the current density increases in the peripheral part of the contact area, where the plate and ground electrode touch, in the conventional technique described above. As a result, the temperature of the connecting elements begins to rise from the edge of the plate, and a melting zone expands from this edge, the connection strength in the center of the plate could be lower than that near the edge.

[0008] To ensure the bond strength in the center of the plate, if the heating level is further increased by an increased current supply between the electrodes, the bonding elements are locally overheated and scattering of the melt is likely, so that variations in the weld nugget diameter and impurities around the plate are likely to occur.

[0009] The present invention was made to solve the aforementioned problem. The subject matter of the present invention is therefore a method for manufacturing the spark plug which is capable of ensuring the bond strength in the center of the disc and simultaneously suppressing the occurrence of scattering. SOLUTION TO THE PROBLEM AND THE EFFECTS OF THE INVENTION

[0010] To achieve the objective, a method for manufacturing a spark plug with a ground electrode to which a bonding surface of a plate is welded comprises: a machining process of machining the ground electrode such that an area located within a border of a part to be welded, where the plate is welded to the ground electrode, and which includes a center of the part to be welded, is rougher than a section excluding the area of ​​the part to be welded; and a joining process of joining the plate to the part of the ground electrode to be welded, which has undergone the machining process, by resistance welding.

[0011] Furthermore, a method for manufacturing a spark plug with a ground electrode to which a bonding surface of a plate is welded comprises: a machining process of machining the plate such that an area located within a border of the bonding surface of the plate and enclosing a center of the bonding surface is rougher than a section excluding the area of ​​the bonding surface; and a joining process of joining the bonding surface of the plate subjected to the machining process to a part intended for welding, wherein the plate is welded onto the ground electrode by resistance welding. EFFECTS OF THE INVENTION

[0012] According to the method for manufacturing the spark plug described in claim 1, the machining process involves machining the area located within the perimeter of the part intended for welding, where the plate is welded onto the ground electrode, and encompassing the center of the part intended for welding, such that it is rougher than the section that does not have this roughened area. Furthermore, the plate is joined to the portion of the ground electrode intended for welding, which has undergone the machining process, by resistance welding. During the joining process, the current density flowing through the roughened area, including the center of the part intended for welding and the plate, can be high compared to that in its surroundings when the initial current is applied.This makes it possible to increase the temperature of the joining elements (the plate and the ground electrode) from the area including the center of the part to be welded and to expand the melting zone without increasing the current flowing through the electrodes. This suppresses the occurrence of scattering and ensures a weld strength in the center of the plate.

[0013] According to the method for manufacturing the spark plug described in claim 2, the machining process roughens the area located within the edge of the plate's contact surface and including the center of the contact surface, making it rougher than the section that does not have this contact surface. Furthermore, the machining process joins the contact surface of the plate to the part intended for welding by resistance welding, where the plate is welded to the ground electrode. During the initial current application in the joining process, the current density flowing through the roughly machined area, including the center of the plate and the ground electrode, can be high compared to that in its surroundings.This makes it possible to increase the temperature of the connecting elements (the plate and the ground electrode) from the area including the center of the plate and to expand the melting zone without increasing the current flowing through the electrodes. This suppresses scattering and ensures a strong connection in the center of the plate.

[0014] According to the method for manufacturing the spark plug mentioned in claim 3, if the area is divided into two parts, namely the first section on the side of one end section and the second section on the side of the other end section, by the plane that runs perpendicular to the axis extending from one end section to the other end section of the ground electrode, one end section of which is connected to the metal housing and the other end section of which is provided with the welded plate, and passes through the center of the part intended for welding or through the center of the joining surface which has been subjected to the joining process, the area of ​​the first section is larger than that of the second section.This makes it possible to increase the temperature of the joining elements (the plate and the ground electrode) of the first section, which is located on the side of one end section of the ground electrode connected to the metal housing, and thus to expand the melting area. Since a cross-section of the weld nugget can be easily ensured on one side of the end section, which is located close to the metal housing and is therefore difficult to increase the temperature there during resistance welding, a joint strength of the first section (on one end section side of the ground electrode) can be ensured in addition to an effect of claim 1 or 2.

