JPH0655280A - Treatment before welding of aluminum alloy sheet for resistance spot welding - Google Patents

Abstract

PURPOSE:To improve a continuous spotting property and to reduce a welding cost by subjecting both surfaces of an aluminum alloy sheet to a chemical etching treatment to remove oxide films, then subjecting this alloy sheet to a heating oxidation treatment in the atm., thereby forming uniform oxide films on both surfaces. CONSTITUTION:Both surfaces 21A, 21B of the aluminum alloy sheet 1A (1B) of an Al-Mg system or Al-Mg-Si system are subjected to the chemical etching treatment to remove the oxide films 7 on both surfaces. The aluminum alloy sheet 1A (1B) after the chemical etching treatment is then subjected to the heating oxidation treatment in the atm. to form the uniform oxide films 10 on both surfaces. The generation of such an event that local melting is generated by local temp. rising and that electrodes are deteriorated is prevented if resistance spot welding is executed by using the aluminum alloy sheets 1A, 1B having the uniform oxide films 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy sheet used for an automobile body sheet or the like, particularly an Al--Mg type or Al--Mg--Si type aluminum alloy sheet for resistance for assembling an automobile body. The present invention relates to a pretreatment method performed in advance before resistance spot welding when performing spot welding.

[0002]

2. Description of the Related Art In the past, steel sheets were often used for automobile body sheets, but in recent years, aluminum alloy sheets have been often used from the viewpoint of weight reduction of the vehicle body. Resistance spot welding is usually used for assembling an automobile body using a steel sheet as a body sheet, but when an aluminum alloy plate is used instead of the steel sheet, resistance spot welding is applied to the aluminum alloy plate. Often.

By the way, aluminum alloys have lower electric resistance and higher thermal conductivity than steel,
Furthermore, the physical properties are different from steel, such as the presence of a non-conductive hard and brittle oxide film on the plate surface. Due to these differences, in resistance spot welding of an aluminum alloy plate, it is usually necessary to flow a welding current of about three times as large as that of a steel plate.
Therefore, at the time of resistance spot welding of the aluminum alloy plate, the heat generation of the electrode becomes excessive, and fusion is likely to occur between the aluminum alloy plate to be welded and the electrode, and the fusion zone moves to the plate side. Unevenness such as chipping occurs on the electrode, and a brittle intermetallic compound layer is generated on the electrode surface,
Deterioration of the electrode causes continuous spotting, that is, the extent to which spot welding can be continuously performed without maintaining or replacing the electrode (the number of continuous spot welding spots) is 1 /
There is a problem that it is only about 10 to 1/100.

Such a problem has heretofore been known as a problem in resistance spot welding of an aluminum alloy plate, but spot welding of the aluminum alloy plate has not been used so far, and therefore, it has been particularly important in the past. Didn't. However, recently, since aluminum alloys have been applied to mass-produced automobiles as described above, this problem has come to be regarded as a major drawback.

Various methods have been proposed as methods for solving such problems. For example, some kind of pretreatment may be applied to the electrode surface, the material of the electrode may be changed, the shape of the electrode may be modified, or the spot shape may be different between the electrode and the plate or between the plate and the plate. Insert a thin plate made of metal, control the rising speed of the current, and further anodize only one side of the plate to increase the oxide film thickness and its anodized surface on the interface side of the plate and the plate. Methods such as setting to be located are known. However, none of these methods has been able to surely and sufficiently solve the above-mentioned problems, and has a problem that it causes technical difficulties in other aspects and increases costs.

[0006]

The situation in resistance spot welding of an aluminum alloy sheet and the temperature distribution in the sheet thickness direction at that time are shown in FIGS. In FIG. 2A, the aluminum alloy plate 1 to be welded
A and 1B are sandwiched between electrodes 2A and 2B,
When pressure is applied and current is applied by 2B, a nugget (welding metal) 3 is formed between the aluminum alloy plates 1A and 1B by electric resistance heat generation, and spot welding is performed. At this time, in the vicinity of the mutual interface 4 between the aluminum alloy plates 1A and 1B and near the interface 5 between the respective electrodes 2A and 2B and the respective aluminum alloy plates 1A and 1B, in addition to the contact resistance of the interfaces, Due to the non-conductive oxide film that is present, the electrical resistance is significantly higher than that of the aluminum alloy base metal part. Therefore, the temperature distribution in the plate thickness direction during energization is as shown in Fig. 2B. Become. Here, the high temperature near the interface 4 between the aluminum alloy plates 1A and 1B contributes to melting the aluminum alloy near the interface 4 to form the weld nugget 3, but the electrodes 2A and 2B and Aluminum alloy plate 1A,
The high temperature in the vicinity of the interface 5 with 1B causes unnecessary local melting in this vicinity, which causes the deterioration of the electrode as described above and causes the deterioration of the continuous spotting property.

