JP6410003B1 - Method of manufacturing resistance spot welded joint - Google Patents

Abstract

Resistance spot welding of a plate assembly in which at least one type of steel plate selected from plated steel plate, cold-rolled steel plate and hot-rolled steel plate and an 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 In manufacturing the joint, a first energization step of energizing the energization time t1 (ms) with the current I1 (kA) and an energization deactivation step of stopping energization for the energization stop time tc (ms) after the first energization step. And a second energization step of energizing the current I2 (kA) for an energization time t2 (ms) after the energization suspending step, and the total thickness of the steel plates to be superposed is TFe (mm), an electrode that contacts the steel plate When the radius of curvature of the tip is RFe (mm) and the tip diameter of the electrode to be brought into contact with the steel plate is DFe (mm), all of the relationships of formulas (1) to (6) are satisfied. I1 <I2 (1) t1> t2 (2) t1 ≧ 40 (3) tc ≧ 5 (4) 3 + 0.04 × √ (I12 × t1 × TFe / DFe) ≦ tc ≦ 495 + √ (I12 × t1 × TFe / DFe) (5) RFe ≧ 20 (6)

Description

  The present invention relates to a method for manufacturing a resistance spot welded joint of dissimilar metal materials. Specifically, a resistance that produces a resistance spot welded joint by joining a plate assembly in which at least one kind of steel plate selected from a plated steel plate, a cold-rolled steel plate, and a hot-rolled steel plate and an aluminum plate are overlapped by resistance spot welding. The present invention relates to a method for manufacturing a spot welded joint.

  In recent automobile industries, for the purpose of improving fuel efficiency by reducing the weight of the vehicle body, the application of light metals such as aluminum alloys to the vehicle body has been promoted. 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 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 by resistance heating. It is.

  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, especially the cross, can be reduced by reducing the thickness of the soft aluminum plate by pressing the electrode and forming a brittle intermetallic compound at the joint interface. There is a problem that the peeling strength cannot be secured when a load in the peeling direction represented by tension or the like is generated.

  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 applied 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.

  Moreover, in patent document 3, 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. 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 exists a problem that the board set which can be applied is limited only to the board set of a cold-rolled steel plate and a 6000 series aluminum alloy plate. 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, the appropriate condition range for pre-energization is extremely high depending on the plate assembly. There is also a problem of narrowing.

  This invention is made | formed in view of the above situations, It is a resistance spot welded joint of a steel plate and an aluminum plate, Comprising: It is between a steel plate and an aluminum plate irrespective of the component and board set of a steel plate or an aluminum plate. It is an object of the present invention to provide a resistance spot welded joint manufacturing method capable of manufacturing a resistance spot welded joint having a good peel 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 a current distribution at the initial stage of energization in resistance spot welding. FIG. 1 shows the distribution of current (welding current) when a plate assembly in which a steel plate 11 and an aluminum plate 12 are stacked is sandwiched between a pair of electrodes 13 and 14 and energized while being pressed. It is represented by reference numeral 21.

  In order to have a good peel strength in resistance spot welding of a steel plate and an aluminum plate, it is important to obtain a joint diameter (nugget diameter) that is as large as possible in order to reduce the stress applied to the joint, It is to suppress the growth of fragile intermetallic compounds at the bonding interface (the mating surface of the steel plate and the aluminum plate). Increasing the welding current and energizing time is generally effective to increase the joint diameter, but increasing the welding current and energizing time increases the heat input and facilitates the growth of intermetallic compounds at the joint interface. turn into. For the reasons described above, it has been difficult to ensure that the resistance spot welded joint between the steel plate and the aluminum plate has a good peel strength (ensure the peel strength).

  Therefore, the present inventors have examined welding conditions such as energization patterns and electrodes that achieve both expansion of the joint diameter and reduction of heat input. In general, in resistance spot welding of a steel plate and an aluminum plate, a steel plate having a high specific resistance first generates heat, and the aluminum plate is melted by heat transfer from the steel plate to achieve joining. Therefore, the present inventors considered that it is important how to heat a wide range of steel plates in a short time in order to achieve both expansion of the joint diameter and reduction of heat input.

