JP7201570B2 - Resistance spot welding method - Google Patents

Description

The present invention relates to a resistance spot welding method. In particular, the present invention relates to an improvement in a resistance spot welding method for overlapping and joining three or more steel sheets, including at least one steel sheet having a different electrical resistance value with respect to other steel sheets.

2. Description of the Related Art Conventionally, resistance spot welding is used as a means for joining a plurality of metal plates to each other in manufacturing automobile bodies and the like. In this resistance spot welding, a pair of electrodes hold a plurality of metal plates (materials to be welded) while energizing them. It is to be joined.

In order to obtain good welding quality in resistance spot welding, it is necessary to suppress the occurrence of spatter (spattering of molten metal).

Patent Literature 1 discloses a resistance spot welding method for galvanized high-strength steel sheets for the purpose of suppressing the generation of spatter. In this patent document 1, for the purpose of suppressing the generation of spatter during resistance spot welding, when initial energization is performed prior to main energization between electrodes, the total thickness of each metal plate (steel plate) overlapped and the electrode Determining the relationship with the center-to-center distance of the pair is disclosed.

JP 2016-41441 A

However, when three or more steel plates including at least one steel plate having a different electrical resistance value (hereinafter, sometimes simply referred to as resistance) from other steel plates are superimposed on each other and resistance spot welding is performed, initial energization is required. At the start, at the interface between the steel sheets where the difference in material resistance is large, the amount of heat generated increases as the resistance becomes greater than at the interface between the other steel sheets. As a result, the formation of the weld nugget begins earlier around this interface than around other interfaces. Since the material resistance increases as the temperature rises, the weld nugget expands further at the portion where the weld nugget is formed at this early stage (the portion where the amount of heat generated is large) as the main energization is started thereafter. , spatter may occur.

FIG. 9 is a diagram showing changes in welding current value (upper diagram in FIG. 9) in conventional resistance spot welding, and an example of accompanying changes in electrical resistance value and temperature (lower diagram in FIG. 9). be. A thick solid line in the lower diagram indicates changes in electrical resistance, and a thin solid line indicates changes in temperature. As shown in FIG. 9, in resistance spot welding, after initial energization is performed for a predetermined period (timings t1 to t2 in the figure), main energization is continuously performed (timings t2 to t4 in the figure). In the initial energization, the welding current value is gradually increased. Further, the main energization is performed at a current value higher than the maximum current value during the energization period of the initial energization. In such a transition of the conventional welding current value, as described above, the formation of the welding nugget is started early around the interface between the steel plates where the material resistance difference is large, and the main energization is started. As a result, the welding nugget further expands, and there is a possibility that spatter will occur during the period of main energization due to an increase in the amount of heat generated due to the increase in resistance. In FIG. 9, spatter occurs at timing t3 during the period of main energization.

Therefore, in order to suppress the generation of spatter, it is necessary to keep the welding current value in the main energization low. In other words, the spatter generation limit (the upper limit of the welding current value in main energization for suppressing the generation of spatter) in the welding current value in main energization becomes low, and the time required for resistance spot welding becomes long.

For this reason, in the prior art, it was difficult to form a weld nugget efficiently while suppressing the generation of spatter.

The present invention has been made in view of the above points, and its object is to provide a resistance spot welding method capable of efficiently forming a weld nugget while suppressing the generation of spatter. be.

A solution of the present invention for achieving the above object is to stack three or more steel plates including at least one steel plate having a different electrical resistance value with respect to other steel plates and sandwich these steel plates between electrodes. After the initial energization is performed by gradually increasing the welding current value between the electrodes, the steel sheets are melted by performing the main energization at a current value higher than the maximum current value in the energization period of the initial energization. The premise is a resistance spot welding method that joins by In this resistance spot welding method, the iron constituting the base material of each steel plate is melted by the initial energization and then cooled to form a shape . The outer dimensions of the welding nugget whose outer dimensions in the direction parallel to the extension direction expand , and the constituent material of the plating layer formed on the outside of the welding nugget and existing on the surface of each steel plate are the iron The relationship between the outer dimensions of the alloy layer , which is generated by increasing the solidus temperature determined by the two components and increases the outer dimensions in the direction as the initial energization continues, is Equation (1) below
1.2 ≤ external dimension of alloy layer / external dimension of welding nugget ≤ 1.5 (1)
By stopping the energization between the electrodes in this state, an interval, which is a period during which energization is stopped, is provided between the initial energization and the main energization.

