JP7410396B2 - Spot welding joint manufacturing method, spot welding equipment and program - Google Patents

Description

The present invention relates to a method of manufacturing a spot welded joint, a spot welding device, and a program.

For example, in the body of a car, a plate assembly is formed by stacking three steel plates, consisting of a thin plate and two thick plates, with the thin plate positioned at the outermost position, and this plate assembly is sandwiched between a pair of electrodes. spot welded joints are applied.

In the conventional spot welding process for such spot welded joints, melting starts at the center of the total plate thickness of the plate set, which is the location farthest from the pair of water-cooled electrodes. Therefore, depending on the total plate thickness of the plate set and the strength of the steel plate, the boundary between the thin plate and the thick plate, which is far from the center of the total plate thickness and close to one of the water-cooled electrodes, is melted. It is generally considered difficult to form a nugget in the area.

Therefore, in order to form a nugget at the boundary between a thin plate and a thick plate, the following first technique and second technique have been proposed.

That is, in the first technique (for example, see Patent Document 1), a plate assembly is sandwiched between a first electrode and a second electrode, and an application is applied to a thin plate located at the outermost part of the plate assembly and contacted by the second electrode. Electricity is applied between the first electrode and the second electrode while the pressure member is brought into contact with the plate assembly and the plate assembly is pressed from the thin plate side by the pressure member.

In this first technique, the total pressing force of the second electrode and the pressing member is balanced with the pressing force of the first electrode, so the pressing force of the second electrode is smaller than the pressing force of the first electrode. . Therefore, the force that acts on the contact surface between the thin plate that the second electrode is in contact with and the thick plate that overlaps the thin plate is smaller than the force that acts on the contact surface between the remaining thick plates. The contact resistance between the plates becomes larger than the contact resistance between the remaining thick plates. This allows a nugget to be formed at the boundary between the thin plate and the thick plate.

In addition, in the second technique (for example, see Patent Document 2), the pressing force of the second electrode that contacts the thin plate is made smaller than the pressing force of the first electrode that contacts the thick plate on the opposite side of the thin plate. , energizes between the first electrode and the second electrode. In this second technique, the contact resistance between the thin plate and the thick plate is greater than the contact resistance between the remaining plates, so that a nugget can be formed at the interface between the thin plate and the thick plate. It is said that

JP2012-66284A Japanese Patent Application Publication No. 2003-251469

However, in the first technique described above, since the thin plate is pressed by the pressure member together with the second electrode, the pressure member is necessary. In addition, in the second technique described above, the pressing force of the first electrode in contact with the thick plate on the opposite side of the thin plate is greater than the pressing force of the second electrode in contact with the thin plate, and the thickness of the first electrode in contact with the first electrode is Since the plate assembly is deformed so that the plate has a concave shape, the shape of the plate assembly changes before and after welding.

The present invention has been made in view of the above-mentioned problems, and eliminates the need for a pressure member, and avoids changes in the shape of the plate assembly before and after welding, while also providing space between a plurality of metal plates constituting the plate assembly. An object of the present invention is to provide a method for manufacturing a spot welded joint, a spot welding device, and a program that can form a nugget at the boundary between the two.

A first aspect of the present invention includes a plurality of first metal plates and a second metal plate thinner than each of the plurality of first metal plates, and the second metal plate is located at the outermost position. Manufacturing a spot welded joint by spot welding a plate set in which the plurality of first metal plates and second metal plates are overlapped with each other by sandwiching them between opposing first and second electrodes to obtain a spot welded joint. In the method, the first electrode is placed on the opposite side to the second metal plate, and the second electrode is placed on the side of the second metal plate, and the first electrode is pressed against the plate set, a first step of deforming the plate assembly so that the side of the second metal plate has a convex shape; and a step of deforming the plate assembly with the first electrode and the second electrode sandwiching the plate assembly; a second step of applying current between two electrodes to form a nugget at a boundary between a first metal plate overlapping the second metal plate of the plurality of first metal plates and the second metal plate; With the plate set sandwiched between the first electrode and the second electrode, electricity is applied between the first electrode and the second electrode so that the plate set is softened, and the second electrode is connected to the plate set. a third step of deforming the board assembly so that the convex shape returns to its original shape; and a second step of deforming the board assembly so that the convex shape returns to its original shape; a fourth step of passing current between the first electrode and the second electrode at a current value larger than the current value at the time of the spot welding joint, and forming a nugget at the boundary between the plurality of first metal plates. This is a manufacturing method.

A second aspect of the present invention includes a plurality of first metal plates and a second metal plate thinner than each of the plurality of first metal plates, and the second metal plate is located at the outermost position. A spot welding device for spot welding a plate set in which a plurality of first metal plates and a plurality of second metal plates are overlapped, the first electrode and the second electrode facing each other, the first electrode and the second metal plate facing each other. a drive section that moves the first electrode and the second electrode in a direction in which the two electrodes face each other; a power supply section that supplies electricity between the first electrode and the second electrode; and a power supply section that includes the drive section and the power supply section. a control unit for controlling, the control unit including the first electrode disposed on the opposite side to the second metal plate, and the second electrode disposed on the side of the second metal plate, a first control unit that controls the drive unit to press the first electrode against the plate assembly and deform the plate assembly so that the second metal plate side has a convex shape; and the first electrode and a first metal that overlaps the second metal plate among the plurality of first metal plates by applying electricity between the first electrode and the second electrode with the plate set sandwiched between the second electrodes. a second control unit that controls the power supply unit to form a nugget at the boundary between the plate and the second metal plate; and the plate assembly is sandwiched between the first electrode and the second electrode. In this state, the power supply section is controlled to supply electricity between the first electrode and the second electrode so that the plate assembly is softened, and the second electrode is pressed against the plate assembly to form the convex shape. a third control section that controls the driving section to deform the board assembly so that the plate assembly returns to its original shape; The control is performed such that a current is passed between the first electrode and the second electrode at a current value larger than the current value during the control by the second control unit, and a nugget is formed at the boundary between the plurality of first metal plates. The spot welding apparatus includes a fourth control section that controls the power supply section.