  In order to reduce heat input in order to suppress the growth of intermetallic compounds, it is effective to shorten the time during which the bonding interface is in a high temperature state. For this purpose, it is 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, and as shown in FIG. Current tends to concentrate at the center of the joint where the oxide film is destroyed by pressurization. Therefore, excessively shortening the time and increasing the current increase and promote the heat input to the center of the joint, so there is a limit to shortening the energization time and increasing the current.

  Based on these findings, the present invention has been completed with further studies, and the gist is as follows.

[1] A plate assembly in which at least one type of steel plate selected from a plated steel plate, a cold-rolled steel plate and a hot-rolled steel plate and an 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. In order to produce a resistance spot welded joint, sandwiched by a pair of electrodes and joined by resistance spot welding,
A first energization step of energizing with an electric current I ₁ (kA) for an energization time t ₁ (ms);
An energization stop step of stopping energization for an energization stop time t _c (ms) after the first energization step;
A second energization step of energizing with an electric current I ₂ (kA) for an energization time t ₂ (ms) after the energization stop step;
The total thickness of the steel sheet superposing _T Fe (mm), a tip radius of curvature of the electrode is contacted with the steel sheet _R Fe (mm), when the tip diameter of the electrode contacting the steel sheet was _D Fe (mm),
The method of manufacturing a resistance spot welded joint, wherein the first energization step, the energization stop step, and the second energization step satisfy all the relationships of the following formulas (1) to (6).
I ₁ <I ₂ (1)
t ₁ > t ₂ (2)
t ₁ ≧ 40 (3)
t _c ≧ 5 (4)
_{3 + 0.04 × √ (I 1} 2 × t 1 × T Fe / D Fe) ≦ t c ≦ 495 + √ (I 1 2 × t 1 × T Fe / D Fe) (5)
R _Fe ≧ 20 (6)

[2] 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 further satisfy a relationship of the following formula (7).
R _Fe ≧ 50 (7)

[3] When the tip curvature radius of the electrode brought into contact with the aluminum plate is R _Al (mm),
The resistance spot-welded joint manufacturing method according to [1] or [2], wherein the first energization step, the energization stop step, and the second energization step further satisfy a relationship of the following formula (8).
R _Al ≧ 50 (8)

  [4] The resistance spot welded joint manufacturing method according to any one of [1] to [3], wherein in the first energization step, at least a part of the aluminum plate in contact with the electrode is melted.

  According to the present invention, a resistance spot welded joint between a steel plate and an aluminum plate having good peel strength can be produced regardless of the components and the plate set of the steel plate and the aluminum plate.

FIG. 1 is a diagram schematically showing a current distribution at the initial stage of energization in resistance spot welding. FIG. 2 is a diagram schematically showing resistance spot welding. FIG. 3 is a diagram illustrating an energization pattern. FIG. 4 is a diagram schematically illustrating a current distribution during energization in the second energization process. FIG. 5 is a diagram showing the tip curvature radius and tip diameter of the electrode.

In the method of manufacturing a resistance spot welded joint according to the present invention, at least one type of steel plate selected from a cold-rolled steel plate, a hot-rolled steel plate and a plated steel plate and an aluminum plate are overlapped and one of the plates arranged on the outermost side is a steel plate. the other is a plate set which is an aluminum plate, sandwiched by a pair of electrodes, and joined by resistance spot welding, in producing a resistance spot welded joint, is energized during the current I _{1 (kA)} energization time t _{1 (ms)} A first energization step, an energization deactivation step in which energization is suspended during an energization deactivation time t _c (ms) after the first energization step, and an energization time t ₂ (ms) with a current I ₂ (kA) after the energization deactivation step. second and a conduction step, the total thickness of the steel sheet superposing T _{Fe (mm),} a tip radius of curvature of the electrode is contacted with the steel sheet R _{Fe (mm),} the electrode contacting the steel tip is energized during When was the _D Fe (mm), the first current supply step, the energization pause step, and the second energizing step is to satisfy all the relationships of the following formulas (1) to (6). In the present invention, the resistance spot welded joint is a generic name including a test piece used for strength test and cross-sectional observation, an automobile member joined by resistance spot welding, and the like.
I ₁ <I ₂ (1)
t ₁ > t ₂ (2)
t ₁ ≧ 40 (3)
t _c ≧ 5 (4)
_{3 + 0.04 × √ (I 1} 2 × t 1 × T Fe / D Fe) ≦ t c ≦ 495 + √ (I 1 2 × t 1 × T Fe / D Fe) (5)
R _Fe ≧ 20 (6)

