JP2008093707A - Resistance spot welding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a resistance spot welding method capable of forming a nugget of a sufficient size at an appropriate position in a plate assembly having a large plate thickness ratio, and capable of performing the welding work, which is accompanied by formation of the nugget, with remarkably simple equipment and control. <P>SOLUTION: A plate assembly 3 consists of a plurality of metal plates 4, 5 superimposed on each other, and a thinnest plate 4 among the metal plates is arranged as one of the outermost layers of the assembly 3. Resistance spot welding is applied to the assembly 3 by energizing it while holding it between a pair of electrodes 1, 2 and pressurizing it. The tip end of the electrode 1 is formed into the shape of a convex curve surface, and a depression 6 is formed on the tip end of the electrode 2. The pressure energization is performed while bringing the electrode 2 with the depression 6 into contact with a thick plate 5 that is arranged as the other of the outermost layers of the assembly 3. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a resistance spot welding method, and more particularly to a resistance spot welding method that enables good nugget formation on a plate assembly formed by superposing a plurality of metal plates having different plate thicknesses.

  In general, a resistance spot welding method, which is a kind of a lap resistance welding method, is used for joining a plurality of stacked metal plates. This welding method is a means for obtaining a welded portion called a nugget by sandwiching a plate assembly formed by superposing a plurality of metal plates between a pair of electrodes, and applying a pressure current with the pair of electrodes. For this reason, in recent years, there is a tendency to be used favorably in fields that require a large number of weldings, such as automobile body welding, and where high-speed welding work is required.

  Also, in recent years, in the field of automobile manufacturing, etc., due to the increasing demand for improved collision safety of the vehicle body, for example, a structure in which a reinforcement is sandwiched between a floor panel and a member constituting a floor portion of a vehicle. Has been adopted. In this structure, unlike the conventional simple two-ply steel plate spot-welded, it is required that three or more steel plates be superposed and spot-welded.

Furthermore, recently, with the demand for further improvement in the collision safety of the car body, the reinforcement and the thickening of reinforcement etc. have progressed, and a floor panel (thin plate) with a thin plate thickness has been placed on one side, In many cases, it is necessary to spot weld a plate assembly in which one or a plurality of thick members and reinforcements (thick plates) are combined.

  Spot welding is performed on a plate assembly with such a large thickness ratio (= total thickness of the plate assembly / thickness of the thinnest metal plate) while maintaining a constant value of applied pressure and energization as in the past. In this case, it is difficult to form a nugget of a necessary size between the thin plate and the thick plate on the outermost side (the side in contact with the electrode). Therefore, the actual situation is that sufficient bonding strength cannot be obtained for the above-described plate assembly.

  As a means for solving such a problem, for example, in Japanese Patent Application Laid-Open No. 2003-251468, when spot welding is performed by sandwiching a workpiece in which a thin plate is stacked on two thick plates with a pair of electrodes, A resistance spot welding method is proposed in which welding is performed such that the contact area between one electrode located on the workpiece and the workpiece is smaller than the contact area between the other electrode located on the thick plate side and the workpiece. (See Patent Document 1).

In addition, in JP 2006-55898 A, a plate assembly in which a thin metal plate is superimposed on one of two or more stacked thick metal plates is sandwiched between a pair of electrodes and welded by resistance spot welding. One of the pair of electrodes, the electrode in contact with the thin metal plate is an electrode whose tip is a curved surface having a predetermined radius of curvature R1, and the electrode in contact with the other thick metal plate is an electrode with the tip in contact with the flat or thin metal plate The electrode is a curved surface having a radius of curvature larger than the radius of curvature R1 of the tip, and resistance spot welding is a two-stage welding of the first stage and the second stage, and the second stage welding is compared to the first stage welding. A resistance spot welding method characterized by high-pressure welding is proposed (see Patent Document 2).
JP 2003-251468 A JP 2006-55898 A

  However, the method described in Patent Document 1 is intended for welding a plate assembly including three metal plates including a thin plate. Therefore, simply by making the tip diameter of the electrode in contact with the thin plate smaller than the tip diameter of the electrode in contact with the thick plate, a plate assembly having a large plate thickness ratio, for example, when the plate thickness ratio exceeds 5, It is difficult to form a nugget that reaches the thin plate side.

