Resistance spot welding method and resistance spot welding device

Abstract

The present disclosure relates to a resistance spot welding method that uses a resistance spot welding device to weld a workpiece formed by stacking a plurality of metal plates. A first energization control is performed on the pair of electrodes, and in the first energization control, a first current is caused to flow between the pair of electrodes that sandwich the workpiece. The pair of electrodes is then subjected to a second energization control in which the current flowing between the pair of electrodes is reduced from the first current to a second current that is smaller than the first current. Then, the pair of electrodes is subjected to third energization control, and in the third energization control, the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current. Then, the pair of electrodes is subjected to fourth energization control, and in the fourth energization control, a fourth current is caused to flow between the pair of electrodes.

Description

Technical Field

[0001] The present disclosure relates to a resistance spot welding method and a resistance spot welding apparatus. Background Art

[0002] Regarding resistance spot welding, a technique of applying a constant current before main energization is known in order to effectively form a nugget and suppress the generation of welding spatter. Japanese Patent Application Laid-Open No. 2021-079410 discloses applying a constant current for a certain period after initial energization in which the welding current gradually increases, and then performing energization control to apply a larger constant current. Summary of the Invention

[0003] Regarding resistance spot welding, when welding a workpiece formed by stacking a plurality of metal plates, the welding quality sometimes deteriorates due to differences in heating between the metal plates. Differences in resistance between the metal plates may cause differences in heating. For example, differences in the tensile strength and / or plate thickness between the metal plates may cause resistance differences.

[0004] If a heating difference occurs between the metal plates, it is possible to cause non-uniform nuggets to form in the workpiece, resulting in insufficient melting of a part of the metal plates.

[0005] Therefore, according to one aspect of the present disclosure, it is desirable to provide a technique that can achieve high-quality welding when performing resistance spot welding on a workpiece formed by stacking a plurality of metal plates.

[0006] According to one embodiment of the present disclosure, there is provided a resistance spot welding method for welding a workpiece formed by stacking a plurality of metal plates using a resistance spot welding apparatus. The resistance spot welding apparatus includes a pair of electrodes.

[0007] The resistance spot welding method includes: performing first energization control on the pair of electrodes, in which, with the workpiece in a state of being clamped by the pair of electrodes on both sides in the stacking direction of the plurality of metal plates, a first current is caused to flow between the pair of electrodes. The resistance spot welding method further includes: performing second energization control on the pair of electrodes, in which the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current. The second energization control may be performed after the first energization control.

[0008] The resistance spot welding method may further include: performing third energization control on the pair of electrodes, in which the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current. The third energization control may be performed after the second energization control.

[0009] The resistance spot welding method may further include: performing a fourth energization control on a pair of electrodes, in which a fourth current flows between the pair of electrodes. The fourth energization control may be performed after the third energization control. The fourth current may be a constant current. The first current may be a constant current smaller than the fourth current. The third current may be greater than the fourth current.

[0010] According to the above resistance spot welding method, the warping of the metal plates can be promoted through the first energization control and the second energization control. The warping helps to produce a state where the current conduction path is concentrated in a narrow area. In addition, through the third energization control, by temporarily applying a large current, the current can be made to shunt to the surface of the metal plates, thereby enabling large-area melting in the stacking direction. Through this melting, the difference in material properties in the stacking direction can be suppressed. Therefore, when the workpiece welding is completed in the fourth energization control, the difference in melting between the plurality of metal plates can be suppressed.

[0011] Therefore, according to the above resistance spot welding method, high-quality welding of a workpiece formed by stacking a plurality of metal plates can be achieved. Further, in the fourth energization control, good welding can be achieved only by controlling the current to the relatively small fourth current. Therefore, according to the above resistance spot welding method, both welding spatter can be suppressed and high-quality welding can be achieved.

[0012] According to one aspect of the present disclosure, the plurality of metal plates may include at least two metal plates having different resistances, tensile strengths, or plate thicknesses from each other. If the plurality of metal plates include metal plates having different resistances, due to the different resistances, there may be differences in heat generation and melting between the plurality of metal plates.

[0013] Metal plates with different strengths have different resistances from each other. Metal plates with different thicknesses also have different resistances from each other. Therefore, if the plurality of metal plates include metal plates having different tensile strengths or plate thicknesses, there may be differences in melting between the plurality of metal plates.

[0014] According to one aspect of the present disclosure, through the above first energization control, second energization control, third energization control, and fourth energization control, the difference in melting between the plurality of metal plates due to differences in resistance, tensile strength, or plate thickness can be suppressed. Thereby improving the welding quality.

[0015] According to one aspect of the present disclosure, the workpiece may include the following workpiece, wherein two metal plates located at both ends in the stacking direction among the plurality of metal plates have different resistances, tensile strengths, or plate thicknesses from each other. The above resistance spot welding method can effectively improve the welding quality of the above workpiece.

[0016] According to an aspect of the present disclosure, a workpiece may include a workpiece in which a plurality of metal plates are stacked in such a manner that the resistance, tensile strength, or plate thickness increases or decreases in the stacking direction. According to an aspect of the present disclosure, a workpiece may include a workpiece in which the tensile strength of a first metal plate located at a first end in the stacking direction among the plurality of metal plates is different from the tensile strength of a second metal plate located at the center in the stacking direction, and the tensile strength of a third metal plate located at a second end, which is the opposite side of the first end in the stacking direction, is less than the sum of the tensile strengths of the first metal plate and the second metal plate. The tensile strength of the second metal plate located at the center in the stacking direction is: the tensile strength of one metal plate sandwiched between the first metal plate and the third metal plate located at both ends in the stacking direction among the plurality of metal plates, or: the sum of the tensile strengths of one or more metal plates sandwiched between the first metal plate and the third metal plate. The above resistance spot welding method can improve the welding quality of workpieces that meet the above conditions.

