JP6037018B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a resistance spot welding method, and specifically to a resistance spot welding method for a plate assembly in which steel plates and aluminum plates are overlapped. In the present invention, the aluminum plate is a general term for a pure aluminum plate and an aluminum alloy plate.

  In recent years, in the automobile industry, for the purpose of improving the fuel efficiency by reducing the weight of the vehicle body, the thinning of the steel plate by the application of a high-strength steel plate and the application of a light metal material such as an aluminum alloy plate to the vehicle body are being promoted.

  Currently, the resistance spot welding method, which is superior in cost and efficiency compared to other welding methods, is most often used for joining steel plates in a vehicle body, and the number of hit points per unit is 3000 to 6000 points. It extends to. In this method, two or more stacked steel plates are sandwiched and pressed with a pair of electrodes from above and below, and a high-current welding current is passed between the upper and lower electrodes for a short time to join them by resistance heating.

  From the viewpoint of maintaining the cost and efficiency of the production process of the vehicle body, it is effective to use the resistance spot welding method also in joining when aluminum plates are mixed, as in the case of steel plates. However, in dissimilar material joining of steel plate and aluminum plate, the soft aluminum plate is greatly reduced in thickness during welding due to the pressurization of the electrode and the heat generated during energization, and fragile intermetallic compounds are formed at the joint interface. Therefore, there is a problem that the joint strength cannot be secured.

  In order to deal with the above problems, the following techniques have been proposed.

  For example, Patent Document 1 discloses resistance spot welding in which welding is performed in an extremely short time and a high current is applied to disperse the melted part to the surroundings, thereby achieving normal contact between metal surfaces and joining by atomic diffusion. A method is described.

  Further, Patent Document 2 discloses that an iron / aluminum clad thin plate is inserted between a steel plate and an aluminum plate so that the same kind of materials face each other, thereby minimizing the amount of aluminum deposited on the electrode while maintaining high strength even at a low current. A resistance spot welding method is described which provides a joint of

  Further, Patent Document 3 discloses that welding is performed by attaching one or more contact plates on both sides of a steel plate and an aluminum plate, so that the interface between the contact plate and the material to be bonded generates resistance heat, and the steel and aluminum are resistance diffusion bonded. In addition, a resistance spot welding method capable of obtaining a high-strength joint is described.

  Patent Document 4 describes that the occurrence of scattering can be suppressed while obtaining a large nugget diameter by optimizing the amounts of Mn and Si in the steel sheet and the steel sheet surface oxide film.

  Moreover, in patent document 5, in the welding of the several metal body from which melting | fusing point differs, by forming with the electrode which provided the recessed part in the front-end | tip surface as an electrode which contact | connects a metal body with low melting | fusing point, formation of the burr | flash by scattering is prevented. A welding method for obtaining a high-productivity and high-quality weld has been proposed.

  Moreover, in patent document 6, by making the tip curvature radius of the convex electrode used for the aluminum material side at the time of spot welding of GA steel plate and aluminum material 2-6 times larger than a steel plate side, the fusion | melting part of an aluminum is carried out. There has been proposed a method of ensuring bonding strength by making it difficult to eliminate, facilitating formation of a molten pool on a bonded surface, and relatively easily removing molten galvanized plating from the bonded surface.

JP 2004-17148 A JP-A-7-136774 JP-A-9-155561 JP 2005-152958 A JP-A-11-342477 JP 2008-200747 A

  However, in the resistance spot welding method described in Patent Document 1, it is necessary to apply a large current compared to the resistance spot welding method between steel plates, and therefore a power source for a welding transformer generally used in an automobile production line There is a problem that the current value is insufficient in the capacity.

  In addition, the resistance spot welding methods described in Patent Documents 2 and 3 require the use of a backing plate or a clad thin plate that is not necessary for the structure of the vehicle body, and further the process change of the production line of the vehicle body. And there is a problem that weight cannot be reduced sufficiently.

  Moreover, in patent document 4, since it is necessary to limit the amount and distribution of the alloy element in a steel plate and an oxide film, there exists a subject that the use of the steel plate which satisfy | fills a required performance is restrict | limited, especially in recent steel plates, it is high strength. Under the circumstances where high alloying is progressing with the progress of application, its application is extremely limited.

  Further, in Patent Documents 5 and 6, as the number of welding points increases, the wear of the electrode tip on the side in contact with the low melting point material or the aluminum material proceeds, but the tip wear of the low melting point material or the aluminum material side electrode at that time is remarkably increased. As it progresses, the welding itself becomes unstable. As a result, there is a problem that the electrode needs to be replaced very frequently.

