JP5045238B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a method of resistance spot welding of stacked steel plates.

  In general, a resistance spot welding method, which is a kind of a lap resistance welding method, is used for joining stacked steel plates. For example, in the manufacture of automobiles, several thousand spots are welded per vehicle. This welding method is a method in which two or more steel plates are overlapped and sandwiched between upper and lower electrodes and a large current is applied between the upper and lower electrodes for a short time to join them. A spot-like weld is obtained by utilizing resistance heat generated by passing a large welding current. This point-like weld is called a nugget and is a part where both steel plates melt and solidify at the contact points of both steel plates when current is passed through both steel plates. The

  Since the joint strength of the resistance spot welded portion depends on the nugget diameter, it is important to secure a nugget diameter equal to or larger than a predetermined diameter. In general, when the applied pressure and energization time are constant, the nugget diameter gradually increases as the welding current increases. However, when the value exceeds a certain value, a phenomenon occurs in which the molten metal scatters between the steel plates. The occurrence of scatter is dangerous and scatters around the weld and deteriorates the appearance, causing variations in the nugget diameter and joint tensile strength, resulting in unstable joint quality.

  Looking at the parts structure of an automobile, for example, a center pillar employs a structure in which a reinforcement is sandwiched between an outer and an inner. In this structure, unlike the case of spot welding of a simple two-ply steel plate, it is required to superimpose three or more steel plates for spot welding.

  Furthermore, recently, with the demand for further improvement in collision safety of the car body, reinforcement and other strength has been increased, and a thin outer (thin plate) has been placed on the outside, and the thickness has been increased on the inside. Often, it is necessary to spot weld a plate assembly that combines thick inners and reinforcements (thick plates). Here, among thin steel plates, a thin steel plate is referred to as a thin plate, a relatively large steel plate is referred to as a thick plate, and so on. It is said that.

  In such a plate assembly with a large plate thickness ratio (= total thickness of the plate assembly / thickness of the thinnest plate), spot welding that keeps the applied pressure and welding current at a constant value as in the past. It is known that a nugget of a necessary size is difficult to be formed between the thin plate and the thick plate on the outermost side (the side in contact with the electrode tip). This tendency is particularly strong when the plate thickness ratio exceeds 5.

  This is because the temperature hardly rises between the outermost thin plate and the thick plate due to cooling by the electrode tip. The nugget is formed by volume resistance heat generation due to the specific resistance of the steel plate from the center between the electrodes, but before the nugget grows to a thin plate, between the thick plate and the thick plate located near the center between the electrodes The nugget grows at a large distance, and it is difficult to obtain a nugget of a necessary size between the thin plate and the thick plate without the occurrence of scattering, because the nugget grows greatly and cannot be suppressed by pressing with an electrode.

  Further, when the outermost thin plate is the outer, formability is more important than strength, so the steel plate used is often mild steel. On the other hand, the thick plate is a strength reinforcing member, and a high-tensile steel plate is often used. In such a plate assembly, the position where heat is generated is biased toward the high-tensile steel plate having a high specific resistance, so that nuggets are more difficult to be formed between the thick plate and the thin plate (mild steel). Moreover, when the steel plate used becomes a plated steel plate, the plating layer melted at a low temperature expands the current-carrying path between the steel plates, thereby reducing the current density and making it more difficult to form a nugget on the thin plate side.

  With respect to such a problem, for example, in Patent Document 1, in a plate assembly having a large plate thickness ratio in which a thin plate is further overlapped with two stacked thick plates, a part of the thin plate is to be welded. Forms a seat surface that is one step higher than the general part, and the electrode that opposes the thin plate is formed with a spherical tip at the beginning, with a low applied pressure at the initial stage of welding, crushing the seat surface of the thin plate, and adjacent to the thin plate There has been proposed a technique for obtaining a necessary nugget between a thin plate and a thick plate by welding the thick plate and then welding the two thick plates with high pressure.

