WO2018181232A1

WO2018181232A1 - Production method for resistance spot welded joint

Info

Publication number: WO2018181232A1
Application number: PCT/JP2018/012260
Authority: WO
Inventors: 央海澤西; 松田　広志; 池田　倫正
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2017-03-31
Filing date: 2018-03-27
Publication date: 2018-10-04
Also published as: KR102276817B1; CN110475642B; KR20190126099A; JPWO2018181232A1; CN110475642A; JP6399266B1

Abstract

Provided is a production method for a resistance spot welded joint that comprises a three-layer stack of two steel sheets and one aluminum sheet. The method makes it possible to produce a resistance spot welded joint that has favorable joint strength, regardless of the components and combination of steel sheets and aluminum sheets. The method involves stacking two steel sheets and one aluminum sheet such that one of the outermost sheets is a steel sheet and the other is the aluminum sheet and then joining the set of sheets by resistance spot welding. The method includes: a first electrification step that is for applying current to form, between the two steel sheets, a nugget that, when the sheet thickness of the thinner of the two steel sheets is tFe (mm), has a diameter of at least 2√tFe (mm); an electrification suspension step that is for suspending electrification after the first electrification step; and a second electrification step that is for applying current after the electrification suspension step.

Description

Method of manufacturing resistance spot welded joint

The present invention relates to a method for manufacturing a resistance spot welded joint of dissimilar metal materials. Specifically, the present invention relates to a resistance spot welded joint manufacturing method in which a plate set in which two steel plates and one aluminum plate are overlapped is joined by resistance spot welding to produce a resistance spot welded joint.

In recent years, in the automobile industry, 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 weight of the vehicle body. At present, 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 hits per vehicle is 3000 to 6000. To the point. The resistance spot welding method is a method in which two or more stacked steel plates are sandwiched between a pair of electrodes, and a high current welding current is shortened between the upper and lower electrodes while pressing with a pair of electrodes from above and below the sandwiched steel plates. This is a method of energizing for a time and 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 joining in the case of steel plates. In the following description, the aluminum plate is a general term for a pure aluminum plate and an aluminum alloy plate. However, in joining dissimilar metal materials of steel and aluminum, the strength of the joint cannot be ensured because the soft aluminum plate is greatly reduced in thickness by pressurization of the electrode or a brittle intermetallic compound is formed at the joint interface. There is a problem. In particular, in a three-ply plate assembly in which two steel plates and one aluminum plate are overlapped, 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 is further difficult to ensure the strength.

In order to solve the above problems, a resistance spot welding method as described below has been proposed. For example, Patent Document 1 describes a resistance spot welding method in which 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 so that a high-strength joint can be obtained even at a low current. Yes.

In Patent Document 2, by welding with one or more contact plates on both sides of a steel plate and an aluminum plate, 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 that provides a high strength joint.

In Patent Document 3, when spot welding a steel material and an aluminum material, by optimizing each amount of Mn and Si in a steel plate and a steel plate surface oxide film, it is possible to suppress the occurrence of scattering while obtaining a large nugget diameter. It is described.

Patent Document 4 describes a dissimilar metal joining method that suppresses the growth of intermetallic compounds at the joining interface by optimizing the conditions for pulsation energization and increasing the pressure after completion of energization.

Patent Document 5 discloses a spot that can suppress the generation of dust from the surface of the steel sheet by optimizing the pre-energization and the subsequent energization conditions, reduce the welding current as much as possible, and obtain a dissimilar material joint having high joint strength. A welding method is described.

Japanese Patent No. 3117053 Japanese Patent No. 3504790 JP 2005-152958 A Japanese Patent No. 5624901 Japanese Patent No. 5572046

However, the resistance spot welding methods described in Patent Documents 1 and 2 require the use of a backing plate or a clad thin plate that is not necessary for the structure of the vehicle body, so that a significant increase in cost and weight cannot be achieved. There's a problem.

Further, in Patent Document 3, since it is necessary to limit the amount and distribution of alloy elements in the steel sheet and the oxide film, there is a problem that the use of the steel sheet that satisfies the required performance is restricted. In particular, the application of the invention of Patent Document 3 is extremely limited under the circumstances where high alloying is progressing with high strength in recent steel plates.