At the interface between steel plates that have a large material resistance difference due to the inclusion of at least one steel plate that has a different electrical resistance value than the other steel plates, the difference between the other steel plates Resistance becomes larger than the interface. Along with this, when the initial energization is started, the amount of heat generated around this interface (the interface between the steel plates having a large material resistance difference) increases. As a result, the formation of the weld nugget begins earlier around this interface than around other interfaces. In such a situation, when the above formula (1) is established, the ratio between the outer dimensions of the alloy layer and the outer dimensions of the welding nugget is properly maintained (the joint strength between the steel plates is sufficiently secured). However, the weld nugget does not become too large), thereby making it difficult for spatter to occur. In this state, the energization between the electrodes is stopped, and the start of the main energization is delayed until the predetermined interval, which is the period during which the energization is stopped, has passed. will be cooled to In other words, the electrical resistance value in the area where this weld nugget is formed becomes low. Therefore, even if main energization is started after that, local expansion of the welding nugget due to the high electrical resistance is suppressed, and the occurrence of spatter can be suppressed. In other words, it is possible to increase the spatter generation limit at the welding current value in the main energization (the spatter generation limit can be increased compared to the conventional technology), and the welding nugget can be formed efficiently while suppressing the generation of spatter. it becomes possible to

In the present invention, in the initial energization, the following formula (1)
1.2 ≤ external dimension of alloy layer / external dimension of welding nugget ≤ 1.5 (1)
By stopping the energization between the electrodes in this state, an interval, which is the energization stop period, is provided between the initial energization and the main energization. As a result, the weld nugget is cooled during the interval, and the electrical resistance in the area where the weld nugget is formed can be lowered. Therefore, even if main energization is started after that, local expansion of the welding nugget is suppressed, and the occurrence of spatter can be suppressed. Therefore, it is possible to increase the spatter generation limit at the welding current value in the main energization, and it is possible to efficiently form a welding nugget while suppressing the generation of spatter.

1 is a schematic configuration diagram showing a welding gun of a resistance spot welding device according to an embodiment; FIG. 1 is a diagram showing a schematic configuration of a control device for a welding gun; FIG. FIG. 4 is a cross-sectional view showing a state in which each steel plate is sandwiched between an upper electrode and a lower electrode; FIG. 4 is a cross-sectional view of each steel plate joined by resistance spot welding; It is a figure which shows an example of transition of the welding current value in embodiment, and the accompanying change of an electrical resistance value and temperature. FIG. 7 is a cross-sectional view of each steel plate showing an example of changes in the welding current value and shapes of the alloy layer and the weld nugget when no interval is provided between the initial energization and the main energization as a comparative example. Cross-sectional view of each steel plate showing an example of transition of welding current value, alloy layer and weld nugget shape when an interval is provided between the initial energization and the main energization and the interval is relatively short. is. Cross-sectional view of each steel plate showing an example of transition of welding current value, alloy layer and weld nugget shape when an interval is provided between the initial energization and the main energization and the interval is relatively long. is. FIG. 10 is a diagram showing an example of changes in welding current value and accompanying changes in electrical resistance and temperature in the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. This embodiment is a case where three steel sheets having a zinc-based coating layer (more specifically, a hot-dip galvanizing layer) are superimposed and resistance spot welded (hereinafter, sometimes simply referred to as welding). will be described as an example. An example is the case where three steel plates are resistance spot welded together in the manufacture of automobile bodies.

- Configuration of Resistance Spot Welding Equipment -
Before describing the resistance spot welding method, an outline of a resistance spot welding apparatus for carrying out this resistance spot welding method will be described.

FIG. 1 is a schematic configuration diagram showing a welding gun G of a resistance spot welding apparatus according to this embodiment. 2 is a diagram showing a schematic configuration of a control device 10 used for controlling the welding gun G. As shown in FIG.

