Resistance spot welding method and resistance spot welding apparatus

Abstract

To provide a technique that can achieve high-quality welding, in resistance spot welding to a work-piece.SOLUTION: A work-piece having a plurality of metal plates overlapped with each other is welded using a resistance spot welding device. First energization control is performed to a pair of electrodes so that first current flow between the pair of electrodes holding the work-piece. Subsequently, second energization control is performed to the pair of electrodes so that the current flowing between the pair of electrodes reduces from the first current to second current smaller than the first current. Subsequently, third energization control is performed to the pair of electrodes so that the current flowing between the pair of electrodes increases from the second current to third current larger than the second current. Subsequently, fourth energization control is performed to the pair of electrodes so that fourth current flows between the pair of electrodes.SELECTED DRAWING: Figure 3

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, in order to efficiently form a weld nugget while suppressing the generation of spatter, a technique of flowing a constant current before this energization is already known. Patent Document 1 discloses performing energization control so as to flow a constant current for a certain period after an initial energization in which the welding current is gradually increased, and then flowing an even larger constant current.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Regarding resistance spot welding, when welding a workpiece in which a plurality of metal plates are overlapped, the welding quality may deteriorate due to variations in heat generation between the metal plates. Variations in heat generation can occur due to differences in resistance between the metal plates. The resistance difference can occur, for example, due to differences in tensile strength or plate thickness between the metal plates.

[0005] When there are variations in heat generation between the metal plates, non-uniform weld nuggets are formed in the workpiece, and insufficient penetration may occur for some of the metal plates.

[0006] Therefore, according to one aspect of the present disclosure, it is desirable to provide a technique capable of achieving high-quality welding with respect to resistance spot welding of a workpiece in which a plurality of metal plates are overlapped.

Means for Solving the Problems

[0008] A resistance spot welding method includes performing a first current control on a pair of electrodes so that a first current flows between the pair of electrodes that sandwich the workpiece. The resistance spot welding method may further include performing a second current control on the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. The second current control may be performed following the first current control.

[0009] The resistance spot welding method may further include performing a third current control on a pair of electrodes such that the current flowing through the pair of electrodes increases from a second current to a third current greater than the second current. The third current control may be performed following the second current control.

[0010] The resistance spot welding method may further include a fourth current control to a pair of electrodes such that a fourth current flows between the pair of electrodes. The fourth current control may be performed following the third current 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 a current larger than the fourth current.

[0011] This resistance spot welding method allows for the warping of the metal plate through the first and second current control mechanisms, concentrating the current path in a narrow area. Furthermore, the third current control mechanism, by temporarily applying a large current, promotes current distribution to the metal plate surface, enabling melting over a wide area in the lamination direction. This melting also suppresses variations in material properties in the lamination direction, thus reducing variations in melting between multiple metal plates when finishing the welding of the workpiece in the fourth current control mechanism.

[0012] Therefore, this resistance spot welding method makes it possible to achieve high-quality welding on workpieces where multiple metal plates are stacked. Furthermore, because a third current control is performed, the fourth current in the fourth current control can be kept relatively small to achieve good welding. Thus, this resistance spot welding method makes it possible to achieve high-quality welding while suppressing spatter.

[0013] According to one aspect of this disclosure, the plurality of metal plates may include at least two metal plates having different resistances, tensile strengths, or thicknesses. When the plurality of metal plates include metal plates with different resistances, variations in heat generation and melting may occur among the plurality of metal plates due to these differences in resistance.

[0014] Metal sheets with different strengths will have different resistances to each other. Metal sheets with different thicknesses will also have different resistances to each other. Therefore, if a group of metal sheets includes sheets with different tensile strengths or thicknesses, variations in melting may occur among the metal sheets.

[0015] According to one aspect of this disclosure, the first, second, third, and fourth energization controls described above can suppress variations in melting between multiple metal plates caused by differences in resistance, tensile strength, or plate thickness, thereby improving welding quality.

[0016] According to one aspect of this disclosure, a workpiece may include a workpiece in which two metal plates located at opposite ends in the lamination direction have different resistance, tensile strength, or thickness from each other. The resistance spot welding method described above is effective in improving the welding quality for such workpieces.

[0017] According to one aspect of this disclosure, a workpiece may include a workpiece in which multiple metal plates are stacked such that resistance, tensile strength, or plate thickness increases or decreases in the stacking direction. According to one aspect of this disclosure, a workpiece may include a workpiece in which, among the multiple metal plates, the tensile strength of a first metal plate located at the first end in the stacking direction is different from the tensile strength of a second metal plate located in the center in the stacking direction, and the tensile strength of a third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is less than the sum of the tensile strengths of the first and second metal plates, and the tensile strength of a second metal plate located in the center in the stacking direction is the tensile strength of one metal plate sandwiched between the first and third metal plates located at both ends in the stacking direction, or the sum of the tensile strengths of one or more metal plates sandwiched between the first and third metal plates. The resistance spot welding method described above can improve the welding quality for workpieces that satisfy such conditions.

