JP2024110656A - Spot welding method and spot welding device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a high-precision weld part having no failure such as crack to be formed with less energy even if a superposed plate having a plurality of metal plates superposed on each other includes a high strength steel plate and a plated steel plate.

SOLUTION: A spot welding method includes: executing a first power feeding step of feeding first electric currents 4 for forming a weld part 10 to a superposed plate 20 through a first electrode 2A pressurizing, in a plate thickness direction, the superposed plate 20, in forming the point-like weld part for welding steel plates 21 and 22 to each other in the superposed plate 20 having two steel plates (high-strength steel plates) 21 and 22 superposed on each other; and then executing a second power feeding step of feeding second electric currents which are smaller in an electric current value than the first electric currents to the superposed plate 20 through a second electrode 2A pressurizing, in the plate thickness direction, a predetermined position closer to the outside in a radial direction than a pressurized position by the first electrode 2A in the superposed plate 20, while keeping the superposed plate 20 in non-contact with the first electrode 2A.

SELECTED DRAWING: Figure 2

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a spot welding method and a spot welding device.

One method for joining metal plates, such as steel plates, in a plate assembly in which multiple metal plates are stacked together is resistance spot welding (hereinafter simply referred to as "spot welding"). Spot welding is a method in which an electric current is supplied to the plate assembly via an electrode pressed against the plate assembly (pressing the plate assembly in the plate thickness direction) to melt the contact area between the metal plates, and then the molten part is quenched and solidified to form a spot-like weld that joins the metal plates together. A widely used spot welding method is direct spot welding, in which a weld is formed between a pair of positive and negative electrodes that are coaxially arranged so that their tips (tip surfaces) face each other across the plate assembly.

Note that the "weld" referred to in this specification conforms to the definition of "weld" described in JIS Z3001-1:2018. In other words, a "weld" is a general term that includes the weld metal (nugget) and the heat-affected zone. The above "weld metal" refers to "a part of the weld that melts and solidifies during welding," and a "heat-affected zone" refers to "an unmelted part of the base material where the structure, metallurgical properties, mechanical properties, etc. have changed due to heat from welding, cutting, etc." The heat-affected zone is also called the "HAZ."

In recent years, for example, in the plate assemblies that constitute the body of an automobile, there have been an increasing number of cases in which high-strength steel plates, such as high-tensile steel plates and ultra-high-tensile steel plates, are used for the purpose of reducing fuel consumption/electricity consumption (weight reduction) and improving collision safety (for example, Patent Document 1). However, there is a problem in that it is more difficult to obtain a weld (nugget) with the desired joint strength in plate assemblies that include high-strength steel plates, compared to plate assemblies that do not include high-strength steel plates. Therefore, when spot welding plate assemblies that include high-strength steel plates, measures are taken such as increasing the pressure applied to the plate assembly in the plate thickness direction and applying large energy to the plate assembly (increasing the current value of the current supplied to the plate assembly, lengthening the current flow time, etc.) compared to spot welding plate assemblies that do not include high-strength steel plates.

JP 2019-116223 A Japanese Patent Application Publication No. 7-68388

However, when spot welding a plate assembly including high-strength steel sheets, if a high pressure and large energy are applied to the plate assembly, large stresses are likely to occur at the boundary between the weld metal (nugget) and the surrounding heat-affected zone, which have different metal structures (mechanical properties, hardness, etc.), making it easier for defects such as cracks to occur in the weld. Also, when spot welding a plate assembly including plated steel sheets such as galvanized steel sheets, if a high pressure and large energy are applied to the plate assembly, the constituent elements of the plating (e.g. zinc) tend to agglomerate at the grain boundaries in the heat-affected zone, making it easier for defects such as cracks to occur in the weld, as described above.

As a method for preventing the occurrence of defects as described above, it is known to soften (anneal or temper) the weld by supplying current again to the plate assembly through the electrode used to form the weld after the weld is formed (for example, Patent Document 2). However, since the metal structure of the weld, particularly the nugget, is a solidified structure formed by rapid cooling from a molten state, a lot of energy (sufficient current flow time and current amount) is required to soften the nugget. This inevitably increases the total time and cost required to spot weld the plate assembly.

In view of the above situation, the present invention aims to provide a technical means that makes it possible to form high-precision welds without defects such as cracks using little energy, even when the assembly of multiple overlapping metal sheets includes high-strength steel sheets or plated steel sheets.

