Resistance spot welding method

Abstract

Provided is a resistance spot welding method capable of efficiently generating as large as possible nuggets while suppressing the occurrence of sputtering. In the resistance spot welding method, at least two steel sheets having a plating layer having zinc as the main component or an Al-Si plating layer are superposed, the at least two steel sheets are sandwiched by a pair of electrodes, pre-energization is performed between the pair of electrodes, and then formal energization is performed, whereby adjacent steel sheets are melted and joined to each other. The resistance spot welding method performs pre-energization in a manner including an energization mode in which the current value of the welding current at the start of energization of the pre-energization is set to the maximum value, and the current value sequentially decreases in a stepped manner from the maximum value as the energization time elapses from the start of energization.

Description

Technical Field

[0001] The present invention relates to a resistance spot welding method. Background Art

[0002] In resistance spot welding, it is known that the welding current is passed between electrodes in a divided manner into a main energization and a preliminary energization (also referred to as an initial energization) before the main energization.

[0003] Patent document 1 discloses the following resistance spot welding method: pre-energization is set to two-stage energization, the current value I1 in the first stage of energization is set to a range that satisfies Vm×1.1≤I1≤15.0kA in relationship to the current value Im in formal energization, and the current value I2 in the second stage of energization is set to no energization or low current energization that satisfies 0≤I2≤Vm×0.7.

[0004] Prior art literature

[0005] Patent Document 1: International Publication No. 2015 / 083381 Summary of the invention

[0006] Problems to be solved by the invention

[0007] However, in the welding method disclosed in Patent Document 1, the time for pulsed power-on is long, so a nugget is generated during the pulsed power-on, and the ratio of the alloy layer diameter to the nugget diameter becomes poor. In particular, in the case of welding with a pressure of 4 kN or less that can be performed using a small welding gun, the nugget grows rapidly during the pulsed power-on, which causes sputtering. In addition, in the welding method disclosed in Patent Document 1, the current rises rapidly during the formal power-on, so it is difficult to increase the likelihood of sputtering during the formal power-on.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a resistance spot welding method capable of efficiently generating a nugget as large as possible while suppressing the occurrence of spatter.

[0009] Means for solving problems

[0010] In order to solve the above-mentioned problems, the resistance spot welding method of the present invention overlaps at least two steel plates having a coating with zinc as a main component or an Al-Si coating, clamps the at least two steel plates by a pair of electrodes, performs pre-energization between the pair of electrodes and then performs formal energization, thereby causing the adjacent steel plates to melt and join with each other. The resistance spot welding method is characterized in that the pre-energization is performed in a manner including the following energization mode: the current value of the welding current at the start of the pre-energization is set to a maximum value, and from the start of the energization, the current value decreases in a step-like manner from the maximum value as the energization time passes.

[0011] As described above, the resistance spot welding method of the present invention allows a welding current with a maximum current value to flow at the start of pre-energization when the steel plate is relatively hard and the contact area between the steel plate and a pair of electrodes and the contact area between adjacent steel plates are relatively small, thereby enabling the temperature of the weld to rise in a short time at the start of pre-energization. In addition, the resistance spot welding method of the present invention can efficiently generate an alloy layer and a nugget at an early stage of pre-energization while efficiently raising the temperature of the weld by finely controlling the current value so that the current value of the welding current decreases in a step-like manner from the maximum value, and can also expand the nugget diameter corresponding to the expansion of the alloy layer diameter to suppress the generation of sputtering. Thus, the resistance spot welding method of the present invention can suppress the generation of sputtering and efficiently generate a nugget as large as possible.

[0012] Effects of the Invention

[0013] According to the present invention, it is possible to provide a resistance spot welding method capable of efficiently forming a nugget as large as possible while suppressing the occurrence of spatter. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a figure explaining the resistance spot welding apparatus for implementing the resistance spot welding method of this embodiment.

[0015] Figure 2 It is a cross-sectional view schematically showing a state where a plurality of steel plates are sandwiched by a pair of electrodes.

[0016] Figure 3 Yes means Figure 2 A cross-sectional view showing a state where a plurality of steel plates are joined.

[0017] Figure 4 It is a diagram showing an example of the energization pattern of the present embodiment and the comparative example.

[0018] Figure 5 This is a graph showing the energization time required for the ratio of the alloy layer diameter to the nugget diameter to reach 1.2 in the energization patterns of the present embodiment and the comparative example.

