Resistance spot welding method

Abstract

To suppress generation of spatter in resistance spot welding.SOLUTION: The resistance spot welding method includes a pre-current application step of applying a current to the workpiece to form a pressure welded section having a predetermined size in the workpiece, and a main current application step of applying a current to the workpiece after the pre-current application step to form a nugget section having a predetermined size inside the pressure welded section, wherein a relationship between a diameter dp of the pressure welded section at the end of the pre-current application step and a lower limit value Ln of a diameter of the nugget section after the end of the main current application step satisfies the following formula (1): 0.7 * Ln ≤ dp ≤ 2.5 * Ln (1) SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a resistance spot welding method. [Background technology]

[0002] In resistance spot welding, a technique is known in which spatter is suppressed by providing a period during which a constant current is applied between a pre-energization step and a main energization step (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-079410 Summary of the Invention [Problem to be solved by the invention]

[0004] There is room for further innovation in suppressing spatter generation during resistance spot welding. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to a first aspect of the present disclosure, there is provided a resistance spot welding method, comprising: a pre-energization step of passing current through a workpiece to form a pressure-welded portion of a predetermined size in the workpiece; and a main energization step of passing current through the workpiece after the pre-energization step to form a nugget portion of a predetermined size inside the pressure-welded portion. The relationship between the diameter dp of the pressure-welded portion at the end of the pre-energization step and the lower limit Ln of the diameter of the nugget portion at the end of the main energization step satisfies the following formula (1): 0.7×Ln≦dp≦2.5×Ln (1) This resistance spot welding method allows the periphery of the nugget to be properly sealed by the pressure weld, thereby preventing molten metal from splashing out of the nugget and causing spatter during the main current application step. (2) A second aspect of the present disclosure provides a resistance spot welding method including a pre-energization step of passing current through a workpiece to form a pressure-welded portion of a predetermined size in the workpiece, and a main energization step of passing current through the workpiece after the pre-energization step to form a nugget of a predetermined size inside the pressure-welded portion. The joint portion of the workpiece is composed of three or more metal plates and has a first joint interface which is the joint interface where the diameter of the nugget portion is maximum, and a second joint interface which is the joint interface where the diameter of the nugget portion is minimum, and the diameter dp1 of the pressure-welded portion at the first joint interface at the end of the pre-current application process, the lower limit value Ln1 of the diameter of the nugget portion at the first joint interface after the end of the main current application process, and the thickness T of the metal plates which determines the lower limit value Ln1 satisfy the relationship of the following formula (2), and the diameter dn2 of the nugget portion at the second joint interface at the end of the pre-current application process and the lower limit value Ln2 of the diameter of the nugget portion at the second joint interface after the end of the main current application process satisfy the relationship of the following formula (3). 1.2×Ln1+0.5×(T) 1 / 2 ≦dp1≦1.2×Ln1+1.5×(T) 1 / 2 ···(2) dn2 ≥ 0.7 × Ln2 (3) This resistance spot welding method allows the periphery of the nugget to be properly sealed by the pressure weld, even when welding workpieces made of three or more metal plates, thereby preventing molten metal from splashing out of the nugget and causing spatter during the main current application step. (3) In each of the above-described embodiments of the resistance spot welding method, the pre-energization step may include pulsation energization, which alternates between energization at a current value higher than a predetermined threshold and energization at a current value equal to or lower than the threshold, and at least one of constant current energization, which energizes at a constant current value, and upslope energization, which increases the current value over time. According to the resistance spot welding method of this aspect, the time required to form the pressure-welded portion in the pre-energization step can be shortened. (4) In the resistance spot welding method according to any of the above aspects, the main current supplying step may include constant current supplying in which current is supplied at a constant value, and upslope current supplying in which the current value is increased over time. According to the resistance spot welding method of this aspect, it is possible to reduce the time required to form the nugget portion while suppressing the generation of spatter during the main current application step. The present disclosure may be realized in various forms other than a resistance spot welding method, for example, a resistance spot welding apparatus. [Brief explanation of the drawings]