By the way, the reason why the temperature becomes high in the vicinity of the interface 5 between each electrode 2A, 2B and each aluminum alloy plate 1A, 1B and local melting is caused is that not only the contact resistance exists between the two as described above. In particular, it is considered that an aluminum alloy, in particular, has an oxide film on the surface of the plate, so that the electrical resistance at that part is significantly increased. However, when the inventors further conducted experiments and studies on the phenomenon, it was not only the fact that an oxide film was present, but rather that the oxide film was rather non-uniform. There was found. This is especially the case of Al-Mg-based alloys such as JIS 5000 series alloys and Al-Mg-S such as JIS 6000 series alloys.
It has been found that this is remarkable in the case of the i-based alloy.

That is, Al-Mg alloy or Al-Mg
Since the Si-based alloy contains a large amount of easily oxidizable Mg, the ingot is subjected to homogenization treatment after face-shaping, hot rolling process,
Further, the plate surface is easily oxidized by the subsequent heat treatment such as intermediate annealing. And the oxide film formed in these processes will be stretched by cold rolling, but since the oxide is hard and brittle, it is often divided and separated by rolling, and therefore is subjected to resistance spot welding. Stage Al
In a -Mg-based or Al-Mg-Si-based aluminum alloy plate, the thickness of the oxide film on the surface thereof and the state of oxide presence are usually non-uniform. A typical example of the situation of such a non-uniform oxide film 7 is schematically shown in FIG. In FIG. 3, the oxide film 7 has flakes or other shapes of non-adhesive oxide pieces 7B that are peeled or nearly peeled on the outermost surface of the oxide layer 7A that is in close contact with the aluminum alloy base metal portion 6. Has been present in. Of course, in reality, there are various complicated situations other than the situation shown in FIG. 3, and in any case, the thickness of the oxide film 7 and the state of oxide presence are usually non-uniform. . If the film thickness and the oxide existence state in the oxide film 7 are non-uniform in this way, the electric resistance in the film thickness direction will necessarily be non-uniform.

Thus, the oxide film is non-uniform and the electric resistance in the film thickness direction is non-uniform.
When the Si-based aluminum alloy plate is directly subjected to resistance spot welding according to the conventional method, as shown in FIG. 4, each electrode 2A, 2B and each aluminum alloy plate 1A, 1
At the interface 5 with B, the welding current 8 flows non-uniformly, so that the plate surface temperature locally rises at the portion 9 where the current density is locally high and local melting occurs at that portion 9,
A melting reaction occurs with the electrode and deteriorates the electrode. When such a situation is repeated at continuous spotting, the electrode deteriorates at an early stage, and the continuous spotting property deteriorates.

Furthermore, the relationship between the thickness of the oxide film of the aluminum alloy plate and the continuous spotting property in resistance spot welding is not unique, and two factors mainly influence it. That is, the thicker the oxide film on the surface of the aluminum alloy plate on the side of the interface 5 between the electrode and the aluminum alloy plate, the higher the temperature in the vicinity of the interface, which causes the deterioration of the electrode as described above and the continuous dot property. On the other hand, if the oxide film on the interface 4 side between the laminated aluminum alloy plates is thick to some extent, a sufficient amount of heat can be obtained to form a welding nugget at that part, which is good. Continuous dot performance is obtained. FIG. 5 shows the results of experiments conducted by the present inventors on such a relationship. As is clear from FIG. 5, when both the continuous spotting property with respect to the interface 4 and the continuous spotting property with respect to the interface 5 are taken into consideration, there is an oxide film thickness for obtaining an overall good continuous spotting property. It is clear that there is a proper area. However,
As described above, in the Al-Mg-based or Al-Mg-Si-based aluminum alloy plate, not only the oxide film thickness is not uniform, but also the oxide film thickness varies depending on the manufacturing process conditions. It is difficult to secure the thickness, and this has also been a cause of deteriorating continuous dot performance.

The present invention has been made in view of the above circumstances, and in particular, Al-Mg system or Al-Mg-Si.
It is an object of the present invention to provide a welding pretreatment method for making it possible to significantly improve continuous spotting property in resistance spot welding of a series aluminum alloy sheet.

[0012]

In order to solve the above-mentioned problems, the present invention basically requires that prior to resistance spot welding an Al--Mg system or Al--Mg--Si system aluminum alloy plate, Both surfaces of each aluminum alloy plate are chemically etched to remove the oxide film on both surfaces, and then heated and oxidized in the atmosphere to form a uniform oxide film on both surfaces.