  Further, in Patent Document 2, in a method in which a workpiece in which a thin plate with low rigidity is superimposed on two thick plates with high rigidity is sandwiched between a pair of electrode tips, the thin plate with the smallest rigidity is brought into contact. By making the tip diameter of the electrode tip smaller than the tip diameter of the electrode tip contacting the thick plate, the contact area between the thin plate and the electrode tip is made smaller than the contact area between the thick plate and the electrode. A technique for obtaining a nugget between a thin plate and a thick plate has been proposed.

Further, in Patent Document 3, in a method of spot welding a workpiece to be welded having a large thickness ratio, after applying a first pressing force to the workpiece and flowing a welding current, the energization is temporarily stopped and the workpiece to be welded is applied. With the body sandwiched between them, a second pressing force larger than the first pressing force is applied and a welding current is flowed again. Preferably, the current value of the welding current in the first step is set to a first value. While changing from the first stage to the third stage, the current value of the second stage is made smaller than the current values of the first stage and the third stage, the joint strength of the welded body having a large plate thickness ratio is increased. The spot welding method of improving is proposed.
JP 2003-071569 A JP 2003-251468 A JP 2004-358500 A

  However, in the resistance spot welding method described in Patent Document 1, in this case, a nugget is formed, but a problem that a step of forming a seat surface that is one step higher than the general part in advance by a press or the like is required in the part to be welded of the thin plate. There is.

  Moreover, in the resistance spot welding method described in Patent Document 2, the thin plate and the electrode tip are formed by making the tip diameter of the electrode tip that contacts the thin plate having the smallest rigidity smaller than the tip diameter of the electrode tip that contacts the thick plate. Nugget is obtained between the thin plate and the thick plate by making the contact area with the plate smaller than the contact area between the thick plate and the electrode, but the contact area between the thin plate and the electrode tip is small. As a result, there is a problem that the range in which pressure is applied becomes narrow, and when a large nugget is formed between the thick plate and the thick plate, scattering occurs.

  In addition, in the resistance spot welding method described in Patent Document 3, it is difficult to obtain a high-strength steel plate with high accuracy by pressing, and in an actual workpiece, there is a gap (gap) between the thick plate and the thick plate. Since there are many cases, in that case, in the resistance spot welding method described in Patent Document 3, it is necessary to energize in a state where the applied pressure is low in the initial stage. Conceivable.

  The present invention has been made in view of the above-described circumstances, and when resistance spot welding is performed on a plate assembly having a large plate thickness ratio in which a thin plate is superimposed on at least one of two or more stacked thick plates. In addition, regardless of the presence or absence of a cap between any of the steel plates in the plate assembly, an appropriate current range (a current that can form a nugget having a required diameter without causing scattering is formed between all the steel plates. It is an object of the present invention to provide a resistance spot welding method capable of performing resistance spot welding with a wide range.

  In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting nugget formation in resistance spot welding. A nugget of the required size between the thin plate and the adjacent thick plate and between the thick plate and the thick plate in spot welding in the case of a large plate thickness ratio where the thin plate is overlapped with one of the two or more stacked thick plates When forming a nugget, it is effective to form a nugget between a thin plate and a thick plate. For this purpose, it is effective to increase the applied pressure during welding by reducing the applied pressure at the initial stage of welding. I found out that there was. And it discovered that it was effective as the means to fix the position of the electrode tip under welding.

  That is, when welding with high pressure from the beginning of welding, the current-carrying area between the thin plate and the thick plate and between the thick plate and the thick plate is widened and the current density is low, so it is difficult to generate heat and the plate has a large plate thickness ratio. In the case of the assembly, the thin plate-thick plate is close to the electrode and is cooled, so that it is difficult to generate heat. For this reason, even if a nugget of a necessary size is formed between the thick plate and the thick plate, no nugget is formed between the thin plate and the thick plate, and the current is increased until the nugget is formed between the thin plate and the thick plate. Then, scattering occurs between the thick plate and the thick plate. Therefore, when the plate thickness ratio is large, the appropriate current range in which the nuggets necessary between the thin plate and the thick plate and between the thick plate and the thick plate can be formed without being scattered becomes narrow.

