RESISTANCE SPOT WELDING METHOD

Abstract

A resistance spot welding method of squeezing parts to be welded, which are a plurality of overlapping metal sheets, by a pair of electrodes and passing a current while applying an electrode force to join the parts to be welded comprises: a first step of passing a current by constant current control to form a fusion zone having a diameter of not less than 2t, expressed in mm, between the metal sheets, where t, expressed in mm, is a sheet thickness of a thinnest metal sheet of the metal sheets; a second step of cooling the fusion zone to have a diameter of not greater than 80% of D, where D, expressed in mm, is a diameter of the fusion zone formed in the first step; and a third step of performing adaptive control welding by controlling a current passage amount according to a target that is set.

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: a first step of passing a current by constant current control to form a fusion zone having a diameter of not less than 2t, expressed in mm, between the metal sheets, where t, expressed in mm, is a sheet thickness of a thinnest metal sheet of the metal sheets; a second step of cooling the fusion zone to have a diameter of not greater than 80% of D, where D, expressed in mm, is a diameter of the fusion zone formed in the first step; and a third step of performing adaptive control welding by controlling a current passage amount according to a target that is set.

2. The resistance spot welding method according to claim 1, wherein test welding for setting the target is performed beforehand, in the test welding, first to third test steps corresponding to the first to the third steps are performed by constant current control, to derive a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume at least in the third test step, and in the third step, the target is set to 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 derived in the third test step, welding is performed using the time variation curve as the target, and, in the case where an amount of time variation of an instantaneous amount of heat generated per unit volume in the third step differs from the time variation curve, adaptive control welding is performed to control the current passage amount in order to compensate for the difference within a remaining welding time in the third step so that a cumulative amount of heat generated in the third step matches the cumulative amount of heat generated that is derived beforehand in the third test step.

3. The resistance spot welding method according to claim 1, wherein in the first step, the current is passed to satisfy a relationship tx/t00.95, where tx, expressed in mm, is a total thickness of the fusion zone formed between the metal sheets in the first step, and t0, expressed in mm, is a total thickness of the parts to be welded.

4. The resistance spot welding method according to claim 2, wherein in the first step, the current is passed to satisfy a relationship tx/t00.95, where tx, expressed in mm, is a total thickness of the fusion zone formed between the metal sheets in the first step, and t0, expressed in mm, is a total thickness of the parts to be welded.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the accompanying drawings:

[0043] FIG. 1 is a diagram schematically illustrating an example of the case of welding a sheet combination having a sheet gap according to one of the disclosed embodiments; and

[0044] FIG. 2 is a diagram schematically illustrating another example of the case of welding a sheet combination having a sheet gap according to one of the disclosed embodiments.

DETAILED DESCRIPTION

[0045] The present disclosure is described below, by way of embodiments.

[0046] 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 comprises: a first step of passing a current by constant current control to form a fusion zone having a diameter of not less than 2t (mm) between the metal sheets, where t (mm) is the sheet thickness of the thinnest metal sheet of the metal sheets; a second step of cooling the fusion zone to have a diameter of not greater than 80% of D, where D (mm) is the diameter of the fusion zone formed in the first step; and a third step of performing adaptive control welding by controlling a current passage amount according to a set target.

[0047] 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.

[0048] The basic structure of the resistance spot welding method according to one of the disclosed embodiments is described below, for each step.

[0049] First Step

[0050] In the first step, a current is passed by constant current control to form a fusion zone having a diameter of not less than 2t (mm) between the metal sheets, where t (mm) is the sheet thickness of the thinnest metal sheet of the parts to be welded.

[0051] By forming a fusion zone having a diameter of not less than 2t (mm) between the metal sheets in the first step, the steel sheets expand by melting, and also soften as a result of heating around the electrodes. The contact area between the electrodes and the steel sheets thus increases. Consequently, the difference in contact area from the test welding pattern due to the presence or absence of a sheet gap and the magnitude of the sheet gap at the start of the below-mentioned third step, i.e. adaptive control welding, decreases. This enables the heat pattern of the weld in the adaptive control welding to follow the heat pattern in the test welding, even in the case where the effects of disturbances are particularly significant. A desired nugget diameter can thus be obtained while effectively preventing expulsion.

[0052] For example, the current passage conditions for forming a fusion zone having a diameter of not less than 2t (mm) between the metal sheets of the parts to be welded in the first step can be determined as follows.

