JP6360056B2 - 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 automobile industries, the application of light metal materials such as aluminum alloy plates to the vehicle body has been promoted for the purpose of improving fuel efficiency by reducing the vehicle body weight.

  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

  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 joined generates resistance heat, and the steel and aluminum are resistance diffusion bonded. A resistance spot welding method is described in which a high strength joint is obtained.

  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.

JP 2004-17148 A JP-A-7-136774 JP-A-9-155561 JP 2005-152958 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.

  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. It is an object of the present invention to provide a resistance spot welding method that can secure joint characteristics between a steel plate and an aluminum plate by the same production process as that for joining steel plates.

  In order to solve the above problems, the present invention has the following features.

[1] One or two or more steel plates and one or two or more aluminum plates are directly stacked, and a plate assembly in which one of the outermost plates is a steel plate and the other is an aluminum plate is resistance spot welded. 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 resistance spot welding method characterized by satisfy | filling.

  [2] The resistance according to [1], wherein the tip diameter DFe of the electrode in contact with the steel plate is 4 mm or more and the tip diameter DAl of the electrode in contact with the aluminum plate is 3 mm or more and 16 mm or less. Spot welding method.

  [3] The resistance spot welding method according to [1] or [2], wherein the tip curvature radius RAl of the electrode in contact with the aluminum plate exceeds 40 mm.

[4] In each aluminum plate in contact with the steel plate, the diameter x1 at which the thickness t1 of the aluminum plate after welding is 20% or less of the plate thickness t0 before welding, and the joint diameter between the steel plate after welding and the aluminum plate The relationship with x2 is
x1 <0.8 × x2
The resistance spot welding method according to any one of [1] to [3], wherein:

[5] The relationship between the thickness t0 (mm) of the aluminum plate before welding and the joining diameter x2 (mm) of the steel plate after welding and the aluminum plate is
3√t0 <x2
The resistance spot welding method according to [4], wherein:

  [6] The resistance spot welding method according to any one of [1] to [5], wherein a current value during welding is 20 kA or less.

  In the present invention, joint characteristics between a steel plate and an aluminum plate can be ensured in the current range that can be output by a welding transformer generally used in an automobile production line, and by the same production process as the joining of steel plates. .

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. It is a figure explaining the method of evaluating the thickness reduction ratio of an aluminum plate. It is a figure explaining the method of calculating the thickness reduction rate of a fracture part.

  Embodiments of the present invention are described below.

  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 an aluminum plate 1 having a plate thickness t0 and a steel plate 2 having a plate thickness ts are directly superimposed is sandwiched between a pair of electrodes (upper electrode 4 and lower electrode 5) and pressed between the upper and lower electrodes 4 and 5. A high-current welding current is applied for a short time and joined by resistance heating to obtain a resistance spot welded joint between the aluminum plate 1 and the steel plate 2.

  In that case, what is important in securing the joint characteristics of the aluminum plate 1 and the steel plate 2 is to reduce the thickness of the aluminum plate 1 by reducing the molten thickness of aluminum while expanding the joining range. That is.

  Therefore, in this embodiment, when resistance plate welding is performed on a plate assembly (material to be welded) 3 in which the aluminum plate 1 and the steel plate 2 are overlapped, the tip diameter DFe of the electrode 5 in contact with the steel plate 2 and the aluminum plate 1 The tip diameter DAl of the electrode 4 in contact with the electrode 4 satisfies the relationship DFe> DAl.

  As a result, it is possible to suppress heat removal by the electrode 4 on the aluminum plate 1 side while ensuring a wide heat generation diameter on the steel plate 2 side, and to raise the temperature of a wide range of the steel / aluminum interface even at a low current. . In addition, since the heat generation diameter on the steel side is widened, the aluminum plate 1 side can be prevented from being excessively heated locally, so the molten thickness of aluminum is reduced. Therefore, there is an effect that the thickness reduction of the aluminum plate 1 due to pressurization is suppressed.

  Regarding the tip diameters of the electrodes 4 and 5, the tip diameter DFe of the electrode 5 in contact with the steel plate 2 is 4 mm or more, and the tip diameter DAl of the electrode 4 in contact with the aluminum plate 1 is 3 to 16 mm. The effect can be obtained effectively.