  The present invention will be specifically described below with reference to FIGS. FIG. 2 is a diagram schematically showing resistance spot welding. FIG. 3 is a diagram illustrating an energization pattern. FIG. 4 is a diagram schematically illustrating a current distribution during energization in the second energization process. The present invention is a method of manufacturing a resistance spot welded joint, wherein a resistance spot welded joint is obtained by resistance spot welding in which a plate assembly in which a plurality of plates are stacked is sandwiched between a pair of electrodes and energized while being pressurized and joined (welded joint). is there.

  First, the steel plate 11 and the aluminum plate 12 are overlapped to form a plate assembly. At this time, as shown in FIG. 2, one of the plates arranged on the outermost side is a steel plate 11 and the other is an aluminum plate 12. In other words, the plates 13 and 14 are brought into contact with each other so as to become the steel plate 11 and the aluminum plate 12, respectively. In addition, in FIG. 2, although the example of the resistance spot welded joint of the board set of 2 sheets which piled up the steel plate 11 and the aluminum plate 12 one by one was shown, between the steel plate 11 and the aluminum plate 12, Furthermore, it is good also as a plate | board set of 3 or more sheets which pinched | interposed another 1 or more steel plate or aluminum plate.

  In the present invention, the plates constituting the resistance spot welded joint, that is, the plates to be resistance spot welded, are at least one kind of steel plate selected from a plated steel plate, a cold rolled steel plate and a hot rolled steel plate, and an aluminum plate. 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. Moreover, as Al system plating, Al-Si system plating (For example, Al-Si system plating containing 10-20 mass% Si) etc. can be illustrated. The component of the steel plate 11 is not particularly limited. Further, the strength of the steel plate 11 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. The steel sheet has a tensile strength of 270 MPa to 1800 MPa (270 MPa to 1800 MPa class). The component of the aluminum plate is not particularly limited, either 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. , Al-Zn-Mg-Cu system) and the like. An oxide film is formed on the surface of the aluminum plate. The plate thickness of the steel plate 11 or the aluminum plate 12 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, the plate set in which the steel plate 11 and the aluminum plate 12 are overlapped is sandwiched between a pair of welding electrodes (electrode 13 and electrode 14) and energized while being pressed, and then the electrode is 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 pressurization mechanism (air cylinder, servo motor, etc.) or type (stationary, robot gun, 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.

  In this manner, the steel plate 11 and the aluminum plate 12 are stacked so that one of the plates arranged on the outermost side is the steel plate 11 and the other is the aluminum plate 12 to form a plate set. The plate assembly is sandwiched between a pair of welding electrodes (electrode 13 and electrode 14), energized while being pressurized, a nugget is formed by resistance heat generation, and the overlapped steel plate 11 and aluminum plate 12 are joined together, thereby providing resistance. A spot welded joint is obtained.

In the present invention, this energization is a specific pattern. That is, in the energization pattern of the present invention, for example, as shown in FIG. 3, the first energization step of energizing with the current I ₁ (kA) for the energization time t ₁ (ms) and the energization pause time after the first energization step. an energization stop process for stopping energization for t _c (ms), and a second energization process for energizing for the energization time t ₂ (ms) with the current I ₂ (kA) after the energization stop process. When the total plate thickness of the steel plate is T _Fe (mm), the tip curvature radius of the electrode 13 in contact with the steel plate is R _Fe (mm), and the tip diameter of the electrode 13 in contact with the steel plate is D _Fe (mm), the above formula All the relationships (1) to (5) are satisfied. 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.

As described above, the energization path is limited by the oxide film on the surface of the aluminum plate 12 at the beginning of energization. Therefore, in the present invention, first, in the first energization process (first stage energization), energization is performed for a longer time and at a lower current than in the second energization process (see the above formulas (1) and (2)). . Thereby, by destroying the oxide film on the surface of the aluminum plate 12, an energization path between the steel plate 11 and the aluminum plate 12 is secured, and an excessive increase in heat input is prevented. Further, the energization time t ₁ of the first conducting step (see above formula (3)) by the above 40 ms, the current path between the steel plate 11 and aluminum plate 12 can be sufficiently secured.

