Resistance spot welding method

Abstract

A stored time variation curve and cumulative amount of heat generated of each step are each used as a target. In the case where a time variation of an instantaneous amount of heat generated per unit volume differs from the time variation curve in any of the steps, a current passage amount is controlled in order to compensate for the difference within a remaining welding time in the step so that a cumulative amount of heat generated per unit volume in actual welding matches the stored cumulative amount of heat generated in the test welding. Further, in the case where expulsion is detected in any of the steps, then in subsequent welding, the cumulative amount of heat generated per unit volume used as the target is reduced, and the current passage amount is adjusted in accordance with the reduced cumulative amount of heat generated per unit volume.

Claims

1. A resistance spot welding method of squeezing parts to be welded by a pair of electrodes, and passing a current while applying an electrode force to join the parts to be welded, the parts to be welded being a plurality of overlapping metal sheets, the resistance spot welding method comprising: performing test welding; and performing actual welding after the test welding, wherein in each of the test welding and the actual welding, a current pattern is divided into two or more steps, in the test welding, an amount of time variation of an instantaneous amount of heat generated per unit volume as a time variation curve and a cumulative amount of heat generated per unit volume are stored for each of the steps, both the amount of time variation and the cumulative amount of heat being calculated from an electrical property between the electrodes in the case of forming an appropriate nugget by passing a current by constant current control, in the actual welding, the stored time variation curve and cumulative amount of heat generated of each of the steps are each used as a target and, adaptive control welding is performed to: in the case where a time variation of the instantaneous amount of heat generated per unit volume differs from the time variation curve in any of the steps, control a current passage amount in order to compensate for the difference within a remaining welding time in the step so that a cumulative amount of heat generated per unit volume in the actual welding matches the stored cumulative amount of heat generated in the test welding; and, in the case where expulsion is detected in any of the steps, then in subsequent welding, reduce the cumulative amount of heat generated per unit volume used as the target and adjust the current passage amount in accordance with the reduced cumulative amount of heat generated per unit volume, and Qs[T] is set in a range of the following expression in relation to Q0[T], Qf, and R:
Qf+(Q0[T]Qf)(1(5R/100))Qs[T]Qf+(Q0[T]Qf)(1(0.1R/100)) where Qs[T] in J is the cumulative amount of heat generated per unit volume in a welding time T used as the target after the detection of the expulsion, Q0[T] in J is the cumulative amount of heat generated per unit volume in the welding time T stored in the test welding, Qf in J is the cumulative amount of heat generated per unit volume until the detection of expulsion, and R in % is a rate of decrease in voltage between electrodes or resistance between electrodes per cycle at 50 Hz upon the detection of the expulsion.

2. The resistance spot welding method according to claim 1, wherein the expulsion is detected based on a change in any of voltage between electrodes, resistance between electrodes, distance between electrodes, servo torque of a welding gun, and strain of the welding gun.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1A is a diagram schematically illustrating an example of the relationship between the welding time and the voltage between electrodes according to one of the disclosed embodiments;

(3) FIG. 1B is a diagram schematically illustrating an example of the relationship between the welding time and the welding current according to one of the disclosed embodiments;

(4) FIG. 1C is a diagram schematically illustrating an example of the relationship between the welding time and the cumulative amount of heat generated according to one of the disclosed embodiments;

(5) FIG. 2 is a diagram schematically illustrating an example of a resistance spot welding device;

(6) FIG. 3A is a diagram schematically illustrating the case of welding a sheet combination of three sheets having a sheet gap according to one of the disclosed embodiments; and

(7) FIG. 3B is a diagram schematically illustrating the case of welding a sheet combination of two sheets having a sheet gap according to one of the disclosed embodiments.

DETAILED DESCRIPTION

(8) Detailed description is given below.

(9) The disclosure relates to a resistance spot welding method of squeezing, by a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets, and passing a current while applying an electrode force to join the parts to be welded. The disclosure particularly relates to a suitable resistance spot welding method in the case of welding a sheet combination that is significantly affected by disturbances and is prone to expulsion and for which an appropriate welding condition is very limited, such as a sheet combination with a sheet thickness ratio ((the total thickness of the sheet combination)/(the sheet thickness of the thinnest metal sheet in the sheet combination)) of more than 3.0 or further 5.0 or more in which a thin sheet is overlapped on at least one face of two or more overlapping thick sheets. The upper limit of the sheet thickness ratio is not limited, but is typically 12.0.