[0017] According to an aspect of the present disclosure, the ratio H1 / H2 between the total plate thickness H1 and the thickness H2 of the metal plate with the smaller thickness among the two metal plates located at both ends in the stacking direction of the plurality of metal plates may be 3.5 or more, where the total plate thickness H1 is the sum of the thicknesses of the plurality of metal plates in the stacking direction of the workpiece. The above resistance spot welding method can improve the welding quality of workpieces that meet the above conditions.

[0018] According to an aspect of the present disclosure, a workpiece may include a workpiece having a strength difference of 445 MPa or more between both sides. According to an aspect of the present disclosure, a workpiece may include a workpiece having a strength difference of 255 MPa or more and a strength ratio of 4.29 or more between both sides. Here, the strength difference between both sides is: the difference in tensile strength between the two metal plates located at both ends in the stacking direction of the workpiece. The strength ratio is: the value obtained by dividing the sum of the tensile strengths of the plurality of metal plates in the workpiece by the tensile strength of the metal plate with the smaller tensile strength among the two metal plates located at both ends in the stacking direction. The above resistance spot welding method can improve the welding quality of workpieces that meet the above conditions.

[0019] According to an aspect of the present disclosure, the plurality of metal plates may include high-tensile strength steel plates. According to the resistance spot welding method of the present disclosure, the welding quality of workpieces including high-tensile strength steel plates can be improved.

[0020] According to an aspect of the present disclosure, a resistance spot welding device may be provided for welding a workpiece formed by stacking a plurality of metal plates. The resistance spot welding device may include a pair of electrodes and a control unit. The pair of electrodes may be configured to clamp the workpiece on both sides in the stacking direction of the plurality of metal plates.

[0021] The control unit can be configured to control the energization between a pair of electrodes. The control unit can perform a first energization control on the pair of electrodes, in which a first current flows between the pair of electrodes that hold the workpiece.

[0022] The control unit can perform a second energization control on the pair of electrodes, in which the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current. The second energization control can be connected after the first energization control.

[0023] The control unit can perform a third energization control on the pair of electrodes, in which the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current. The third energization control can be connected after the second energization control.

[0024] The control unit can perform a fourth energization control on the pair of electrodes, in which a fourth current flows between the pair of electrodes. The fourth energization control can be connected after the third energization control. The fourth current can be a constant current. The first current can be a constant current smaller than the fourth current. The third current can be larger than the fourth current.

[0025] According to the above resistance spot welding device, similar to the above resistance spot welding method, the welding quality can be improved. Brief Description of the Drawings

[0026] Figure 1 is a schematic diagram of the resistance spot welding device.

[0027] Figure 2 is a block diagram showing the electrical configuration of the resistance spot welding device.

[0028] Figure 3 is a diagram showing a typical current curve.

[0029] Figure 4 is a flowchart showing the welding process including current control.

[0030] Figure 5 is a diagram showing a specific example of the current curve for a specific workpiece.

[0031] Figure 6A is a schematic cross-sectional view of the workpiece for explaining the first stage of the initial energization; Figure 6B is a schematic cross-sectional view of the workpiece for explaining the second stage of the initial energization.

[0032] Figure 7A is a schematic cross-sectional view of the workpiece for explaining the melting condition under high current energization; Figure 7B and 7C is a cross-sectional view of the workpiece for explaining the growth of the nugget under the main energization.

[0033] Figure 8A 、 8B 8C is a diagram illustrating a modified example of the current curve.

[0034] Figure 9A 、 9B 9C is a diagram illustrating a modified example of the current curve. Detailed implementation mode

[0035] The following describes an exemplary embodiment of the present disclosure with reference to the accompanying drawings as an example.

[0036] Figure 1 The shown resistance spot welding device 1 is configured to weld a workpiece W formed by stacking a plurality of metal plates by resistance spot welding.

[0037] The workpiece W may include a plurality of steel plates as the plurality of metal plates. The workpiece W may include at least two steel plates having different resistances from each other. The workpiece W may include at least two steel plates having different tensile strengths from each other. The workpiece W may include at least two steel plates having different plate thicknesses from each other. The higher the tensile strength of the steel plate, the greater the resistance. The greater the plate thickness of the steel plate, the greater the resistance.

[0038] Figure 1 The shown exemplary workpiece W is a workpiece formed by stacking three steel plates, including a first steel plate P1, a second steel plate P2, and a third steel plate P3.

[0039] Not limited thereto, the first steel plate P1 may be, for example, a hot-dip galvanized steel plate. For example, the first steel plate P1 may be a high-tensile strength steel plate with a tensile strength of 440 MPa (megapascals) or more. For example, the first steel plate P1 may be a steel plate of the SCGA440 standard with a thickness of 1.4 mm.