  Moreover, even if it is the method of patent document 2, the plate | board thickness ratio which can be welded is about 6-7. Therefore, when a nugget having an appropriate size reaching the thin plate side is formed on a plate assembly having a larger plate thickness ratio (for example, about 10), this method is insufficient. In addition, since the above-mentioned method requires a change in the applied pressure and control, such as increasing the initial applied pressure, it takes time to set the control and increases the equipment for applying a high applied pressure. Invite. An increase in equipment space leads to restrictions on the welding location, which may lead to a reduction in design freedom.

  In view of the above circumstances, in the present invention, a nugget having a sufficient size can be formed at an appropriate position even for a plate assembly having a large plate thickness ratio, and the welding operation accompanied by the formation of this nugget is extremely performed. It is a technical problem to provide a resistance spot welding method that can be performed with simple equipment and control.

  In order to solve the above-mentioned problem, the present invention encloses a plate assembly in which the thinnest metal plate is arranged on one outermost layer among a plurality of stacked metal plates between a pair of electrodes, and energizes while applying pressure. In the resistance spot welding method in which welding is carried out, one of the pair of electrodes, one of which has a convex curved end on one electrode, and the other electrode whose depression is recessed, and Provided is a resistance spot welding method characterized in that pressurization energization is performed in a state where the provided electrode is in contact with the other outermost layer of the plate assembly.

  As described above, the present invention uses an electrode having a convex curved surface at the tip and an electrode provided with a dent at the tip when the plate assembly is pressurized and energized, and the electrode on the side provided with the dent is arranged on the thick plate side. The point is a technical feature. That is, at the initial stage of energization, on the thin plate side, the electrode having the convex curved tip contacts in a dotted manner, whereas on the thick plate side, the recessed peripheral edge of the tip contacts the metal plate surface, so that the electrode contacts in an annular shape. easy. In this case, the contact area with the electrode is smaller on the thin plate side than on the thick plate side, and the current density is increased accordingly, so heat is generated preferentially on the thin plate side. Therefore, the nugget is easily formed even on the thin plate side. In addition, an annular contact state occurs on the thick plate side, so that a portion that becomes the nucleus of the nugget is formed and grows through a process different from the conventional one. Therefore, as a result, the nugget grows over a wide range, and a reliable and strong bonding occurs on the thin plate side, and a nugget with a very large diameter (a large surface area) can be obtained.

  Also, with this method, it is possible to form an appropriate nugget even in a state where the applied pressure and the energization amount (current amount) are kept constant, so the variation in welding quality for each product is reduced, High quality welding work can be performed stably. In addition, the conventional method requires labor and cost for adjusting the pressure and setting the control. However, with this method, no special control or equipment is required for the above-mentioned reasons. Welding can be easily performed with control. In addition, such a method does not require a high pressurizing force, so that the space required for the pressurizing equipment can be reduced. Moreover, since there is no restriction on the welding location, the degree of freedom in design can be improved.

  As described above, when a plate assembly is welded using an electrode provided with a recess, the surface of the thick plate that is located in the outermost layer of the plate assembly and contacts the electrode provided with the recess is usually called a welding mark. A protrusion is formed. At this time, since a certain correlation is recognized between the height of the welding mark and the nugget diameter (welding quality), the inventors have, for example, the shape of the welding mark formed according to the depression. The inventors have obtained the knowledge that the quality of welding (whether or not a nugget reaching the thin plate side is formed) can be judged by visual observation, more precisely, by the height of the welding mark.

  Therefore, when the nugget is properly formed, the depth of the recess is preferably set to a size corresponding to the height of the welding mark formed on the contact surface with the electrode provided with the recess. When the nugget is appropriately formed by performing welding using an electrode provided with such a dent, a welding mark is formed in a shape following the dent. Therefore, the quality of welding can be easily confirmed visually. Preferably, the depression is formed so that a pattern corresponding to a marker (a kind of mark) formed on the bottom of the depression is formed on the welding mark. By forming a mark corresponding to the marker provided on the bottom surface of the recess in the welding mark, it is possible to more easily confirm the quality of the welding by visual observation. Here, “appropriate nugget is formed” means that the nugget is formed over all the metal plates constituting the plate assembly including the thin plate.