[0018] According to one aspect of this disclosure, the ratio H1 / H2 between the total plate thickness H1, which is the sum of the thicknesses of multiple metal plates in the stacking direction of a workpiece, and the thickness H2 of the thinner of the two metal plates located at both ends in the stacking direction of the multiple metal plates, may be 3.5 or greater. The above-described resistance spot welding method can improve the welding quality for workpieces that satisfy such conditions.

[0019] According to one aspect of this disclosure, the workpiece may include a workpiece having a difference in strength between its two ends of 445 MPa or more. According to another aspect of this disclosure, the workpiece may include a workpiece having a difference in strength between its two ends of 255 MPa or more and a strength ratio of 4.29 or more. Here, the difference in strength between its two ends is the difference in tensile strength between two metal plates located at both ends of the workpiece in the lamination direction, and the strength ratio is the value obtained by dividing the sum of the tensile strengths of multiple metal plates in the workpiece by the tensile strength of the metal plate with the lower tensile strength among the two metal plates located at both ends of the lamination direction. The resistance spot welding method described above can improve the welding quality for workpieces that satisfy these conditions.

[0020] According to one aspect of the present disclosure, the plurality of metal plates may include high-tensile steel plates. According to the resistance spot welding method of the present disclosure, the welding quality for a workpiece including high-tensile steel plates can be improved.

[0021] According to one aspect of the present disclosure, a resistance spot welding apparatus for welding a workpiece in which a plurality of metal plates are stacked may be provided. The resistance spot welding apparatus may include a pair of electrodes and a control unit. The pair of electrodes may be arranged so as to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates.

[0022] The control unit may be configured to control energization between the pair of electrodes. The control unit may execute first energization control for the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the workpiece.

[0023] The control unit may execute second energization control for the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. The second energization control may be performed following the first energization control.

[0024] The control unit may execute third energization control for the pair of electrodes so that the current flowing through the pair of electrodes increases from the second current to a third current larger than the second current. The third energization control may be performed following the second energization control.

[0025] The control unit may execute fourth energization control for the pair of electrodes so that a fourth current flows between the pair of electrodes. The fourth energization control may be performed following 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 a current larger than the fourth current.

[0026] According to this resistance spot welding apparatus, similarly to the resistance spot welding method described above, the welding quality can be improved.

Brief Description of the Drawings

[0027] [Figure 1] This is a schematic diagram of a resistance spot welding machine. [Figure 2] This is a block diagram showing the electrical configuration of a resistance spot welding machine. [Figure 3] This graph shows a typical current profile. [Figure 4] This is a flowchart illustrating the welding process, including current control. [Figure 5] This graph shows a specific example of the current profile for a particular workpiece. [Figure 6] Figure 6A is a schematic cross-sectional view of the workpiece illustrating the first stage of initial energization, and Figure 6B is a schematic cross-sectional view of the workpiece illustrating the second stage of initial energization. [Figure 7] Figure 7A is a schematic cross-sectional view of a workpiece illustrating melting under high current application, and Figures 7B and 7C are cross-sectional views of a workpiece illustrating the growth of the weld nugget under this current application. [Figure 8] Figures 8A, 8B, and 8C illustrate variations of the current profile. [Figure 9] Figures 9A, 9B, and 9C illustrate variations of the current profile. [Modes for carrying out the invention]

[0028] Exemplary embodiments of the present disclosure are described below with reference to the drawings. The resistance spot welding apparatus 1 shown in Figure 1 is configured to weld a workpiece W, which consists of multiple overlapping metal plates, using resistance spot welding.

[0029] Workpiece W may comprise multiple steel plates as multiple metal plates. Workpiece W may comprise at least two steel plates with different resistances. Workpiece W may comprise at least two steel plates with different tensile strengths. Workpiece W may comprise at least two steel plates with different thicknesses. Steel plates with higher tensile strength have greater resistance. Steel plates with greater thickness have greater resistance. Resistance as used herein refers to electrical resistance.

[0030] The exemplary workpiece W shown in Figure 1 is a workpiece in which three steel plates are stacked, comprising a first steel plate P1, a second steel plate P2, and a third steel plate P3.

[0031] While not limited to these, the first steel sheet P1 could be, for example, a hot-dip galvanized steel sheet. For example, the first steel sheet P1 could be a high-tensile steel sheet with a tensile strength of 440 MPa (megapascals) or more. For example, the first steel sheet P1 could be a 1.4 mm thick SCGA440 standard steel sheet.

