JP6094079B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a method for resistance spot welding of a workpiece in which a plurality of steel plates are overlapped, particularly a plate assembly having a large plate thickness ratio (= total thickness of plate assembly / plate thickness of the thinnest plate).

  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 high current 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 high-current welding current. This spot-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 an electric current is passed through the stacked steel plates. Be joined.

  Looking at the parts structure of an automobile, for example, the 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.

  In addition, recently, with the demand for further improvement in the collision safety of the car body, the reinforcement and the like of the reinforcement have been increased, and the thinner outer (thin plate) disposed on the outside, In many cases, it is necessary to spot-weld a plate assembly combining a thick inner and reinforcement (thick plate). Here, among the steel plates assembled in a plate, a steel plate having a relatively small thickness is referred to as a thin plate, a steel plate having a relatively large thickness is referred to as a thick plate, and the following is also the same description. To do.

  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), and it is difficult to obtain a sound joint. 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 at the surface grows too much and can not be suppressed by the pressure applied by the electrode, but it can be scattered (a phenomenon in which the base material is locally overheated and melted and scattered). It becomes difficult to obtain between thick plates, and the thin plates are easy to peel off when a destructive test is performed.

  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).

  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 superimposed on two stacked thick plates, a general part is partially located at a position where the thin plate is to be welded. The electrode facing the thin plate is formed with a higher stepped surface, the tip of the electrode is formed into a spherical surface, and the thin plate and the thick plate adjacent to it are crushed by the low pressure during the first stage of welding. And then, two thick plates are welded together with high pressure, and a technique for obtaining a necessary nugget between a thin plate and a thick plate has been proposed.

  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 tip. Thus, 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. The 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 tip, but the contact area between the thin plate and the electrode tip is small. The range in which pressure is applied by the electrode is narrow, and there is a problem that scattering occurs when a large nugget is formed between the thick plate and the thick plate.

  Further, in the resistance spot welding method described in Patent Document 3, it is possible to form a nugget between a thin plate and a thick plate. However, if a gap exists between the steel plates, it becomes difficult to form the nugget, and the thin plate— It is considered difficult to obtain a sound joint between the planks.

  The present invention has been made in view of the circumstances as described above, and in a plate assembly having a large plate thickness ratio in which a thin plate is further superimposed on at least one of the outer sides of two or more thick plates superimposed, It is an object of the present invention to provide a resistance spot welding method capable of obtaining a sound joint between a thin plate and a thick plate even when there is a gap between the plates.

  In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting nugget formation in resistance spot welding.

  First, when spot welding is performed on a plate assembly with a large plate thickness ratio in which a thin plate is further overlapped on one of the two or more thick plates that are overlapped, a nugget of the required size is formed between the thin plate and the thick plate. It is known that it is difficult to be applied, and it was found that it is effective to reduce the applied pressure in the first stage of welding and to increase the applied pressure during welding.

  That is, when high pressure is applied from the first stage of welding, the current-carrying area between the electrode and thin plate, between the thin plate and thick plate, and between the thick plate and thick plate becomes large, and the current density becomes low, so it is difficult to generate heat. In the case of a plate assembly having a large thickness ratio, the space between the thin plate and the thick plate is close to the electrode, so that it is cooled and is in a state where it is more 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 thin plate is easily peeled off when a destructive test is performed.

  On the other hand, when welding is performed at a low pressure in the first stage 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 is small, and the current density is high and the heat is easily generated. In particular, by setting the applied pressure low so that the contact diameter between the electrode and the thin plate is reduced, the current density at the contact portion between the electrode and the thin plate increases, but the heat generation between the electrode and the thin plate is caused by the cooling action of the electrode. In the first half of energization, the heat generation in the vicinity of the electrode slightly away from the electrode is the largest. In a plate assembly with a large plate thickness ratio, the heat generation area near the electrode and the boundary between the thin plate and the thick plate are close to each other. By reducing the diameter and increasing the current density, a nugget is formed between the thin plate and the thick plate.