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 of which are short. Required. Therefore, when the specific resistance of the steel plate is high or when the plate thickness is large, there is a concern that scattering occurs on the surface of the steel plate.

In Patent Document 5, there is a problem that applicable plate sets are limited to plate sets of cold-rolled steel plates and 6000 series aluminum alloy plates. In Patent Document 5, it is necessary to perform pre-energization under conditions that do not melt the aluminum alloy plate. However, since the aluminum alloy plate has a lower melting point than a steel plate, depending on the plate assembly, the appropriate condition range of pre-energization may be. There is also the problem of becoming very narrow.

In addition, all of these documents are intended to solve the problems in the two-layer plate assembly of steel plates and aluminum plates. In a three-layer plate assembly in which one aluminum plate is superimposed on two steel plates, -A method for obtaining a desired nugget diameter not only between aluminum plates but also between steel plates and steel plates is not described.

This invention is made | formed in view of the above situations, Comprising: It is a resistance spot welded joint of 2 sheets of steel plates and 1 sheet of aluminum plates, Comprising: It depends on the component and board assembly of a steel plate or an aluminum plate. It aims at providing the manufacturing method of the resistance spot welding joint which can manufacture the resistance spot welding joint which has favorable joint strength.

As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge. FIG. 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 overlaid. It is a figure which clamps a set with a pair of

electrodes

14 and 15. FIG. Moreover, FIG. 2 is a figure which shows typically the electric current distribution of the energization initial stage of resistance spot welding. FIG. 2 shows an initial current (welding current) when a plate assembly in which two

steel plates

11 and 12 and an aluminum plate 13 are stacked is sandwiched between a pair of

electrodes

14 and 15 and energized while being pressed. Is represented by reference numeral 21 in FIG.

In order to ensure good joint strength in resistance spot welding of a three-layered set of two steel plates and one aluminum plate, between steel plates and between steel plates and between steel plates and aluminum plates as shown in FIG. It is effective to obtain nuggets each having a large nugget diameter (nugget 16 between steel plate and steel plate, nugget 17 between steel plate and aluminum plate). In addition, excessive heat input causes a decrease in joint strength due to the growth of brittle intermetallic compounds at the bonding interface between the steel plate and aluminum plate (the mating surface between the steel plate and the aluminum plate), so the heat input must also be reduced. I must.

In order to suppress the growth of this intermetallic compound, it is generally effective to shorten the energization pattern and increase the current, but the energization path is limited because a strong oxide film exists on the surface of the aluminum plate. As shown in FIG. 2, at the initial stage of energization, the current tends to concentrate at the joint center 23 where the oxide film is destroyed by pressurization. Therefore, excessively short time and high current increase and promote heat input between the steel plate and the aluminum plate at the center of the joint (nugget). Further, excessive shortening of time and increase in current tend to cause scattering between the steel sheet and the steel sheet, making it difficult to ensure the nugget diameter between the steel sheet and the steel sheet. In addition, compared with a two-layered plate set using one steel plate and one aluminum plate, the total thickness of the steel plates is larger in a three-layered plate set using two steel plates and one aluminum plate. For this reason, insufficient heat removal from the electrodes tends to cause scattering not only between the steel plates but also on the steel plate surface. For the above reasons, there is a limit to shortening the energization time and increasing the current.

The present invention has been completed by further studies based on these findings, and the gist is as follows.

[1] A plate assembly in which two steel plates and one aluminum plate are overlapped and one of the plates arranged on the outermost side is a steel plate and the other is an aluminum plate is joined by resistance spot welding to form a resistance spot welded joint In manufacturing
When the thickness of the thinner one of the two steel plates is defined as t _Fe (mm), a first nugget having a nugget diameter of 2√t _Fe (mm) or more is energized between the two steel plates. Energization process;
An energization stop step of stopping energization after the first energization step;
A method of manufacturing a resistance spot welded joint including a second energization step of energizing after the energization suspending step.