[0015] According to the method for manufacturing the spark plug described in claim 4, the first processing step involves roughening the area by irradiating it with a laser beam, and the second processing step involves removing the oxide layer formed in the first step by irradiating the oxide layer with a laser beam of lower power than that used in the first step. Therefore, in addition to the effect described in any one of claims 1 to 3, the oxide layer has little effect on the formation and growth of the weld lens during the joining process.

[0016] According to the method for manufacturing the spark plug described in claim 5, during the joining process at least the part intended for welding on the ground electrode has no plating layer. This prevents the plating layer from melting before the ground electrode melts. Thus, in addition to the effects described in any one of claims 1 to 4, the plating layer has no influence on the formation and growth of the weld nugget. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a one-sided cross-section of a spark plug according to an embodiment of the present invention. Fig. Figure 2 is a cross-section through a metal housing and a ground electrode. Fig. Figure 3 is a perspective view of a plate and the ground electrode after a machining process according to a first embodiment of the present invention. Fig. 4A is a cross-section of the ground electrode in a first step of the machining process. Fig. 4B is a cross-section of the ground electrode in a second step of the machining process. Fig. Figure 5 is a drawing showing a side view of the plate and the ground electrode between the electrodes. Fig. Figure 6 is a perspective view of a plate and the ground electrode after a machining process according to a second embodiment of the present invention. FORMS OF IMPLEMENTATION FOR THE EXECUTION OF THE INVENTION

[0017] Preferred embodiments of the present invention are explained below with reference to the drawings. Fig. Figure 1 is a one-sided cross-section of a spark plug 10 according to an embodiment of the present invention. Fig. In Figure 1, the lower side of the drawing is designated as the front end of the spark plug 10, and the upper side of the drawing is designated as the rear end of the spark plug 10. The spark plug 10 has an insulator 11, a metal housing 16, and a ground electrode 17.

[0018] The insulator 11 is a cylindrical, tubular element made of aluminum oxide, etc., which exhibits excellent mechanical properties and insulating performance at high temperatures. The insulator 11 is provided with an axial hole 12 that penetrates the insulator 11 along an axis O. A central electrode 13 is inserted into the axial hole 12 at a front end.

[0019] The central electrode 13 is a rod-shaped element extending along axis O. A core of the central electrode 13, made of copper or primarily copper, is coated with nickel or a nickel-based alloy. The central electrode 13 is supported by the insulator 11, and its tip is exposed through the axial hole 12. A precious metal-containing plate 14 is connected to the tip of the central electrode 13.

[0020] A metal terminal 15 is a rod-shaped element to which a high-voltage cable (not shown) is attached. The metal terminal 15 is made of a conductive metal material (e.g., low-carbon steel). The metal terminal 15 is attached to a rear end of the insulator 11, with a front end of the metal terminal 15 inserted into the axial hole 12 of the insulator 11. The metal housing 16 is attached to an outer circumference of the insulator 11.

[0021] The metal housing 16 is an essentially cylindrical, tubular element made of conductive metal material (e.g., low-carbon steel). The ground electrode 17 is connected to a front end section of the metal housing 16. The ground electrode 17 is a rod-shaped element made of metal (e.g., a nickel-based alloy) to which a precious metal plate 30 is connected. One end section 18 of the ground electrode 17 is connected to the metal housing 16, and the plate 30 is welded to the other end section 19 of the ground electrode 17. In the present embodiment, the other side of the end section 19 of the ground electrode 17 is bent or curved. A spark gap forms between the plate 30 connected to the other end section 19 of the ground electrode 17 and the center electrode 13 (the plate 14).

[0022] The spark plug 10 is manufactured, for example, according to the following procedure. First, the center electrode 13, which is fitted with the plate 14 previously attached to its tip, is inserted into the axial hole 12 of the insulator 11 and positioned so that the tip of the center electrode 13 is exposed to the outside of the axial hole 12. After the metal terminal 15 is inserted into the axial hole 12 and the electrical connection between the metal terminal 15 and the center electrode 13 is secured, the metal housing 16, to which the ground electrode 17 has previously been connected, is attached to the outer circumference of the insulator 11. After the plate 30 has been connected to the other end section 19 of the ground electrode 17 by resistance welding, the spark plug 10 is formed by bending or curving the ground electrode 17 so that the plate 30 faces the center electrode 13 (the plate 14).

