Joint product between steel product and aluminum material, spot welding method for the joint product, and electrode chip for use in the joint product

Abstract

To provide a joint product of a steel product and an aluminum material, and a spot welding method for the joint product, ensuring that spot welding with high bonding strength can be performed. In one embodiment, a steel product 1 having a sheet thickness t1 of 0.3 to 3.0 mm and an aluminum material 2 having a sheet thickness t2 of 0.5 to 4.0 mm are joined together by spot welding to form a joint product of a steel product and an aluminum produce. In this joint product, the nugget area in the joint part is from 20×t20.5 to 100×t20.5 mm2, the area of a portion where the thickness of the interface reaction layer is from 0.5 to 3 μm is 10×t20.5 mm2 or more, and the difference between the interface reaction layer thickness at the joint part center and the interface reaction layer thickness at a point distant from the joint part center by a distance of one-fourth of the joint diameter Dc is 5 μm or less. According to this construction, there can be provided a dissimilar material joint product with excellent bonding strength, which can be formed by an existing spot welding apparatus at a low cost without newly using other materials such as clad material or without newly adding a separate step, and a spot welding method for the dissimilar material joint product.

Description

TECHNICAL FIELD[0001]The present invention relates to a technique of joining a steel product with an aluminum material by spot welding, which is required in the assembling process of a transit vehicle such as automobile and railway vehicle, a mechanical part, a building structure or the like.BACKGROUND ART[0002]Spot welding generally joins together similar metal members with each other. However, if it is applicable to joining of dissimilar metal members (dissimilar material joint product), for example, joining of an iron-based material (hereinafter simply referred to as a “steel product”) and an aluminum-based material (a generic term for pure aluminum and aluminum alloys; hereinafter simply referred to as an “aluminum material”), this can greatly contribute to weight saving and the like.[0003]However, in joining together a steel product and an aluminum material, a brittle intermetallic compound is readily produced at the joint part and it is very difficult to obtain a joint part ha...

AI Technical Summary

Benefits of technology

[0067]According to the present invention constructed as above, at the dissimilar material joining of a steel product with an aluminum material by spot welding, an interface reaction layer having an optimal thickness range in a large area can be formed while obtaining a relatively large nugget area and the bonding strength of the dissimilar material joint product can be enhanced. Accordingly, when forming a dissimilar material joint product of a steel product and an aluminum material by spot welding, an existing spot welding apparatus can be used, unlike conventional techniques, without newly using other materials or newly adding a separate step and in turn, a great cost reduction can be realized.

[0068]Consequently, the joint product of the present invention can be very usefully applied as various structural members in a transit vehicle such as automobile and railway vehicle, a mechanical part, a building structure and the like. Therefore, the usage of a dissimilar material joint product of a steel product and an aluminum material can be greatly expanded by the present invention.

[0069]Also, the present inventors have found that in joining together dissimilar materials, that is, a steel product and an aluminum material, by spot welding, it is important to control the area or thickness distribution of the interface reaction layer formed and for obtaining high bonding strength, to effect suppression control of the time spent to form an interface reaction layer between the steel and the aluminum material. For the suppression control of the interface reaction layer-forming time, previous formation of a suppression layer on the material is found to be important.

[0070]In the present invention, a suppression layer not only effecting suppression control of the interface reaction layer-forming time but also enabling suppression of the dissimilar metal contact corrosion is selected as the suppression layer. As such a suppression layer, in the present invention, two films, that is, a Zn or Al metal film in the specific range and an organic resin adhesive film, are previously provided between a joining surface of the steel product and the aluminum material to be joined together.

[0071]In order to obtain high bonding strength by suppression control of the interface reaction layer-forming time, a metal film having a melting point close to that of the aluminum material is required so that the metal film can work out to an intermediate layer between the aluminum material and the steel product when put into contact with the molten aluminum. In this respect, the Zn or Al metal film in the specific range of the present invention has a melting point close to that of the aluminum material and exerts a function of effecting suppression control of the interface reaction layer-forming time at the spot welding to obtain high bonding strength.

[0072]Also, in order to suppress the dissimilar metal contact corrosion, the suppression layer after spot welding needs to become an electrically insulating layer intervening in a broad range or entirely between the steel product and the aluminum material. On the other hand, in order to enable spot welding and obtain high bonding strength of the spot welded part, the suppression layer needs to allow for electrical conduction between the steel product and the aluminum material at the spot welding. In this respect, the organic resin adhesive film has a function of effecting suppression control of the interface reaction layer-forming time at the spot welding as an electrically insulating layer and further suppressing the dissimilar metal contact corrosion after the spot welding.

Problems solved by technology

However, in joining together a steel product and an aluminum material, a brittle intermetallic compound is readily produced at the joint part and it is very difficult to obtain a joint part having reliably high strength (bonding strength).

Accordingly, joining for such a dissimilar material joint product (joint product of dissimilar metal members) has been conventionally performed using a bolt, a rivet or the like, but this has a problem in the coupling joint reliability, airtightness, cost and the like.

However, all of these methods involve not a simple spot welding but a spot welding in multiple layers, require a separate step such as plating or working, raise an issue that new equipment must be incorporated into the existing welding line, or incur a rise of the welding cost.

Furthermore, these methods all have many problems in view of operation, such as severe limitations on the welding conditions.

However, in Patent Document 8, the strength is 80 kgf and is insufficient, though the breaking is a base material rupture.

Merely by an anchor effect owing to such microscopic bonding, tensile shear strength may be ensured but cross tensile strength (peel strength) cannot be maintained, and the usage is limited to special cases involving only tensile shear stress.

In Patent Document 10, a resistive element needs to be further inserted and this not only raises the above-described issue that new equipment must be incorporated into the existing welding line, but also incurs a rise in the cost.

Certainly, the interface reaction layer which is an intermetallic compound of steel and aluminum is brittle, but according to the present inventors, it is found that if the interface reaction layer is not present at all as in Patent Document 10, an adherence layer by interdiffusion is in turn not formed and low bonding strength results.

Accordingly, similarly to Patent Document 9, merely by microscopic bonding, sufficiently high cross tensile strength cannot be maintained, and the demand for a higher-strength joint product cannot be responded to.

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