RESISTANCE SPOT WELDING METHOD AND WELD MEMBER PRODUCTION METHOD

Abstract

A resistance spot welding method comprises: performing test welding; and performing actual welding after the test welding, wherein in main current passage in the actual welding, adaptive control welding is performed, and in subsequent current passage in the actual welding, current passage is performed by constant current control with a current determined based on an electrical property between electrodes in each of main current passage in the test welding and the main current passage in the actual welding.

Claims

1. 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 resistance spot welding method comprising: performing test welding; and performing actual welding after the test welding, wherein (a) in the test welding, main current passage for nugget formation and subsequent current passage for subsequent heat treatment are performed, in the main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the subsequent current passage in the test welding, current passage is performed by constant current control, and (b) thereafter, in the actual welding, main current passage for nugget formation and subsequent current passage for subsequent heat treatment are performed, in the main current passage in the actual welding, the time variation curve 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 main current passage in the test welding are set as a target, and adaptive control welding is performed to control a current passage amount according to the target, and in the subsequent current passage in the actual welding, current passage is performed by constant current control with a current determined based on an electrical property between the electrodes in each of the main current passage in the test welding and the main current passage in the actual welding.

2. The resistance spot welding method according to claim 1, wherein
0.8Itp(RBtm/RBam)Iap1.2Itp(RBtm/RBam), where RBtm is an average value of a resistance between the electrodes in the main current passage in the test welding, RBam is an average value of a resistance between the electrodes in the main current passage in the actual welding, Itp is a current in the subsequent current passage in the test welding, and Iap is the current in the subsequent current passage in the actual welding.

3. The resistance spot welding method according to claim 1, wherein in the adaptive control welding in the main current passage in the actual welding, in the case where an amount of time variation of an instantaneous amount of heat generated per unit volume differs from the time variation curve of the instantaneous amount of heat generated per unit volume set as the target, the current passage amount is controlled in order to compensate for the difference from the time variation curve within a remaining welding time in the main current passage in the actual welding so that a cumulative amount of heat generated per unit volume in the main current passage in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

4. The resistance spot welding method according to claim 1, wherein a cooling time is set between the main current passage and the subsequent current passage in the actual welding, and the number of repetitions of a welding interval for the cooling time and the subsequent current passage after the main current passage is two or more times.

5. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 1.

6. The resistance spot welding method according to claim 2, wherein in the adaptive control welding in the main current passage in the actual welding, in the case where an amount of time variation of an instantaneous amount of heat generated per unit volume differs from the time variation curve of the instantaneous amount of heat generated per unit volume set as the target, the current passage amount is controlled in order to compensate for the difference from the time variation curve within a remaining welding time in the main current passage in the actual welding so that a cumulative amount of heat generated per unit volume in the main current passage in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

7. The resistance spot welding method according to claim 2, wherein a cooling time is set between the main current passage and the subsequent current passage in the actual welding, and the number of repetitions of a welding interval for the cooling time and the subsequent current passage after the main current passage is two or more times.

8. The resistance spot welding method according to claim 3, wherein a cooling time is set between the main current passage and the subsequent current passage in the actual welding, and the number of repetitions of a welding interval for the cooling time and the subsequent current passage after the main current passage is two or more times.

9. The resistance spot welding method according to claim 6, wherein a cooling time is set between the main current passage and the subsequent current passage in the actual welding, and the number of repetitions of a welding interval for the cooling time and the subsequent current passage after the main current passage is two or more times.

10. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 2.

11. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 3.

12. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 4.

13. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 6.

14. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 7.

15. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 8.

16. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the accompanying drawings:

[0043] FIG. 1A is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0044] FIG. 1B is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0045] FIG. 1C is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0046] FIG. 1D is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0047] FIG. 1E is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0048] FIG. 1F is a diagram illustrating an example of a current pattern in main current passage in test welding;

[0049] FIG. 2A is a diagram illustrating an example of a current pattern in test welding in the case where main current passage is one-step current passage;

[0050] FIG. 2B is a diagram illustrating an example of a current pattern in test welding in the case where main current passage is two-step current passage;

[0051] FIG. 3A is a diagram illustrating an L-shaped tensile test piece used in examples in the case where the number of overlapping metal sheets is two and there is no existing weld;

[0052] FIG. 3B is a diagram illustrating an L-shaped tensile test piece used in examples in the case where the number of overlapping metal sheets is two and there is an existing weld;

[0053] FIG. 4A is a diagram illustrating an L-shaped tensile test piece used in examples in the case where the number of overlapping metal sheets is three and there is no existing weld; and

[0054] FIG. 4B is a diagram illustrating an L-shaped tensile test piece used in examples in the case where the number of overlapping metal sheets is three and there is an existing weld.