  However, even if it is energized for a long time with low pressure applied, the molten part formed in the vicinity of the electrode grows up to the surface of the steel sheet, and surface scattering (melted metal scatters between the steel sheet and the electrode) occurs. . In addition, since the applied pressure is small between the thick plate and the thick plate, middle scattering (molten metal scatters between the steel plates) occurs before the necessary nugget diameter is obtained.

  Therefore, by starting welding with a low pressure and increasing the pressure during welding, the above problems can be solved, and even between a thin plate and a thick plate, even between a thin plate and a thick plate, We thought that welding to form a nugget with a diameter required for the welding would be possible. A nugget is formed between a thin plate and a thick plate by applying a high current for a short time with a low pressure in the first stage of welding. After that, by increasing the applied pressure, the nugget formation near the electrodes was stopped 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 heating area was near the center between the electrodes. The nugget is formed between the planks and the planks. 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 having a large plate thickness ratio, resistance spot welding that can obtain nuggets of a necessary size between the thin plate and the thick plate and between the thick plate and the thick plate is possible.

  However, if there is a gap between the stacked steel plates, in the welding under the low pressure in the first stage of welding, the gap may not be eliminated. In this case, the welding current is the adjacent welding point, etc. The current is diverted to the contact point between the steel plates at a distant position. In this case, the welding current density immediately below the electrode is lower than that without a gap, and it is difficult to form a nugget between the thin plate and the thick plate. On the other hand, if the welding conditions at low pressure in the first half of welding are selected as appropriate when there is a gap, if there is no gap or if the gap does not collapse during welding, The current density between the plate and the thick plate becomes excessive, and scattering occurs.

  Therefore, the present inventors have further studied, and it is possible to form a nugget between a thin plate and a thick plate and between a thick plate and a thick plate even in a plate assembly having a large plate thickness ratio regardless of the presence or absence of a plate gap. And found a technique.

  That is, the above method is applied after applying a conductive resin between two or more stacked thick plates. Between the thin plate and the thick plate stacked on at least one side, the plate gap between the thin plate and the thick plate is crushed and contacted directly under the electrode even with a low pressurizing force before the welding. On the other hand, between two or more thick plates that are overlapped, the gap is difficult to collapse in low-pressure welding in the first stage of welding, but by applying a conductive resin, the gap is reduced. Even when it does not collapse, there is little current to be shunted, and the welding current is in a state of flowing linearly between the electrodes, and the current density between the thin plate and the thick plate can be increased. A nugget can be formed between the plates. Thereafter, by increasing the pressing force, the resin between the thick plate and the thick plate is pushed out from directly under the electrode, and a nugget can be formed between the thick plate and the thick plate by welding with a large pressing force in the latter stage of welding.

  In addition, reducing the diversion in a place other than just below the electrode between the thin plate and the thick plate is effective for forming a nugget between the thin plate and the thick plate during welding under low pressure in the first stage of welding. It is not essential to apply a resin between the thin plate and the thick plate, but it is more preferable to apply a resin having no conductivity. By applying a non-conductive resin between the thin plate and the thick plate, the resin is extruded under pressure by the electrode just below the electrode to secure a current path, and the outside of the thin plate is made of non-conductive resin. Since the thick plates are insulated, it is easy to increase the current density immediately below the electrode between the thin plates and the thick plates, and it is easy to form a healthy nugget.

  In the conventional resistance spot welding method, it is necessary to grow a nugget formed near the center between the electrodes until the gap between the thin plate and the thick plate is melted, and an excessive current is required. On the other hand, in the case of the above method, a sound joint can be formed between the thin plate and the thick plate in the first stage of welding. What is necessary is just to select an appropriate welding current value according to the applied pressure in the latter stage of welding. The applied pressure in the latter stage of welding may be sufficient to squeeze the gap between the thick plates and extrude the resin, and this depends on the strength of the steel plate and the rigidity of the gap.