  On the other hand, when welding with a low pressure at the initial stage of welding, the current-carrying area between the thin plate and thick plate and between the thick plate and thick plate is reduced, and the current density is high and heat is easily generated even at a low current. Are also heated to high temperatures. However, since the pressurizing range becomes narrow when the pressure is kept low, scattering occurs when the nugget is small, and an appropriate current range in which a nugget of a necessary size can be obtained without scattering is narrowed.

  Therefore, by starting welding with a low applied pressure and increasing the applied pressure during welding, the above problems can be solved, and welding with a wide appropriate current range can be achieved even with a plate assembly having a large thickness ratio. By energizing with low pressure at the initial stage of welding, both the thin plate and the thick plate and between the thick plate and the thick plate are heated to a high temperature. After that, the energized area is increased by increasing the pressing force, and the current density between the thin plate and the thick plate decreases.However, because the specific resistance of the steel plate is greatly increased by being heated to a high temperature, it generates heat. The nugget is easy to grow between the thin plate and the thick plate. In addition, since the pressurizing force is large, the pressurization area by the electrode is widened, and even if the nugget grows greatly, it becomes difficult for scattering to occur.

  As described above, even in a plate assembly with a large plate thickness ratio, resistance spot welding with a wide appropriate current range that can obtain a nugget of a necessary size between the thin plate and the thick plate and between the thick plate and the thick plate without being scattered. However, further investigations have been made, and even when there is a gap between the steel plates, a method has been found that enables resistance spot welding with a wide appropriate current range by increasing the applied pressure during welding.

  Specifically, for example, as shown in FIG. 1, a plate assembly having a large plate thickness ratio in which the thin plates 11 are superposed on the upper surfaces of the two superposed thick plates 12 and 13, and the thick plate 12 -thickness In the case of a plate assembly in which a gap 17 exists between the plates 13, the work 10 is sandwiched between the two electrodes 21 and 22, and energization is started with a preset low initial pressure P and a low current. Since current flows through the point contact portion 16 between the thin plate 11 and the thick plate 12, the current density is increased and heat generation proceeds in this portion. On the other hand, due to the presence of the gap 17 between the thick plate 12 and the thick plate 13, as shown by the arrow in FIG. To flow. Due to this bypassed current, the temperature of the steel plate rises near the contact portion between the thick plate 12 and the thick plate 13 and is easily deformed, and the gap 17 that could not be closed only by the low initial pressure P is closed, and the gap between the electrodes The distance decreases. Then, as shown in FIG. 2, when the thick plate 12 and the thick plate 13 are in contact with each other and the distance between the electrodes is reduced, the positions of the electrodes 21 and 22 are fixed, and the main energization is started. This is because thermal expansion occurs when energization is started and the steel sheet is heated. However, by fixing the positions of the electrodes 21 and 22 and maintaining the distance between the electrodes, this thermal expansion is suppressed and the plate assembly is applied. It takes advantage of the increase in true pressure.

  The present invention has been conceived based on the above concept and has the following characteristics.

[1] When resistance spot welding is performed while a plate assembly in which a thin plate is superimposed on at least one of two or more stacked thick plates is sandwiched between a pair of electrodes and a pressing force is applied, the pressing force reaches a set value P After that, the pre-energization at a low current is started, and after the stacked plates are in contact with each other, the position of the electrode is fixed, and then the main energization is started, and the main energization is started by fixing the electrode position. The pressing force is increased by suppressing the thermal expansion of the subsequent plate assembly, and the set pressing force P and the pressing force maximum value Pm are
Pm ≧ 1.5 × P
P ≦ 4kN
A resistance spot welding method characterized by:

  [2] The pressurizing force is applied by a servo motor, and it is determined by reading an electrode position with an encoder built in the servo motor that the stacked plates are in contact with each other. The resistance spot welding method according to [1].

  [3] The resistance spot welding method according to [1], wherein the determination that all the stacked plates are in contact with each other is made based on a change in voltage in preliminary energization at a low current. .