[0023] A method for connecting the plate 30 to the ground electrode 17 is described using the Fig. 2, Fig. 3, Fig. 4 to Fig. 5 explained. First, the ground electrode 17 is connected to the plate 30 using the Fig. 2, Fig. 3 to Fig. 4 explained. Fig. Figure 2 is a cross-section of the metal housing 16 and the ground electrode 17 including the axis O. Fig. Figure 2 shows a state before the metal housing 16 is attached to the insulator 11 and before the ground electrode 17 is bent. Before the plate 30 is connected, the ground electrode 17 is formed into a linear shape along an axis 20 extending from one end section 18 to the other end section 19. The ground electrode 17 in Fig. 2 is shown as a cross-section intersected by a cross-sectional surface with axis 20.

[0024] As in Fig. As shown in Figure 2, a plating layer 21 is formed on a surface of the metal housing 16 to which the ground electrode 17 is connected. The plating layer 21 is a surface treatment layer primarily used to improve the corrosion resistance of the metal housing 16. The plating layer 21 consists mainly of, for example, zinc, chromium-treated zinc, or nickel. The plating layer 21 is formed on the metal housing 16, to which the ground electrode 17 is connected, by drum plating.

[0025] This plating process forms the plating layer 21 not only on the surface of the metal housing 16 but also on a surface of the ground electrode 17. When the plate 30 with the plating layer 21 formed on the surface of the ground electrode 17 is resistance welded, the plating layer 21 affects the weld quality of the plate 30, since the plating layer 21 melts before the ground electrode 17. In the present embodiment, in order to avoid having to consider the melting of the plating layer 21 during resistance welding, the plating layer 21 is removed from at least one part 22 intended for welding, to which the plate 30 is welded. The plating layer 21 is partially removed by a physical removal process such as ion etching and shot peening, or by a chemical removal process such as soaking the ground electrode 17 in a pickling solution.

[0026] Although at least the plating layer 21 of the part 22 of the ground electrode 17 intended for welding must be removed, it is self-evident that the plating layer 21 formed on the same surface as the part 22 intended for welding can be removed extensively, extending almost to the metal housing 16. In this case, it is preferable to remove the plating layer 21 of the ground electrode 17 to a position beyond any portion of the ground electrode 17 that bends towards the center electrode 13. If the ground electrode 17 is bent towards the center electrode 13 with the plating layer 21 forming or remaining on the ground electrode 17, there is a risk that part of the plating layer 21 will detach due to the bending. Then a spark discharge occurs between a part where the plating layer 21 is detaching and the center electrode 13, and the ignition capability may deteriorate.This problem can be suppressed by removing the plating layer 21 from the bending part of the ground electrode 17.

[0027] Fig. Figure 3 is a perspective view of the plate 30 and the ground electrode 17 after a machining process according to a first embodiment. The machining process is a process that increases the surface roughness of a portion of the part 22 intended for welding, to which the plate 30 is to be welded. The part 22 intended for welding is a section against which a joining surface 31 of the plate 30 is pressed against the ground electrode 17 during resistance welding. Therefore, the part 22 intended for welding has the same size and shape as the joining surface 31 of the plate 30. In the present embodiment, the plate 30 has a disc shape, and the part 22 intended for welding is circular.

[0028] In the machining process, the part 22 intended for welding is machined such that a region 26, located within a rim 23 of the part 22 intended for welding and enclosing a center 24 of the part 22 intended for welding, is coarser or rougher than a section 29 excluding the region 26 of the part 22 intended for welding. In the present embodiment, the shape of the region 26 resembles the area of ​​the joining surface 31 of the plate 30. Methods for roughening the region 26 include deformation machining (or plastic machining) using a transfer die, grinding, machining by electrical discharge, blasting, hammering, laser irradiation or electron beam irradiation, and etching. In the present embodiment, a case is described in which the region 26 is roughened by laser beam irradiation, which facilitates micromachining and can be performed in the atmosphere.

[0029] If the area 26 is divided into two parts by a plane 25 that is perpendicular to the axis 20 of the ground electrode 17 and passes through the center 24 of the part 22 intended for welding, one of the two parts is a first section 27 on one side of one end section 18 that is connected to the metal housing 16 (see Fig. 2), and the other part is a second section 28 on one side of the other end section 19 (see Fig. 2) The position of area 26 in relation to the center 24 of the part 22 intended for welding is determined such that an area of ​​the first section 27 is larger than that of the second section 28.

