JP2008290099A - Resistance spot welding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a resistance spot welding method capable of increasing the strength of weld joints between a thin plate and a thick plate and between thick plates irrespective of presence/absence of any gap between any steel plates of a plate assembly when performing the resistance spot welding of the plate assembly having the large thickness ratio in which a thin plate is superposed on at least any one of two or more superposed thick plates. <P>SOLUTION: When performing the resistance spot welding while holding a workpiece of a plate assembly in which a thin plate 11 is superposed on superposed two thick plates 12, 13 by a pair of electrodes 21, 22 and applying the pressure thereto, the workpiece 10 is fixed, the electrode 21 on the side in contact with the thin plate 11 forms a fixed electrode of a welding gun, an electrode 22 on the side in contact with the thick plate 13 forms a movable electrode, and the welding is performed in two-staged processes. The welding is performed with low pressure and high current in the first process, and the welding is performed with the pressure higher than that in the first process. <P>COPYRIGHT: (C)2009,JPO&INPIT

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 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 greatly, and it is difficult to obtain a nugget of a necessary size between the thin plate and the thick plate without causing the occurrence of scattering, because the nugget grows and cannot be suppressed by the pressure applied by the 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. A technique has been proposed in which a nugget is formed between a thin plate and a thick plate to increase the welding strength of the workpiece by making the pressure applied to the workpiece of the electrode tip smaller than the pressure applied to the electrode tip workpiece contacting the thick plate. .

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 thickness ratio is increased. The spot welding method of improving is proposed.
JP 2003-071569 A JP 2003-251469 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.

  Further, in the resistance spot welding method described in Patent Document 2, by making the pressure applied to the workpiece of the electrode tip that contacts the thin plate having the smallest rigidity smaller than the pressure applied to the workpiece of the electrode tip that contacts the thick plate, Nugget is also obtained between the thin plate and the thick plate, but the contact area between the thin plate and the electrode tip is reduced because the applied pressure of the electrode tip contacting the thin plate is small, so that the electrode is pressed by the electrode. The range is narrow, and it is considered that scattering tends to occur when a large nugget is formed between the thick plate and the thick plate. Furthermore, since a difference is caused in the applied pressure by forcibly moving the welding gun body to which the electrode is attached after the work is sandwiched between the electrodes, there is a possibility that a large distortion occurs in the work.

  Further, in the resistance spot welding method described in Patent Document 3, it is difficult to obtain accuracy in the thick plate of the high-tensile steel plate by press working, and there are many gaps between the thick plate and the thick plate in an actual work. In this 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, but it is considered that it is often difficult to implement due to the existence of the gap.

  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 superimposed on one of two or more stacked thick plates, To provide a resistance spot welding method for increasing the strength of a welded joint between a thin plate and a thick plate and between a thick plate and a thick plate regardless of the presence or absence of a gap between any steel plate of the plate assembly. It is the purpose.

  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.

  That is, when pressurized with high pressure from the beginning of welding, the current-carrying area between the electrode and the thin plate, between the thin plate and the thick plate, and between the thick plate and the thick plate becomes wide and the current density becomes low, so it is difficult to generate heat. Furthermore, in the case of a plate assembly with a large plate thickness ratio, the thin plate-thick plate is close to the electrode and thus cooled, and it becomes 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, it is difficult to form the nugget between the thin plate and the thick plate. For this reason, at present, welding is performed with an excessive welding current, and welding is performed while generating severe scattering, so that the necessary joint strength can be obtained even between a thin plate and a thick plate.

  On the other hand, when welding is performed at a low pressure in the initial stage of welding, the current-carrying areas between the electrode and the thin plate, between the thin plate and the thick plate, and between the thick plate and the thick plate are small, and the current density is high and the heat is easily generated. In particular, immediately after the start of welding, heat generation near the electrode slightly away from the electrode increases due to the influence of current fringing. In a plate assembly with a large plate thickness ratio, the heat generation region near this electrode and the thin plate-thick plate boundary are close to each other, and by setting the applied pressure low, this region is thermally expanded and deformed. Due to this thermal expansion / deformation, the contact diameter between the thin plate and the thick plate is further reduced, the current density between the thin plate and the thick plate is further increased, and a molten part is formed. However, if the pressure is kept low, the melted portion formed in the vicinity of the electrode grows up to the surface of the steel plate even when the current is applied for a long time, and surface scattering occurs. Since the applied pressure is small between the thick plate and the thick plate, scattering occurs before the necessary nugget diameter is obtained.

