Spot welding method

Abstract

To enable an outermost thin plate in particular and a thick plate contacting the outermost thin plate to be easily and tightly joined to each other, in plate-assembling with a high thickness ratio.SOLUTION: In a spot welding method, which welds an assembled plate 20 constituted of a thin plate 21 and a plurality (here, two) of thick plates 22 and 23 overlapped with one side of the thin plate, by carrying currents between a pair of electrodes 5 and 6 while pressing the assembled plate with the plate sandwiched between the pair of electrodes 5 and 6, currents are carried between the pair of electrodes 5 and 6 while making the electrode 6 which functions as a positive electrode of the pair of electrodes 5 and 6 contact the thin plate 21.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spot welding method.

Background Art

[0002] Spot welding is a method of joining metal plates by sandwiching a stack of two or more metal plates (e.g., steel plates) in the plate thickness direction, applying pressure, passing an electric current between a pair of electrodes (positive and negative pair), melting the contact portion between the metal plates, and then solidifying the molten portion to form a nugget. Spot welding is not only capable of easily and firmly joining metal plates, but also has the feature that it hardly affects the appearance quality. Therefore, it is frequently used, for example, when manufacturing the outer plate (outer panel) of an automobile.

[0003] By the way, for the purpose of weight reduction of automobiles and improvement of collision safety, etc., a stack of plates serving as the outer panel of an automobile often uses a stack of three or more metal plates, specifically, a stack of plates with a large plate thickness ratio (a stack of plates with a high plate thickness ratio) composed of a thin plate and a plurality of thick plates stacked on one side thereof. Here, the "plate thickness ratio" is a value obtained by dividing the total thickness of all the metal plates stacked (total plate thickness) by the thickness of the thinnest plate arranged on the outermost side (= total plate thickness / thickness of the outermost thin plate).

[0004] However, when joining a stack of plates with a high plate thickness ratio as described above by spot welding, the amount of molten metal penetrating into the outermost thin plate becomes insufficient, and it is considered difficult to form an appropriate nugget at the contact portion between the outermost thin plate and the thick plate in contact therewith (to ensure a predetermined joining strength between the two). Therefore, for example, in Patent Document 1 below, in order to enable strong joining of metal plates (particularly, the outermost thin plate and the thick plate in contact therewith) even in a stack of plates with a high plate thickness ratio, it has been proposed to change the pressing force applied to the stack of plates during spot welding and the value of the current flowing between the pair of electrodes in a predetermined manner.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2004-358500 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the spot welding method described in Patent Document 1 requires that the preferred mode of change of the applied pressure to be applied to the plate assembly (a plate assembly with a high plate thickness ratio; the same applies hereafter in this paragraph) and the preferred mode of change of the current value to be flowed between the pair of electrodes be derived each time according to the number of metal plates constituting the plate assembly, the plate thickness, and the material, and then precisely adjusted. For this reason, it is not easy to apply the spot welding method described in Patent Document 1 to situations where spot welding needs to be performed on various plate assemblies with different numbers of metal plates, plate thicknesses, and materials, such as in a manufacturing line for automobile body panels.

[0007] Therefore, the present invention aims to provide a spot welding method that enables easy and strong joining of the outermost thin plate and the thick plate adjacent to it in plate assemblies with a high plate thickness ratio. [Means for solving the problem]

[0008] The inventors, when joining a plate assembly with a high plate thickness ratio consisting of a thin plate and multiple thick plates superimposed on one side by spot welding, focused on the polarity of a pair of positive and negative electrodes that pressurize the plate assembly in the thickness direction and found that the amount of molten metal that penetrates into the thin plate (outermost thin plate) constituting the plate assembly, that is, the joint strength between the thin plate and the thick plate in contact with it, changes depending on the arrangement of the positive and negative electrodes. Then, comparing the case where current is passed between a pair of electrodes with the positive electrode in contact with the thin plate of the plate assembly and the case where current is passed between a pair of electrodes with the negative electrode in contact with the thin plate of the plate assembly, under the same welding conditions, the inventors found that the former can rapidly increase the amount of molten metal that penetrates into the thin plate compared to the latter, leading to the creation of the present invention.

