KR102276817B1 - Method of Manufacturing Resistance Spot Welded Joints - Google Patents

Abstract

As a resistance spot welded joint of overlapping three sheets of two steel plates and one aluminum plate, it is possible to manufacture a resistance spot welded joint with good joint strength regardless of the composition or plate structure of the steel or aluminum plate. A manufacturing method is provided. In manufacturing a resistance spot welded joint by bonding two steel plates and one aluminum plate to each other, one of the plates disposed on the outermost side is a steel plate and the other is an aluminum plate, joined by resistance spot welding , a first energizing step of forming a nugget having _{a nugget diameter of 2√t Fe} (mm) or more between the two steel sheets by energizing when the thickness of the thinner steel sheet among the two steel sheets is t _{Fe (mm);} , a energization stop step of stopping energization after the first energization step, and a second energization step of energization after the energization stop step.

Description

Method of Manufacturing Resistance Spot Welded Joints

The present invention relates to a method of manufacturing a resistance spot weld joint of dissimilar metallic material. Specifically, manufacturing a resistance spot welded joint in which a sheet set in which two steel plates and one aluminum plate are stacked on each other is joined by resistance spot welding to produce a resistance spot welded joint it's about how

In the recent automobile industry, for the purpose of improving fuel efficiency by reducing the weight of the vehicle body, the application of light metals such as aluminum alloy to the vehicle body is progressing. Currently, the resistance spot welding method, which is superior to other welding methods in terms of cost and efficiency, is most used for joining steel sheets in a vehicle body, and the number of RBI points per vehicle reaches 3000 to 6000 points. In the resistance spot welding method, two or more overlapping steel sheets are sandwiched by a pair of electrodes, and a high-current welding current is passed between the upper and lower electrodes for a short time while pressurizing the sandwiched steel sheets from the top and bottom to the pair of electrodes. It is a method of joining 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 in the joining in the case where aluminum plates are mixed, as in the case of joining steel plates to each other. In addition, in the following description, an aluminum plate is a generic name of a pure aluminum plate and an aluminum alloy plate. However, in the bonding of dissimilar metal materials between steel and aluminum, the thickness of the soft aluminum plate is greatly reduced by the pressure of the electrode, or a weak intermetallic compound is formed at the bonding interface, so that the joint strength cannot be secured. there is In particular, in a three-ply stacking plate in which two steel plates and one aluminum plate are stacked on each other, it is necessary to obtain a desired joint diameter (nugget diameter) between the steel plate and the aluminum plate and between the steel plate and the steel plate. , it becomes more difficult to secure the strength of the joint.

In order to solve the said subject, the resistance spot welding method as described below is proposed. For example, Patent Document 1 describes a resistance spot welding method in which a high-strength joint is obtained even at low current by inserting an iron/aluminum clad sheet between a steel plate and an aluminum plate so that the same materials face each other. has been

In Patent Document 2, by adding one or more backing plates to both sides of the steel plate and the aluminum plate and welding, the interface between the backing plate and the material to be joined generates resistance heat, and the steel and aluminum are resistance diffusion bonded to obtain a high-strength joint. A method of spot welding is described.

Patent Document 3 states that in spot welding steel and aluminum materials, by optimizing the respective amounts of Mn and Si in the oxide film on the surface of the steel sheet and the steel sheet, the occurrence of expulsion can be suppressed while obtaining a large nugget diameter. is described.

Patent Document 4 describes a dissimilar metal bonding method in which growth of an intermetallic compound at a bonding interface is suppressed by optimizing the pulsation energization conditions and increasing the pressing force after energization is completed.

In Patent Document 5, by optimizing the pre-energization and subsequent energization conditions, the occurrence of blowout from the surface of the steel sheet can be suppressed, and the welding current can be made as small as possible, and a dissimilar material having high bonding strength A method of spot welding in which a joint is obtained is described.

Japanese Patent Publication No. 3117053 Japanese Patent Publication No. 3504790 Japanese Laid-Open Patent Publication No. 2005-152958 Japanese Patent Publication No. 5624901 Japanese Patent Publication No. 5572046

However, in the resistance spot welding method described in Patent Documents 1 and 2, since it is necessary to use unnecessary backing plates or thin clad plates in view of the structure of the vehicle body, there are problems such as a significant increase in cost and insufficient weight reduction.