[0007] [Figure 1] Schematic diagram of a resistance spot welding device. [Figure 2] 3 is a flowchart showing the steps of a resistance spot welding method. [Figure 3] FIG. [Figure 4] FIG. 2 is an explanatory diagram showing a current pattern in a resistance spot welding method. [Figure 5] FIG. 10 is an explanatory diagram showing a pressure-welded portion and a nugget portion in the case of three-piece welding. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: 1 is a schematic diagram of a resistance spot welding apparatus 1 according to a first embodiment. In this embodiment, the resistance spot welding apparatus 1 includes a welding gun 10, a power supply device 30, a current sensor 53, a voltage sensor 54, and a control device 80.

[0009] The resistance spot welding apparatus 1 joins two or more overlapping metal plates by resistance spot welding. In the following description, each metal plate joined by resistance spot welding is referred to as the workpiece W. In this embodiment, the workpiece W includes a first metal plate W1 and a second metal plate W2. The first metal plate W1 and the second metal plate W2 are steel plates. The steel plates included in the workpiece W may have a surface layer made of hot-dip galvanized, alloyed hot-dip galvanized, zinc oxide, or an aluminum-silicon alloy. However, the metal plates included in the workpiece W are not limited to steel plates and may be, for example, aluminum alloy plates.

[0010] The welding gun 10 is attached, for example, to the tip of a robot arm (not shown). The gun body 11 is moved by the robot arm to the target joint WP of the workpiece W. The welding gun 10 includes the gun body 11, a movement mechanism 20, a pair of opposing electrodes 21, 22, and a pressure device 40. In the following description, one of the pair of electrodes 21, 22 will be referred to as the first electrode 21, and the other will be referred to as the second electrode 22. In this embodiment, the first electrode 21 is a movable electrode and is attached to the upper part of the gun body 11. The second electrode 22 is a fixed electrode and is attached to the lower part of the gun body 11 in a position opposite the first electrode 21. However, the first electrode 21 may be a fixed electrode, and the second electrode 22 may be a movable electrode.

[0011] The movement mechanism 20 raises and lowers the first electrode 21, which is a movable electrode. The movement mechanism 20 is equipped with a servo motor (not shown), converts the rotational force of the servo motor into a linear movement force in the lifting and lowering direction, and transmits the converted linear movement force to the first electrode 21, thereby raising and lowering the first electrode 21. The power supply unit 30 supplies a welding current of a target current value between the first electrode 21 and the second electrode 22. The pressure device 40 is equipped with a cylinder (not shown), and with the workpiece W sandwiched between the first electrode 21 and the second electrode 22, the cylinder presses the first electrode 21 against the workpiece W, thereby applying pressure to the workpiece W.

[0012] Current sensor 53 detects the current value of the welding current supplied from power supply device 30 at predetermined time intervals and transmits a signal indicating the current value to control device 80. A toroidal coil, for example, is used as current sensor 53. Voltage sensor 54 detects the voltage value between first electrode 21 and second electrode 22 at predetermined time intervals and transmits a signal indicating the voltage value to control device 80.

[0013] The control device 80 is configured by a computer including a processor such as a CPU, memory such as ROM and RAM, an input / output interface, and an internal bus. The processor, memory, and input / output interface are connected via the internal bus to enable bidirectional communication. The input / output interface is connected to the servo motor of the movement mechanism 20, the power supply device 30, the cylinder of the pressure device 40, the current sensor 53, and the voltage sensor 54. The control device 80 controls each component of the resistance spot welding apparatus 1 by the processor executing a computer program pre-stored in the memory. Specifically, the control device 80 comprehensively controls the current value, current application time, pressure applied by the electrodes 21 and 22, current application timing, pressure application timing, etc. Note that at least some of the functions of the control device 80 may be implemented by hardware circuits.

[0014] Fig. 2 is a flowchart showing the procedure of the resistance spot welding method according to this embodiment. Fig. 3 is an explanatory diagram showing a pressure-welded portion PP and a nugget portion PN formed in a workpiece W by the resistance spot welding method. Fig. 3 shows the pressure-welded portion PP and the nugget portion PN when viewed perpendicular to the weld interface WS of the workpiece W.