The welding gun G has a gun body 1 held by a robot arm RA, an upper electrode 2, a lower electrode 3 erected on a lower portion 1a of the gun body 1, and an electric motor that holds and raises and lowers the upper electrode 2. An upper electrode lifting device (hereinafter, simply referred to as an electrode lifting device) 4, an electrode position detection device 5, and a welding current value (hereinafter, sometimes simply referred to as a current value) that flows between the upper electrode 2 and the lower electrode 3 are measured. It is configured with a current adjusting device 6 for adjustment as a main component.

In FIG. 1, the three steel plates W1, W2, and W3 to be welded are, for example, the steel plate W1 positioned on the uppermost side is a hot stamp material (ultra-high tensile strength steel plate) having a hot dip galvanized layer, and the lower side The two steel sheets W2 and W3 positioned are so-called GA materials having an alloyed hot-dip galvanized layer. Further, in this embodiment, the plate thickness dimensions of the steel plates W1, W2, and W3 are all the same (for example, 1.2 mm). In addition, in the present invention, the types of the steel plates W1, W2, and W3 are not limited to those described above, and can be appropriately selected. Further, the plate thickness dimensions of the steel plates W1, W2, and W3 do not necessarily have to be the same. For example, steel plates W1, W2, and W3 in which the ratio of the thickness dimension of the other steel plates W2 and W3 to the thickness dimension of a specific steel plate W1 is in the range of 0.7 to 1.5, respectively, can be applied. is.

Further, each galvanized layer is not limited to being formed by hot-dip plating, and may be formed by electroplating. In other words, the steel sheets W1, W2, and W3 may be hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electro-galvanized steel sheets, or electro-alloy galvanized steel sheets, and the resistance spot welding method according to the present invention is applied. It is possible to In the present invention, the types and number of steel plates W1, W2, and W3 are not limited to these. In addition, the material (zinc in this embodiment) constituting the plating layer is a material having a melting point lower than that of iron (Fe) constituting the base material of each of the steel sheets W1, W2, and W3. .

As shown in FIG. 1, the gun body 1 is a substantially U-shaped member, and a lower electrode 3 is detachably erected on the upper surface of its lower portion 1a. An electrode elevating device 4 is attached to the tip of the upper portion 1b of the gun body 1. As shown in FIG.

The electrode lifting device 4 includes a servomotor 41 attached to the tip of the upper portion 1b of the gun body 1, and a lifting member 42 coupled to a drive shaft (not shown) of the servomotor 41. The upper electrode 2 is detachably attached to the lower end portion 42 a of the lifting member 42 .

The electrode position detection device 5 is composed of an encoder, for example, and is attached to the upper end portion 41 a of the servomotor 41 . Then, the detected value is transmitted to the control device 10 .

The current adjusting device 6 adjusts the value of current flowing between the upper electrode 2 and the lower electrode 3 according to the current command value transmitted from the control device 10 . As the current adjusting device 6, a known device such as one having a variable resistor or a converter is applied.

The control device 10 includes an input unit 11 that acquires information from an input device 7 (see FIG. 2) for inputting the plate thickness of each steel plate W1, W2, W3, etc. , a current value calculator 13 that calculates the current value when energizing between the upper electrode 2 and the lower electrode 3, and the pressure required for welding (the upper electrode 2 and the lower electrode A pressing force setting unit 14 for setting the pressing force applied to the steel plates W1, W2, and W3 by the electrode 3; An output unit 15 for outputting pressure information is provided as a main part.

The control device 10 comprises a CPU, a ROM, a RAM, an input/output interface, etc., and is realized by storing programs corresponding to the above functions in the ROM. Further, the RAM temporarily stores information such as detection values from the electrode position detection device 5 and plate thickness. The rest of the configuration of the control device 10 is the same as that conventionally used for the welding gun G, so detailed description thereof will be omitted.

－Resistance spot welding method－
Next, a resistance spot welding method, which is a feature of this embodiment, will be described.

In this resistance spot welding method, the steel plates W1, W2, and W3 are pressed between the upper electrode 2 and the lower electrode 3 with a predetermined pressure (pressure set by the pressure setting unit 14) by the operation of the electrode lifting device 4. By sandwiching, the steel plates W1, W2, and W3 are in close contact with each other without gaps (close contact within the tip diameters of the electrodes 2 and 3), and the current value calculated by the current value calculation unit 13 is applied. I do.