  [4] The resistance spot according to any one of [1] to [3], wherein a current higher than a preset main energization setting current is supplied in the initial several cycles after the main energization is started. Welding method.

  According to the present invention, a thin plate is superposed on at least one of two or more superposed thick plates, and a plate assembly having a large plate thickness ratio in which a gap exists between any of the steel plates is a resistance spot. Even in the case of welding, it is possible to easily perform resistance spot welding with a wide appropriate current range (a current range in which a nugget having a necessary diameter can be formed between all steel plates without causing scattering).

  Embodiments of the present invention will be described below.

  In the present invention, for example, as shown in FIG. 1, a plate assembly having a large plate thickness ratio in which the thin plates 11 are superposed on the upper surfaces of the two superposed thick plates 12 and 13, and the thick plate 12 -thickness is used. When resistance plate welding is performed while applying a pressing force by sandwiching a plate assembly having a gap 17 between the plates 13 between a pair of electrodes (electrode tips) 21 and 22, first, as a first step, a low initial value set in advance The energization is started with a pressure P and a low current, heat is generated between the thin plate 11 and the thick plate 12, and the thick plates 12 and 13 are made to flow by the current between the thick plate 12 and the thick plate 13 flowing around the gap 17. By making it easy to deform, the gap 17 between the thick plate 12 and the thick plate 13 is closed. The distance between the electrodes at this time is monitored, and as shown in FIG. 2, when the distance between the electrodes decreases until the thick plate 12 and the thick plate 13 come into contact with each other and the contact portion shape is stabilized, the decrease in the distance between the electrodes stops. Therefore, the electrodes 21 and 22 are fixed at the positions, and the main energization is started as the second step. The applied pressure at the start of the main energization remains the initial applied pressure P and is a low applied pressure, but the pressure is increased by suppressing the thermal expansion of the plate assembly after the start of the main energization by fixing the positions of the electrodes 21 and 22. Is gradually increased so that the maximum value Pm of the applied pressure satisfies Pm ≧ 1.5 × P.

  Thus, in addition to generating heat between the thin plate 11 and the thick plate 12 by energization in the first step, since the applied pressure is low at the initial stage of the main energization, it is between the electrode 21 and the thin plate 11 and between the thin plate 11 and the thick plate. The contact diameter between the thin plates 11 is kept small, the temperature between the thin plate 11 and the thick plate 12 is high, and the nugget 18 that grows from the vicinity of the central portion between the electrodes in this energization also grows between the thin plate 11 and the thick plate 12. It has become easier.

  In this way, welding is started at a low pressure, and by increasing the pressure during welding, even in a plate assembly having a large thickness ratio, an appropriate current range (necessary nugget diameter D1, The nugget 18 having D2 can be welded with a wide range of currents that can be formed between the thin plate 11 and the thick plate 12, and between the thick plate 12 and the thick plate 13, respectively.

  As a method for fixing the position of the electrode, a mechanical stopper may be provided at a position where the movable electrode (for example, the electrode 21) is fixed, but the electrode pressing mechanism is a servo motor pressing type. If there is, a method of feeding back the position information of the tip of the electrode 21 in the servo motor is also conceivable. It is not necessary to fix the electrode position so that the electrode 21 does not move at all, and it is sufficient that the increase in the distance between the electrodes can be suppressed. Even if the position of the movable electrode 21 is fixed, the distance between the electrodes changes depending on the rigidity of the arm portion of the gun and the like, and the amount of increase in the pressing force also changes depending on the rigidity of the arm portion of the gun. Although it is desirable that the gun has a high rigidity, the maximum value Pm of the pressurizing force due to the increase in the pressurizing force during welding is set to Pm ≧ 1.5 × P with respect to the initial set pressurizing force P. What is necessary is just to satisfy and P <= 4kN. When P> 4 kN, since the initial pressure is large, the current-carrying area between the thin plate 11 and the thick plate 12 is widened, and the influence of cooling by the electrode is increased by increasing the contact diameter with the electrode. Even if the current is increased, it is difficult to sufficiently heat the thin plate 11 and the thick plate 12. Further, when Pm <1.5 × P, the effect of suppressing the occurrence of scattering due to an increase in pressure is small.