[0013]

The process of the pretreatment method of the present invention will be described with reference to FIGS.
The operation of the present invention will be described based on FIG.

First, Al-Mg as shown in FIG.
Type or Al-Mg-Si type aluminum alloy plate 1A
Both surfaces 21A and 21B of (1B) are chemically etched to remove the oxide film 7 on each surface as shown in FIG. 1 (B). Then, the aluminum alloy plate 1A (1B) after the chemical etching treatment is subjected to a heat oxidation treatment in the atmosphere to form a uniform oxide film 10 on both surfaces as shown in FIG. 1 (C).

Here, the oxide film 7 before the chemical etching treatment is caused by the presence of a non-adhesive oxide piece 7B which is peeled or is about to be peeled off on the outermost surface thereof as shown in FIG. 1 (A). Then, the entire film thickness and the presence state of oxide are usually non-uniform, but the non-uniform oxide film 7 is removed by the chemical etching process, and the heat-oxidation process is appropriately performed again. As a result, a uniform oxide film 10 is formed. The new oxide film 10 has not only a uniform film thickness but also a uniform oxide existence state. Further, after the uniform oxide film 10 is formed in this way, since the expansion processing such as rolling is not performed,
The uniform state is maintained until resistance spot welding.

Since the uniform oxide film as described above has a uniform electric resistance in the film thickness direction, the uniform oxide film 1
When the resistance spot welding as described above is performed using the aluminum alloy plates 1A and 1B having 0, the deviation of the current is prevented from occurring at the interface 5 between the electrode and the aluminum alloy plate, and the welding current flows uniformly. Therefore, the current density does not locally increase in the vicinity of the interface 5, so that it is possible to effectively prevent the occurrence of a situation in which local melting causes local melting and deteriorates the electrode.

Further, as described above, the oxide film having an initial non-uniform thickness is removed, and an oxide film having a uniform thickness is again generated by an appropriate heat oxidation treatment. The thickness can be adjusted to be within the optimum thickness range for the continuous hitting property as shown in FIG.

As the chemical etching treatment for removing the oxide film on the surface of the aluminum alloy plate, pickling using an acid solution such as nitric acid, hydrochloric acid or sulfuric acid, or alkali cleaning using an alkali solution such as caustic soda, etc. It may be applied, but it is not limited to these, and the point is that the oxide film on the plate surface can be removed.

Regarding the conditions for the heat oxidation treatment in the atmosphere for forming a uniform oxide film again, for example, an ordinary electric furnace may be used and the temperature may be maintained within the range of 250 to 390 ° C. for 0 to 5 hours. . The optimum thickness of the new oxide film generated by this heat oxidation treatment is welding conditions, plate thickness,
Although it depends on the alloy composition, etc., it is usually 100 to 10
It may be about 00 angstrom.

The welding method and welding conditions for performing resistance spot welding using the aluminum alloy sheet treated by the welding pretreatment method of the present invention are not particularly limited, and are applied to ordinary aluminum alloy sheets. The method and conditions described above may be applied.

Further, the Al-M object of the present invention
The component composition of the g-based alloy or the Al-Mg-Si-based alloy is not particularly limited, but normally the Mg content is 0.2 to
6.0%, and if necessary, Mn 0.1-2.
0%, Cu 0.05-1.0%, Si 0.1-2.0
%, Cr 0.01 to 0.2%, Ti 0.01 to 0.3%
An alloy containing one or more of the above and the balance consisting of Al and unavoidable impurities is suitable. If the Mg content is less than 0.2%, there is a problem that an excessive welding current is required from the viewpoint of welding, the spot welding strength is too low, and the strength is insufficient as an automobile body sheet or the like. . Further, if Mg exceeds 6.0%, it becomes difficult to manufacture a plate. Furthermore, Mn, Cu, Si, C
The addition of r and Ti has the effect of improving the strength or formability of the alloy. If the respective lower limits are exceeded, the above effects will not be sufficiently obtained, and if the respective upper limits are exceeded, the above effects will be saturated. On the contrary, there is a possibility that the moldability is lowered. In addition, as a practical alloy satisfying the above-described compositional range, JIS A 5052 alloy, 51
82 alloy, 5082 alloy, 2036 alloy, 6009 alloy, 6010 alloy and the like, but it is needless to say that they are not limited to these.