[0019] Figure 6 It means in Figure 2 FIG. 1 is a diagram showing changes in the alloy layer and the nugget diameter generated at the boundary between the second steel plate and the third steel plate.

[0020] Figure 7 It is a cross-sectional view showing the state of the alloy layer and the nugget generated by the current-conducting pattern of the present embodiment and the comparative example.

[0021] Figure 8 It means in Figure 2 FIG. 1 is a diagram showing changes in the diameter of a nugget generated at the boundary between the first steel plate and the second steel plate.

[0022] Fig. 9 is a schematic representation of Figure 2 A cross-sectional view showing a state where a plurality of steel plates of different forms are sandwiched by a pair of electrodes.

[0023] Fig.10 It means in Fig. 9 FIG. 1 is a diagram showing changes in the alloy layer diameter and the nugget diameter of the alloy layer and the nugget generated at the boundary between the first steel plate and the second steel plate.

[0024] Fig.11 It means in Fig. 9 FIG. 1 is a diagram showing changes in the diameter of a nugget generated at the boundary between the second steel plate and the third steel plate. DETAILED DESCRIPTION

[0025] Hereinafter, embodiments of the present invention will be described using the drawings. Configurations denoted by the same reference numerals in each embodiment have the same functions in each embodiment unless otherwise specified, and description thereof will be omitted.

[0026] Figure 1 It is a diagram for explaining a resistance spot welding device 1 for carrying out the resistance spot welding method according to the present embodiment.

[0027] The resistance spot welding device 1 includes a welding gun 2 held by a robot arm 3 and a pair of electrodes 4 and 5 provided on the welding gun 2. The pair of electrodes 4 and 5 are composed of an upper electrode 4 and a lower electrode 5. In addition, the resistance spot welding device 1 includes: a lifting device 6 for lifting and lowering the upper electrode 4, a position detection device 7 for detecting the position of the pair of electrodes 4 and 5, and a current adjustment device 8 for adjusting the current value of the welding current flowing between the pair of electrodes 4 and 5. In addition, the resistance spot welding device 1 includes a control device 9 for controlling the respective components of the resistance spot welding device 1 to control the position of the pair of electrodes 4 and 5, the pressure applied to the welding object, and the current value of the welding current.

[0028] use Figure 2~Figure 3 Next, a welding target of the resistance spot welding method according to the present embodiment will be described. Figure 2 It is a cross-sectional view schematically showing a state where a plurality of steel plates are sandwiched by a pair of electrodes 4 and 5 . Figure 3 Yes means Figure 2 A cross-sectional view showing a state where a plurality of steel plates are joined.

[0029] In the resistance spot welding method of the present embodiment, a plurality of steel plates formed by overlapping at least two steel plates having a coating layer with zinc as a main component are used as welding objects. In the present embodiment, as a plurality of steel plates, a first steel plate W1, a second steel plate W2, and a third steel plate W3 are overlapped. The first steel plate W1 and the second steel plate W2 may be, for example, SCGA1180. The third steel plate W3 may also be, for example, SCGA440. The hardness of the steel plates is different in the first steel plate W1, the second steel plate W2, and the third steel plate W3. The thickness of each of the first steel plate W1 to the third steel plate W3 may be, for example, 1.4 mm.

[0030] In the present embodiment, it is assumed that there is a plate gap at the boundary between adjacent steel plates, and a gap G is provided to simulate the plate gap. The gap G is provided by, for example, placing a spacer S with a height of 2 mm between the second steel plate W2 and the third steel plate W3.

[0031] In the resistance spot welding method of this embodiment, a pair of electrodes 4 and 5 are used to weld the Figure 2 In the state where the plurality of steel plates shown are clamped and pressed, a welding current is passed between a pair of electrodes 4 and 5, thereby melting and joining the adjacent steel plates. Figure 3 As shown, a nugget N and an alloy layer A surrounding the periphery of the nugget N are generated in the plurality of steel plates after joining. The nugget N is generated by the iron constituting the base material of each steel plate being melted and then solidified. The alloy layer A is generated by the high-temperature reaction of the zinc constituting the plating layer of each steel plate with the iron constituting the base material of each steel plate. The alloy layer diameter D1 is the outer dimension of the alloy layer A along the direction of each steel plate. The nugget diameter D2 is the outer dimension of the nugget N along the direction of each steel plate.