[0030] Fig. 4A is a cross-section of the ground electrode 17 in a first step of the machining process. Fig. 4B is a cross-section of the ground electrode 17 in the second stage of the machining process. In the first stage, in Fig. In the process shown in Figure 4A, the part 22 of the ground electrode 17 intended for welding is irradiated with a laser beam 41 from an irradiation head 40. As the part 22 intended for welding is irradiated with the laser beam 41, it is partially melted and becomes indented or hollowed out, forming a liquid phase. Since the laser beam 41 moves along the part 22 intended for welding, the liquid phase flows (or is fluidized) and solidifies due to the effect of surface tension, forming irregularities (an uneven surface, protrusions, and depressions) along which the depressions continue. This allows for a surface roughness of the area 26 (see Figure 4A). Fig. 3) the part of the part 22 intended for welding, is increased. Depending on the power of the laser beam 41, an oxide layer 42 forms on the uneven surface.

[0031] In the second in Fig. In the process shown in Figure 4B, the oxide layer 42 formed in the first process is melted or sublimated by irradiating the oxide layer 42 with the laser beam 41, which has a lower power than that of the first process. The surface roughness from the first process is retained, and the oxide layer 42 formed on the part 22 intended for welding is removed. Pulsed wave and continuous wave lasers can be used as the laser beam 41.

[0032] The resistance welding of the ground electrode 17 and the plate 30 subjected to the machining process is described by Fig. 5 explained. Fig. Figure 5 is a drawing showing a side view of the plate 30 and the ground electrode 17, which are arranged between the electrodes (a first electrode 43 and a second electrode 44). Fig. Figure 5 shows a part of the ground electrode 17 as a partial cross-section, which was cut by a sectioning surface including the axis 20 (see Fig. 2).

[0033] As in Fig. As shown in Figure 5, the ground electrode 17 is previously connected to the metal housing 16, which is attached to the outer circumference of the insulator 11. In a joining process, the plate 30 is placed onto the part 22 intended for welding (see Figure 5). Fig. 3) the ground electrode 17, and the ground electrode 17 and the plate 30 (hereinafter referred to as connecting elements) are inserted between the first electrode 43 and the second electrode 44. Before applying current, the connecting elements are pressed together by the first electrode 43 and the second electrode 44. After the pressure has stabilized, the current is applied between the first electrode 43 and the second electrode 44. Since current tends to expand or spread out once it is in the connecting elements due to a scattering phenomenon, the current density near the edge 23 of the part 22 intended for welding decreases (see Fig. 3) and a rim 32 of the connecting surface 31 of the plate 30 tends towards.

[0034] However, the ground electrode 17 undergoes the processing process in such a way that the area 26, which is located within the edge 23 of the part 22 intended for welding (see Fig. 3) and includes the center 24 of the part 22 intended for welding, is coarser or rougher than the section 29 outside the area 26 of the part 22 intended for welding. Therefore, the contact resistance of the area 26 may be higher than that of the section 29. As a result, with an initial current application, the current density of the area 26, including the center 24 of the part 22 intended for welding, increases, and the temperature of the area 26 begins to rise. Subsequently, the temperature of the area 26 continues to rise, and the temperature of the section 29 around the area 26 also rises. When the temperatures of the area 26 and the section 29 exceed the melting points of the fasteners, the fasteners begin to melt, and a melting area expands due to and in proportion to an amount of heat applied, i.e., with an increase in current and a duration of current application.When the current application is stopped, the melting area solidifies, and a weld nugget is formed by the melting and solidification of the plate 30 and the ground electrode 17.

[0035] Here, in the joining process, increasing the contact force exerted on the joining elements by the first electrode 43 and the second electrode 44 makes it easier to bring the joining surface 31 of the plate 30 into contact with the minute irregularities formed in the area 26 of the ground electrode 17. Therefore, the contact area between the irregularities of surface 26 and the joining surface 31 is increased, the current density decreases, and consequently, the heating level is reduced. To increase the heating level, it is necessary to increase the amount of current flowing through the first electrode 43 and the second electrode 44. Conversely, decreasing the contact force exerted on the joining elements by the first electrode 43 and the second electrode 44 reduces the contact area between the irregularities of surface 26 and the joining surface 31 and increases the current density of surface 26.Therefore, the heating degree in the center of the connection surface 31 of the plate 30 is higher than in its surroundings.

