Method for manufacturing electrodes for resistance spot welding and resistance spot welding joints

Abstract

The present invention provides a resistance spot welding electrode and a method for manufacturing a resistance spot welding joint that can obtain a sound joint between a thin plate and a thick plate and between two thick plates, even when there is a gap between the plates, in a plate assembly with a large plate thickness ratio in which a thin plate is superimposed on at least one of two or more superimposed thick plates. [Solution] The electrode for resistance spot welding according to the present invention comprises a first electrode 21 having a structure in which an annular pressurizing member 24 insulated from the electrode body 23 is attached around the tip portion 23a of the electrode body 23 through which the welding current flows, and a second electrode 22, wherein the diameter of the tip portion 22a of the second electrode 22 is larger than the diameter of the tip portion 23a of the electrode body 23 of the first electrode 21, and the tip portion 24a of the annular pressurizing member 24 of the first electrode 22 is positioned at a recess amount g1 of 0 mm or more and 1.0 mm or less from the peripheral edge of the tip portion 23a of the electrode body 23.

Description

Technical Field

[0001] The present invention relates to an electrode for resistance spot welding and a method for manufacturing a resistance spot welded joint. More specifically, the present invention relates to an electrode for resistance spot welding and a method for manufacturing a resistance spot welded joint for welding a workpiece in which a plurality of steel plates are stacked, particularly a plate assembly having a large plate thickness ratio.

Background Art

[0002] Generally, for joining overlapped steel plates, a resistance spot welding method, which is a type of overlap resistance welding method, is used. For example, in the manufacture of automobiles, thousands of spot welds are made per vehicle. This welding method is a method in which two or more steel plates are overlapped, and while sandwiching their surfaces directly with upper and lower electrodes and applying pressure, a high-current welding current is passed between the upper and lower electrodes for a short time to join them. By utilizing the resistance heat generated by flowing a high-current welding current, a dot-shaped welded portion is obtained. This dot-shaped welded portion is called a nugget, and it is a portion where both steel plates are melted and solidified at the contact portion of both steel plates when current is passed through both steel plates, and thereby the two steel plates are joined in a dot shape.

[0003] Moreover, looking at the component structure of automobiles, for example, in the center pillar, a structure in which a reinforcement is sandwiched between an outer and an inner is adopted. In this structure, different from the case of spot welding simply two-layered steel plates, it is required to spot weld three or more steel plates stacked together. [[ID=1十七]]

[0004] Furthermore, recently, with the demand for further improvement in the collision safety of vehicle bodies, the strength and thickness of reinforcements and the like have been increasing. It is often necessary to spot weld a plate assembly in which an outer (thin plate) with a relatively small plate thickness is arranged on the outside, an inner with a relatively large plate thickness on the inside, and a reinforcement (thick plate) is combined between them. Here, among the assembled steel plates, the steel plate with a relatively small plate thickness is described as a thin plate, and the steel plate with a relatively large plate thickness is described as a thick plate, and the same description will be used hereinafter.

[0005] In plate assemblies with a large plate thickness ratio (total plate thickness of the assembly / thickness of the thinnest plate), it is known that if spot welding is performed using conventional methods with constant pressure and welding current, it is difficult to form a nugget of the required size between the outermost thin plate (the side that contacts the electrode tip) and the thick plate, making it difficult to obtain a sound joint. This tendency is particularly strong in plate assemblies with a plate thickness ratio exceeding 5.

[0006] This is because the temperature does not rise easily between the outermost thin plate and the thick plate due to cooling by the electrode tip. The nugget is formed by volume resistivity heating due to the resistivity of the steel plate, starting from near the center between the electrodes. However, as the nugget grows down to the thin plate, the growth of the nugget between the thick plates located near the center between the electrodes is significant, and this cannot be suppressed by the pressure from the electrodes, resulting in scattering. Therefore, it becomes difficult to obtain a nugget of the required size between the thin plate and the thick plate without scattering, and the thin plate is prone to delamination when destructive testing is performed.