  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 applying a large current for a short time with a low pressure at the initial stage of welding, a molten portion is formed in the vicinity of the electrode, and a nugget is formed between the thin plate and the thick plate. After that, by increasing the applied pressure, the nugget formation stops near the electrodes due to the expansion of the current-carrying area between the electrode and the thin plate and the increase of the cooling action by the electrodes, and this time the heat generation area moves near the center between the electrodes. Then, a nugget is formed between the thick 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, a nugget of a necessary size can be formed between a thin plate and a thick plate and between a thick plate and a thick plate, and resistance spot welding with high joint strength can be performed. When there is a gap between them, it is difficult to form a nugget between the thin plate and the thick plate even if the above method is used. As shown in FIG. 1 (a), two electrodes 21 and 22 are used for a plate assembly 10 having a large plate thickness ratio in which a thin plate 11 is superimposed on the upper surface of two stacked thick plates 12 and 13. When performing resistance spot welding by sandwiching, when there is no gap between the steel plates, the contact areas of the contact portions 16a to 16d between the steel plates can be limited to a small size when pressed, respectively. As shown in (b), for example, when there is a gap 17 between the thick plate 12 and the thick plate 13, the gap 17 is crushed by pressurization by the electrodes 21 and 22, and the steel plate is deformed, so that the electrode 21-thin plate Since the contact area of the contact portion 16a between 11 and the contact portion 16b between the thin plate 11 and the thick plate 12 is greatly increased, it is difficult to sufficiently increase the current density and it is difficult to form a nugget. . If the rigidity of the gap is high or the electrode pressure is low, the deformation of this steel sheet is small, but the steel sheet softens when energized and becomes easy to deform, resulting in deformation in the direction in which the gap is crushed. Since the contact area is greatly increased between the electrode 21 and the thin plate 11 and between the thin plate 11 and the thick plate 12, it is difficult to sufficiently increase the current density.

  Therefore, the present inventors have further studied and found a method capable of increasing resistance force during welding and enabling resistance spot welding with a wide appropriate current range even when a gap exists between steel plates. .

  Specifically, the position of a workpiece with a large plate thickness ratio obtained by superimposing a thin plate on one of two or more superposed plates is fixed with a clamper or the like, and the electrode on the side in contact with the thin plate is fixed to the welding gun. The test piece is sandwiched with the electrode on the side in contact with the thick plate as the movable electrode, and the above welding is started. Since the electrode on the thin plate side is a fixed electrode, even if there is a gap, deformation of the thin plate and the thick plate in contact with the thin plate is suppressed to a small extent, and between the electrode and the thin plate due to deformation when the gap is crushed, between the thin plate and the thick plate There is no significant increase in the contact diameter. By welding at a low pressure and high current at the initial stage of welding, and then welding at a high pressure, melting between the thin plate and thick plate, and between the thick plate and thick plate, respectively. A part is formed and welding with high joint strength is possible.

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

  [1] When performing resistance spot welding while applying a pressing force by sandwiching a pair of electrodes by placing a thin plate on one of two or more thick plates superimposed, The electrode on the side in contact with the thin plate is used as a fixed electrode of the welding gun, the electrode on the side in contact with the thick plate is used as a movable electrode, welding is performed in two steps, and welding is performed with low pressure and high current in the first step. A resistance spot welding method comprising welding in a second step with a pressure greater than the pressure in the first step.

  [2] The tip of the electrode in contact with the thin plate is a curved surface, and the tip of the electrode in contact with the thick plate is a curved surface having a larger radius of curvature than the tip of the electrode in contact with the thin plate. The resistance spot welding method according to [1] above.

