JPH07178565A - Resistance welding method for dissimilar metals - Google Patents

Abstract

(57) [Summary] [Purpose] Dissimilar metals are easily joined by resistance welding. [Configuration] bonding metal A, if B melting point difference is less 300K (mp T _B melting point T _A <metal B of the metal A), melting point (T _A -300) K or T _B following intermediate layer X is interposed between the metals A and B. Melting point and (T _A -300) K or T _A following intermediate layer X, by the melting point using a following intermediate layer Y T _A or T _B, even if the melting point difference is more than 300K, high joint strength Can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to dissimilar metals such as aluminum and steel, steel and titanium, in which a brittle intermetallic compound is formed at the joint interface when welding is performed and it is difficult to obtain high joint strength. Method of resistance welding.

[0002]

In the welding of dissimilar metals such as aluminum and steel, aluminum and titanium, titanium and steel,
It is known that a brittle intermetallic compound is formed at the joint interface and high joint strength cannot be obtained. In the present specification, when simply referred to as aluminum, it means pure aluminum and aluminum alloy, and similarly, titanium and the like mean the pure metal and alloy.

Conventionally, such dissimilar metals have been joined by mechanical joining methods such as screws, bolts, fitting, solid-state joining methods such as explosion welding, hot rolling and friction rolling, and further by bonding. Methods are being considered. However, mechanical joining and joining by adhesion have problems in reliability, airtightness, workability, and the like. Further, the solid phase bonding method has problems that the shape of the bonding material is largely restricted and the workability is low.

From the above, it is expected to develop a simpler and easier work method for joining dissimilar metals. In particular,
Since the joining of aluminum and steel is an indispensable technology for weight reduction of automobiles, establishment of a joining method using simple and efficient resistance welding is desired.

The present state and problems of resistance welding of dissimilar metals will be described below by taking spot welding of aluminum and steel, which is attracting attention from the viewpoint of weight reduction of a vehicle body.

[0006] In spot welding of aluminum and steel, there is a problem in that physical properties such as melting point, electric resistance and thermal conductivity are greatly different. For example, if aluminum and steel sheets are stacked and simply spot welded,
Since the melting point of aluminum is 1/2 or less of the melting point of steel, and since aluminum has a higher thermal conductivity, heat generated by resistance welding is conducted from the steel side to the aluminum side,
Since the aluminum is unilaterally melted, the aluminum-side plate surface is largely damaged by welding. Further, in this process, an intermetallic compound is formed at the bonding interface, and the nugget is also formed unevenly. Therefore, high joint strength cannot be obtained.

As one of the countermeasures, it is considered to interpose an intermediate layer between aluminum and steel. For example, in Japanese Patent Laid-Open No. 4-143083, Mg is used as the intermediate layer.
A technique for interposing a foil is disclosed. In addition, JP-A-4
No. 251676 and Japanese Patent Application Laid-Open No. 6-39558 disclose a technique of forming a plating layer on the surface of steel in contact with aluminum.

[0008]

However, these conventional techniques cannot stabilize high joint strength.
This is because the conventional technique uses the intermediate layer as a mere insert material interposed between aluminum and steel, and does not sufficiently consider the large difference in the physical property values of aluminum and steel when designing the intermediate layer.

That is, the melting point difference between aluminum and steel is 3
Much more than 00K. According to the investigation by the present inventors, in the case of dissimilar metals having such a large difference in melting point, one intermediate layer cannot obtain stable joint strength. Conversely, if the melting point difference is 300K or less, such as steel and titanium, 1
Stable joint strength can be obtained even with two intermediate layers.

In other words, the above-mentioned conventional technique is effective for joining steel and titanium, but when it is used for joining aluminum and steel, which is the original object of joining, it cannot exhibit a sufficient effect. is there.

An object of the present invention is to provide a resistance welding method for dissimilar metals which can stably obtain high joint strength regardless of the kind of the joining material.

[0012]

An intermediate layer is considered to be essential for resistance welding of dissimilar metals. In the process of investigating the behavior of the intermediate layer, its ideal function became clear. The ideal function of the intermediate layer found by the present inventors will be described with reference to FIG.