  In addition, as a method of monitoring the distance between the electrodes in the process of closing the gap between the thick plate 12 and the thick plate 13 to determine whether or not the gap 15 between the thick plate 12 and the thick plate 13 is crushed and lost, It is easiest to do this by reading the electrode position with an encoder built in the servo motor. That is, when the electrode position is read by the encoder and the change in the electrode position is eliminated, it is determined that all the overlapped plates are in contact with each other. This can be determined in the same manner whether or not there is a gap between the stacked plates.

  In addition, there may be a method in which a laser displacement meter or the like is attached outside and the distance between the electrodes is monitored.

  Furthermore, if it is a resistance spot welder with constant current control, it is possible to determine whether or not the gap 17 is closed by monitoring the welding voltage in the process of closing the gap between the thick plate 12 and the thick plate 13. It is. This is because when the energization is performed while the gap 17 is not closed, the welding current flows around the gap 17 and the resistance increases due to the temperature rise in the bypass. This increases the voltage in the resistance spot welder under constant current control. When the gap 17 is closed, a current also flows directly under the electrode, so that the resistance decreases as the energization area increases. Since the voltage decreases at this time, it is possible to determine whether or not the gap 17 is closed by monitoring the welding voltage. That is, if the voltage in the preliminary energization at a low current stops changing, it is determined that all the stacked plates are in contact with each other. This can be determined in the same manner whether or not there is a gap between the stacked plates.

  In the main energization, it is more effective to flow a current higher than a preset main energization set current in the initial several cycles. This is because the applied pressure is low at the start of main energization, so the contact area between the electrode and the steel plate and the steel plate and the steel plate is small, and heat is likely to be generated near the electrode and between the steel plate and the steel plate. At this time, by flowing a high current, the temperature between the thin plate 11 and the thick plate 12 can be raised to a high temperature, and thereby, the nugget can easily grow between the thin plate 11 and the thick plate 12. is there.

  Further, the tip of the electrode tip 21 on the thin plate 11 side is a curved surface, and the tip of the electrode tip 22 on the thick plate 13 side is a flat or thin plate 11 side having a necessary nugget diameter between the thick plate 12 and the thick plate 13. It is more preferable that the curved surface has a larger radius of curvature than the electrode tip 21. By making the tip of the electrode tip 21 on the thin plate 11 side a curved surface and making the tip of the electrode tip 22 on the thick plate 13 side flatter, deformation of the thin plate 11 can be suppressed to be small and the energization area becomes narrow. Therefore, the current density between the thin plate 11 and the thick plate 12 increases, and the nugget 18 easily grows between the thin plate 11 and the thick plate 12. In addition, by making the tip of the electrode 21 on the thin plate 11 side a curved surface, by increasing the pressurizing force during welding, the range in which the electrode tip 21 on the thin plate 11 side can be applied increases, and the occurrence of scattering is suppressed. Thus, a nugget 19 having a necessary diameter between the thick plate 12 and the thick plate 13 can be formed.

  Incidentally, the welding apparatus used in the present invention needs to have a pressurizing mechanism that suppresses fluctuations in the distance between electrodes during energization, and it is necessary to be able to observe the distance between electrodes. Thus, a servo motor pressurizing type welding machine can be easily performed. In addition, if the part to be welded is sandwiched between a pair of upper and lower electrode tips, and pressurizes and energizes, the shape of the pressurization mechanism (stationary, robot gun), type of power source (single-phase AC, AC inverter, DC) Inverter) and the like are not particularly limited.

  In addition, the steel plates to be welded can be applied regardless of the strength level (soft steel, high-strength steel plate) or with or without surface treatment (no surface treatment, plated steel plate). The present invention can be applied even when the plate thickness ratio exceeds 5.

  As Example 1 of the present invention, an example of the present invention and a comparative example were implemented.