  The present invention advantageously solves the above-described problem. When resistance plate welding is performed on a plate assembly in which a steel plate and an aluminum plate are overlapped, a current that can be output by a welding transformer generally used in an automobile production line. Provided is a resistance spot welding method that can secure the joint characteristics of a steel plate and an aluminum plate in the range and the same production process as the joining of steel plates, and can ensure the stability of welding even at the time of electrode wear during continuous welding For the purpose.

  In order to solve the above-mentioned problems, the present inventors have studied the relationship between the electrode tip shape, the state of indentation (thinning) of the steel plate and aluminum plate after welding, and the joint strength. As a result, it is found that a sound weld can be obtained by appropriately selecting the electrode in contact with the steel plate and the electrode in contact with the aluminum plate and optimizing the amount of indentation (thickness reduction) of the weld after welding. did.

  The present invention has been completed through further studies based on the above findings, and the gist of the present invention is as follows.

[1] When performing resistance spot welding on a plate assembly in which a plurality of steel plates and aluminum plates are stacked and one of the plates arranged on the outermost side is a steel plate and the other is an aluminum plate,
The relationship between the tip diameter DFe of the electrode in contact with the steel plate and the tip diameter DAl of the electrode in contact with the aluminum plate is
DFe <DAl
The filling,
The resistance spot welding is characterized in that the tip shape of the electrode in contact with the aluminum plate is either a flat shape, a convex shape having a tip curvature radius of 170 mm or more, or a concave shape having a tip curvature radius of 10 mm or more. Method.

  [2] The tip shape of the electrode in contact with the aluminum plate is either a convex shape having a tip curvature radius of 170 mm or more and 1500 mm or less, or a concave shape having a tip curvature radius of 10 mm or more and 1000 mm or less. 1] The resistance spot welding method according to item 1.

[3] The relationship between the tip diameter DAl of the electrode in contact with the aluminum plate and the thickness tAl of the aluminum plate is
DAl ≧ 3tAl
The resistance spot welding method according to [1] or [2], wherein:

  [4] The above-described [1] to [1], wherein a welded portion having an aluminum plate indentation amount of 0.8 or more in thickness reduction ratio (plate thickness at the center of the joint / original plate thickness) is formed. The resistance spot welding method according to [3].

  In the present invention, when resistance spot welding is performed on a plate assembly in which a steel plate and an aluminum plate are overlapped, a current range that can be output by a welding transformer generally used in an automobile production line and the same as the joining of steel plates By the production process, the joint characteristics between the steel plate and the aluminum plate can be secured, and the welding stability can be secured even when the electrode is worn during continuous welding.

  That is, according to the present invention, in a general automobile assembly line in which steel plates are resistance spot welded together, the steel plate and the aluminum plate can be stably joined without causing an increase in cost due to the introduction of new equipment. Is possible.

It is a figure explaining the resistance spot welding method in one Embodiment of this invention. It is a figure explaining the tip diameter D and the tip curvature radius R of the electrode used in one Embodiment of this invention.

  The present invention will be described in detail by its embodiments.

  FIG. 1 is a diagram illustrating a resistance spot welding method according to an embodiment of the present invention. A plate assembly 3 in which a steel plate 1 having a plate thickness tFe and an aluminum plate 2 having a plate thickness tAl are directly overlapped is pressed between a pair of electrodes (upper electrode 4 and lower electrode 5) and pressed between the upper and lower electrodes 4 and 5. A current welding current is applied for a short time and joined by resistance heating to obtain a resistance spot welded joint between the steel plate 1 and the aluminum plate 2.

  When the resistance spot welded joint between the steel plate 1 and the aluminum plate 2 obtained as described above is deformed, the fracture occurs at the softer aluminum plate 2 side or at the joint interface between the steel plate 1 and the aluminum plate 2. For this reason, in order to ensure sufficient joint strength, it is necessary to suppress the thickness reduction of the aluminum plate 2 at the welded portion (joined portion) and to suppress the generation of fragile intermetallic compounds at the joining interface as much as possible. In particular, it is important to suppress the thickness reduction of the aluminum plate 2 at the weld. This is because the aluminum plate 2 is reduced in thickness even when a sufficient bonding area can be obtained while suppressing the formation of intermetallic compounds at the bonding interface and sufficient strength of the bonding interface can be secured. This is because the aluminum plate 2 is broken from the reduced thickness portion, and as a result, sufficient strength cannot be secured.