A third aspect of the present invention includes a first electrode and a second electrode facing each other, a drive unit that moves the first electrode and the second electrode in a direction in which the first electrode and the second electrode face each other, and A power supply section that supplies current between the first electrode and the second electrode, and a control section that controls the drive section and the power supply section, and includes a plurality of first metal plates and a plurality of first metal plates. a second metal plate having a thickness thinner than each of the plurality of first metal plates and the second metal plate, the plurality of first metal plates and the second metal plates being stacked on top of each other such that the second metal plate is located at the outermost position, A program applied to a spot welding device that performs spot welding by being sandwiched between the first electrode and the second electrode, wherein the first electrode is placed on the opposite side from the second metal plate in the computer as the control unit. and, with the second electrode disposed on the second metal plate side, press the first electrode against the plate assembly so that the second metal plate side has a convex shape. A first step of controlling the drive unit to deform it, and applying electricity between the first electrode and the second electrode in a state where the plate assembly is sandwiched between the first electrode and the second electrode. , a second step of controlling the power supply unit to form a nugget at a boundary between a first metal plate that overlaps the second metal plate of the plurality of first metal plates and the second metal plate; and controlling the power source unit to conduct electricity between the first electrode and the second electrode so that the plate assembly is softened when the plate assembly is sandwiched between the first electrode and the second electrode. a third step of controlling the drive unit to press the second electrode against the plate assembly and deform the plate assembly so that the convex shape returns to its original shape; With the plate assembly sandwiched between the electrodes and the second electrode, current is passed between the first electrode and the second electrode at a current value larger than the current value at the time of the second step, and the plurality of This is a program for executing a fourth step of controlling the power supply unit to form a nugget at the boundary between the first metal plates.

According to the present invention, a pressure member is not required, and a nugget can be formed at the boundary between a plurality of metal plates constituting a plate assembly while avoiding changes in the shape of the plate assembly before and after welding.

1 is a diagram showing the configuration of a spot welding device according to an embodiment of the present invention. FIG. 2 is a block diagram showing the configuration of a control section in FIG. 1. FIG. 2 is a flowchart showing the flow of processing by a control unit in FIG. 1. FIG. It is a figure explaining the manufacturing method of the spot welding joint concerning one embodiment of the present invention. It is a figure explaining the modification of the manufacturing method of the spot welding joint concerning one embodiment of the present invention.

An embodiment of the present invention will be described below.

FIG. 1 is a diagram showing the configuration of a spot welding apparatus 10 according to an embodiment of the present invention. This spot welding apparatus 10 is an apparatus that spot-welds the plate assembly 70 by sandwiching the plate assembly 70 between a first electrode 31 and a second electrode 32 facing each other to obtain a spot welded joint 80.

The plate assembly 70 to be welded includes, for example, two thick plates 71 and 72 and a thin plate 73 thinner than each of the two thick plates 71 and 72. As an example, the two thick plates 71 and 72 have the same thickness, but may have different thicknesses. In this plate set 70, two thick plates 71 and 72 and a thin plate 73 are stacked on top of each other such that the thin plate 73 is located at the outermost position. The thick plates 71 and 72 are an example of a "first metal plate," and the thin plate 73 is an example of a "second metal plate." The thickness of the metal plate targeted by the present invention is in the range of about 0.4 mm to 4.5 mm. The thick plate as used in the present invention does not refer to a so-called thick steel plate having a thickness of 6.0 mm or more, but refers to a relatively thick plate in a set of plates.

The two thick plates 71 and 72 and the thin plate 73 are, for example, steel plates. Examples of the steel plates used in the plate set 70 include electroplated steel plates, hot-dip plated steel plates, and alloyed hot-dip plated steel plates.

Note that the plate assembly 70 may be a laminate in which two thick plates 71 and 72 and a thin plate 73 are stacked with other steel plates. Moreover, the metal plate used for the plate set 70 may be other than steel plate as long as it can conduct electricity. The type, form, mechanical properties, etc. of the material of the metal plate are not particularly limited.

Further, the plate thickness ratio of the plate set 70 is not particularly limited, but is preferably 4.0 or more, for example. The plate thickness ratio is the ratio of the total plate thickness of the plate set 70 to the thickness of the thinnest metal plate among the plurality of metal plates constituting the plate set 70.

Although the board set 70 is not limited to being arranged horizontally, in this embodiment, a case where the board set 70 is arranged horizontally will be described as an example. This plate assembly 70 is restrained by a plurality of restraint parts 90. The plurality of restraint parts 90 include a pair of upper restraint parts 90 and a pair of lower restraint parts 90. The upper pair of restraint parts 90 are arranged side by side in the horizontal direction. Similarly, the pair of lower restraining parts 90 are arranged side by side in the horizontal direction.

Note that the plate assembly 70 may be a panel component of a size that requires restraint by a plurality of restraint parts 90, but may also be formed as a part of a large panel component such as a body panel component for an automobile. When the plate assembly 70 is formed in a part of a large panel component in this way, the entire panel component is restrained without having to restrict the periphery of the panel assembly 70 with a plurality of restraining parts 90. The board assembly 70 can be restrained in the same restraint state as when the plurality of restraint parts 90 are restrained.

As shown in FIG. 1, a spot welding device 10 according to an embodiment of the present invention includes a welding gun 11, a welding robot 12, a power supply section 13, and a control section 14.

The welding gun 11 includes a gun body 21 , an electrode drive section 22 , a first electrode 31 , and a second electrode 32 . The gun body 21 is formed in a C-shape when viewed from the side. The electrode drive unit 22 is, for example, a servo motor mechanism, and is fixed to the upper part of the gun body 21. The first electrode 31 is a movable electrode, and is connected to the movable part of the electrode drive section 22. The second electrode 32 is a fixed electrode and is fixed to the lower part of the gun body 21.

The first electrode 31 and the second electrode 32 are opposed to each other in the vertical direction of the welding gun 11. The electrode drive unit 22 is configured to move the first electrode 31 up and down in the direction in which the first electrode 31 and the second electrode 32 face each other, that is, in the up and down direction of the welding gun 11. As the first electrode 31 moves up and down, the first electrode 31 moves in the vertical direction of the welding gun 11 relative to the fixed second electrode 32. The first electrode 31 is arranged between the upper pair of restraint parts 90, and the second electrode 32 is arranged between the lower pair of restraint parts 90.

The welding robot 12 is, for example, a six-axis vertical articulated robot. This welding robot 12 includes a robot arm 24 having a plurality of joints 23 and a plurality of joint drive units 25 that drive each of the plurality of joints 23. Be prepared. The electrode drive section 22 and the plurality of joint drive sections 25 are examples of "drive sections."