In order to achieve the above object, the present invention provides a method for forming spot welds for joining metal plates in a plate assembly in which a plurality of metal plates are overlapped, comprising the steps of:
A first current supplying step is carried out in which a first current for forming a weld is supplied to the plate assembly via a first positive electrode that presses the plate assembly in a plate thickness direction, and then
The method is characterized in that, with the plate assembly and the first positive electrode in a non-contact state, a second power supply process is carried out in which a second current having a current value smaller than the first current is supplied to the plate assembly via a second positive electrode which applies pressure in the plate thickness direction to a predetermined position of the plate assembly that is radially outward of the pressure position applied by the first positive electrode.

By carrying out the second power supply process as described above following the first power supply process, it is possible to keep the heat-affected zone around the nugget warm (slow the cooling rate of the heat-affected zone) until the nugget formed near the center of the weld formed at the contact part between the metal plates in the first power supply process completely cools and solidifies. As a result, when the sheet assembly includes high-strength steel sheets, it is possible to suppress or prevent the occurrence of large stress at the boundary between the nugget and the heat-affected zone in the weld formed in the sheet assembly, so that the occurrence of defects such as cracks caused by the above-mentioned stress can be prevented as much as possible. In addition, when the sheet assembly includes plated steel sheets, it is possible to prevent the constituent elements of the plating from agglomerating at the grain boundaries of the heat-affected zone, so that the occurrence of defects such as cracks caused by the agglomeration of the constituent elements of the plating can be prevented as much as possible.

In the second power supply process, the second current is not supplied to the plate assembly to soften the solidified (cooled) weld, but is supplied to the plate assembly to slow the cooling rate of part of the weld (the heat-affected zone around the nugget). Therefore, the current value of the second current can be smaller than the current value of the first current, and the amount of the second current supplied to the plate assembly (the time that the current is passed through the second positive electrode) can be small. As a result, a high-precision weld without defects such as cracks can be formed with little energy.

In the second power supply process, the second positive electrode applies pressure to the plate assembly in the plate thickness direction, and then the contact state between the plate assembly and the first positive electrode is released, and then a second current can be supplied to the plate assembly via the second positive electrode.

The above object can also be achieved by a spot welding device according to the present invention. That is, the spot welding device according to the present invention forms spot-like welds for joining metal plates to each other in a plate assembly in which a plurality of metal plates are overlapped, and
a first positive electrode capable of supplying a first current for forming a welded portion to the plate assembly while applying a pressure force in a plate thickness direction to the plate assembly; and a second positive electrode disposed radially outside the first positive electrode and capable of supplying a second current having a current value smaller than the first current to the plate assembly while applying a pressure force in the plate thickness direction to the plate assembly.
The device is characterized by comprising a linear motion mechanism that moves the first positive electrode and the second positive electrode back and forth along the thickness direction of the board assembly so that when one of the first positive electrode and the second positive electrode comes into contact with the board assembly, the other comes out of contact with the board assembly.

As described above, according to the present invention, even when a sheet assembly in which multiple metal sheets are stacked includes high-strength steel sheets or plated steel sheets, it is possible to form high-precision welds that are free of defects such as cracks using little energy.

FIG. 1A is a conceptual diagram showing a main part of a spot welding device according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along line AA in FIG. 2A to 2D are diagrams showing a welding procedure (procedure for implementing the spot welding method according to the present invention) using the spot welding apparatus of FIG. 1, in which FIG. 2A shows an initial stage of a first power supply process, FIG. 2B shows an intermediate stage of the first power supply process, FIG. 2C shows an initial stage of a second power supply process, and FIG. 2D shows an intermediate stage of the second power supply process. 1A and 1B are partial cross-sectional views of a spot welding device according to another embodiment of the present invention. FIG. 13 is a diagram conceptually showing a main part of a spot welding device according to another embodiment of the present invention.

The following describes an embodiment of the present invention with reference to the drawings.

First, an overview of a spot welding device 1 used to carry out a spot welding method according to an embodiment of the present invention will be described with reference to Figures 1(a) and (b). The spot welding device 1 is used when spot welding a plate assembly 20 in a flat (horizontal) position, which is made up of multiple overlapping metal plates, and includes a first electrode set 2 consisting of a pair of first electrodes 2A, 2B, and a second electrode set 3 consisting of a pair of second electrodes 3A, 3B.

The first electrodes 2A and 2B constituting the first electrode set 2 are made of general round bar electrodes or square bar electrodes [round bar electrodes in this case; see Fig. 1(b)] and are arranged coaxially so that their tips, which are formed into a convex spherical shape, face each other across the plate set 20.

