JP2017060994A - Resistance spot weld joint, resistance spot welding method, and manufacturing method for resistance spot weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure an excellent joint characteristic between a steel plate and an aluminum plate, in a current range outputtable by a welding apparatus used generally in an automobile production line, and in the same production process as joint between each steel plate.SOLUTION: In a resistance spot weld joint, which is a joint obtained by performing resistance spot welding of a plate set formed by piling a steel plate and an aluminum plate, the area ratio of a blow hole is 50% or less in the range of 1 mm to the inside from the outer periphery of nugget of the aluminum plate on a mating surface between the steel plate and the aluminum plate.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a resistance spot welded joint, a resistance spot welding method, and a method of manufacturing a resistance spot welded joint.

  In recent automobile industries, the application of light metals such as aluminum alloys 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 commonly used for joining steel plates in the car body, and the number of hits per car is 3000 to 6000. To the point. 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 the joining in the case where aluminum plates are mixed, as in the case of steel plates. In the following description, the aluminum plate means a generic name of a pure aluminum plate and an aluminum alloy plate. However, in the joining of dissimilar materials of steel and aluminum, there is a problem that the strength of the joint cannot be secured because the soft aluminum plate is greatly reduced in thickness by pressing the electrode or a brittle intermetallic compound is formed at the joint interface There is.

  In order to solve the above problems, the following techniques have been proposed. For example, in Patent Document 1, a very short time is required for welding, and a high melting point is formed by applying a high current, and at the same time, a high pressure is applied to disperse the molten portion to the surroundings to obtain a clean metal. A resistance spot welding method is described that achieves bonding by plane contact and atomic diffusion.

  Patent Document 2 describes a resistance spot welding method in which a high-strength joint can be obtained even at a low current by inserting an iron / aluminum clad thin plate between a steel plate and an aluminum plate so that the same kind of materials face each other.

  In Patent Document 3, 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. A resistance spot welding method is described that provides a high strength joint.

  In Patent Document 4, when spot welding a steel material and an aluminum material, it is possible to suppress the occurrence of scattering while obtaining a large nugget diameter by optimizing the amounts of Mn and Si in the steel plate and the steel plate surface oxide film. It is described.

Patent No. 394001 Japanese Patent No. 3117053 Japanese Patent No. 3504790 JP 2005-152958 A

  However, in the resistance spot welding method described in Patent Document 1, it is necessary to apply a large current as compared with a resistance spot welding method between general steel plates, so that a welding transformer used in an existing automobile production line is used. However, there is a problem in that the current value is insufficient with the power supply 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. The application of the invention is extremely limited under the circumstances where the alloying with the strengthening is progressing.

  The present invention advantageously solves the above-mentioned problem, in a current range that can be output by a welding apparatus generally used in an automobile production line, and by the same production process as the joining of steel plates, An object of the present invention is to provide a resistance spot welded joint, a resistance spot welded method, and a method of manufacturing a resistance spot welded joint that can ensure good joint characteristics of an aluminum plate.

In order to achieve the above object, the present invention has the following features.
[1] A joint obtained by resistance spot welding of a plate assembly in which a steel plate and an aluminum plate are overlapped,
A resistance spot welded joint with a blowhole area ratio of 50% or less in the range from the outer periphery to the inner side of the aluminum plate nugget on the mating surface of the steel plate and aluminum plate.
[2] The aluminum plate is disposed on the outermost side of the stacked plate assembly,
When the thickness of the aluminum plate arranged on the outermost side before joining is t0 and the thickness after joining is t,
In the range from the outer periphery of the aluminum plate nugget on the mating surface of the steel plate and the aluminum plate to 1 mm inward, t / t0 is 0.6 or more,
The resistance spot welded joint according to [1], wherein t / t0 is 0.5 or more inside the range.
[3] A resistance spot welding method for producing the resistance spot welded joint of [1] or [2],
Stack the steel plate and the aluminum plate so that the aluminum plate is the outermost side of the stacked plate assembly,
A resistance spot welding method in which an electrode having a curved surface with a radius of curvature exceeding 40 mm is brought into contact with the outermost aluminum plate, the plate assembly is sandwiched between the electrodes and pressed, and the electrodes are energized for welding.
[4] The resistance spot welding method according to [3], wherein an electrode having a recess at a tip is brought into contact with a steel plate arranged on the outermost side, and a current is applied between the electrodes while sandwiching and pressing the plate assembly.
[5] A method of manufacturing a resistance spot welded joint using the resistance spot welding method according to [3] or [4].