FIG. 3 is a cross-sectional view showing a state in which the steel plates W1, W2, W3 are sandwiched between the upper electrode 2 and the lower electrode 3. As shown in FIG. In this state, initial energization (also called pre-energization) and main energization are performed as energization between the upper electrode 2 and the lower electrode 3 .

In the initial energization, for example, when an oxide film (a film with high electrical resistance) exists on the surface of the steel plates W1, W2, and W3, the oxide film is removed or reduced to melt the steel plates W1, W2, and W3 in the main energization. It is implemented for the purpose of facilitating Also, this initial energization is performed to start forming an alloy layer, which will be described later, at the contact portions of the steel plates W1, W2, and W3, and to start forming a weld nugget.

Further, the main energization is for melting and joining the steel plates W1, W2, W3, and is performed for growing the alloy layer and the welding nugget. Also, the welding current value in this main energization is set higher than the welding current value in the initial energization. The form (current value and transition timing) of each of these initial energization and main energization will be described later.

FIG. 4 is a cross-sectional view showing a state in which the steel plates W1, W2, and W3 are joined together by performing the initial energization and the main energization. As shown in FIG. 4, in the joint portion (welded portion) of each of the steel plates W1, W2, and W3, there are a weld nugget N and a weld nugget N formed on the outer peripheral side of the weld nugget N (outer periphery of the weld nugget N). There is an alloy layer A formed so as to surround the The weld nugget N is formed by cooling after melting the iron (Fe) forming the base material of each of the steel plates W1, W2, and W3. The alloy layer A is formed by a high-temperature reaction between zinc forming the coating layer on the surface of each of the steel sheets W1, W2 and W3 and iron forming the base material of each of the steel sheets W1, W2 and W3. (The formation process and growth of these weld nuggets N and alloy layer A will be described later). Note that the dimension T1 in FIG. 4 is the outer dimension of the alloy layer, and the dimension T2 is the outer dimension of the weld nugget.

The outline of the forming process of the alloy layer A and the weld nugget N accompanying the initial energization and the main energization in order will be described below. Fig. 5 shows transitions in the welding current value (the current value in the initial energization and the current value in the main energization) (the upper diagram in Fig. 5), and the accompanying electric resistance value and temperature in the forming region of the weld nugget N. 6 is a diagram showing an example of change in (lower diagram in FIG. 5). FIG. In the lower diagram, the electrical resistance value in this embodiment is indicated by a thick solid line, and the temperature is indicated by a thin solid line. In addition, in order to facilitate comparison with the conventional technology, the upper diagram shows the transition of the welding current value in the conventional technology with a dashed line, and the lower diagram shows the electric resistance value in the conventional technology with a thick dashed line. Temperature is indicated by a thin dashed line.

First, the initial energization is started (timing t10 in FIG. 5), the steel plates W1, W2, W3 are heated, and the boundaries between the steel plates W1, W2, W3 are formed in the region between the upper electrode 2 and the lower electrode 3. The temperature of the part rises. During this initial energization, the welding current value is gradually increased (gradually increased) according to a preset gradient (timings t10 to t11 in FIG. 5). At this time, actually, the amount of heat generated increases as the resistance increases at the interface between the steel plates W1 and W2, where the difference in material resistance increases.

As a specific example of this initial energization, energization is started from a predetermined current value in the range of 1.5 to 10 kA, preferably 2 to 8 kA, for a period of 1 to 50 cyc (cycle), preferably 2 to 40 cyc. , the current value is gradually increased to about 1.05 to 2.5 times the current value at the start of the initial energization. These values are not limited to these, and are appropriately set through experiments and simulations so that the sizes and growth rates of the alloy layer A and the welding nugget N required during the energization period of the initial energization can be achieved.

As the temperatures at the boundaries between the steel sheets W1, W2, and W3 rise due to this initial energization, the zinc forming the coating layers on the surfaces of the steel sheets W1, W2, and W3 and the steel sheets W1, W2, and W3 An alloy is produced by a high-temperature reaction with the iron that constitutes the base material. Specifically, zinc, which is the main component of the surface layer (coating layer), is diffused into the base material (iron) of each of the steel sheets W1, W2, and W3, and the solidus temperature determined by both components is increased. will be

By continuing this initial energization, the alloy layer A formed by this alloy grows and the external dimensions of the alloy layer A increase.