(10) Any welding device that includes a pair of upper and lower electrodes and is capable of freely controlling each of the electrode force and the welding current during welding may be used in the resistance spot welding method according to the disclosure. The force mechanism (air cylinder, servomotor, etc.), the type (stationary, robot gun, etc.), the electrode shape, and the like are not limited.

(11) The term thin sheet means a metal sheet with relatively small sheet thickness and the term thick sheet means a metal sheet with relatively large sheet thickness, of the metal sheets used in the parts to be welded. Typically, the sheet thickness of the thin sheet is or less of that of the metal sheet (thick sheet) with the largest sheet thickness.

(12) In the resistance spot welding method according to the disclosure, the current pattern in actual welding is divided into two or more steps to perform welding.

(13) The point at which the current pattern is divided into steps may be set as appropriate depending on, for example, the material, sheet thickness, and number of overlapping metal sheets of the parts to be welded.

(14) For example, in the case of welding a sheet combination with a sheet thickness ratio of more than 3.0 or further 5.0 or more in which a thin sheet is overlapped on at least one face of two or more overlapping thick sheets, nugget formation may be performed on one of the part between the thick sheets and the part between the thin and thick sheets in the first step and the other one of the part between the thick sheets and the part between the thin and thick sheets in the second step.

(15) In the case of welding the parts to be welded which are made up of two metal sheets, the current pattern may be divided into a step of forming a stable current path (weld fusion zone) between the metal sheets and a step of subsequent nugget growth, in order to perform adaptive control welding highly responsive to changes in phenomenon during welding. In the case where the parts to be welded are coated steel sheets, the welding process may be divided into three steps of: a step of melting the coating to rapidly expand the current conducting area; a step of forming a stable current path (weld fusion zone) between the electrodes by subsequent current passage; and a step of subsequent nugget growth.

(16) The magnitude relationship between the welding currents in the respective steps is not limited. A cooling time may be provided between the steps.

(17) Before the actual welding, test welding is performed with the current pattern being divided into steps at the same point as in the actual welding. In the test welding, the amount of time variation of the instantaneous amount of heat generated per unit volume as a time variation curve and the cumulative amount of heat generated per unit volume are stored for each of the steps, where both the amount of time variation and the cumulative amount of heat are calculated from the electrical property between the electrodes in the case of forming an appropriate nugget by passing a current by constant current control.

(18) For the test welding, a welding test with the same steel type and thickness as the parts to be welded is performed by constant current control under various conditions in the state where there is no current shunting to an existing weld or sheet gap, to find an optimal condition in the test welding. Then, for each step, the time variation of the instantaneous amount of heat generated per unit volume is stored as a time variation curve, and the cumulative amount of heat generated per unit volume is stored. Both the time variation and the cumulative amount are calculated from the electrical property between the electrodes during the welding performed under the aforementioned condition. The electrical property between the electrodes means the resistance between electrodes or the voltage between electrodes.

(19) After the test welding, the actual welding is performed. In the actual welding, the time variation curve stored in the test welding for each step is used as a target. If the time variation of the instantaneous amount of heat generated per unit volume follows the stored time variation curve, the welding is continued without change and completed. If the time variation of the instantaneous amount of heat generated per unit volume differs from the stored time variation curve, on the other hand, the adaptive control welding of controlling the current passage amount depending on the difference is carried out to compensate for the difference within the remaining welding time of the step so that the cumulative amount of heat generated per unit volume in the actual welding matches the stored cumulative amount of heat generated per unit volume in the test welding.

(20) Thus, the required cumulative amount of heat generated can be ensured to obtain an appropriate nugget diameter even in the state where the effects of disturbances such as electrode tip wear, current shunting, or a sheet gap are significant.

(21) The disturbances mentioned here include not only the presence of an existing weld or a contact point of the parts to be welded near the current welding point, but also electrode wear.

(22) The method of calculating the cumulative amount of heat generated per unit volume is not limited. PTL 3 describes an example of the method, which may be used in this disclosure. The following is the procedure of calculating the cumulative amount Q of heat generated per unit volume according to this method.