A welding gun 11 is fixed to the tip of the robot arm 24. By driving each of the plurality of joints 23 by the plurality of joint drive units 25, the welding robot 12 can freely move the welding gun 11. This movement of the welding robot 12 allows the entire welding gun 11, including the first electrode 31 and the second electrode 32, to move up and down, as will be described later.

The power supply unit 13 is, for example, a power supply circuit, and is configured to conduct electricity between the first electrode 31 and the second electrode 32. The control unit 14 is, for example, a computer, and is configured to control the electrode drive unit 22, the plurality of joint drive units 25, and the power supply unit 13.

FIG. 2 is a block diagram showing the configuration of the control section 14 in FIG. 1. As shown in FIG. 2, the control unit 14 includes a processor 41 and a memory 42 as a hardware configuration. The processor 41 is configured by, for example, a CPU (Central Processing Unit), and the memory 42 is configured by, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), and a storage.

The memory 42 has a non-volatile storage section, and this non-volatile storage section stores a program 43 for executing a method for manufacturing a spot welded joint, which will be described later.

Further, the control unit 14 includes a first control unit 51, a second control unit 52, a third control unit 53, a fourth control unit 54, and a fifth control unit 55 as functional configurations. The first control section 51, the second control section 52, the third control section 53, the fourth control section 54, and the fifth control section 55 are realized by the processor 41 executing the program 43. . The functions of the first control section 51 and the like will be explained together with a method for manufacturing a spot welded joint, which will be described later.

Next, a method for manufacturing a spot welded joint according to an embodiment of the present invention will be described.

FIG. 3 is a flowchart showing the flow of processing by the control unit 14 in FIG. Moreover, FIG. 4 is a figure explaining the manufacturing method of the spot weld joint based on one Embodiment of this invention. In FIG. 4, (A) is a diagram explaining the operation of the first electrode 31 and the second electrode 32, and (B) is a diagram illustrating the pressurizing force of the first electrode 31 and the second electrode 32, the first electrode 31 and the second electrode 32, It is a graph which shows the relationship between the current value passed between and time.

A method for manufacturing a spot welded joint according to an embodiment of the present invention involves spot welding the plate set 70 in a restrained state as described above by sandwiching it between the first electrode 31 and the second electrode 32 facing each other. This is a method of obtaining the joint 80. This method of manufacturing a spot welded joint is carried out using the spot welding apparatus 10 described above. In the following description, reference will be made to FIGS. 1 and 2 for the configuration of the spot welding apparatus 10.

In the method for manufacturing a spot welded joint according to an embodiment of the present invention, for example, the first electrode 31 is placed on the side of the thick plate 71 (the side opposite to the thin plate 73) and brought into contact with the thick plate 71, and The initial state is such that the two electrodes 32 are arranged on the side of the thin plate 73 and a clearance 92 is provided between the second electrode 32 and the thin plate 73.

The method for manufacturing this spot welded joint is started from the above-mentioned initial state. This method for manufacturing a spot welded joint includes a first step, a second step, a third step, a fourth step, and a fifth step, as explained in order below.

(first step)
The first step is to deform the plate set 70 so that the side of the thin plate 73 has a convex shape. In this first step, the first control section 51 controls the electrode drive section 22 and the plurality of joint drive sections 25. At this time, the first electrode 31 is lowered and the welding gun 11 is also lowered, thereby pressing the first electrode 31 against the plate assembly 70 and controlling the plate assembly 70 to deform the plate assembly 70 so that the thin plate 73 side becomes convex. is performed for the electrode drive section 22 and the plurality of joint drive sections 25.

At this time, the first electrode 31 is pressed against the plate assembly 70, and the plate assembly 70 is deformed so that the side of the thin plate 73 becomes convex, so that the second electrode 32 comes into contact with the thin plate 73, and the plate assembly 70 The state is sandwiched between the first electrode 31 and the second electrode 32. Then, pressurization of the plate assembly 70 is started by the first electrode 31 and the second electrode 32. The pressing force applied to the plate assembly 70 is kept substantially constant until the fifth step, which will be described later, is completed.

In this way, when the first electrode 31 is pressed against the plate assembly 70 and the plate assembly 70 is deformed so that the thin plate 73 side becomes convex, the contact area between the thick plate 71 and the first electrode 31 is This is larger than the contact area with the second electrode 32.

In this first step, when the contact area between the thick plate 71 and the first electrode 31 is S1, and the contact area between the thin plate 73 and the second electrode 32 is S2, the relationship between the contact area S1 and the contact area S2 is determined. However, as an example, the board set 70 is deformed so as to satisfy the equation (1). The reason for this will be described later.
0.1<S2/S1<0.9...(1)

In the example shown in FIG. 4, the initial state is such that the first electrode 31 is in contact with the thick plate 71 and a clearance 92 is provided between the second electrode 32 and the thin plate 73. The initial state may be a state in which the plate assembly 70 is sandwiched between the electrode 31 and the second electrode 32 (pressurized state). In the first step, the welding gun 11 is lowered by the plurality of joint drive units 25 while the electrode drive unit 22 is stopped, and the first electrode 31 is pressed against the plate set 70, so that the side of the thin plate 73 becomes convex. The plate assembly 70 may be modified so that

In addition, in the first step, when deforming the plate assembly 70 so that the side of the thin plate 73 has a convex shape, the first electrode 31 and the plate assembly 70 are sandwiched between the first electrode 31 and the second electrode 32. Current may be passed between the second electrodes 32 at a current value I1. In this case, the current value I1 that is passed between the first electrode 31 and the second electrode 32 is set so that the plate assembly 70 is not welded but softened.

In this way, the plate assembly 70 can be softened when deforming the plate assembly 70, so that the plate assembly 70 can be easily deformed. In this case, the plate assembly 70 may be deformed while softening the plate assembly 70 by passing current between the first electrode 31 and the second electrode 32, or the plate assembly 70 may be deformed by passing electricity between the first electrode 31 and the second electrode 32. The plate assembly 70 may be deformed after being energized to soften the plate assembly 70. Further, preferably, electricity is passed between the first electrode 31 and the second electrode 32 to soften the plate assembly 70, but the electricity supply for softening the plate assembly 70 may be omitted.