[0007] Furthermore, when the outermost thin plate is the outer layer, formability is more important than strength, so mild steel is often used. On the other hand, the thick plates are strength-reinforcing members, and high-tensile steel is often used. In such a plate assembly, the heat generation is biased towards the high-tensile steel plate with higher resistivity, making it even more difficult for nuggets to form between the thick plate and the thin plate (mild steel).

[0008] Against this backdrop, Patent Document 1 proposes a method for spot welding a workpiece consisting of two rigid thick plates with a low-rigidity thin plate on top of it, by sandwiching it between a pair of electrode tips. In this method, the tip diameter of the electrode tip that contacts the thin plate with the lowest rigidity is made smaller than the tip diameter of the electrode tip that contacts the thick plate. This makes the contact area between the thin plate and the electrode tip smaller than the contact area between the thick plate and the electrode, thereby obtaining a nugget between the thin plate and the thick plate.

[0009] Furthermore, Patent Document 2 proposes a technique for obtaining a nugget between a thin metal plate and a thick metal plate by sandwiching a plate assembly, which consists of two or more thick metal plates stacked on top of one of them, with a pair of electrode tips and welding them together by resistance spot welding. In this technique, one of the pair of electrode tips, the electrode tip in contact with the thin metal plate, has a curved tip with a predetermined radius of curvature R1, and the other electrode tip in contact with the thick metal plate has a flat tip or a curved tip with a radius of curvature greater than the radius of curvature R1 of the electrode tip in contact with the thin metal plate. The resistance spot welding is performed in two stages, a first stage and a second stage, with the second stage welding being performed with higher pressure than the first stage welding, and the current value of the second stage welding being less than or equal to the current value of the first stage welding.

[0010] Furthermore, Patent Document 3 proposes a technique for welding plate assemblies with a plate thickness ratio of 5 or more, in which the electrode on the thin plate side is configured such that the electrode tip that conducts current and the insulating pressure member arranged around the electrode tip can each control the pressure independently, and the pressure applied to the plate assemblies from the electrode tip is controlled to be smaller than the pressure applied from the electrode on the opposite side (thick plate side) of the plate assemblies, thereby enabling welding of plate assemblies with a high plate thickness ratio. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2003-251468 [Patent Document 2] Japanese Patent Publication No. 2006-55898 [Patent Document 3] International Publication No. 2019 / 098305 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, in the resistance spot welding method described in Patent Document 1, the contact area between the thin plate and the electrode tip is small, resulting in a narrow range of pressure applied by the electrode. This leads to the problem of spatter occurring when attempting to form a large nugget between two thick plates.

[0013] Furthermore, in the resistance spot welding method described in Patent Document 2, it is possible to form a nugget between a thin plate and a thick plate, but the pressure applied during the first stage of current application is set low. Therefore, if there is a gap between the steel plates, the clamping pressure on the gap may be insufficient, making spattering more likely. Also, the plates may deform significantly due to the gap, and the contact diameter between the thin plate and the thick plate may increase, making it difficult to form a nugget between the thin plate and the thick plate.

[0014] Furthermore, in the resistance spot welding method described in Patent Document 3, at least the electrode on the thin plate side must be double-acting, requiring separate control of the electrode tip and the pressurizing member. This complicates the equipment configuration and significantly increases equipment costs.

[0015] The present invention has been made in view of the above circumstances, and aims to provide a resistance spot welding electrode and a method for manufacturing a resistance spot welding joint that can obtain good joints between thin plates and thick plates and between thick plates, even when there are gaps between plates in a plate assembly with a large plate thickness ratio in which a thin plate is superimposed on at least one of two or more superimposed thick plates. [Means for solving the problem]

[0016] To achieve the above objectives, the inventors diligently investigated various factors affecting nugget formation in resistance spot welding. First, regarding the problem of difficulty in forming a nugget of the required size between the thin and thick plates when resistance spot welding is performed in a plate assembly with a large thickness ratio, where a thin plate is superimposed on one of two or more overlapping thick plates, and there is a gap between the thick plates, the inventors found that to solve this problem, it is effective to apply an electrode to the thin plate side that reduces the contact diameter with the thin plate, i.e., an electrode with a smaller diameter at the electrode tip than conventional electrodes, and to apply an electrode to the corresponding thick plate side that increases the contact diameter between the electrode on the thick plate side and the thick plate, i.e., an electrode with a larger diameter at the electrode tip, thereby making the diameter of the electrode tip on the thick plate side larger than the diameter of the electrode tip on the thin plate side.