[0036] During the joining process, by adjusting the contact force exerted on the joining elements by the first electrode 43 and the second electrode 44, the current density flowing through the roughly machined area 26, including the center 24 of the part 22 intended for welding and the plate 30, can be high compared to that in its surroundings during the initial current application. It is therefore possible to increase the temperature of the joining elements (the plate 30 and the ground electrode 17) in the area 26, including the center 24 of the part 22 intended for welding, and thus expand the fusion zone, without increasing the amount of current flowing through the first electrode 43 and the second electrode 44. This suppresses the occurrence of scattering (molten metal dispersion) and ensures a weld strength in the center of the joining surface 31 of the plate 30.

[0037] If the weld strength in the center of the contact surface 31 of the plate 30 is lower than near the edge 32 of the contact surface 31, and the weld nugget is also worn away from the periphery in a situation where the plate 30 has a large surface area and is eroded by ignition sparks, and the cross-section of the weld nugget is insufficient, there is a risk that the plate 30 will detach from the ground electrode 17. In contrast, in the present embodiment, since the weld strength in the center of the contact surface 31 of the plate 30 can be maintained, even if the plate 30 has a large surface area and is eroded by ignition sparks, the cross-section of the weld nugget can be maintained. This prevents the plate 30 from detaching from the ground electrode 17.

[0038] In the machining process, the area 26 in the part 22 intended for welding is formed such that, when divided in two by the plane 25 running perpendicular to the axis 20 of the ground electrode 17 through the center 24 of the part 22 intended for welding, the area of ​​the first section 27 on the side of one end section 18 of the ground electrode 17 connected to the metal housing 16 is larger than that of the second section 28 on the side of the other end section 19. This makes it possible in the joining process to increase the temperature of the joining elements (the plate 30 and the ground electrode 17) from the first part 27 and to expand the melting area.Since a cross-section of the weld nugget on one side of the end section 18, which is close to the metal housing 16 and where it is therefore difficult to increase the heating degree during resistance welding, can be easily secured, a connection strength of the first section 27 (on one side of the end section 18 of the ground electrode 17) can be maintained.

[0039] Since the shapes of the contact surface 31 of the plate 30 and the surface 26 are similar, the weld nugget formed at the ground electrode 17 and the plate 30 (the connecting elements) can easily be shaped to match the shape of the contact surface 31 of the plate 30. This makes it easy to maintain the contact thickness of the plate 30.

[0040] During the processing process, area 26 is roughened by irradiation with laser beam 41 in the first process, and the oxide layer 42 formed in the first process is removed in the second process by irradiation with laser beam 41, which has a lower power than that used in the first process. This allows the oxide layer 42, which influences the contact resistance between the weldable part 22 of the ground electrode 17 and the joining surface 31 of the plate 30, to be maintained in a consistent state throughout the process. Since the surface roughness of the weldable part 22 is preserved in the second process, the contact resistance between the weldable part 22 of the ground electrode 17 and the joining surface 31 of the plate 30 remains stable. This reduces the occurrence of variations in the formation behavior and growth of the weld nugget during the joining process.

[0041] Since the majority of the irregularities (the protrusions and depressions) are formed in area 26, and the size of a depression (a recess) is smaller than the area of ​​the contact surface 31 of the plate 30, the majority of the protrusions contact the plate 30 when the plate 30 is placed on the part 22 intended for welding. When the pressing force is applied to these plates 30 and the part 22 intended for welding by the first electrode 43 and the second electrode 44, the protrusions are elastically or plastically deformed, and a predetermined contact area can be achieved. When the current is passed through this section by the first electrode 43 and the second electrode 44, the current flows concentrated through the contact area. Since the resistance of the contact area is high compared to other parts, the contact area is heated, softened, and crushed, thus creating a new contact area.Since the current can easily flow through the new contact surface, the new contact surface heats up. As the contact surface expands during heating and the melting zone is formed in this way, the occurrence of variations in the shape and growth behavior of the weld nugget during the joining process can be reduced.