[0009] Based on the above findings, the present invention provides a spot welding method for joining a plate assembly consisting of a thin plate and a plurality of thick plates superimposed on one side thereof by sandwiching it between a pair of electrodes and applying pressure while passing an electric current between the pair of electrodes, characterized in that the positive electrode of the pair of electrodes is in contact with the thin plate while an electric current is passed between the pair of electrodes.

[0010] In this way, as described above, the amount of molten metal that melts into the thin sheet constituting the plate assembly consisting of a thin sheet and multiple thick sheets superimposed on one side of it, i.e., a plate assembly with a high plate thickness ratio, can be rapidly increased, making it easy to obtain a plate assembly with a high plate thickness ratio in which the thin sheet and the thick sheet in contact with it are firmly joined. [Effects of the Invention]

[0011] As described above, the spot welding method according to the present invention makes it possible to easily and strongly join the outermost thin plate and the adjacent thick plate, even in so-called high-thickness ratio plate assemblies. Therefore, it is possible to easily obtain high-thickness ratio plate assemblies with enhanced joint strength between adjacent metal plates. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of a spot welding machine. [Figure 2] Figures (A) to (C) are cross-sectional views showing spot welding of a plate assembly with a high plate thickness ratio. [Figure 3] This is a schematic diagram illustrating the electron flow and molten metal growth direction when the spot welding method according to the present invention is performed. [Figure 4] This is a schematic diagram illustrating the electron flow and molten metal growth direction when a spot welding method different from the present invention is performed. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0014] Figure 1 is a schematic diagram of a spot welding apparatus 1. The spot welding apparatus 1 shown in the figure is used, for example, when spot welding a plate assembly 20 made by overlapping multiple metal plates as shown in Figure 2, and mainly comprises a welding robot 2 and a control device 10.

[0015] The welding robot 2 comprises a robot arm 3 and a welding gun 4 attached to the tip of the robot arm 3. The robot arm 3 is a so-called multi-joint arm, and the robot arm 3 in the illustrated example is a 6-axis arm having a first rotation axis 3a to a sixth rotation axis 3f. With this configuration, the welding gun 4 attached to the tip of the robot arm 3 can be positioned at any three-dimensional position and in any orientation.

[0016] The welding gun 4 mainly comprises a pair of electrodes 5 and 6, a pressurizing mechanism 7, a transformer 8, and a support arm 9. One of the pair of electrodes 5 and 6 (electrode 5, positioned on the upper side in the figure) is attached to the pressurizing mechanism 7 and moves toward and away from the other electrode (electrode 6, positioned on the lower side in the figure), which is fixedly supported by the support arm 9 (movable up and down movement). For this reason, electrodes 5 and 6 will also be referred to as the "movable electrode 5" and the "fixed electrode 6" below. The movable electrode 5 and the fixed electrode 6 are coaxially arranged so that their tips face each other (see also Figure 2), and are constantly cooled by a coolant flowing through them during welding of the plate assembly 20.

[0017] The pressurizing mechanism 7 applies a pressure in the thickness direction to the plate assembly 20 positioned between the two electrodes 5 and 6 by moving the movable electrode 5 up and down. The pressurizing mechanism 7 is preferably one that can precisely adjust the amount of up and down movement of the movable electrode 5, i.e., the pressure to be applied to the plate assembly 20, such as a linear actuator driven by a servo motor.

[0018] The transformer 8 is electrically connected to a power source (not shown) and generates a high current for welding between the movable electrode 5 and the fixed electrode 6. In the present embodiment, the fixed electrode 6 is electrically connected to the positive terminal of the transformer 8 via a support arm 9 formed of a conductive metal such as an aluminum alloy and a conductive member (for example, a flexible conductor also referred to as a shunt) (not shown). The movable electrode 5 is electrically connected to the negative terminal of the transformer 8 via a conductive member such as a shunt (not shown). Therefore, the movable electrode 5 and the fixed electrode 6 of the present embodiment constitute a negative electrode and a positive electrode, respectively [see Fig. 2(A)].

[0019] The control device 10 includes a first control unit 11 and a second control unit 12.