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

  [1] Resistance spot welding with a pair of electrodes sandwiched between at least one of the two or more thick steel plates that are superposed on each other and a thin steel plate that is superposed with a pair of electrodes. In carrying out the process, a conductive resin is interposed between the two or more thick steel plates that are superposed, and the welding process is divided into the first and second stages. A resistance spot welding method characterized in that welding is performed with a low applied pressure satisfying ≦ 3 kN, and then welding is performed by energizing under high pressure in the latter stage of welding.

  [2] The resistance spot welding method according to [1], wherein welding is performed with a resin having no conductivity interposed between the thin steel plate and the thick steel plate.

  According to the present invention, in the case of resistance spot welding a plate assembly having a large plate thickness ratio in which two or more stacked thick steel plates are overlapped with a thin steel plate on at least one of the outer sides, Even when there is a gap between the steel plates, it is possible to perform resistance spot welding that forms a sound joint between the thin plate and the thick plate and between the thick plate and the thick plate.

It is a figure which shows the state (plate assembly) before the welding start in one Embodiment of this invention. It is a figure which shows the nugget formation state of the welding first term in one Embodiment of this invention. It is a figure which shows the nugget formation state of the welding latter stage in one Embodiment of this invention. It is a figure which shows the board assembly in the Example of this invention.

  One embodiment of the present invention is described below.

  In one embodiment of the present invention, for example, as shown in FIG. 1, a thin steel plate (thin plate) 11 is stacked on the upper surface of two stacked thick steel plates (thick plates) 12 and 13. Furthermore, the thickness ratio is such that there is a gap between the steel plates (the first gap 21 between the thin plate 11 and the thick plate 12 and the second gap 22 between the thick plate 12 and the thick plate 13). When a large plate assembly (workpiece) 10 is sandwiched between a pair of electrodes (electrode tips) 16 and 17 and is subjected to resistance spot welding while applying pressure, between the two thick plates 12 and 13 (plate A conductive resin 26 is applied to the gap 22), and the welding process is divided into the first and second stages. In the first stage of welding, pressurization is performed with a low pressure, and the gap 21 is crushed. By energizing, current flows linearly between the electrodes 16 and 17 through the conductive resin 26, It is possible to increase the current density directly under the poles 16 and 17, as shown in FIG. 2, the nugget 18 is formed between the thin plate 11 thick 12.

  Further, it is preferable to apply a resin having no conductivity to the gap 21. Since the thin plate 11 is thin, the thin plate 11 can be crushed even with a low pressure in the first stage of welding. At this time, the non-conductive resin applied to the plate gap 21 is extruded from directly below the electrodes 16 and 17, and the electrode 16 , Insulate the parts other than 17 directly below. As a result, the current-carrying area immediately below the electrodes 16 and 17 is kept narrow, and the formation of the nugget 18 in the first stage of welding can be made easier.

  Then, after the energization in the first half of welding is completed, in the latter half of welding, the plate gap 22 is completely crushed under high pressurization under high pressure, and energization in the second half of the welding is performed, as shown in FIG. The nugget 19 can also be stably formed between the 12-thick plate 13.

  Here, the resin used between the thin plate 11 and the thick plate 12 may be any resin as long as it does not have conductivity. For example, the resin used as a sealant in the manufacture of automobiles and the adhesive used in the weld bond method. It's okay. In the case of an adhesive, the effect of improving the fatigue characteristics and strength of the welded portion and the corrosion resistance of the welded portion can also be obtained. For example, a thermosetting epoxy resin adhesive can be applied. On the other hand, the resin 26 used between the thick plate 12 and the thick plate 13 needs to have conductivity. A non-conductive resin used between the thin plate 11 and the thick plate 12 mixed with a conductive substance can be used. For example, a mixture of carbon black or graphite can be used.

  In this way, in this embodiment, in the first stage of welding, welding is performed with a low pressure, so that only the current density in the gap 21 is increased to form the nugget 18, and then the pressure is increased in the second stage of welding. Thus, by extruding the resin 26 in the gap 22, the nugget 19 can be stably formed between the thick plate 12 and the thick plate 13 by welding in the latter stage of welding, and the thickness ratio with the gaps 21 and 22 is Even with the large plate assembly 10, the nuggets 18 and 19 having the necessary nugget diameter 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, and can be welded to form a sound joint. It becomes.