  Therefore, in this embodiment, the above-mentioned problem is solved by limiting the tip diameter and tip shape of the electrode as described below.

  That is, the relationship between the tip diameter DFe of the electrode (upper electrode) 4 in contact with the steel plate 1 and the tip diameter DAl of the electrode (lower electrode) 5 in contact with the aluminum plate 2 satisfies the condition DFe <DAl. By pressing and supporting in a wider area than the welded portion and sufficiently stably removing heat from the electrode (lower electrode 5) on the aluminum plate 2 side, the surface of the aluminum plate 2 and the extreme inside It becomes possible to suppress softening and to suppress the thickness reduction of the welded portion on the aluminum 2 side.

  Furthermore, the tip shape of the electrode 5 on the aluminum plate 2 side is a flat shape (tip radius of curvature R is infinite), or the tip radius of curvature R is 170 mm or more in the case of a convex shape, or in the case of a concave shape. The above-mentioned effect can be obtained stably by setting the radius of curvature R of the tip to 10 mm or more. At the same time, even when the electrode is worn, the energization on the aluminum plate 2 side at the time of welding, particularly at the initial stage of energization is joined. It can be made into a ring shape in a considerably wider range than the part, the thickness reduction of the aluminum plate 2 can be stably suppressed, and it becomes easy to secure a stable welded part.

  The above effect is achieved by making the tip shape of the electrode 5 on the aluminum plate 2 side either a convex shape with a tip curvature radius of 170 mm or more and 1500 mm or less, or a concave shape with a tip curvature radius of 10 mm or more and 1000 mm or less. , Even more. Thereby, stable welding is possible even in a state in which the straightness (the center axis of the electrode is orthogonal to the welding surface) is not necessarily obtained with high accuracy in an actual process such as an assembly line.

  This is due to the following reason. That is, when the tip shape of the electrode 5 on the aluminum plate 2 side is a convex shape with a tip radius of curvature exceeding 1500 mm, when the electrode 5 is new or just after dressing, the surface straightness accuracy during welding (weld surface and electrode) If the degree of orthogonality with respect to the central axis of the electrode 5 is not adjusted with high accuracy, the tip of the electrode 5 may hit the surface of the aluminum plate 2 and the expected effect may not be obtained. On the other hand, when the tip shape of the electrode 5 is a convex shape having a tip curvature radius of 170 mm or more and 1500 mm or less, the tip of the electrode 5 has a relatively sharp shape. It is possible to accurately suppress the one-piece contact that occurs in a state (when it is new or immediately after dressing). Preferably it is the range of 200 mm or more and 1000 mm or less. Further, when the tip shape of the electrode 5 is a concave shape with a tip curvature radius of 10 mm or more and 1000 mm or less, even if the tip of the electrode 5 hits the surface of the aluminum plate 2, the molten aluminum remains inside the electrode 5 ( The welding is stabilized because it is pushed to the center axis side). Preferably it is the range of 40 mm or more and 500 mm or less.

  As for the tip diameter D and the tip curvature radius R of the electrode, in FIG. 2, (a-1) and (a-2) are flat shapes (tip curvature radius R is infinite), (b- Examples where 1) and (b-2) are convex shapes and examples where (c-1) and (c-2) are concave shapes are shown (JIS C 9304), respectively. Incidentally, (c-1) is a concave shape in which the periphery of the concave portion is flat.

  Therefore, when the tip radius of curvature of the electrode in contact with the aluminum plate is RAl and the tip radius of curvature of the electrode in contact with the steel plate is RFe, the electrode shown in FIG. 2 is the tip diameter DFe of the electrode 4, and the tip radius of curvature R shown in FIG. 2 is the tip radius of curvature RFe of the electrode 4. Similarly, when the electrode shown in FIG. 2 is used as the electrode 5 in contact with the aluminum plate 2, the tip diameter D shown in FIG. 2 becomes the tip diameter DAl of the electrode 5, and is shown in FIG. The tip radius of curvature R is the tip radius of curvature RAl of the electrode 5.

  For this effect, the relationship between the tip diameter DAl of the electrode 5 in contact with the aluminum plate 2 and the thickness tAl of the aluminum plate 2 is DAl ≧ 3tAl, which is sufficient for the thickness of the aluminum plate 2. The strength can be obtained stably.