  According to the present invention, in the case of resistance spot welding a plate assembly having a large plate thickness ratio in which a thin plate is superposed on one of two or more superposed plates, even if a gap exists between the steel plates, Nugget) can be formed between all the steel plates, and the strength of the welded joint between the steel plates can be increased.

  One embodiment of the present invention is described below.

  FIG. 2 is a configuration diagram of a workpiece and a spot welding gun according to an embodiment of the present invention.

  As shown in FIG. 2, the workpiece 10 is a three-layer stack of a thin plate 11, a thick plate 12, and a thick plate 13, and is a plate assembly having a large plate thickness ratio, with a gap between the three steel plates at the welding position. Is assumed. The positions of these workpieces 10 are fixed by clamps 25.

  The spot welding gun 30 is attached to the wrist 31a of the robot 31, and includes a transformer 32 for supplying a welding current, a pressurizing mechanism 33 using a servo motor as a drive source, and a C-type extending upward attached to the gun body. The arm 34, the fixed electrode 21 fixed to the C-shaped arm 34, the movable electrode 22 moved and pressurized by the pressurizing mechanism 33, the cooling mechanism (not shown) of the transformer 32 and the electrodes 21 and 22, and the like.

  Using this spot welding gun 30, spot welding of the workpiece 10 is performed as follows.

  First, the fixed electrode 21 is brought into contact with the thin plate 11 by position control of the robot 31, and then the movable electrode 22 is moved to sandwich the workpiece 10 between the electrodes 21 and 22. The workpiece 10 is fixed by a clamp 25, and even if there is a gap 17a between the thin plate 11 and the thick plate 12, the thin plate 11 is often made of mild steel. In addition, the gap 17a is closed, and the contact diameter between the thin plate 11 and the thick plate 12 at that time becomes small, and there is no problem in forming a nugget between the thin plate 11 and the thick plate 12 during welding. Further, since the workpiece 10 is fixed by the clamp 25, the positional relationship between the fixed electrode 21 and the thin plate 11 does not change when the workpiece 10 is sandwiched by the movable electrode 22, and the deformation due to the electrode pressing force is the same as that of the movable electrode 22. The thick plate 13 in contact only moves so as to close the gap 17b between the thick plate 12 and the thick plate 13. Accordingly, the thick plate 12 is hardly deformed and remains flat, and the contact diameter between the fixed electrode 21 and the thin plate 11 and between the thin plate 11 and the thick plate 12 is limited to be small as in the case where there are no gaps 17a and 17b. Is possible.

  First, by applying a high current in a short time with a low applied pressure, a nugget 18a is formed between the thin plate 11 and the thick plate 12, as shown in FIG. At this time, if the energization path is secured somewhere even if the gap 17b between the thick plate 12 and the thick plate 13 is not crushed, the nugget 18a is interposed between the thin plate 11 and the thick plate 12, as shown in FIG. Formation is possible. Thereafter, the applied pressure is increased and the gap 17b between the thick plate 12 and the thick plate 13 is reliably crushed. By sufficiently increasing the applied pressure, the contact diameter between the fixed electrode 21 and the thin plate 11 is also increased. As a result, heat is generated near the center between the electrodes, and as shown in FIG. 13 is formed between the thin plate 11 and the thick plate 12 and between the thick plate 12 and the thick plate 13, and the joint strength can be obtained even in a workpiece having a large thickness ratio. It is possible to obtain a resistance spot welded joint excellent in.

  In the contact operation of the fixed electrode 21 and the thin plate 11 by the position control of the robot 31, depending on the press accuracy of the work 10, it is assumed that the teaching position and the actual position of the work 10 are greatly different. In order to perform the contact operation at this time without any problem, a strain sensor or a stress sensor is attached to the arm 34, the pressure applied to the thin plate 11 by the fixed electrode 21 by the contact operation is monitored, and the value becomes a constant value. It is more preferable to add feedback control so that it is determined that the contact operation is completed.

  Further, a position control mechanism by a servo motor 33 that can freely move the gun body in the pressurizing direction of the gun 30 is added to the attachment portion between the servo gun 30 and the robot 31, and a contact operation is performed by the mechanism. It is also conceivable to add a control to monitor the torque applied to the servomotor 33 so as to determine that the contact operation is completed when the value reaches a certain value.