Moreover, in patent document 3, since it is necessary to limit the amount and distribution of alloy elements in a steel plate and an oxide film, there exists a subject, such as use of the steel plate which satisfy|fills required performance being restrict|limited. In particular, the application of the invention of Patent Document 3 is very limited under the circumstances in which the high alloying accompanying the strengthening of the steel sheet in recent years is advancing.

In Patent Document 4, the energization time for pre-energization is 20 ms or less, and the energization time for pulsation energization is 10 ms or less, both for a short time. do. Therefore, when the specific resistance of a steel plate is high, or when a plate|board thickness is large, there exists a possibility of fluttering in the steel plate surface.

In Patent Document 5, there is a problem that the applicable sheet sheet is limited only to the sheet sheet of a cold rolled steel sheet and a 6000 series aluminum alloy sheet. Further, in Patent Document 5, it is necessary to conduct energization under conditions that do not melt the aluminum alloy plate. However, since the aluminum alloy plate has a lower melting point compared to the steel plate, the range of suitable conditions for energization is very narrow depending on the plate set. There is also the problem of

Incidentally, all of these documents are intended to solve the problems in the stacking of two steel plates and aluminum plates, and in the stacking of three stacks of two steel plates and one aluminum plate overlapping each other, between the steel plate and the aluminum plate. In addition, there is no description of a method for obtaining a desired nugget diameter between the steel plate and the steel plate.

The present invention has been made in view of the above circumstances, and is a resistance spot weld joint of overlapping three sheets of two steel sheets and one aluminum sheet, which has good joint strength regardless of the composition or plate structure of the steel sheet or aluminum sheet. An object of the present invention is to provide a method for manufacturing a resistance spot welded joint capable of manufacturing a resistance spot welded joint.

MEANS TO SOLVE THE PROBLEM In order to achieve said objective, the present inventors acquired the following recognition as a result of repeating earnest examination. 1 is a diagram schematically showing resistance spot welding of the present invention, in which two steel plates (middle plate) 11 and a steel plate (lower plate) 12 and an aluminum plate (upper plate) 13 are superimposed on each other, It is a view to be fitted with a pair of electrodes (14, 15). In addition, FIG. 2 is a figure which shows schematically the electric current distribution at the beginning of energization of resistance spot welding. In Fig. 2, the current at the initial stage of energization when a plate set in which two steel plates 11 and 12 and an aluminum plate 13 are overlapped with each other is sandwiched by a pair of electrodes 14 and 15 and energized while being pressed (welding). current) is indicated by reference numeral 21 in FIG. 2 .

In order to ensure good joint strength in the resistance spot welding of a three-ply stack of two steel plates and one aluminum plate, the nugget diameter between steel plates and steel plates or between steel plates and aluminum plates, respectively, as shown in FIG. It is effective to obtain a large nugget (the nugget 16 between a steel plate and a steel plate, and the nugget 17 between a steel plate and an aluminum plate). In addition, since excessive heat input causes a decrease in joint strength due to growth of a fragile intermetallic compound at the bonding interface between the steel sheet and the aluminum sheet (the abutment surface of the steel sheet and the aluminum sheet), the reduction in heat input must not strive

In general, shortening the energization pattern and increasing the current to a higher current is effective for inhibiting the growth of this intermetallic compound. However, since a strong oxide film exists on the surface of the aluminum plate, the conduction path is limited, and as shown in FIG. The current tends to concentrate on the center 23 of the junction where the oxide film is destroyed by pressurization. Therefore, excessively shortening the time and increasing the current increases and promotes heat input between the steel sheet and the aluminum sheet toward the center of the joint (nugget). Furthermore, due to excessively shortening the time and increasing the current, the occurrence of blown between the steel sheet and the steel sheet is also likely to occur, so that it is difficult to secure the nugget diameter between the steel sheet and the steel sheet. In addition, in comparison with the stacking of two sheets using one steel sheet and one aluminum plate, the total thickness of the steel sheet is large in the stacking of three sheets using two steel sheets and one aluminum plate. For this reason, since heat removal to an electrode becomes inadequate, it becomes easy to generate|occur|produce not only between a steel plate - a steel plate, but also the generation|occurrence|production of the blowout at the steel plate surface. From the above reasons, there is a limit to shortening the energization time and increasing the current.