[0032] In the resistance spot welding method of the present embodiment, when the welding current is passed between the pair of electrodes 4 and 5, pre-energization is performed before the formal energization, and the formal energization is performed after the pre-energization. The inventors of the present application have conducted in-depth research on what kind of energization mode is set as the control mode of the current value of the welding current in the pre-energization and the formal energization, and created an energization mode that efficiently generates as large a nugget as possible while suppressing the generation of sputtering. The energization mode indicates how the current value of the welding current is changed as the energization time passes. In addition, welding conditions other than the energization mode can also be the same as before.

[0033] use Figure 4~Figure 8 The current conduction pattern of the welding current in the resistance spot welding method according to the present embodiment will be described. Figure 4 (a) is a diagram showing an example of a energization pattern according to the present embodiment. Figure 4 (b) is a diagram showing an example of a energization pattern of a comparative example.

[0034] exist Figure 4 (a) and Figure 4 In (b), time point T0 indicates the time point at which power is started. Time point T1 indicates the time point after 100 ms from time point T0. Time point T2 indicates the time point after 150 ms from time point T1. Time point T3 indicates the time point after 118 ms from time point T2. The period from time point T0 to time point T2 indicates the pre-power-on period. The period from time point T2 to time point T3 indicates the actual power-on period.

[0035] The power-on mode of this embodiment is as follows Figure 4 As shown in (a), different energization patterns are adopted in the first period starting from the start of energization of pre-energization and the second period starting after the first period. That is, in the first period of pre-energization from time point T0 to time point T1, a energization pattern is adopted in which the current value of the welding current at the start of energization of pre-energization (time point T0) is set to the maximum value, and from the start of energization, the current value decreases stepwise from the maximum value as the energization time passes. In the second period of pre-energization from time point T1 to time point T2, an upslope energization pattern is adopted in which the current value gradually increases from the current value at the end of the first period (time point T1) as the energization time passes. In the formal energization period from time point T2 to time point T3, a energization pattern is adopted in which the current value is constant from the end of the second period of pre-energization (time point T2) to the end of the formal energization period (time point T3).

[0036] The step-shaped energization pattern used in the first period of pre-energization is formed by combining a plurality of sets of current values ​​of the welding current and the energization time at the current values. The current values ​​constituting the step-shaped energization pattern used in the first period of pre-energization, the energization time at the current values, and the number (number of steps) of the current values ​​vary depending on the type of steel plate to be welded and the assembly method thereof, and can be determined in advance by experiments, etc.

[0037] exist Figure 4 In the example shown in (a) of FIG. 1 , the step-shaped energization pattern adopted in the first period of pre-energization is the following energization pattern. That is, Figure 4 In the step-shaped energization pattern shown in (a), energization of a first current value (14 kA) indicating the maximum current value of the welding current continues for a first energization time (24 ms) from the start of energization of the pre-energization. Figure 4 In the step-like energization pattern shown in (a), energization at a second current value (11 kA) smaller than the first current value continues for a second energization time (6 ms) from the elapsed time of the first energization time. Figure 4 In the step-shaped energization pattern shown in (a), energization at a third current value (10 kA) which is smaller than the second current value continues for a third energization time (6 ms) from the elapsed time point of the second energization time. Figure 4In the step-like energization pattern shown in (a), energization at a fourth current value (9 kA) smaller than the third current value continues for a fourth energization time (18 ms) from the elapsed time point of the third energization time. Figure 4 In the step-like energization pattern shown in (a), energization at a fifth current value (8 kA) which is smaller than the fourth current value continues for a fifth energization time (46 ms) from the elapsed time point of the fourth energization time.

[0038] Each combination of the current value of the step-shaped power-on pattern used in the first period of pre-power-on and the power-on time at the current value is a combination of the upper limit current value (sputtering limit current value) that can suppress the occurrence of sputtering and the upper limit power-on time that can suppress the occurrence of sputtering during power-on at the upper limit current value. This is equivalent to the time point when each current value of the step-shaped power-on pattern used in the first period of pre-power-on changes to the next current value becoming the time point before sputtering is about to occur due to the power-on of each current value.