  As described above, in order to form the melted portion (nugget) 18 between the thin plate 11 and the thick plate 12, by passing a high welding current with a low pressurizing force in the first stage of welding, the gap between the thin plate 11 and the thick plate 12 in the vicinity of the electrode. It is important to heat the neighborhood intensively. At this time, in this embodiment, since the conductive resin 26 is applied to the gap 22 between the thick plate 12 and the thick plate 13, even if the welding pressure at the initial stage of welding is low, it is between the electrodes 16 and 17. Since the energization path can be secured, it is not necessary to set the pressurizing force high in order to crush the gap 22. Specifically, 3 kN or less is sufficient, and the lower the pressurization, the nugget formation between the thin plate 11 and the thick plate 12 occurs. Becomes easy. On the contrary, when the applied pressure exceeds 3 kN, the plate gap 22 between the thick plate 12 and the thick plate 13 is greatly deformed in a direction to be crushed. As a result, the thin plate 11 is also largely deformed. The current density decreases due to the large expansion of the contact area. Even if the current is increased in order to increase the current density, surface scattering occurs and a healthy nugget cannot be formed. Therefore, the applied pressure in the first half of welding is 3 kN or less.

  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 about the required nugget diameter between the thick plate 12 and the thick plate 13. More preferably, the electrode tip 16 is a curved surface having a larger radius of curvature.

  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 amount of pushing of the electrode 16 into the thin plate 11 is reduced at a low pressure, and the current-carrying area is reduced. Therefore, the current density between the thin plate 11 and the thick plate 12 becomes high, and the nugget 18 is easily formed 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 19 having a necessary diameter between the thick plate 12 and the thick plate 13 can be formed.

  By the way, the welding apparatus used in the present invention can be any type of pressurizing mechanism (such as an air cylinder or servomotor) as long as it sandwiches a portion to be welded by a pair of upper and lower electrode tips, and pressurizes and energizes. The shape (stationary type, robot gun), the type of power source (single-phase AC, AC inverter, DC inverter) and the like are not particularly limited.

  Moreover, the steel plate to be welded can be applied regardless of the strength level (soft steel, high-tensile steel plate), and can also be applied to a hot press material or a warm press material. Moreover, it can be applied not only to a bare steel plate but also to many types of plated steel plates such as an electrogalvanized steel plate, a hot dip galvanized steel plate, an alloyed hot dip galvanized steel plate, and an Al-based plated steel plate. The plate assembly can be applied not only to three sheets but also to four sheets.

  In order to confirm the effect of the present invention, the present invention example and the comparative example were carried out.

  The plate assembly is the plate assembly shown in FIGS. 4A and 4B and Table 1. That is, as shown in FIG. 4A, the plate assembly a is composed of (first steel plate (thin plate) 11 + second steel plate (thick plate) 12 + third steel plate (thick plate) 13). As shown in FIG. 4B, the plate assembly b is (first steel plate (thin plate) 11 + second steel plate (thick plate) 12 + third steel plate (thick plate) 13+. 4 sheets of 4th steel plate (thick plate) 14). Then, the steel plate spacers 31, 32, 33 are inserted between the steel plates with a predetermined interval W as appropriate, and the first plate gap 21 having a depth G1 between the thin plate 11 and the thick plate 12, and the thick plate 12-thick plate. 13, a second gap 22 having a depth G <b> 2 and a third gap 23 having a depth G <b> 3 are formed between the thick plate 13 and the thick plate 14.

  In each case, the shape of the upper and lower electrode tips, the applied pressure in the first stage of welding (first stage), the energizing time, the welding current, the applied pressure in the latter stage of welding (second stage), the energizing time, and the welding current are 2 and those shown in Table 3. In addition, regarding the presence or absence of application of resin in each of the plate gaps 21, 22, 23, the case where a conductive resin is applied is ◯, the case where a non-conductive resin is applied is Δ, and any resin is applied. The case where there was not was described by x. The welding conditions were set such that the nugget diameter between the thick plate 12 and the thick plate 13 was 4√t (t: the thickness of the thinner one of the two thick plates 12 and 13 (mm)) mm. The welding machine used was a single-phase AC servo motor pressure resistance spot welder.