[0032] While not limited to these, the second steel sheet P2 could be, for example, a hot-dip galvanized steel sheet. For example, the second steel sheet P2 could be a high-tensile steel sheet with a tensile strength of 1180 MPa or higher. For example, the second steel sheet P2 could be a 1.4 mm thick steel sheet conforming to the SCGA1180 standard. Such high-tensile steel sheets with extremely high tensile strength are also called ultra-high-tensile steel.

[0033] While not limited to these, the third steel sheet P3 could be, for example, a cold-rolled steel sheet. For example, the third steel sheet P3 could be a high-tensile steel sheet with a tensile strength of 1470 MPa or more. For example, the third steel sheet P3 could be a steel sheet conforming to the SPC1470 standard with a thickness of 2 mm.

[0034] The exemplary workpiece W shown in Figure 1 consists of a first steel plate P1, a second steel plate P2, and a third steel plate P3 stacked in sequence. Hereinafter, the direction in which the multiple metal plates constituting the workpiece W are aligned will be referred to as the stacking direction. 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 also 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.

[0035] The resistance spot welding apparatus 1 includes a resistance welding machine 20. The resistance welding machine 20 welds multiple metal plates, arranged as workpieces W, in the stacking direction by resistance spot welding.

[0036] The resistance welding machine 20 comprises a first electrode 21 and a second electrode 22. The first electrode 21 is positioned below the workpiece W. The second electrode 22 is positioned above the workpiece W, together with the first electrode 21, so as to sandwich the workpiece W in the stacking direction. The first electrode 21 is movable in the vertical direction relative to the second electrode 22.

[0037] The first electrode 21 and the second electrode 22 each contact the workpiece W during welding. The first electrode 21 contacts the third steel plate P3, which is the bottommost metal plate of the workpiece W. The second electrode 22 contacts the first steel plate P1, which is the topmost metal plate of the workpiece W. The first electrode 21 and the second electrode 22 clamp the workpiece W so as to apply pressure to both sides in the stacking direction. In this state, a welding current is supplied between the first electrode 21 and the second electrode 22 and flows through the workpiece W. The workpiece W is welded by the resistance heating generated by the welding current.

[0038] The resistance spot welding apparatus 1, as an electrical system for the resistance welding machine 20, includes a welding power supply 30, a current sensor 40, and a control unit 50, as shown in Figure 2. The welding power supply 30 is configured to supply welding current between the first electrode 21 and the second electrode 22. The current sensor 40 is installed in the line between the welding power supply 30 and the first electrode 21 or the second electrode 22, and is configured to detect the current I supplied between the first electrode 21 and the second electrode 22, and to input the detection signal to the control unit 50. The current I is the welding current described above.

[0039] The control unit 50 controls the current flow between the first electrode 21 and the second electrode 22 by controlling the welding power supply 30. Specifically, the control unit 50 is configured to control the current I flowing between the first electrode 21 and the second electrode 22 so that the current I flowing between them changes according to the current profile shown in Figure 3.

[0040] In detail, 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. Hereafter, the first electrode 21 and the second electrode 22 will be collectively referred to as a pair of electrodes 21 and 22.

[0041] When the control unit 50 receives a welding start instruction for the workpiece W via an operation unit (not shown), it starts the control process shown in Figure 4 and controls the current I between the pair of electrodes 21 and 22 so that the melting current changes according to the current profile shown in Figure 3. This control enables good resistance spot welding of the workpiece W.

[0042] Specifically, the control unit 50 performs a first energization control on a pair of electrodes 21 and 22 so that a first current I1 flows between the pair of electrodes 21 and 22 that sandwich the workpiece W during a period C1 from the start of welding T0 to a first time point T1 after a predetermined time has elapsed (S110). The first current I1 is a constant current.

[0043] Following the first energization control (S110), the control unit 50 performs a second energization control on the pair of electrodes 21 and 22 during a period C2 from the first time point T1 to the second time point T2, such that the current I flowing between the pair of electrodes 21 and 22 decreases from the first current I1 to a second current I2 which is smaller than the first current I1 (S120). The second current I2 is greater than zero.

[0044] Following the second energization control (S120), the control unit 50 performs a third energization control on the pair of electrodes 21 and 22 during a period C3 from the second time point T2 to the third time point T3, such that the current I flowing through the pair of electrodes 21 and 22 increases from the second current I2 to a third current I3 which is greater than the second current (S130).

[0045] The control unit 50 further performs a third energization control on the pair of electrodes 21 and 22 during the period C4 from the third time point T3 to the fourth time point T4, such that the current I flowing through the pair of electrodes 21 and 22 maintains the third current I3 (S130).