[0039] Specifically, the combination of the upper limit current value and the upper limit energization time that can suppress the occurrence of sputtering is a combination in which the ratio (D1 / D2) of the alloy layer diameter D1 to the nugget diameter D2 is greater than a preset reference value (a value within a range of 1.2 to 1.5). The reference value will be described later.

[0040] exist Figure 4 In the example shown in (a), the combination of the first current value and the first power-on time is a combination of the upper limit current value that can suppress the generation of sputtering even if the first current value is applied, and the upper limit power-on time that can suppress the generation of sputtering even if the power-on at the first current value is continued for the first power-on time. The same is true for the combination of the second current value and the second power-on time, the combination of the third current value and the third power-on time, the combination of the fourth current value and the fourth power-on time, and the combination of the fifth current value and the fifth power-on time.

[0041] On the other hand, the energization pattern of the comparative example is as follows Figure 4 As shown in (b), during the pre-energization period from time point T0 to time point T2, an upslope energization pattern is adopted in which the current value of the welding current gradually increases with the passage of energization time from the current value of the welding current at the start of the pre-energization (time point T0). During the formal energization period from time point T2 to time point T3, an energization pattern is adopted in which the current value is maintained at a constant value from the start of the formal energization period (time point T2) to the end of the formal energization period (time point T3). However, as Figure 4 As shown in (b) of FIG. 1 , the current value maintained constant during the main energization period becomes a value larger than the maximum value of the current value during the pre-energization period.

[0042] Figure 5This is a graph showing the energization time required for the ratio (D1 / D2) of the alloy layer diameter D1 to the nugget diameter D2 to reach a reference value = 1.2 in the energization patterns of the present embodiment and the comparative example.

[0043] Sputtering is not easy to occur when the nugget N is surrounded by the alloy layer A, but it is easy to occur if the nugget N is exposed from the alloy layer A. That is, the alloy layer A acts as a partition wall to suppress the occurrence of sputtering. When the ratio of the alloy layer diameter D1 to the nugget diameter D2 is lower than the preset reference value = 1.2, the nugget N is exposed from the alloy layer A, and sputtering is easy to occur. When the ratio of the alloy layer diameter D1 to the nugget diameter D2 is equal to the reference value = 1.2, the state of the nugget N surrounded by the alloy layer A can be maintained, and the occurrence of sputtering can be suppressed.

[0044] In addition, when the ratio of the alloy layer diameter D1 to the nugget diameter D2 exceeds the reference value = 1.2, there is room for expanding the nugget N. In this case, the heat input to the weld (the boundary between adjacent steel plates) is insufficient, and if the purpose is to suppress the generation of sputtering and grow the nugget N as much as possible, the energization time will be unnecessary. That is, in this case, it can be said that inefficient energization is performed.

[0045] It is therefore considered that if the energization time required for the ratio of the alloy layer diameter D1 to the nugget diameter D2 to reach the reference value = 1.2 is short, the occurrence of sputtering can be suppressed and the nugget N as large as possible can be efficiently generated.

[0046] That is, the reference value associated with the ratio of the alloy layer diameter D1 to the nugget diameter D2 represents the value of the ratio that distinguishes whether it is possible to take into account both the suppression of the generation of sputtering and the efficient generation of the nugget N. The reference value varies depending on the type of steel plate to be welded and the assembly method thereof, and can be determined in advance through experiments, etc. The inventors of the present application have conducted in-depth research on the optimal value of the reference value for various types of steel plates and their assembly methods, and have created a method that can set the reference value to a predetermined value within a range of 1.2 to 1.5. Figure 2 In the type of steel plate and its assembly method shown in FIG. 1 , the reference value is set to 1.2. However, the resistance spot welding method of this embodiment is not limited to this, and the reference value can be set to a predetermined value within the range of 1.2 or more and 1.5 or less according to the type of steel plate to be welded and its assembly method.

[0047] like Figure 5As shown, in the energization pattern of the comparative example, the ratio of the alloy layer diameter D1 to the nugget diameter D2 reaches the reference value = 1.2 at the end of the pre-energization period (time point T2), which requires a energization time of 250 ms. This is because the purpose of the energization pattern of the comparative example is to reliably suppress the generation of sputtering by generating an alloy layer A of a certain size before the generation of the nugget N.