[0020] The first control unit 11 is electrically connected to a motor (not shown) for driving the rotation axis of the robot arm 3, and controls the operation of the motor for driving the rotation axis according to a program taught, for example, by manually operating the robot arm 3. The first control unit 11 is also electrically connected to a servo motor that constitutes the pressing mechanism 7, and controls the operation of the servo motor according to a predetermined pressing pattern (temporal change of the pressing force) determined in advance according to the plate thickness and number of metal plates that constitute the plate assembly 20.

[0021] The second control unit 12 is electrically connected to the transformer 8, and controls the current value flowing between the movable electrode 5 and the fixed electrode 6 according to a predetermined energization pattern (temporal change of the current value to be passed between the two electrodes 5 and 6 during welding of the plate assembly 20) determined in advance according to the plate thickness and material of the metal plates that constitute the plate assembly 20.

[0022] Hereinafter, a spot welding method according to an embodiment of the present invention, which is executed using the spot welding device 1 having the above configuration, will be described.

[0023] First, an example of a plate assembly 20 as a welding target will be explained based on Figure 2(A). The plate assembly 20 shown in the figure consists of a thin plate 21 and multiple thick plates (here, two thick plates 22 and 23) superimposed on one side (upper side in the figure) of the thin plate 21. Both the thin plate 21 and the thick plates 22 and 23 are conductive metal plates such as steel plates, and the thick plates 22 and 23 are thicker than the thin plate 21. For the thin plate 21, for example, a mild steel plate with a tensile strength of 300 MPa or less is used, and for the thick plates 22 and 23, for example, a high-tensile steel plate with a tensile strength of 490 MPa or more, or an ultra-high-tensile steel plate with a tensile strength of 980 MPa or more is used. The plate thickness ratio of the plate assembly 20 (= total plate thickness T of the plate assembly 20 / plate thickness t of the thin plate 21) is at least 4, and may be 7 or more.

[0024] The plate assembly 20 is positioned horizontally with the thin plate 21 on the bottom (the thick plate 23 on the top) between the movable electrode 5 and the fixed electrode 6, and is clamped between the two electrodes 5 and 6. In other words, when the robot arm 3 (or its rotation axis drive motor) and the pressurizing mechanism 7 of the welding gun 4 (or the servo motors that make up the mechanism) are driven based on the operation command output from the first control unit 11 of the control device 10, the plate assembly 20 is clamped in the thickness direction between the movable electrode 5, which acts as a negative electrode with its tip in contact with the thick plate 23, and the fixed electrode 6, which acts as a positive electrode with its tip in contact with the thin plate 21. After the plate assembly 20 is clamped in this way, welding of the plate assembly 20 is performed while changing the pressure applied to the plate assembly 20 and the current value flowing between the two electrodes 5 and 6 according to the pressurizing pattern pre-stored in the first control unit 11 and the current supply pattern pre-stored in the second control unit 12.

[0025] When current is passed between the two electrodes 5 and 6 that are pressurizing the plate assembly 20, electrons flow between the two electrodes 5 and 6. First, heat is generated at the point of high electrical resistance in the plate assembly 20, in this case, at the contact point C between the thick plates 22 and 23. This generates a nugget M (formed when the constituent metals of the thick plates 22 and 23 melt and mix together, and then harden) at the contact point C [see Figure 2(A)]. Subsequently, as current is passed between the two electrodes 5 and 6 that are pressurizing the plate assembly 20 in the thickness direction, the molten metal (nugget M) grows [see Figure 2(B)]. Then, as shown in Figure 2(C), when the grown molten metal melts into the thin plate 21 to a predetermined depth, and a nugget M spanning from the thin plate 21 to the thick plate 23 is generated, the current is stopped between the two electrodes 5 and 6. This results in a plate assembly 20 in which the thin plate 21 and the thick plate 22, and the thick plates 22 and 23 are joined together.