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. FIG. 4 is a diagram schematically illustrating a current distribution during energization in the second energization process. As shown in FIG. 4, since the current density is high in the contact end vicinity 22 between the steel plate 11 and the electrode 13, heat generation in the contact end vicinity 22 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 12 can be melted over a wide area, thereby expanding the bonding diameter. 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, since the re-heated from the hot junction center as was briefly and high current of the power supply of the second current supply step, heat input becomes excessively large easy.
Therefore, in the present invention, the energization stop time t _c is 5 ms or more (see the above equation (4)) and the energization stop step satisfying the above equation (5) is performed, and then the second energization step with a high current and a short time is performed. To do. 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 22 between the steel plate 11 and the electrode 13 having a high current density. The nugget diameter can be expanded while suppressing. Here, it is effective to set the lower limit value of t _{c according to} the heat generation amount in the first energization process and the heat removal amount to the electrode after the first energization process. For this reason, as the lower limit value of t _c, the relational expression (5) (that is, the left side of the above expression (5)) consisting of I ₁ , t ₁ , T _Fe , and D _Fe that are parameters affecting them was determined. If T _Fe is large, the amount of heat generated by the steel sheet in the first energization process is increased, and it is difficult for heat to be removed to the electrode, so the lower limit of t _c is large. In addition, when D _Fe is large, the contact area with the electrode increases, and heat removal from the electrode is promoted, so the lower limit value of t _c becomes small. Further, since an excessive increase in t _c decreases production efficiency, the relational expression (5) (that is, the right side of the above expression (5)) including similar parameters is set for the upper limit value of t _c . Incidentally, the energization time _{t 2} in the second energizing step, for example 5~100ms are preferred.

Further, in the present invention, in addition to the specific energization pattern described above, the electrode 13 that satisfies the above formula (6), that is, the electrode 13 that is brought into contact with the steel plate 11 has a tip curvature radius R _Fe of 20 mm or more. Must be used. This is because the contact area between the steel plate 11 and the electrode 13 is increased by increasing the tip curvature radius R _Fe of the electrode 13 to be brought into contact with the steel plate 11, thereby increasing the energization area in the second energization step, and the heat generation range of the steel plate 11. This is because the bonding diameter can be increased. In addition, since the contact area between the steel plate 11 and the electrode 13 is increased, heat removal to the electrode 13 is promoted, and the energization stop time t _c after the end of the first energization process can be shortened. Moreover, the effect which suppresses generation | occurrence | production of the scattering from the steel plate surface is also acquired by preventing the excessive increase in the current density in the contact end vicinity 22 of the steel plate 11 and the electrode 13. The electrode 13 is brought into contact with the steel plate 11, tip curvature radius R _Fe satisfies a relationship represented by the following formula (7), that the tip curvature radius R _Fe it is preferable to use an electrode is at least 50mm. The reason is suppression of surface scattering by enlarging the contact area of an electrode and a steel plate, and reducing a current density.
R _Fe ≧ 50 (7)
The type of the tip of the electrode 13 is, for example, DR type (dome radius type), R type (radius type), or D type (dome type) described in JIS C 9304: 1999. FIG. 5 shows the tip radius of curvature R and the tip diameter D of the electrode. FIG. 5A is a diagram showing the tip radius of curvature R and the tip diameter D of the radius electrode, and FIG. 5B is a diagram showing the tip radius of curvature R and the tip diameter D of the dome radius electrode. . As shown in FIG. 5 (b), the dome radius electrode has a curved surface on the tip side with a two-step curvature. The radius of curvature of the tip of the electrode is a portion (center) that first contacts the plate to be resistance spot welded. Radius of curvature of the curved surface on the side.
The tip diameter D _Fe of the electrode 13 brought into contact with the steel plate 11 is preferably 4 mm to 16 mm, for example, from the viewpoint of securing the contact area between the electrode and the steel plate. The tip diameter D _Fe of the electrode 13 is more preferably 6 mm to 16 mm, and further preferably 8 mm to 16 mm.