Based on these recognition, this invention is completed by adding examination further, The summary is as follows.

[1] Two steel plates and one aluminum plate are stacked on top of each other to form a resistance spot welded joint by joining a plate set in which one of the plates is a steel plate and the other is made of an aluminum plate. In manufacturing,

A first energizing step of forming a nugget having _{a nugget diameter of 2√t Fe} (mm) or more between the two steel plates by energizing when the thickness of the thinner one among the two steel plates is t _{Fe (mm);}

an energization stop step of stopping energization after the first energization step;

A method for manufacturing a resistance spot welded joint having a second energization step of energizing after the energization stop step.

[2] Let the radius of curvature of the tip of the electrode in contact with the steel plate be R _Fe (mm), the current in the first energization step is I ₁ (kA), and the energization time is t ₁ (ms), and the energization is stopped. When the energization pause time of the process is t _c (ms), the current of the second energization process is I ₂ (kA), and the energization time is t ₂ (ms),

The method for manufacturing a resistance spot welded joint according to [1], wherein the first energization step, the energization stop step, and the second energization step satisfy all of the relationships of the following formulas (1) to (5).

I ₁ ＜ I ₂ (1)

t ₁ > t ₂ (2)

t ₁ ≥40 (3)

t _c ≥5 (4)

R _Fe ≥20 (5)

[3] When the radius of curvature of the tip of the electrode brought into contact with the steel plate is R _Fe (mm) and the diameter of the tip is D _Fe (mm),

The resistance spot weld joint according to [1] or [2], wherein the first energization step, the energization stop step, and the second energization step further satisfy the relation of the following formulas (6) and (7). manufacturing method.

R _Fe ≥50 (6)

D _Fe ≥3 (7)

According to the present invention, as a resistance spot welded joint of overlapping three sheets of two steel plates and one aluminum plate, it is possible to manufacture a resistance spot welded joint having good joint strength regardless of the composition or plate composition of the steel plate or aluminum plate. .

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematically resistance spot welding of this invention.
Fig. 2 is a diagram schematically showing the current distribution at the initial stage of energization in resistance spot welding.
3 : is a figure explaining an electricity supply pattern.
Fig. 4 is a diagram showing a tip radius of curvature and a tip diameter of an electrode.
5 is a diagram for explaining a tensile test (tensile shear test) of an Example of the present invention.
Fig. 6 is a view for explaining a tensile test (cross tensile test) of an embodiment of the present invention.

(Form for implementing the invention)

In the method for manufacturing a resistance spot welded joint of the present invention, two steel plates and one aluminum plate are superimposed on each other, and one of the plates disposed at the outermost side is a steel plate and the other is an aluminum plate, and the plate set is subjected to resistance spot welding. In the production of a resistance spot welded joint by joining by means of an _{electric current, when the thickness of the thinner steel sheet among the two steel sheets is t Fe} (mm), the nugget diameter between the two steel sheets is 2√t _Fe ( mm) or larger, a first energization step of forming a nugget, a energization stop step of stopping energization after the first energization step, and a second energization step of energization after the energization stop step. In the present invention, the resistance spot welded joint is a generic term including test pieces used for strength testing, cross-sectional observation, and the like, and automobile members joined by resistance spot welding.

The present invention will be specifically described below with reference to Figs. The present invention provides a resistance spot weld joint by resistance spot welding, in which a plate set in which two steel plates and one aluminum plate are stacked on each other is sandwiched by a pair of electrodes and joined (welded joint) by energizing while pressing. is a method for producing a resistance spot welded joint.

First, the steel plate 11, the steel plate 12, and the aluminum plate 13 are superimposed on each other, and it is set as the plate structure. At this time, as shown in FIG. 1, one side of the board arrange|positioned on the outermost becomes the steel plate 12, and it is made to become the aluminum plate 13 on the other side. In other words, the plates for contacting the electrodes 14 and 15 overlap each other so as to become the steel plate 12 and the aluminum plate 13, respectively.