[0048] On the other hand, in the energization pattern of the present embodiment, the ratio of the alloy layer diameter D1 to the nugget diameter D2 reaches the reference value = 1.2 at the end of the first period of pre-energization (time point T1), which only requires a energization time of 100 ms. That is, the energization pattern of the present embodiment requires only 40% of the energization time of the energization pattern of the comparative example. In addition, the energization pattern of the present embodiment is the same as the energization pattern of the comparative example, in which the alloy layer diameter D1 is sufficiently larger than the nugget diameter D2, and the nugget N is maintained surrounded by the alloy layer A. Therefore, it can be seen that the pre-energization of the present embodiment includes a energization pattern that decreases in a step-like manner as in the first period, which can suppress the generation of sputtering and generate as large a nugget N as possible in a shorter time than the comparative example.

[0049] Figure 6 It means in Figure 2 FIG. 1 is a graph showing the changes in the alloy layer diameter D1 and the nugget diameter D2 of the alloy layer A and the nugget N generated at the boundary between the second steel sheet W2 and the third steel sheet W3. Figure 6 , changes in the alloy layer diameter D1 and the nugget diameter D2 are shown when a plate gap exists at the boundary between adjacent steel plates.

[0050] Figure 7 (a)~ Figure 7 (c) is a cross-sectional view showing the state of the alloy layer A and the nugget N generated by the current-carrying pattern of the present embodiment. Figure 7 (a) is a cross-sectional view showing the state of the alloy layer A and the nugget N at the end of the first period of pre-energization (time point T1). Figure 7 (b) is a cross-sectional view showing the state of the alloy layer A and the nugget N at the end of the second period of the preliminary energization (time point T2). Figure 7 (c) is a cross-sectional view showing the state of the alloy layer A and the nugget N when the main energization period ends (time point T3).

[0051] like Figure 6As shown, in the energization mode of the present embodiment, in the first period of pre-energization, even if there is a plate gap, the alloy layer diameter D1 and the nugget diameter D2 are expanded in a state where the alloy layer diameter D1 is sufficiently larger than the nugget diameter D2. That is, it is possible to suppress the generation of sputtering and generate the alloy layer A and the nugget N in a state where the nugget N is surrounded by the alloy layer A. At the start of pre-energization (time point T0), the steel plate is relatively hard, and the contact area between the steel plate and the pair of electrodes 4 and 5 and the contact area between adjacent steel plates are small, so the temperature of the weld is most likely to rise. Therefore, in the energization mode of the present embodiment, at the start of pre-energization, a welding current with a maximum current value is passed, thereby causing the temperature of the weld to rise in a short time. Furthermore, during the first period of pre-energization, the current value of the welding current is finely controlled so that it decreases stepwise from the maximum value, thereby efficiently increasing the temperature of the weld part while efficiently generating the alloy layer A and the nugget N, and correspondingly increasing the diameter D2 of the nugget to suppress the generation of sputtering. Figure 7 In (a), it can be confirmed that the nugget N surrounded by the alloy layer A is generated at the end of the first period of pre-energization (time point T1).

[0052] Furthermore, in the energization pattern of the present embodiment, during the second period of pre-energization, the current value of the welding current is gradually increased from the end of the first period (time point T1), thereby expanding the alloy layer diameter D1 and the nugget diameter D2 while the alloy layer diameter D1 is sufficiently larger than the nugget diameter D2. Figure 7 In (b), it can be confirmed that at the end of the second period of pre-energization (time point T2), the nugget N is surrounded by the alloy layer A, and the alloy layer A and the nugget N are larger than Figure 7 (a) grows more.

[0053] Furthermore, in the energization mode of the present embodiment, during the main energization period, the current value of the welding current is kept constant from the end of the second period of the pre-energization (time point T2), thereby expanding the alloy layer diameter D1 and the nugget diameter D2 while maintaining the alloy layer diameter D1 sufficiently larger than the nugget diameter D2. Figure 7 In (c), it can be confirmed that at the end of the formal energization period (time point T3), the nugget N is maintained surrounded by the alloy layer A, and the alloy layer A and the nugget N are larger than Figure 7 (b) further growth.