[0026] Incidentally, when current is flowing between the two electrodes 5 and 6, electrons flow from the movable electrode 5, which acts as the negative electrode, to the fixed electrode 6, which acts as the positive electrode, avoiding the molten metal (nugget M) generated in the contact area C and its vicinity, as schematically shown in Figure 3. Therefore, in this embodiment, where the movable electrode 5, which acts as the negative electrode, is in contact with the thick plate 23 and the fixed electrode 6, which acts as the positive electrode, is in contact with the thin plate 21, and current is flowing between the two electrodes 5 and 6, as the molten metal (nugget M) grows, heat generation is induced and promoted on the downstream side in the direction of electron flow, that is, on the thin plate 21 side. As a result, the growth rate of the nugget M increases, making it possible to reach the thin plate 21 and melt the molten metal while the contact force (contact area) of the fixed electrode 6 with respect to the thin plate 21 is small, that is, to secure the amount of molten metal required to melt into the thin plate 21.

[0027] Therefore, when spot welding the plate assembly 20 with a high plate thickness ratio as described above, if the fixed electrode 6, which is the positive electrode of the pair of positive and negative electrodes 5 and 6, is brought into contact with the thin plate (outermost thin plate) 21 and current is passed between the pair of electrodes 5 and 6, the amount of molten metal that penetrates into the thin plate 21 can be rapidly increased. Consequently, a nugget M can be quickly and appropriately formed that firmly joins not only the thick plates 22 and 23 together, but also the thin plate 21 and the thick plate 22.

[0028] In this embodiment, when spot welding the plate assembly 20, the applied pressure (stroke amount of the movable electrode 5) and the current value flowing between the two electrodes 5 and 6 are changed over time. However, according to the present invention, a nugget M in which thin plates 21 to thick plates 23 are firmly joined can be appropriately formed, so it is not necessarily required to precisely control the applied pressure and the current value flowing between the two electrodes 5 and 6 when spot welding the plate assembly 20. Therefore, a plate assembly 20 with a high plate thickness ratio and enhanced joint strength between adjacent metal plates can be easily and quickly obtained.

[0029] For reference, unlike the embodiments of the present invention described above, we will now describe the case in which current is passed between a pair of electrodes 5 and 6 with the movable electrode 5 as the negative electrode in contact with the thin plate 21 of the plate assembly 20 (with the plate assembly 20 inverted), as shown in Figure 4. In this case, it may not be possible to form a predetermined nugget M on the plate assembly 20. This is because, as mentioned above, when current is passed between a pair of electrodes 5 and 6, electrons flow from the negative electrode side to the positive electrode side. Therefore, the direction of electron flow between electrodes 5 and 6 during current application and the direction of growth of the molten metal (nugget M) generated at the contact area between the thick plates 22 and 23 are in opposite directions. In short, in this case, the current application time and pressurization time required for the molten metal to reach the thin plate 21 are longer, which increases the amount of penetration (contact area) of the movable electrode 5 into the thin plate 21, and thus reduces the current density.

[0030] Although one embodiment of the present invention has been described above, the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention.

[0031] For example, in the embodiment described above, the movable electrode 5 connected to the pressurizing mechanism 7 was used as the negative electrode, and the fixed electrode 6 supported by the support arm 9 was used as the positive electrode. However, conversely, the movable electrode 5 connected to the pressurizing mechanism 7 may be used as the positive electrode, and the fixed electrode 6 supported by the support arm 9 may be used as the negative electrode. However, even in this case, when spot welding the plate assembly 20 with a high plate thickness ratio, it is important to apply current between the pair of electrodes 5 and 6 with the movable electrode 5, which is the positive electrode, in contact with the thin plate 21.

[0032] Although not shown in the diagram, the spot welding apparatus 1 may also be equipped with a switching mechanism that allows arbitrary selection of the output terminals (positive and negative terminals) of the transformer 8 to be electrically connected to the movable electrode 5 and the fixed electrode 6, respectively (in other words, the polarity of the movable electrode 5 and the fixed electrode 6 can be arbitrarily changed). The switching mechanism may be electrical or mechanical, and can be provided, for example, between the transformer 8 and the shunt mentioned above. By providing such a switching mechanism, the polarity of both electrodes 5 and 6 can be easily and quickly changed without having to provide the spot welding apparatus 1 with a large-scale mechanism, such as one that allows the welding gun 4 to be inverted, thus dramatically increasing the versatility of the spot welding apparatus 1. [Examples]