DETAILED DESCRIPTION

[0055] One of the disclosed embodiments 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 resistance spot welding method comprising: performing test welding; and performing actual welding after the test welding, wherein (a) in the test welding, main current passage for nugget formation and subsequent current passage for subsequent heat treatment are performed, in the main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the subsequent current passage in the test welding, current passage is performed by constant current control, and (b) thereafter, in the actual welding, main current passage for nugget formation and subsequent current passage for subsequent heat treatment are performed, in the main current passage in the actual welding, the time variation curve 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 main current passage in the test welding are set as a target, and adaptive control welding is performed to control a current passage amount according to the target, and in the subsequent current passage in the actual welding, current passage is performed by constant current control with a current determined based on an electrical property between the electrodes in each of the main current passage in the test welding and the main current passage in the actual welding.

[0056] 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 one of the disclosed embodiments. The force mechanism (air cylinder, servomotor, etc.), the type (stationary, robot gun, etc.), the electrode shape, and the like are not limited. Herein, the electrical property between the electrodes means the resistance between the electrodes or the voltage between the electrodes.

[0057] The test welding and the actual welding in the resistance spot welding method according to one of the disclosed embodiments will be described below. [0058] Test welding

[0059] In the test welding, main current passage for nugget formation and subsequent current passage for subsequent heat treatment are each performed by constant current control.

[0060] In the main current passage in the test welding, a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored.

[0061] The test welding may be performed in a state in which there is no disturbance, or performed in a state in which there is a disturbance such as current shunting or a sheet gap (i.e. a state assuming that there is a disturbance).

[0062] The current pattern in the main current passage in the test welding may be a current pattern of a constant current throughout the current passage. The current pattern may be a current pattern divided into two or more steps each of which has a constant current, as illustrated in FIGS. 1A and 1B. The current pattern may be a current pattern of two or more steps with a cooling time being provided therebetween, as illustrated in FIG. 1C. The current pattern may be a current pattern in slope form, as illustrated in FIGS. 1D to 1F. The current pattern may be any combination of these patterns.

[0063] Herein, the term constant current control includes not only a current pattern of a constant current throughout the current passage, but also the current patterns illustrated in FIGS. 1A to 1F, and any current patterns combining these current patterns. The same applies to constant current control performed in the subsequent current passage in each of the test welding and the actual welding.

[0064] A preferable range of the current in the main current passage in the test welding varies depending on which sheet combination is used as the parts to be welded. For example, in the case of using a sheet combination of steel sheets of 980 MPa-grade in tensile strength (TS) and 1.2 mm to 1.6 mm in sheet thickness, the current in the main current passage in the test welding is preferably in a range of 3.0 kA to 12.0 kA.

[0065] The total welding time in the main current passage in the test welding excluding the cooling time is preferably in a range of 60 ms to 1000 ms.

[0066] In the subsequent current passage in the test welding, current passage is performed by constant current control to carry out subsequent heat treatment.

[0067] A time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume in the subsequent current passage in the test welding may or may not be stored.

[0068] A preferable range of the current in the subsequent current passage in the test welding varies depending on which sheet combination is used as the parts to be welded. For example, in the case of using a sheet combination of steel sheets of 980 MPa-grade in tensile strength (TS) and 1.2 mm to 1.6 mm in sheet thickness, the current in the subsequent current passage in the test welding is preferably in a range of 3.0 kA to 15.0 kA.

[0069] The welding time per one subsequent current passage in the test welding is preferably in a range of 10 ms to 200 ms.

[0070] Actual Welding

[0071] After the test welding, the actual welding is performed.

[0072] In the main current passage in the actual welding, the time variation curve 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 main current passage in the test welding are set as a target, and adaptive control welding is performed to control a current passage amount according to the target.