  According to the present invention, it is possible to ensure good joint characteristics between a steel plate and an aluminum plate in a current range that can be output by a welding apparatus generally used in an automobile production line, and in the same production process as the joining of steel plates. can do.

It is a figure which shows the resistance spot welding method which concerns on embodiment of this invention. It is a figure which shows the resistance spot welding joint which concerns on embodiment of this invention. It is a figure which shows the resistance spot welding joint which concerns on embodiment of this invention. It is a figure which shows the resistance spot welding joint which concerns on embodiment of this invention. It is sectional drawing and the top view which show the example of an electrode used with the resistance spot welding method of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a diagram showing a resistance spot welding method according to an embodiment of the present invention. Moreover, FIG. 2 is a figure which shows the resistance spot welded joint which concerns on embodiment of this invention. In the present invention, the upper and lower electrodes are pressed while sandwiching and pressing 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 overlapped by a pair of electrodes (upper electrode 4 and lower electrode 5). Between 4 and 5, a welding current in a current range (20 kA or less) that can be output by a commonly used welding apparatus is energized for a short time and joined by resistance heating, and resistance between the aluminum plate 1 and the steel plate 2 A spot welded joint is obtained.

  In this case, in order to ensure good joint characteristics (joint strength) between the aluminum plate 1 and the steel plate 2, it occurs at the time of welding at the joint interface between the aluminum plate 1 and the steel plate 2 (a mating surface between the aluminum plate 1 and the steel plate 2). It is important to reduce the area ratio of the pores (blow holes) of the aluminum plate to be made. By reducing the area ratio of the blowhole, it is possible to prevent a crack generated at the joint interface from being propagated when stress is applied to the joint, so that a resistance spot welded joint with high joint strength is obtained.

  Therefore, in the present invention, the range (hereinafter also referred to as “outer peripheral portion”) of the aluminum plate 1 and the steel plate 2 from the outer periphery to the inner side of the nugget 6 of the aluminum plate 1 on the mating surface (hereinafter also referred to as “mating surface”). )), That is, the area ratio of the blowhole existing in the outer peripheral portion of the mating surface with respect to the outer peripheral portion of the mating surface is defined as 50% or less. The reason why the blowhole area ratio in the range from the outer periphery to the inner side of the nugget 6 of the aluminum plate 1 to 1 mm is limited to 50% or less is generally that in the resistance spot welded joint, the outer periphery of the nugget is most likely to concentrate stress. This is because if there are many blow holes in the outer peripheral portion of the nugget, cracks are likely to propagate and the joint is liable to break. The “nugget” is a melted and solidified portion generated in a welded portion in lap resistance welding. In this specification, a melted portion that becomes a nugget when solidified (that is, a melted portion before solidifying) is also called a nugget. There is a case.

  In order to reduce the stress concentration, it is important to secure a bonding area by setting the blowhole area ratio to 50% or less at the outer peripheral portion of the nugget 6 of the aluminum plate 1 at the mating surface of the steel plate 2 and the aluminum plate 1. When the required joint strength is high, the blow hole area ratio is preferably 40% or less. The area ratio of the blowhole can be calculated by observing a cross section in the plate thickness direction of the welded joint. The weld joint is peeled off at the interface (mating surface), the surface of the aluminum plate 1 on the side bonded to the steel plate 2 is observed, and the total area of the blowhole area at the outer peripheral portion is divided by the area of the outer peripheral portion. The area ratio calculated from the above is the same value as the blowhole area ratio calculated by the cross-sectional observation in the plate thickness direction.