[0046] Following the third energization control (S130), the control unit 50 performs a fourth energization control on the pair of electrodes 21 and 22 during a period C5 from the fourth time point T4 to the fifth time point T5, such that a fourth current I4 smaller than the third current I3 flows between the pair of electrodes 21 and 22 (S140).

[0047] During the period C5 from the fourth time point T4 to the fifth time point T5, current control is performed so that a constant current, the fourth current I4, flows between the pair of electrodes 21 and 22. During the period C1 from the welding start time T0 to the first time point T1, current control is also performed so that a constant current, the first current I1, flows between the pair of electrodes 21 and 22. However, the first current I1 is smaller than the fourth current I4 for reasons that will be explained later.

[0048] During the period C3 and C4 from the second time point T2 to the fourth time point T4, for reasons to be explained later, the current between the pair of electrodes 21 and 22 is controlled so that a third current I3, which is larger than the fourth current I4, flows between the pair of electrodes 21 and 22.

[0049] The control unit 50 performs these first, second, third, and fourth energization controls continuously as a series of energization controls from time T0, and stops the energization between the pair of electrodes 21 and 22 at the fifth time T5. After that, the control process is terminated. At the fifth time T5, resistance spot welding to the workpiece W is completed.

[0050] The purpose of the current control according to the current profile described above will now be explained. In this embodiment, the current applied from the welding start time T0 to the first time point T1 corresponds to the first stage of initial current application and is performed to heat the workpiece W to an extent that the workpiece W does not melt due to resistive heating within the workpiece W. The first current I1 and the duration C1 from time point T0 to time point T1 are determined within a range that does not cause the workpiece W to melt.

[0051] Figure 5 shows a specific example of a current profile that can be used when the workpiece W is a workpiece (hereinafter referred to as the "workpiece of interest") that comprises a first steel plate P1 having a thickness of 1.4 mm and conforming to the SCGA440 standard, a second steel plate P2 having a thickness of 1.4 mm and conforming to the SCGA1180 standard, and a third steel plate P3 having a thickness of 2 mm and conforming to the SPC1470 standard. The applied pressure between the pair of electrodes 21, 21 is 5.51 kN (kilonewtons).

[0052] In Figure 5, the horizontal axis represents time, and the vertical axis represents current I. The graph in Figure 5 shows in detail the ratio of time lengths between time points T0, T1, T2, T3, T4, and T5, and the ratio between currents I1, I2, I3, and I4, assuming that time and current I are zero at the origin.

[0053] Figure 6A conceptually illustrates the heating of the inside of the workpiece W during the first stage of initial energization. The dashed lines in Figure 6A schematically show the heated areas 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 areas indicated by the dashed lines correspond to the resistance centers of the workpiece W. Examples of these combinations of the first steel plate P1, second steel plate P2, and third steel plate P3 include the combinations of the first steel plate P1, second steel plate P2, and third steel plate P3 in the workpiece of interest described above.

[0054] The first stage of initial energization is performed not only to heat the resistance center, but also to cause warping in the multiple metal plates that make up the workpiece W, thereby limiting the current path within the workpiece W to a narrow area.

[0055] The curvature of the metal plate referred to here is such that, as you move away from the center of the current-carrying path connecting the first electrode 21 and the second electrode 22, adjacent metal plates in the stacking direction are spaced further apart in the stacking direction.

[0056] 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 in a region away from the center of the current path. Similarly, due to the warping of the third steel plate P3, the second steel plate P2 and the third steel plate P3 are separated in a region away from the center of the current path.

[0057] This separation limits the current path, causing current to flow concentratedly within the workpiece W. Below, current concentration refers to the concentrated flow of current at a desired current density between a pair of electrodes 21 and 22 through a limited area within the workpiece W.

[0058] The energization from the first time point T1 to the second time point T2 corresponds to the second stage of initial energization and is performed with the aim of gradually heating the area within the workpiece W sandwiched between the first electrode 21 and the second electrode 22 by applying heat to the workpiece W.

[0059] The dashed line in Figure 6B conceptually illustrates that in the second stage of initial energization, heat spreads from the center of resistance, and the entire region within the workpiece W sandwiched between the first electrode 21 and the second electrode 22 is heated.

[0060] This heating plays a role in stably forming the warp of the metal plate. That is, by providing a process to gradually decrease the current I, such as current control from the first time point T1 to the second time point T2, the warp of the metal plate is stably generated and current concentration is stably achieved.

[0061] In this embodiment, the control unit 50 controls the current flowing between the pair of electrodes 21 and 22 based on the detection signal from the current sensor 40. However, if the warping of the metal plate cannot be stably achieved, the current density changes when the current flows in the stacking direction of the workpiece W, and the manner of heat generation changes. Therefore, in order to achieve stable current concentration, it is important to perform downslope current control (S120) between the first time point T1 and the second time point T2.

