JPH05320932A - Aluminum sheet excellent in spot resistance weldability, workability and corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the service life of an electrode at the time of spot resistance welding, i.e., continuous spottability, the press formability and corrosion resistance (crevice corrosion resistance) of an aluminum sheet without deteriorating its coating properties. CONSTITUTION:Among the surfaces removing the oxidized film of an aluminum sheet, on the surface on the side of the joined face by spot welding, a chromate film with 5 to 200mg/m<2> chromium coating weight expressed in terms of metal chromium is formed, and on its surface, a lubricating inorganic compound layer is formed. On the surface of the side of the face in contact with an electrod, a chromate film is formed in such a manner that the chromium coating weight is regulated to <=100mg/m<2> and regulated to <=2/3 of the chromium coating weight on the side of the joined face by the welding, and on its surface or directly on the surface of aluminum on which a chromate film is not formed, a water-soluble lubricating inorganic compound layer is formed. Thus, the aluminum sheet excellent in spot resistance weidability, workability and corrosion resistance is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum or aluminum alloy sheet mainly used as a lightweight material for automobiles, and particularly to an aluminum sheet excellent in spot resistance weldability, press workability and corrosion resistance.

[0002]

2. Description of the Related Art In recent years, in order to improve fuel efficiency of automobiles from the viewpoint of global environmental problems, in particular, measures against global warming due to CO ₂ , the weight reduction of automobile bodies has been actively promoted. Aluminum alloys have been spotlighted in terms of reliability, processability, recycling, etc., and are recently being applied to internal and external boards of automobile bodies.

However, most of these aluminum alloy sheets are spot resistance welded when they are assembled into an automobile body. However, since the aluminum alloy sheets are extremely inferior in spot resistance welding property, especially electrode life, to the steel sheet, The current situation is that it has caused a significant decrease in production efficiency.

That is, in spot resistance welding of an aluminum alloy plate, the welding phenomenon is likely to occur and the number of welding spots at which an appropriate nugget can be obtained with the same electrode is extremely short, so that proper nugget cannot be performed before welding occurs. You can arrange the electrode shape by dressing before,
Alternatively, the frequency with which the electrode is replaced with a new one also increases, which has a great influence on the welding efficiency and, consequently, the production efficiency of the automobile body.

Since aluminum alloys have only recently been put to practical use in mass-produced products by spot resistance welding such as automobiles, there are very few proposals for proper means for solving the above problems, but Kaisho 60-18
Japanese Patent No. 7483 proposes an improvement method by controlling the thickness of the oxide film on the aluminum plate joint surface by anodizing. Although not intended to be applied to aluminum alloy plates for automobiles, JP-A-53-6252, JP-A-53-48954, and JP-A-53-48955 disclose Zn and Ti on the aluminum bonding surface. And an improved method by interposing a stainless steel thin film is disclosed in JP-A-57-
Japanese Patent No. 4389 discloses an improvement method by plating with Cr, Ni, Cu, and Ag.

However, the improvement method by controlling only the anodized film is not practical because uniform treatment is difficult and its effect is unstable. Further, the improvement method by interposing Zn, Ti, and stainless steel thin films is difficult to apply to automobiles, and the improvement method by applying Cr, Ni, Cu, Ag plating is the adhesion of these platings to aluminum alloys. Is extremely bad and not practical.

Further, aluminum has a serious drawback in that it has a small elongation and is soft as compared with a steel sheet, so that it tends to cause galling and is inferior in press formability.

Furthermore, these aluminum plates are originally excellent in corrosion resistance, and this corrosion resistance is maintained by an oxide film on the surface, but in the gaps such as hem portions formed by spot welding or bending work. There is a danger that this oxide film will be destroyed and cause so-called crevice corrosion.

[0009]

On the other hand, it is premised that the above-mentioned drawbacks of the aluminum plate are eliminated and that the paintability of the molded product obtained by press-molding the aluminum plate is not lost. Thus, an object of the present invention is to provide an aluminum plate excellent in electrode life during spot resistance welding, that is, continuous spotting property, press formability, and corrosion resistance (crevice corrosion resistance), without losing paintability. To do.

[0010]

The above object is the general concept of the present invention, that is, of the surface of the aluminum plate from which the oxide film has been removed, the amount of chromium deposited on the surface on the spot welding joint surface side in terms of metallic chromium. A chromate film of 5 to 200 mg / m ² is formed and a lubricating inorganic compound layer is formed on the chromate film, and the surface of the electrode contact surface side has a chromium adhesion amount of 100 mg / m ² or less and the welding joint surface side. 2/3 of the amount of chromium deposited
It is characterized in that a water-soluble lubricating inorganic compound layer is formed on a chromate film formed on the aluminum surface or directly on the aluminum surface without forming a chromate film so as to have the following adhesion amount. It is achieved by an aluminum plate having excellent spot resistance weldability, workability, and corrosion resistance.

