JP5082249B2 - 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 the surface is directly sandwiched between the upper and lower electrodes and a large welding 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 resistance spot welds depends on the nugget diameter, it is important to ensure a nugget diameter of a predetermined diameter or greater, especially when high joint strength is required for automobile parts and the like. . 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.

  Further, in the resistance spot welding method described in Patent Document 3, it is necessary to change the pressurizing force and current in multiple stages, and in order to make this possible, it is considered that a complicated and expensive control device is required. In addition, since there are many items to be controlled, it is considered difficult to determine appropriate welding conditions, and management becomes difficult.

  The present invention has been made in view of the circumstances as described above, and in the case of resistance spot welding a plate assembly having a large plate thickness ratio in which a thin plate is overlapped on at least one of two or more stacked thick plates. To provide a resistance spot welding method capable of easily performing 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). It is intended.

  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 reduce the initial pressure during welding and increase the pressure during welding. I found out. And it discovered that it was effective as the means to fix the position of the electrode tip under welding.

  That is, when high pressure is applied 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 thickness ratio is large. In the case of a plate assembly, the space between the thin plate and the thick plate is close to the electrode, so that it is cooled and becomes less likely 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. When the current is increased until the nugget is formed between the thin plate and the thick plate, scattering occurs between the thick plate and the thick plate. Therefore, when the plate thickness ratio is large, no scattering occurs, and 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 scattering is narrowed.

  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 becomes small, the current density is high and heat is easily generated even at a low current, and between the thin plate and thick plate. Although it is heated to a high temperature, since the pressurizing range becomes narrow if the pressure is kept low, scattering occurs with a small nugget. Therefore, the appropriate current range in which a nugget of a necessary size can be obtained without occurrence of 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 pressing force is large, the pressurization area by the electrode is widened, and even if the nugget grows greatly, it is difficult for scattering to occur.

  As described above, even in a plate assembly with a large plate thickness ratio, a resistance spot 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 occurrence of scattering. Welding is possible, but further investigations have been made, and a method has been found that allows the above-mentioned idea to be easily implemented without using a complicated control system for increasing the welding pressure during welding. Specifically, a method is used in which a test piece is sandwiched between two electrodes, the position of the movable electrode is fixed when energization is started, and the change in the interelectrode distance is suppressed. This is because the thermal expansion occurs when energization is started and the steel sheet is heated, but the fact that this thermal expansion is suppressed by fixing the distance between the electrodes increases the true pressure applied to the plate assembly. To do.

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

[1] In carrying out resistance spot welding while applying a pressing force by sandwiching a plate assembly in which a thin steel plate is superposed on at least one of two or more thick high-strength steel plates stacked together , The tip of each electrode is curved, and the position of the pair of electrodes is fixed when the applied pressure reaches a set value P and the thermal expansion of the plate assembly after energization is suppressed by fixing the electrode position. The pressurizing force is increased with the initial set pressurizing force P and the maximum pressurizing force value Pm.
6.5kN ≧ Pm ≧ 1.5 × P
P ≦ 4kN
A resistance spot welding method characterized by welding using a resistance welding gun having high rigidity so that
[2] The resistance spot welding method according to [1], wherein the tip of the electrode on the thick plate side has a larger radius of curvature than the tip of the electrode on the thin plate side .

  According to the present invention, even when resistance plate welding is performed on a plate assembly having a large thickness ratio in which a thin plate is superimposed on at least one of two or more thick plates that are overlapped, an appropriate current range (necessary without causing scattering) It is possible to easily perform resistance spot welding with a wide current range) in which a nugget having a diameter can be formed between all the steel plates.

  Embodiments of the present invention will be described below.

  In the present invention, for example, as shown in FIGS. 1 and 2, a plate assembly having a large plate thickness ratio in which the thin plates 11 are superimposed on the upper surfaces of the two thick plates 12 and 13 that are superimposed is formed as a pair of electrodes ( In the case of resistance spot welding while applying pressure between the electrode tips 16 and 17, the positions of the electrode tips 16 and 17 are fixed when the pressure reaches the set value P. The pressing force is increased by suppressing the thermal expansion of the plate assembly after the start of energization by fixing the positions 16 and 17, so that the maximum value Pm of the pressing force becomes Pm ≧ 1.5 × P. .

  In this way, welding is started at a low pressure, and the pressure is increased during welding, so that even in a plate assembly having a large thickness ratio, an appropriate current range (thin plate 11-thick plate 12 without causing scattering). Wide range of current that can secure the necessary nugget diameters 14 and 15 between the thick plate 12 and the thick plate 13), and it is easy to carry out without requiring the use of a complicated control system. Can do.

  As a method for fixing the position of the electrode, a mechanical stopper may be provided at a position for fixing the movable electrode (for example, the electrode 16). However, 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 16 in the servo motor is also conceivable. It is not necessary to fix the electrode position so that the electrode 16 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 16 is fixed, the distance between the electrodes changes depending on the rigidity of the arm part of the gun, and therefore the increase in the applied pressure also changes depending on the rigidity of the arm part 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, the initial applied pressure is large, so the current-carrying area between the thin plate and thick plate is widened, and the influence of cooling by the electrode is increased by increasing the contact diameter with the electrode. Even if it is increased, it becomes difficult to sufficiently heat the thin plate to the thick plate. Further, when Pm <1.5 × P, the effect of suppressing the occurrence of scattering due to an increase in pressure is small.