[0022]

EXAMPLES Aluminum alloys shown by alloy numbers 1 to 4 in Table 1 were cast by a DC casting method according to a conventional method, subjected to chamfering and homogenization treatment, then hot rolled, and further cooled while sandwiching intermediate annealing. It was subjected to hot rolling to obtain a 1 mm plate. As a chemical etching treatment for removing the oxide film, each plate was etched by immersing it in hydrofluoric acid having a concentration of 1% at room temperature for 1 minute, washed with water, dried, and then dried in air at 350 ° C x 2 A heat oxidation treatment for a period of time was performed.

With respect to each of the plates after the heating and oxidation treatment, two sheets were piled up, and an electric resistance spot welding test was conducted by a continuous welding system using a three-phase rectification type spot welding machine. The welding conditions at this time are as follows: welding current 32k
A, the pressing force was 300 kg, the electrification time was 0.1 sec for one dot, and the continuous dot cycle was 20 times / min.

In such a resistance spot welding test of continuous spots, the evaluation of continuous spot properties was performed as follows. That is, the number of dots was evaluated up to the limit at which welding can be performed continuously without cleaning or replacing the electrode tip. Specifically, in general, the electrode deteriorates with the progress of continuous hitting points, which gradually reduces the nugget diameter. In this welding test, the initial nugget diameter was 6.0 mmφ.
The above-mentioned limit was set at the stage where the nugget diameter became smaller than 4.8 mmφ as the continuous hitting point progressed. Table 2 shows the results obtained by repeating the resistance spot welding test twice for each plate for the number of hit points up to the limit.

As a comparative example, JIS A 5182
Table 2 also shows the results of examining the limit number of points when the alloy plate was subjected to only the thermal oxidation treatment without the chemical etching treatment under the same conditions as described above.

[0026]

[Table 1]

[0027]

[Table 2]

From Table 2, in the example of the present invention to which the welding pretreatment method of the present invention is applied, the limit continuous dot number is about twice or more as compared with the case of the comparative example in which only the heat oxidation treatment is applied. It is clear that the continuous dot performance has been dramatically improved.

[0029]

EFFECT OF THE INVENTION Al-Mg system or Al-Mg-Si
Before performing resistance spot welding on an aluminum alloy plate made of a system alloy, if the aluminum alloy plate to be welded is subjected to welding pretreatment by the method of the present invention, the electrode may be formed due to the nonuniformity of the oxide film. The possibility of uneven current density near the interface between the aluminum alloy plate and the aluminum alloy plate is reduced, and as a result, there is a risk of electrode deterioration due to local melting due to local temperature rise due to the above uneven current density. By adjusting the heat oxidation treatment conditions, it is possible to form an oxide film that is uniform and has an optimum thickness for continuous dot formation.
Therefore, for these reasons, the continuous spotting property in resistance spot welding can be significantly improved as compared with the conventional one.
Therefore, the frequency of electrode cleaning and replacement in resistance spot welding can be reduced, welding workability can be significantly improved as compared with the prior art, and welding costs can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing stepwise a welding pretreatment method of the present invention.

FIG. 2 is a schematic diagram showing an example of a conventional resistance spot welding condition on an aluminum alloy plate and an example of a temperature distribution in a plate thickness direction during welding.

FIG. 3 is a cross-sectional view schematically showing an example of a situation near the surface of a conventional aluminum alloy plate.

FIG. 4 is a schematic diagram schematically showing a distribution of welding current when performing resistance spot welding on the conventional aluminum alloy plate shown in FIG.

FIG. 5 is a graph showing the relationship between the thickness of an oxide film and the continuous spotting property when resistance spot welding is performed on an aluminum alloy plate.

[Explanation of symbols]

 1A, 1B Aluminum alloy plate 2A, 2B Electrode 7 Oxide film 10 Oxide film

Claims (2)

[Claims] 1. An Al-Mg system or Al-Mg-Si
Prior to resistance spot welding of aluminum-based aluminum alloy sheets, both sides of each aluminum alloy sheet are chemically etched to remove the oxide film on both sides, and then heat-oxidized in the atmosphere to uniformly oxidize both sides. A method for pre-welding an aluminum alloy plate for resistance spot welding, which comprises forming a film. 2. The aluminum alloy plate contains Mg in an amount of 0.2 to 6.0% (weight%, the same applies hereinafter), and if necessary, Mn of 0.1 to 2.0% and Cu of 0.05 to
1.0%, Si 0.1-2.0%, Cr 0.01-0.
2. An aluminum alloy plate for resistance spot welding according to claim 1, wherein an alloy containing 2% and one or more of 0.01 to 0.3% Ti and the balance Al and inevitable impurities is used. Welding pretreatment method.