The steel sheets 11 and 12 are not particularly limited, and examples thereof include a cold rolled steel sheet, a hot rolled steel sheet, and a plated steel sheet. A plated steel sheet is a steel sheet having a metal plating layer on the surface, and examples of the metal plating layer include a Zn-based plating layer and an Al-based plating layer. Examples of the Zn-based plating include general hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), electro-galvanizing (EG), and Zn-Ni-based plating (eg, Zn- containing 10 to 25 mass% of Ni). Ni-based plating), Zn-Al-based plating, Zn-Mg-based plating, Zn-Al-Mg-based plating, and the like can be exemplified. Moreover, as Al-type plating, Al-Si type plating (For example, Al-Si type plating containing 10-20 mass % of Si) etc. can be illustrated. The components of the steel plates 11 and 12 are not particularly limited. In addition, the strength of the steel plates 11 and 12 is also not particularly limited, for example, a JIS 5 tensile test piece is produced from the steel sheet in a direction parallel to the rolling direction, and a tensile test is performed in accordance with the regulations of JIS Z 2241:2011. A high-strength steel sheet having a calculated tensile strength of 590 MPa or more (590 MPa class or more), and further, 980 MPa or more (980 MPa class or more) can be used. The steel sheet 11 and the steel sheet 12 may have the same or different components, and even if they have the same or different strength or sheet thickness, there is no problem at all, and a plated steel sheet having a metal plating layer and a steel sheet having no metal plating layer may be used. The component of the aluminum plate 13 is not specifically limited, either, A pure aluminum plate or an aluminum alloy plate may be sufficient. As an aluminum alloy plate, 5000 series (Al-Mg system), 6000 series (Al-Mg-Si system), 2000 series (Al-Cu system), 7000 series (Al-Zn-Mg system) prescribed|regulated by JIS are, for example. , Al-Zn-Mg-Cu) and the like. In addition, an oxide film is formed on the surface of the aluminum plate 13 . In addition, although there is no limitation in particular also about the plate thickness of the steel plates 11 and 12 or the aluminum plate 13, it is preferable that it is the range (about 0.5-4.0 mm) which can be used for a general automobile body.

Next, the plate set in which the steel plates 11 and 12 and the aluminum plate 13 are overlapped with each other is sandwiched by a pair of welding electrodes (electrodes 14 and 15) and energized while pressurized, then the electrode is freed from the steel plate. As a welding apparatus usable in the resistance spot welding method of the present invention, a welding apparatus having a pair of upper and lower electrodes and capable of arbitrarily controlling a pressing force and a welding current during welding can be used. The pressurization mechanism (air cylinder, servo motor, etc.) of the welding apparatus, the type (stationary type, robot gun, etc.), the shape of an electrode, etc. are not specifically limited. In addition, the present invention can be applied to either direct current or alternating current, and the type of power source (single-phase alternating current, alternating current inverter, direct current inverter) is not particularly limited. Here, in the case of alternating current, "current" means "effective current". In addition, resistance spot welding is always performed in a water-cooled state.

In this way, the steel plates 11 and 12 and the aluminum plate 13 are stacked so that one of the plates arranged on the outermost side becomes the steel plate 12 and the other becomes the aluminum plate 13 to form a plate set. This plate group is sandwiched by a pair of welding electrodes (electrodes 14 and 15), and current is applied while pressing, and the nuggets 16 and 17 are formed by resistance heating and steel plates overlapped with each other ( By joining 11 and 12 and the aluminum plate 13, a resistance spot weld joint is obtained.

In this invention, let this energization be a specific pattern. That is, in the conduction pattern of the present invention, for example, as shown in FIG. 3 , when the thickness of the thinner steel sheet among the two steel sheets is t _Fe (mm), electricity is passed through the two steel sheets 11, 12) A first energization step of forming a nugget having a nugget diameter of 2√t _Fe (mm) or more between them, a energization stop step of stopping energization after the first energization step, and a second energization step of energizing after the energization stop step has After the second energization step is completed, if necessary, energization of one or more stages is performed, for example, after energization of the third stage and subsequent stages for the purpose of post heat treatment or the like, energization is stopped.