A and B are different metals to be joined, and the melting point T _A of the metal _A is lower than the melting point T _{B of the} metal B. That is, T _A <T _B. X is an intermediate layer, and the metal A, B
Intervenes in between. The intermediate layer X is sandwiched between the metals A and B, and the metals A and B are sandwiched between the electrodes to carry out pressurization and energization, whereby the metals A and B are resistance welded (here, spot welding).

At this time, the action of the intermediate layer X suppresses the excessive melting points of the metals A and B, so that the metal A
It prevents the direct contact of B and prevents the formation of intermetallic compounds at the joint interface. After the joint surfaces of the metals A and B are heated to a required temperature, the intermediate layer X is completely melted and discharged from the joint interface by a pressing force. By these, the joining surfaces of the metals A and B heated to an appropriate temperature can be instantly overlapped and directly joined.

This is the ideal function of the intermediate layer X. That is, the intermediate layer X is not used as a mere insert material, but is made to function as a medium for controlling the timing of heating and melting the metals A and B. More specifically, due to the function of the intermediate layer X, the heating and melting timings of the joining surfaces of the metals A and B are adjusted, and when the joining surfaces are in the most suitable state for joining, the intermediate portions from the joining surfaces are brought into contact with each other. The layers are discharged and the metals A and B are directly joined.

In order to make such an ideal function in the intermediate layer X, it is necessary to reflect the difference in the physical property values of the metals A and B in the design of the intermediate layer X, which is investigated by the present inventors. As a result, it was found that this can be achieved by giving the intermediate layer X a certain condition based on the physical properties of the metals A and B.

Further, when the melting points of the metals A and B are greatly different from each other, such as aluminum and steel, if only the intermediate layer X is used, the metals A and B react with each other to form a brittle intermetallic compound, which is high. It was found that the bonding strength could not be obtained. This is because when the melting points of the metals A and B are largely different, the intermediate layer X is heated before the metal B having a high melting point is heated to an appropriate temperature.
Is melted and discharged, and as a result, the molten metals A and B are kept in contact with each other until the joining is completed, and a brittle intermetallic compound is formed. In other words, the method using only the intermediate layer X is effective only when the melting point difference between the metals A and B is small.

Therefore, as a result of investigating combinations of various intermediate layers, when the melting points of the metals A and B are significantly different, it is necessary to use another intermediate layer Y having different physical properties as shown in FIG. 1 (B). It turned out that there is.

The present invention has been made on the basis of the above findings, and has as its gist the following two resistance welding methods for dissimilar metals.

[0020] 1) dissimilar metals A melting point difference is 300K or less, B (in the resistance welding of _{the melting point T B) of the} melting point T _A <metal _B of the metal A, melting point (T _A -300) K or T _B below By interposing the intermediate layer X between the metals A and B, the intermediate layer X is instantaneously melted by resistance heating during resistance welding, and the molten intermediate layer X is discharged from the joint interface by a pressing force. Then, the metals A and B are directly joined.

[0021] 2) different melting points dissimilar metals A, in the resistance welding of B (melting point T _B melting point T _A <metal B of the metal A), the intermediate layer is not more than the melting point (T _A -300) K or T _A X and an intermediate layer Y having a melting point of T _A or more and T _B or less,
By interposing between the metals A and B so that the intermediate layer X is located on the metal A side and the intermediate layer Y is located on the metal B side, the intermediate layers X and Y are instantaneously melted by resistance heating during resistance welding. At the same time, the melted intermediate layers X and Y are discharged from the bonding interface by a pressing force, and the metals A and B are directly bonded.

The thickness of the intermediate layer is 0 <R _X × d _X ^1/4 <R _X × 0.5 ^1/4 (1) Z _AB × 3500 <R _Y × d _Y ^1/4 <R _Y × 0.5 ^1/4 (2) Z _AB = (R _A / K _A ) × (K _B / R _B ) × (1 / T _B ) 0 <d _A , d _B ≦ 3 d _i : Thickness (Mm) R _i : specific resistance (μΩ · mm) K _i : thermal conductivity (cal / cm ² / cm / s / ° C.) is desirable. However, in the first method that does not use the intermediate layer Y, the condition (2) is unnecessary.

The welding current and welding _{time, I O = α × (Z} AB × T B) × (1 / t 1/2) +9 ... (3) Z AB = (R A / K A) × (K B / R _B ) × (1 / T _B ) 350 <α <550 0 <t _O <100 I _O : Welding current (kA) t _O : Welding time (ms) R _i : Specific resistance (μΩ · mm) K _i : Thermal conductivity (cal / cm ² / cm / s / ° C.) α: It is desirable that it is a constant.