[0017] In other words, if electrodes of the same shape (diameter of the electrode tip) are used on both the thin plate and the thick plate side, as in the conventional method, the contact diameter between the thin plate and the electrode will be large, the current density between the thin plate and the thick plate will be low, and it will be difficult to form a nugget. On the other hand, on the thick plate side, the contact diameter between the electrode and the thick plate tends to be small, and consequently, heat generation will increase between the thick plates. As a result, scattering is more likely to occur between the thick plates, and it is thought that forming a nugget between the thin plate and the thick plate will become even more difficult.

[0018] In this regard, if an electrode with a smaller contact diameter with the thin plate (i.e., an electrode with a smaller tip diameter than conventional electrodes) is used as the electrode on the thin plate side, and an electrode with a larger contact diameter with the thick plate (i.e., an electrode with a larger tip diameter) is used as the corresponding electrode on the thick plate side, the contact between the electrode and the thin plate will remain small. As a result, the current density from the electrode on the thin plate side to the space between the thin plate and the thick plate will remain high, making it easier for a nugget to form between the thin plate and the thick plate. On the other hand, on the thick plate side, the contact diameter between the electrode and the thick plate will be larger, resulting in a lower current density on the thick plate side, and also increasing heat dissipation by the electrode. This causes the heat balance to be biased towards the thin plate side, and the formation of the molten area tends to be biased towards the thin plate side.

[0019] However, even when an electrode with a smaller contact diameter with the thin plate (i.e., an electrode with a smaller tip diameter than conventional electrodes) is used as the electrode on the thin plate side, and an electrode with a larger contact diameter with the thick plate (i.e., an electrode with a larger tip diameter) is used as the corresponding electrode on the thick plate side, if there is a gap between the plates, especially between thick plates in a plate assembly with a high plate thickness ratio, the plates deform when the gap is compressed by the electrodes, and the contact diameter between the thin plate and the thick plate expands significantly. As a result, even if the shape of the electrode tip is changed between the thin plate side and the thick plate side, making the tip diameter of the electrode on the thin plate side smaller and the tip diameter of the electrode on the thick plate side larger, the current density between the thin plate and the thick plate remains low, making it difficult to form a molten area. Furthermore, because the tip diameter of the electrode on the thin plate side is smaller, when the diameter of the molten area between the thick plates becomes larger, the pressure force from the electrodes does not easily act on the edge of the molten area, making spattering more likely, and it becomes difficult to secure a large nugget diameter while suppressing spattering.

[0020] Therefore, the inventors conducted further studies and found a method that makes it possible to form nuggets between thin plates and thick plates, and between thick plates, even in plate assemblies with a large plate thickness ratio, regardless of the presence or absence of gaps between plates. Specifically, the inventors use an electrode (first electrode) which has a structure in which an annular pressing member, insulated from the electrode body, is attached around the tip of the electrode body through which the welding current flows, and which has a shape that suppresses deformation of the surrounding plate. By using such an electrode, the flow of electricity at the contact point between the electrode and the thin plate is limited to a narrow area on the electrode tip surface, and a high current density can be maintained in the region extending from the thin plate side electrode to the area between the thin plate and the thick plate. Furthermore, the annular pressing member suppresses deformation around the electrode even when there are gaps between plates, making it possible to suppress the expansion of the current-carrying area between the thin plate and the thick plate, and making it easier to form nuggets between the thin plate and the thick plate.

[0021] In addition, the annular pressure member can apply a pressure force from the electrode over a wide range, and even if a large molten part is formed between thick plates, the occurrence of scattering can be suppressed, and the problems when using an electrode with a small diameter at the tip of the electrode on the thin plate side electrode can be avoided.