Furthermore, in the above example, the plurality of joints 23 are driven by the plurality of joint drive units 25 to lower the welding gun 11, but as shown in FIG. In the case where the initial state is a state in which . That is, in the first step, while the plurality of joint drive units 25 are stopped, the first electrode 31 is lowered by the electrode drive unit 22 and pressed against the plate set 70, and the plate is moved so that the thin plate 73 side has a convex shape. The set 70 may also be modified.

(Second step)
The second step is to form a nugget 74 at the boundary between the thick plate 72 and the thin plate 73. In this second step, the second control unit 52 controls the first electrode 31 and the second electrode 32 so that electricity is supplied between the first electrode 31 and the second electrode 32 while the plate assembly 70 is sandwiched between the first electrode 31 and the second electrode 32. Control over the power supply unit 13 is performed by.

Here, when the current value I2 flowing between the first electrode 31 and the second electrode 32 is large, melting starts from the center of the total plate thickness of the plate assembly 70. On the other hand, if the current value I2 is small, the contact area between the thin plate 73 and the second electrode 32 is smaller than the contact area between the thick plate 71 and the first electrode 31 in the second step, so the current density on the thin plate 73 side becomes higher than the current density on the thick plate 71 side, and the nugget 74 is preferentially formed at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72. .

In this second step, the current value I2 is set such that the nugget 74 is preferentially formed at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72. Set. This current value I2 is set to a value smaller than a current value I4 that forms a nugget 75 at the boundary between the thick plates 71 and 72 in a fourth step to be described later.

In this second step, it is sufficient that a nugget 74 is formed at the boundary between the thick plate 72 and the thin plate 73, and a nugget may be formed at the boundary between the thick plate 71 and the thick plate 72. Either one is fine.

In this second step, when a nugget is formed at the boundary between the thick plate 71 and the thick plate 72, the nugget at the boundary between the thick plate 71 and the thick plate 72 is The current value I2 is set so that the nugget diameter is smaller than the nugget diameter at the boundary between the thin plate 73 and the nugget 74. The nugget diameter in this case is the maximum value of the diameter measured along the direction parallel to each boundary.

In the example shown in FIG. 4, as an example, in the second step, no nugget is formed at the boundary between the thick plate 71 and the thick plate 72, but at the boundary between the thick plate 72 and the thin plate 73. Only a nugget 74 is formed. When the thickness of the thin plate 73, which is the thinner one of the thick plate 72 and the thin plate 73, is t (mm), in order to ensure the bonding strength between the thick plate 72 and the thin plate 73, , it is desirable to form a nugget 74 having a nugget diameter of 4√t to 5√t.

In this second step, the current value I2 is set within an appropriate current range. The appropriate current range is the current value _{I4√t that forms a nugget with a nugget diameter equal to or larger than the reference value (generally 4√t), and the current value I4√t that forms a nugget with a nugget diameter greater than or equal to the reference value (generally 4√t)} , and the current value I4√t that forms a nugget with a nugget diameter greater than or equal to the reference value (generally 4√t), and the current value I4√t that forms a nugget with a nugget diameter greater than the standard value (generally 4√t) This refers to the width (I _max −I _4√t ) from the upper limit current value I _max at which no scattering phenomenon occurs.

In this second step, the nugget 74 is preferentially formed at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72, so that the current value I _4√t is the current value required to form the nugget 74 having a nugget diameter equal to or larger than the reference value. The current value I2 under the above conditions is determined experimentally in advance.

Here, in this embodiment, as described above, the nugget 74 is preferentially formed at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72. Make it. On the other hand, as a reference example, in the second step, a nugget is also formed at the boundary between the thick plate 71 and the thick plate 72, and the nugget diameter of this nugget is the same as that between the thick plate 72 and the thin plate 73. It is conceivable to make the nugget diameter larger than the nugget diameter of the nugget 74 formed at the boundary between them.

However, this reference example has the following disadvantages. That is, in the above reference example, when the thickness of the thinner of the thick plates 71 and 72 (or each thick plate if the thicknesses are the same) is t, the current value I _{4√ t} is the current value necessary to form a nugget having a nugget diameter equal to or larger than a reference value (generally 4√t) at the boundary between the thick plates 71 and 72. Therefore, in the case of this reference example, since the current value I _4√t becomes high, there is a disadvantage that the appropriate current range in the second step becomes narrow.

In addition, in the second step, since the first electrode 31 is pressed against the plate assembly 70 and the plate assembly 70 is deformed so that the thin plate 73 side has a convex shape, the application of the second electrode 32 to the thin plate 73 is The pressure becomes weaker than the pressure applied by the first electrode 31 to the thick plate 71. Therefore, if the current value I2 is set within the appropriate current range of the above reference example, the current value I2 will be close to the upper limit current value I _max , and molten metal will not scatter between the thick plate 72 and the thin plate 73. There is a possibility that this may occur.

In this regard, in the method for manufacturing a spot welded joint according to an embodiment of the present invention, in the second step, the boundary between the thick plate 72 and the thin plate 73 is The nugget 74 is preferentially formed in the area. Therefore, the current value I _4√t in the second step can be set to the current value necessary to form the nugget 74 having a nugget diameter larger than the reference value, so the current value I _4√t can be made low. I can do it. Therefore, since the appropriate current range in the second step can be widened, practicality can be ensured.

In addition, in this second step, the appropriate current range can be widened, so by making the current value I2 sufficiently lower than the upper limit current value _Imax , molten metal can be inserted between the thick plate 72 and the thin plate 73. It is possible to suppress the occurrence of scattering.

Note that the second step is preferably executed after the first step from the following viewpoint. That is, in the first step described above, in addition to the pressing force exerted by the first electrode 31 and the second electrode 32, a force for lowering the welding gun 11 is applied to the plate assembly 70 by the welding robot 12. For this reason, it is assumed that the actual pressing force by the second electrode 32 will be weaker than the actual pressing force by the first electrode 31, and the holding of the plate assembly 70 will become unstable.

If current is applied between the first electrode 31 and the second electrode 32 in a state where the actual pressing force by the second electrode 32 is weaker than the actual pressing force by the first electrode 31, the thick plate 72 and the thin plate 73 There is a risk of molten metal scattering between the two. Therefore, the second step is preferably performed after the first step.