[0053] In a state without disturbances such as a sheet gap or current shunting to an existing weld or in a state simulating expected disturbances, a preliminary welding test is performed by constant current control under various conditions using metal sheets of the same steel type and thickness as the parts to be welded used in the actual welding.

[0054] The conditions (welding current, welding time, electrode force) for forming a fusion zone having the target fusion zone diameter (not less than 2t (mm)) between the metal sheets of the parts to be welded in both of the state without disturbances and the state with disturbances are then determined, and a current is passed by constant current control under the determined conditions. The formation state of the fusion zone can be checked, after solidification, by a peel test or by cross-sectional observation at the nugget center (by etching with a saturated picric acid solution). The diameter of the fusion zone mentioned here is the length, on an overlapping line of the metal sheets, of the fusion zone formed between the metal sheets in a cross section of the weld taken at the center in the sheet transverse direction.

[0055] The current is preferably passed under the conditions for forming a fusion zone having a diameter of not less than 2.5t (mm) between the metal sheets of the parts to be welded, in the case where the effects of disturbances are particularly significant, such as:

[0056] (1) when a large nugget diameter needs to be ensured (e.g. the target nugget diameter is 4.5t or more);

[0057] (2) an existing weld is present immediately nearby (e.g. the distance between the welding point and the existing weld is 7 mm or less) or many existing welds are present around the welding point (e.g. three or more existing welds are present around the welding point); or

[0058] (3) the effect of a sheet gap is considerable (e.g. there is a sheet gap of 2.5 mm or more in at least one location between the metal sheets, or the sheet gap distance is less than 40 mm).

[0059] In terms of preventing expulsion in the first step, the diameter of the fusion zone formed in the first step is preferably not greater than 80% of the final target nugget diameter. The diameter of the fusion zone formed in the first step is more preferably not greater than 70% of the final target nugget diameter.

[0060] Moreover, in the first step, the current is preferably passed to satisfy the relationship tx/t00.95, where tx (mm) is the total thickness of the fusion zone formed between the metal sheets in the first step, and t0 (mm) is the total thickness of the parts to be welded (before welding start).

[0061] This is because, since the thickness of the fusion zone decreases as a result of cooling in the below-mentioned second step, the difference in the thickness of the fusion zone from the test welding pattern due to the presence or absence of a sheet gap and the magnitude of the sheet gap at the start of the adaptive control welding can be reduced more, and consequently expulsion in the adaptive control welding can be prevented more advantageously to obtain a nugget of an appropriate diameter. In the case where the effects of disturbances are particularly significant, it is more preferable to satisfy the relationship tx/t00.90. In terms of further reducing the difference in the thickness of the fusion zone from the test welding pattern due to the presence or absence of a sheet gap and the magnitude of the sheet gap at the start of the adaptive control welding, tx/t0 is preferably 0.10 or more. tx/t0 is more preferably 0.30 or more.

[0062] To form a fusion zone so as to satisfy the relationship tx/t00.95, for example, the same preliminary welding test as described above may be performed to determine such conditions that form a fusion zone satisfying the relationship tx/t00.95.

[0063] The total thickness of the fusion zone mentioned here is the maximum total thickness of the fusion zone formed between the metal sheets in a cross section of the weld taken at the center in the sheet transverse direction. This can be checked, for example, by cross-sectional observation at the nugget center (by etching with a saturated picric acid solution).

[0064] Second Step

[0065] Next, the fusion zone is cooled to have a diameter of not greater than 80% of D, where D (mm) is the diameter of the fusion zone formed in the first step. Herein, the fusion zone denotes a portion in a molten state, and the diameter of the fusion zone changes as appropriate depending on the molten state.

[0066] No lower limit is placed on the diameter of the fusion zone in the cooling in the second step, and the diameter of the fusion zone in the cooling in the second step may be 0% (completely solidified state).

[0067] The fusion zone cooling method is, for example, a method of providing a cooling time C2 (ms) during which current passage is stopped. The cooling time C2 varies depending on the sheet combination and the nugget diameter. For example, the same preliminary welding test as described above may be performed to determine such a cooling time that allows the fusion zone to have a diameter of not greater than 80% of D.