By forming a nugget having a diameter (nugget diameter) of _{2√t Fe} (mm) or more between the steel plates 11 and 12 in the first energizing step (first energization), in the second energizing step (second energizing) The effect of preventing the reduction of the nugget diameter of the nuggets 16 and 17 due to the occurrence of excessive flying is obtained. In addition, the steel plate 11 is deformed due to the thermal effect of energization in the first energization step of forming a nugget having a diameter of _{2√t Fe (mm) or more, and the contact area between the steel plate 12 and the electrode 14 is enlarged.} Therefore, the current density in the vicinity of the surface of the steel plate 12 in the energization of the second energization step is reduced, and the effect of suppressing the occurrence of flutter from the surface of the steel plate 12 can also be obtained. In addition, the oxide film on the surface of the aluminum plate 13 is destroyed by the energization of the first energization step and its thermal effect, so that an energization path between the steel plate and the aluminum plate is secured. Therefore, the increase in heat input accompanying an excessive increase in the current density toward the center of the junction in the energization of the second energization step is suppressed. In addition, the nugget may or may not be formed between the steel plate and the aluminum plate by this first energization step. However, from the viewpoint of suppressing the growth of the intermetallic compound due to excessive heat input, when the plate thickness of the aluminum plate 13 is t _Al (mm), the nugget of the nugget formed between the steel plate and the aluminum plate in the first energizing step. The diameter (mm) _{is preferably 6√t Al} or less. More preferably, the lower limit of the nugget diameter (mm) of the nugget formed between the steel plate and the aluminum plate in the first energizing step is 1.5√t _Al or more.

Further, the plate thickness t t _Al or _Fe are used as the unit ㎜, and the thickness t t _Fe or _Al in FIG ㎜ unit of 2√t _Fe or _Al 6√t assignment. The nugget diameter of the nugget formed between the steel plates refers to the abutment surface (joint surface) of the steel plate 12 brought into contact with the electrode 14 and the steel plate (steel plate 11 in FIG. 1 ) in contact with the steel plate 12 . , is the maximum diameter of the nugget of the steel plate 12. In addition, the nugget diameter of the nugget formed between the steel plate and the aluminum plate refers to the abutment surface (joint surface) of the aluminum plate 13 and the plate in contact with the aluminum plate 13 (the steel plate 11 in FIG. 1 ). , is the maximum diameter of the nugget 17 of the aluminum plate 13. The nugget diameters of the nuggets 16 and 17 are shown in FIG. 1 . In addition, a "nugget" is a molten and solidified portion formed in a welded portion in lap resistance welding, but in this specification, a molten portion that becomes a nugget upon solidification (that is, a molten portion before solidification) may also be called a nugget.

In the present invention, by energizing in the specific energization pattern described above, the occurrence of excessive blowout is suppressed to prevent a decrease in the nugget diameter of the nuggets 16 and 17, and the oxide film is destroyed to increase the current density. The increase in heat input accompanying an increase is suppressed. Therefore, in this invention, the resistance spot weld joint of the steel plate 11, the steel plate 12, and the aluminum plate 13 which have favorable joint strength can be manufactured. In addition, as described above, irrespective of the composition or plate composition of the steel or aluminum plate, specifically, the presence or absence and thickness of the metal plating layer on the surface of the steel or aluminum plate, the composition and thickness of the oxide film, the strength of the base metal, the plate It can be applied regardless of thickness.

In the present invention, the resistance spot welded joint having good joint strength refers to a resistance spot welded joint having high either or both of tensile shear strength (TSS) and cross tensile strength (CTS).

Regarding this conduction pattern, as shown in FIG. 3 , the radius of curvature at the tip of the electrode 14 brought into contact with the steel plate 12 is R _Fe (mm), and the current in the first conduction step is I ₁ (kA) , the energization time is t ₁ (ms), the energization pause time of the energization stop step is t _c (ms), the current of the second energization process is I ₂ (kA), and the energization time is t ₂ (ms) In this case, the first energization step, the energization stop step, and the second energization step satisfy all of the relationships of the above formulas (1) to (5), so that the effects of the present invention can be more effectively obtained.

In detail, as shown in the above formulas (1) to (2), in the first energizing step, the second energizing process conducts energization at a lower current for a longer period of time than in the second energizing process, thereby suppressing the fly-off between the steel sheets as described above, while suppressing 2√t _{Secure the nugget diameter of Fe} (mm) or more. In addition, as shown in Fig. 2, the energization path is limited by the oxide film on the surface of the aluminum plate 13 at the initial stage of energization, but the oxide film is destroyed by energization at a low current for a long period of time, so that the steel plates 11 and 12 are energized. While securing an energization path between the aluminum plate 13 and the aluminum plate 13, excessive heat input increase is prevented.