  When the tip diameter DFe of the electrode 5 is less than 4 mm, the heat generation diameter on the steel plate 2 side is insufficient. If the tip diameter DAl of the electrode 4 is less than 3 mm, the current density in the aluminum plate 1 is too large, which increases the risk of scattering. If the tip diameter DAl of the electrode 4 exceeds 16 mm, heat removal by the electrode 4 on the aluminum plate 1 side is performed. Therefore, the current value required for joining must be increased.

  Further, considering the balance between the heat generation diameter on the steel plate 2 side and the heat removal on the aluminum plate 1 side, the tip diameter DFe of the electrode 5 is preferably 6 mm or more, and the tip diameter DAl of the electrode 4 is preferably 4 mm or more and 14 mm or less. More preferably, it is 4.5 mm or more and 12 mm or less.

  Note that the upper limit of the tip diameter DFe of the electrode 5 is preferably 20 mm in consideration of handling properties, device restrictions, and the like. More preferably, it is 18 mm or less.

    Further, the relationship between the tip diameter DFe of the electrode 5 in contact with the steel plate 2 and the plate thickness tFe of the steel plate 2 satisfies 3tFe ≦ DFe, and the tip diameter DAl of the electrode 4 in contact with the aluminum plate 1 and the plate thickness tAl of the aluminum plate 1 More preferably, the relationship of 2tAl ≦ DAl ≦ 14tAl is satisfied. Incidentally, in FIG. 1, tAl = t0 and tFe = ts.

  Further, from the viewpoint of suppressing the thickness reduction of the aluminum plate 1, when the tip curvature radius RAl of the electrode 4 in contact with the aluminum plate 1 exceeds 40 mm, the above effect can be obtained effectively. If the tip curvature radius RAl of the electrode 4 is 40 mm or less, the stress applied to the region in contact with the vicinity of the center of the electrode 4 in the aluminum plate 1 becomes excessively large, and the thickness reduction of the aluminum plate 1 in this region becomes significant. It is.

  Further, from the viewpoint of preventing the current density in the aluminum plate 1 from becoming too large, the tip curvature radius RAl of the electrode 4 is preferably 50 mm or more. More preferably, it is 55 mm or more.

  Further, from the viewpoint of suppressing heat removal by the electrode 4 on the aluminum plate 1 side, the tip curvature radius RAl of the electrode 4 is preferably 400 mm or less.

  Further, in a plate assembly having a large current value required for obtaining a high-strength joint, it is more preferable that the tip curvature radius RAl of the electrode 4 is 200 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 are shown (JIS C 9304).

  That is, when the electrode shown in FIG. 2 is used as the electrode 4 in contact with the aluminum plate 1, the tip diameter D shown in FIG. 2 becomes the tip diameter DAl of the electrode 4 and is shown in FIG. The tip curvature radius R becomes the tip curvature radius RAl of the electrode 4. Similarly, when the electrode shown in FIG. 2 is used as the electrode 5 in contact with the steel plate 2, the tip diameter D shown in FIG. 2 becomes the tip diameter DFe of the electrode 5, which is shown in FIG. The tip curvature radius R becomes the tip curvature radius RFe of the electrode 5.

  In addition, about the thickness reduction of the aluminum plate 1, as shown in FIG. 3, the plate | board thickness t1 of the aluminum plate 1 after welding becomes 20% or less of the plate thickness t0 before welding, and the steel plate 2 after welding. As long as the relationship between the joining diameter (nugget diameter) x2 of the aluminum plate 1 satisfies x1 <0.8 × x2, there is no problem even if the thickness of the aluminum plate 1 is reduced.

  This is because, in a peel test and a joint strength test, the fracture of the joint occurs from the outer periphery of the welded portion, and thus the thickness reduction near the welded center has little effect on the joint characteristics.

  In a state where the peeling load is large, it is preferable to satisfy the relationship of x1 <0.6 × x2. Furthermore, it is more preferable to satisfy the relationship of x1 <0.5 × x2.

  In order to secure the joint strength, the relationship between the thickness t0 (mm) of the aluminum plate 1 before welding and the welded steel plate / aluminum plate joining diameter x2 (mm) is 3√t0 <x2. It is preferable to satisfy. Furthermore, it is more preferable to satisfy the relationship of 3.5√t0 <x2.