(23) Let t be the total thickness of the parts to be welded, r be the electrical resistivity of the parts to be welded, V be the voltage between electrodes, I be the welding current, and S be the contact area of the electrodes and the parts to be welded. In this case, the welding current passes through a columnar portion whose cross-sectional area is S and thickness is t, to generate heat by resistance. The amount q of heat generated per unit volume and per unit time in the columnar portion is given by the following Equation (1):
q=(V.Math.I)/(S.Math.t)(1).

(24) The electrical resistance R of the columnar portion is given by the following Equation (2):
R=(r.Math.t)/S(2).

(25) Solving Equation (2) for S and substituting the solution into Equation (1) yields the amount q of heat generated as indicated by the following Equation (3):
q=(V.Math.I.Math.R)/(r.Math.t.sup.2)=(V.sup.2)/(r.Math.t.sup.2)(3).

(26) As is clear from Equation (3), the amount q of heat generated per unit volume and per unit time can be calculated from the voltage between electrodes V, the total thickness t of the parts to be welded, and the electrical resistivity r of the parts to be welded, and is not affected by the contact area S of the electrodes and the parts to be welded. Although the amount of heat generated is calculated from the voltage between electrodes V in Equation (3), the amount q of heat generated may be calculated from the interelectrode current I. The contact area S of the electrodes and the parts to be welded need not be used in this case, either. By cumulating the amount q of heat generated per unit volume and per unit time for the welding time, the cumulative amount Q of heat generated per unit volume for the welding is obtained. As is clear from Equation (3), the cumulative amount Q of heat generated per unit volume can also be calculated without using the contact area S of the electrodes and the parts to be welded.

(27) Although the above describes the case of calculating the cumulative amount Q of heat generated by the method described in PTL 3, the cumulative amount Q may be calculated by any other method.

(28) In the resistance spot welding method according to the disclosure, it is very important to, in the case where expulsion is detected in any of the steps, then in subsequent welding, reduce the cumulative amount of heat generated per unit volume used as the target and adjust the current passage amount, i.e. the welding current and the voltage between electrodes, in accordance with the reduced cumulative amount of heat generated per unit volume, as illustrated in FIGS. 1A to 1C.

(29) Once expulsion has occurred during welding, molten metal splatters on the surroundings. Consequently, the sheet thickness of the weld decreases, and the voltage between electrodes decreases sharply. If welding is continued using, as the target, the time variation of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are stored in the test welding, such control that excessively increases the welding current or the voltage between electrodes will end up being performed. This facilitates further expulsion.

(30) Hence, in the resistance spot welding method according to the disclosure, in the case where expulsion is detected in any of the steps, then in subsequent welding, the cumulative amount of heat generated per unit volume used as the target is reduced, and the current passage amount, i.e. the welding current and the voltage between electrodes, is adjusted in accordance with the reduced cumulative amount of heat generated per unit volume.

(31) The reason why the desired nugget diameter can be obtained even when the target of the cumulative amount of heat generated per unit volume is reduced appears to be as follows: The sheet thickness of the weld decreases due to expulsion, and as a result the amount of heat that needs to be generated to obtain the desired nugget diameter decreases, too.

(32) Regarding the method of newly setting the target of the cumulative amount of heat generated per unit volume after expulsion, for example, Qs[T]is preferably set in the range of the following expression in relation to Q0[T], Qf, and R:
Qf+(Q0[T]Qf)(1(5R/100))Qs[T]Qf+(Q0[T]Qf)(1(0.1R/100))

(33) where Qs[T] (J) is the cumulative amount of heat generated per unit volume in welding time T newly used as the target after the detection of expulsion, Q0[T] (J) is the cumulative amount of heat generated per unit volume in the welding time T stored in the test welding, Qf (J) is the cumulative amount of heat generated per unit volume until the detection of expulsion, and R (%) is the rate of decrease in voltage between electrodes or resistance between electrodes per cycle (50 Hz (hereafter all time units are expressed by the number of cycles at 50 Hz)) upon expulsion.

(34) By setting the target of the cumulative amount of heat generated in this range, even in the case where expulsion is detected during welding, a nugget of the desired diameter corresponding to the sheet thickness of the weld after expulsion can be obtained without further expulsion. The welding time T mentioned here is the time elapsed from when the current passage starts.