Although not shown in the figure, the first electrodes 2A, 2B are electrically connected to a power source via a transformer to generate a high current for spot welding (for forming a welded portion) between the two electrodes 2A, 2B. In this embodiment, as shown in FIG. 2(a), the first electrode 2A arranged on the upper side of the plate assembly 20 is a positive electrode electrically connected to the positive terminal of the transformer, and the first electrode 2B arranged on the lower side of the plate assembly 20 is a negative electrode electrically connected to the negative terminal of the transformer.

Both second electrodes 3A and 3B constituting the second electrode group 3 are cylindrical [see FIG. 1(b)] and are arranged coaxially so that their tips face each other across the plate assembly 20 at a predetermined radially outer side than the first electrode group 2 (the first electrodes 2A and 2B constituting the first electrode group 2). Specifically, as shown in FIG. 2(c) and (d), the size and position of the second electrodes 3A and 3B are adjusted so that the current (second current 5) generated when the current is applied between the second electrodes 3A and 3B flows through the heat-affected zone 12 formed around the nugget 11 of the welded portion 10 formed in the plate assembly 20. Note that the size of the nugget 11 (nugget diameter) can be controlled by controlling the time of application of current between the first electrodes 2A and 2B and the current value of the current (first current 4) flowing between the first electrodes 2A and 2B, so that the position where the heat-affected zone 12 is formed can be easily estimated.

Although not shown in the figure, the second electrodes 3A and 3B are also electrically connected to the power source. Here, the second electrode 3A arranged on the upper side of the board assembly 20 is a positive electrode electrically connected to the positive terminal of the power source, and the second electrode 3B arranged on the lower side of the board assembly 20 is a negative electrode electrically connected to the negative terminal of the power source. The second electrode set 3 may be electrically connected to the power source via a transformer like the first electrode set 2, or may be electrically connected to the power source without a transformer. This is because the current value of the second current 5 supplied to the board assembly 20 (generated between the second electrodes 3A and 3B) when the second electrode set 3 is energized is intentionally made smaller than the current value of the first current 4 supplied to the board assembly 20 (generated between the first electrodes 2A and 2B) when the first electrode set 2 is energized. Specifically, the current value of the second current 5 is set to 50% or less of the current value of the first current 4. However, the current values of the first current 4 and the second current 5 and the time for which current is passed through the first electrode set 2 and the second electrode set 3 are set according to the number of overlapping metal plates that make up the plate assembly 20, the material of the metal plates, the thickness of the metal plates, etc.

The first electrode 2A and the second electrode 3A, which are arranged coaxially on the upper side of the board assembly 20, are insulated by insulating means (not shown), and the first electrode 2B and the second electrode 3B, which are arranged coaxially on the lower side of the board assembly 20, are also insulated by insulating means (not shown).

The spot welding device 1 includes a linear motion mechanism that moves the first electrode 2A and the second electrode 3A forward and backward in the thickness direction (vertical direction) of the plate assembly 20 so that when one of the two electrodes 2A, 3A arranged coaxially on the upper side of the plate assembly 20 contacts the plate assembly 20 (pressing the plate assembly 20 in the plate thickness direction), the other electrode does not contact the plate assembly 20, and a linear motion mechanism that moves the first electrode 2B and the second electrode 3B forward and backward in the plate thickness direction of the plate assembly 20 so that when one of the two electrodes 2B, 3B arranged coaxially on the lower side of the plate assembly 20 contacts the plate assembly 20, the other electrode does not contact the plate assembly 20. The linear motion mechanism can precisely adjust the relative positions of the electrodes 2A, 2B, 3A, 3B with respect to the plate assembly 20, and more specifically, the pressure force in the plate thickness direction to be applied to the plate assembly 20, for example, a linear motion actuator driven by a servo motor.

The first electrode set 2 and the second electrode set 3 described above, and the linear motion mechanism that moves them forward and backward in the plate thickness direction of the plate assembly 20, are held, for example, by a welding gun attached to the tip of a three-dimensionally movable robot arm provided on a welding robot. In other words, the spot welding method according to the present invention can be performed automatically using a welding robot.

Next, the plate assembly 20, which is the object to be welded and is made by stacking multiple metal plates, will be described. The plate assembly 20 includes at least one so-called high-strength steel plate made of 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, or a plated steel plate having a plating layer such as zinc plating or aluminum plating on the surface. The plate assembly 20 shown in FIG. 1 and the like is made by stacking two high-strength steel plates 21 and 22, but it may also be made by stacking three or more metal plates. The thickness of each steel plate 21 and 22 is not particularly limited, but when the plate assembly is to form, for example, the body of an automobile, steel plates 21 and 22 with a thickness of 0.5 mm to 2.5 mm are used. The steel plates 21 and 22 may be the same type of steel plate with the same material composition, or may be different types of steel plates with different material compositions.