[0073] For example, in the adaptive control welding in the main current passage in the actual welding, welding is performed with the time variation curve 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 main current passage in the test welding being set as the target. If the amount of time variation of the instantaneous amount of heat generated per unit volume follows the time variation curve, the welding is continued without change and completed. If the amount of time variation of the instantaneous amount of heat generated per unit volume differs from the time variation curve, the current passage amount is controlled in order to compensate for the difference within a remaining welding time in the main current passage in the actual welding so that the cumulative amount of heat generated per unit volume in the main current passage in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

[0074] A method of calculating the amount of heat generated is not limited. PTL 5 describes an example of the method, which may be used herein. The following is the procedure of calculating the amount q of heat generated per unit volume and per unit time and the cumulative amount Q of heat generated per unit volume according to this method.

[0075] 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 the 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).

[0076] The electrical resistance R of the columnar portion is given by the following Equation (2):

R=(r.Math.t)/S(2).

[0077] 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).

[0078] 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 V between the electrodes, 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 V between the electrodes 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.

[0079] Although the above describes the case of calculating the cumulative amount Q of heat generated by the method described in PTL 5, the cumulative amount Q may be calculated by any other method.

[0080] In the subsequent current passage in the actual welding, it is important to perform current passage by constant current control with a current determined based on the electrical property between the electrodes in each of the main current passage in the test welding and the main current passage in the actual welding.

[0081] As mentioned earlier, if the subsequent current passage is performed by adaptive control in the presence of a disturbance, the current density distribution of the weld and thus the heat generation pattern change due to the disturbance in some cases, making it impossible to achieve the predetermined heat treatment effect. In particular, if adaptive control is performed with a shorter welding time in the subsequent current passage in a state in which the effect of current shunting is considerable, current control by adaptive control lags behind and a target amount of heat cannot be generated in some cases.

[0082] If the subsequent current passage in the actual welding is performed by constant current control with the current determined based on the electrical property between the electrodes in each of the main current passage in the test welding and the main current passage in the actual welding, on the other hand, the predetermined heat treatment effect can be achieved even in the case where there is a disturbance and the welding time is short.

[0083] It is therefore important to, in the subsequent current passage in the actual welding, perform current passage by constant current control with the current determined based on the electrical property between the electrodes in each of the main current passage in the test welding and the main current passage in the actual welding.

[0084] For example, the predetermined heat treatment effect can be achieved even in the case where there is a disturbance and the welding time is short, by performing the subsequent current passage in the actual welding by constant current control under the condition that Iap satisfies

0.8Itp(RBtm/RBam)Iap1.2Itp(RBtm/RBam)

[0085] where RBtm is an average value of the resistance between the electrodes in the main current passage in the test welding, RBam is an average value of the resistance between the electrodes in the main current passage in the actual welding, Itp is the current in the subsequent current passage in the test welding, and Iap is the current in the subsequent current passage in the actual welding.

[0086] This is because, from the ratio (RBtm/RBam) between the resistance between the electrodes in the main current passage in the test welding and the resistance between the electrodes in the main current passage in the actual welding, a current necessary for the subsequent current passage can be estimated with the effect of a disturbance being taken into consideration.

[0087] More preferably, Iap satisfies

0.9Itp(RBtm/RBam)Iap1.1Itp(RBtm/RBam).

[0088] In the main current passage in each of the test welding and the actual welding, in the case where a cooling time is provided during the current passage as illustrated in FIG. 1C, a time average value of the resistance between the electrodes during the current passage excluding the cooling time is taken to be the average value of the resistance between the electrodes.

[0089] In detail, a value obtained by dividing a time integration value of the resistance between the electrodes in the main current passage by the total welding time in the main current passage excluding the cooling time is taken to be the average value of the resistance between the electrodes in the main current passage. The same applies to the subsequent current passage.

[0090] Instead of the average value of the resistance between the electrodes in the main current passage in each of the test welding and the actual welding, an average value of the current in the main current passage in each of the test welding and the actual welding may be used.

[0091] In this case, the predetermined heat treatment effect can be achieved even in the case where there is a disturbance and the welding time is short, by performing the subsequent current passage in the actual welding by constant current control under the condition that lap satisfies

0.8Itp(IBam/IBtm)Iap1.2Itp(IBam/IBtm),

[0092] where IBtm is an average value of the current in the main current passage in the test welding, and IBam is an average value of the current in the main current passage in the actual welding.

[0093] More preferably, lap satisfies

0.9Itp(IBam/IBtm)Iap1.1Itp(IBam/IBtm).

[0094] In the main current passage in each of the test welding and the actual welding, in the case where a cooling time is provided during the current passage as illustrated in FIG. 1C, a time average value of the current during the current passage excluding the cooling time is taken to be the average value of the current.