[0068] Especially, it is preferable to satisfy the relationship C220.Math.t0/R, where t0 is the total thickness (mm) of the parts to be welded, and R is the electrode tip diameter (mm). In the case where the effects of disturbances are particularly significant, it is more preferable to satisfy the relationship C230.Math.t0/R.

[0069] No upper limit is placed on the cooling time C2, yet the cooling time C2 is preferably 1000 ms or less in terms of productivity.

[0070] Third Step

[0071] In the third step, adaptive control welding is performed by controlling the current passage amount according to the set target.

[0072] For example, test welding for setting the target is performed beforehand, and the target is set to 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 derived in a third test step in the test welding corresponding to the third step. Welding is performed using the time variation curve as the target, and, in the case where the amount of time variation of the instantaneous amount of heat generated in the third step differs from the time variation curve, adaptive control welding is performed to control the current passage amount in order to compensate for the difference within the remaining welding time in the third step so that the cumulative amount of heat generated in the third step matches the cumulative amount of heat generated that is derived beforehand in the third test step.

[0073] If the amount of time variation of the instantaneous amount of heat generated follows the time variation curve, the welding is continued without change and completed.

[0074] By performing welding in the first to third steps under the conditions described above, even in the case where the effects of disturbances are particularly significant and further the electrode tips wear, it is possible to reduce the difference in the contact area between the electrodes and the steel sheets and further the difference in the thickness of the fusion zone from the test welding pattern due to the presence or absence of a sheet gap and the magnitude of the sheet gap at the start of current passage in the third step, i.e. the adaptive control welding.

[0075] This enables the heat pattern of the weld in the adaptive control welding to follow the heat pattern in the test welding, and avoids false recognition that the amount of heat generated has increased locally or the amount of heat generated is insufficient. A desired nugget diameter can thus be obtained while effectively preventing expulsion.

[0076] The test welding is described below.

[0077] (Test Welding)

[0078] It is preferable that, in the test welding, welding is performed in first to third test steps corresponding to the first to third steps by constant current control, to derive the time variation of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume at least in the third test step.

[0079] For example, a welding test with the same steel type and thickness as the parts to be welded is performed with various conditions by constant current control under conditions corresponding to the conditions set in the first and second steps in a state without a sheet gap or current shunting to an existing weld, i.e. in a state without disturbances, to find optimal conditions in the test welding (optimal conditions in the test step corresponding to the third step).

[0080] Welding is then performed in the first to third test steps by constant current control, under the above-mentioned optimal conditions. In this way, the electrical property between the electrodes in the case where an appropriate nugget is formed in a state without disturbances is determined. From this electrical property between the electrodes, the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume at least in the third test step are derived, and set as the target in the third step. The test welding may be performed in a state with disturbances such as a sheet gap or current shunting, within the range in which the nugget diameter is substantially unchanged.

[0081] Herein, the electrical property between the electrodes means the interelectrode resistance or the interelectrode voltage.

[0082] The 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.

[0083] 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 interelectrode voltage, 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).

[0084] The electrical resistance R of the columnar portion is given by the following Equation (2):

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

[0085] 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).

[0086] As is clear from Equation (3), the amount q of heat generated per unit volume and per unit time can be calculated from the interelectrode voltage 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 interelectrode voltage 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.

[0087] 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.

[0088] The parts to be welded used in the resistance spot welding method according to one of the disclosed embodiments are not limited. The resistance spot welding method may be used for 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 three or more overlapping steel sheets.

[0089] The first step and the third step may further be divided into a plurality of current passage steps, or an upslope or a downslope may be added. Moreover, a subsequent current may be applied to heat-treat the weld after the current for nugget formation. The current passage conditions in this case are not limited, and also the magnitude relationships with the welding currents in the preceding steps are not limited.

[0090] The electrode force need not be constant, and may be divided into multiple levels as with the welding current.

Examples

[0091] For each sheet combination of two or three overlapping metal sheets listed in Table 1 and illustrated in FIGS. 1 and 2, resistance spot welding was performed under the conditions listed in Table 2 to produce a weld joint. As illustrated in FIGS. 1 and 2, spacers 15 were inserted between the metal sheets 11 to 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 tg and sheet gap distances Lg (for a sheet combination of three overlapping metal sheets, the sheet gap thickness tg and the sheet gap distance Lg between the metal sheets 11 and 12 and the sheet gap thickness tg and the sheet gap distance Lg between the metal sheets 12 and 13 are the same). In the drawings, reference sign 14 is an electrode.