In this way, when the second step is executed after the first step, the actual pressing force acting on the plate assembly 70 from the first electrode 31 and the second electrode 32 is equal, and the gap between the first electrode 31 and the second electrode 32 is Since electricity can be applied, scattering of molten metal between the thick plate 72 and the thin plate 73 can be suppressed.

(Third step)
The third step is a step of deforming the board assembly 70 so that the convex shape of the board assembly 70 returns to its original shape. In this third step, with the plate assembly 70 sandwiched between the first electrode 31 and the second electrode 32, the first electrode 31 and the second electrode are The third control section 53 controls the power supply section 13 so that the power supply section 32 is energized. In this third step, the current value I3 that is applied between the first electrode 31 and the second electrode 32 is set so that the plate assembly 70 is not welded but is softened. In FIG. 4, the softened region of the board assembly 70 in the third step is indicated by the reference numeral 70A.

Further, in the third step, the plurality of joint drive units 25 are controlled by the third control unit 53 while passing current between the first electrode 31 and the second electrode 32 at the current value I3. At this time, by raising the entire welding gun 11 including the first electrode 31 and the second electrode 32, the second electrode 32 is pressed against the plate assembly 70 so that the convex shape of the plate assembly 70 returns to its original shape. Control for deforming the plate assembly 70 is performed on the plurality of joint drive units 25. That is, by controlling the plurality of joint drive units 25, the welding gun 11 is raised so that the first electrode 31 is at the same position as in the initial state.

As a result, the convex shape of the board assembly 70 is returned to its original shape, that is, as an example, a flat shape. At this time, it is difficult to completely return the board assembly 70 to its original shape, and it is assumed that the board assembly 70 becomes approximately flat. Further, it is assumed that pressure contact marks of the first electrode 31 from the first step and the second step remain on the surface of the thick plate 71. For this reason, the original shape includes cases where it is approximately flat, cases where pressure contact marks of the first electrode 31 remain on the surface of the thick plate 71, etc. Note that the original shape of the board set 70 may be other than the flat shape, and in the third step, the board set 70 may be deformed so as to be returned to the original shape other than the flat shape.

In this way, when the convex shape of the plate assembly 70 is returned to its original shape (substantially the original shape), the contact area between the thick plate 71 and the first electrode 31 and the contact area between the thin plate 73 and the second electrode 32 are The contact area becomes equal (substantially equal).

In this third step, when deforming the plate assembly 70, electricity is passed between the first electrode 31 and the second electrode 32 to soften the plate assembly 70, so that the plate assembly 70 can be easily deformed.

In addition, in the third step, as described above, the plate assembly 70 may be deformed while supplying electricity between the first electrode 31 and the second electrode 32 to soften the plate assembly 70. The plate assembly 70 may be deformed after the plate assembly 70 is softened by passing current between the second electrodes 32 . Further, preferably, electricity is passed between the first electrode 31 and the second electrode 32 to soften the plate assembly 70, but the electricity supply for softening the plate assembly 70 may be omitted.

In the third step, the welding gun 11 is raised with the plate set 70 sandwiched between the first electrode 31 and the second electrode 32, so that the plate set 70 is moved so that the convex shape of the plate set 70 returns to its original shape. may be modified as follows. That is, after controlling the electrode drive section 22 to raise the first electrode 31 and providing a clearance between the first electrode 31 and the thick plate 71, the plurality of joint drive sections 25 are controlled to raise the welding gun 11. May be increased. Thereby, the second electrode 32 may be pressed against the plate assembly 70, and the plate assembly 70 may be deformed so that the convex shape of the plate assembly 70 returns to its original shape.

(Fourth step)
The fourth step is a step of forming a nugget 75 at the boundary between the thick plates 71 and 72. In this fourth step, with the plate assembly 70 sandwiched between the first electrode 31 and the second electrode 32, the first electrode 31 and the second electrode are connected at a current value I4 larger than the current value I2 at the second step. The fourth control section 54 controls the power supply section 13 so that the power supply section 32 is energized. At this time, because the current value I4 is large and the contact area between the thick plate 71 and the first electrode 31 is equal to the contact area between the thin plate 73 and the second electrode 32, the gap between the thick plate 72 and the thin plate 73 is The nugget 75 is preferentially formed at the boundary between the thick plates 71 and 72, which is closer to the center of the total thickness of the plate set 70, than at the boundary between the plates 71 and 72.

In the fourth step, a current value I4 to be passed between the first electrode 31 and the second electrode 32 is set so that a nugget 75 is formed at the boundary between the thick plate 71 and the thick plate 72. .

In this fourth step, it is sufficient that the nugget 75 is formed at the boundary between the thick plate 71 and the thick plate 72, and the nugget 75 formed at the boundary between the thick plate 71 and the thick plate 72 is It may or may not be integrated with the nugget 74 formed at the boundary between the thick plate 72 and the thin plate 73.

In this fourth step, the nugget diameter is set so that the nugget 75 at the boundary between the thick plate 71 and the thick plate 72 has a larger nugget diameter than the nugget 74 at the boundary between the thick plate 72 and the thin plate 73. , current value I4 is set. The nugget diameter in this case is the maximum value of the diameter measured along the direction parallel to each boundary.

In the example shown in FIG. 4, as an example, a nugget 75 formed at the boundary between the thick plate 71 and the thick plate 72 is replaced by a nugget 74 formed at the boundary between the thick plate 72 and the thin plate 73. It is integrated with. The connection between the thick plate 71 and the thick plate 72, where t is the thickness of the thinner of the thick plate 71 and the thick plate 72 (or each thick plate if the thickness is the same) In order to ensure strength, it is desirable to form the nugget 75 with a nugget diameter of 4√t to 5√t. In this fourth step, the current value I4 is set within the appropriate current range.

Here, in this embodiment, as described above, in the second step, the nugget 74 is preferentially placed at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72. is formed. On the other hand, as a reference example, in the second step, a nugget 75 is formed at the boundary between the thick plate 71 and the thick plate 72, and the nugget diameter of the nugget 75 is different from that between the thick plate 72 and the thin plate 73. It is conceivable to make the nugget diameter larger than the nugget diameter of the nugget 74 formed at the boundary between the two.