  In the example of the present invention, a servo motor pressure spot welding apparatus was used. The target plate assembly is plate assembly A (thin plate 11 + thick plate 12 + thick plate 13) shown in FIG. 3 and Table 1, and the plate thickness ratio is 7.6. Then, steel plate spacers 15 and 15 having a thickness of 1.2 mm are inserted between the thick plate 12 and the thick plate 13 with an interval W of 45 mm, and the portion of the steel plate spacer 15 is welded in advance to simulate the gap 17. Formed. The welding current in the process of closing the gap 17 was 6 kA, the energization time for main energization was 20 cycles (50 Hz), and the main energization setting current was 7 kA. The electrode was a DR type (tip diameter 8 mm, R40). The applied pressure is set to 3.4 kN at the start of welding, the electrode position is fixed after the gap 17 is crushed, and the actual applied pressure is increased so that the maximum applied pressure Pm is set to 6.3 kN. .

  On the other hand, in the comparative example, the conventional air pressurization type resistance spot welder was used. The target plate assembly is the same as that of the present invention example, but the gap 17 is not provided. Then, welding was performed with a constant pressure of 6.3 kN.

  Here, the proper current range is such that no scattering occurs during welding in the main energization, and the cross section of the nugget is observed, and the thickness of the thinner one of the two adjacent steel plates is defined as t. The welding current range was such that a nugget satisfying that the diameter was 4√t or more was obtained.

  FIG. 4 shows the relationship between the welding current and nugget diameter of the main welding in the present invention example and the appropriate current range. FIG. 5 shows the relationship between the welding current and the nugget diameter of the main welding in the comparative example.

  First, as shown in FIG. 5, in the comparative example, a high current is required to form a nugget between the thin plate 11 and the thick plate 12 in spite of the plate assembly having no gap, and the size is 4√t or more. You can't get your nuggets without splashing.

  On the other hand, as shown in FIG. 4, in the present invention example, it is understood that nuggets are formed with a low current between the thin plate 11 and the thick plate 12 and have a wide appropriate current range of 1 kA or more.

  FIG. 6 shows a measurement result when the welding current of the main welding is 9 kA as an example of measuring the temporal change of the applied pressure during welding and the temporal change of the interelectrode distance in the inventive example.

  First, when energization is started at a low current, current flows through the spacer 15 and the steel plate temperature rises, so that the steel plate is easily deformed, and the distance between the electrodes gradually decreases. When the gap 17 between the thick plate 12 and the thick plate 13 is crushed and the thick plate 12 and the thick plate 13 contact each other, the current that has been flowing through the spacer 15 so far flows between the electrodes at the shortest distance. And more heat is generated between the electrodes. This facilitates deformation at the portion to be welded between the electrodes, and the distance between the electrodes is greatly reduced. In this state, energization was performed for several cycles, and when the familiarity between the thick plate 12 and the thick plate 13 was improved, the position of the movable electrode was fixed and the main energization was started. When the main energization is started, since the electrode position is fixed, the actual pressurizing force gradually increases. Since the applied pressure during the main welding is low in the initial stage of welding and high in the latter stage of welding, it becomes easy to obtain nuggets of a necessary size between the thin plate 11 and the thick plate 12 and between the thick plate 12 and the thick plate 13. .

  Furthermore, as Example 2 of the present invention, the present invention example and a comparative example were implemented by enlarging the target plate assembly.

  The plate assembly is the plate assembly shown in FIGS. 7 (a) and 7 (b). That is, the plate assembly shown in FIG. 7A is a three-layer stack of (thin plate 11 + thick plate 12 + thick plate 13), and the depth G1 is appropriately set between the thin plate 11 and the thick plate 12 using the steel plate spacer 15. A gap having a depth G2 is provided between the thick plate 12 and the thick plate 13. Further, the plate assembly shown in FIG. 7B is a four-layer stack of (thin plate 11 + thick plate 12 + thick plate 13 + thin plate 14), and a depth between the thin plate 11 and the thick plate 12 is appropriately determined using a steel plate spacer 15. A gap of depth G1, a gap of depth G2 between the thick plate 12 and the thick plate 13, and a gap of depth G3 between the thick plate 13 and the thin plate 14 are provided.