Moreover, as shown in said Formula (3), the electricity supply path|route between the steel plates 11 and 12 and the aluminum plate 13 can fully be ensured by _{the electricity supply time t 1 of the 1st electricity supply process being 40 ms or more.}

Then, following the first energization step, energization is stopped for a predetermined energization pause time t _c (ms) (energization interruption process), and following this energization interruption process, in the second energization process (energization of the second stage), the first It conducts energization at a higher current for a shorter time than the energization step. Accordingly, it is possible to instantaneously generate heat over a wide area. In the vicinity of the contact end of the steel plate 12 and the electrode 14, since the current density is high, heat generation in the vicinity of the contact end is promoted as the current becomes higher. Therefore, the high current in the second energization step is effective in generating heat over a wide area by energization for a short time, melting the wide range of aluminum plates 13, and thereby increasing the bonding diameter. In addition, as described above, since the contact diameter between the steel plate 12 and the electrode 14 is enlarged due to the energization in the first energization step, blown-out from the surface of the steel plate 12 is less likely to occur. However, at the time of completion of the energization of the first energization step, the oxide film is initially destroyed in the first energization step, and the center of the junction where energization is started becomes higher. Therefore, when the energization pause time t _c in the energization pause step is short, even if the energization in the second energization step is made to be energized for a short time and high current, since the temperature is raised again from the center of the high-temperature junction portion, the heat input tends to be excessive. Then, as shown in the above formula (4), _{after performing the energization pause step with an energization pause time t c} of 5 ms or more, the second energization step with a high current and a short time is performed. As a result, since the temperature at the center of the junction is temporarily lowered in the energization stop step, heat generation in the vicinity of the contact end between the high current density steel sheet 12 and the electrode 14 can be promoted in the energization of the second energization step. It is possible to efficiently raise the temperature over a wide area, and it is possible to achieve both securing of the nugget diameter of the aluminum plate 13 and reducing heat input. In addition, in the second energization step, blowout may occur to such an extent that excessive reduction in the diameter of the nugget does not occur. Further, the energization time t ₂ in the second energization step is preferably 5 to 100 ms, for example, from the viewpoint of securing the nugget diameter and suppressing excessive heat input. _{The energization time t 2} in the second energization step is more preferably 5-90 ms, still more preferably 10-80 ms.

In addition to the energization pattern described above, as shown in the above formula (5), _{by using an electrode having a tip radius of curvature R Fe} of 20 mm or more as the electrode 14 in contact with the steel plate 12, the effect of the present invention is more effective. can be obtained It is because the contact diameter of an electrode and a steel plate can be ensured and it is easy to suppress the fluttering from the steel plate surface by making front-end|tip curvature radius R _{Fe into 20 mm or more.}

Further, as the electrode 14 in contact with the steel plate 12 , when the tip diameter of the electrode 14 in contact with the steel plate 12 is D _Fe (mm), the relationship between the following formulas (6) and (7) It is preferable to use an electrode that satisfies This is because the contact area between the steel plate 12 and the electrode 14 is increased due to the expansion of the radius of curvature R _Fe at the tip of the electrode 14 in contact with the steel plate 12, so that the energization area in the second energization step is increased. , because the heat generation range and joint diameter of the steel sheet can be expanded. In addition, by preventing an excessive increase in the current density in the vicinity of the contact end between the steel plate 12 and the electrode 14 , the effect of easily suppressing the occurrence of blowout from the surface of the steel plate 12 is also obtained.

R _Fe ≥50 (6)

D _Fe ≥3 (7)

In addition, the tip radius of curvature R _Fe of the electrode 14 is more preferably 100 mm or more. The upper limit of the front-end|tip curvature radius R _Fe is not specifically prescribed|regulated. Further, the tip diameter D _Fe of the electrode 14 is more preferably 4 mm or more, still more preferably 5 mm or more. Although the upper limit in particular of the tip diameter D _Fe is not prescribed|regulated, it is preferable to set it as 20 mm or less from a viewpoint of ensuring a current density and a surface pressure.