  Further, the tip of the fixed electrode 21 on the thin plate 11 side is a curved surface, and the tip of the movable electrode 22 on the thick plate 13 side is a flat or thin plate 11 side having a diameter of about the required nugget diameter between the thick plate 12 and the thick plate 13. More preferably, the curved surface has a larger radius of curvature than the fixed electrode 21. The tip of the fixed electrode 21 on the thin plate 11 side is a curved surface, and the tip of the movable electrode 22 on the thick plate 13 side is flattened, so that the welding between the fixed electrode 21 and the thin plate 11 when performing welding with low pressure is performed. Since the current-carrying area between the 11 and the thick plate 12 becomes narrow, the current density increases, and the nugget 18a is easily formed between the thin plate 11 and the thick plate 12.

  In addition, by making the tip of the fixed electrode 21 on the thin plate 11 side a curved surface, by increasing the pressing force during welding, the range in which the fixing electrode 21 on the thin plate 11 side can be applied increases, and scattering occurs. The nugget 18b having a necessary diameter between the thick plate 12 and the thick plate 13 can be formed.

  Incidentally, the type of the 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.

  Examples of the present invention are described below.

  Here, the present invention example and the comparative example were carried out using a servo motor pressurization type C gun type single-phase AC resistance spot welding apparatus. The target plate assembly is a three-layered plate assembly of (thin plate 11 + thick plate 12 + thick plate 13) as shown in FIG. 6 and Table 1, and a steel plate spacer 15 is appropriately placed between the steel plates with a predetermined interval W. The spacer 15 is pre-welded to form a gap having a depth G1 between the thin plate 11 and the thick plate 12 and a gap having a depth G2 between the thick plate 12 and the thick plate 13 in a simulated manner. .

  In each case, whether the workpiece position is fixed, the shape of the electrode tips on the thin plate side and the thick plate side, the type of thin plate side electrode (fixed electrode or movable electrode), the first and second stages of welding The pressure and energization time are as shown in Table 2.

  In each case, the welding current is low and the welding is performed until severe scattering occurs, the cross-section of the nugget is observed, and the thin one of the two adjacent steel plates without occurrence of intense scattering When the thickness of the steel sheet was t, the nugget satisfying that the nugget diameter was 4√t or more was obtained (good), and the nugget was not obtained (x).

  As a result, as shown in Table 2, in the example of the present invention, the nugget can be formed between the thin plate and the thick plate and between the thick plate and the thick plate regardless of the presence or absence of the gap. I couldn't get a nugget in between. Thus, the effectiveness of the present invention can be confirmed.

It is a figure explaining the welding state of resistance spot welding. It is a figure explaining one Embodiment of this invention. It is a figure explaining the welding state in one Embodiment of this invention. It is a figure explaining the welding state in one Embodiment of this invention. It is a figure explaining the welding state in one Embodiment of this invention. It is a figure which shows the board assembly in the Example 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)
15 Steel plate spacers 16a to 16d Contact portions between steel plates 17a, 17b Gap 18a, 18b Nugget 21 Electrode (electrode tip)
22 Electrode (electrode tip)
25 Clamp 30 Welding Gun 31 Robot 31a Robot Wrist 32 Transformer 33 Pressure Mechanism (Servo Motor)
34 C-type arm

Claims (2)

  When resistance spot welding is performed while applying a pressing force by sandwiching a workpiece of a plate assembly in which a thin plate is superimposed on one of two or more thick plates that are overlapped, a thin plate is brought into contact with the fixed workpiece. The electrode on the side is the fixed electrode of the welding gun, the electrode on the side in contact with the thick plate is the movable electrode, welding is a two-step process, welding is performed with low pressure and high current in the first process, A resistance spot welding method, wherein welding is performed with a pressing force larger than the pressing force in the first step.   The tip of the electrode in contact with the thin plate is a curved surface, and the tip of the electrode in contact with the thick plate is a curved surface having a larger radius of curvature than the tip of the electrode in contact with the thin plate. The resistance spot welding method according to 1.

Images