[0022] By using the above electrode, it is possible to form a nugget between a thin plate and a thick plate in the welding of a plate assembly with a high plate thickness ratio. Furthermore, by devising the energization pattern, it is possible to more stably form the nugget between the thin plate and the thick plate. Specifically, the resistance spot welding is a two-stage welding consisting of the first stage and the second stage, and the welding in the first stage uses an energization pattern with a short time and a high current compared to the welding in the second stage. By energizing with a short time and a high current in the first-stage energization, it becomes possible to effectively utilize the heat generation in a state where the current density directly under the electrode is high, and a molten part can be formed between the thin plate and the thick plate. Then, in the second-stage welding, by performing welding under conditions where the required nugget diameter between the thick plates can be obtained, the required nugget diameters between the thin plate and the thick plate and between the thick plates can be easily formed. On the other hand, when using a one-stage energization pattern for a long time, a molten part is formed due to the balance between resistance heat generation and heat extraction by the electrode. Therefore, the cooling effect is large between the thin plate and the thick plate located near the electrode. Although it is possible to form a molten part by using the electrode of the present invention in which the diameter of the tip of the electrode is limited to be small and an insulated annular pressure member covers the periphery, it becomes an unfavorable condition for obtaining deep penetration.

[0023] The present invention was conceived based on the above concept and has the following features.

[0024] [1] An electrode for resistance spot welding, A first electrode having a structure in which an annular pressure member insulated from the electrode body is attached around the tip of the electrode body through which the welding current flows, A second electrode, Comprising, The diameter of the tip of the second electrode is larger than the diameter of the tip of the electrode body in the first electrode. A resistance spot welding electrode in which the tip of the annular pressure member in the first electrode is disposed at a position with a recess amount g1 of 0 mm or more and 1.0 mm or less from the peripheral edge of the tip of the electrode body.

[0025] [2] A method for manufacturing a resistance spot welding joint, in which a stack of plates obtained by stacking a steel plate with a smaller thickness on one of two or more steel plates with a larger thickness is sandwiched between a pair of electrodes, and resistance spot welding is performed while applying a pressing force. As the pair of electrodes, A first electrode having a structure in which an annular pressure member insulated from the electrode body is attached around the tip of the electrode body through which a welding current flows, A second electrode, are provided. The diameter of the tip of the second electrode is larger than the diameter of the tip of the electrode body in the first electrode. An electrode in which the tip of the annular pressure member in the first electrode is disposed at a position with a recess amount g1 of 0 mm or more and 1.0 mm or less from the peripheral edge of the tip of the electrode body is used. The first electrode is disposed on the side of the steel plate with a smaller thickness, the second electrode is disposed on the side of the steel plate with a larger thickness, and the stack of plates is sandwiched and pressed between the first electrode and the second electrode, and a welding current is passed between the first electrode and the second electrode to obtain the resistance spot welding joint. A method for manufacturing a resistance spot welding joint.

[0026] [3] The method for manufacturing a resistance spot welding joint according to [2], wherein the spot welding is performed in two stages of a first stage and a second stage, and the welding in the first stage uses a current application pattern with a short time and a high current compared to the welding in the second stage.

[0027] [4] The method for manufacturing a resistance spot welded joint according to [2], wherein the spot welding is a two-stage welding process consisting of a first stage and a second stage, and the first stage welding is a welding process that repeats short-duration, high-current current application and short-duration cooling compared to the second stage welding. [Effects of the Invention]

[0028] According to the present invention, in a plate assembly with a large plate thickness ratio in which a thin plate is superimposed on at least one of two or more superimposed thick plates, a sound joint can be obtained between the thin plate and the thick plate and between the thick plates, even if there is a gap between the plates. [Brief explanation of the drawing]

[0029] [Figure 1] This figure shows an example of an electrode according to the present invention and the arrangement of the electrode and plate assembly. [Figure 2] This is a magnified view of the electrode shown in Figure 1. [Figure 3] This diagram illustrates nugget formation during a single-stage welding process. [Figure 4] This diagram illustrates nugget formation when welding is performed in two stages. [Figure 5] This diagram illustrates the details of the board assembly used in the example. [Modes for carrying out the invention]

[0030] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a diagram showing an example of an electrode according to the present invention and the arrangement of the electrode and plate assembly. Figure 2 is an enlarged view of the electrode shown in Figure 1. The electrode according to the present invention comprises a pair of electrodes, namely a first electrode 21 and a second electrode 22. The first electrode 22 has an electrode body 23 and an annular pressurizing member 24, and has a structure in which the annular pressurizing member 24, which is electrically insulated from the electrode body 23, is attached around the tip portion 23a of the electrode body 23.