[2] The radius of curvature of the tip of the electrode brought into contact with the steel plate is R _Fe (mm), the current in the first energization step is I ₁ (kA), the energization time is t ₁ (ms), and the energization pause time and _t c (ms), the current of the second current step _I 2 (kA), when the energization time was _t 2 (ms),
The method of 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 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 tip curvature radius of the electrode brought into contact with the steel sheet is R _Fe (mm) and the tip diameter is D _Fe (mm),
In the resistance spot welded joint according to [1] or [2], the first energization process, the energization stop process, and the second energization process further satisfy a relationship of the following formulas (6) and (7): Production method.
R _Fe ≧ 50 (6)
D _Fe ≧ 3 (7)

According to the present invention, a resistance spot welded joint consisting of two steel plates and one aluminum plate, which has a good joint strength regardless of the components and the plate set of the steel plates and aluminum plates. Can be manufactured.

FIG. 1 is a diagram schematically showing resistance spot welding of the present invention. FIG. 2 is a diagram schematically showing a current distribution in the initial stage of energization of resistance spot welding. FIG. 3 is a diagram illustrating an energization pattern. FIG. 4 is a diagram showing the tip curvature radius and tip diameter of the electrode. FIG. 5 is a diagram for explaining a tensile test (tensile shear test) of an example of the present invention. FIG. 6 is a diagram for explaining a tensile test (cross tensile test) of an example of the present invention.

The method of manufacturing a resistance spot welded joint according to the present invention comprises a plate assembly in which two steel plates and one aluminum plate are overlapped and one of the plates arranged on the outermost side is a steel plate and the other is an aluminum plate. When producing a resistance spot welded joint by welding, when the thickness of the thinner steel plate of the two steel plates is t _Fe (mm), the nugget diameter is 2 between the two steel plates when energized. a first power supply step of forming a √t _{Fe (mm)} or more of the nugget, and has a current pause step of pausing the current after the first current step, and a second current supply step of energizing after the energization pause step . In the present invention, the resistance spot welded joint is a generic name including a test piece used for strength test, cross-sectional observation, an automobile member joined by resistance spot welding, and the like.

The present invention will be specifically described below with reference to FIGS. The present invention obtains a resistance spot welded joint by resistance spot welding in which a plate assembly in which two steel plates and one aluminum plate are overlapped is sandwiched between a pair of electrodes and energized while being pressed (welded). It is a manufacturing method of a resistance spot welded joint.

First, the steel plate 11, the steel plate 12, and the aluminum plate 13 are overlapped to form a plate assembly. At this time, as shown in FIG. 1, one of the plates arranged on the outermost side is a steel plate 12 and the other is an aluminum plate 13. In other words, the plates that contact the

electrodes

14 and 15 are overlapped so as to become the steel plate 12 and the aluminum plate 13, respectively.

The

steel plates

11 and 12 are not particularly limited, and examples thereof include cold rolled steel plates, hot rolled steel plates, and plated steel plates. The 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), electrogalvanizing (EG), and Zn—Ni-based plating (for example, Zn containing 10 to 25% by mass of Ni). -Ni-based plating), Zn-Al-based plating, Zn-Mg-based plating, Zn-Al-Mg-based plating, and the like. Examples of the Al plating include Al—Si plating (for example, Al—Si plating containing 10 to 20% by mass of Si). The components of the

steel plates

11 and 12 are not particularly limited. The strength of the

steel plates

11 and 12 is not particularly limited. For example, a JIS No. 5 tensile test piece is produced from the steel plate in a direction parallel to the rolling direction, and a tensile test is performed in accordance with the provisions of JIS Z 2241: 2011. Thus, a high-strength steel sheet having a tensile strength of 590 MPa or more (590 MPa class or more), or 980 MPa or more (980 MPa class or more) can be used. The steel plate 11 and the steel plate 12 may have the same or different components, and there is no problem whether the strength and the thickness are the same or different, and a plated steel plate having a metal plating layer and a steel plate having no metal plating layer are used. May be. The components of the aluminum plate 13 are not particularly limited, and may be a pure aluminum plate or an aluminum alloy plate. Examples of the aluminum alloy plate include 5000 series (Al-Mg series), 6000 series (Al-Mg-Si series), 2000 series (Al-Cu series), and 7000 series (Al-Zn-Mg series) defined by JIS. And Al-Zn-Mg-Cu-based). An oxide film is formed on the surface of the aluminum plate 13. Further, the thickness of the

steel plates

11 and 12 and the aluminum plate 13 is not particularly limited, but is preferably in a range (about 0.5 to 4.0 mm) that can be used for a general automobile body.