[0062] The energization from the second time point T2 to the fourth time point T4 is performed to promote current diversion to the surface of the metal plate, which has relatively low resistance, thereby achieving heating and melting over the entire surface of the workpiece W from top to bottom between the pair of electrodes 21 and 22. Hereafter, the period C3 and C4 from the second time point T2 to the fourth time point T4 will also be referred to as the high-current section L.

[0063] When a relatively low-resistance steel plate, such as the first steel plate P1 of the workpiece in question, is present on the surface of the workpiece W, if a large current is not passed, sufficient Joule heat may not be generated in the low-resistance steel plate, potentially resulting in insufficient welding to the first steel plate P1. By passing a large current, it is possible to suppress the deterioration of welding quality caused by insufficient heat generation in the relatively low-resistance steel plate.

[0064] Figure 7A shows that in the third current control, applying a large current causes widespread melting between the pair of electrodes 21 and 22, resulting in material fusion between adjacent steel plates in the first steel plate P1, the second steel plate P2, and the third steel plate P3. This fusion reduces the resistance difference between the metal plates.

[0065] The energization from the fourth time point T4 to the fifth time point T5 corresponds to the main energization and is performed to finish the welding of the workpiece W. The fourth current I4 and the duration of the period C5 from time point T4 to time point T5 (i.e., the main energization time) are determined so that a suitable weld nugget G is formed within the workpiece W.

[0066] With this current application, the melt within the workpiece W spreads from the resistance center outwards, forming an appropriate weld nugget G within the workpiece W. Due to the mitigation of the resistance difference caused by the penetration resulting from the large current applied beforehand, the melt within the workpiece W spreads more uniformly than without the third current control. As a result, an appropriate weld nugget G is formed within the workpiece W.

[0067] Figures 7B and 7C show that the welding nugget G gradually grows as the current is applied. As shown in Figure 7C, good welding is achieved as the welding nugget G spreads throughout the multiple metal plates (first steel plate P1, second steel plate P2, third steel plate P3) within the workpiece W.

[0068] Furthermore, in this embodiment, current concentration is achieved, enabling efficient heating and melting of the metal plate, and making it possible to form an appropriate welding nugget G with a relatively small current.

[0069] Furthermore, in this embodiment, stable current concentration can be achieved in the process up to the second stage of initial energization, thereby increasing the current margin in the main energization. In other words, a relatively wide current range can be allowed as the fourth current I4 for achieving proper welding. The thick arrows attached to the fourth current I4 in Figure 5 indicate that the margin of the fourth current I4 is wide.

[0070] Furthermore, in this embodiment, the third current control enables penetration over a wide area in the stacking direction of the workpiece W, so that in this current application, spatter can be suppressed and good welding can be achieved with a small current.

[0071] The resistance spot welding apparatus 1 and resistance spot welding method of this embodiment have been described above. This welding method is particularly useful when welding a workpiece W made up of multiple metal plates with different resistances stacked on top of each other. Examples of multiple metal plates with different resistances include multiple metal plates with different tensile strengths and multiple metal plates with different thicknesses.

[0072] When there is a resistance difference between metal plates in a workpiece W, the resistive heating is smaller in the low-resistance areas compared to other areas. In this embodiment, a large current can be used to promote current diversion to the relatively low-resistance metal plates. Furthermore, material penetration between adjacent metal plates can be caused, mitigating the resistance difference. Therefore, it is possible to successfully weld multiple metal plates with different resistances.

[0073] The resistance spot welding method according to the current profile including the above-described high current section L functions particularly effectively when a workpiece W including three metal plates, such as the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3, satisfies at least any one of the first condition, the second condition, the third condition, and the fourth condition represented by the following inequalities. · First condition: X1 < X3 · Second condition: X1 > X2 and (X1 + X2) > X3 · Third condition: X1 < X2 and (X1 + X2) > X3 · Fourth condition: X1 ≧ X2 > X3

[0074] Here, it is assumed that the first metal plate in the workpiece W is a metal plate with a tensile strength X1, the second metal plate is a metal plate with a tensile strength X2, and the third metal plate is a metal plate with a tensile strength X3.

[0075] The first metal plate and the third metal plate are metal plates located at both ends in the stacking direction of the workpiece W, and the second metal plate is a metal plate sandwiched between the first metal plate and the second metal plate. The first metal plate corresponds to the first steel plate P1 illustrated in FIG. 1. 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 be associated with the third steel plate P3, the second steel plate P2, and the first steel plate P1 in this order, respectively. That is, in understanding the first condition, the second condition, the third condition, and the fourth condition, the first metal plate, the second metal plate, and the third metal plate may be understood as being stacked from top to bottom or from bottom to top. In other words, the first metal plate and the tensile strength X1 may be read as the third metal plate and the tensile strength X3, and the third metal plate and the tensile strength X3 may be read as the first metal plate and the tensile strength X1.