[0031] As shown in Figure 1, a plate assembly (workpiece) with a large plate thickness ratio is created by overlapping two steel plates with relatively large thicknesses (thick plates) 12 and 13, with a relatively small thickness (thin plate) 11 placed on top of the upper surface, and further leaving gaps between the plates (a first gap 31 between the thin plate 11 and the thick plate 12, and a second gap 32 between the thick plate 12 and the thick plate 13). This assembly is then resistance spot welded by sandwiching it between a pair of electrodes 21 and 22 and applying pressure. In this process, the electrode 21 that contacts the thin plate 11 is an electrode having a structure in which an annular pressurizing member 24 is attached to the electrode body 23 through which the welding current flows, via an insulator 25, so that the electrode body 23 and the annular pressurizing member 24 are electrically insulated.

[0032] The tip portion 23a of the electrode body 23 has a convex shape with a large radius of curvature to prevent uneven contact, and the diameter (diameter) d1 of the tip portion 23a is smaller than the diameter (diameter) d3 of the tip portion 22a of the electrode 22 on the thick plate side. It is preferable that the diameter d1 of the tip portion 23a be 3 mm or more and 6 mm or less. By setting d1 to 3 mm or more, it is possible to suppress the occurrence of surface scattering due to excessively high current density in the part where the tip portion 23a of the electrode body 23 and the thin plate 11 come into contact. Also, by setting d1 to 6 mm or less, it is possible to obtain the high current density necessary to form a molten area between the thin plate and the thick plate.

[0033] Furthermore, an annular pressurizing member 24 is attached to the periphery of the tip portion 23a of the electrode body 23, with a height not exceeding that of the periphery of the tip portion 23a of the electrode body 21. The amount of recess g1 from the periphery of the tip portion 23a of the electrode body 23 is set to be between 0 mm and 1.0 mm. If the tip portion 24a of the annular pressurizing member 24 protrudes from the tip portion 23a of the electrode body 23 (i.e., the amount of recess g1 is a negative value), it will prevent contact between the electrode body 23 and the thin plate 11. On the other hand, if the amount of recess g1 exceeds 1.0 mm, the suppression of deformation of the thin plate 11 will be insufficient, and the expansion of the contact diameter between the thin plate 11 and the thick plate 12 cannot be suppressed, making it unsuitable for forming a molten portion between the thin plate 11 and the thick plate 12. Therefore, the amount of recess g1 is set to be between 0 mm and 1.0 mm. The above-mentioned recess amount g1 is the amount of recess at the tip portion 24a of the annular pressurizing member 24 closest to the thin plate 11, with reference to the tip portion 23a of the electrode body 23 closest to the thin plate 11 when the electrode 21 is brought into contact with the thin plate 11.

[0034] Regarding the material of the annular pressure member 24, it is assembled to suppress the deformation of the thin plate 11. Therefore, the material is not limited as long as it is electrically insulated from the electrode body 23, and it may be a metal such as chromium copper or steel, or even an engineering plastic. In the case of a non-conductive material, it may be integrally molded with the insulator 25. Furthermore, the annular pressure member 24 can suppress the contact diameter between the electrode 21 and the thin plate 11, and has the effect of promoting the formation of a molten area between the thin plate 11 and the thick plate 12. A larger outer diameter d2 is more effective in suppressing the deformation of the thin plate 11. However, if the outer diameter d2 is too large, welding of the narrow flange portion becomes impossible, so an appropriate size should be selected depending on the part in which it is used.

[0035] Furthermore, the shape of the surface S2 of the annular pressure member 24 does not need to be flat; it may be curved or inclined. Alternatively, the tip portion 23a of the electrode body 23 and the tip portion 24a of the annular pressure member 24 may be processed together using a tip dresser with a tip radius of curvature sufficient to suppress deformation of the thin plate 11.