When the initial energization is further continued, iron melts in the central portion of the alloy layer A, thereby starting the formation of a weld nugget N. That is, a weld nugget N (molten iron) made of iron is formed in the central portion of the molten metal, and an alloy layer A is formed on the outer peripheral side of the weld nugget N, and the weld nugget N and the alloy layer A expand. I will go. The temperature in the area of the welding nugget N (molten iron) is approximately 1400°C.

Then, the initial energization ends at timing t11 in FIG. 5, and the energization between the electrodes is stopped from this point. The purpose of stopping the current flow between the electrodes is to provide a cooling period for the formed weld nugget N, thereby lowering the electrical resistance in the region where the weld nugget N is formed. As can be seen from the lower part of FIG. 5, the temperature begins to drop when the initial energization is completed and the energization between the electrodes is stopped, and the electrical resistance is accordingly lowered.

As for the timing (timing t11 in FIG. 5) to end this initial energization, the relationship between the external dimensions of the welding nugget N and the external dimensions of the alloy layer A in the direction parallel to the extending direction of each of the steel plates W1, W2, and W3 is , the following equation (1)
1.2 ≤ external dimension of alloy layer / external dimension of welding nugget ≤ 1.5 (1)
It is set so that the initial energization is terminated in this state.

Specifically, when the formation of the weld nugget N is started as described above, the weld nugget N grows together with the growth of the alloy layer A. At the initial stage when forming of the weld nugget N is started, the external dimensions of the alloy layer A are significantly larger than the external dimensions of the weld nugget N. Then, after a predetermined period of time has passed since the welding nugget N started to be formed, the speed of expansion of the external dimensions of the weld nugget N becomes greater than the speed of expansion of the external dimensions of the alloy layer A, and the welding nugget N The ratio of the outer dimensions of the alloy layer A to the outer dimensions of the alloy layer A is 1.5 or less. In this embodiment, the initial energization is terminated when the ratio of the external dimensions of the alloy layer A to the external dimensions of the welding nugget N is in the range of 1.2 or more and 1.5 or less. For example, the initial energization is terminated when the ratio becomes 1.3. The timing for ending the initial energization is not limited to the timing when the ratio becomes 1.3, but can be set to any value within the range of 1.2 or more and 1.5 or less as described above. be.

The reason why the ratio of the external dimensions of the alloy layer A to the external dimensions of the welding nugget N at the timing of ending the initial energization is defined as in formula (1) is as follows. This is because there is a possibility that spatter will occur due to further expansion of the welding nugget N with the start of main energization, and this value is obtained by experiments and simulations. In addition, when this ratio exceeds 1.5, it becomes difficult to form a sufficiently large welding nugget N by main energization, and the bonding strength between the steel plates W1, W2, and W3 is sufficiently secured. This value is also obtained through experiments and simulations.

Further, the outer dimensions of the welding nugget N required at the end of the initial energization are experimentally set in advance according to the material, plate thickness, required joint strength, etc. of the steel plates W1, W2, and W3. . For example, the initial energization is continued until the outer dimensions of the welding nugget N to be finally obtained (the outer dimensions at the end of the main energization) reach about 1/2. This value is not limited to this, and is set as appropriate.

In this way, the initial energization is terminated when the ratio is in the range of 1.2 or more and 1.5 or less, and the energization between the electrodes is stopped from this point. In other words, an interval is provided between the initial energization and the main energization. This interval (the period during which energization is stopped; timings t11 to t12 in FIG. 5) is set to, for example, 5 to 20 cyc, preferably 8 to 15 cyc. These values are not limited to this.

Then, after a predetermined interval has passed, main energization is started (timing t12). The current value in this main energization is set to a current value higher than the maximum current value in the energization period of the initial energization. That is, as described above, the main energization is started by further increasing the current value with respect to the final current value of the current value of the initial energization which is increased at a predetermined gradient. Specifically, the current value in this main energization is a predetermined value set from the range of 0.1 to 8.0 kA, preferably 1.0 to 5.0 kA, with respect to the final current value of the initial energization. set to a high value. As a specific example of this final current value, it is preferable to use a small value in the case of a material with high surface resistance. This value is not limited to this value, and is appropriately set through experiments and simulations so that the alloy layer A and the welding nugget N of a predetermined size are formed within a predetermined period of time.