[0095] In detail, a value obtained by dividing a time integration value of the current in the main current passage by the total welding time in the main current passage excluding the cooling time is taken to be the average value of the current in the main current passage. The same applies to the subsequent current passage.

[0096] The welding time per one subsequent current passage in the actual welding is preferably in a range of 10 ms to 200 ms.

[0097] In each of the test welding and the actual welding, a cooling time may be set between the main current passage and the subsequent current passage. The cooling time is preferably in a range of 20 ms to 2000 ms.

[0098] In each of the test welding and the actual welding, the number of times a welding interval for the cooling time and the subsequent current passage are performed after the main current passage may be two or more times, as illustrated in FIGS. 2A and 2B (the number of times the welding interval for the cooling time and the subsequent current passage are performed after the main current passage is defined as the number of repetitions N). In this way, the predetermined heat treatment effect can be achieved more favorably.

[0099] Even if heat is generated excessively in the first subsequent current passage and remelting occurs, by performing heat treatment in the second subsequent current passage, the effect of improving joint strength can be achieved. No upper limit is placed on the number of repetitions, but the upper limit of the number of repetitions is preferably about 10. The welding time, the cooling time, and the current may be different each time.

[0100] In the case where the number of repetitions of the welding interval for the cooling time and the subsequent current passage after the main current passage is two or more times, the current Itp in the subsequent current passage in the test welding and the current Iap in the subsequent current passage in the actual welding are each a value obtained by dividing a time integration value of the current in the subsequent current passage by the total welding time in the subsequent current passage excluding the cooling time.

[0101] The conditions in the actual welding other than those described above may be basically the same as the conditions in the test welding.

[0102] The parts to be welded or the sheet combination used is not limited. The resistance spot welding method may be used for steel sheets and coated steel sheets having various strengths from mild steel to ultra high tensile strength steel. The resistance spot welding method may also be used for a sheet combination of three or more overlapping steel sheets, and is particularly advantageous in the case where one or more steel sheets of the sheet combination has a tensile strength of 590 MPa or more.

[0103] In each of the test welding and the actual welding, the electrode force in the current passage may be constant, or be changed as appropriate. A preferable range of the electrode force varies depending on which sheet combination is used as the parts to be welded. For example, in the case of using a sheet combination of two overlapping steel sheets of 980 MPa-grade in tensile strength (TS) and 1.2 mm to 1.6 mm in sheet thickness, the electrode force is preferably in a range of 1.5 kN to 10.0 kN.

[0104] By joining a plurality of overlapping metal sheets by the resistance spot welding method described above, various high-strength weld members, in particular weld members of automotive parts and the like, are produced while stably ensuring a desired nugget diameter by effectively responding to variations in the disturbance state.

EXAMPLES

[0105] The presently disclosed techniques will be described below, by way of examples. The conditions in the examples are one example of conditions employed to determine the operability and effects of the presently disclosed techniques, and the present disclosure is not limited to such example of conditions. Various conditions can be used in the present disclosure as long as the object of the present disclosure is fulfilled, without departing from the scope of the present disclosure.

[0106] Test welding was performed under the conditions listed in Table 2 for each sheet combination of two or three overlapping metal sheets listed in Table 1, and then actual welding was performed under the conditions listed in Table 3 for the same sheet combination, to produce a weld joint (L-shaped tensile test piece).

[0107] FIGS. 2A and 2B illustrate current patterns in the test welding. FIG. 2A illustrates a current pattern in the case where the main current passage is one-step current passage, and FIG. 2B illustrates a current pattern in the case where the main current passage is two-step current passage.

[0108] The test welding was performed in a state in which there was no disturbance, as illustrated in FIGS. 3A and 4A. The actual welding was performed in a state in which there was no disturbance as in the test welding, and in a state in which there was a disturbance as illustrated in FIGS. 3B and 4B.

[0109] FIG. 3A illustrates a state in which the number of overlapping metal sheets is two and there is no existing weld. FIG. 3B illustrates a state in which the number of overlapping metal sheets is two and there is an existing weld. The welding spot spacing L (center-to-center spacing) between the existing weld and the welding point (current welding point) was varied.

[0110] FIG. 4A illustrates a state in which the number of overlapping metal sheets is three and there is no existing weld. FIG. 4B illustrates a state in which the number of overlapping metal sheets is three and there is an existing weld.