[0036] On the other hand, the electrode 22 on the thick plate 13 side only needs to have a shape in which the diameter d3 of its tip portion 22a is larger than the diameter of the tip portion 23a of the electrode body 23 of the electrode 21 on the thin plate 11 side. It is preferable that the radius of curvature of its tip portion 22a be large so that the contact diameter with the thick plate 12 during welding is large. Specifically, if the radius of curvature of the tip portion 22a is less than 40 mm, even if the thick plate 13 deforms during welding, the contact area between the thick plate 13 and the tip portion 22a of the electrode 22 will be small, making it difficult to shift the heat balance towards the thin plate 11 side. On the other hand, if the radius of curvature of the tip portion 22a exceeds 200 mm, the contact area is easily limited when the electrode 22 is not in contact with the thick plate 13 perpendicularly. For this reason, it is preferable that the tip portion 22a of the electrode 22 has a radius of curvature of 40 mm or more and 200 mm or less.

[0037] Furthermore, by performing resistance spot welding using the electrodes 21 and 22 according to the present invention as described above, as shown in Figure 3, a nugget 41 spanning between the thin plate 11 and the thick plate 12, and between the thick plate 12 and the thick plate 13 can be formed in good condition, thereby manufacturing a resistance spot welded joint. Even if there is a gap between the steel plates, the deformation of the thin plate 11 is suppressed by the annular pressurizing member 24 attached to the electrode 21 on the thin plate 11 side, the diameter of the tip portion 23a of the electrode body 23 through which the welding current flows is kept small, and the current density between the thin plate 11 and the thick plate 12 is kept high. As a result, even plate assemblies where nugget formation was difficult with conventional resistance spot welding can be welded using a normal single-stage current flow pattern. Moreover, by using the electrodes 21 and 22 according to the present invention, the area of ​​the thin plate 11 in contact with the electrode with high thermal conductivity (for example, a copper electrode) is reduced, which reduces the effect of heat dissipation by the electrode 21, making it easier to obtain deeper penetration in the thin plate 11. Furthermore, because the annular pressurizing member 24 applies pressure over a wide area, scattering can be suppressed even when the nugget diameter between the thick plate 12 and the thick plate 13 becomes larger.

[0038] Furthermore, by using electrodes 21 and 22 with the above configuration and performing more appropriate multi-stage current application, it becomes possible to further stabilize the formation of a nugget between the thin plate 11 and the thick plate 12. For example, by performing spot welding in two stages, a first stage and a second stage, as shown in Figure 4, in the first stage welding, a short-time, high-current weld is performed to form a nugget 42 between the thin plate 11 and the thick plate 12. Then, in the second stage welding, welding is performed for a current application time equivalent to that of normal resistance spot welding to form a nugget 43 of the required diameter between the thick plate 12 and the thick plate 13. As a result, in high-thickness-ratio triple-layer welding, a more stable nugget can be formed between the thin plate 11 and the thick plate 12. When using the electrode 21 according to the present invention, the area in contact between the electrode 21 and the thin plate 11 can be reduced, and the deformation of the thin plate 11 is reduced, which also suppresses the expansion of the contact diameter between the thin plate 11 and the thick plate 12. Therefore, in the first stage of welding, the formation of a molten zone between the thin plate 11 and the thick plate 12 is promoted. By applying a high current for a short time, it becomes possible to more effectively utilize the heat generated between the thin plate 11 and the thick plate 12 to form a molten zone between them before being significantly affected by the cooling effect of the electrode 21. Between the first stage of welding and the second stage of welding, cooling without applying current may be performed, in which case the effect of suppressing spatter generation in the second stage of welding can be obtained.

[0039] Furthermore, the first stage welding using a short-duration, high-current current pattern is effective in forming a large molten area between the thin plate 11 and the thick plate 12, even if the current pattern involves repeating short-duration, high-current current application and short-duration cooling several times. The cooling here is for suppressing spatter generation during the first stage welding and does not necessarily require a state of no current; a current value lower than the current value during short-duration, high-current current application is sufficient.

[0040] Incidentally, the welding apparatus that can be used in the present invention is not particularly limited in terms of the type of pressurizing mechanism (using an air cylinder or a servo motor), the type of model (stationary or robotic gun), or the type of power supply (single-phase AC, AC inverter, DC inverter), as long as it clamps the part to be welded between a pair of upper and lower electrode tips and applies pressure and current.