[0042] The plating layer 21 is not formed on the part 22 intended for welding on the ground electrode 17. This prevents the plating layer 21 from melting on the part 22 intended for welding before the ground electrode 17 melts. If the plating layer 21 were to melt, the contact area would increase and the current density would decrease, thus reducing the degree of heating. This could cause variations in the formation and growth of the weld nugget. In contrast, in the present embodiment, since the plating layer 21 is not formed on the part 22 intended for welding, it does not influence the shape and growth behavior of the weld nugget.

[0043] Next, a second embodiment will be described using the following methods: Fig. Section 6 explains. In the first embodiment, the case is described in which the surface roughness of a portion of the part 22 of the ground electrode 17 intended for welding is increased. In contrast, the second embodiment describes a case in which the surface roughness of a portion of the joining surface 51 of a plate 50 is increased. The same elements or components as in the first embodiment are designated by the same reference numerals, and their explanation is omitted. Fig. Figure 6 is a perspective view of the plate 50 and the ground electrode 17 after a machining process according to the second embodiment.

[0044] As in Fig. As shown in Figure 6, the plate 50 has a rectangular parallelepiped shape. The plate 50 contains precious metal. A machining process is a method that increases the surface roughness of a portion of the joining surface 51 that is welded to the ground electrode 17 of the plate 50. The joining surface 51 of the plate 50 is pressed against the ground electrode 17 during resistance welding. A weldable portion 58 of the ground electrode 17 is a section against which the joining surface 51 of the plate 50 is pressed against the ground electrode 17 during resistance welding. Therefore, the weldable portion 58 has the same size and shape as the joining surface 51 of the plate 50. In the present embodiment, the joining surface 51 of the plate 50 has a rectangular shape.The plate 50 is placed on the ground electrode 17 such that one longitudinal side of the connection surface 51 is perpendicular to the axis 20 of the ground electrode 17.

[0045] In the machining process, the plate 50 is machined such that a region 54, located within a border 52 of the joining surface 51 and enclosing a center 53 of the joining surface 51, is coarser or rougher than a section 57 outside the region 54 of the joining surface 51. The center 53 of the joining surface 51 is an intersection of diagonal lines connecting the vertices of the joining surface 51. In the present embodiment, the shape of region 54 resembles the joining surface 51 of the plate 50. The method for roughening region 54 is the same as in the first embodiment; therefore, its explanation is omitted here.

[0046] If the area 54 is divided into two parts by a plane 25, which is perpendicular to the axis 20 of the ground electrode 17 and passes through a center 59 of the part 58 intended for welding, one of the two parts is a first section 55 on one side of the one end section 18, which is connected to the metal housing 16 (see Fig. 2), and the other a second section 56 on one side of the other end section 19 (see Fig. 2) A position of the center 59 of the part 58 intended for welding corresponds to a position of the center 53 of the joining surface 51 of the plate 50 after a joining process. In the machining process, a position of the area 54 is determined with respect to the center 53 of the joining surface 51 such that the area of ​​the first section 55 is larger than that of the second section 56.

[0047] In the joining process, the plate 50 is placed on the part 58 of the ground electrode 17 intended for welding, and the ground electrode 17 and the plate 50 (called joining elements) are inserted between the first electrode 43 and the second electrode 44 (see Fig. 5) Before applying current, the connecting elements are pressed together by the first electrode 43 and the second electrode 44. After the pressure has stabilized, the current is applied between the first electrode 43 and the second electrode 44. Since the current tends to expand or spread out due to the scattering phenomenon as soon as it enters the connecting elements, the current density tends to increase near the edge 52 of the connecting surface 51.

[0048] However, the plate 50 is machined such that the area 54, which lies within the edge 52 of the contact surface 51 and includes the center 53 of the contact surface 51, is coarser or rougher than the section 57, which does not include the area 54 of the contact surface 51. Therefore, the contact resistance of the area 54 can be higher than that of the section 57. As a result, when an initial current is applied, the current density of the area 54, including the center 53 of the contact surface 51, increases, and the temperature of the area 54 begins to rise. Subsequently, the temperature of the area 54 continues to rise, and the temperature of the section 57 around the area 54 also rises. When the temperatures of the area 54 and the section 57 exceed the melting points of the connecting elements, the connecting elements begin to melt, and a melting area expands around and in accordance with a quantity of heat applied, i.e.,with an increase in current and application time. When the application is stopped, the molten area solidifies, and a weld nugget forms, resulting from the melting and solidification of the plate 50 and the ground electrode 17.