  The current range (for example, 20 kA or less) that can be output by a welding device commonly used in automobile production lines, and resistance plate welding of steel plates and aluminum plates in the same production process as joining of steel plates Even when welding, as specified by the present invention, if the area ratio of the blowhole in the range from the outer periphery of the nugget of the aluminum plate to the inside 1 mm at the mating surface of the steel plate and the aluminum plate is 50% or less Good joint characteristics between the steel plate and the aluminum plate can be ensured.

  Furthermore, in order to obtain a joint with high strength, it is preferable to relieve the stress at the outer periphery of the nugget where stress tends to concentrate. In order to reduce the stress concentration, as shown in FIG. 2, the thickness of the aluminum plate 1 before joining (before resistance spot welding) is t0, and the thickness after joining (after resistance spot welding) (that is, in the welded joint). When the thickness of the aluminum plate 1 is t, t / t0 is 0.6 or more in the range from the outer periphery of the nugget 6 of the aluminum plate 1 to 1 mm inward on the mating surface. T / t0 is effectively 0.5 or more. Note that t / t0 in the range from the outer periphery of the nugget 6 to 1 mm inward is equal to or greater than t / t0 inside this range.

  In the range from the outer periphery of the nugget 6 of the aluminum plate 1 on the mating surface to 1 mm inward, t / t0 is partly less than 0.6, or partly inside this part, t / t0 is less than 0.5. Then, even if the bonding interface is well bonded, the thinned aluminum plate 1 is likely to be broken at the base material portion, that is, the aluminum plate 1 itself is easily broken, and the joint strength is good. Is difficult to obtain. When the required joint strength is high, it is preferable that t / t0 is 0.7 or more in the range from the outer periphery of the nugget 6 of the aluminum plate 1 on the mating surface to 1 mm inward.

  The nugget 7 of the steel plate 2 may be formed at the interface with the aluminum plate 1 as shown in FIG. 3 depending on the plate assembly and welding conditions, or at a position not in contact with the aluminum plate 1 as shown in FIG. Sometimes formed. Moreover, the nugget of the steel plate 2 may not be formed as shown in FIG. 3 and 4 are diagrams showing resistance spot welded joints according to embodiments of the present invention, respectively.

  Next, a resistance spot welding method for manufacturing the resistance spot welding joint configured as described above will be described with reference to FIGS.

  First, the steel plate 2 and the aluminum plate 1 are overlapped to form a plate assembly 3. Then, the electrode 4 is brought into contact with the aluminum plate 1, the electrode 5 is brought into contact with the steel plate 2, and the plate assembly 3 is sandwiched between the electrodes 4 and 5, and the electrodes are energized. Resistive heat is generated by energization, a part of the welded portion of the aluminum plate 1 is melted to form a molten portion, and the molten portion is solidified to form a nugget 6 and the aluminum plate 1 and the steel plate 2 are joined. Resistance spot welded joints can be manufactured.

  The electrode 4 to be brought into contact with the aluminum plate 1 is an electrode having a curved surface whose tip exceeds a curvature radius of 40 mm. When the radius of curvature of the tip of the electrode 4 to be in contact with the aluminum plate 1 is 40 mm or less, the area where the electrode 4 and the aluminum plate 1 are in contact with each other is narrowed, and the stress applied to the nugget center is increased. The generated blow hole is easily pushed out to the outer periphery of the nugget. As a result, there is a possibility that the area ratio of blowholes on the outer periphery of the nugget increases. Therefore, it is assumed that the radius of curvature of the tip of the electrode 4 in contact with the aluminum plate 1 exceeds 40 mm. The type of the tip of the electrode 4 is, for example, DR type (dome radius type), R type (radius type), and D type (dome type) described in JIS C 9304: 1999. Note that the DR-shaped electrode has a curved surface on the tip side with a two-step curvature, but the radius of curvature defined as exceeding 40 mm is the radius of curvature at the tip, so the portion that first contacts the plate to be resistance spot welded (center Radius of curvature).

  Similarly, if the area where the electrode 4 and the aluminum plate 1 are in contact with each other is small, the temperature rise during welding and the thickness of the aluminum plate 1 due to pressurization tend to occur. In order to ensure the joint strength, it is necessary to obtain a large nugget diameter. In such a case, it is preferable that the radius of curvature of the tip of the electrode 4 in contact with the aluminum plate 1 is 50 mm or more.