[0041] Furthermore, this technique can be applied to steel plates of any strength level (mild steel, high-tensile steel) to be welded, and can also be applied to hot-pressed and warm-pressed materials. In addition to bare steel plates, it can also be applied to many types of plated steel plates, such as electro-galvanized steel plates, hot-dip galvanized steel plates, alloyed hot-dip galvanized steel plates, and Al-based plated steel plates. Moreover, it is an effective technique not only for three-layer stacks such as thin plate 11, thick plate 12, and thick plate 13, but also for plate assemblies with a large thickness ratio where the thinnest plate is stacked on the outside of two or more stacked steel plates, and can be applied to plate assemblies of four or more layers such as thin plate-thick plate-thick plate-thick plate, or thin plate-thick plate-thick plate.

[0042] Furthermore, in order to improve the mechanical properties of the welded joint (such as cross tensile strength) and to suppress LME cracking that may occur in plated steel sheets, a currenting pattern in which a nugget of the required diameter is formed in the second stage of welding, followed by additional cooling and current application, can be considered. However, as long as a molten zone is formed between the thin and thick plates of a high plate thickness ratio plate assembly in the first and second stages of welding, this does not depart from the present invention. [Examples]

[0043] To confirm the effects of the present invention, examples and comparative examples were carried out. However, the conditions in the examples are merely examples adopted to confirm the effects of the present invention, and various conditions can be adopted as long as the present invention does not depart from the spirit of the invention and achieves the objective of the present invention.

[0044] Figure 5 shows the plate assembly used in this embodiment. The plate assembly shown in Figure 5 is a three-layer steel plate assembly consisting of a thin plate made of 0.6 mm thick, 270 MPa class GA steel plate, a thick plate made of 1.4 mm thick, 1470 MPa class cold-rolled steel plate, and a thick plate made of 1.6 mm thick, 980 MPa class cold-rolled steel plate. In this plate assembly, a gap was formed between two thick plates by inserting a 1.0 mm thick mild steel spacer with a 50 mm gap, and the spacer portion was clamped. The welding position was set to the center position between the spacers. In Figure 5, electrode A corresponds to the first electrode 21 in Figure 1, and electrode B corresponds to the second electrode 22. Table 1 shows the electrode configuration and the welding results in that case. In electrode A, a 1 mm thick insulator was placed between the tip of the electrode body and the annular pressure member to electrically insulate the electrode body from the annular pressure member. Furthermore, electrode B, which does not have an annular pressurizing member, was a DR-type CrCu electrode with a base diameter of 16 mm. The welding conditions were a single stage energization with a pressing force of 3920 N and an energization time of 0.4 sec. A welding robot with an inverter DC type C gun was used as the welding machine. The welding results were observed by increasing the welding current by 0.5 kA increments, and cutting the welded sample at the center of the weld area at -1.0 kA from the current at which spatter occurred. A nugget diameter of 3.1 mm or more was obtained between the thin plate and the thick plate, and 4.7 mm or more was obtained between the two thick plates. A nugget diameter of 4.7 mm or more was obtained on both sides.

[0045] [Table 1]

[0046] In the electrode configuration conforming to the present invention, the necessary nugget diameter was secured between the thin plate and the thick plate. However, when using a conventional DR-type electrode without an annular pressurizing member, a nugget of the required diameter was not formed between the thin plate and the thick plate. Furthermore, even when an annular pressurizing member is attached, if the diameter of the tip of electrode B (second electrode) is smaller than the diameter of the tip of electrode A (first electrode), it can be seen that a nugget of the required diameter is not obtained between the thin plate and the thick plate. [Examples]

[0047] Similar to Example 1, resistance spot welding was performed on a plate assembly consisting of three steel plates. The plate assembly and electrode configuration were the same as in Example 1a (Table 1a), and the spot welding consisted of two stages, a first stage and a second stage. The first stage welding was performed for a shorter time and with a higher current than the second stage welding, and the effect was verified. The applied pressure throughout the two stages of welding was 3920N. Two current application patterns for the first stage welding, (A) and (B), were also verified. In the first current application pattern (A), the current application time was 0.04 sec, and the welding current value was (X+4)kA, where XkA was the welding current value in the second stage welding, followed by a cooling time of 0.08 sec. The second current application pattern (B) for the first stage welding was a current application pattern that repeated the first current application pattern (A) three times. In both current application patterns (A) and (B), the current application time for the second stage of welding was set to 0.4 seconds.