[0040] Not limited thereto, the second steel plate P2 may be, for example, a hot-dip galvanized steel plate. For example, the second steel plate P2 may be a high-tensile strength steel plate with a tensile strength of 1180 MPa or more. For example, the second steel plate P2 may be a steel plate of the SCGA1180 standard with a thickness of 1.4 mm. The above high-strength steel plate with extremely high tensile strength is also called an ultra-high tensile strength material.

[0041] Not limited thereto, the third steel plate P3 may be, for example, a cold-rolled steel plate. For example, the third steel plate P3 may be a high-tensile strength steel plate with a tensile strength of 1470 MPa or more. For example, the third steel plate P3 may be a steel plate of the SPC1470 standard with a thickness of 2 mm.

[0042] Figure 1The exemplary workpiece W shown includes a first steel plate P1, a second steel plate P2, and a third steel plate P3 stacked in sequence. Hereinafter, the stacking direction represents the arrangement direction of the plurality of metal plates constituting the workpiece W. The stacking direction corresponds to the normal direction of the first steel plate P1, the second steel plate P2, and the third steel plate P3, as well as the normal direction of the workpiece surface. The stacking direction corresponds to the thickness direction of the first steel plate P1, the second steel plate P2, and the third steel plate P3, as well as the thickness direction of the workpiece.

[0043] The resistance spot welding device 1 has a resistance welding machine 20. The resistance welding machine 20 welds a plurality of metal plates configured as the workpiece W in the stacking direction by resistance spot welding.

[0044] The resistance welding machine 20 includes a first electrode 21 and a second electrode 22. The first electrode 21 is disposed below the workpiece W. The second electrode 22 is disposed to clamp the workpiece W together with the first electrode 21 in the stacking direction above the workpiece W. The first electrode 21 can move relative to the second electrode 22 in the vertical direction.

[0045] During welding, the first electrode 21 and the second electrode 22 are in contact with the workpiece W respectively. The first electrode 21 is in contact with the third steel plate P3, which is the metal plate located at the lowermost layer of the workpiece W. The second electrode 22 is in contact with the first steel plate P1, which is the metal plate located at the uppermost layer of the workpiece W. The first electrode 21 and the second electrode 22 clamp the workpiece W in a manner of pressing both sides of the workpiece W in the stacking direction. In this state, a welding current is provided between the first electrode 21 and the second electrode 22 and flows through the workpiece W. The workpiece W is welded by the resistance heat generated by the welding current.

[0046] As Figure 2 shown, the resistance spot welding device 1 includes a welding power source 30, a current sensor 40, and a control unit 50 as elements of the electrical system of the resistance welding machine 20.

[0047] The welding power source 30 is configured to provide a welding current between the first electrode 21 and the second electrode 22. The current sensor 40 is provided on the line between the welding power source 30 and the first electrode 21 or between the welding power source 30 and the second electrode 22, and the current sensor 40 is configured to detect the current I supplied between the first electrode 21 and the second electrode 22 and input the detection signal to the control unit 50. The current I is the above-mentioned welding current.

[0048] The control unit 50 controls the energization between the first electrode 21 and the second electrode 22 by controlling the welding power source 30. Specifically, the control unit 50 is configured to control the current I flowing between the first electrode 21 and the second electrode 22 as energization control so that the current I flowing between the first electrode 21 and the second electrode 22 follows Figure 3The current curve shown changes.

[0049] Specifically, the control unit 50 is configured to feedback-control the current I flowing between the first electrode 21 and the second electrode 22 based on the current I detected by the current sensor 40. Hereinafter, the first electrode 21 and the second electrode 22 are collectively referred to as a pair of electrodes 21, 22.

[0050] When a welding start instruction for the workpiece W is input through an operation unit (not shown), the control unit 50 starts Figure 4 the control process shown. In the control process, the control unit 50 controls the current I between the pair of electrodes 21, 22 so that the melting current changes according to Figure 3 the current curve shown. Through the above control, good resistance spot welding of the workpiece W can be achieved.

[0051] Specifically, in the period C1 from the welding start time point T0 of the workpiece W to the first time point T1 after a predetermined time, the control unit 50 performs first energization control (S110) on the pair of electrodes 21, 22. During C1, the first energization control is performed in such a way that the first current I1 flows between the pair of electrodes 21, 22 that hold the workpiece W. The first current I1 is a constant current.

[0052] Subsequent to the first energization control (S110), the control unit 50 performs second energization control (S120) on the pair of electrodes 21, 22 in the period C2 from the first time point T1 to the second time point T2. During the period C2, the second energization control is performed in such a way that the current I flowing between the pair of electrodes 21, 22 decreases from the first current I1 to the second current I2, where the second current I2 is less than the first current I1. The second current I2 is greater than zero.

[0053] Subsequent to the second energization control (S120), the control unit 50 performs third energization control (S130) on the pair of electrodes 21, 22 in the period C3 from the second time point T2 to the third time point T3 and in the period C4 from the third time point T3 to the fourth time point T4.

[0054] During the period C3, the third energization control is performed in such a way that the current I flowing between the pair of electrodes 21, 22 increases from the second current I2 to the third current I3, where the third current I3 is greater than the second current I2. During the period C4, the third energization control is performed in such a way that the current I flowing between the pair of electrodes 21, 22 maintains the third current I3.

[0055] Following the third energization control (S130), the control unit 50 performs fourth energization control (S140) on the pair of electrodes 21 and 22 during a period C5 from a fourth time point T4 to a fifth time point T5. During the period C5, the fourth energization control is performed in such a manner that a fourth current I4 smaller than the third current I3 flows between the pair of electrodes 21 and 22.