  The electrode 5 brought into contact with the steel plate 2 is preferably an electrode having a concave portion at the tip, and for example, an electrode having a cylindrical hole (concave) at the tip as shown in FIG. FIG. 5 is a cross-sectional view (FIG. 5A) and a top view (FIG. 5B) showing an example of an electrode used in the resistance spot welding method of the present invention. As shown in FIG. 5, the electrode 10 has a cylindrical recess 11 at the center of the tip that is brought into contact with the steel plate 2. Since the pressure applied to the nugget center is reduced by using the electrode 10 having the concave portion at the tip as the electrode 5 to be brought into contact with the steel plate 2, the pores generated at the nugget center portion are suppressed from being pushed out to the outside. The effect of preventing blowholes from occurring in the outer periphery of the nugget that is formed is obtained. Although there is no restriction | limiting in the dimension of the recessed part 11 of the electrode 10, in order to acquire the said effect effectively, even when the electrode 10 is worn out by a continuous hitting point, the diameter of the recessed part 11 (when the recessed part is cylindrical is shown in FIG. X) is preferably 1 mm or more and 10 mm or less, and the depth of the recess (Y shown in FIG. 5 when the recess is cylindrical) is preferably 2.5 mm or more. As the electrode 5, an electrode having no recess at the tip can be used. That is, as the type of the tip of the electrode 5, for example, there is no concave portion such as DR type (dome radius type), R type (radius type), D type (dome type) described in JIS C 9304: 1999. Although a thing may be sufficient, what the recessed part was formed in the front-end | tip of this DR type (dome radius type), R type (radius type), and D type (dome shape) is preferable. The DR-shaped electrode has a curved surface on the tip side with a two-step curvature. However, when the concave portion is formed at the tip, the concave portion is in a portion (center-side curved surface) that first comes into contact with the plate to be resistance spot welded. Provide. Moreover, there is no restriction | limiting in particular about the curvature radius of the front-end | tip of the electrode 5 which contacts the steel plate 2. As shown in FIG.

  There is no problem even if the electrode 4 in contact with the aluminum plate 1 is formed with the same recess, but the diameter of the recess of the electrode 4 is 2 mm or less in order to prevent the aluminum plate 1 from scattering from the plate surface due to thermal expansion. It is desirable to do.

  The tip diameters of the electrode 4 and the electrode 5 are not particularly limited, but the tip diameter is preferably 4 mm or more and 12 mm or less.

  As mentioned above, in this embodiment, although the welded joint which welded the plate | board assembly 3 of the 2 sheets pile of the aluminum plate 1 and the steel plate 2 was demonstrated as an example, this invention is further one or more sheets between the two plates. The present invention can also be applied to a welded joint of three or more stacked plates sandwiching steel plates or aluminum plates. In that case, it is preferable that the above relationship is established in all the aluminum plates in contact with the steel plate.

  In order to reduce the stress concentration on the outer periphery of the nugget in a welded joint consisting of three or more stacked plates, the steel plate and the aluminum plate are overlapped so that the aluminum plate is on the outermost side of the stacked plate assembly. When the thickness of the outermost aluminum plate before joining is t0, and the thickness after joining is t, t / t0 is 0.6 or more in the range from the outer periphery of the nugget on the mating surface to 1 mm inward, In addition, t / t0 may be 0.5 or more inside this range.

  In addition, there are no particular restrictions on the current value, energization time, and applied pressure during welding, but it is preferable that the current value be 10 kA to 20 kA, the energization time be 20 ms to 100 ms, and the applied pressure be 1 kN to 5 kN. is there. Further, the current value and the applied pressure may be changed in two or more stages during welding.

  Further, 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 according to 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. Note that the plate assembly, welding conditions, and electrode shape used in this example are examples applied to show the effects of the present invention, and it goes without saying that other conditions may be used.