  It is also possible to quantitatively determine the quality of welding from the resistance of the workpiece (plate assembly) during welding. That is, when resistance spot welding is performed using an electrode with a depression, the resistance (electrical resistance) of the workpiece gradually decreases from the start of energization, and tends to reach an equilibrium state when the nugget is properly formed. It has been known. Therefore, by making a determination based on this knowledge, specifically, by confirming that the resistance value measured during welding has reached an equilibrium state, it is determined that proper nugget formation has been performed properly. can do. In addition, this determination work can be easily and automatically performed based on the measured value of electrical resistance by obtaining a determination criterion for the equilibrium state in advance. According to such means, it is possible to easily and accurately determine the quality of welding, and consequently the quality control of the welded portion, and therefore it is very suitable for mass production processing. Of course, this determination means may be used in combination with the visual determination means described above.

  As described above, according to the present invention, a sufficiently large nugget is placed at an appropriate position even for a plate assembly having a large thickness ratio, for example, a plate assembly having a plate thickness ratio of 10. Can be formed. In addition, the welding operation involving the formation of the nugget can be performed with very simple equipment and simple control, thereby stabilizing the joint quality and improving the degree of design freedom.

  In addition, by setting the depth of the recess provided in the electrode provided with the recess according to the height of the weld mark formed on the surface of the metal plate when the nugget is properly formed, quality control of the welded portion Can be performed easily and with high accuracy.

  Hereinafter, an embodiment of a resistance spot welding method according to the present invention will be described with reference to the drawings.

  FIG. 1 conceptually shows a welding method according to an embodiment of the present invention. As shown in the figure, this welding method is to perform spot welding by sandwiching a plate assembly 3 as a work made of a plurality of metal plates with a pair of electrodes 1 and 2 and energizing them while applying pressure. .

  The plate assembly 3 to be welded is formed by superposing a plurality of metal plates, specifically a plurality of metal plates 4 and 5 having different plate thicknesses. Here, among the superposed metal plates 4 and 5, the thinnest plate (hereinafter referred to as the thin plate 4) is arranged in one outermost layer of the plate assembly 3, and the other outermost layer. A metal plate other than the thin plate 4 (hereinafter referred to as a thick plate 5) is disposed. Here, the plate assembly 3 is composed of two or more metal plates having different plate thicknesses, and the thin plate 4 may be disposed on one outermost layer, and the number of the thick plates 5 is not limited. Further, when a plurality of thick plates 5 are used, the thicknesses of the thick plates 5 and 5 may be different.

  The pair of electrodes 1 and 2 is disposed at a position sandwiching the plate assembly 3 in the thickness direction through a predetermined pitch, and is energized while applying pressure to the plate assembly 3 by an appropriate pressure control means (not shown). It is like that. Of the pair of electrodes 1 and 2, one electrode 1 having a convex curved end is used, and the other electrode 2 having a depression 6 at the tip is used. In this embodiment, both the electrodes 1 and 2 have a convex spherical tip, and only the other electrode 2 is provided with a circular recess 6 at the center (top) thereof.

  Of the electrodes 1 and 2 having the above configuration, one electrode 1 is in contact with the surface 5 a of the thick plate 5 constituting the outermost layer of the plate assembly 3, and the electrode 2 provided with the recess 6 is the surface of the thin plate 4. It arrange | positions so that 4a may be contacted.

  From this state, the pair of electrodes 1 and 2 are pressed (pressurized) against the plate assembly 3 by a pressure control means (not shown), and the plate assembly 3 is sandwiched between the pair of electrodes 1 and 2 by a current control means (not shown). Energize. Here, one electrode 1 and the surface 4a of the thin plate 4 are in the contact state shown in FIG. 2 (a) (in the same figure, they are in contact with each other at a region indicated by C1). Further, the other electrode 2 provided with the depression 6 at the tip and the surface 5a of the thick plate 5 located in the outermost layer are in a contact state shown in FIG. 2B (in the region indicated by C2 in the same figure). Contact). That is, in the initial stage of energization, the electrode 1 having a convex curved tip contacts the thin plate 4 in a point-like manner, whereas the electrode 2 provided with the recess 6 contacts the thick plate 5 at the periphery of the recess 6. Therefore, the contact area with the electrode is smaller on the thin plate 4 side than on the thick plate 5 side, and the current density is increased on the thin plate 4 side. Furthermore, by forming the annular contact area C2 on the thick plate 5 side, the formation and growth of the nugget also proceeds according to the contact area C2. For these reasons, as shown in FIG. 3, a large-diameter nugget 7 that reaches the thin plate 4 side can be obtained, whereby a strong joint can be obtained between the thin plate 4 and the thick plate 5. It becomes.