[0015] As shown in FIG. 2, in this embodiment, the resistance spot welding method includes a pre-energization step (step S100) and a main energization step (step S200) that is performed after the pre-energization step. As shown in FIG. 3, a pressure-welded portion PP and a nugget portion PN are formed in the workpiece W by the resistance spot welding method. The pressure-welded portion PP is an unmelted portion of the joint portion WP of the workpiece W that is joined by solid-state welding. The nugget portion PN is a portion of the joint portion WP of the workpiece W that is joined by melting and solidifying. In the following description, the diameter of the pressure-welded portion PP is referred to as the pressure-welded diameter, and the diameter of the nugget portion PN is referred to as the nugget diameter. The pressure-welded portion PP can also be referred to as the alloy layer, and the diameter of the alloy layer can also be referred to as the alloy layer diameter.

[0016] In the pre-energization process, current is applied to the workpiece W while pressurizing it with the first electrode 21 and the second electrode 22 so that a pressure-welded portion PP of a size within a target range is formed on the workpiece W. In the pre-energization process, at least one of pulsation energization, constant current energization, and up-slope energization is performed. Pulsation energization is a method of alternately energizing a high current, in which a current value exceeding a predetermined threshold is applied for a short period of less than 10 milliseconds, and a low voltage, in which a current value equal to or less than the threshold is applied for a short period of less than 10 milliseconds. Constant current energization is a method of energizing a constant current value for a predetermined period of time. Up-slope energization is a method of energizing a current value such that the current value monotonically increases over time. The pre-energization process forms a pressure-welded portion PP and a nugget portion PN on the workpiece W. The nugget portion PN is formed inside the pressure-welded portion PP. However, the nugget portion PN may not be formed on the workpiece W at the end of the pre-energization process. 3 shows the pressure welding diameter dp at the end of the pre-current process and the nugget diameter dn at the end of the pre-current process. The pressure welding diameter dp at the end of the pre-current process is larger than the nugget diameter dn at the end of the pre-current process.

[0017] In the main current application process, current is applied to the workpiece W while pressurizing it with the first electrode 21 and the second electrode 22 so that a nugget portion PN of a size within a target range is formed on the workpiece W. In the main current application process, at least one of constant current application and upslope current application is performed. The main current application process increases the size of the nugget portion PN formed on the workpiece W. The main current application process also increases the size of the press-welded portion PP formed on the workpiece W. However, the size of the press-welded portion PP may remain approximately the same as at the end of the pre-current application process. FIG. 3 shows the press-welded diameter Dp after the main current application process and the nugget diameter Dn after the main current application process. The nugget diameter Dn after the main current application process is larger than the nugget diameter dn at the end of the pre-current application process. The press-welded diameter Dp after the main current application process is equal to or larger than the press-welded diameter dp at the end of the pre-current application process and is larger than the nugget diameter Dn after the main current application process. The difference between the pressure-welded diameter Dp and the nugget diameter Dn after the completion of the main current-passing step is smaller than the difference between the pressure-welded diameter dp and the nugget diameter dn at the completion of the pre-current-passing step.

[0018] 3 shows the pressure welding diameter Dp and nugget diameter Dn after the main current application process, as well as the lower limit nugget diameter Ln after the main current application process. The lower limit nugget diameter Ln is the lower limit of the nugget diameter Dn after the main current application process that is acceptable in terms of welding quality, and if the nugget diameter Dn after the main current application process is less than the lower limit nugget diameter Ln, it is determined that the welding is defective. In this embodiment, the resistance spot welding method is performed so that the pressure welding diameter dp at the end of the pre-current application process and the lower limit nugget diameter Ln after the main current application process satisfy the relationship shown in Equation (1) below. 0.7×Ln≦dp≦2.5×Ln (1)

[0019] 4 is an explanatory diagram showing a current pattern in the resistance spot welding method according to this embodiment. In FIG. 4, the horizontal axis represents time, and the vertical axis represents current. In this embodiment, in the pre-current step, pulsation current, constant current current, and up-slope current are applied in this order, and in the main current step, up-slope current and constant current current are applied in this order.