[0076] The first condition is that 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 opposite end of the workpiece W. In other words, the two metal plates located at both ends of the workpiece W have different tensile strengths X1 and X3.

[0077] The second condition is that the tensile strength X1 of the first metal plate located at the first end in the stacking direction of the workpiece W is greater than the tensile strength X2 of the second metal plate located in the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end in the stacking direction of the workpiece W is less than the sum of the tensile strengths X1 of the first metal plate and X2 of the second metal plate. The second end in the stacking direction of the workpiece W is the end opposite to the first end in the stacking direction of the workpiece W.

[0078] The third condition is that 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 in 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 strengths X1 of the first metal plate and X2 of the second metal plate.

[0079] The second and third conditions can be summarized as follows: X1 ≈ X2 and (X1 + X2) > X3 In other words, the second and third conditions are that 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 in 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 strengths X1 of the first metal plate and X2 of the second metal plate.

[0080] The fourth condition is that multiple metal plates are stacked such that the tensile strength increases or decreases in the stacking direction. The stacking direction 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.

[0081] The tensile strengths X1, X2, and X3 in the first, second, third, and fourth conditions may be reinterpreted as resistances X1, X2, and X3. The tensile strengths X1, X2, and X3 may also be reinterpreted 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.

[0082] If the first condition is satisfied, the resistance center of the workpiece W shifts toward the third metal plate in the stacking direction of the workpiece W due to the influence of the third metal plate, which has high resistance. If the fourth condition is satisfied, the resistance center of the workpiece W shifts toward the first metal plate in the stacking direction of the workpiece W due to the influence of the first metal plate, which has high resistance.

[0083] Therefore, applying a conventional welding method to a workpiece W that satisfies any of these conditions, simply by growing a weld nugget G from the resistance center, may result in insufficient welding of the metal plate located at the edge of the workpiece W.

[0084] According to this embodiment, as shown in Figure 7A, it is possible to melt the first metal plate, which has relatively low resistance, with a large current, and to properly weld the workpiece W. Therefore, spatter can also be suppressed.

[0085] The first, second, third, and fourth conditions described above can also be applied when the workpiece W is a stack of four or more metal plates. When the above conditions are satisfied by replacing "second metal plate" with "stack of multiple metal plates" and replacing the tensile strength X2 of the second metal plate with the sum of the tensile strengths of the metal plates constituting the "stack of multiple metal plates", the resistance spot welding method of this embodiment functions effectively for good welding of the workpiece W. When the workpiece W is a stack of four or more metal plates, the "second metal plate located in the center of the workpiece W" in the second and third conditions 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 in 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.

[0086] Welding tests in various environments revealed that the resistance spot welding method of this embodiment contributes to the proper welding of the workpiece W even when the workpiece W satisfies either of the following fifth and sixth conditions. • Fifth condition: The difference in strength between the two sides, ΔX, is 445 MPa or greater. • Sixth condition: The difference in strength between the two sides ΔX is 255 MPa or more, and the strength ratio R is 4.29 or more.

[0087] The difference in strength between the two sides ΔX, as used here, is the difference in tensile strength between the metal plates located at both ends in the stacking direction of the workpiece W. In other words, the difference in strength between the two sides ΔX is the difference in tensile strength between the two metal plates in the workpiece W that are in contact with a pair of electrodes 21 and 22. Using the tensile strengths X1, X2, and X3 described above, the difference in strength between the two sides ΔX can be expressed by the equation ΔX = |X1 - X3|.

[0088] The strength ratio R corresponds to the ratio R = R1 / R2 between the sum of the tensile strengths R1 of the multiple metal plates constituting the workpiece W in the stacking direction and the tensile strength R2 of the metal plate with the lower tensile strength among the two metal plates located at both ends in the stacking direction of the multiple metal plates. Using the tensile strengths X1, X2, and X3 described above, R1 can be expressed by the formula R1 = (X1 + X2 + X3), and R2 can be expressed using the MIN function by the formula R2 = min{X1, X3}. In this case, the strength ratio R = (X1 + X2 + X3) / min{X1, X3}.

[0089] The larger the strength difference ΔX between the two sides, the less the metal plate with relatively low tensile strength will melt due to low resistance. The larger the strength ratio R, the more uneven the weld nugget G will be. Therefore, the resistance spot welding method of this embodiment is highly useful for workpieces W that satisfy the fifth or sixth condition described above.