[0056] During the period C5, the fourth energization control is performed in such a manner that a constant current flows as the fourth current I4 between the pair of electrodes 21 and 22. As described above, during a period C1 from a welding start time point T0 to a first time point T1, the current between the pair of electrodes 21 and 22 is also controlled in such a manner that a constant current flows as the first current I1. However, for the following reasons, the first current I1 is smaller than the fourth current I4.

[0057] During periods C3 and C4 from a second time point T2 to a fourth time point T4, for the following reasons, the current between the pair of electrodes 21 and 22 is controlled in such a manner that a current larger than the fourth current I4 flows as the third current I3.

[0058] The control unit 50 continuously performs the above-described first energization control, second energization control, third energization control, and fourth energization control as a series of energization controls starting from the time point T0, and stops the energization between the pair of electrodes 21 and 22 at the fifth time point T5. Thereafter, the control process ends. At the fifth time point T5, the resistance spot welding of the workpiece W is completed.

[0059] The purpose of the energization control performed according to the above current curve will be described. In the present embodiment, the energization from the welding start time point T0 to the first time point T1 corresponds to the first stage of the initial energization, and its implementation purpose is to heat the workpiece W to a degree where the workpiece W will not melt by resistance heating in the workpiece W. The first current I1 and the duration of the period C1 from the time point T0 to the time point T1 are set within a range where the workpiece W will not melt.

[0060] Figure 5 A specific example of a current curve that can be adopted when the workpiece W is a workpiece (hereinafter referred to as the workpiece of interest) having a steel plate of SCGA440 standard with a thickness of 1.4 mm as the first steel plate P1, a steel plate of SCGA1180 standard with a thickness of 1.4 mm as the second steel plate P2, and a steel plate of SPC1470 standard with a thickness of 2 mm as the third steel plate P3 is shown. The pressing force between the pair of electrodes 21 and 21 is 5.51 kN (kiloNewton).

[0061] Figure 5 In, the horizontal axis represents time and the vertical axis represents the current I. Figure 5The time and current I at the origin are both set to zero, and the ratio of the durations between time points T0, T1, T2, T3, T4, and T5, as well as the ratio between currents I1, I2, I3, and I4, are shown in detail.

[0062] Figure 6A Schematically shows the situation where the inside of the workpiece W is heated in the first stage of the initial energization. Figure 6A The dashed line in shows schematically the heated part in the workpiece W in the case where the resistance of the second steel plate P2 is higher than that of the first steel plate P1 and the resistance of the third steel plate P3 is higher than that of the second steel plate P2. The part shown by the dashed line corresponds to the resistance center of the workpiece W. An example of the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3 includes the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3 in the workpiece of interest described above.

[0063] The first stage of the initial energization is not only for preheating the resistance center but also for causing warping of the plurality of metal plates constituting the workpiece W, thereby restricting the current path in the workpiece W to a narrow area.

[0064] The warping of the metal plate mentioned here means that the metal plate warps in such a way that the farther away from the center of the current path connecting the first electrode 21 and the second electrode 22, the more the adjacent metal plates in the stacking direction are separated in the stacking direction.

[0065] Figure 6A Shows that due to the warping of the first steel plate P1, the first steel plate P1 and the second steel plate P2 are separated from each other in the area far from the center of the current path. Similarly, Fig. 6 shows that due to the warping of the third steel plate P3, the second steel plate P2 and the third steel plate P3 are separated from each other in the area far from the center of the current path.

[0066] Since the above separation defines the current path, the current flows concentratedly inside the workpiece W. Hereinafter, current concentration means the situation where the current flows concentratedly in the area where the current is restricted in the workpiece W between a pair of electrodes 21 and 22 with an ideal current density.

[0067] The energization from the first time point T1 to the second time point T2 corresponds to the second stage of the initial energization, and its purpose is to heat the entire area in the workpiece W clamped by the first electrode 21 and the second electrode 22 by gradually inputting heat to the workpiece W.

[0068] Figure 6B The dashed line in conceptually shows the situation where in the second stage of the initial energization, heat diffuses from the resistance center and the entire area in the workpiece W clamped by the first electrode 21 and the second electrode 22 is heated.

[0069] This heating plays a role in stably forming the warping of the metal sheet. That is, by controlling the current from the first time point T1 to the second time point T2, a process of gradually decreasing the current I is set, so as to stably form the warping of the metal plate and achieve stable current concentration.

[0070] In the present embodiment, the control unit 50 controls the current flowing between the pair of electrodes 21 and 22 according to the detection signal from the current sensor 40. However, if the warping of the metal plate cannot be stably achieved, the current density when the current flows along the stacking direction of the workpiece W will change, and thus the heating method will also change. Therefore, in order to achieve a stable current density, it is very important to perform the downslope power-on control (S120) between the first time point T1 and the second time point T2.

[0071] The power-on from the second time point T2 to the fourth time point T4 is to apply a large current to cause the current to shunt to the surface of the metal plate with relatively low resistance, so as to achieve overall heating and melting of the workpiece W from the upper surface to the lower surface between the pair of electrodes 21 and 22. Hereinafter, the period C3 and C4 from the second time point T2 to the fourth time point T4 are represented by the large current range L.