[0020] In the pulsation energization, at least one high current energization and at least one low current energization are performed. In this embodiment, the pulsation energization starts with a high current energization and ends with a high current energization. However, the pulsation energization may start with a low current energization instead of starting with a high current energization, and may end with a low current energization instead of ending with a high current energization. In this embodiment, the pulsation energization includes three high current energizations and two low current energizations. In each high current energization, energization is performed at a predetermined current value I1 for a predetermined current energization time T1, and in each low current energization, energization is performed at a predetermined current value I2 for a predetermined current energization time T2. The current value I1 in each high current energization may be the same or different. The current value I2 in each low current energization may be the same or different. The current energization time T1 in each high current energization may be the same or different. The energization time T2 for each low current energization may be the same or different. The current value I1 for the high current energization is preferably in the range of 8.0 kA or more and 15.0 kA or less. The current value I2 for the low current energization is preferably at least one-third of the current value I1 for the high current energization and not more than two-thirds of the current value I1 for the high current energization. The number of high current energizations performed in pulsation energization is preferably in the range of 2 to 5 times. The energization time T1 for each high current energization is preferably in the range of 4 milliseconds to 8 milliseconds. The energization time T2 for each low current energization is preferably in the range of 4 milliseconds to 8 milliseconds. The total energization time Tps for pulsation energization is preferably in the range of 20 milliseconds to 40 milliseconds.

[0021] In the constant current energization in the pre-energization step, energization is performed so that a constant current value I3 is maintained for a predetermined energization time T3. The current value I3 is less than the current value I1. The current value I3 may be equal to or greater than the current value I2, or may be less than the current value I2.

[0022] In the upslope current conduction in the pre-current conduction step, current is supplied so that the current value monotonically increases from a predetermined current value I3 to a predetermined current value I4 during a predetermined current conduction time T4. Current value I4 is less than current value I1. Current value I4 may be equal to or greater than current value I2, or may be less than current value I2.

[0023] In the up-slope current conduction of the main current conduction step, current is supplied so that the current value monotonically increases from a predetermined current value I4 to a predetermined current value I5 during a predetermined current conduction time T5. The rate of increase in the current value per unit time in the up-slope current conduction of the main current conduction step is greater than the rate of increase in the current value per unit time in the up-slope current conduction of the pre-current conduction step. The current value I5 is equal to or greater than the current value I2. The current value I5 may be equal to or greater than the current value I1, or may be less than the current value I1.

[0024] In the constant current energization of the main energization step, energization is performed so that a constant current value I5 is maintained for a predetermined energization time T6. The current value I5 is equal to or greater than the current value I2 and is higher than the current value IE at the end of the pre-energization step. The welding conditions, including the current values ​​I1 to E5 and the energization times T1 to T6, that can satisfy the relationship of the above-mentioned formula (1) can be determined, for example, from the results of tests or simulations that are performed in advance.

[0025] In the case of two-sheet welding in which two metal sheets W1 and W2 are joined by resistance spot welding, the relationship shown in the above-mentioned formula (1) can be satisfied by welding under the following conditions, for example. <Materials and thickness of each metal plate> The first metal plate W1 is a cold-rolled steel plate (SPC440), and the thickness t1 of the first metal plate W1 is 1.20 mm. The second metal plate W2 is a cold-rolled steel plate (SPC1180), and its thickness t2 = 1.60 mm. The gap between the first metal plate W1 and the second metal plate W2 is 2.00 mm. <Pressure> The pressure is constant at 3430N from the start of the pre-energization process to the end of the main energization process. <Power supply conditions> Current application pattern for the pre-current application process and the main current application process: As shown in Figure 4. Pulsation energization in the pre-energization process: high current value I1 = 8.0 kA, high current energization time T1 = 6 ms, low current value I2 = 3.0 kA, low current energization time T2 = 6 ms. Pre-energization process: constant current I3 = 6.6 kA, energization time T3 = 1 ms. Up-slope current application in the pre-energization step: starting current I3 = 6.6 kA, ending current I4 = 6.8 kA, current application time T4 = 150 ms. · Upslope current flow in this current flow process: starting current value I4 = 6.8 kA, ending current value I5 = 7.8 kA, current flow time T5 = 34 ms. · Constant current energization in this energization process: current value I5 = 7.8 kA, energization time T6 = 34 ms.