The shape of the tip of the electrode 14 is, for example, a DR type (dome radius type), R type (radial type), or D type (dome type) described in JIS C 9304:1999. 4 shows the tip radius of curvature R and the tip diameter D of the electrode. Fig. 4(a) is a diagram showing the tip radius of curvature R and the tip diameter D of the radial electrode, and Fig. 4(b) is a diagram showing the tip radius of curvature R and the tip diameter D of the dome-radial electrode. . In addition, as shown in Fig. 4(b), the curved surface on the tip side of the dome-radius type electrode has two stages of curvature, but the radius of curvature at the tip side of the electrode is the portion ( The radius of curvature R of the curved surface on the center side).

The shape of the tip of the electrode 15 brought into contact with the aluminum plate 13 is, for example, DR type (dome radial type), R type (radial type), D type (dome type) described in JIS C 9304:1999. )to be.

_{The tip diameter D Al} of the electrode 15 brought into contact with the aluminum plate 13 is preferably set to, for example, 4 to 16 mm from the viewpoint of suppressing the deformation of the aluminum plate and securing the surface pressure. More preferably, it is 6-16 mm. _{Further, the radius of curvature R Al} at the tip of the electrode 15 brought into contact with the aluminum plate 13 is preferably set to, for example, 30 mm or more from the viewpoint of suppressing the deformation of the aluminum plate and securing the surface pressure. More preferably, it is 40 mm or more. The upper limit of the front-end|tip curvature radius R _Al is not specifically prescribed|regulated.

In addition, when it is desired to expand the appropriate current range for application by energization in the second energization step, it is effective to secure a sufficient energization path between the steel plates 11 and 12 and the aluminum plate 13 in the first energization step. Therefore, it is preferable to set it as 50 ms or more, and, as for the energization time t _{1 of a 1st energization process, it is more preferable to set it as 60 ms or more.} On the other hand, from the viewpoint of suppressing an increase in tact time, t ₁ is preferably 800 ms or less, more preferably 600 ms or less.

In addition, in a sheet plate where heat generation to the electrode is unlikely to occur, such as when the steel sheet has a large thickness, the energization pause time t _c is preferably set to 10 ms or more, more preferably 20 ms or more. On the other hand, from the viewpoint of suppressing an increase in tact time, t _c is preferably 600 ms or less, and more preferably 400 ms or less.

The welding current (current at the time of energization) in the present invention is not particularly limited, and the welding current is, for example, 4 to 40 kA. However, in terms of construction, it is necessary to obtain a predetermined nugget diameter, and since an excessive current value causes a blowout, the current I ₁ in the first energizing step is, for example, 5 to 20 kA, and the current I 1 in the second energizing step is The current value I ₂ is, for example, 10 to 40 kA.

The pressing force during welding is not particularly limited. For example, the pressing force is, for example, 2.0 kN to 7.0 kN, and the pressing force may be changed during welding and before and after welding.

Moreover, there is no problem at all even if a control method of monitoring parameters such as resistance value and voltage value during welding and changing the current value and the energization time according to the fluctuation is used.

Example

Examples of the present invention are shown below. In addition, it goes without saying that the plate set, welding conditions, and electrode shapes used in this example are examples applied to show the effect of the present invention, and therefore other conditions may be used.

(Invention Examples and Comparative Examples)

As test materials, the steel plates 11 and 12 and the aluminum plate 13 shown in Table 1 were used. The tensile strength of the steel sheets 11 and 12 shown in Table 1 was obtained by producing a JIS 5 tensile test piece from the steel sheet in a direction parallel to the rolling direction, and performing a tensile test in accordance with JIS Z 2241:2011. The steel plates 11 and 12 and the aluminum plate 13 shown in Table 1 are, as shown in Fig. 1, subjected to resistance spot welding, and consist of a three-ply stacking plate in which one aluminum plate and two steel plates are stacked on each other. A resistance spot weld joint was fabricated. In addition, the oxide film was formed on the surface of the aluminum plate 13 used. The welding machine used an inverter DC type resistance spot welding machine, and the conditions shown in Table 2 were the tip curvature radius and tip diameter of the electrodes 14 and 15, and the energization pattern. For the electrodes 14 and 15, DR type electrodes made of chromium copper were used. Resistance spot welding was performed at room temperature (20°C), and the electrode was always water-cooled. In addition, the pressing force was made constant over the 1st energization process, the energization stop process, and the 2nd energization process.