The spot welding device 1 has the above-mentioned general configuration, and by sequentially carrying out the first power supply process and the second power supply process, a spot-shaped weld 10 is (automatically) formed at a planned welding location of the plate assembly 20 in which two overlapping steel plates 21, 22 are assembled. The first and second power supply processes are described below with reference to Figures 2(a) to (d).

[First power supply step]
In the first power supply step, first, as shown in Fig. 2(a), the plate assembly 20 to be welded is placed in a flat position, and then the linear motion mechanism is driven to press the tips of the first positive electrode 2A and the first negative electrode 2B, which are arranged opposite each other across the planned welding point of the plate assembly 20, against the upper and lower surfaces of the plate assembly 20. As a result, the plate assembly 20 is sandwiched in the plate thickness direction by the pair of first electrodes 2A and 2B, and the two steel plates 21 and 22 come into contact between the first electrodes 2A and 2B. During the first power supply step, the second electrodes 3A and 3B constituting the second electrode set 3 are not in contact with the plate assembly 20.

Next, when the first electrodes 2A and 2B apply pressure to the plate assembly 20 in the plate thickness direction by the first electrodes 2A and 2B, a first current 4 is supplied to the plate assembly 20 through the first electrode 2A, which is the positive electrode, as shown in FIG. 2(b). As a result, when electrons flow between the first electrodes 2A and 2B (in the plate assembly 20 interposed therebetween), heat is generated at a location of high electrical resistance in the plate assembly 20, in this case the contact portion between the steel plates 21 and 22 formed between the first electrodes 2A and 2B arranged opposite each other, and a molten metal 13 is generated in which the constituent metals of the steel plates 21 and 22 melt and mix at the contact portion. Thereafter, as the current continues to flow between the first electrodes 2A and 2B that are applying pressure to the plate assembly 20, the molten metal 13 grows. Then, when the molten metal 13 grows to a predetermined size, the current to the first electrodes 2A and 2B is stopped. The first electrodes 2A and 2B are designed to allow a coolant to flow through them at least while they are applying pressure to the plate assembly 20. Therefore, when the current between the first electrodes 2A and 2B is stopped, the molten metal 13 is rapidly cooled and solidified, forming a weld metal (nugget) 11 that joins the steel plates 21 and 22 together.

As electricity flows between the first electrodes 2A, 2B, a heat-affected zone 12, also known as a HAZ, is formed around the molten metal 13. The heat-affected zone 12 conforms to the definition of "heat-affected zone" specified in JIS Z 3001-1:2018 and is the part of the unmelted base material (steel plates 21, 22) whose structure and mechanical properties have changed due to the heat generated during welding (when electricity flows between the first electrodes 2A, 2B).

[Second power supply step]
When the current between the first electrodes 2A and 2B is stopped, the linear motion mechanism is driven to raise the first electrode 2A to eliminate the contact state between the board assembly 20 and the first electrode 2A (releasing the pressure force by the first positive electrode 2A), while the second electrode 3A is lowered to press against the upper surface of the board assembly 20, and the first electrode 2B is lowered to eliminate the contact state between the board assembly 20 and the first electrode 2B (releasing the pressure force by the first electrode 2B), while the second electrode 3B is raised to press against the lower surface of the board assembly 20 [see FIG. 2(c)]. At this time, the drive mechanism moves each of the electrodes 2A, 2B, 3A, and 3B up and down so that the contact state between the first electrodes 2A and 2B and the board assembly 20 is eliminated after the second electrodes 3A and 3B are pressed against the board assembly 20 (after the second electrodes 3A and 3B press the board assembly 20 in the plate thickness direction).

Next, while pressing the second electrodes 3A and 3B against the plate assembly 20 (while applying pressure to the plate assembly 20 in the plate thickness direction by the second electrodes 3A and 3B), a current (second current 5) is supplied to the plate assembly 20 via the positive second electrode 3A, as shown in FIG. 2(d). Since the pressure position of the plate assembly 20 by the second electrodes 3A and 3B and the current value of the second current 5 are adjusted as described above, the second current 5 supplied to the plate assembly 20 via the electrode 3A flows in the plate thickness direction within the plate assembly 20 to keep the heat-affected zone 12 formed around the nugget 11 of the plate assembly 20 warm (to prevent the heat-affected zone 12 from being rapidly cooled). This makes it possible to suppress or prevent large stresses from occurring at the boundary between the nugget 11 and the heat-affected zone 12 during the process in which the nugget 11, which is formed across the two steel plates 21 and 22, completely cools and solidifies, thereby preventing defects such as cracks and fissures caused by the above stress from occurring in the welded zone 10.