However, this reference example has the following disadvantages. That is, in the above reference example, in the second step, the contact area between the thin plate 73 and the second electrode 32 is smaller than the contact area between the thick plate 71 and the first electrode 31, so the current density on the thin plate 73 side is The current density becomes higher than that on the plate 71 side, and the molten metal at the boundary between the thick plates 71 and 72 becomes insufficient. For this reason, if it is attempted to form the nugget 75 at the boundary between the thick plates 71 and 72 in the second step, there is a possibility that the nugget diameter of the nugget 75 will be smaller than the desired nugget diameter.

In this regard, in the method for manufacturing a spot welded joint according to an embodiment of the present invention, in the first step, the plate assembly 70 is deformed so that the side of the thin plate 73 has a convex shape, and then in the second step, A nugget 74 is formed at the boundary between the thick plate 72 and the thin plate 73 by passing current between the first electrode 31 and the second electrode 32 at a current value I2. Then, in a third step, the convex shape of the plate assembly 70 is returned to its original shape, and in a fourth step, electricity is passed between the first electrode 31 and the second electrode 32 at a current value I4. A nugget 75 is formed at the boundary between the thick plate 71 and the thick plate 72.

Here, in the third step, by returning the convex shape of the plate set 70 to its original shape, the contact area between the thick plate 71 and the first electrode 31 and the contact area between the thin plate 73 and the second electrode 32 are reduced. The contact area becomes equal. Furthermore, in the fourth step, the current value I4 is made larger than the current value I2. As a result, in the fourth step, priority is given to the boundary between the thick plate 71 and the thick plate 72, which is closer to the center of the total plate thickness of the plate set 70, than the boundary between the thick plate 72 and the thin plate 73. Since the nugget 75 is formed between the thick plates 71 and 72, the nugget diameter of the nugget 75 formed at the boundary between the thick plates 71 and 72 can be secured.

Note that the fourth step is preferably executed after the third step from the following viewpoint. That is, in the third step described above, in addition to the pressure applied by the first electrode 31 and the second electrode 32, a force for lifting the welding gun 11 is applied to the plate assembly 70 by the welding robot 12. For this reason, it is assumed that the actual pressing force by the first electrode 31 will be weaker than the actual pressing force by the second electrode 32, and the holding of the plate assembly 70 will become unstable.

If current is applied between the first electrode 31 and the second electrode 32 in such a state that the actual pressing force by the first electrode 31 is weaker than the actual pressing force by the second electrode 32, the thick plate 71 and the thick plate There is a possibility that molten metal may be scattered between the parts and the parts 72. Therefore, the fourth step is preferably performed after the third step.

In this way, when the fourth step is executed after the third step, the actual pressing force acting on the plate assembly 70 from the first electrode 31 and the second electrode 32 is equal, and the gap between the first electrode 31 and the second electrode 32 is Since electricity can be applied, scattering of molten metal between the thick plates 71 and 72 can be suppressed.

(Fifth step)
The fifth step is to maintain the pressurized state of the plate assembly 70 until the nuggets 74 and 75 are cooled and solidified. In this fifth step, the fifth control section 55 controls the electrode driving section 22 to pressurize the plate assembly 70 by the first electrode 31 and the second electrode 32 until the time for cooling and solidifying the nuggets 74 and 75 has elapsed. It is done against. In the manner described above, the plate set 70 is spot welded, and a spot welded joint 80 is obtained.

(action and effect)
As detailed above, in the method for manufacturing a spot welded joint according to an embodiment of the present invention, in the first step, the first electrode 31 is pressed against the plate assembly 70 so that the second metal plate side has a convex shape. The board assembly 70 is deformed. Subsequently, in a second step, electricity is applied between the first electrode 31 and the second electrode 32 while the plate assembly 70 is sandwiched between the first electrode 31 and the second electrode 32.

At this time, the contact area between the thin plate 73 and the second electrode 32 is smaller than the contact area between the thick plate 71 and the first electrode 31, so the current density on the thin plate 73 side is higher than the current density on the thick plate 71 side. Therefore, the nugget 74 can be formed preferentially at the boundary between the thick plate 72 and the thin plate 73 rather than at the boundary between the thick plate 71 and the thick plate 72.

Next, in the third step, the second electrode 32 is pressed against the plate set 70 to deform the plate set 70 so that the convex shape returns to its original shape. Then, in the fourth step, with the plate assembly 70 sandwiched between the first electrode 31 and the second electrode 32, the first electrode 31 and the second electrode 32 and energized.

At this time, since the contact area between the thick plate 71 and the first electrode 31 and the contact area between the thin plate 73 and the second electrode 32 are equal, the plate assembly 70 The nugget 75 can be preferentially formed at the boundary between the thick plates 71 and 72 near the center of the total plate thickness.

Moreover, in the method for manufacturing a spot welded joint according to an embodiment of the present invention, when forming the nugget 74 at the boundary between the thick plate 72 and the thin plate 73 in the second step, and in the fourth step, When forming the nugget 75 at the boundary between the plate assembly 71 and the thick plate 72, a pressure member for adjusting the pressure applied to the plate assembly 70 is not required.

In addition, in the third step, the second electrode 32 is pressed against the plate assembly 70 and the plate assembly 70 is deformed so that the convex shape returns to its original shape, so that the shape of the plate assembly 70 does not change before and after welding. It can be avoided.

As described above, according to the method for manufacturing a spot welded joint according to an embodiment of the present invention, there is no need for a pressure member, and while avoiding changes in the shape of the plate assembly 70 before and after welding, it is possible to A nugget 74 can be formed at the boundary between the thin plate 72 and the thin plate 73, and a nugget 75 can be formed at the boundary between the thick plate 71 and the thick plate 72.

(About the relationship between contact area S1 and contact area S2)
Subsequently, in the first step described above, the relationship between the contact area S1 between the thick plate 71 and the first electrode 31 and the contact area S2 between the thin plate 73 and the second electrode 32 satisfies equation (1). , the reason for deforming the board assembly 70 will be explained.

The present inventors have proposed a method that includes a pair of thick plates 71 and 72 and a thin plate 73 that is thinner than each of the pair of thick plates 71 and 72, and the pair of thick plates are arranged such that the thin plate 73 is located at the outermost position. The spot welding of the plate set 70 in which the thin plates 71, 72 and the thin plates 73 are stacked has been extensively studied. As a result, the relationship between the contact area S1 between the thick plate 71 and the first electrode 31 on the side opposite to the thin plate 73 among the pair of thick plates 71 and 72, and the contact area S2 between the thin plate 73 and the second electrode 32 is determined. The following findings were obtained regarding this.