  In addition, about said diameter x1, when it is not a circle | round | yen, what is necessary is just to set it as the diameter x1 of the equivalent circle | round | yen with equal area. The same applies to the diameter x2.

  Further, by setting the current value during welding to 20 kA or less, it becomes possible to output with a general-purpose welding transformer, and the above-described effect can be specifically realized.

  As described above, in this embodiment, a two-layered 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. In that case, it is preferable that the above relationship is satisfied in all the aluminum plates in contact with the steel plate.

  Further, the current value, energization time, and applied pressure during welding need not be constant, and the current value and applied pressure may be changed in two or more stages.

  There is no problem even if a control method is used in which parameters such as resistance value and voltage value during welding are monitored and the current value and energization time are changed in accordance with the fluctuation.

  Further, the present invention can be applied regardless of the presence / absence and thickness of the surface of the steel plate and the aluminum plate, the composition and thickness of the oxide film, the strength of the base material, and the plate thickness.

  Examples of the present invention are shown below.

  As test materials, mild steel was used for the steel plate, and a 5000 series aluminum alloy plate was used for the aluminum plate. As the welding machine, an inverter DC resistance spot welding machine was used. The welding time was 40 msec, the applied pressure was 2.5 kN, and the electrode shape and current value were changed for welding.

  After welding, the joint is cut at the center of the welded portion to observe the cross section. The diameter x1 of the aluminum plate after welding is 20% or less of the plate thickness t0 before welding, and the steel plate / aluminum plate after welding. The joint diameter x2 was determined. Moreover, the peel diameter (mm) was calculated | required by the peel test prescribed | regulated to JISZ3144. For a joint that has been broken into a button shape in a peel test, as shown in FIG. 4, the thickness t2 of the aluminum plate at the fractured part is measured and divided by the base metal plate thickness (plate thickness before welding) t0. The thickness reduction ratio (%) of the part was obtained.

  As the evaluation criteria for joint characteristics, the plate thickness t0 before welding of the aluminum plate is defined as the reference plate thickness t (mm), the peel diameter is 4√t or more, the thickness reduction rate of the fracture portion is 30% or more, and the current value is 20 kA or less. ◎ for some, peel diameter of 3.5√t or more, thickness reduction rate of fractured portion 30% or more and current value 20 kA or less, ○, peel diameter 3.5√t or more and thickness reduction of fractured portion When the rate is 30% or more and the current value is higher than 20 kA, the peel diameter is 3.5√t or more and the thickness reduction rate of the fracture portion is less than 30%, or the peel diameter is less than 3.5√t Was marked with x. Table 1 shows the results.

  In the examples of the present invention, the evaluation was ○ to ◎ in all cases.

DESCRIPTION OF SYMBOLS 1 Aluminum plate 2 Steel plate 3 Board assembly 4 Electrode which contacts an aluminum plate (upper electrode)
5 Electrode in contact with steel plate (lower electrode)

Claims (5)

When one or more of the steel plates and one or more of the aluminum plates are directly stacked, one of the plates arranged on the outermost side is a steel plate and the other is an aluminum plate, and when resistance spot welding is performed,
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 tip diameter DFe of the electrode in contact with the steel plate is 4 mm or more, and the tip diameter DAl of the electrode in contact with the aluminum plate is 9 mm or more and 16 mm or less,
Further, the resistance spot welding method, wherein the tip radius of curvature RAl of the electrode in contact with the aluminum plate exceeds 40 mm.   2. The resistance spot welding method according to claim 1, wherein the tip curvature radius RAl of the electrode in contact with the aluminum plate is 45 mm or more. In each aluminum plate in contact with the steel plate, a diameter x1 at which the thickness t1 of the aluminum plate after welding is 20% or less of the plate thickness t0 before welding, and a joining diameter x2 between the steel plate after welding and the aluminum plate Relationship
x1 <0.8 × x2
The resistance spot welding method according to claim 1, wherein: The relationship between the plate thickness t0 (mm) before welding of the aluminum plate and the joint diameter x2 (mm) between the steel plate after welding and the aluminum plate is
3√t0 <x2
The resistance spot welding method according to claim 3, wherein:   The resistance spot welding method according to claim 1, wherein a current value during welding is 20 kA or less.

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