[0049] As described above, during the joining process, in the initial current application, the current density flowing through the roughly machined area 54, including the center 53 of the joining surface 51 of the plate 50 and the ground electrode 17, can be high. It is therefore possible to increase the temperature of the joining elements (the plate 50 and the ground electrode 17) from the area 54, including the center 53 of the joining surface 51 of the plate 50, and thus expand the melting area, without increasing the amount of current flowing through the first electrode 43 and the second electrode 44. This, as in the first embodiment, suppresses the occurrence of scattering (the spreading of the melt) and ensures a bond strength in the center of the joining surface 51 of the plate 50.

[0050] Although the present invention has been explained on the basis of the embodiments mentioned above, the present invention is not necessarily limited to the embodiments mentioned above, and the present invention may be modified within the scope of the technical ideas of the present invention.

[0051] The embodiments described above illustrate the case where the plate 30 has a cylindrical column shape and the joining surface 31 has a round shape, as well as the case where the plate 50 has a rectangular parallelepiped shape and the joining surface 51 has a rectangular shape. However, the shapes of the plate and the joining surface are not necessarily limited to these shapes but can be arbitrarily defined. In a case where the shapes of the plate and the joining surface are arbitrarily defined, the centers of the joining surface and the weld area coincide with the centroids of the plane figures of the joining surface and the part intended for welding, respectively.

[0052] The above description explains the case where the roughly machined areas 26 and 54, which are part of the weldable portion 22 of the ground electrode 17 and part of the joining surface 51 of the plate 50, are similar to the weldable portion 22 and the joining surface 51, respectively. However, the shapes and sizes of areas 26 and 54 are not limited to these. The shapes and sizes of areas 26 and 54 can be suitably adjusted according to the pressure and current conditions of the specific resistance welding application.

[0053] The above description describes the case where the plating layer 21 formed on the ground electrode 17 is removed before the metal housing 16, to which the ground electrode 17 is connected, is attached to the insulator 11. However, the sequence for removing the plating layer 21 is not necessarily limited to this order. Naturally, the plating layer 21 formed on the ground electrode 17 can also be removed after the metal housing 16, to which the ground electrode 17 is connected, has been attached to the insulator 11.

[0054] The above descriptions explain the case in which the plating layer 21 covering the ground electrode 17 is chemically removed by immersing a front end of the ground electrode 17 in the paint stripping solution (not shown), and the case in which the plating layer 21 covering the ground electrode 17 is removed by physical removal methods such as ion etching and shot peening. However, the method for removing the plating layer 21 is not necessarily limited to these methods. Of course, plating can also be carried out after covering a front end section of the ground electrode 17 with a rubber tube, etc.Since in this case the front end section (including the parts 22 and 58 intended for welding) of the ground electrode 17 does not come into contact with the plating solution through the mask, the plating layer 21 is formed on a part with the exception of at least the parts 22 and 58 intended for welding.

[0055] The above description describes the case where the plating layer 21 on the metal housing 16 and the ground electrode 17 is formed by drum plating of the metal housing 16 to which the ground electrode 17 is connected. However, the sequence for connecting the ground electrode 17 and the plating is not necessarily limited to this order. The plating layer 21 can be formed on the metal housing 16 by rack plating or drum plating before the ground electrode 17 is connected. In this case, the ground electrode 17, without the plating layer 21 being formed, can then be connected to the metal housing 16. Removing the plating layer 21 formed on the ground electrode 17 is not necessary.Of course, the plates 30 and 50 can be connected here to the plating layer 21 formed on the ground electrode 17, although these explanations have been omitted in the above statements.

[0056] The above description describes the case of direct resistance welding, in which plates 30 and 50 are placed on the parts 22 and 58 of the ground electrode 17 intended for welding, respectively, and current is applied between the first electrode 43 and the second electrode 44, with the back sides of the parts 22 and 58 of the ground electrode 17 intended for welding being in contact with the first electrode 43 and plates 30 and 50 being in contact with the second electrode 44. However, resistance welding is not necessarily limited to direct resistance welding.