[0026] According to the resistance spot welding method of the present embodiment described above, resistance spot welding is performed so that the pressure weld diameter dp at the end of the pre-energization process and the lower limit nugget diameter Ln after the main energization process satisfy the relationship shown in Equation (1). Here, the pressure weld portion PP has the function of containing the molten metal within the nugget portion PN to prevent the molten metal from spilling out of the nugget portion PN. Therefore, if the pressure weld diameter dp at the end of the pre-energization process is too small, the molten metal is more likely to spill out of the nugget portion PN, and spatter is more likely to occur during the main energization process. On the other hand, if the pressure weld diameter dp at the end of the pre-energization process is too large, it becomes difficult to press the entire pressure weld portion PP with the electrodes 21 and 22, and spatter is more likely to occur during the main energization process. Furthermore, if the pressure weld diameter dp at the end of the pre-energization process is too large, the pressure weld portion PP is more likely to protrude from the edge of the workpiece W. If the pressure weld portion PP protrudes beyond the edge of the workpiece W, the function of the pressure weld portion PP to contain the molten metal in the nugget portion PN is impaired, making spatter more likely to occur during the main current application process. To address these issues, in this embodiment, resistance spot welding is performed so that the pressure weld diameter dp at the end of the pre-current application process and the lower limit nugget diameter Ln after the main current application process satisfy the relationship shown in Equation (1). This allows the pressure weld portion PP to adequately seal the periphery of the nugget portion PN. This prevents molten metal from spilling out of the nugget portion PN during the main current application process, causing spatter. Furthermore, fluctuations in the magnitude of the welding current due to disturbances during the main current application process can easily cause spatter. However, in this embodiment, the pressure weld portion PP can adequately seal the periphery of the nugget portion PN, widening the allowable range of fluctuations in the welding current.

[0027] In addition, in this embodiment, pulsation current is applied in the pre-energization step, so that the generation of spatter is suppressed by the low current application, while the high current application raises the temperature of the joint WP to a temperature range where the press-welded portion PP is easily formed. Furthermore, in this embodiment, at least one of up-slope current application and constant current application is applied after the pulsation current application in the pre-energization step, so that the press-welded portion PP can be enlarged while maintaining the balance between the size of the press-welded portion PP and the size of the nugget portion PN within an appropriate range. Therefore, the press-welded portion PP of the desired size can be formed in a short time while suppressing the generation of spatter.

[0028] Furthermore, in this embodiment, upslope current supply and constant current supply are performed in the main current supply step. By performing upslope current supply in the main current supply step, it is possible to suppress a sudden increase in the temperature of the joint WP and a sudden expansion of the nugget portion PN, compared to when only constant current supply is performed in the main current supply step. By performing constant current supply in the main current supply step, it is possible to grow the nugget portion PN in a shorter time, compared to when only upslope current supply is performed in the main current supply step. Therefore, it is possible to form a nugget portion PN of a desired size in a short time while suppressing the generation of spatter.

[0029] B. Second embodiment: 5 is a cross-sectional view showing a joint WP of a workpiece W joined by a resistance spot welding method according to a second embodiment. The second embodiment differs from the first embodiment in that the workpiece W is not made up of two metal plates W1 and W2, but is made up of three metal plates W1 to W3. Unless otherwise specified, the second embodiment is the same as the first embodiment.