[0024]

The first method is effective only when the melting point difference between the metals A and B is 300 K or less, and is therefore applied only in this case. Specifically, for example, the metal A is steel and the metal B is titanium. When the metal A is steel and the metal B is titanium, examples of the intermediate layer X include Ni-Zn, Zn, Ti.
-Mn, Al and alloys thereof, Mg and alloys thereof and the like can be used.

In the second method, the melting point difference between the metals A and B is 300K.
It is effective in the case where the melting point exceeds, but can be naturally used in the case where the melting point difference is 300 K or less (for example, joining of steel and titanium). As a combination of the metals A and B having a melting point difference exceeding 300 K, for example, the metal A may be aluminum and the metal B may be steel. In this case, the intermediate layer X is Al
-Si, Zn, etc., and Al-Mn as the intermediate layer Y,
Ni-Zn, Cu, Ni or the like can be used.

FIG. 2 shows the results of a tensile test of the joint portion when the material and thickness of the intermediate layer X were changed in the first method (spot welding), using the thickness as a parameter, the melting point and the specific resistance × (thickness) ¹ It is organized according to the relationship with ^{/ 4} . Metal A is steel (thickness 1.0 mm), metal B is titanium (thickness 1.0 m)
m). Welding conditions are 1ms-32kA-1960N
And The white mark is the base material fracture, and the black mark is the interface peeling.

As can be seen from the figure, when the melting point difference between the metals A and B is 300 K or less, the melting point T of the metal B is not less than the melting point T _A -300 K of the metal A as the intermediate layer X.
_B or less must be used. A proper intermediate layer X is
Here, there are three types of Ni-Zn, Zn, and Ti-Mn.

When the melting point of the intermediate layer X exceeds the melting point T _{A of the} metal B, the bonding surface of the metal A is abnormally heated during bonding, and
The metal B is melted rather than the intermediate layer X, and the bonding interface of the intermediate layer X remains after bonding. Therefore, high joint strength cannot be obtained. Further, the melting point of the intermediate layer X is (the melting point of the metal A T _A −3
When it is less than 00) K, the intermediate layer X is melted and discharged from between the bonding surfaces at the initial stage of bonding, the bonding surface of the metal B cannot be sufficiently heated, and the metals A and B come into contact with each other to form a bonding interface. Since an intermetallic compound is formed in the joint, high joint strength cannot be obtained.

The particularly desirable melting point of the intermediate layer X is T _A or more and T _B or less.

FIG. 3 shows the tensile test results of the joints when the materials and thicknesses of the intermediate layers X and Y were changed in the second method (spot welding), the melting point and the specific resistance x with the thickness as a parameter.
(Thickness) It is arranged in relation to ^1/4 . Metal A is aluminum (thickness 1.0 mm), metal B is steel (thickness)
1.0 mm). Welding conditions are 1ms-50kA-19
It was set to 60N. The white mark is the base material fracture, and the black mark is the interface peeling.

As can be seen from the figure, the intermediate layer X on the metal A side
Having a melting point of (metal A's melting point T _A −300) K or more and metal A's melting point T _A or less, and a metal B-side intermediate layer Y having a melting point T _A or more of metal _A and a metal B melting point T _B. By using the following, the melting point difference between the metals A and B is 300
Even if it exceeds K, high joint strength can be obtained.
Suitable intermediate layers here are two types of X, Zn and Al-Si, and four types of Y, Al-Mn, Cu, Ni-Zn and Ni.

In the second method, the intermediate layer X having the lowest melting point is in contact with the metal A on the low melting point side, and this intermediate layer X is formed at the initial stage of welding.
Melting occurs. After the intermediate layer X is melted, the surface of the metal A having the next highest melting point is melted, the oxide layer on the surface is destroyed, and the active surface of the metal A appears. Subsequently, the intermediate layer Y having the next highest melting point is melted, and the active surface of the metal B appears. Then, finally, the oxide films existing on the surfaces of the melted intermediate layers X and Y and the metal A are extruded from the joint interface by the welding pressure, and the surfaces of the active metal A and the metal B are in a solid state. When superposed, the metals A and B having high strength are joined together.