[0072] When there is a steel plate with relatively low resistance, such as the first steel plate P1 of the workpiece of interest, on the surface of the workpiece W, if a large current is not applied, sufficient Joule heat may not be generated on the low-resistance steel plate, which may lead to insufficient welding of the first steel plate P1. By applying a large current, the decline in welding quality due to insufficient heating of the steel plate with relatively low resistance can be suppressed.

[0073] Figure 7A It is shown that by applying a large current in the third power-on control, extensive melting is generated between the pair of electrodes 21 and 22, and thus the situation where material melting occurs between the steel plates adjacent to each other in the first steel plate P1, the second steel plate P2, and the third steel plate P3. Through melting, the resistance difference between the metal plates is reduced.

[0074] The power-on from the fourth time point T4 to the fifth time point T5 corresponds to the main power-on for completing the welding of the workpiece W. The fourth current I4 and the duration of the period C5 from the time point T4 to the time point T5 (i.e., the main power-on time) are set such that a suitable nugget is formed in the workpiece W.

[0075] Through the main power-on, the melting in the workpiece W spreads from the resistance center to the surroundings, and a suitable nugget is formed in the workpiece W. Since the melting generated by the previous large current alleviates the resistance difference, the melting in the workpiece W is more uniform than when there is no third power-on control. Thus, a suitable nugget can be formed in the workpiece W.

[0076] Figure 7B and Figure 7C shows the case where the nugget gradually increases through the main energization. As Figure 7C shown, good welding can be achieved by the overall expansion of the nugget through multiple metal plates (the first steel plate P1, the second steel plate P2, and the third steel plate P3) in the workpiece W.

[0077] Furthermore, in the present embodiment, since current concentration is achieved, effective heating and melting of the metal plates can be realized, and thus a suitable nugget can be formed with a relatively small current.

[0078] Furthermore, in the present embodiment, since current concentration can be stably achieved in the process up to the second stage of the initial energization, the current margin of the main energization can be increased. That is to say, as the fourth current I4 for achieving sufficient welding, a relatively wide current range can be allowed. Figure 5 The thick arrow attached to the fourth current I4 in the figure indicates that the margin of the fourth current I4 is wide.

[0079] Furthermore, in the present embodiment, through the third energization control, a large range of melting can be achieved in the stacking direction of the workpiece W. Therefore, in the main energization, welding spatter can be suppressed with a small current, and thus good welding can be achieved.

[0080] The above has described the resistance spot welding apparatus 1 and the resistance spot welding method of the present embodiment, and this welding method plays a beneficial role when welding a workpiece W formed by stacking a plurality of metal plates having different resistances from each other. Examples of the plurality of metal plates having different resistances from each other include a plurality of metal plates having different tensile strengths from each other and a plurality of metal plates having different plate thicknesses from each other.

[0081] When there is a resistance difference between the metal plates in the workpiece W, the resistance heating at the low-resistance part is lower than that at other parts. In the present embodiment, a large current can promote the current to shunt to the metal plate with a relatively low resistance. In addition, the large current can also melt the material between adjacent metal plates, thereby reducing the resistance difference. Therefore, a plurality of metal plates having different resistances from each other can be welded well.

[0082] When the workpiece W having three metal plates exemplified by the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3 satisfies at least one of the first condition, the second condition, the third condition, and the fourth condition represented by the following inequalities, the resistance spot welding method including the current curve of the large current interval L is particularly effective.

[0083] First condition: X1 < X3.

[0084] Second condition: X1 > X2 and (X1 + X2) > X3.

[0085] Third condition: X1 < X2 and (X1 + X2) > X3

[0086] Fourth condition: X1 ≥ X2 > X3.

[0087] Here, it is assumed that the first metal plate in the workpiece W is a metal plate with a tensile strength of X1, the second metal plate has a tensile strength of X2, and the third metal plate is a metal plate with a tensile strength of X3.

[0088] The first metal plate and the third metal plate are the metal plates located at both ends of the workpiece W in the stacking direction. The second metal plate is the metal plate sandwiched between the first metal plate and the third metal plate. The first metal plate corresponds to Figure 1 the first steel plate P1 shown in. The second metal plate corresponds to the second steel plate P2. The third metal plate corresponds to the third steel plate P3. However, the first metal plate, the second metal plate, and the third metal plate may also correspond to the third steel plate P3, the second steel plate P2, and the first steel plate P1 in sequence. That is, when understanding the first condition, the second condition, the third condition, and the fourth condition, it can be understood that the first metal plate, the second metal plate, and the third metal plate are stacked from top to bottom, or it can be understood that the first metal plate, the second metal plate, and the third metal plate are stacked from bottom to top. In other words, the first metal plate and the tensile strength X1 can be understood as the third metal plate and the tensile strength X3, and the third metal plate and the tensile strength X3 can be understood as the first metal plate and the tensile strength X1.

[0089] The first condition is as follows: the tensile strength X1 of the first metal plate located at one end of the workpiece W is less than the tensile strength X3 of the third metal plate located at the other end of the workpiece W. In other words, the condition is that the two metal plates at both ends of the workpiece W have different tensile strengths X1 and X3.

[0090] The second condition is as follows: the tensile strength X1 of the first metal plate located at the first end of the workpiece W in the stacking direction is greater than the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end of the workpiece W in the stacking direction is less than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate. The second end of the workpiece W in the stacking direction is the end opposite to the first end of the workpiece W in the stacking direction.