  In the present invention, by using the specific electrode as the electrode 13 that is made in the specific energization pattern and brought into contact with the steel plate 11, excessive heat generation can be prevented while melting the aluminum plate 12 in a wide range. It is possible to achieve both the expansion of the bonding diameter and the suppression of growth of the intermetallic compound at the bonding interface by reducing the heat input. Therefore, in this invention, the resistance spot welded joint of the steel plate 11 and the aluminum plate 12 which has favorable peeling 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.

The electrode 14 in contact with the aluminum plate 12 satisfies the following formula (8) when the tip curvature radius of the electrode 14 in contact with the aluminum plate 12 is R _Al (mm), that is, the tip curvature radius R _Al is It is preferable that the electrode be 50 mm or more. Thereby, the effect of the present invention can be obtained more effectively. This is because the effect of suppressing the thickness reduction of the aluminum plate 12 due to energization can be obtained by reducing the surface pressure applied to the aluminum plate 12.
R _Al ≧ 50 (8)
The tip radius of curvature R _Al is more preferably 80 mm or more.
The type of the tip of the electrode 14 brought into contact with the aluminum plate 12 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 14 brought into contact with the aluminum plate 12 is preferably 4 mm to 16 mm, for example, from the viewpoint of reducing the surface pressure applied to the aluminum alloy plate. The tip diameter D _Al of the electrode 14 is more preferably 6 mm to 16 mm, and further preferably 8 mm to 16 mm.
Also, there is no particular relationship between the tip diameter D _Al of the electrode 14 on the aluminum plate 12 side and the tip diameter D _Fe of the electrode 13 on the steel plate 11 side, and both may be the same or different. Also good.

In the present invention, when it is desired to expand the current range that can be applied by energization in the second energization process, it is effective to secure a sufficient energization path between the steel plate 11 and the aluminum plate 12 in the first energization process. . Therefore energization time _{t 1} of the first energizing step is preferably set to 50ms or more, and more preferably, 60ms or more. The upper limit of the energization time t ₁ of the first energizing step is not particularly defined, in view of the tact time shortened, preferably, the energization time t ₁ is less than 600 ms.
Moreover, it is preferable that at least a part of the aluminum plate 12 is melted during energization in the first energization process. Since the aluminum plate 12 is melted during energization in the first energization step, the oxide film on the surface of the aluminum plate 12 is completely removed, so that the energization path can be stabilized. However, in order to prevent excessive heat input, when the thickness of the aluminum plate 12 located on the outermost side and brought into contact with the electrode 14 is T (mm), the electrode 14 formed by energization in the first energization step is used. The nugget diameter of the outermost aluminum plate 12 to be contacted is preferably 6√T (mm) or less, and more preferably 5√T (mm) or less. Moreover, it is preferable that the nugget diameter of the outermost aluminum plate 12 formed by energization in the first energization process and brought into contact with the electrode 14 is 2√T (mm) or more. In addition, the thickness T of the aluminum plate 12 uses mm as a unit, and the unit of 6√T or 5√T into which T is substituted is also mm. Here, the nugget diameter of the aluminum plate 12 is the maximum diameter of the nugget of the aluminum plate 12 at the mating surface (joint surface) between the aluminum plate 12 and the plate (steel plate 11 in FIG. 2) in contact with the aluminum plate 12. is there. 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.

Also, like the plate thickness of the steel sheet is large, in the heat removal is less likely to occur 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. And, energization pause time t _c, it is preferable to satisfy the relation of formula (9), more preferably satisfies the following formula (10). The reason for this is that once the weld is cooled, a wide range of heat generation is promoted in the second energization step, and the joint strength is improved.
_{8 + 0.04 × √ (I 1} 2 × t 1 × T Fe / D Fe) ≦ t c ≦ 495 + √ (I 1 2 × t 1 × T Fe / D Fe) (9)
_{15 + 0.04 × √ (I 1} 2 × t 1 × T Fe / D Fe) ≦ t c ≦ 495 + √ (I 1 2 × t 1 × T Fe / D Fe) (10)

The welding current (current during energization) 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, and an excessive current value causes scattering, the current I _{1 in} the first energization process is, for example, 4 to 20 kA, and the current in the second energization process the value _{I 2} is 10~40kA example.

  Moreover, there is no restriction | limiting in particular in the applied pressure during welding, for example, an applied pressure is 2.0 kN-7.0 kN, for example, You may change an applied pressure during welding and before and after welding.