[0111] The welding time in subsequent current passage in each of the test welding condition in Table 2 and the actual welding condition in Table 3 is the welding time per one subsequent current passage. The cooling time, the current in subsequent current passage, and the welding time in subsequent current passage in each of the test welding condition in Table 2 and the actual welding condition in Table 3 were the same for each subsequent current passage.

[0112] In the control method of main current passage in the actual welding condition in Table 3, constant current control indicates constant current control performed under the same condition as in the test welding. In the current determination method in constant current control in Table 3,

[0113] Formula (A), Formula (B), and Formula (C) respectively indicate determining the current Iap in the subsequent current passage in the actual welding according to the following Formulas (A), (B), and (C) that are each within the foregoing range of 0.8Itp(RBtm/RBam) Iap 1.2Itp(RBtm/RBam):

Iap=0.8Itp(RBtm/RBam)Formula (A):

Iap=1.0Itp(RBtm/RBam)Formula (B):

Iap=1.2Itp(RBtm/RBam).Formula (C):

[0114] In the case where actual welding was performed in a state in which there was an existing weld, the below-described tensile test was conducted after cutting off the part of the existing weld from the L-shaped tensile test piece.

[0115] 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.

[0116] Each obtained L-shaped tensile test piece was used to conduct a tensile test at a tension rate (in longitudinal direction) of 10 mm/min, and the joint strength (L-shape tensile strength (LTS)) was measured. Based on whether expulsion occurred in welding and the joint strength, evaluation was performed in the following three levels: [0117] A: LTS was 2.0 kN or more regardless of welding spot spacing L, and no expulsion occurred. [0118] B: LTS was 2.0 kN or more when there was no existing weld or when welding spot spacing L10 mm, LTS was less than 2.0 kN when welding spot spacing L<10 mm, and no expulsion occurred. [0119] F: LTS was less than 2.0 kN when there was no existing weld or when welding spot spacing L10 mm, or expulsion occurred.

TABLE-US-00001 TABLE 1 Sheet combination Intermediate ID Upper sheet sheet Lower sheet A1 1180 MPa-grade 1180 MPa-grade cold-rolled steel cold-rolled steel sheet sheet (sheet thickness: (sheet thickness: 1.6 mm) 1.6 mm) A2 980 MPa-grade 980 MPa-grade cold-rolled steel cold-rolled steel sheet sheet (sheet thickness: (sheet thickness: 1.6 mm) 1.6 mm) A3 980 MPa-grade 1470 MPa-grade cold-rolled steel cold-rolled steel sheet sheet (sheet thickness: (sheet thickness: 1.4 mm) 1.2 mm) A4 980 MPa-grade 1470 MPa-grade GA GA steel sheet steel sheet (sheet thickness: (sheet thickness: 1.2 mm) 2.0 mm) A5 1470 MPa-grade 1470 MPa-grade GA GA steel sheet steel sheet (sheet thickness: (sheet thickness: 1.4 mm) 1.4 mm) A6 1800 MPa-grade 980 MPa-grade GA noncoated hot steel sheet stamp steel sheet (sheet thickness: (sheet thickness: 1.2 mm) 1.4 mm) A7 1800 MPa-grade 1180 MPa-grade ZnNi-coated cold-rolled steel sheet hot stamp steel (sheet thickness: sheet (sheet 1.4 mm) thickness: 1.6 mm) B1 270 MPa-grade 1470 MPa-grade 1470 MPa-grade GA steel sheet GA steel sheet GA steel sheet (sheet thickness: (sheet thickness: (sheet thickness: 0.7 mm) 1.4 mm) 1.4 mm)