  The results of evaluating the joints in each case are also shown in Tables 2 and 3. The evaluation is based on the evaluation based on the presence or absence of the occurrence of scattering and the peel test specified in JIS Z 3001, and the case where the plug is formed between the thin plate 11 and the thick plate 12 is good (◯), and the plug is not formed. What fractured at the interface was regarded as defective (x).

  As a result, in the example of the present invention, plug breakage occurs between the thin plate 11 and the thick plate 12 and the welding is good, but in the comparative example, peeling occurs between the thin plate 11 and the thick plate 12 or the occurrence of scattering is observed. As a result, a good weld joint could not be obtained. Thereby, the effectiveness of the present invention can be confirmed.

  Although the case where the nugget diameter between the thick plate 12 and the thick plate 13 is 4√t is evaluated here, in the case of carrying out the present invention, the nugget diameter between the thick plate 12 and the thick plate 13 is May be performed with the nugget diameter required between the thick plate and the thick plate.

  Furthermore, in this invention, the effect at the time of apply | coating nonelectroconductive resin between the thin plate 11-thick board 12 was also confirmed. The plate assembly at that time is the plate assembly shown in FIG.

  In each case, the shape of the upper and lower electrode tips, the welding pressure in the first stage of welding (first stage), the energizing time, the welding current, the welding pressure in the latter stage of welding (second stage), the energizing time, It was shown in Table 4. Regarding the presence / absence of application of the resin in each of the plate gaps 21 and 22, ◯ when the conductive resin is applied, △ when the non-conductive resin is applied, and × when no resin is applied It was described in. The welding conditions were set such that the nugget diameter between the thick plate 12 and the thick plate 13 was 4√t (t: the thickness of the thinner one of the two thick plates 12 and 13 (mm)) mm. The welding machine used was a single-phase AC servo motor pressure resistance spot welder.

  Table 4 shows the results of evaluating the joints in each case. The evaluation was performed based on the evaluation based on the presence or absence of occurrence of scattering and the peel test defined in JIS Z 3001, and the plug diameter between the thin plate 11 and the thick plate 12 was measured.

  As a result, as shown in Table 4, all of the examples of the present invention (Nos. 36 to 44) are plug ruptures, which are good results, but among them, the thin plate 11-the thick plate 12 When a non-conductive resin is applied between them (No. 37, 40, 43), the plug diameter between the thin plate 11 and the thick plate 12 is larger than in other conditions. Thereby, in this invention, the effectiveness at the time of apply | coating nonelectroconductive resin between the thin plate 11-thick board 12 was able to be confirmed.

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 (thick plate)
16 Thin plate side electrode (electrode tip)
17 Thick plate side electrode (electrode tip)
18 Nugget between thin plate and thick plate 19 Nugget between thick plate and thick plate 21 First plate gap 22 Second plate gap 23 Third plate gap 26 Conductive resin 31 Steel plate spacer 32 Steel plate spacer 33 Steel plate spacer

Claims (2)

Overlaid two or more the thickness of the thick steel plate was, on the outside of at least one further superposed thin steel plate thickness, sandwiched Itakumi the thickness ratio exceeds 5 by a pair of electrodes, the pressure In addition, when conducting resistance spot welding, an electrically conductive resin is interposed between the two or more thick steel plates stacked together, and the welding process is divided into the first and second stages. A resistance spot welding method characterized in that welding is performed at a low pressure satisfying a pressure P of P ≦ 3 kN, and thereafter, welding is performed by energizing under high pressure in the latter stage of welding.   The resistance spot welding method according to claim 1, wherein welding is performed by interposing a resin having no conductivity between the thin steel plate and the thick steel plate.

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