[0054] That is, in the energization pattern of the present embodiment, during the second period of pre-energization and the period of main energization, the alloy layer A and the nugget N can be grown while suppressing the generation of sputtering. Figure 6As shown in the figure, during the second period of pre-energization and the period of formal energization, the ratio of the alloy layer diameter D1 to the nugget diameter D2 can be maintained at a reference value = 1.2. This is because, by adopting the above-mentioned energization pattern during the second period of pre-energization and the period of formal energization, the rapid temperature rise of the weld can be suppressed, the generation of sputtering can be suppressed, and the heat input to the weld at the sputtering limit current value can be maximized, so that the alloy layer A and the nugget N can be efficiently grown. The sputtering limit current value refers to the limit current value that can suppress the generation of sputtering. In the case where there is no up-ramp energization such as the second period of pre-energization, and the welding current is energized at a constant current value, if the current value is too high, sputtering will occur, and if the current value is too low, the growth rate of the nugget N will slow down, so it is difficult to control the growth of the nugget N. In addition, when the welding current is energized at a constant current value without energizing at a constant current value such as during the formal energization, it is easier to control the growth of the nugget N than energizing at a constant current value, but it is difficult to maximize the heat input to the weld at the sputtering limit current value.

[0055] Thus, the energization pattern of this embodiment adopts the energization pattern described above in the first and second periods of pre-energization and in the main energization period, and can efficiently generate the largest possible nugget N while suppressing the occurrence of sputtering even if there is a plate gap.

[0056] Figure 7 d)~ Figure 7 (f) is a cross-sectional view showing the state of an alloy layer and a nugget generated by the energization pattern of the comparative example. Figure 7 (d) is a cross-sectional view showing the state of the alloy layer A and the nugget N when the current-carrying time has elapsed by 100 ms (the same time point as the time point T1). Figure 7 (e) is a cross-sectional view showing the state of the alloy layer A and the nugget N at the end of the pre-energization period (time point T2). Figure 7 (f) is a cross-sectional view showing the state of the alloy layer A and the nugget N when the main energization period ends (time point T3).

[0057] In the energization mode of the comparative example, at the same time point as the end of the first period of pre-energization in the present embodiment (time point T1), as shown in FIG. Figure 7 As shown in (d), it can be confirmed that although the alloy layer A is generated, the nugget N is not generated. In the energization pattern of the comparative example, at the end of the pre-energization period (time point T2), as shown in FIG. Figure 7 As shown in (e), it can be confirmed that a nugget N surrounded by the alloy layer A is generated, but the size of the alloy layer A and the nugget N is larger than that of the alloy layer A. Figure 7 In the power-on mode of the comparative example, at the end of the formal power-on period (time point T3), as shown in FIG. Figure 7As shown in (f), it can be confirmed that the nugget N is surrounded by the alloy layer A, but the size of the alloy layer A and the nugget N is larger than that of the alloy layer A. Figure 7 That is, it can be confirmed that the energization pattern of the present embodiment can generate a larger nugget N than that of the comparative example.

[0058] Figure 8 It means in Figure 2 1 and 2 are graphs showing changes in the nugget diameter D2 of the nugget N generated at the boundary between the first steel plate W1 and the second steel plate W2.

[0059] exist Figure 8 , the change of the nugget diameter D2 is shown when there is no plate gap between the boundaries of the adjacent steel plates. Figure 8 As shown, the energization pattern of the present embodiment can generate a larger nugget N than the comparative example not only at the boundary portion where the plate gap exists but also at the boundary portion where the plate gap does not exist.

[0060] use Figure 9~Figure 11 The result of applying the current conduction pattern of the present embodiment to a plurality of steel plates in which a spacer S is arranged between a first steel plate W1 and a second steel plate W2 as welding objects will be described. Fig. 9 is a schematic representation of Figure 2 A cross-sectional view showing a state where a plurality of steel sheets of different forms are sandwiched between a pair of electrodes 4 and 5 . Fig.10 It means in Fig. 9 1 and 2 are graphs showing changes in the alloy layer diameter D1 and the nugget diameter D2 of the alloy layer A and the nugget N generated at the boundary between the first steel sheet W1 and the second steel sheet W2. Fig.11 It means in Fig. 9 1 is a diagram showing changes in the nugget diameter D2 of the nugget N generated at the boundary between the second steel plate W2 and the third steel plate W3.