  When welding is performed in the above-described state, it is possible to form a welded portion in which the indentation amount on the aluminum plate 2 side at the joint center is 0.8 or more in thickness reduction ratio (plate thickness at the center of the joint / original plate thickness). This is possible and is extremely effective in securing joint strength.

  In this embodiment, a two-layer plate assembly (welded joint) of the steel plate 1 and the aluminum plate 2 has been described as an example, but one or more other steel plates or aluminum plates are arranged in any order between the two plates. It can also be applied to a plate assembly (welded joint) of three or more layers sandwiched between two.

  Moreover, as long as the provisions of the present invention are satisfied, the current value, energization time, and applied pressure during welding need not be constant, and there is no problem even if the current value and applied pressure are changed in two or more stages.

  Also, a control method may be used in which parameters such as resistance value and voltage value during welding are monitored, and the current value and energization time are changed according to the fluctuation.

  In addition, the steel plate made into the object by the joining method of this invention will not be specifically limited if it is steel plates, such as soft steel to high strength steel, stainless steel, and high alloy steel. Furthermore, Zn, Al, Mg and its alloys are hot-plated, alloyed hot-plated, electroplated on the surface, and the surface is further chromated, or a resin film is formed. Is also included. The same applies to the aluminum plate and is not particularly limited.

  The invention is explained in more detail by means of examples. The following examples are not intended to limit the present invention, and are all included in the technical scope of the present invention as long as the gist of the present invention is satisfied.

  As test materials, mild steel was used for the steel plate, and a 5000 series aluminum alloy plate was used for the aluminum plate. And it was set as the board assembly which piled up these directly. As the welding machine, an inverter DC resistance spot welding machine was used. The welding pressure was 2.5 kN, and welding was performed while changing the electrode shape, welding current, and energization time.

  After welding, cut the joint at the center of the weld and observe the cross section, measure the thickness at the center of the joint of the aluminum plate, and reduce the thickness from the ratio of the original plate thickness (plate thickness at the center of the joint / original plate thickness). ) The joint quality was judged by measuring the peel diameter (mm) by a peel test specified in JIS Z 3144. When peeling did not occur in a circular shape at the time of peeling, the equivalent circle diameter of the area remaining in the plug shape was determined as the peel diameter.

  As the evaluation criteria for joint characteristics, the thickness of the aluminum plate before welding is defined as the reference thickness t (mm), and the peel diameter is 4√t or more and the thickness reduction rate is 0.8 or more. A circle having a diameter of 4√t or more and a thickness reduction ratio of 0.7 or more and less than 0.8, and a peel diameter of less than 4√t or a thickness reduction ratio of less than 0.7. It was.

  Table 1 shows the results when the surface perpendicularity error is 0%, and Table 2 shows the results when the surface perpendicularity error is 5%. As shown in Tables 1 and 2, in the examples of the present invention, the evaluations were ○ to ◎ in all cases.

1 Steel plate 2 Aluminum plate 3 Board assembly 4 Electrode in contact with steel plate (upper electrode)
5 Electrode in contact with aluminum plate (lower electrode)

Claims (4)

A plurality of steel plates and aluminum plates are stacked, and resistance spot welding is performed on a plate assembly in which one of the outermost plates is a steel plate and the other is an aluminum plate (except for energization time of 200 msec or more during welding). On the occasion
The relationship between the tip diameter DFe of the electrode in contact with the steel plate and the tip diameter DAl of the electrode in contact with the aluminum plate is
DFe <DAl
The filling,
The resistance spot welding is characterized in that the tip shape of the electrode in contact with the aluminum plate is either a flat shape, a convex shape having a tip curvature radius of 170 mm or more, or a concave shape having a tip curvature radius of 10 mm or more. Method.   The tip shape of the electrode in contact with the aluminum plate is either a convex shape having a tip radius of curvature of 170 mm to 1500 mm, or a concave shape having a tip radius of curvature of 10 mm to 1000 mm. Resistance spot welding method. The relationship between the tip diameter DAl of the electrode in contact with the aluminum plate and the thickness tAl of the aluminum plate is
DAl ≧ 3tAl
The resistance spot welding method according to claim 1, wherein:   4. The welded portion according to claim 1, wherein an indentation amount of the aluminum plate in the center of the joint is formed by a thickness reduction ratio (plate thickness in the center of the joint / original plate thickness) of 0.8 or more. Resistance spot welding method.

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