By starting this main energization, the welding nugget N and the alloy layer A further expand. In addition, since the above-described interval is provided prior to the main energization to reduce the temperature and the electrical resistance, an excessive increase in temperature and electrical resistance can be suppressed even during the period of the main energization. It will be.

By continuing this energization for a predetermined period (timings t12 to t13 in FIG. 5), the welding nugget N and the alloy layer A having predetermined external dimensions are formed. For example, if the total plate thickness of each of the steel plates W1, W2, and W3 is t, main energization is continued until the external size of the welding nugget N reaches 3√t or more. After that, the main energization is terminated, the weld nugget N and the alloy layer A are solidified, and a predetermined welded portion is formed.

In the main energization performed after the interval has elapsed, a constant welding current is energized that is slightly higher than the maximum current value in the energization period of the initial energization as described above. , the weld nugget N and the alloy layer A further expand. The value of the welding current value in the main energization at this time is such that the above formula (1) continues to hold, that is, the ratio of the external dimensions of the alloy layer A to the external dimensions of the welding nugget N is less than 1.2. It is set in advance by experiments or simulations so that there is no

In the prior art, when three or more steel plates including at least one steel plate having a different electrical resistance value with respect to other steel plates are superimposed and resistance spot welding is performed, when the initial energization is started, the material At the interface between steel plates with a large resistance difference, the amount of heat generated increases as the resistance increases compared to the interface between other steel plates. As a result, the formation of the weld nugget begins earlier around this interface than around other interfaces. Since the material resistance increases as the temperature rises, the weld nugget expands further at the portion where the weld nugget is formed at this early stage (the portion where the amount of heat generated is large) as the main energization is started thereafter. , there was a possibility that spatter would occur. Therefore, in order to suppress the generation of spatter, it was necessary to keep the welding current value in the main energization low. In other words, the spatter generation limit at the welding current value in the main energization is lowered, making it difficult to form the welding nugget efficiently.

On the other hand, in the present embodiment, as described above, when the formula (1) is established, the ratio between the outer dimension T1 of the alloy layer A and the outer dimension T2 of the weld nugget N is Spatter is less likely to occur because the welding nugget N is properly maintained (the welding nugget N is not excessively large while ensuring sufficient bonding strength between the steel plates W1, W2, and W3). In this state, the energization between the electrodes is stopped, and the start of the main energization is delayed until the predetermined interval, which is the period during which the energization is stopped, has passed. It will be cooled in between. In other words, the electric resistance value in the region where this weld nugget N is formed becomes low. Therefore, even if the main energization is started after that, local expansion of the welding nugget N due to the high electrical resistance value is suppressed, and the occurrence of spatter can be suppressed. . In other words, it is possible to increase the spatter generation limit at the welding current value in the main energization (the spatter generation limit can be increased compared to the conventional technology), and the welding nugget N can be efficiently removed while suppressing the generation of spatter. It is possible to mold.

-Experimental example-
Next, an example of an experiment conducted to confirm the above effects will be described. In this experimental example, the shapes of the alloy layer A and the welding nugget N were compared between the case where the interval was not provided and the case where the interval was provided.

FIG. 6 shows transitions in welding current values (FIG. 6(a)) and an example of the shapes of the alloy layer A and the welding nugget N when no interval is provided between the initial energization and the main energization as a comparative example. It is sectional drawing (FIG.6(b)) of the steel plates W1, W2, and W3. Moreover, FIG. 7 shows the transition of the welding current value (FIG. 7(a)) and the alloy layer A when an interval is provided between the initial energization and the main energization, and the interval is relatively short. 7A and 7B are cross-sectional views of the steel plates W1, W2, and W3 showing an example of the shape of the welding nugget N (FIG. 7B). Moreover, FIG. 8 shows the transition of the welding current value (FIG. 8(a)) and the alloy layer A when an interval is provided between the initial energization and the main energization, and the interval is relatively long. 8A and 8B are cross-sectional views of steel plates W1, W2, and W3 showing an example of the shape of a welding nugget N (FIG. 8B). Further, in these comparative examples and each embodiment, the initial energization period is set to be the same (for example, 20 cyc), and the main energization period is also set to be the same (for example, 30 cyc).