[0061] like Fig.10 As shown, even when a spacer S is arranged between the first steel plate W1 and the second steel plate W2, the energization pattern of the present embodiment can suppress the generation of sputtering during the first period of pre-energization, and efficiently generate the alloy layer A and the nugget N. Furthermore, the energization pattern of the present embodiment can suppress the rapid temperature rise of the weld during the second period of pre-energization and the period of formal energization, suppress the generation of sputtering, and maximize the heat input to the weld at the sputtering limit current value, so that the alloy layer A and the nugget N grow efficiently. In addition, as Fig.11 As shown, even when a spacer S is arranged between the first steel plate W1 and the second steel plate W2, the energization pattern of this embodiment can generate a larger nugget N than the comparative example not only at the boundary portion where there is a plate gap, but also at the boundary portion where there is no plate gap.

[0062] In addition, in the above description, the case where a gasket S is arranged between the first steel plate W1 and the second steel plate W2 or between the second steel plate W2 and the third steel plate W3 is described as an example. However, the resistance spot welding method of this embodiment can be applied even if a gasket S is arranged between the first steel plate W1 and the second steel plate W2 or between the second steel plate W2 and the third steel plate W3. That is, the resistance spot welding method of this embodiment can be applied even if there are plate gaps at all the boundaries between adjacent steel plates.

[0063] In addition, in the above description, a plurality of steel plates formed by overlapping at least two steel plates having a coating layer with zinc as a main component are described as an example of welding objects. However, the resistance spot welding method of this embodiment can also be applied to a plurality of steel plates formed by overlapping at least two steel plates having an Al-Si coating layer as welding objects. In addition, the resistance spot welding method of this embodiment can also be applied to a plurality of steel plates formed by overlapping at least two steel plates having a zinc oxide layer as a surface layer as welding objects.

[0064] As described above, the resistance spot welding method of the present embodiment overlaps at least two steel plates having a zinc-based plating layer or an Al-Si plating layer, clamps the at least two steel plates by a pair of electrodes, performs pre-energization between the pair of electrodes, and then performs formal energization, thereby melting and joining the adjacent steel plates. The resistance spot welding method of the present embodiment performs pre-energization in a manner including the following energization mode: the current value of the welding current at the start of energization of the pre-energization is set to the maximum value, and the current value is sequentially reduced in a step-like manner from the maximum value as the energization time passes from the start of energization.

[0065] As described above, in the resistance spot welding method of the present embodiment, at the start of pre-energization when the steel plate is relatively hard and the contact area between the steel plate and a pair of electrodes and the contact area between adjacent steel plates is relatively small, a welding current having a maximum current value is passed, thereby at the start of pre-energization, the temperature of the weld can be raised in a short time. In addition, the resistance spot welding method of the present embodiment can efficiently generate the alloy layer A and the nugget N at an early stage of pre-energization while efficiently raising the temperature of the weld, and can expand the nugget diameter D2 corresponding to the expansion of the alloy layer diameter D1 to suppress the generation of sputtering. As a result, the resistance spot welding method of the present embodiment can suppress the generation of sputtering and efficiently generate the nugget N as large as possible.

[0066] Furthermore, in the resistance spot welding method of the present embodiment, the step-shaped energization pattern adopted in the first period of pre-energization is formed by combining a plurality of sets of current values ​​and energization times at the current values. Each combination of the current value and the energization time at the current value constituting the step-shaped energization pattern is a combination of an upper limit current value capable of suppressing the generation of sputtering and an upper limit energization time capable of suppressing the generation of sputtering during energization at the upper limit current value.

[0067] Thus, the resistance spot welding method of the present embodiment can reliably suppress the generation of sputtering, and can efficiently generate the alloy layer A and the nugget N at an early stage of pre-energization. Thus, the resistance spot welding method of the present embodiment can reliably suppress the generation of sputtering, and can efficiently generate the nugget N as large as possible.

[0068] Furthermore, in the resistance spot welding method of the present embodiment, a nugget N is generated at a boundary portion between adjacent steel plates by pre-energization, and an alloy layer A is generated which is composed of a material of the plating layer and a material of the base material of the steel plate and surrounds the nugget N. Each combination of step-shaped energization patterns is a combination in which the ratio (D1 / D2) of the outer dimensions of the alloy layer A (alloy layer diameter D1) in the direction along the steel plate to the outer dimensions of the nugget N (nugget diameter D2) is a reference value within a range of 1.2 or more and 1.5 or less.