[0030] FIG. 5 shows a cross section of the joint WP at the end of the pre-energization process. In this embodiment, the workpiece W includes a first metal plate W1, a second metal plate W2, and a third metal plate W3. At the joint WP of the workpiece W, the first metal plate W1, the second metal plate W2, and the third metal plate W3 overlap in this order. The nugget portion PN is ideally elliptical, but in reality, due to disturbances and other factors, it becomes triangular as shown in FIG. 5. Therefore, the nugget diameter at the joint interface between the first metal plate W1 and the second metal plate W2 is larger than the nugget diameter at the joint interface between the second metal plate W2 and the third metal plate W3. In the following description, of the multiple joint interfaces present at the joint WP of the workpiece W, the joint interface with the largest nugget diameter is referred to as the first joint interface WS1, and the joint interface with the smallest nugget diameter is referred to as the second joint interface WS2. In FIG. 5, the joint interface between the first metal plate W1 and the second metal plate W2 is the first joint interface WS1, and the joint interface between the second metal plate W2 and the third metal plate W3 is the second joint interface WS2.

[0031] FIG. 5 shows the pressure welding diameter dp1 of the first bonding interface WS1 at the end of the pre-current process, the pressure welding diameter dp2 of the second bonding interface WS2 at the end of the pre-current process, the nugget diameter dn1 of the first bonding interface WS1 at the end of the pre-current process, the nugget diameter dn2 of the second bonding interface WS2 at the end of the pre-current process, the lower limit nugget diameter Ln1 of the first bonding interface WS1 at the end of the main current process, and the lower limit nugget diameter Ln2 of the second bonding interface WS2 at the end of the main current process. In this embodiment, the resistance spot welding method is performed so that the pressure welding diameter dp1 of the first joint interface WS1 at the end of the pre-current flow process, the lower limit nugget diameter Ln1 of the first joint interface WS1 at the end of the main current flow process, and the sheet thickness T that determines the lower limit nugget diameter Ln1 of the first joint interface WS1 satisfy the relationship of the following formula (2), and the nugget diameter dn2 of the second joint interface WS2 at the end of the pre-current flow process and the lower limit nugget diameter Ln2 of the second joint interface WS2 at the end of the main current flow process satisfy the relationship of the following formula (3). Here, the sheet thickness T that determines the lower limit nugget diameter Ln1 of the first joint interface WS1 is the sheet thickness of the smallest metal sheet among the three metal sheets W1 to W3 that make up the joint WP. 1.2×Ln1+0.5×(T) 1 / 2 ≦dp1≦1.2×Ln1+1.5×(T) 1 / 2 ···(2) dn2 ≥ 0.7 × Ln2 (3)

[0032] In the case of a three-plate welding in which three metal plates W1 to W3 are joined by resistance spot welding, the relationship shown in the above-mentioned formula (2) and the relationship shown in the above-mentioned formula (3) can be satisfied by welding under the following conditions, for example. <Materials and thickness of each metal plate> The first metal plate W1 is a hot-dip galvanized steel plate (SCGA780), and the thickness of the first metal plate W1 is t1 = 1.40 mm. The second metal plate W2 is a hot-dip galvanized steel plate (SCGA980), and the thickness of the second metal plate W2 is t2 = 1.60 mm. The third metal plate W3 is a hot-dip galvanized steel plate (SCGA1180), and the thickness of the third metal plate W3 is t3 = 2.00 mm. The gap G1 between the first metal plate W1 and the second metal plate W2 is 0.00 mm. The gap G2 between the second metal plate W2 and the third metal plate W3 is 2.00 mm. <Pressure> The pressure is constant at 3920N from the start of the pre-energization process to the end of the main energization process. <Power supply conditions> Current application pattern for the pre-current application process and the main current application process: As shown in Figure 4. Pulsation energization in the pre-energization process: high current value I1 = 11.5 kA, high current energization time T1 = 6 ms, low current value I2 = 6.5 kA, low current energization time T2 = 6 ms. Pre-energization process: constant current I3 = 7.0 kA, energization time T3 = 1 ms. Up-slope current application in the pre-energization step: starting current I3 = 7.0 kA, ending current I4 = 7.6 kA, current application time T4 = 400 ms. · Upslope current flow in this current flow process: starting current value I4 = 7.6 kA, ending current value I5 = 8.2 kA, current flow time T5 = 34 ms. · Constant current energization in this energization process: current value I5 = 8.2 kA, energization time T6 = 34 ms.