Next, a plate assembly in which the

steel plates

11 and 12 and the aluminum plate 13 are overlapped is sandwiched between a pair of welding electrodes (electrode 14 and electrode 15), and energized while being pressed, and then the electrodes are released from the steel plate. As a welding apparatus that can be used in the resistance spot welding method of the present invention, a welding apparatus that includes a pair of upper and lower electrodes and can arbitrarily control the pressure and welding current during welding can be used. There are no particular limitations on the pressure mechanism (air cylinder, servo motor, etc.), type (stationary, robot gun, etc.), electrode shape, etc. of the welding apparatus. Further, the present invention can be applied to both direct current and alternating current, and the type of power source (single-phase alternating current, alternating current inverter, direct current inverter) and the like are not particularly limited. Here, in the case of alternating current, “current” means “effective current”. In addition, resistance spot welding is performed in a state that is always water-cooled.

Thus, the

steel plates

11 and 12 and the aluminum plate 13 are overlapped so that one of the plates arranged on the outermost side is the steel plate 12 and the other is the aluminum plate 13 to form a plate set. The plate assembly is sandwiched between a pair of welding electrodes (electrode 14 and electrode 15), energized while being pressed, and

nuggets

16 and 17 are formed by resistance heating, and the stacked

steel plates

11 and 12 and the aluminum plate 13 are joined. By doing so, a resistance spot welded joint is obtained.

In the present invention, this energization is a specific pattern. That is, the energization pattern of the present invention is, for example, as shown in FIG. 3, when the thickness of the thinner one of the two steel plates is t _Fe (mm), the two

steel plates

11 and 12 are energized. a first power supply step of nugget diameter forms a 2√t _{Fe (mm)} or more of the nugget between the energization pause step of pausing the current after the first energization step, a second current supply step of energizing after the energization pause step And have. In addition, after the second energization process is completed, energization of one or more stages is performed as necessary, and then energization is performed after the third stage for the purpose of post heat treatment, for example, and then the 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 energization process (first stage energization), the second energization process (second stage energization) The effect of preventing the nugget diameters of the

nuggets

16 and 17 from being reduced due to excessive scattering during energization is obtained. In addition, since the steel plate 11 is deformed due to the thermal effect of energization in the first energization step for forming a nugget having a diameter of 2√t _Fe (mm) or more, the contact area between the steel plate 12 and the electrode 14 is increased. The current density in the vicinity of the surface of the steel plate 12 during energization in the two energization process is reduced, and the effect of suppressing the occurrence of scattering from the surface of the steel plate 12 can also be obtained. Furthermore, the oxide film on the surface of the aluminum plate 13 is destroyed by the energization of the first energization process and the heat effect, and an energization path between the steel plate and the aluminum plate is secured. Therefore, an increase in heat input due to an excessive increase in current density at the center of the joint is suppressed in energization in the second energization process. In this first energization step, a nugget may or may not be formed between the steel plate and the aluminum plate. However, from the viewpoint of suppressing the growth of intermetallic compounds due to excessive heat input, the nugget formed between the steel plate and the aluminum plate in the first energization step when the thickness of the aluminum plate 13 is t _Al (mm). The nugget diameter (mm) is desirably 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 energization step is set to 1.5√t _Al or more.
Incidentally, the plate thickness _{t Fe} and _{t Al} is used mm as the unit, also with mm unit of 2√T _Fe and 6√T _Al obtained by substituting the thickness _{t Fe} and _{t Al.} The nugget diameter of the nugget formed between the steel plates is the nugget of the steel plate 12 at the mating surface (joint surface) between the steel plate 12 brought into contact with the electrode 14 and the steel plate 12 (steel plate 11 in FIG. 1). Is the maximum diameter. The nugget diameter of the nugget formed between the steel plate and the aluminum plate is the aluminum plate at the mating surface (joint surface) between the aluminum plate 13 and the plate in contact with the aluminum plate 13 (steel plate 11 in FIG. 1). The maximum diameter of 13 nuggets 17. The nugget diameters of the