As described above, the intermediate layers X and Y control the heat generation and melting phenomenon in resistance welding to suppress the melting of the base material, the destruction of the oxide film on the aluminum surface, the discharge of the molten layer and the oxide film, and the active surface. It contributes to the formation of the process of joining and gives high strength to the joining interface.

When the melting points of the intermediate layers X and Y are both higher than the melting point of the metal A, the surface of the metal A is melted at the initial stage of melting, and thereafter, the metal A is thicker than the intermediate layers X and Y. Is large, the amount of welding heat is mainly consumed for melting the metal A, and as a result, a nugget biased toward the metal A is formed. Moreover, since the melting points of the intermediate layers X and Y are higher than that of the metal A,
The intermediate layers X and Y are insufficiently melted, and the intermediate layers X and Y are not completely discharged from the bonding interface. Therefore, high strength cannot be obtained even if joining is performed.

When the melting points of the intermediate layers X and Y are lower than the melting point of the metal A, the intermediate layers X and Y are formed at the initial stage of welding in the same manner as in the method disclosed in JP-A-4-251676. Is melted and discharged, and the joining is completed without sufficiently exerting the effect of the intermediate layer, so that high joining strength cannot be obtained.

When the melting point of the intermediate layer Y on the high melting point metal B side is lower than the melting point of the metal A and the melting point of the intermediate layer X on the low melting point metal side is higher than the melting point of the metal A, the intermediate layer Y at the initial stage of welding. Melts and the intermediate layer X is also discharged from the bonding interface due to the melting, so that the effect of the film cannot be obtained.

That is, in the second method, the heat generation at the bonding interface is concentrated by the low melting point intermediate layer X located on the low melting point metal A side, and after the intermediate layer X is melted, the metal A is melted. , By activating the bonding surface of the metal A and melting the high melting point intermediate layer Y located on the high melting point metal B side,
Efficient discharge of these molten layers, active metal A,
The joining surface of B is superposed and directly joined. Therefore, the timing of melting the metals A and B and the intermediate layers X and Y is important, and it is necessary to have the above melting point relationship. It goes without saying that such an effect cannot be obtained when the number of intermediate layers is one.

As for the brittle intermetallic compound formed at the joining interface, which is most concerned with joining the metals A and B, as described above, the molten layer is completely discharged from the joining interface and the active surface is directly superposed. Therefore, the intermetallic compound is not formed.

In the present invention, from the viewpoint of further activating the surface of the metal A having a low melting point, it is effective to provide the intermediate layer Z having a low melting point on the bonding surface of the metal A. The melting point of the intermediate layer Z needs to be lower than that of the metal A for the reasons described above, and a sufficient effect can be obtained even with the same components as those of the low melting point intermediate layer X described above.

A particularly desirable melting point of the intermediate layer Y is (T _B −
It is 100) K or less and (T _A +50) K or more. When the melting point of the intermediate layer Y approaches the melting point T _B of the metal B, the melting of the metal B increases, and when the melting point of the intermediate layer Y approaches the melting point T _A of the metal A, the melting of the metal A increases. The strength may not be obtained.

The desirable melting point of the intermediate layer X is (T _A -5
0) K or less, and (T _A -300) K or more. Middle layer X
When the melting point of the metal A approaches the melting point T _A of the metal A, the melting of the metal A becomes large, and when the melting point of the metal A is too far from the melting point T _{A of the} metal A, the intermediate layer X is melted and discharged in the initial stage of welding. May not be effective enough.

Metals A and B to which the welding method of the present invention can be applied
Table 1 lists representative examples of suitable intermediate layers in each combination and each combination.

[0043]

[Table 1]

In the present invention, because of resistance welding, the melting of the metals A, B and the intermediate layer is related to the melting point as well as the specific resistance and thickness.

For example, even if the melting point of the intermediate layer satisfies the above conditions, if the melting point is higher than the melting point T _A of the metal A, the heat generated in the intermediate layer is transmitted to the metal A and the melting of the intermediate layer is insufficient. Becomes

More specifically, for example, the second method [FIG.
When Cu is used as the intermediate layer Y on the metal B side in (B)], its thermal conductivity is large and its specific resistance is small.
When the layer thickness is small, the heat generated at the interface is small, and the intermediate layer Y does not melt, so the heating of the joint surface of the metal B is insufficient, and the intermediate layer Y remains at the joint interface, reducing the joint strength. (Fig. 3).