[0033] To demonstrate the usefulness of the present invention, when spot welding a high-thickness ratio plate assembly consisting of a thin plate and two thick plates (first and second thick plates) superimposed on one side of it (details of each metal plate will be described later), we confirmed the difference in welding quality when applying the method according to the present invention, in which the positive electrode is in contact with the outermost thin plate and current is passed through a pair of electrodes (see Figure 3), and when applying a different method from the present invention, in which the negative electrode is in contact with the outermost thin plate and current is passed through a pair of electrodes (see Figure 4), under the same welding conditions. In this confirmation test, (1) Of the nuggets produced by spot welding the above plate assembly, the maximum diameter [unit: mm] of the portion formed on the outermost thin plate, and (2) The penetration rate of the molten metal into the outermost thin sheet σ [unit: %] I confirmed it. The "penetration rate σ" mentioned above is the percentage obtained by dividing the penetration depth of the molten metal into the outermost thin sheet (the thickness of the portion of the nugget formed on the outermost thin sheet) d by the thickness t of the outermost thin sheet [σ = (d / t) × 100)].

[0034] The details of the board assembly used in this verification test are as follows. • Thin sheet: Cold-rolled steel sheet with a thickness of 0.55 mm (tensile strength 340 MPa). • First thick plate: Cold-rolled steel sheet with a thickness of 1.6 mm (tensile strength 980 MPa). • Second thick plate: Cold-rolled steel sheet with a thickness of 2.0 mm (tensile strength 980 MPa). Plate thickness ratio: 7.54 (=4.15 / 0.55)

[0035] Table 1 shows the results of the verification tests. In Table 1, the verification items (1) and (2) above are indicated as "nugget diameter of the outermost thin plate" and "penetration rate of the outermost thin plate," respectively. "Example 1" in Table 1 refers to the result when the above plate assembly is spot-welded using the method according to the present invention with a predetermined energizing pattern and pressure pattern (Figure 3), and "Comparative Example 1" refers to the result when the above plate assembly is spot-welded using a method different from the present invention (Figure 4), with the same energizing pattern and pressure pattern as Example 1. "Example 2" refers to the result when the above plate assembly is spot-welded using the method according to the present invention, with a different energizing pattern and pressure pattern than Example 1 (and Comparative Example 1), and "Comparative Example 2" refers to the result when the above plate assembly is spot-welded using a method different from the present invention, with the same energizing pattern and pressure pattern as Example 2.

[0036] [Table 1]

[0037] As is clear from Table 1, the spot welding method according to the present invention makes it possible to simultaneously increase the amount of molten metal that penetrates into the outermost thin plate constituting a plate assembly with a high plate thickness ratio, and to enlarge the nugget diameter in the outermost thin plate. Therefore, it becomes possible to easily obtain a plate assembly with a high plate thickness ratio in which the joint strength between the thin plate and the thick plate in contact with it is particularly enhanced. [Explanation of Symbols]

[0038] 1 Spot welding machine 2. Welding robots 3. Robot Arm 4. Welding gun 5 electrodes (movable side electrode) 6 electrodes (fixed side electrode) 7. Pressurization mechanism 8 transformers 10 Control device 20 board assembly 21 thin plate 22,23 Thick plates t (thin sheet) thickness M Nuggets

Claims

[Claim 1] In a spot welding method in which a plate assembly consisting of a thin plate and multiple thick plates superimposed on one side is joined by sandwiching it between a pair of electrodes and applying pressure while passing an electric current between the pair of electrodes, The system includes a transformer that generates a high welding current in a predetermined pattern between the pair of electrodes, a control device having a first control unit and a second control unit, and a switching mechanism, the switching mechanism allowing arbitrary selection of the positive and negative terminals of the transformer to be electrically connected to each of the pair of electrodes. A spot welding method characterized in that, when current is passed between the pair of electrodes with the positive electrode of the pair in contact with the thin plate, the pressure applied to the plate assembly via the pair of electrodes is changed over time according to a pressure pattern pre-stored in the first control unit according to the configuration of the plate assembly, and the current value flowing between the pair of electrodes is changed over time according to a current pattern pre-stored in the second control unit according to the configuration of the plate assembly.

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