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

  In the present specification, each of the above formulas defines a relationship of only numerical values.

  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 plate 11 and the aluminum plate 12 shown in Table 1-1 and Table 1-2 were used. The tensile strength of each steel plate shown in Table 1-1 and Table 1-2 is that the JIS No. 5 tensile test piece is produced from the steel plate in the direction parallel to the rolling direction, and conforms to the provisions of JIS Z 2241: 2011. The tensile test was performed. As shown in FIG. 2, the steel plate 11 and the aluminum plate 12 shown in Table 1-1 were subjected to resistance spot welding to produce a resistance spot welded joint consisting of a two-layer plate set. Although not shown, the steel plate and the aluminum plate shown in Table 1-2 are similarly subjected to resistance spot welding, and another steel plate between the steel plate 11 (lower plate) and the aluminum plate 12 (upper plate). A resistance spot welded joint made of a three-layered plate assembly sandwiching 15 (medium plate) was produced. The aluminum plate 12 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 13 and 14 and the energization pattern were set as shown in Table 2. The electrodes 13 and 14 were all DR type electrodes made of chromium copper. Resistance spot welding was performed at room temperature (20 ° C.), and the electrodes 13 and 14 were always water-cooled. The applied pressure was constant throughout the first energization process, the energization stop process, and the second energization process. In the first energization process, a part of the aluminum plate 12 was melted.

  About the obtained resistance spot welded joint, the cross tension test based on JISZ3137 was implemented, and peeling strength was evaluated. When the cross tensile strength (CTS) is CTS ≧ 0.9 kN, A, 0.9 kN> CTS ≧ 0.8 kN, B, 0.8 kN> CTS ≧ 0.7 kN, C, 0.7 kN> Each case of CTS was evaluated as F and evaluated. The evaluation results are shown in Table 2. In the example of the present invention, the evaluation was any one of A to C.

  Moreover, the 1st electricity supply process was performed on the conditions similar to the above, the cross-section observation of the junction part was performed, and the nugget diameter (mm) of the aluminum plate 12 formed by electricity supply of the 1st electricity supply process was calculated | required. In addition, the nugget diameter of the aluminum plate 12 is the aluminum at the joint surface between the aluminum plate 12 and the plate in contact with the aluminum plate 12 (steel plate 11, steel plate 15 in the case of a three-layered plate set shown in Table 1-2). The maximum diameter of the nugget on the plate 12 was measured. The measurement results are shown in Table 2.

11, 15 Steel plate 12 Aluminum plate 13, 14 Electrode 21 Welding current 22 Near contact end of steel plate and electrode

Claims (4)

A pair of plates in which at least one plate selected from a plated steel plate, a cold-rolled steel plate and a hot-rolled steel plate and an 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, In order to produce a resistance spot welded joint, sandwiched by electrodes and joined by resistance spot welding,
A first energization step of energizing with an electric current I ₁ (kA) for an energization time t ₁ (ms);
An energization stop step of stopping energization for an energization stop time t _c (ms) after the first energization step;
A second energization step of energizing with an electric current I ₂ (kA) for an energization time t ₂ (ms) after the energization stop step;
When the total plate thickness of the steel plates to be stacked is T _Fe (mm), the tip curvature radius of the electrode in contact with the steel plate is R _Fe (mm), and the tip diameter of the electrode in contact with the steel plate is D _Fe (mm),
The method of manufacturing a resistance spot welded joint, wherein the first energization step, the energization stop step, and the second energization step satisfy all the relationships of the following formulas (1) to (6).
I ₁ <I ₂ (1)
t ₁ > t ₂ (2)
t ₁ ≧ 40 (3)
t _c ≧ 5 (4)
_{3 + 0.04 × √ (I 1} 2 × t 1 × T Fe / D Fe) ≦ t c ≦ 495 + √ (I 1 2 × t 1 × T Fe / D Fe) (5)
R _Fe ≧ 20 (6) 2. 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 formula (7).
R _Fe ≧ 50 (7) When the radius of curvature of the tip of the electrode brought into contact with the aluminum plate is R _Al (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 formula (8).
R _Al ≧ 50 (8)   The method of manufacturing a resistance spot welded joint according to any one of claims 1 to 3, wherein in the first energization step, at least a part of the aluminum plate in contact with the electrode is melted.

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