  Table 1 shows the plate assembly No. used. The plate thickness configuration, plate thickness ratio, and gap size (gap width W, gap depths G1, G2, and G3) in each of the plate assemblies A to L are shown.

  Table 2 shows the plate assembly No., the shape of the upper and lower electrode tips, the current of pre-energization for crushing the gap, the presence or absence of a 20% increase in welding current between 5 cycles / 50 Hz from the start of main energization, It shows whether or not the welding energizing time, the set pressure P at the start of main welding, the maximum pressure Pm due to the increase in pressure during welding, and Pm ≧ 1.5P are satisfied.

  Table 2 shows the results of investigation and evaluation of the appropriate current range ΔI in each case. By the way, the evaluation is that no scattering occurs at the time of welding, the cross section of the nugget is observed, and the thickness of the thinner steel plate of two adjacent steel plates is t, and the nugget diameter is 4√t or more. A welding current range in which a nugget satisfying certain conditions is obtained is defined as an appropriate current range ΔI, a case where the proper current range ΔI is 1 kA or more is good (◯), and a case where the proper current range ΔI is less than 1 kA is defective (× ).

  As a result, in the example of the present invention, the appropriate current range ΔI is 1 kA or more and good welding is achieved, but in the comparative example, the appropriate current range ΔI is less than 1 kA, which is not suitable for practical use. Thereby, the effectiveness of the present invention can be confirmed.

  Here, the proper current range ΔI is defined as a range in which nuggets having a size of 4√t or more can be obtained without being scattered, but the present invention is not limited to this, and the reference nugget diameter matches the user's standard. It may be 3√t or 5√t. What is needed is the strength of the joint, and the nugget diameter is specified as one of the criteria for judging whether the strength required by the user can be secured. In addition, the present invention proposes a technique for forming a joint that can provide sufficient strength even between a thin plate and a thick plate even when a gap exists between the steel plates in resistance spot welding of a plate assembly having a large plate thickness ratio. Therefore, the present invention does not deviate from the present invention even if welding is performed while causing scattering.

It is a figure explaining the welding state in embodiment of this invention. It is a figure explaining the welding state in embodiment of this invention. It is a figure which shows the board assembly in Example 1 of this invention. It is explanatory drawing of the example of this invention in Example 1 of this invention. It is explanatory drawing of the comparative example in Example 1 of this invention. It is explanatory drawing of the example of this invention in Example 1 of this invention. It is a figure which shows the board assembly in Example 2 of this invention.

Explanation of symbols

10 Board assembly (work)
11 First steel plate (thin plate)
12 Second steel plate (thick plate)
13 Third steel plate (thick plate)
14 Fourth steel plate (thin plate)
15 Steel plate spacer 16 Point contact part between thin plate and thick plate 17 Gap between thick plate and thick plate 18 Nugget 21 Electrode (electrode tip)
22 Electrode (electrode tip)

Claims (4)

In performing resistance spot welding while sandwiching a plate assembly in which a thin plate is superimposed on at least one of two or more thick plates that are overlapped and applying a pressing force, the pressing force reaches a set value P, Pre-energization at a low current is started, and the position of the electrode is fixed when all the overlapped plates are in contact with each other. Then, the main energization is started, and the plate after the start of main energization is performed by fixing the electrode position. The pressurizing force is increased by suppressing the thermal expansion of the assembly, and the set pressurizing force P and the maximum value Pm of the pressurizing force are
Pm ≧ 1.5 × P
P ≦ 4kN
A resistance spot welding method characterized by:   2. The pressurizing force is applied by a servo motor, and it is determined by reading an electrode position with an encoder built in the servo motor that the overlapping plates are in contact with each other. The resistance spot welding method as described.   2. The resistance spot welding method according to claim 1, wherein the determination that all the superposed plates are in contact with each other is made based on a change in voltage during preliminary energization at a low current.   The resistance spot welding method according to any one of claims 1 to 3, wherein a current higher than a preset main energization setting current is passed in the initial several cycles after the main energization is started.

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