About the obtained resistance spot welded joint, the tensile shear test based on JISZ3136 was implemented, and the joint strength was evaluated. In FIG. 5, the figure explaining the tensile test (tensile shear test) of an Example is shown. As shown in FIG. 5 , a tensile shear test was performed between the steel plate and the aluminum plate by pulling in the direction of the arrow from both sides of the resistance spot welded joint. And the maximum load (tensile shear strength) when the junction part of a steel plate and an aluminum plate fracture|ruptures was computed.

Tensile shear strength (TSS) evaluated the case of TSS≥1.5kN as A, the case of 1.5kN>TSS≥1.0kN as B, and the case of 1.0kN>TSS as F, respectively, and evaluated. Table 2 shows the evaluation results. In the example of this invention, evaluation of tensile shear strength (TSS) was either A or B.

Furthermore, about the resistance spot welded joint obtained by the welding conditions shown in Table 3, the cross tension test based on JISZ3137 was implemented and joint strength was evaluated. Fig. 6 is a diagram for explaining the tensile test (cross tensile test) of the example. As shown in Fig. 6, a cross tensile test between a steel plate and an aluminum plate was performed by pulling in the direction of the arrow from both sides of the resistance spot weld joint. And the maximum load (cross tensile strength) when the junction part of a steel plate and an aluminum plate fracture|ruptures was computed.

Cross tensile strength (CTS) evaluated the case of CTS≥0.7kN as A, and the case of 0.7kN>CTS as F, respectively. Table 3 shows the evaluation results. In the example of this invention, evaluation of cross tensile strength (CTS) was A. In addition, the result of the tensile shear test performed by the method similar to the above was also shown in Table 3 together. In the example of this invention, evaluation of TSS was A.

In addition, the nugget diameter 16 between the steel plate 11 and the steel plate 12 formed by the energization in the first energization process by observing the cross-section of the joint by performing the first energization process under the same conditions as above, the steel plate 11 and the aluminum The nugget diameters 17 between the plates 13 were respectively obtained. In addition, the nugget diameter 17 is the maximum diameter of the nugget 17 of the aluminum plate 13 in the joint surface of the aluminum plate 13 and the plate (steel plate 11) in contact with the aluminum plate 13 ( mm) was measured. The nugget diameter 16 was measured by measuring the maximum diameter (mm) of the nugget 16 at the joint surface of the steel plate 12 and the plate (steel plate 11) in contact with the steel plate 12 . The measurement results are shown in Tables 2 and 3.

11, 12: steel plate
13: aluminum plate
14, 15: electrode
16: Nugget between steel plate and steel plate
17: nugget between steel plate and aluminum plate

Claims (3)

Resistance spot welding joint by joining a sheet set in which two steel plates and an aluminum plate are superimposed on each other, one of which is a steel plate and the other of which is an aluminum plate, is joined by resistance spot welding. In manufacturing,
A first energizing step of forming a nugget having _{a nugget diameter of 2√t Fe} (mm) or more between the two steel plates by energizing when the thickness of the thinner one among the two steel plates is t _{Fe (mm);}
an energization stop step of stopping energization after the first energization step;
a second energization step of energizing after the energization stop step;
Let the radius of curvature of the tip of the electrode in contact with the steel sheet be R _Fe (mm), the current in the first energization step is I ₁ (kA), and the energization time is t ₁ (ms), and the energization in the energization stop step When the rest time is t _c (ms), the current of the second energization step is I ₂ (kA), and the energization time is t ₂ (ms),
The method for manufacturing a resistance spot welded joint, wherein the first energization step, the energization stop step, and the second energization step satisfy all of the relationships of the following formulas (1) to (5).
I ₁ ＜ I ₂ (1)
t ₁ > t ₂ (2)
t ₁ ≥40 (3)
t _c ≥5 (4)
R _Fe ≥20 (5) According to claim 1,
When the radius of curvature of the tip of the electrode brought into contact with the steel sheet is R _Fe (mm) and the diameter of the tip is D _Fe (mm),
The first energization step, the energization stop step, and the second energization step further satisfy the relation of the following formulas (6) and (7).
R _Fe ≥50 (6)
D _Fe ≥3 (7) delete

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