JPH0417980A - Electric resistance spot welding method for aluminum alloy plates - Google Patents

Abstract

PURPOSE:To obtain excellent continuous spot weldability and to remarkably improve welding workability by bringing plates on which different kind of metallic films are formed by plating on one side to come into contact with electrodes of the aluminum alloy plates into contact with each other via the nonplated surfaces and executing energizing between the above-mentioned electrodes to come into contact with the plated surfaces. CONSTITUTION:When only the plate surfaces 3 and 3' to come into contact with the electrodes 1 are plated to form the plated films 14 and 14', the plated films 14 and 14' instead of electrically nonconductive hard and brittle oxide films come into contact with the electrodes. These plated films 14 and 14' consist of electrically conductive metal except impurities of a part. Since electricity flows uniformly on the plated surface, there is no local heat generation on the plate surface, so deterioration in accordance with the local temperature rise of the electrode 1 interfaces is remarkably reduced. Meanwhile, aluminum oxide films 8 and 8' existing in the joined surfaces of the aluminum alloy plates facilitates the formation of a nugget 6 by the temperature rise on the joined surfaces.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an electric resistance spot welding method for aluminum alloy plates having excellent continuous spot weldability.

[Conventional technology] In recent years, the demand for lighter vehicle bodies has become stronger due to fuel efficiency regulations for automobiles, and as a result, aluminum alloy sheets are increasingly being used for automobile outer panels instead of steel sheets. . In order to assemble aluminum alloy plates into the car body, it is necessary to perform electric resistance spot welding, just like with steel plates. By the way, compared to steel, aluminum alloys have properties such as (1) low electrical resistivity, (2) high thermal conductivity, and (3) the presence of a non-conductive, hard, brittle, and dense oxide film on the plate surface. are different. Therefore, in electrical resistance spot welding of aluminum alloy plates, it is necessary to flow approximately three times as much welding current as in the case of steel plates due to the above (1) and (2).

Hereinafter, the electric resistance spot welding phenomenon will be explained with reference to FIGS. 2 to 4.

In the figure, l is an electrode, 2.2' is an aluminum alloy plate, and 3.
3' is the electrode side surface, 6 is the nugget, and 7 is the temperature distribution between the electrodes.

When electricity is applied between the electrodes 1, the electrical resistance at the contact surface between the plates 2.2' and the contact surface between the electrode 1 and the plate 2.2' becomes higher than that of the aluminum base, and electrical resistance heat generation occurs. The temperature distribution 7 at the welded part is as shown in Fig. 2 (B).
Due to the above property (3), electric charge is stored in the non-conductive oxide film 8 (see Figure 3) on the electrode side surface 3.3', and then dielectric breakdown occurs, causing partial damage during welding. A phenomenon occurs in which a crack 9 is formed in the surface and electricity is passed only through this portion (arrow in FIG. 3). For this reason, during welding, heat generation at the electrode contact area becomes locally excessive, melting the aluminum alloy plate, and the generated molten A°C passes through the crack 9 to the plate side 3.
3' to the electrode 1, and by metallurgically forming a low melting point compound with Cu of the electrode material, a local molten part 12 is also formed in the electrode. It is believed that this melted portion 12 causes the electrode 1 to chip and deteriorate.

In electric resistance spot welding of aluminum alloy plates, the number of continuous welding points that can be continuously welded without electrode maintenance was 1/10 to 1/100 of that of steel plates.

Furthermore, it has been conventionally recognized that electrode dressing is necessary when spot welding aluminum plates. However, as shown in item 2, the continuous hitting ability is poor and it is necessary to dress every few dozen points.
This has not been considered a particular problem in applications where spot welding is not used frequently. However, as aluminum alloy plates came to be used in mass-produced automobiles as described above, the method of frequently dressing the electrodes could no longer be adopted.

[Problem to be solved by the invention]

Various methods have been proposed to solve these problems. For example, the electrode surface may be pretreated, the material of the electrode may be changed from general CrCu to alumina-dispersed copper, the shape of the electrode may be modified, the current rise speed may be controlled, etc.

However, none of these proposals has achieved a level of practical satisfaction on automobile assembly lines, and the reality is that the welding workability of aluminum alloy plates is inferior to that of steel plates.

The object of the present invention is to solve the above-mentioned problems and to significantly improve the workability of welding aluminum alloy plates.

[Means to solve the problem]

The present invention has discovered a unique phenomenon in which the Zn plating treatment on the electrode/plate interface side, which was conventionally thought to significantly deteriorate the continuous spot welding performance of spot welding on steel sheets, actually improves the continuous spot welding performance on aluminum alloy plates. Furthermore, similar effects were found for plated metals other than Zn, and the present invention was completed.

The gist of the present invention is to replace the aluminum oxide film present on the electrode surface of the aluminum alloy plate with a thin film of a different type of plated metal.

That is, the present invention provides a method for electrical resistance spot welding of aluminum alloy plates, in which a dissimilar metal coating is formed on one side of the aluminum alloy plate in contact with an electrode by plating.
It is characterized in that the electrodes are brought into contact with each other through the non-plated surface, and electricity is applied between the electrodes that are in contact with the plated surface.

Hereinafter, the configuration of the present invention will be explained in detail.

In the present invention, the aluminum alloy is not particularly limited, but for example, JIS A 5052, 5182
．． 5082.20366009.6010 etc. can be preferably applied.