[0092] The result in the case where the control mode is constant current in Table 2 indicates the result of performing welding by constant current control under the welding conditions listed in Table 2. The result in the case where the control mode is adaptive control in Table 2 indicates the result of, after performing test welding by constant current control in the absence of disturbances such as a sheet gap under the welding conditions listed in Table 2 to derive the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume, performing adaptive control welding of adjusting the current using the derived values as the target (the current in Table 2 in the case of adaptive control is the current in the test welding). The conditions such as welding time, electrode force, and cooling time were the same in the test welding and the actual welding.

[0093] 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.

[0094] For each obtained joint, the weld was cut and etched in section, and then observed with an optical microscope. Each sample in which each nugget diameter between the metal sheets was not less than 4.5t as a target diameter (t: the sheet thickness (mm) of the thinner metal sheet of adjacent two metal sheets) and no expulsion occurred was evaluated as good. Each sample in which any nugget diameter was less than 4.5t or expulsion occurred was evaluated as poor.

TABLE-US-00001 TABLE 1 Sheet combination Steel sheet of reference sign Steel sheet of reference sign Steel sheet of reference sign t 2t No. 11 in the drawings 12 in the drawings 13 in the drawings (mm) (mm) A 980 MPa-grade cold rolled 980 MPa-grade cold rolled 1.0 2.00 steel sheet steel sheet (sheet thickness: 1.0 mm) (sheet thickness: 1.0 mm) B 980 MPa-grade cold rolled 980 MPa-grade cold rolled 1.6 2.53 steel sheet steel sheet (sheet thickness: 1.6 mm) (sheet thickness: 1.6 mm) C 980 MPa-grade cold rolled 980 MPa-grade cold rolled 2.0 2.83 steel sheet steel sheet (sheet thickness: 2.0 mm) (sheet thickness: 2.0 mm) D 1180 MPa-grade cold rolled 1180 MPa-grade cold rolled 1.6 2.53 steel sheet steel sheet (sheet thickness: 1.6 mm) (sheet thickness: 1.6 mm) E 270 MPa-grade cold rolled 980 MPa-grade cold rolled 0.7 1.67 steel sheet steel sheet (sheet thickness: 0.7 mm) (sheet thickness: 1.6 mm) F 1470 MPa-grade cold rolled 1470 MPa-grade cold rolled 1.4 2.37 steel sheet steel sheet (sheet thickness: 1.4 mm) (sheet thickness: 1.4 mm) G 590 MPa-grade cold rolled 1180 MPa-grade cold rolled 1.4 2.37 steel sheet steel sheet (sheet thickness: 1.4 mm) (sheet thickness: 1.6 mm) H 270 MPa-grade GA steel 980 MPa-grade cold rolled 1.0 2.00 sheet steel sheet (sheet thickness: 1.0 mm) (sheet thickness: 1.6 mm) I 980 MPa-grade GA steel 980 MPa-grade GA steel 1.6 2.53 sheet sheet (sheet thickness: 1.6 mm) (sheet thickness: 1.6 mm) J 270 MPa-grade GI steel sheet 980 MPa-grade cold rolled 1.0 2.00 (sheet thickness: 1.0 mm) steel sheet (sheet thickness: 1.6 mm) K 980 MPa-grade cold rolled 980 MPa-grade cold rolled 980 MPa-grade cold rolled 1.0 2.00 steel sheet steel sheet steel sheet (sheet thickness: 1.0 mm) (sheet thickness: 1.0 mm) (sheet thickness: 1.0 mm) L 270 MPa-grade cold rolled 1180 MPa-grade cold rolled 980 MPa-grade cold rolled 0.7 1.67 steel sheet steel sheet steel sheet (sheet thickness: 0.7 mm) (sheet thickness: 1.4 mm) (sheet thickness: 1.0 mm) M 270 MPa-grade GA steel 980 MPa-grade GA steel 980 MPa-grade GA steel 0.7 1.67 sheet sheet sheet (sheet thickness: 0.7 mm) (sheet thickness: 1.4 mm) (sheet thickness: 1.0 mm) N 440 MPa-grade cold rolled 980 MPa-grade cold rolled 980 MPa-grade cold rolled 1.0 2.00 steel sheet steel sheet steel sheet (sheet thickness: 1.0 mm) (sheet thickness: 1.8 mm) (sheet thickness: 1.6 mm)