(35) Qs[T] is more preferably set in the range of the following expression in relation to Q0[T], Qf, and R:
Qf+(Q0[T]Qf)(1(4R/100))Qs[T]Qf+(Q0[T]Qf)(1(0.3R/100)).

(36) Qs[T] is further preferably set in the range of the following expression in relation to Q0[T], Qf, and R:
Qf+(Q0[T]Qf)(1(3R/100))Qs[T]Qf+(Q0[T]Qf)(1(0.5R/100)).

(37) In the aforementioned respective expressions, the left side of the expression is Qf or less in the following cases:
Qf<Qs[T]Qf+(Q0[T]Qf)(1(0.1R/100))
Qf<Qs[T]Qf+(Q0[T]Qf)(1(0.3R/100))
Qf<Qs[T]Qf+(Q0[T]Qf)(1(0.5R/100)).

(38) The time variation curve of the instantaneous amount of heat generated per unit volume is corrected according to the correction of the target of the cumulative amount of heat generated per unit volume, at the same ratio as the correction of the cumulative amount of heat generated.

(39) As the expulsion detector, the following technique may be used.

(40) When expulsion occurs, the voltage between electrodes and the resistance between electrodes decrease due to a decrease in sheet thickness of the weld. Accordingly, a technique of recognizing the occurrence of expulsion when the rate of decrease R in voltage between electrodes or resistance between electrodes per cycle (50 Hz) exceeds a predetermined threshold may be used as an example. The threshold is preferably 10% to 30%, in terms of distinguishing the decrease from changes in voltage between electrodes or resistance between electrodes in normal adaptive control.

(41) Even in the case where expulsion occurs, the maximum value of the rate of decrease R in voltage between electrodes or resistance between electrodes per cycle (50 Hz) is typically 50%.

(42) The distance between electrodes also decreases sharply due to a decrease in sheet thickness associated with expulsion, and so may be equally used for the expulsion detector.

(43) When expulsion occurs, the reaction force of the steel sheet to the electrode force of the electrode decreases sharply. An attempt to maintain the set electrode force in such a state causes the electrode force to significantly change temporarily, due to a delay in response of an electrode force control device. This may be equally used for the expulsion detector. The method of measuring the electrode force is not limited, and the electrode force may be measured based on the torque of the servomotor of a welding gun, the strain of the welding gun, or the like.

(44) FIG. 2 is a schematic diagram illustrating an example of the structure of a resistance spot welding device suitable for the resistance spot welding method according to the disclosure.

(45) In FIG. 2, reference sign 1 is a resistance spot welding power source, 2 is a controller that provides a control signal to the resistance spot welding power source 1, 3 is a welding current detector that outputs a detected signal to the controller 2, and 4 is a secondary conductor connected to the output of the resistance spot welding power source 1 and also connected to electrodes 7 to pass a current to the electrodes 7. Reference sign 5 is a lower arm and 6 is a force cylinder, which are attached to the respective electrodes 7. The electrodes 7 squeeze parts to be welded 8. Reference sign 9 is a voltage between electrodes detection line attached to the electrodes 7, and connected to the controller 2. The controller 2 can switch between a test welding mode and an actual welding mode.

(46) In the test welding mode, the controller 2 calculates the instantaneous amount of heat from the current received from the welding current detector 3 and the voltage received from the voltage between electrodes detection line 9, and stores the amount of time variation of the instantaneous amount of heat generated.

(47) In the actual welding mode, having started current passage under the welding condition of the test welding, the controller 2 calculates, per sampling time, the instantaneous amount of heat generated from the current received from the welding current detector 3 and the voltage received from the voltage between electrodes detection line 9. The controller 2 then compares the instantaneous amount of heat generated in the actual welding and the stored instantaneous amount of heat generated in the test welding, per unit time. When the two values differ from each other, adaptive control welding of controlling the welding current depending on the difference is performed. Thus, the welding current adaptively controlled by the controller 2 is passed through the parts to be welded 8 so that the cumulative amount of heat generated in the actual welding matches the stored cumulative amount of heat generated in the test welding.

(48) In the case where expulsion is detected from, for example, a sharp decrease of the voltage received from the voltage between electrodes detection line 9, the controller 2 newly sets, as the target, the cumulative amount of heat generated stored in the test welding, and adjusts the current passage amount, i.e. the welding current and the voltage between electrodes, based on the newly set cumulative amount of heat generated.