nuggets

16 and 17 are shown in FIG. 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 the present invention, by energizing with the above-described specific energization pattern, the occurrence of excessive scattering is suppressed to prevent the nugget diameters of the

nuggets

16 and 17 from decreasing, and the oxide film is destroyed to cause excessive current density. Suppresses the increase in heat input that accompanies the increase. Therefore, in this invention, the resistance spot welded joint of the steel plate 11, the steel plate 12, and the aluminum plate 13 which has favorable joint strength can be manufactured. In addition, as described above, regardless of the components and plate set of the steel plate or aluminum plate, specifically, the presence or thickness of the metal plating layer on the surface of the steel plate or aluminum plate, the composition or thickness of the oxide film, the mother It can be applied regardless of material strength and plate thickness.
In the present invention, the resistance spot welded joint having a good joint strength refers to a resistance spot welded joint having either or both of tensile shear strength (TSS) and cross tensile strength (CTS).

For this energization pattern, as shown in FIG. 3, the tip curvature radius of the electrode 14 contacting the steel plate 12 and _R Fe (mm), the current of the first current supply step _I 1 (kA), the energization time _{t 1} (Ms), when the energization stop time of the energization stop 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 effects of the present invention can be obtained more effectively by satisfying all the relationships of the above formulas (1) to (5) in the energization stop process and the second energization process.

More specifically, as shown in the above formulas (1) and (2), the first energization process is conducted for a longer time and at a lower current than the second energization process, thereby causing the scattering between the steel plates as described above. While suppressing, a nugget diameter of 2√t _Fe (mm) or more is secured. 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 this long time and low current energization destroys the oxide film, and the

steel plates

11 and 12 and aluminum While ensuring the energization path between the plates 13, an excessive increase in heat input is prevented.
Moreover, as shown to the said Formula (3), the electricity supply path | route between the

steel plates

11 and 12 and the aluminum plate 13 is fully securable by making the electricity supply time t1 of a _1st electricity supply process into 40 ms or more.

Then, following the first energization process, energization is suspended for a predetermined energization suspension time t _c (ms) (energization suspension process), and following this energization suspension process, the second energization process (second-stage energization process). ) For a shorter time and higher current than in the first energization step. As a result, a wide range of heat can be instantaneously generated. Since the current density is high in the vicinity of the contact end between the steel plate 12 and the electrode 14, heat generation near the contact end is promoted as the current is increased. Therefore, the increase in current in the second energization step is effective in generating heat over a wide area by short-time energization, and the aluminum plate 13 in a wide area can be melted, thereby increasing the bonding diameter. As described above, since the contact diameter between the steel plate 12 and the electrode 14 is increased by energization in the first energization step, the scattering from the surface of the steel plate 12 is unlikely to occur. However, at the end of energization in the first energization process, the center of the joint where the oxide film is initially destroyed in the first energization process and energization starts is at a higher temperature. Therefore, if the energization pause time t _c in the energization pause process is short, even if a short time and high current of the power supply of the second current supply step, to re-heated from the hot junction center, excessive heat input It is easy to become. Therefore, as shown in the equation (4), to perform the high current in a short time in the second energizing step after energization pause time t _c makes a 5ms or more energization pause process. As a result, the temperature at the center of the joint portion once decreases in the energization suspending process, and thus energization in the second energizing process can promote heat generation in the vicinity of the contact end between the steel plate 12 and the electrode 14 having a high current density. It is possible to increase the temperature of the aluminum plate 13 and to ensure the nugget diameter of the aluminum plate 13 and to reduce the heat input. Note that in the second energization step, scattering that does not cause an excessive decrease in the nugget diameter may occur. Further, the energization time t ₂ in the second energizing step is preferably from the viewpoint of the nugget diameter ensuring the excess heat input suppressed, for example, 5 ~ 100 ms. Energizing time _{t 2} in the second energizing step is more preferably 5 ~ 90 ms, more preferably from 10 ~ 80 ms.