  As test materials, a mild steel plate as the steel plate 2 and a 5000 series aluminum alloy plate as the aluminum plate 1 were superposed and resistance spot welded to produce a welded joint. As the welding machine, an inverter DC resistance spot welding machine was used, and welding was performed by changing the tip curvature radius of the electrode 4 in contact with the aluminum alloy plate and the welding conditions. However, the current was 20 kA or less. Table 1 shows the thickness of the mild steel plate and the aluminum alloy plate used, the radius of curvature of the tip of the electrode 4 in contact with the aluminum alloy plate, and the welding conditions. The electrode 4 in contact with the aluminum alloy plate was an electrode made of chromium copper, DR type, tip diameter 8 mm (no recess). As the electrode 5 in contact with the mild steel plate, an electrode made of chrome copper, DR type, tip diameter 8 mm, tip curvature radius 40 mm was used. In addition, in the electrode 5 in contact with the mild steel plate, a cylindrical recess having a diameter of 4 mm and a depth of 5 mm was added to the center of the tip as in the electrode shown in FIG.

  The area ratio of the blowhole and the thickness reduction rate of the aluminum plate 1 (t) in the range from the outer periphery to the inner side of the nugget of the aluminum alloy plate at the mating surface of the mild steel plate (steel plate 2) and the aluminum alloy plate (aluminum plate 1) Various weld joints with different / t0) were produced. The area ratio of the blowhole in the range from the outer periphery to the inner side of the nugget of the aluminum alloy plate at the mating surface of the mild steel plate (steel plate 2) and the aluminum alloy plate (aluminum plate 1) is measured with an optical microscope (100 times magnification). Calculated from cross-sectional observation of the welded joint. Specifically, a cross section in the plate thickness direction of the welded joint and having the longest length on the mating surface of the nugget formed on the aluminum alloy plate was observed with an optical microscope. In the cross section, the total value of the lengths of the blow holes observed at the outer edge (the range from the outer periphery of the aluminum alloy plate nugget to the inner side of 1 mm on the mating surface of the aluminum alloy plate and the mild steel plate) is obtained, and the blow hole Was divided by the total value of the length of the outer edge (ie, 2 mm). Further, the thickness t of the aluminum plate 1 after joining was obtained from the cross section used when obtaining the area ratio of the blowhole.

  After welding, the obtained welded joint was subjected to a tensile shear test to evaluate the joint strength. The tensile shear test method is based on JIS Z 3136: 1999. As the evaluation of the joint strength, the case where the tensile shear strength was 2.5 kN or more was evaluated as ○, and the case where it was less than 2.5 kN was evaluated as ×. The results are shown in Table 1. In the examples of the present invention, the evaluation was good in all cases.

DESCRIPTION OF SYMBOLS 1 Aluminum plate 2 Steel plate 3 Board assembly 4 Upper electrode 5 Lower electrode 6 Aluminum plate nugget 7 Steel plate nugget 10 Electrode 11 Recess X Recess diameter Y Recess depth

Claims (5)

It is a joint in which a plate assembly in which a steel plate and an aluminum plate are superposed is resistance spot welded,
A resistance spot welded joint with a blowhole area ratio of 50% or less in the range from the outer periphery to the inner side of the aluminum plate nugget on the mating surface of the steel plate and aluminum plate. The aluminum plate is placed on the outermost side of the stacked plate assembly,
When the thickness of the aluminum plate arranged on the outermost side before joining is t0 and the thickness after joining is t,
In the range from the outer periphery of the aluminum plate nugget on the mating surface of the steel plate and the aluminum plate to 1 mm inward, t / t0 is 0.6 or more,
The resistance spot welded joint according to claim 1, wherein t / t0 is 0.5 or more inside the range. A resistance spot welding method for producing the resistance spot welded joint according to claim 1 or 2,
Stack the steel plate and the aluminum plate so that the aluminum plate is the outermost side of the stacked plate assembly,
A resistance spot welding method in which an electrode having a curved surface with a radius of curvature exceeding 40 mm is brought into contact with the outermost aluminum plate, the plate assembly is sandwiched between the electrodes and pressed, and the electrodes are energized for welding.   The resistance spot welding method according to claim 3, wherein an electrode having a recess at the tip is brought into contact with a steel plate disposed on the outermost side, and the electrodes are energized while being pressed by sandwiching the plate assembly.   The manufacturing method of the resistance spot welding joint using the resistance spot welding method of Claim 3 or 4.

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