That is, if the relationship between the contact area S1 between the thick plate 71 and the first electrode 31 and the contact area S2 between the thin plate 73 and the second electrode 32 is S2/S1<0.9, the first electrode 31 and When electricity is applied between the first electrode 31 and the second electrode 32 with the plate assembly 70 sandwiched between the second electrodes 32, a nugget 74 can be formed at the boundary between the thick plate 72 and the thin plate 73. It has become clear that an appropriate current range can be secured.

However, it has been found that if S2/S1 is too small, molten metal scatters easily between the thick plate 72 and thin plate 73, and when 0.1<S2/S1, molten metal scatters. It has become clear that this will no longer occur.

In this way, the inventor found that it is preferable that the relationship between the contact area S1 between the thick plate 71 and the first electrode 31 and the contact area S2 between the thin plate 73 and the second electrode 32 satisfies formula (1). found out.

Note that the contact area S1 between the thick plate 71 and the first electrode 31 and the contact area S2 between the thin plate 73 and the second electrode 32 can be measured using pressure-sensitive paper. For example, when measuring the contact area S1, the first electrode 31 is pressed against the thick plate 71 with a predetermined pressure while pressure-sensitive paper is sandwiched between the thick plate 71 and the first electrode 31. Thereafter, the contact area S1 can be determined by separating the first electrode 31 from the thick plate 71, collecting the pressure-sensitive paper, and measuring the area of the discolored portion of the pressure-sensitive paper. The same applies to the contact area S2.

Next, a modification of one embodiment of the present invention will be described.

In the embodiment described above, the pressure applied to the plate assembly 70 is kept approximately constant from the time when the first electrode 31 and the second electrode 32 start applying pressure to the plate assembly 70 until the end of the fifth step. The applied force may be different for each step.

Further, in the above embodiment, the current value I1 in the first step, the current value I2 in the second step, the current value I3 in the third step, and the current value I4 in the fourth step are each constant, but for example, As shown in FIG. 5, in the first step and the second step, the current value may gradually increase toward the current value I2. Similarly, in the third step and the fourth step, the current value may gradually increase toward the current value I4.

In this way, in the first step and the second step, when the current value gradually increases, welding can be started from minute irregularities between the thick plate 72 and the thin plate 73. Similarly, in the third and fourth steps, when the current value gradually increases, welding can be started from minute irregularities between the thick plates 71 and 72.

Further, in the embodiment described above, the spot welding apparatus 10 has the welding robot 12 to move the welding gun 11 up and down, but it may have a drive device other than the welding robot 12.

Next, examples will be described.

Tables 1 and 2 show the conditions of the examples.
Invention Examples 1 to 4 are examples of this embodiment. In Comparative Example 1, the pressing force of the second electrode in contact with the thin plate is smaller than the pressing force of the first electrode in contact with the thick plate on the opposite side of the thin plate, and the pressure between the first electrode and the second electrode is This is an example in which electricity is supplied (for example, see Patent Document 2). In Comparative Example 1, the pressing force of the first electrode in contact with the thick plate on the opposite side of the thin plate is greater than the pressing force of the second electrode in contact with the thin plate, and the thick plate in contact with the first electrode has a concave shape. Since the plate assembly deforms as follows, the shape of the plate assembly changes before and after welding.

Other conditions for each example are as follows.
The energization pattern between the first electrode and the second electrode is as shown in FIG.
The welding conditions are as shown in Tables 1 and 2 and below.
・Power source: Uses 50 Hz commercial power source ・Welding machine: Stationary AC spot welding machine ・Plate assembly: Thin steel plate (plate thickness 0.7mm-SCGA270C)/Thicker thin steel plate (plate thickness 1.6mm-SCGA780DP/Thick plate (plate) Thickness 2.3mm-SCGA980DP)
・First electrode and second electrode: DR6φ-40R, electrode diameter φ16mm, made of Cr-Cu alloy ・Pressing force: 3.92kN
・Squeeze time: 50cyc
・Retention time: 18cyc (fifth step)
・Restraint conditions for plate assembly: The plate assembly is restrained by restraining parts from both sides in the thickness direction. The distance between the left and right restraint parts of the board assembly is 100mm (the board assembly can be freely deformed in the thickness direction between these parts).

As a result of the experiment under the above conditions, the nugget diameter (diameter of the nugget) immediately before molten metal scattering was as shown in Table 3.

The criteria for determining pass/fail in the examples are as follows.
・If the appropriate current range ΔI is 1.5kA or more, the test is passed.
・If the amount of change in shape of the plate assembly before and after welding is 0, it will be passed.

Furthermore, in contrast to this embodiment, a nugget is formed at the boundary between the thin plate and the thick plate without deforming the plate assembly, and then a nugget is formed at the boundary between the thick plate and the thick plate. Let us take a conventional example as an example. The conditions of this conventional example are the same as those of invention examples 1 to 4.

In this conventional example, when forming a nugget at the boundary between a thin plate and a thick plate, the current value at which the nugget diameter becomes 4√t is 8.3 kA, and the current value just before the molten metal scatters. It became 8.8kA. In addition, in this conventional example, when forming a nugget at the boundary between thick plates, the current value at which the nugget diameter becomes 4√t is 5.8 kA, and the current value just before the molten metal scatters. became 8.8kA.

In this conventional example, when forming a nugget at the boundary between thick plates, the appropriate current range is 8.8kA-5.8kA=3.0kA, which means that more than 1.5kA can be secured; The appropriate current range ΔI when forming a nugget at the boundary between the two is 8.8kA-8.3kA=0.5kA, which is less than 1.5kA, so this conventional example was rejected.

In addition, in Comparative Example 1, although the appropriate current range ΔI is 1.5 kA or more, the plate assembly is deformed so that the thick plate in contact with the second electrode has a concave shape, and the shape of the plate assembly changes before and after welding. Therefore, it was judged as a failure on this point.

On the other hand, invention examples 1 to 4 all had an appropriate current range ΔI of 1.5 kA or more and passed the test. Further, invention examples 1 to 4 passed the test because the amount of change in shape of the plate assembly before and after welding was 0.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above, and it is of course possible to implement various modifications other than the above without departing from the spirit thereof. It is.