[0033] According to the resistance spot welding method of the present embodiment described above, even in the case of three-sheet welding, it is possible to prevent molten metal from flying out of the nugget portion PN during the main current application process and to prevent spatter from occurring.

[0034] C. Other Embodiments: (C1) In the resistance spot welding method of each of the above-described embodiments, the pre-energization step includes pulsation energization. However, the pre-energization step does not necessarily have to include pulsation energization. In this case, the pre-energization step may include at least one of constant current energization and up-slope energization.

[0035] (C2) In the resistance spot welding method of each of the above-described embodiments, the main current supplying step includes upslope current supplying and constant current supplying. However, the main current supplying step may include upslope current supplying without constant current supplying, or may include constant current supplying without upslope current supplying.

[0036] (C3) In the resistance spot welding method of the second embodiment described above, the joint WP of the workpiece W is composed of three metal plates W1 to W3. In contrast to this, in the resistance spot welding method of the second embodiment, the joint WP of the workpiece W may be composed of four or more metal plates.

[0037] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0038] 1...resistance spot welding device, 10...welding gun, 11...gun body, 20...moving mechanism, 21...first electrode, 22...second electrode, 30...power supply, 40...pressure device, 53...current sensor, 54...voltage sensor, 80...control device, dn, Dn...nugget diameter, dp, Dp...pressure welding diameter, Ln...lower limit nugget diameter, PN...nugget portion, PP...pressure welding portion, W...workpiece, W1...first metal plate, W2...second metal plate, W3...third metal plate, WP...joint portion, WS...joint interface

Claims

1. 1. A resistance spot welding method comprising: a pre-energization step of energizing a workpiece to form a pressure-welded portion of a predetermined size on the workpiece; a main current application process in which current is applied to the workpiece after the pre-current application process to form a nugget portion of a predetermined size inside the pressure-welded portion; and a relationship between a diameter dp of the pressure welded portion at the end of the pre-energization step and a lower limit Ln of a diameter of the nugget portion at the end of the main energization step satisfies the following formula (1): 0.7 × Ln ≦ dp ≦ 2.5 × Ln (1)

2. 1. A resistance spot welding method comprising: a pre-energization step of energizing a workpiece to form a pressure-welded portion of a predetermined size on the workpiece; a main current application process in which current is applied to the workpiece after the pre-current application process to form a nugget portion of a predetermined size inside the pressure-welded portion; and the joint portion of the workpiece is formed of three or more metal plates and has a first joint interface which is a joint interface where the diameter of the nugget portion is maximum, and a second joint interface which is a joint interface where the diameter of the nugget portion is minimum, a diameter dp1 of the pressure-welded portion at the first joint interface at the end of the pre-current flow process, a lower limit Ln1 of the diameter of the nugget portion at the first joint interface after the end of the main current flow process, and a thickness T of the metal plate that determines the lower limit Ln1 satisfy the relationship of the following formula (2), and a diameter dn2 of the nugget portion at the second joint interface at the end of the pre-current flow process, and a lower limit Ln2 of the diameter of the nugget portion at the second joint interface after the end of the main current flow process satisfy the relationship of the following formula (3). 1.2×Ln1+0.5×(T) １／２ ≦dp1≦1.2×Ln1+1.5×(T) １／２ ・・・(2) dn2 ≧ 0.7 × Ln2 (3)

3. 3. The resistance spot welding method according to claim 1 or 2, The pre-energization step includes: Pulsation energization is performed by alternately energizing the electric current at a value higher than a predetermined threshold and energizing the electric current at a value equal to or lower than the threshold; At least one of a constant current energization in which a current is energized at a constant value and an up-slope energization in which the current value is increased over time; A resistance spot welding method comprising:

4. 3. The resistance spot welding method according to claim 1 or 2, The main current application step includes: Constant current energization, which energizes at a constant current value; Up-slope current conduction, which increases the current value over time, A resistance spot welding method comprising:

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