If the resistivity and thickness of the intermediate layer are large,
The intermediate layer does not melt sufficiently and cannot fulfill its function.

As a result of organizing the experimental results from these viewpoints, the conditions (1) and (2) were obtained. The proper thickness of the intermediate layer obtained from these conditions is shown in Table 2 in detail.

Incidentally, Ni-Zn, Ti-Mn, Al-S
The melting points of alloys such as i and Al-Mn can be adjusted in a wide range by changing the component ratio.

[0050]

[Table 2]

If the resistivity and thickness of the intermediate layer X are too large in the first method, the melting will be insufficient and it will be difficult to discharge from the bonding interface. The lower limit of the intermediate layer X is not particularly limited because it may function as a barrier for preventing contact between the metals A and B. Also in the second method, only the upper limit is required for the resistivity and thickness of the intermediate layer X for the same reason. When the resistivity and thickness of the intermediate layer Y are excessively large, the melting is insufficient and discharge from the bonding interface becomes difficult. When the intermediate layer Y is excessively small, the bonding surface of the metal B on the high melting point side is insufficient. Since it is not heated, high joint strength cannot be obtained.

As the intermediate layers X and Y, any of powder, foil, plating and the like can provide sufficient effect, and each intermediate layer may have a different or similar multi-layer structure.

When the intermediate layers X and Y are collectively formed on one of the joining surfaces of the metals A and B, the intermediate layer is not required on the joining surface of the other metal, and the handling of the intermediate layers X and Y is required for joining. Becomes unnecessary. For example, the intermediate layers X and Y on the surface of the steel plate
It is a surface-treated steel sheet formed by plating or the like.

The thicknesses d _A and d _B of the metals A and B are set to 0.3 mm or less in order to perform resistance welding with a small welding heat input.

As the welding method, not only spot welding but also resistance welding such as seam welding, DC butt welding and projection welding can be applied.

Welding conditions are also important in the present invention. If the welding current is too high even with the proper intermediate layer,
Since the intermetallic compound is formed by the reaction of the metals A and B, high joint strength cannot be obtained, and when the amount is too small, the intermediate layer is insufficiently melted and the metal B on the high melting point side is joined. Again, high joint strength cannot be obtained because the surfaces are not heated sufficiently.

FIG. 4 shows that in the first method (spot welding), metal A is steel (thickness 1.0 mm) and metal B is titanium (thickness).
When the thickness is 1.0 mm, the appropriate intermediate layer X (Ni, thickness
0.1 mm) and the proper welding conditions at this time were investigated.

FIG. 5 shows that in the second method (spot welding), when the metal A is aluminum (thickness 1 mm) and the metal B is steel (thickness 1 mm), the appropriate intermediate layer X (Zn,
Thickness 0.1 mm) and intermediate layer Y (Al-Mn, 0.01 m)
m) is used to examine appropriate welding conditions at this time.

As a result of arranging these experimental results, the condition (3) was obtained. Table 3 shows the proper welding current I _O (the proper welding current when the welding time is 1 ms) obtained from these conditions.
Shown in.

[0060]

[Table 3]

The welding time t _{O is set} to 100 ms or less in order to suppress the formation of the intermetallic compound at the bonding interface by performing the bonding for a short time.

[0062]

EXAMPLES Examples of the present invention will be shown below, and the effects of the present invention will be clarified by comparison with Comparative Examples.

When the melting point difference between the metals A and B is 300 K or less , a thickness of 0 mm is required when spot-welding the metal A, which is a 1 mm thick steel plate (SPCD), and the metal B, which is a 1 mm thick titanium plate (TP25). 0.005-0.5 mm Pb, Z
n, Al-Mn, Cu, Ni-Zn, Ni, Zr, Mo
Was used. The breakdown of the middle layer is shown in Tables 4 and 5.

Welding time 1 to 400 ms, welding current 5 to 8
Spot welding was performed under the conditions of 0 kA and a pressing force of 1960 N, and the joint strength was evaluated from the fracture position in the cross tension test. The results are shown in Tables 6-9. Among these results, FIG. 2 shows the effect of the type and thickness of the intermediate layer on the joint strength, and FIG. 4 shows the effect of welding current.