[0090] The resistance spot welding method of this embodiment also functions effectively when welding a workpiece W whose plate thickness satisfies the following seventh condition. • Seventh condition: The plate thickness ratio H is 3.5 or greater.

[0091] The thickness ratio H referred to here is the ratio H1 / H2 between the total thickness H1, which is the sum of the thicknesses of the multiple metal plates constituting the workpiece W in the stacking direction, and the thickness H2 of the smaller of the two metal plates located at both ends in the stacking direction of the multiple metal plates.

[0092] [Differentiation] The current profile is not limited to the example shown in Figure 3, but may be changed to any of the current profiles in Figures 8A, 8B, 8C, 9A, 9B, and 9C. As can be seen from comparing Figure 3 with Figures 8A, 8B, 8C, 9A, 9B, and 9C, the initial current including the downslope and the large current before the main current are particularly beneficial for good welding.

[0093] In the current profile shown in Figure 8A, the current waveform in the high-current section L1 is triangular with an upslope and a sharp peak, rather than a trapezoid as shown in Figure 3. In the current profile shown in Figure 8B, the current waveform in the high-current section L2 is rectangular. In the current profile shown in Figure 8C, the current waveform in the high-current section L3 is triangular with a downslope starting from the peak.

[0094] In the current profile shown in Figure 9A, the current waveform in the high-current section L4 is trapezoidal with an upslope and a downslope. In the current profile shown in Figure 9B, the current waveform in the high-current section L5 is triangular with an upslope and a downslope and a sharp peak. In the current profile shown in Figure 9C, the current waveform in the high-current section L6 is trapezoidal, starting from the peak.

[0095] [Other embodiments] This disclosure is not limited to the embodiments described above, and various forms can be adopted. For example, the resistance spot welding apparatus 1 and resistance spot welding method described above may be used to weld a workpiece W in which two metal plates are stacked together, as multiple metal plates.

[0096] The function of one component in the above embodiment may be distributed among multiple components. The functions of multiple components may be integrated into one component. Some parts of the configuration of the above embodiment may be omitted. At least some parts of the configuration of the above embodiment may be added to or replaced by the configuration of other above embodiments. Any aspect of the technical concept specified by the wording of the claims constitutes an embodiment of the present disclosure.

[0097] [Technical Concept Disclosed in This Specified Specification] This specification can be understood to disclose the following technical concepts: [Item 1] A resistance spot welding method for welding a workpiece, which is made up of multiple stacked metal plates, using a resistance spot welding apparatus equipped with a pair of electrodes that sandwich the workpiece on both sides in the stacking direction of the multiple metal plates, A first current supply control is performed on the pair of electrodes so that a first current flows between the pair of electrodes that sandwich the workpiece. Following the first energization control, a second energization control is performed on the pair of electrodes such that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. Following the second energization control, a third energization control is performed on the pair of electrodes such that the current flowing through the pair of electrodes increases from the second current to a third current that is greater than the second current. Following the third current control, a fourth current control is performed on the pair of electrodes so that a fourth current flows between them. Includes, The fourth current mentioned above 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. Resistance spot welding method. [Item 2] The resistance spot welding method according to item 1, wherein the plurality of metal plates comprises at least two metal plates having different resistances, tensile strengths, or thicknesses. [Item 3] The resistance spot welding method according to item 1 or item 2, wherein the workpiece includes a workpiece in which two metal plates located at both ends in the stacking direction have different resistance, tensile strength, or plate thickness from each other. [Item 4] The aforementioned workpiece is A workpiece in which the plurality of metal plates are stacked such that resistance, tensile strength, or plate thickness increases or decreases in the stacking direction, and A workpiece in which, among the plurality of metal plates, the tensile strength of the first metal plate located at the first end in the stacking direction is different from the tensile strength of the second metal plate located in the center in the stacking direction, and the tensile strength of the third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is smaller than the sum of the tensile strengths of the first and second metal plates, and the tensile strength of the second metal plate located in the center in the stacking direction is the sum of the tensile strength of one metal plate sandwiched between the first and third metal plates located at both ends in the stacking direction, or the sum of the tensile strengths of one or more metal plates sandwiched between the first and third metal plates. A resistance spot welding method according to any one of items 1 to 3, comprising at least one of the above. [Item 5] The resistance spot welding method according to any one of items 1 to 4, wherein the ratio H1 / H2 between the total plate thickness H1, which is the sum of the thicknesses of the plurality of metal plates in the workpiece in the stacking direction, 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, is 3.5 or more. [Item 6] The aforementioned workpiece is Workpieces with a strength difference of 445 MPa or more on both sides, and The workpiece in which the difference in strength between the two sides is 255 MPa or more, and the strength ratio is 4.29 or more. Including at least one of the following, The resistance spot welding method according to any one of items 1 to 5, wherein the difference in strength between both sides is the difference in tensile strength between two metal plates located at both ends in the stacking direction of the workpiece, 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 lower tensile strength among the two metal plates located at both ends in the stacking direction. [Item 7] The aforementioned plurality of metal plates include high-tensile steel plates, and the resistance spot welding method is according to any one of items 1 to 6. [Item 8] A resistance spot welding apparatus for welding a workpiece in which multiple metal plates are stacked, A pair of electrodes are arranged to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates, A control unit configured to control the current flow between the pair of electrodes, Equipped with, The control unit, A first current supply control is performed on the pair of electrodes so that a first current flows between the pair of electrodes that sandwich the workpiece. Following the first energization control, a second energization control is performed on the pair of electrodes such that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. Following the second energization control, a third energization control is performed on the pair of electrodes such that the current flowing through the pair of electrodes increases from the second current to a third current that is greater than the second current. Following the third current control, a fourth current control is performed on the pair of electrodes so that a fourth current flows between them. The fourth current mentioned above 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. Resistance spot welding equipment. [Explanation of Symbols]