  In addition, with this method, for example, the nugget 7 can be appropriately formed by pressurization energization with a constant pressure and energization amount (current amount). Therefore, it is possible to reduce the variation in quality of the welded portion and stably form a high-quality welded portion. Further, according to this method, since welding can be easily performed with simple equipment and control, for example, the space required for the pressure equipment can be reduced. Therefore, the welding operation can be performed without being restricted by the welding location, and it is possible to improve the degree of freedom of welding and consequently to expand the types of weldable products.

  Regarding the energization pattern, even the simplest single-stage energization can be welded without any particular problems with the above-described welding method, but energization is performed by changing the amount of current during the energization period, such as multi-stage energization. It doesn't matter.

  Further, in the case where pressurization energization is performed using the electrodes 1 and 2 having the above-described configuration, a welding mark 8 is formed at a position corresponding to the depression 6 on the surface 5a of the thick plate 5 in contact with the electrode 2 as shown in FIG. Is formed protruding. Here, as shown in FIG. 4, when the nugget 7 is appropriately formed with a depth dp of the depression 6, a welding mark formed on the contact surface (surface 5 a) with the electrode 2 provided with the depression 6. If it is set to a size corresponding to a height h of 8, the formation quality of the nugget 7 can be determined by the height h of the welding mark 8. That is, when the nugget 7 is appropriately formed by performing welding using the electrode 2 provided with the depression 6 having such a size (depth dp = height h) on the thick plate 5 side, It is formed in a shape that follows the recess 6. Therefore, the quality of welding can be easily confirmed visually.

  In this case, by forming the bottom surface 6a of the recess 6 flat, for example, as shown in FIG. 4, a welding mark 8 having a flat surface 8a following the bottom surface 6a is formed. Accordingly, the bottom surface 6a of the recess 6 functions as a kind of marker for the welding mark 8, and the quality of the welding can be more easily confirmed visually. The shape of the marker is not particularly limited, but it is preferable to use the flat bottom surface 6a as a marker because it is very easy to measure the height h of the welding mark 8 and confirm the mark.

  As described above, after the welding operation, it is possible to determine the quality of the welding based on the information obtained from the welding mark 8, but in addition to this, the quality of the welding is determined based on the resistance of the workpiece (plate assembly 3) during welding. It is also possible. The concept of this determination means can be described using, for example, a correlation diagram between energization time and electrical resistance shown in FIG. That is, as shown in FIG. 5, the electrical resistance measured between the plate assemblies 3 gradually decreases with the start of energization, but the nugget 7 is properly formed as described above, and this has been dealt with. When the weld mark 8 is formed, the resistance reaches an equilibrium state. Therefore, in this welding pass / fail judgment means, it is possible to confirm that the nugget 7 is properly formed by checking the fluctuation of resistance during energization using the point where the electrical resistance reaches the equilibrium state as a judgment criterion. If it is this determination means, since determination can be performed based on the measured value of the electrical resistance between the plate assemblies 3, such pass / fail determination can be performed automatically and further during welding. Therefore, it is possible to easily and highly accurately determine the quality of the welded part, and hence the quality control of the welded part, which is very suitable for mass production processing.

  Of course, if it is desired to perform quality control of the welded part more reliably and with high accuracy, the determination means based on the resistance can be used in combination with the determination means based on the aspect of the weld mark 8 described above. At this time, utilizing the fact that a certain correlation is seen between the resistance being in an equilibrium state and the mark corresponding to the welding mark being formed by the marker provided on the bottom surface of the electrode recess. The quality of welding can also be determined.

  In addition, when performing the quality determination of welding using such a determination means, it becomes a problem how to set the determination criterion of an equilibrium state. Specifically, it is necessary to define the equilibrium state by the vertical width (variation range) of the resistance value and the period (period indicated by Te in the figure) regarded as the equilibrium state. 4. It varies depending on the type and thickness of the thick plate 5) or the order of superposition thereof. Therefore, it is preferable to calibrate in advance according to each welding mode, and to determine a determination criterion for an equilibrium state in an actual welding operation based on the calibration.