(49) The magnitude relationship of each condition such as electrode force or welding time between the steps is not limited, and may be set as appropriate depending on, for example, the material, sheet thickness, and number of overlapping metal sheets of the parts to be welded.

(50) For example, suppose, when welding a sheet combination with a sheet thickness ratio of more than 3.0 or further 5.0 or more in which a thin sheet is overlapped on at least one face of two or more overlapping thick sheets, the part between the thick sheets is molten in the first step and the part between the thin and thick sheets is molten in the second step. Then, the electrode force F1 (kN) in the first step and the electrode force F2 (kN) in the second step preferably satisfy the relationship F1>F2. Further preferably, the welding times T1 and T2 in the first and second steps are 5 cycles to 50 cycles and 1 cycle to 20 cycles, respectively. In this case, a cooling time of 1 cycle or more and 100 cycles or less is preferably provided between the current passage in the first step and the current passage in the second step, in terms of preventing expulsion in the second step.

(51) The metal sheets as the parts to be welded in the resistance spot welding method according to the disclosure are not limited. The resistance spot welding method may be used for the welding of steel sheets and coated steel sheets having various strengths from mild steel to ultra high tensile strength steel and light metal sheets of aluminum alloys and the like. The resistance spot welding method may also be used for a sheet combination of four or more overlapping steel sheets.

Examples

(52) For each sheet combination of two or three overlapping metal sheets listed in Table 1 and illustrated in FIGS. 3A and 3B, resistance spot welding was performed under each condition listed in Table 2 to produce a joint. In the drawings, reference sign 11 is a metal sheet (thin sheet), 12 and 13 are each a metal sheet (thick sheet), 14 is an electrode, and 15 is a spacer.

(53) The result in the case where the control mode is adaptive control (without expulsion correction) in Table 2 indicates the result of, after performing test welding in the absence of disturbances such as a sheet gap under the welding condition in Table 2 and storing the time variation of the instantaneous amount of heat generated per unit volume, performing adaptive control spot welding of adjusting the current with reference to the time variation curve of the instantaneous amount of heat generated per unit volume obtained in the test welding regardless of whether or not expulsion occurred. The result in the case where the control mode is adaptive control (with expulsion correction) in Table 2 indicates the result of, when the rate of decrease in voltage between electrodes per unit time exceeded a threshold (20%), determining that expulsion occurred, and, in subsequent welding, reducing the cumulative amount of heat generated per unit volume used as the target, as presented in Table 2, based on the rate of decrease in voltage between electrodes or resistance between electrodes per cycle upon expulsion detection, and adjusting the current passage amount in accordance with the reduced cumulative amount of heat generated per unit volume. The conditions such as welding time and electrode force were the same in the test welding and the actual welding.

(54) As illustrated in FIGS. 3A and 3B, the spacers 15 (inter-spacer distance: 60 mm) were inserted between the thick sheets 12 and 13, and the sheet combination was clamped from above and below (not illustrated), to create a sheet gap of any of various sheet gap thicknesses.

(55) An inverter DC resistance spot welder was used as the welder, and chromium copper electrodes with 6 mm face diameter DR-shaped tips were used as the electrodes.

(56) For each obtained joint, the weld was cut and etched in section, and then observed with an optical microscope to measure each of the nugget diameter d1 (mm) between the thin and thick sheets and the nugget diameter d2 (mm) between the thick sheets. Each sample in which the nugget diameters d1 and d2 were both 4t or more (t: the sheet thickness (mm) of the thinner metal sheet of the adjacent two metal sheets) and the minimum thickness of the weld was 40% or more of the total thickness of the sheet combination before welding was evaluated as good. Each sample in which any of the nugget diameters d1 and d2 was less than 4t or the minimum thickness of the weld was less than 40% of the total thickness of the sheet combination before welding was evaluated as poor.