TABLE-US-00002 TABLE 2 Test welding condition Main current passage Average of resistance between Subsequent current passage Current Welding Current Welding Average time in Current Welding 1 in time 1 2 in time 2 of current electrodes in time in Elec- main in main main in main in main in main subsequent subsequent Sheet trode current current current current current current Cooling current current Number of combi- force passage passage passage passage passage passage time passage passage repetitions nation F I.sub.tm1 t.sub.tm1 I.sub.tm2 t.sub.tm2 Ibtm RB.sub.tm t.sub.c I.sub.tp t.sub.tp N No. ID (kN) (kA) (ms) (kA) (ms) (kA) () (ms) (kA) (ms) (times) Remarks 1 1-1 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 Ex. 1-2 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 1-3 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 2 2-1 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 Ex. 2-2 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 2-3 A1 4.5 6.0 320 6.0 200 160 9.5 40 2 3 3-1 A1 4.5 6.0 320 6.0 200 80 11.5 40 1 Ex. 3-2 A1 4.5 6.0 320 6.0 200 80 11.5 40 1 3-3 A1 4.5 6.0 320 6.0 200 80 11.5 40 1 4 4-1 A1 4.5 6.0 320 6.0 200 80 6.0 40 1 Comp. 4-2 A1 4.5 6.0 320 6.0 200 80 6.0 40 1 Ex. 4-3 A1 4.5 6.0 320 6.0 200 80 6.0 40 1 5 5-1 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 Comp. 5-2 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 Ex. 5-3 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 6 6-1 A1 4.5 4.0 100 6.0 240 5.4 220 160 9.5 40 2 Ex. 6-2 A1 4.5 4.0 100 6.0 240 5.4 220 160 9.5 40 2 6-3 A1 4.5 4.0 100 6.0 240 5.4 220 160 9.5 40 2 7 7-1 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 Comp. 7-2 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 Ex. 7-3 A1 4.5 6.0 320 6.0 200 160 6.0 40 1 8 8-1 A1 4.5 6.0 320 6.0 200 160 10.0 60 2 Ex. 8-2 A1 4.5 6.0 320 6.0 200 160 10.0 60 2 8-3 A1 4.5 6.0 320 6.0 200 160 10.0 60 2 9 9-1 A1 4.5 6.0 320 6.0 200 160 9.5 40 10 Ex. 9-2 A1 4.5 6.0 320 6.0 200 160 9.5 40 10 9-3 A1 4.5 6.0 320 6.0 200 160 9.5 40 10 10 10-1 A2 3.5 5.5 280 5.5 210 80 8.5 40 2 Ex. 10-2 A2 3.5 5.5 280 5.5 210 80 8.5 40 2 10-3 A2 3.5 5.5 280 5.5 210 80 8.5 40 2 11 11-1 A3 5.5 6.2 260 6.2 180 80 10.0 40 2 Ex. 11-2 A3 5.5 6.2 260 6.2 180 80 10.0 40 2 11-3 A3 5.5 6.2 260 6.2 180 80 10.0 40 2 12 12-1 A4 6.0 6.5 300 6.5 190 80 10.0 40 2 Ex. 12-2 A4 6.0 6.5 300 6.5 190 80 10.0 40 2 12-3 A4 6.0 6.5 300 6.5 190 80 10.0 40 2 13 13-1 A5 5.0 7.0 280 7.0 160 80 10.5 40 2 Ex. 13-2 A5 5.0 7.0 280 7.0 160 80 10.5 40 2 13-3 A5 5.0 7.0 280 7.0 160 80 10.5 40 2 14 14-1 A6 5.0 6.0 400 6.0 190 80 9.0 40 2 Ex. 14-2 A6 5.0 6.0 400 6.0 190 80 9.0 40 2 14-3 A6 5.0 6.0 400 6.0 190 80 9.0 40 2 15 15-1 B1 5.0 7.0 320 9.0 200 80 9.0 40 2 Ex. 15-2 B1 5.0 7.0 320 9.0 200 80 9.0 40 2 15-3 B1 5.0 7.0 320 9.0 200 80 9.0 40 2 16 16-1 B1 5.0 4.0 160 7.5 200 5.9 180 80 8.5 40 2 Ex. 16-2 B1 5.0 4.0 160 7.5 200 5.9 180 80 8.5 40 2 16-3 B1 5.0 4.0 160 7.5 200 5.9 180 80 8.5 40 2 17 17-1 A7 5.5 5.0 240 7.2 240 6.1 190 100 8.5 40 2 Ex. 17-2 A7 5.5 5.0 240 7.2 240 6.1 190 100 8.5 40 2 17-3 A7 5.5 5.0 240 7.2 240 6.1 190 100 8.5 40 2