  In the above description, as the electrode for resistance spot welding, the electrodes 1 and 2 having the shape shown in FIG. 1, that is, both the electrodes 1 and 2 have a convex spherical tip, and only the other electrode 2 has its center (top). ) Is provided with a circular recess 6, but it is not necessary to be limited to this. Regarding the electrode 1 used on the thin plate 4 side, an electrode having an arbitrary shape can be used as long as the tip has a convex curved surface shape. Further, as far as the electrode 2 used on the thick plate 5 side is concerned, as long as the dent 6 is provided at the tip, an electrode of any shape (for example, the dent 6 provided on a flat surface without any limitation) May be used). Of course, as in the above-described embodiment, if a good nugget is formed by one-stage energization without increasing the applied pressure so much, it is preferable that the current density be appropriately increased. That is, as shown in FIG. 1, if the tip shape of the electrode 2 is a convex curved surface and the center portion (top portion) has a recess, the peripheral portion of the recess 6 is preferentially brought into contact during initial energization. And the contact area (area C2 shown in FIG. 2) at that time can be suppressed to an appropriate size.

  In order to verify the usefulness of the present invention, the quality of the nugget was determined for the joined body obtained by the welding method according to the present invention and the joined body obtained by the conventional method. Details are described below.

  In both of the method according to the present invention and the conventional method, a plate as a work is overlapped with four metal plates as thick plates (both SCGA590 thickness 1.6 mm), and one metal as a thin plate. A plate (SCGA270 thickness: 0.65 mm) and a further overlapped plate were used. The thickness ratio in this case was 10.8.

  Moreover, as an electrode used in the method according to the present invention, an electrode having a tip shape shown in FIG. 6 was used as an electrode arranged on the thick plate side. The electrode used by being arranged on the thin plate side has a tip shape shown in FIG. 6 and has a shape (a tip is a convex spherical shape as shown by a broken line) from which a portion corresponding to a depression is removed. On the other hand, as the electrodes used in the conventional method, those having a shape from which the depressions in FIG. 6 were removed were used for both electrodes.

  Further, regarding the pressurization energization conditions, both the method according to the present invention and the conventional method were applied pressure: 300 kgf, one-stage energization (energization time: 28 cycle 1 cycle = 1/60 sec). Regarding the current value at the time of one-stage energization, the method of the present invention: 9.6 kA and the conventional method: 7.4 kA, respectively. The current value was the current value immediately before scattering occurred.

  FIG. 7 shows a cross-sectional photograph of the nugget formed inside the plate assembly welded by the conventional method, and FIG. 8 shows a cross-sectional photograph of the nugget welded by the method according to the present invention. From these results, the nugget obtained by the conventional method is formed without reaching the thin plate located in the outermost layer, whereas the nugget obtained by the method according to the present invention is not only a thick plate. It can be seen that the plate is formed over the entire thickness direction until reaching the outermost thin plate. In addition, regarding the nugget diameter (usually the size in the extending direction of the plate), the nugget obtained by the method according to the present invention is the largest on the thin plate side, and is sufficient even in the center in the thickness direction where the nugget diameter is the smallest. It can be seen that it has a large nugget diameter.

It is sectional drawing which shows notionally the resistance spot welding method which concerns on one Embodiment of this invention. (A) is the figure which looked at the contact part with the electrode of a thin plate from the direction of arrow a in FIG. 1, (b) is the figure which looked at the contact part with the electrode of a thick plate from the direction of arrow b in FIG. . It is sectional drawing of the board assembly when an appropriate nugget is formed. It is an enlarged view which shows the relationship between the hollow of an electrode and the welding trace formed by this. It is a correlation diagram which shows the relationship between electricity supply time and an electrical resistance. It is a side view which shows the front-end | tip shape of the electrode used for an Example. It is a cross-sectional photograph of the nugget obtained by the conventional method. It is a cross-sectional photograph of the nugget obtained by the method according to the present invention.

Explanation of symbols

1 Electrode (thin plate side)
2 electrodes (thick plate side)
3 Plate assembly 4 Thin plate 5 Thick plate 6 Recess 6a Bottom surface 7 Nugget 8 Weld mark C1 Contact area (thin plate)
C2 contact area (thick plate)

Claims (2)

In the resistance spot welding method of performing welding by energizing while sandwiching a plate assembly formed by arranging the thinnest metal plate in one outermost layer among a plurality of superimposed metal plates,
Of the pair of electrodes, one electrode having a tip with a convex curved surface, the other electrode having a recessed tip, and
A resistance spot welding method, wherein the electrode provided with the depression is pressed and energized in a state where the electrode is in contact with the other outermost layer of the plate assembly.   The resistance according to claim 1, wherein the depth of the recess is set to a size corresponding to a height of a welding mark formed on a contact surface with the electrode provided with the recess when the nugget is appropriately formed. Spot welding method.