As for the welding current value, specifically, in the comparative example of FIG. and In the embodiment of FIG. 7, the starting current value of the initial energization was 2.0 kA, the final current value was 5.0 kA, and the current value of the main energization was 6.0 kA. Also, the interval was set to 9 cyc. In the embodiment of FIG. 8, the starting current value for initial energization was 2.0 kA, the final current value was 5.0 kA, and the current value for main energization was 7.0 kA. Also, the interval was set to 10 cyc. The current value of main energization in each of the comparative example and each of the embodiments is defined as a value corresponding to the limit of spatter generation.

Comparing the shapes of the weld nuggets N in each figure, the larger weld nugget N is formed in the embodiment of FIG. 7 than in the comparative example of FIG. Specifically, in the comparative example of FIG. 6, the outer dimension of the weld nugget N is about 4.0 mm, and in the embodiment of FIG. 7, the outer dimension of the weld nugget N is about 5.2 mm. . That is, in the comparative example of FIG. 6, the current value in the main energization can be increased by only 0.5 kA with respect to the final current value in the initial energization in order not to generate spatter, whereas the embodiment of FIG. In the case, the current value in the main energization can be increased by 1.0 kA relative to the final current value in the initial energization, and as a result, a large-sized (large external dimension) welding nugget N can be formed. .

Also, the weld nugget N is formed larger in the embodiment of FIG. 8 than in the embodiment of FIG. Specifically, in the embodiment of FIG. 8, the outside dimension of the weld nugget N was approximately 5.8 mm. That is, in the embodiment shown in FIG. 8, the current value in the main energization can be increased by 2.0 kA relative to the final current value in the initial energization. can be molded.

In this way, when the interval is provided compared to when the interval is not provided, the current value at the spatter generation limit becomes higher, so that the size of the welding nugget N can be increased. It has been confirmed that the welding nugget N can be further enlarged by increasing the limit current value. As a result, the effect of this embodiment could be confirmed.

-Other embodiments-
It should be noted that the present invention is not limited to the above-described embodiments, and all modifications and applications within the scope of the claims and their equivalents are possible.

For example, in the above-described embodiment, the case where the present invention is applied to resistance spot welding used for manufacturing automobile bodies has been described. The present invention is not limited to this, and can be applied to resistance spot welding used for manufacturing other products.

Further, in the above-described embodiment, the case where the steel sheets W1, W2, and W3 having the plating layer containing zinc as a main component are resistance spot-welded has been described. The present invention is not limited to this, and can also be applied to resistance spot welding of steel plates having Al—Si plating layers. It can also be applied to resistance spot welding of steel sheets having a zinc oxide film as a surface layer.

Moreover, in the above-described embodiment, all the steel sheets W1, W2, and W3 have a plated layer on their surfaces. The present invention is not limited to this, and only some steel sheets may have a plating layer on the surface thereof.

INDUSTRIAL APPLICABILITY The present invention is applicable to a resistance spot welding method in which three or more steel plates including at least one steel plate having an electrical resistance value different from that of another steel plate are overlapped and joined together.

2 upper electrode 3 lower electrode 6 current regulator 10 controller 13 current value calculator W1, W2, W3 steel plate N welding nugget A alloy layer

Claims (1)

Three or more steel plates including at least one steel plate having an electrical resistance value different from that of other steel plates are superimposed on each other, these steel plates are sandwiched between electrodes, and the welding current value is gradually increased between the electrodes. In a resistance spot welding method in which the steel plates are melted and joined together by performing a main energization at a current value higher than the maximum current value in the energization period of the initial energization after performing several initial energizations,
Due to the initial energization, the iron constituting the base material of each steel plate is melted and then cooled to be shaped, and with the continuation of the initial energization , the The outer dimensions of the welding nugget whose outer dimensions are increasing , and the constituent material of the plating layer formed on the outside of the welding nugget and existing on the surface of each steel plate are diffused into the iron and are determined from both components. The relationship between the outer dimensions of the alloy layer , which is generated by increasing the solidus temperature and whose outer dimensions in the above direction increase with the continuation of the initial energization, is expressed by the following equation (1).
1.2 ≤ external dimension of alloy layer / external dimension of welding nugget ≤ 1.5 (1)
A resistance spot welding method characterized by providing an interval, which is an energization stop period, between the initial energization and the main energization by stopping the energization between the electrodes in this state.

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