[0091] The third condition is as follows: the tensile strength X1 of the first metal plate located at the first end of the workpiece W is less than the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end of the workpiece W is less than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate.

[0092] The second condition and the third condition can be summarized as follows.

[0093] X1≈X2 and (X1 + X2) > X3

[0094] That is to say, the second and third conditions are as follows: the tensile strength X1 of the first metal plate located at the first end of the workpiece W is different from the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end of the workpiece W is less than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate.

[0095] The fourth condition is as follows: a plurality of metal plates are stacked in such a manner that the tensile strength increases or decreases in the stacking direction. The stacking direction mentioned here includes the direction from the first metal plate to the third metal plate and the direction from the third metal plate to the first metal plate.

[0096] The tensile strengths X1, X2, and X3 of the first condition, the second condition, the third condition, and the fourth condition can also be understood as resistances X1, X2, and X3. The tensile strengths X1, X2, and X3 can also be understood as plate thicknesses X1, X2, and X3. The greater the tensile strength, the greater the resistance. The greater the plate thickness, the greater the tensile strength.

[0097] When the first condition is satisfied, due to the influence of the third metal plate with a relatively large resistance, the resistance center of the workpiece W shifts toward the third metal plate side in the stacking direction of the workpiece W. When the fourth condition is satisfied, due to the influence of the first metal plate with a relatively large resistance, the resistance center of the workpiece W shifts toward the first metal plate side in the stacking direction of the workpiece W.

[0098] Therefore, if a conventional welding method is used for the workpiece W that satisfies any one of the above conditions and the nugget is simply enlarged from the resistance center, this method may cause insufficient welding of the metal plates located at the ends of the workpiece W.

[0099] According to the present embodiment, as Figure 7A shown, by applying a large current, the first metal with a relatively low resistance can also be melted, thereby ensuring the welding of the workpiece W. Therefore, welding spatter can be suppressed.

[0100] When the workpiece W is formed by stacking four or more metal plates, the above first condition, second condition, third condition, and fourth condition can also be applied. When "formed by stacking multiple metal plates" is used instead of "the second metal plate", and the sum of the tensile strengths of the metal plates that make up "formed by stacking multiple metal plates" is used instead of the tensile strength X2 of the second metal plate, if the above conditions are met, the resistance spot welding method of the present embodiment can play an effective role in achieving good welding of the workpiece W. When the workpiece W is formed by stacking four or more metal plates, the "second metal plate located at the center of the workpiece W" in the second condition and the third condition is one or more metal plates sandwiched between two metal plates located at both ends of the workpiece W. In this case, the "tensile strength X2 of the second metal plate located at the center of the workpiece W" is the sum of the tensile strengths of one or more metal plates sandwiched between two metal plates located at both ends of the workpiece W.

[0101] Through welding tests in various environments, it has been found that the resistance spot welding method of the present embodiment is also helpful for achieving proper welding of the workpiece W when the workpiece W satisfies any one of the following fifth condition and sixth condition.

[0102] Fifth condition: The strength difference ΔX between the two sides is 445 MPA or more.

[0103] Sixth condition: The strength difference ΔX between the two sides is 255 MPA or more, and the strength ratio R is 4.29 or more.

[0104] The strength difference ΔX between the two sides mentioned here refers to the difference in the tensile strengths of the metal plates located at both ends of the workpiece W in the stacking direction. That is to say, the strength difference ΔX between the two sides is the difference in the tensile strengths of the two metal plates contacted by a pair of electrodes 21 and 22 in the workpiece W. Using the above tensile strengths X1, X2, and X3, the strength difference ΔX between the two sides can be expressed by the formula ΔX = |X1 - X3|.

[0105] The strength ratio R corresponds to the ratio R = R1 / R2 between the sum R1 of the tensile strengths of the multiple metal plates that make up the workpiece W in the stacking direction and the tensile strength R2 of the metal plate with the smaller tensile strength among the two metal plates located at both ends of the multiple metal plates in the stacking direction. Using the above tensile strengths X1, X2, and X3, R1 can be expressed by the formula R1 = (X1 + X2 + X3), and R2 can be expressed by the formula R2 = min{X1, X3} using the MIN function. In this case, the strength ratio R = (X1 + X2 + X3) / min{X1, X3}.

[0106] The greater the strength difference ΔX between the two sides, the slower the melting progress of the metal plate with relatively lower tensile strength due to the smaller resistance. The greater the strength ratio R, the greater the offset of the fusion core. Therefore, the resistance spot welding method of the present embodiment has high utilization value for the workpiece W that satisfies the above fifth condition or sixth condition.

[0107] When welding a workpiece W whose plate thickness satisfies the following seventh condition, the resistance spot welding method of the present embodiment can also basically function effectively.

[0108] Seventh condition: The plate thickness ratio H is 3.5 or more.

[0109] The thickness ratio H mentioned here refers to the ratio H1 / H2 between the total plate thickness H1 and the thickness H2 of the thinner of the two metal plates at both ends in the stacking direction among the multiple metal plates. Here, the total plate thickness H1 is the sum of the thicknesses of the multiple metal plates constituting the workpiece W in the stacking direction.

[0110] [Modification example]

[0111] The current curve is not limited to Figure 3 the example shown, and can also be changed to Figure 8A , Figure 8B , Figure 8C , Figure 9A , Figure 9B and Figure 9C any one of the current curves in. By comparing Figure 3 and Figure 8A , 8B , 8C, 9A, 9B, 9C, it can be understood that the initial energization including the downhill and the large current before the main energization are particularly beneficial for achieving good welding.