In addition to the energization pattern described above, as shown in the above formula (5), the effect of the present invention can be obtained more effectively 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. be able to. This is because the contact radius between the electrode and the steel plate can be ensured by setting the tip curvature radius R _Fe to 20 mm or more, and scattering from the steel plate surface can be easily suppressed.
Furthermore, as the electrode 14 in contact with the steel plate 12, it is preferable to use an electrode satisfying the relationship of the following formulas (6) and (7) when the tip diameter of the electrode 14 in contact with the steel plate 12 is D _Fe (mm). . This is because the contact area between the steel plate 12 and the electrode 14 is increased due to the increase in the tip curvature radius R _Fe of the electrode 14 in contact with the steel plate 12, so that the energization area in the second energization process is increased. This is because the diameter can be expanded. Further, 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, an effect of easily suppressing the occurrence of scattering from the surface of the steel plate 12 can be obtained.
R _Fe ≧ 50 (6)
D _Fe ≧ 3 (7)
The tip curvature radius R _Fe of the electrode 14 is more preferably 100 mm or more. The upper limit of the tip curvature radius R _Fe is not particularly specified. Further, the tip diameter D _Fe of the electrode 14 is more preferably 4 mm or more, and further preferably 5 mm or more. Although not specified tip diameter D limit of _Fe is particularly in view of ensuring the current density and the surface pressure is preferably set to 20mm or less.
The type of the tip of the electrode 14 is, for example, DR type (dome radius type), R type (radius type), or D type (dome type) described in JIS C 9304: 1999. FIG. 4 shows the tip radius of curvature R and the tip diameter D of the electrode. 4A is a diagram showing the tip radius of curvature R and the tip diameter D of the radius-shaped electrode, and FIG. 4B is a diagram showing the tip radius of curvature R and the tip diameter D of the dome radius-shaped electrode. . As shown in FIG. 4B, the dome radius-shaped electrode has a curved surface on the tip side with a two-step curvature, but the radius of curvature of the tip of the electrode is a portion (center) that is first in contact with the plate to be resistance spot welded. Radius of curvature of the curved surface on the side.

The type of the tip of the electrode 15 brought into contact with the aluminum plate 13 is, for example, DR type (dome radius type), R type (radius type), or D type (dome type) described in JIS C 9304: 1999.
The tip diameter D _Al of the electrode 15 brought into contact with the aluminum plate 13 is preferably 4 to 16 mm, for example, from the viewpoint of suppressing deformation of the aluminum plate and securing the surface pressure. More preferably, it is 6 to 16 mm. Similarly, the tip radius of curvature R _Al 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 deformation of the aluminum plate and securing the surface pressure. More preferably, it is 40 mm or more. The upper limit of the tip curvature radius R _Al is not particularly defined.

Further, when it is desired to expand the appropriate current range applied by energization in the second energization process, 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 process. It is preferable that its reason energization time _{t 1} of the first energizing step 50ms or more, and more preferably to 60ms or more. On the other hand, from the viewpoint of suppressing an increase in tact time, t ₁ is preferably 800 ms or less, and more preferably 600 ms or less.

Also, like the plate thickness of the steel sheet is large, in the heat removal occurs hard plate sets to the electrodes, energization pause time t _c is preferably greater than or equal to 10 ms, and more preferably to more than 20ms. 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 when energized) in the present invention is not particularly limited, and the welding current is, for example, 4 to 40 kA. However, since it is necessary to obtain a predetermined nugget diameter in construction, an excessive current value causes scattering, so the current I _{1 in} the first energization process is, for example, 5 to 20 kA, and the current in the second energization process the value _{I 2} is, for example, 10 ~ 40 kA.

Also, there is no particular limitation on the applied pressure during welding. For example, the applied pressure is, for example, 2.0 kN to 7.0 kN, and the applied pressure may be changed during welding and before and after welding.

Also, there is no problem 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.

Examples of the present invention are shown below. The plate set, 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.