[0098] 1... Resistance spot welding device, 20... Resistance welding machine, 21... First electrode, 22... Second electrode, 30... Welding power supply, 40... Current sensor, 50... Control unit.

Claims

1. A resistance spot welding method for welding a workpiece, which is made up of multiple stacked metal plates, using a resistance spot welding apparatus equipped with a pair of electrodes that sandwich the workpiece on both sides in the stacking direction of the multiple metal plates, A first current supply control is performed on the pair of electrodes so that a first current flows between the pair of electrodes that sandwich the workpiece. Following the first energization control, a second energization control is performed on the pair of electrodes such that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. Following the second energization control, a third energization control is performed on the pair of electrodes such that the current flowing through the pair of electrodes increases from the second current to a third current that is greater than the second current. Following the third current control, a fourth current control is performed on the pair of electrodes so that a fourth current flows between them. After the completion of the fourth energization control, the energization to the pair of electrodes is stopped, Includes, The fourth current mentioned above 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. Resistance spot welding method.

2. The resistance spot welding method according to claim 1, wherein the plurality of metal plates comprises at least two metal plates having different resistances, tensile strengths, or plate thicknesses.

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

4. The aforementioned workpiece is A workpiece in which the plurality of metal plates are stacked such that resistance, tensile strength, or plate thickness increases or decreases in the stacking direction, and A workpiece in which, among the plurality of metal plates, the tensile strength of the first metal plate located at the first end in the stacking direction is different from the tensile strength of the second metal plate located in the center in the stacking direction, and the tensile strength of the third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is smaller than the sum of the tensile strengths of the first and second metal plates, and the tensile strength of the second metal plate located in the center in the stacking direction is the sum of the tensile strength of one metal plate sandwiched between the first and third metal plates located at both ends in the stacking direction, or the sum of the tensile strengths of one or more metal plates sandwiched between the first and third metal plates. A resistance spot welding method according to claim 1, comprising at least one of the following.

5. The resistance spot welding method according to claim 1, wherein the ratio H1 / H2 between the total plate thickness H1, which is the sum of the thicknesses of the plurality of metal plates in the workpiece in the stacking direction, 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, is 3.5 or more.

6. The aforementioned workpiece is Workpieces with a difference in strength between both sides of 445 MPa or more, The workpiece in which the difference in strength between the two sides is 255 MPa or more, and the strength ratio is 4.29 or more. Including at least one of the following, The resistance spot welding method according to claim 1, wherein the difference in strength between both sides is the difference in tensile strength between two metal plates located at both ends in the stacking direction of the workpiece, 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 lower tensile strength among the two metal plates located at both ends in the stacking direction.

7. The resistance spot welding method according to any one of claims 1 to 6, wherein the plurality of metal plates include high-tensile steel plates.

8. A resistance spot welding apparatus for welding a workpiece in which multiple metal plates are stacked, A pair of electrodes are arranged to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates, A control unit configured to control the current flow between the pair of electrodes, Equipped with, The control unit, A first current supply control is performed on the pair of electrodes so that a first current flows between the pair of electrodes that sandwich the workpiece. Following the first energization control, a second energization control is performed on the pair of electrodes such that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. Following the second energization control, a third energization control is performed on the pair of electrodes such that the current flowing through the pair of electrodes increases from the second current to a third current that is greater than the second current. Following the third current control, a fourth current control is performed on the pair of electrodes so that a fourth current flows between them. After the completion of the fourth energization control, the energization to the pair of electrodes is stopped. The fourth current mentioned above 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. Resistance spot welding equipment.

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