TABLE-US-00003 TABLE 3 Actual welding condition Main current passage Average of resistance Subsequent current passage Average between Current Current Welding of current electrodes determi- Calcu- Calcu- Calcu- in subse- time Number Elec- Control in main in main nation lation lation lation quent in subse- of Sheet trode method current current Control method method in value value value current quent repe- combi- Welding force of main passage passage of subsequent constant of of of Cooling passage current titions nation spot spacing F current IB.sub.am RB.sub.am current current Formula Formula Formula time I.sub.ap passage N LTS Expul- Eval- No. ID L (kN) passage (kA) () passage control (A) (B) (C) (ms) (kA) (ms) (times) (kN) sion uation Remarks 1 1-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Formula (B) 7.6 9.5 11.4 160 9.5 40 2 3.3 None A Ex. weld control control 1-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Formula (B) 7.9 9.9 11.9 160 9.9 40 2 3.1 None control control 1-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (B) 8.2 10.3 12.3 160 10.3 40 2 3.6 None control control 2 2-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Formula (C) 7.6 9.5 11.4 160 11.4 40 2 3.3 None A Ex. weld control control 2-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Formula (C) 7.9 9.9 11.9 160 11.9 40 2 3.1 None control control 2-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (C) 8.2 10.3 12.3 160 12.3 40 2 3.2 None control control 3 3-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Formula (B) 9.2 11.5 13.8 80 11.5 40 1 2.8 None B Ex. weld control control 3-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Formula (B) 9.6 12.0 14.4 80 12.0 40 1 2.9 None control control 3-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (B) 9.9 12.4 14.9 80 12.4 40 1 1.7 None control control 4 4-1 A1 No existing 4.5 Adaptive 6.0 200 Adaptive control 80 40 1 2.8 None F Comp. weld control Ex. 4-2 A1 10 mm 4.5 Adaptive 6.4 192 Adaptive control 80 40 1 1.8 None control 4-3 A1 7 mm 4.5 Adaptive 6.7 185 Adaptive control 80 40 1 1.7 None control 5 5-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Same as test 160 6.0 40 1 2.6 None F Comp. weld control control welding Ex. 5-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Same as test 160 6.0 40 1 1.8 None control control welding 5-3 A1 7 mm 4.5 Adaptive 6.7 186 Constant current Same as test 160 6.0 40 1 1.6 None control control welding 6 6-1 A1 No existing 4.5 Adaptive 5.4 220 Constant current Formula (B) 7.6 9.5 11.4 160 9.5 40 2 3.4 None A Ex. weld control control 6-2 A1 10 mm 4.5 Adaptive 5.8 210 Constant current Formula (B) 8.0 10.0 11.9 160 10.0 40 2 3.5 None control control 6-3 A1 7 mm 4.5 Adaptive 6.2 200 Constant current Formda (B) 8.4 10.5 12.5 160 10.5 40 2 3.3 None control control 7 7-1 A1 No existing 4.5 Constant 6.0 200 Constant current Same as test 4.8 6.0 7.2 160 6.0 40 1 2.6 None F Comp. weld current control welding Ex. control 7-2 A1 10 mm 4.5 Constant 6.0 192 Constant current Same as test 5.0 6.3 7.5 160 6.0 40 1 1.6 None current control welding control 7-3 A1 7 mm 4.5 Constant 6.0 185 Constant current Same as test 5.2 6.5 7.8 160 6.0 40 1 1.4 None current control welding control 8 8-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Formula (A) 8.0 10.0 12.0 160 8.0 60 2 3.4 None A Ex. weld control control 8-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Formula (A) 8.3 10.4 12.5 160 8.3 60 2 3.2 None control control 8-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (A) 8.6 10.8 13.0 160 8.6 60 2 3.5 None control control 9 9-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Formula (B) 7.6 9.5 11.4 160 9.5 40 10 3.2 None A Ex. weld control control 9-2 A1 10 mm 4.5 Adaptive 6.4 192 Constant current Formula (B) 7.9 9.9 11.9 160 9.9 40 10 3.5 None control control 9-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (B) 