(Invention Example and Comparative Example)
As test materials, the

steel plates

11 and 12 and the aluminum plate 13 shown in Table 1 were used. The tensile strengths of the

steel plates

11 and 12 shown in Table 1 were obtained by preparing a JIS No. 5 tensile test piece in a direction parallel to the rolling direction from the steel plate and conducting a tensile test in accordance with the provisions of JIS Z 2241: 2011. Asked. The

steel plates

11 and 12 and the aluminum plate 13 shown in Table 1 are made of a three-layered plate set in which resistance spot welding is performed and one aluminum plate and two steel plates are overlapped as shown in FIG. A resistance spot welded joint was manufactured. The aluminum plate 13 used had an oxide film formed on the surface. An inverter DC resistance spot welder was used as the welding machine, and the tip curvature radius and tip diameter of the

electrodes

14 and 15 and the energization pattern were set as shown in Table 2. The

electrodes

14 and 15 were all made of chromium-copper DR type electrodes. Resistance spot welding was performed at room temperature (20 ° C.), and the electrode was always water-cooled. The applied pressure was constant throughout the first energization process, the energization stop process, and the second energization process.

About the obtained resistance spot welded joint, the tensile shear test based on JISZ3136 was implemented and joint strength was evaluated. In FIG. 5, the figure explaining the tension test (tensile shear test) of an Example is shown. As shown in FIG. 5, a tensile shear 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 welded joint. And the maximum load (tensile shear strength) when the junction part of a steel plate and an aluminum plate fracture | ruptured was computed.
The tensile shear strength (TSS) was evaluated as A when TSS ≧ 1.5 kN, B when 1.5 kN> TSS ≧ 1.0 kN, and F when 1.0 kN> TSS. The evaluation results are shown in Table 2. In the examples of the present invention, the tensile shear strength (TSS) was evaluated as either A or B.
Furthermore, about the resistance spot welded joint obtained on 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 view for explaining the tensile test (cross tensile test) of the example. As shown in FIG. 6, a cross tension 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 welded joint. And the maximum load (cross tensile strength) when the junction part of a steel plate and an aluminum plate fracture | ruptured was computed.
The cross tensile strength (CTS) was evaluated as A when CTS ≧ 0.7 kN and F when 0.7 kN> CTS. The evaluation results are shown in Table 3. In the example of the present invention, the evaluation of the cross tensile strength (CTS) was A. Table 3 also shows the results of the tensile shear test performed by the same method as described above. In the example of the present invention, the evaluation of TSS was A.

Further, the first energization process is performed under the same conditions as described above, and the nugget diameter 16 between the steel sheet 11 and the steel sheet 12 formed by energization of the first energization process by observing the cross section of the joint portion, and between the steel sheet 11 and the aluminum plate 13. Each nugget diameter 17 was determined. In addition, the nugget diameter 17 measured the maximum diameter (mm) of the nugget 17 of the aluminum plate 13 in the joint surface of the aluminum plate 13 and the board (steel plate 11) which contact | connects the aluminum plate 13. FIG. The nugget diameter 16 was determined by measuring the maximum diameter (mm) of the nugget 16 of the aluminum plate 12 at the joint surface between 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

A plate assembly in which two steel plates and one aluminum plate are overlapped and one of the plates arranged on the outermost side is a steel plate and the other is an aluminum plate is joined by resistance spot welding to produce a resistance spot welded joint. Hits the,
When the thickness of the thinner one of the two steel plates is defined as t Fe (mm), a first nugget having a nugget diameter of 2√t Fe (mm) or more is energized between the two steel plates. Energization process;
An energization stop step of stopping energization after the first energization step;
A method of manufacturing a resistance spot welded joint including a second energization step of energizing after the energization suspending step.
The radius of curvature of the tip of the electrode brought into contact with the steel plate is R Fe (mm), the current in the first energization step is I 1 (kA), the energization time is t 1 (ms), and the energization pause time in the energization pause step Is t c (ms), the current in the second energization step is I 2 (kA), and the energization time is t 2 (ms),
The resistance spot welded joint manufacturing method according to claim 1, wherein the first energization step, the energization stop step, and the second energization step satisfy all 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)
When the tip curvature radius of the electrode brought into contact with the steel sheet is R Fe (mm) and the tip diameter is D Fe (mm),
3. The method of manufacturing a resistance spot welded joint according to claim 1, wherein the first energization step, the energization stop step, and the second energization step further satisfy a relationship of the following formulas (6) and (7): .
R Fe ≧ 50 (6)
D Fe ≧ 3 (7)