10 Spot welding device 11 Welding gun 12 Welding robot 13 Power supply section 14 Control section 21 Gun body 22 Electrode drive section (an example of a drive section)
23 Joint 24 Robot arm 25 Joint drive unit (an example of a drive unit)
31 First electrode 32 Second electrode 41 Processor 42 Memory 43 Program 51 First control section 52 Second control section 53 Third control section 54 Fourth control section 55 Fifth control section 70 Plate assembly 71 Thick plate (first metal plate example)
72 Thick plate (an example of the first metal plate)
73 Thin plate (an example of second metal plate)
74 Nugget 75 Nugget 80 Spot weld joint 90 Restraint part 92 Clearance

Claims (6)

The plurality of first metal plates include a plurality of first metal plates and a second metal plate thinner than each of the plurality of first metal plates, and the plurality of first metal plates are arranged such that the second metal plate is located at the outermost position. In a method for manufacturing a spot welded joint, a plate set in which a plate and the second metal plate are stacked is sandwiched between opposing first and second electrodes and spot welded to obtain a spot welded joint,
With the first electrode placed on the opposite side of the second metal plate and the second electrode placed on the side of the second metal plate, the first electrode is pressed against the plate set, and the second a first step of deforming the plate assembly so that the metal plate side has a convex shape;
With the plate set sandwiched between the first electrode and the second electrode, electricity is applied between the first electrode and the second electrode, so that the second metal plate overlaps with the second metal plate among the plurality of first metal plates. a second step of forming a nugget at the interface between the first metal plate and the second metal plate;
With the plate set sandwiched between the first electrode and the second electrode, electricity is applied between the first electrode and the second electrode so that the plate set is softened, and the second electrode is connected to the plate set. a third step of deforming the board set so that the convex shape returns to its original shape by pressing against it;
With the plate assembly sandwiched between the first electrode and the second electrode, current is passed between the first electrode and the second electrode at a current value larger than the current value at the second step, and the a fourth step of forming a nugget at the boundary between the plurality of first metal plates;
A method of manufacturing spot welded joints, including: In the first step, with the plate assembly sandwiched between the first electrode and the second electrode, the first electrode and the second electrode are heated at a current value smaller than the current value at the second step. energizing between them to soften the board assembly;
A method for manufacturing a spot welded joint according to claim 1. In the third step, with the plate assembly sandwiched between the first electrode and the second electrode, the first electrode and the second electrode are heated at a current value smaller than the current value at the second step. energizing between them to soften the board assembly;
A method for manufacturing a spot welded joint according to claim 1 or 2. In the first step, a contact area S1 between the first metal plate on the side opposite to the second metal plate among the plurality of first metal plates and the first electrode, and a contact area S1 between the second metal plate and the first electrode are determined. The relationship between the contact area S2 with the two electrodes satisfies formula (1),
0.1<S2/S1<0.9...(1)
A method for manufacturing a spot welded joint according to any one of claims 1 to 3. The plurality of first metal plates include a plurality of first metal plates and a second metal plate thinner than each of the plurality of first metal plates, and the plurality of first metal plates are arranged such that the second metal plate is located at the outermost position. A spot welding device for spot welding a plate set in which a plate and the second metal plate are stacked,
A first electrode and a second electrode facing each other,
a drive unit that moves the first electrode and the second electrode in a direction in which the first electrode and the second electrode face each other;
a power supply unit that conducts electricity between the first electrode and the second electrode;
a control unit that controls the drive unit and the power supply unit;
Equipped with
The control unit includes:
With the first electrode disposed on the opposite side of the second metal plate and the second electrode disposed on the second metal plate side, the first electrode is pressed against the plate set, and the second metal plate is pressed against the plate assembly. a first control unit that controls the drive unit to deform the plate set so that the two metal plate sides have a convex shape;
With the plate set sandwiched between the first electrode and the second electrode, electricity is applied between the first electrode and the second electrode, and the second metal plate among the plurality of first metal plates is a second control unit that controls the power supply unit to form a nugget at the boundary between the overlapping first metal plate and the second metal plate;
Controlling the power supply unit to conduct electricity between the first electrode and the second electrode so that the plate assembly is softened when the plate assembly is sandwiched between the first electrode and the second electrode. a third control unit that controls the drive unit to press the second electrode against the plate set and deform the plate set so that the convex shape returns to its original shape;
In a state where the plate assembly is sandwiched between the first electrode and the second electrode, a current value larger than the current value under control by the second control unit is applied between the first electrode and the second electrode. a fourth control unit that controls the power supply unit to energize and form a nugget at the boundary between the plurality of first metal plates;
Spot welding equipment including. A first electrode and a second electrode facing each other,
a drive unit that moves the first electrode and the second electrode in a direction in which the first electrode and the second electrode face each other;
a power supply unit that conducts electricity between the first electrode and the second electrode;
a control unit that controls the drive unit and the power supply unit;
Equipped with
The plurality of first metal plates include a plurality of first metal plates and a second metal plate thinner than each of the plurality of first metal plates, and the plurality of first metal plates are arranged such that the second metal plate is located at the outermost position. A program applied to a spot welding device that performs spot welding by sandwiching a plate set in which a plate and the second metal plate are stacked between the first electrode and the second electrode,
A computer as the control unit,
With the first electrode disposed on the opposite side of the second metal plate and the second electrode disposed on the second metal plate side, the first electrode is pressed against the plate set, and the second metal plate is pressed against the plate assembly. A first step of controlling the drive unit to deform the plate assembly so that the two metal plate sides have a convex shape;
With the plate set sandwiched between the first electrode and the second electrode, electricity is applied between the first electrode and the second electrode, and the second metal plate among the plurality of first metal plates is a second step of controlling the power supply unit to form a nugget at the boundary between the overlapping first metal plate and the second metal plate;
Controlling the power supply unit to conduct electricity between the first electrode and the second electrode so that the plate assembly is softened when the plate assembly is sandwiched between the first electrode and the second electrode. a third step of controlling the drive unit to press the second electrode against the plate set and deform the plate set so that the convex shape returns to its original shape;
In a state where the plate set is sandwiched between the first electrode and the second electrode, current is passed between the first electrode and the second electrode at a current value larger than the current value at the second step, a fourth step of controlling the power supply unit to form a nugget at the boundary between the plurality of first metal plates;
A program to run.