[0048] After welding, similar to Example 1, the current value (X) for the second stage of welding was increased by 0.5 kA increments. A sample welded at -1.0 kA from the current value at which spatter occurred was cut in the center of the weld and observed in cross-section. The nugget diameter was then confirmed and compared with the test results of Example 1 (a). As a result, the nugget diameter between the thin plate and the thick plate, and between the thick plates, was satisfactory even when two-stage welding was performed. Furthermore, when two-stage welding was performed, nuggets were formed between the thin plate and the thick plate, and between the thick plates. The penetration depth of the nugget between the thin plate and the thick plate into the thin plate side was approximately 0.1 mm in the one-stage welding, while in the two-stage welding, a deeper penetration of 0.2 mm was confirmed for current pattern (A) and 0.25 mm for current pattern (B). In actual automobile welding, various disturbances tend to make nugget formation unstable. By combining the electrode configuration of the present invention, which ensures deep penetration, with a two-stage welding process in which a high current is applied for a short time in the first stage of welding, it is expected that nugget formation can be achieved more stably. [Industrial applicability]

[0049] According to the present invention, in a plate assembly with a large plate thickness ratio in which a thin plate is superimposed on at least one of two or more superimposed thick plates, a sound joint can be obtained between the thin plate and the thick plate and between the thick plates, even if there is a gap between the plates. [Explanation of Symbols]

[0050] 11 thin plate 12,13 Thick plates 21 Thin plate side electrode (first electrode) 22 Thick plate side electrode (second electrode) 23 Electrode body 23a Tip of the electrode body 24 Annular pressurizing member 24a Tip of the annular pressurizing member 25 Insulator

Claims

1. An electrode for resistance spot welding, A first electrode having a structure in which an annular pressurizing member, insulated from the electrode body, is attached around the tip of the electrode body through which the welding current flows, The second electrode and Equipped with, The diameter of the tip of the second electrode is larger than the diameter of the tip of the electrode body of the first electrode. A resistance spot welding electrode in which the tip of the annular pressurizing member in the first electrode is positioned at a recess amount g1 of 0 mm to 1.0 mm from the peripheral edge of the tip of the electrode body.

2. A method for manufacturing a resistance spot welded joint, in which a plate assembly consisting of two or more overlapping steel plates of greater thickness and one of them overlapping a steel plate of less thickness is sandwiched between a pair of electrodes, and resistance spot welding is performed while applying pressure, As the pair of electrodes, A first electrode having a structure in which an annular pressurizing member, insulated from the electrode body, is attached around the tip of the electrode body through which the welding current flows, The second electrode and Equipped with, The diameter of the tip of the second electrode is larger than the diameter of the tip of the electrode body of the first electrode. Using an electrode in which the tip of the annular pressurizing member in the first electrode is positioned at a recess amount g1 of 0 mm or more and 1.0 mm or less from the peripheral edge of the tip of the electrode body, A method for manufacturing a resistance spot welded joint, comprising: placing the first electrode on the side of the steel plate with a smaller thickness and the second electrode on the side of the steel plate with a larger thickness; sandwiching the plate assembly between the first electrode and the second electrode and applying pressure; and passing a welding current between the first electrode and the second electrode to obtain the resistance spot welded joint.

3. The method for manufacturing a resistance spot welded joint according to claim 2, wherein the spot welding is performed in two stages, a first stage and a second stage, and the first stage welding uses a welding pattern that is shorter in duration and uses a higher current than the second stage welding.

4. The method for manufacturing a resistance spot welded joint according to claim 2, wherein the spot welding is a two-stage welding process consisting of a first stage and a second stage, and the first stage welding uses a pattern that repeats short-duration, high-current current application and short-duration cooling compared to the second stage welding.

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