TABLE-US-00002 TABLE 2 First step Third step Welding Welding Second step Welding Welding Sheet Electrode current time Cooling time current time Joint combination force F I1 T1 C2 I3 T3 No. No. (kN) (kA) (ms) Control mode (ms) (kA) (ms) Control mode 1 A 3.5 4.5 200 Constant current 100 5.5 260 Adaptive control 2 B 5 6.5 320 Constant current 200 7.5 320 Adaptive control 3 C 5.5 7.5 100 Constant current 300 7.5 320 Adaptive control 4 D 4.5 7 140 Constant current 60 6 300 Adaptive control 5 E 4 7 120 Constant current 160 7.5 280 Adaptive control 6 F 5 5 200 Constant current 100 6 300 Adaptive control 7 G 5 5 240 Constant current 100 6 300 Adaptive control 8 H 3.5 6.5 120 Constant current 140 7.5 360 Adaptive control 9 I 4.5 6 260 Constant current 200 7 320 Adaptive control 10 J 4 6 300 Constant current 100 7 300 Adaptive control 11 K 5 5.5 200 Constant current 200 6 300 Adaptive control 12 L 4.5 6 240 Constant current 100 7 400 Adaptive control 13 M 5 7 260 Constant current 120 7.5 320 Adaptive control 14 N 5 6.5 300 Constant current 200 7 360 Adaptive control 15 B 5 7.5 320 Adaptive control 16 D 4 7 340 Adaptive control 17 F 5 3.5 100 Constant current 20 7.5 280 Adaptive control 18 K 4 6.5 300 Adaptive control 19 M 5 7.5 400 Adaptive control 20 N 5 7.5 400 Adaptive control 21 B 5 6.5 320 Constant current 10 8 320 Adaptive control Ratio of diameter of Sheet gap Sheet gap fusion zone in thickness distance Diameter of fusion zone second step Joint tg Lg 2t formed in first step D to D* No. (mm) (mm) (mm) (mm) tx/t0 (%) Evaluation Remarks 1 1.0 80 2.00 2.7 0.60 0 Good Example 2 2.0 40 2.53 4.5 0.65 0 Good Example 3 1.0 60 2.83 3.8 0.50 0 Good Example 4 0.5 80 2.53 3.0 0.70 40.0 Good Example 5 1.0 60 1.67 2.3 0.50 0 Good Example 6 0.5 60 2.37 3.0 0.75 0 Good Example 7 0.5 60 2.37 3.0 0.70 0 Good Example 8 1.6 60 2.00 2.6 0.60 15.0 Good Example 9 1.0 40 2.53 3.5 0.45 0 Good Example 10 1.0 80 2.00 2.6 0.55 20.0 Good Example 11 1.0 80 2.00 Between steel sheets 11 0.60 0 Good Example and 12: 3.0 Between steel sheets 12 and 13: 3.0 12 0.5 60 1.67 Between steel sheets 11 0.74 0 Good Example and 12: 2.0 Between steel sheets 12 and 13: 2.4 13 0.5 60 1.67 Between steel sheets 11 0.80 0 Good Example and 12: 2.2 Between steel sheets 12 and 13: 2.6 14 1.0 80 2.00 Between steel sheets 11 0.83 0 Good Example and 12: 2.5 Between steel sheets 12 and 13: 3.0 15 2.0 40 2.53 Poor Comparative Example 16 2.0 40 2.53 Poor Comparative Example 17 1.6 40 2.37 0 Poor Comparative Example 18 1.6 40 2.00 Poor Comparative Example 19 1.0 40 1.67 Poor Comparative Example 20 1.6 40 2.00 Poor Comparative Example 21 2.0 40 2.53 4.5 0.65 82.0 Poor Comparative Example *0% denotes a state in which each fusion zone was completely solidified in second step.

[0095] In all Examples, no expulsion occurred, and a nugget with a diameter of 4.5t or more was obtained, regardless of the sheet gap thickness tg and the sheet gap distance Lg.

[0096] In all Comparative Examples, on the other hand, expulsion occurred or a nugget with a sufficient diameter was not formed.

REFERENCE SIGNS LIST

[0097] 11, 12, 13 metal sheet [0098] 14 electrode [0099] 15 spacer