[0069] Thus, the resistance spot welding method of the present embodiment can reliably achieve both suppression of the occurrence of sputtering and efficient generation of the nugget N. Thus, the resistance spot welding method of the present embodiment can reliably suppress the occurrence of sputtering and reliably and efficiently generate the nugget N as large as possible.

[0070] In addition, in the resistance spot welding method of the present embodiment, the pre-energization period for pre-energization includes a first period starting from the start of energization and a second period starting after the first period, and the energization pattern of the pre-energization in the first period is the above-mentioned step-like energization pattern, and the energization pattern of the pre-energization in the second period is an upslope energization pattern in which the current value gradually increases from the current value at the end of the first period as the energization time passes.

[0071] Thus, the resistance spot welding method of the present embodiment can suppress the rapid temperature rise of the weld during the second period of pre-energization to suppress the generation of sputtering, and efficiently grow the alloy layer A and the nugget N. Thus, the resistance spot welding method of the present embodiment can further suppress the generation of sputtering, and efficiently generate the nugget N as large as possible.

[0072] In the resistance spot welding method of the present embodiment, the energization pattern of the main energization is a energization pattern in which the current value at the end of the preliminary energization is maintained constant from the end of the preliminary energization to the end of the main energization.

[0073] Thus, the resistance spot welding method of the present embodiment can suppress the rapid temperature rise of the weld during the formal energization period to suppress the generation of sputtering, and efficiently grow the alloy layer A and the nugget N. Therefore, the resistance spot welding method of the present embodiment can further suppress the generation of sputtering and efficiently generate the nugget N as large as possible.

[0074] The embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments, and various changes can be made within the scope of the spirit of the present invention described in the claims. The present invention can add the structure of a certain embodiment to the structure of other embodiments, or replace the structure of a certain embodiment with other embodiments, or delete a part of the structure of a certain embodiment.

[0075] Description of Reference Numerals

[0076] 1…resistance spot welding device, 2…welding gun, 3…robot arm, 4…upper electrode, 5…lower electrode, 6…lifting device, 7…position detection device, 8…current adjustment device, 9…control device, A…alloy layer, D1…alloy layer diameter, D2…nucleus diameter, G…gap, N…nucleus, S…shim, T0~T3…time point, W1…first steel plate, W2…second steel plate, W3…third steel plate.

Claims

1. A resistance spot welding method, wherein at least two steel plates having a zinc-based coating or an Al-Si coating are overlapped, the at least two steel plates are clamped by a pair of electrodes, and a pre-current is applied between the pair of electrodes and then a current is applied therebetween, thereby melting and joining the adjacent steel plates, wherein the resistance spot welding method is characterized in that: The pre-energization is performed so as to include an energization pattern in which a welding current value at the start of energization of the pre-energization is set to a maximum value and the current value is sequentially decreased in steps from the maximum value as energization time passes from the start of energization.

2. The resistance spot welding method according to claim 1, It is characterized in that The step-like energization pattern is formed by combining a plurality of sets of current values ​​and energization times at the current values. Each combination of the current value and the energization time at the current value constituting the step-shaped energization pattern is a combination of an upper limit current value capable of suppressing sputtering generation and an upper limit energization time capable of suppressing sputtering generation during energization at the upper limit current value.

3. The resistance spot welding method according to claim 2, It is characterized in that By the pre-energization, a nugget is generated at a boundary between the adjacent steel sheets, and an alloy layer is generated which is composed of a material of the plating layer and a material of the base material of the steel sheet and surrounds the nugget. The combinations constituting the step-shaped energization patterns are combinations in which a ratio of an outer dimension of the alloy layer to an outer dimension of the nugget in a direction along the steel sheet becomes a reference value within a range of 1.2 to 1.

5.

4. The resistance spot welding method according to claim 1, It is characterized in that The pre-energization period during which the pre-energization is performed includes a first period starting from the start of the energization and a second period starting after the first period. The energization pattern of the pre-energization in the first period is the step-shaped energization pattern, The energization pattern of the pre-energization in the second period is an up-slope energization pattern in which the current value gradually increases from the current value at the end of the first period as the energization time elapses.

5. The resistance spot welding method according to claim 1, It is characterized in that The energization pattern of the main energization is a energization pattern in which the current value at the end of the pre-energization is maintained constant from the end of the pre-energization to the end of the main energization.

Images