[0057] For example, indirect resistance welding could be used, in which the plates 30 and 50 are placed on the parts 22 and 58 of the ground electrode 17 intended for welding, respectively, and the current is applied between the first electrode 43 and the second electrode 44, with the plates 30 and 50 in contact with the second electrode 44 and the respective surfaces, which are positioned on the same surfaces of the parts 22 and 58 of the ground electrode 17 intended for welding, in contact with the first electrode 43. It should be noted that the shapes and sizes of the first electrode 43 and the second electrode 44 can be adjusted as appropriate.

[0058] The above descriptions explain the case where the ground electrode 17, which is connected to the metal housing 16, is curved. However, the shape of the ground electrode 17 is not necessarily limited to this form. Naturally, a linear ground electrode can also be used instead of the curved ground electrode 17. In this case, the linear ground electrode is connected to the metal housing 16 and faces the center electrode 13.

[0059] The above illustrations explain the case where the ground electrode 17 is positioned such that the plates 30 and 50 of the center electrode 13 face along axis O. However, the positional relationship between the ground electrode 17 and the center electrode 13 is not necessarily limited to this positional relationship but can be suitably adjusted. For example, the ground electrode 17 could be positioned such that a side face of the center electrode 13 (plate 14) and the plates 30 and 50 face each other. In this case, the parts 22 and 58 intended for welding are located on an end face of the ground electrode 17, and the plates 30 and 50 are welded to the parts 22 and 58, respectively. EXPLANATION OF THE REFERENCE SYMBOLS 10 Spark plug 17 Ground electrode 18 a final section 19 the other end section 20 axle 21 Plating layer 22, 58 part intended for welding 23 Edge of the part to be welded 24, 59 Middle of the part to be welded Level 25 26 area 27 first section 28 second section 29 Section without area 30, 50 tiles 31, 51 Connection surface 41 Laser beam 42 Oxide layer 52 Edge of the connection surface 53 Center of the connection surface 54 area 55 first section 56 second section Section 57 (without area)

Claims

[1] Method for manufacturing a spark plug (10) wherein the spark plug (10) has a ground electrode (17) to which a connecting surface (31) of a plate (30) is welded, the method comprising: a machining process of machining the ground electrode (17) such that an area (26) located within a rim (23) of a part (22) intended for welding, where the plate (30) is welded onto the ground electrode (17), and which includes a center (24) of the part (22) intended for welding, is rougher than a section (29) that does not include the area (26) of the part (22) intended for welding; and a joining process of connecting the plate (30) with the part (22) of the ground electrode (17) intended for welding, which was subjected to the machining process, by resistance welding. [2] Method for manufacturing a spark plug (10) wherein the spark plug (10) has a ground electrode (17) to which a connecting surface (51) of a plate (50) is welded, the method comprising: a machining process of machining the plate (50) such that an area (54) located within a border (52) of the joining surface (51) of the plate (50) and enclosing a center (53) of the joining surface (51) is rougher than a section (57) that does not include the area (54) of the joining surface (51); and a joining process of connecting the joining surface (51) of the plate, which has been subjected to the machining process, (50) with a part (58) intended for welding, where the plate (50) is welded onto the ground electrode (17) by resistance welding. [3] The method for manufacturing the spark plug (10) according to claim 1 or 2, wherein: the spark plug (10) has a metal housing (16), one end section (18) of the ground electrode (17) is connected to the metal housing (16) and the plate (30; 50) is welded to the other end section (19) of the ground electrode (17), and when the area (26; 54) is divided into two parts, namely a first section (27; 55) on one side of one end section (18) and a second section (28; 56) on one side of the other end section (19), by a plane (25) that is perpendicular to an axis (20) extending from one end section (18) to the other end section (19) of the ground electrode (17) and passing through the center (24) of the part (22; 58) intended for welding or the center (53) of the joining surface (51) that has been subjected to the joining process, an area of ​​the first section (27; 55) is larger than that of the second section (28; 56). [4] The method for manufacturing the spark plug (10) according to any one of claims 1 to 3 above, wherein The processing process includes: a first process of roughening the area (26; 54) by irradiating the area (26; 54) with a laser beam (41); and a second process for removing an oxide layer formed in the first process by irradiating the oxide layer (42) with a laser beam (41) which has a lower power than that in the first process. [5] The method for manufacturing the spark plug (10) according to any one of the preceding claims 1 to 4, wherein at least the part (22; 58) intended for welding on the ground electrode (17) does not have a plating layer (21) on the part (22, 58) intended for welding when it is subjected to the joining process.

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