As can be seen from Tables 6 to 9, metals A and B
If the melting point difference of is less than 300K, the first method is effective,
The second method is, of course, also effective.

When the melting point difference between metals A and B exceeds 300K
Aluminum plate with a thickness of 1 mm, which is a compound metal A (A505
2) and 1 mm thick steel plate (SPCD), which is metal B, are spot welded with P of 0.005 to 0.5 mm thickness.
b, Zn, Al-Mn, Cu, Ni-Zn, Ni, Z
An intermediate layer (Tables 4 and 5) composed of r and Mo was used.

Welding time 1 to 400 ms, welding welding current 5
Spot welding was performed under the conditions of -80 kA and pressure 1960 N, and the joint strength was evaluated from the fracture position in the cross tension test. The results are shown in Tables 10 to 16. Of these results, FIG. 3 shows the effect of the type and thickness of the intermediate layer on the joint strength, and FIG. 5 shows the effect of welding current.

As can be seen from Tables 10 to 16, when the melting point difference between the metals A and B is 300 K or less, the second method is effective, but the first method is not effective.

[0069]

[Table 4]

[0070]

[Table 5]

[0071]

[Table 6]

[0072]

[Table 7]

[0073]

[Table 8]

[0074]

[Table 9]

[0075]

[Table 10]

[0076]

[Table 11]

[0077]

[Table 12]

[0078]

[Table 13]

[0079]

[Table 14]

[0080]

[Table 15]

[0081]

[Table 16]

[0082]

As described above, the resistance welding method for dissimilar metals according to the present invention enables simple and efficient welding by resistance welding even for dissimilar metals which are difficult to join by ordinary welding. It will bring a great industrial advantage to the production fields where the weight reduction of railway cars and railway cars is required.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a welding method of the present invention.

FIG. 2 is a diagram showing the effectiveness of the first method.

FIG. 3 is a diagram showing proper welding conditions in the first method.

FIG. 4 is a diagram showing effectiveness of the second method.

FIG. 5 is a diagram showing appropriate welding conditions in the second method.

[Explanation of symbols]

 A, B Metal to be joined (A is low melting point side, B is high melting point side) X, Y Intermediate layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Uchida 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (5)

[Claims] 1. A dissimilar metal A melting point difference is 300K or less, in the resistance welding of B (melting point T _B melting point T _A <metal B of the metal A), melting point (T _A -300) K or T _B below 2. A resistance welding method for dissimilar metals, characterized in that the intermediate layer X, which is, is interposed between the metals A and B to perform welding. Wherein the melting point is different from dissimilar metals A, in the resistance welding of B (melting point T _B melting point T _A <metal B of the metal A), melting point (T _A -300) intermediate layer is less than K or more T _A X
And an intermediate layer Y having a melting point of T _A or more and T _B or less between the metals A and B such that the intermediate layer X is located on the metal A side and the intermediate layer Y is located on the metal B side, A resistance welding method for dissimilar metals, characterized by performing welding. 3. In resistance welding, the intermediate layer is instantly melted by resistance heating, and the molten intermediate layer is discharged from the joining interface by a pressing force to directly join the metals A and B. The resistance welding method for dissimilar metals according to claim 1 or 2. 4. The thickness of the intermediate layers X and Y is 0 <R _X × d _X ^1/4 <R _X × 0.5 ^1/4 (1) Z _AB × 3500 <R _Y × d _Y ^{1 / _{4 <R Y × 0.5 1/4 ...}} (2) Z AB = (R A / K A) × (K B / R B) × (1 / T B) 0 <d A, d B ≦ 3 d _i : thickness (mm) R _i : specific resistance (μΩ · mm) K _i : thermal conductivity (cal / cm ² / cm / s / ° C.), claim 1, 2, or 3. A resistance welding method for dissimilar metals as described. 5. The welding current is I _O = α × (Z _AB × T _B ) × (1 / t ^1/2 ) +9 (3) Z _AB = (R _A / K _A ) × (K _B / R _B ) × (1 / T _B ) 350 <α <550 0 <t _O <100 I _O : Welding current (kA) t _O : Welding time (ms) R _i : Specific resistance (μΩ · mm) K _i : The resistance welding method for dissimilar metals according to claim 1, 2, 3 or 4, wherein thermal conductivity (cal / cm ² / cm / s / ° C) α is a constant.