(57) TABLE-US-00001 TABLE 1 Sheet Metal sheet (thin sheet) Metal sheet (thick sheet) Metal sheet (thick sheet) Sheet combination of reference sign of reference sign of reference sign thickness No. 11 in the drawings 12 in the drawings 13 in the drawings ratio A 270 MPa-grade cold rolled 980 MPa-grade cold rolled 980 MPa-grade cold rolled 6.7 steel sheet (sheet steel sheet (sheet steel sheet (sheet thickness: 0.7 mm) thickness: 2.0 mm) thickness: 2.0 mm) B 270 MPa-grade GA steel sheet 780 MPa-grade GA steel sheet 780 MPa-grade GA steel sheet 5.6 (sheet thickness: 0.7 mm) (sheet thickness: 1.6 mm) (sheet thickness: 1.6 mm) C 270 MPa-grade GA steel sheet 590 MPa-grade GA steel sheet 590 MPa-grade GA steel sheet 3.9 (sheet thickness: 0.7 mm) (sheet thickness: 1.0 mm) (sheet thickness: 1.0 mm) D 980 MPa-grade cold rolled 980 MPa-grade cold rolled 2.0 steel sheet (sheet steel sheet (sheet thickness: 2.0 mm) thickness: 2.0 mm)

(58) TABLE-US-00002 TABLE 2 First step Second step Electrode Welding Welding Cooling Electrode Welding Welding Sheet force current time time force current time Joint combination F1 I1 T1 Tc F2 I2 T2 Control Sheet gap Expulsion R*1 Qs[T]*2 No. No. (kN) (kA) (cycles) (cycles) (kN) (kA) (cycles) mode thickness detection (%) (J) Evaluation Remarks 1 A 3.5 8.0 20 50 1.0 10.5 5 Adaptive control 1.5 mm First step: detected 20 Qf + (Q0[T]*.sup.3 Good Example (with expulsion Second step: not Qf) 0.8 correction) detected 2 A 5.0 7.0 20 50 1.0 10.5 5 Adaptive control 0.5 mm Not detected Good Reference (with expulsion Example correction) 3 A 5.0 7.0 20 50 1.0 10.5 5 Adaptive control 1.0 mm Not detected Good Reference (with expulsion Example correction) 4 B 3.0 6.5 15 15 1.5 10.0 4 Adaptive control 1.5 mm First step: detected 25 Qf + (Q0[T]*.sup.3 Good Example (with expulsion Second step: not Qf) 0.7 correction) detected 5 C 3.0 9.5 8 3.0 7.0 10 Adaptive control 1.5 mm First step: detected 23 Qf + (Q0[T]*.sup.3 Good Example (with expulsion Second step: not Qf) 0.85 correction) detected 6 D 3.5 8.0 7 3.5 8.5 16 Adaptive control 1.5 mm First step: detected 25 Qf + (Q0[T]*.sup.3 Good Example (with expulsion Second step: not Qf) 0.75 correction) detected 7 A 3.5 8.0 20 50 1.0 10.5 5 Adaptive control 1.5 mm First step: detected Poor Comparative (without expulsion Second step: not sheet thickness Example correction) detected decrease 8 C 3.0 9.5 8 3.0 7.0 10 Adaptive control 1.5 mm First step: detected Poor Comparative (without expulsion Second step: not sheet thickness Example correction) detected decrease 9 A 3.5 8.0 20 Adaptive control 1.5 mm First step: detected Poor Comparative (without expulsion sheet thickness Example correction) decrease 10 D 3.5 8.0 16 Adaptive control 1.5 mm Fust step: detected Poor Comparative (without expulsion sheet thickness Example correction) decrease *1Rate of decrease in voltage between electrodes or resistance between electrodes per cycle (50 Hz) upon expulsion detection. *2Cumulative amount of heat generated per unit volume in welding time T used as the target after expulsion detection. *.sup.3Cumulative amount of heat generated per unit volume in welding time T stored in test welding.

(59) In Examples, the minimum thickness of the weld was sufficient, and a nugget with a diameter of 4t or more was obtained between the thin and thick sheets and between the thick sheets. In Reference Examples with small sheet gap thicknesses, a nugget with a diameter of 4t or more was obtained between the thin and thick sheets and between the thick sheets, without detecting expulsion during the process.

(60) In all Comparative Examples, on the other hand, the minimum thickness of the weld was insufficient.

REFERENCE SIGNS LIST

(61) 1 resistance spot welding power source 2 controller that provides control signal to resistance spot welding power source 3 welding current detector 4 secondary conductor connected to output of resistance spot welding power source 5 lower arm 6 force cylinder 7 electrode 8 part to be welded 9 voltage between electrodes detection line 11 metal sheet (thin sheet) 12, 13 metal sheet (thick sheet) 14 electrode 15 spacer