8.2 10.3 12.3 160 10.3 40 10 3.1 None control control 10 10-1 A2 No existing 3.5 Adaptive 6.0 210 Constant current Formula (B) 6.8 8.5 10.2 80 8.5 40 2 3.1 None A Ex. weld control control 10-2 A2 10 mm 3.5 Adaptive 6.4 190 Constant current Formula (B) 7.5 9.4 11.3 80 9.4 40 2 3.3 None control control 10-3 A2 7 mm 3.5 Adaptive 6.7 180 Constant current Formula (B) 7.9 9.9 11.9 80 9.9 40 2 3.1 None control control 11 11-1 A3 No existing 5.5 Adaptive 6.0 180 Constant current Formula (B) 8.0 10.0 12.0 80 10.0 40 2 2.9 None A Ex. weld control control 11-2 A3 10 mm 5.5 Adaptive 6.4 172 Constant current Formula (B) 8.4 10.5 12.6 80 10.5 40 2 2.8 None control control 11-3 A3 7 mm 5.5 Adaptive 6.7 165 Constant current Formula (B) 8.7 10.9 13.1 80 10.9 40 2 2.7 None control control 12 12-1 A4 No existing 6.0 Adaptive 6.0 210 Constant current Formula (B) 7.2 9.0 10.9 80 9.0 40 2 3.5 None A Ex. weld control control 12-2 A4 10 mm 6.0 Adaptive 6.4 190 Constant current Formula (B) 8.0 10.0 12.0 80 10.0 40 2 3.3 None control control 12-3 A4 7 mm 6.0 Adaptive 6.7 180 Constant current Formula (B) 8.4 10.6 12.7 80 10.6 40 2 3.2 None control control 13 13-1 A5 No existing 5.0 Adaptive 6.0 160 Constant current Formula (B) 8.4 10.5 12.6 80 10.5 40 2 3.2 None A Ex. weld control control 13-2 A5 10 mm 5.0 Adaptive 6.4 155 Constant current Formula (B) 8.7 10.8 13.0 80 10.8 40 2 3.4 None control control 13-3 A5 7 mm 5.0 Adaptive 6.7 149 Constant current Formula (B) 9.0 11.3 13.5 80 11.3 40 2 3.4 None control control 14 14-1 A6 No existing 5.0 Adaptive 6.0 190 Constant current Formula (B) 7.2 9.0 10.8 80 9.0 40 2 2.3 None A Ex. weld control control 14-2 A6 10 mm 5.0 Adaptive 6.4 181 Constant current Formula (B) 7.6 9.4 11.3 80 9.4 40 2 2.5 None control control 14-3 A6 7 mm 5.0 Adaptive 6.7 174 Constant current Formula (B) 7.9 9.8 11.8 80 9.8 40 2 2.4 None control control 15 15-1 B1 No existing 5.0 Adaptive 6.0 200 Constant current Formula (B) 7.2 9.0 10.8 80 9.0 40 2 3.3 None A Ex. weld control control 15-2 B1 10 mm 5.0 Adaptive 6.4 192 Constant current Formula (B) 7.5 9.4 11.3 80 9.4 40 2 3.5 None control control 15-3 B1 7 mm 5.0 Adaptive 6.7 185 Constant current Formula (B) 7.8 9.7 11.7 80 9.7 40 2 3.2 None control control 16 16-1 B1 No existing 5.0 Adaptive 6.0 180 Constant current Formula (B) 6.8 8.5 10.2 80 8.5 40 2 2.6 None A Ex. weld control control 16-2 B1 10 mm 5.0 Adaptive 6.4 171 Constant current Formula (B) 7.2 8.9 10.7 80 8.9 40 2 2.8 None control control 16-3 B1 7 mm 5.0 Adaptive 6.7 162 Constant current Formula (B) 7.6 9.4 11.3 80 9.4 40 2 2.3 None control control 17 17-1 A7 No existing 5.5 Adaptive 6.0 192 Constant current Formula (B) 6.7 8.4 10.1 100 8.4 40 2 2.5 None A Ex. weld control control 17-2 A7 10 mm 5.5 Adaptive 6.3 182 Constant current Formula (B) 7.1 8.9 10.6 100 8.9 40 2 2.3 None control control 17-3 A7 7 mm 5.5 Adaptive 6.8 175 Constant current Formula (B) 7.4 9.2 11.1 100 9.2 40 2 2.2 None control control A: LTS was 2.0 kN or more regardless of welding spot spacing L, and no explusion occurred. B: LTS was 2.0 kN or more when there was no existing weld or when welding spot spacing L 10 mm, LTS was less than 2.0 kN when welding spot spacing L < 10 mm, and no explusion occurred. F: LTS was less than 2.0 kN when there was no existing weld or when welding spot spacing L 10 mm, or explusion occurred.

[0120] As can be seen in Table 3, all Examples (Ex.) were evaluated as A or B. In particular, all Examples in which the number of repetitions of the welding interval for the cooling time and the subsequent current passage after the main current passage was two or more times were evaluated as A.

[0121] All Comparative Examples (Comp. Ex.) not satisfying the appropriate conditions according to the present disclosure were evaluated as F, and could not obtain sufficient joint strength.