[0112] In Figure 8A the current curve shown, the current waveform in the large current interval L1 is not a trapezoid as Figure 3 shown, but a triangle with an uphill and a sharp peak. In Figure 8B the current curve shown, the current waveform in the large current interval L2 is a rectangle. In Figure 8C the current curve shown, the current waveform in the large current interval L3 is a triangle with a downhill starting from the peak.

[0113] In Figure 9A the current curve shown, the current waveform in the large current interval L4 is a trapezoid with an uphill and a downhill. In Figure 9B the current curve shown, the current waveform in the large current interval L5 is a triangle with an uphill and a downhill and a sharp peak. In Figure 9C the current curve shown, the current waveform in the large current interval L6 is a trapezoid starting from the peak.

[0114] [Other embodiments]

[0115] The present disclosure is not limited to the above embodiments and can take various forms. For example, the above resistance spot welding device 1 and resistance spot welding method can also be used to weld a workpiece W formed by stacking two metal plates.

[0116] The functions of one component in the above embodiments can be shared by multiple components, or the functions of multiple components can be integrated into one component. In addition, a part of the configuration of the above embodiments can be omitted. In addition, at least a part of the configuration of the above embodiments can be added to the configuration of the above other embodiments, or at least a part of the configuration of the above embodiments can be replaced with the configuration of the above other embodiments, etc. All modes included in the technical idea determined by the statements recorded in the claims are embodiments of the present disclosure.

[0117] [Technical idea disclosed in this specification]

[0118] It can be understood that this specification discloses the following technical idea.

[0119] [Item 1]

[0120] A resistance spot welding method, in which a resistance spot welding device having a pair of electrodes is used to weld a workpiece formed by stacking a plurality of metal plates. The resistance spot welding method is characterized by including the following steps:

[0121] Perform a first power-on control on the pair of electrodes. In the first power-on control, with the workpiece being clamped by the pair of electrodes on both sides in the stacking direction of the plurality of metal plates, a first current is made to flow between the pair of electrodes;

[0122] Perform a second power-on control on the pair of electrodes. In the second power-on control, following the first power-on control, the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current;

[0123] Perform a third power-on control on the pair of electrodes. In the third power-on control, following the second power-on control, the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current; and

[0124] Perform a fourth power-on control on the pair of electrodes. In the fourth power-on control, following the third power-on control, a fourth current is made to flow between the pair of electrodes, and

[0125] the fourth current is a constant current,

[0126] the first current is a constant current smaller than the fourth current,

[0127] the third current is larger than the fourth current.

[0128] [Item 2]

[0129] The resistance spot welding method according to Item 1, characterized in that

[0130] the plurality of metal plates include at least two metal plates having different resistances, tensile strengths or plate thicknesses from each other.

[0131] [Item 3]

[0132] The resistance spot welding method according to Item 1 or 2, characterized in that

[0133] the workpiece includes the following workpiece, wherein two metal plates at both ends in the stacking direction among the plurality of metal plates have different resistances, tensile strengths or plate thicknesses from each other.

[0134] [Item 4]

[0135] The resistance spot welding method according to any one of Items 1 to 3, characterized in that

[0136] the workpiece includes at least one of the following workpieces:

[0137] (i) A workpiece, wherein the plurality of metal plates are stacked together in such a manner that the resistance, tensile strength or plate thickness increases or decreases in the stacking direction; and

[0138] (ii) A workpiece, wherein the tensile strength of a first metal plate at a first end in the stacking direction among the plurality of metal plates is different from the tensile strength of a second metal plate at the center in the stacking direction, and the tensile strength of a third metal plate at a second end, which is the opposite side of the first end in the stacking direction, is less than the sum of the tensile strengths of the first metal plate and the second metal plate. The tensile strength of the second metal plate at the center in the stacking direction is: the tensile strength of one metal plate sandwiched between the first metal plate and the third metal plate at both ends in the stacking direction among the plurality of metal plates, or: the sum of the tensile strengths of more than one metal plate sandwiched between the first metal plate and the third metal plate.

[0139] [Item 5]

[0140] The resistance spot welding method according to any one of Items 1 to 4, characterized in that

[0141] the ratio H1 / H2 between the total plate thickness H1 and the thickness H2 of the metal plate with the smaller thickness among the two metal plates at both ends in the stacking direction of the plurality of metal plates is 3.5 or more, where the total plate thickness H1 is the sum of the thicknesses of the plurality of metal plates in the stacking direction in the workpiece.

[0142] [Item 6]

[0143] The resistance spot welding method according to any one of Items 1 to 5, characterized in that

[0144] the workpiece includes at least one of a workpiece with a strength difference of 445 MPa or more between both sides and a workpiece with a strength difference of 255 MPa or more between both sides and a strength ratio of 4.29 or more;

[0145] the strength difference between both sides is: the difference in tensile strength between two metal plates at both ends of the workpiece in the stacking direction, and the strength ratio is: the value obtained by dividing the sum of the tensile strengths of the plurality of metal plates in the workpiece by the tensile strength of the metal plate with the smaller tensile strength among the two metal plates at both ends in the stacking direction.

[0146] [Item 7]

[0147] The resistance spot welding method according to any one of Items 1 to 6, characterized in that

[0148] the plurality of metal plates include high-tensile strength steel plates.