  Furthermore, the tip of the electrode tip 16 on the thin plate 11 side is a curved surface, and the tip of the electrode tip 17 on the thick plate 13 side is a flat or thin plate 11 side having a diameter of the 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 16. By making the tip of the electrode tip 16 on the thin plate 11 side a curved surface and making the tip of the electrode tip 17 on the thick plate 13 side flatter, the deformation of the thin plate 11 can be suppressed to a small level and the energization area becomes narrow. Therefore, the current density between the thin plate 11 and the thick plate 12 increases, and the nugget 15 easily grows between the thin plate 11 and the thick plate 12. In addition, by making the tip of the electrode 16 on the thin plate 11 side a curved surface, increasing the pressing force during welding increases the range in which the electrode tip 16 on the thin plate 11 side can apply the pressing force, thereby suppressing the occurrence of scattering. Thus, a nugget 15 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 the electrodes during energization, but sandwiches the portion to be welded by a pair of upper and lower electrode tips. If pressure or current is applied, type of pressurization mechanism (by air cylinder, servomotor), shape (stationary, robot gun), power supply type (single-phase AC, AC inverter, DC inverter), etc. It is 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, in order to confirm the effect of the present invention, the present invention was carried out using a stationary servo motor pressurizing spot welding apparatus (Example of the present invention). For comparison, welding was performed using a stationary air pressurization type spot welding apparatus without controlling the distance between the electrodes (Comparative Example). The target plate assembly is the plate assembly A (thin plate 11 + thick plate 12 + thick plate 13) shown in FIG. And the difference of the appropriate electric current range in this invention example and a comparative example was investigated. Here, the proper current range is such that no scattering occurs during welding, the cross-section of the nugget is observed, and the thickness of the thinner one of the two adjacent steel plates is t, and the nugget diameter is 4√t. It was set as the welding current range which can obtain the nugget which satisfies the above. The plate thickness ratio is 7.6. The energization time was 20 cycles (50 Hz). The electrode was a DR type (tip diameter 10 mm, R40). In the example of the present invention, the set pressure P at the start of welding was 3.4 kN, and the maximum pressure Pm due to the increase in pressure during welding was 6.3 kN. In the comparative example, the maximum pressure Pm (6.3 kN) of the present invention example was set as the set pressure.

  FIG. 4 shows the change over time of the electrode displacement in the example of the present invention, FIG. 5 shows the change over time in welding force at a welding current of 7 kA in the example of the present invention, and FIG. 6 shows the welding current and nugget diameter in the example of the present invention. Each relationship is shown. On the other hand, FIG. 7 shows the temporal change of the electrode displacement in the comparative example, FIG. 8 shows the temporal change of the welding pressure at the welding current 8 kA in the comparative example, and FIG. 9 shows the welding current and the nugget diameter in the comparative example. Each relationship is shown.

  In the comparative example, a high current is required to form a nugget between the thin plate 11 and the thick plate 12, and a nugget having a size of 4√t or more cannot be obtained without being scattered. On the other hand, in the example of the present invention, it can be seen that a nugget is formed between the thin plate 11 and the thick plate 12 with a low current and has a wide appropriate current range of 1 kA or more.

  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. 3A to 3D and Table 1. That is, the plate assemblies A, B, and C are three layers of (thin plate 11 + thick plate 12 + thick plate 13), and the thick plates 12 and 13 are different from each other. The plate assembly D is a four-layer stack of (thin plate 11 + thick plate 12 + thick plate 13 + thin plate 14). Table 2 shows the shape of the upper and lower electrode tips, the energization time, the applied pressure P at the start of welding, and the maximum applied pressure Pm in each case.

  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. In addition, the present invention proposes a method for easily forming a nugget between a thin plate and a thick plate in resistance spot welding of a plate assembly having a large plate thickness ratio. It does not come off.

It is a figure which shows the example of the board assembly 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, 2 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 example of this invention 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 2 of this invention. It is explanatory drawing of the comparative example in Example 2 of this invention. It is explanatory drawing of the comparative example in Example 2 of this invention.

Explanation of symbols

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 Nugget 16 Nugget 17 Upper electrode tip 18 Lower electrode tip

Claims (2)

When carrying out resistance spot welding while sandwiching a plate assembly in which a thin plate of mild steel is superimposed on at least one of two or more stacked high-strength steel plates with a pair of electrodes, each of the pair of electrodes The position of the pair of electrodes is fixed when the pressure reaches a set value P and the thermal expansion of the plate assembly after the start of energization is suppressed by fixing the electrode position. The initial set pressure P and the maximum pressure Pm are
6.5kN ≧ Pm ≧ 1.5 × P
P ≦ 4kN
A resistance spot welding method characterized by welding using a resistance welding gun having high rigidity so that 2. The resistance spot welding method according to claim 1, wherein the tip of the electrode on the thick plate side has a larger radius of curvature than the tip of the electrode on the thin plate side .

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