Types of metals to be plated include Zn, Cu, Ni, Fe,
Cr, Ag, alloys of these metals, etc. can be used. There is no particular limit to the plating thickness, but if it is less than 0.1 μm, it may be An
may be exposed from the plating film, and even if the thickness exceeds 10 μm, the performance of the plating film will not change and it is not economical.

Although sufficient performance can be obtained with a single layer of plating film, it is also possible to plate Zn, which has excellent adhesion to Ag, as a base layer and then plate Fe or Ni as an upper layer.

Further, it is preferable that no aluminum oxide film exists between the plating film and the aluminum base. However, even if the aluminum oxide film is completely dissolved with chemicals, it is inevitable that an extremely thin aluminum oxide film will be formed due to contact between the aluminum alloy plate and the atmosphere until plating. Such an unavoidably generated aluminum oxide film under the plating film is permissible as long as the film thickness is 10 Å or less. Exceeding this range not only makes plating difficult, but also causes local concentration of current as described with reference to FIG. 3, which is undesirable.

The surface to be plated should be limited to only the side that comes into contact with the electrode, and it is not preferable to plate the interface where the aluminum alloy plates come into contact with each other, as this will actually make the welding unstable and reduce the weldability.

The oxide film on the other side, which is not plated, may have a natural oxide film thickness that occurs during the normal plate manufacturing process, but it is possible to actively perform oxidation annealing to reduce the oxide film thickness to, for example, 0.01 to 0.1 μm.
It may be increased to

The type of electric resistance spot welding method is not particularly limited, and usually three-phase rectification type, single-phase rectification type, electrostatic storage type, and three-phase frequency transmission type can be used.

Although various improvements have been made to the material, shape, etc. of the electrodes as described above, the present invention is not limited to these, and ordinary materials can be used.

[Function] In the method of the present invention, as shown in FIGS. 1(A) and 1(B), when plating is performed only on the plate surface 3.3' that contacts the electrode 1 to form a plating film 14.14', A plating film 14.14' is used instead of the non-conductive hard and brittle oxide film 8.
comes into contact with the electrode.

This plating film 14, 14' is made of a conductive metal excluding some impurities, and even if a surface oxide film is formed on the plating layer during storage of the aluminum alloy plate, the aluminum oxide film on the plate surface It is thinner and less dense than 8 (Fig. 3). For this reason, the electric current flows uniformly on the plated surface, so there is no local heat generation on the plate surface, so the temperature distribution is the first.
As shown in Figure (B), it seems that the deterioration caused by the local temperature rise at the interface of the electrode 1 is significantly reduced.

On the other hand, the aluminum oxide films 8,8' present on the joint surfaces of the aluminum alloy plates promote the formation of nuggets 6 due to temperature rise at the joint surfaces.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

[Example] One surface of an A A 6009 alloy plate with a thickness of 1.0 mm was degreased with a sodium phosphate degreaser, washed with water, and desmutted to remove the aluminum oxide film.
Current density 5 A/d in SO4, HF mixed plating bath
Electroplating was performed at m2 to achieve a Zn plating thickness of 3.
g/m''. An HF-containing plating solution was used to remove as much as possible of the thin aluminum oxide film formed on the alloy plate after desmutting.

Two of these plates were stacked with the plated surfaces facing the electrodes, and a continuous welding point electrical resistance spot welding test was conducted using a three-phase rectifier welding machine under the following conditions.

Here, continuous dot performance was evaluated based on the limit at which continuous welding could be performed without cleaning or replacing the electrode tip. This limit is determined by welding so that the initial nugget diameter is 6.0 mmφ, and as the successive dots progress, the nugget diameter decreases, and when this decrease causes the nugget diameter to become less than 4.8 mmφ. was set as the limit.

Current 30KA.

Pressure force 300kg Table 1 shows the results of this limit number of hits.

For comparison, the limit number of dots for a board not plated with Zn and a board plated with Zn on both sides are also shown in this table. However, when these plates were welded under the same conditions as the single-sided plated plates, the same nugget diameter could not be obtained, so the welding current was slightly modified so that the initial nugget diameter was 6.0 mmφ.

Table 1 [Effects of the Invention] As explained above, if the method of the present invention is followed, continuous spot welding performance will be improved! This enables electric resistance spot welding of aluminum alloys with a three-cycle current application time that greatly improves welding workability.

[Brief explanation of drawings]

Figure 1 (A) is a conceptual diagram of the welding method according to the present invention, Figure 1 (B) is a conceptual graph showing the temperature distribution in the vertical direction of the plate surface during welding according to the present invention, Figure 2 (A), (B) is a diagram similar to Figures 1 (A) and (B) for the conventional method. Figure 3 is an explanatory diagram showing the melting of the electrode side aluminum alloy plate during welding in the conventional method. Figure 4 is the molten metal. FIG. 3 is an explanatory diagram of a situation in which the aluminum alloy plate moves from the aluminum alloy plate to the electrode side. 1-electrode, 2.2'-aluminum alloy plate, 3.3'-
Electrode side surface, 6-nugget, 7-temperature distribution between electrodes, 1
2- Molten metal on the electrode side, 13- Plating film

Claims (1)

[Claims] 1. In an electric resistance spot welding method for aluminum alloy plates, plates having a dissimilar metal film formed by plating on one side of the aluminum alloy plate in contact with an electrode are brought into contact with each other via the non-plated surfaces, and are brought into contact with the plated surface. An electric resistance spot welding method for aluminum alloy plates with excellent continuous dot performance, characterized by applying current between electrodes.