[0149] [Item 8]

[0150] A resistance spot welding device for welding a workpiece formed by stacking a plurality of metal plates, the resistance spot welding device is characterized by comprising:

[0151] a pair of electrodes configured to clamp the workpiece on both sides in the stacking direction of the plurality of metal plates; and

[0152] a control unit configured to control energization between the pair of electrodes, and

[0153] the control unit performs the following control:

[0154] perform a first energization control on the pair of electrodes, in which a first current flows between the pair of electrodes clamping the workpiece;

[0155] perform a second energization control on the pair of electrodes, in which, following the first energization control, the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current;

[0156] perform a third energization control on the pair of electrodes, in which, following the second energization control, the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current; and

[0157] Perform a fourth energization control on the pair of electrodes, in which, following the third energization control, a fourth current flows between the pair of electrodes.

[0158] The fourth current is a constant current.

[0159] The first current is a constant current that is less than the fourth current.

[0160] The third current is greater than the fourth current.

Claims

1. A resistance spot welding method, in which a resistance spot welding device having a pair of electrodes is used to weld a workpiece formed by stacking a plurality of metal plates, the resistance spot welding method being characterized in that it comprises the following steps: performing a first energization control on the pair of electrodes, in which a first current is caused to flow between the pair of electrodes while the workpiece is clamped by the pair of electrodes on both sides in the stacking direction of the plurality of metal plates; performing a second energization control on the pair of electrodes, in which, after the first energization control, the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current; performing a third energization control on the pair of electrodes, in which, following the second energization control, the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current; and The pair of electrodes is subjected to a fourth energization control, in which, after the third energization control, a fourth current is caused to flow between the pair of electrodes, and The fourth current is a constant current, The first current is a constant current smaller than the fourth current, The third current is greater than the fourth current.

2. The resistance spot welding method according to claim 1, characterized in that: The plurality of metal plates include at least two metal plates having different resistance, tensile strength or plate thickness from each other.

3. The resistance spot welding method according to claim 1 or claim 2, characterized in that: The workpiece includes a workpiece in which two metal plates located at both ends of the stacking direction among the plurality of metal plates have different electrical resistance, tensile strength, or plate thickness from each other.

4. The resistance spot welding method according to any one of claims 1 to 3, characterized in that: The workpiece includes at least one of the following workpieces: (i) A workpiece, wherein the plurality of metal plates are stacked together in such a manner that the electrical resistance, tensile strength or plate thickness increases or decreases in the stacking direction; and (ii) A workpiece, wherein the tensile strength of a first metal plate located at a first end of the plurality of metal plates in the stacking direction is different from the tensile strength of a second metal plate located in the center of the stacking direction, and the tensile strength of a third metal plate located at the end portion, i.e., the second end, on the opposite side of the first end in the stacking direction is less than the sum of the tensile strength of the first metal plate and the tensile strength of the second metal plate, and the tensile strength of the second metal plate located in the center of the stacking direction is: the tensile strength of a metal plate sandwiched between the first metal plate and the third metal plate located at both ends of the stacking direction among the plurality of metal plates, or is: the sum of the tensile strengths of more than one metal plate sandwiched between the first metal plate and the third metal plate among the plurality of metal plates.

5. The resistance spot welding method according to any one of claims 1 to 4, characterized in that: The ratio H1 / H2 between the total plate thickness H1 and the thickness H2 of the smaller metal plate among the two metal plates located at the two ends of the stacking direction is greater than 3.5, wherein the total plate thickness H1 is the sum of the thicknesses of the multiple metal plates in the workpiece in the stacking direction.

6. The resistance spot welding method according to any one of claims 1 to 5, characterized in that: The workpiece includes at least one of a workpiece having a strength difference of 445 MPa or more on both sides and a workpiece having a strength difference of 255 MPa or more on both sides and a strength ratio of 4.29 or more. The strength difference on both sides is: the difference in tensile strength of the two metal plates located at the two ends of the stacking direction in the workpiece, and the strength ratio is: the value obtained by dividing the sum of the tensile strengths of the multiple metal plates in the workpiece by the tensile strength of the metal plate with smaller tensile strength among the two metal plates located at the two ends of the stacking direction.

7. The resistance spot welding method according to any one of claims 1 to 6, characterized in that: The plurality of metal plates include high tensile strength steel plates.

8. A resistance spot welding device for welding a workpiece formed by stacking a plurality of metal plates, the resistance spot welding device being characterized by comprising: a pair of electrodes configured to clamp the workpiece on both sides of the stacking direction of the plurality of metal plates; and a control unit configured to control the conduction of electricity between the pair of electrodes; and The control unit performs the following control: performing a first energization control on the pair of electrodes, in which a first current is caused to flow between the pair of electrodes that clamp the workpiece; performing a second energization control on the pair of electrodes, in which, subsequent to the first energization control, the current flowing between the pair of electrodes is reduced from the first current to a second current smaller than the first current; performing a third energization control on the pair of electrodes, in which, following the second energization control, the current flowing between the pair of electrodes is increased from the second current to a third current larger than the second current; and performing a fourth energization control on the pair of electrodes, in which, after the third energization control, a fourth current is caused to flow between the pair of electrodes, The fourth current is a constant current, The first current is a constant current smaller than the fourth current, The third current is greater than the fourth current.

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