In this second power supply process, the second current 5 is not supplied to the plate assembly 20 to soften the completely solidified welded portion 10 (nugget 11), but is supplied to the plate assembly 20 to slow down the cooling rate of the heat-affected zone 12 around the nugget 11. Therefore, the current value of the second current 5 can be smaller than the current value of the first current 4, and the amount of the second current 5 supplied to the plate assembly 20 (the time during which current is passed between the second electrodes 3A and 3B) can be small. From the above, according to the spot welding method of the present invention, a high-precision welded portion 10 without defects such as cracks can be formed in the plate assembly 20 with little energy.

Although one embodiment of the present invention has been described above, the embodiment of the present invention is not limited to the above.

For example, the second electrodes 3A, 3B constituting the second electrode set 3 may be, instead of the cylindrical ones described above, arc-shaped ones arranged at intervals in the circumferential direction of the first electrodes 2A, 2B as shown in Figures 3(a) and (b). Figure 3(a) shows a case where four second electrodes 3A each having an arc shape are arranged at intervals in the circumferential direction, and Figure 3(b) shows a case where two second electrodes 3A each having an arc shape are arranged at intervals in the circumferential direction.

In the above, the first electrode 2A and the second electrode 3A arranged on the upper side of the board assembly 20 are both positive electrodes, and the first electrode 2B and the second electrode 3B arranged on the lower side of the board assembly 20 are both negative electrodes, but the polarity of the electrodes may be reversed. The polarities of the first electrode 2A and the second electrode 3A arranged on the upper side of the board assembly 20 may be different from each other, and the polarities of the first electrode 2B and the second electrode 3B arranged on the lower side of the board assembly 20 may be different from each other. As a specific example, the first electrodes 2A and 3A arranged on the upper side of the board assembly 20 are respectively positive and negative electrodes, and the second electrodes 2B and 3B arranged on the lower side of the board assembly 20 are respectively negative and positive electrodes. The polarity of each electrode can be selected according to the configuration of the board assembly 20.

Furthermore, if the second current 5 can be accurately passed through the heat-affected zone 12 in the second power supply process, the negative electrode of the second electrode set 3 (e.g., the electrode arranged on the lower side of the plate assembly 20) may also be the negative electrode (2B) of the first electrodes 2A, 2B constituting the first electrode set 2, as shown in FIG. 4.

In the embodiment described above, both of the two metal plates 21, 22 that make up the plate assembly 20 are high-strength steel plates, but the spot welding method and device according to the present invention can also be preferably applied to the plate assembly 20 that includes plated steel plates such as zinc-plated steel plates when forming a welded portion 10 that joins the metal plates together. In that case, the constituent elements of the plating can be prevented from agglomerating at the grain boundaries of the heat-affected zone 12, so that defects such as cracks caused by the agglomeration of the plating constituent elements can be prevented as much as possible from occurring in the welded portion 10.

The spot welding method according to the present invention can be applied not only to cases where it is performed automatically by a welding robot, but also to cases where a worker manually forms spot welds 10 in a plate assembly 20.

1 Spot welding device 2 First electrode set 2A First electrode (positive electrode)
2B First electrode 3 Second electrode set 3A Second electrode (positive electrode)
3B: second electrode 4: first current 5: second current 10: welded portion 11: nugget (weld metal)
12 Heat-affected zone 13 Molten metal 20 Plate assembly 21, 22 Steel plate (metal plate)

Claims (2)

When forming spot welds that join metal plates together in a plate assembly in which multiple metal plates are overlapped,
A first current supplying step is carried out in which a first current for forming a weld is supplied to the plate assembly via a first positive electrode that presses the plate assembly in a plate thickness direction, and then
a second current supply step of supplying a second current having a smaller current value than the first current to the plate assembly via a second positive electrode that applies pressure in the plate thickness direction to a predetermined position of the plate assembly that is radially outward from the pressure position applied by the first positive electrode while the plate assembly is not in contact with the first positive electrode. A spot welding device that forms spot welds that join a plurality of overlapping metal plates to each other in a plate assembly, comprising:
a first positive electrode capable of supplying a first current for forming a welded portion to the plate assembly while applying a pressure force in a plate thickness direction to the plate assembly; and a second positive electrode disposed radially outside the first positive electrode and capable of supplying a second current having a current value smaller than the first current to the plate assembly while applying a pressure force in the plate thickness direction to the plate assembly;
a linear motion mechanism that moves the first positive electrode and the second positive electrode back and forth along a thickness direction of the plate assembly so that when one of the first positive electrode and the second positive electrode comes into contact with the plate assembly, the other electrode is not in contact with the plate assembly.

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