[0011]

[Operation] Improving weldability When spot resistance welding is continuously repeated on an aluminum alloy plate using the same copper-based electrode, the current-carrying diameter at the electrode tip expands with the spot, and the current density in the plate Although the diameter of the nugget also decreases due to the decrease, and finally the life of the electrode is reached, the aluminum alloy plate is characterized in that the expansion rate of the electrode tip diameter is remarkably higher than that of the steel plate.

As a result of paying attention to this phenomenon and carefully studying the cause thereof, in the case of continuous spot welding of an aluminum alloy plate, molten aluminum adheres to the current-carrying portion of the electrode tip,
The oxide deposits together with the spots, which forms an insulating layer, which eventually causes a spark between the electrode and this insulating layer, where the electrode becomes worm-like and the aluminum alloy at the joint surface. It was found that the insulating oxide film on the plate surface hinders rapid welding. It was found that such a peculiar phenomenon of the aluminum alloy plate promotes the enlargement of the electrode tip diameter and the electrode life is extremely deteriorated.

Based on the above findings of the present invention, it will be understood that the removal of the non-uniform oxide film on the aluminum surface suppresses the generation of sparks on the electrode and the aluminum surface.

Further, by performing a specified amount of chromate treatment on the welded joint surface side, the resistance heat generation amount on the welded joint surface side is secured. On the other hand, on the electrode contact surface, the chromate treatment amount is set to 2/3 or less in terms of chromium adhesion amount as compared with the welded joint surface in consideration of not performing chromate treatment. As a result, heat generation on the electrode surface side is suppressed and generation of sparks is suppressed, aluminum welding to the electrode is prevented, and spot weldability is improved.

Improvement of Press Formability Improvement of press formability is achieved by providing a lubricating inorganic compound layer exemplified by borate of alkali metal. This is based on the finding that the moldability can be improved by improving the lubricity of the surface.

The reason why an inorganic substance such as a borate of an alkali metal is effective for improving slidability is that an aqueous solution containing the same is brought into contact with an aluminum alloy plate and dried by heating to form a film with a specific amount of adhesion. In that case, alkali metal borate, etc. forms a strong film with a network structure on the surface of the aluminum plate, and this film prevents the direct contact between the aluminum alloy plate and the press die and the lubrication property of the film itself causes sliding. It is presumed that the mobility is further improved.

Improvement of corrosion resistance (crevice corrosion resistance) Since the aluminum surface can be protected by performing chromate treatment and resin coating treatment, the corrosion resistance can be remarkably improved by these treatments.

Maintainability of coatability The lubricating inorganic compound layer provided for the purpose of improving press moldability needs to be substantially removed by a degreasing step as a pretreatment step for coating, that is, washing with water or alkaline water. If it remains, it adversely affects the painting process and deteriorates the paintability. Therefore, the paintability of the aluminum plate is maintained by making the lubricating inorganic compound layer substantially removable by the degreasing step.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A more preferred embodiment of the present invention and advantages based on it will become apparent by describing the structure of the present invention in detail below.

Aluminum Plate The aluminum plate in the present invention is aluminum or aluminum and magnesium, manganese, silicon,
It is a plate-shaped material that can be spot resistance welded and is formed of an aluminum alloy with at least one metal selected from chromium, copper, and zinc.

In the present invention, the chromate treatment is performed after removing the non-uniform oxide layer formed on the surface of the aluminum plate during the manufacturing process. As a result, a uniform chromate layer is formed. As a method for removing the oxide layer, a commonly used method can be used. For example, methods such as degreasing and nitric hydrofluoric acid pickling / washing, degreasing and alkali etching washing, and alkali etching and light pickling / washing can be exemplified.

The chromate deposition amount of the chromate layer to the chromate layer weld joint surface ensures a resistance heating of the weld joint surface, yet ensure clearance of the corrosion resistance to be formed after welding joining (crevice corrosion resistance) 5 to 200 mg / m ² reckoned as metal chromium terms, preferably 10-100 mg / m ^2.

Although it is not necessary to perform chromate treatment on the electrode contact surface, it is preferable to perform chromate treatment to form a chromate layer in order to prevent the aluminum from being rusted. From the viewpoint of prevention of welding between the aluminum and the electrode, 100 mg / m ² or less in the chromate layer is a chromium metal basis, of preferably a 1~60mg / m ^2, yet welded surface 2/3
It is formed so as to have the following chromium deposition amount. Of course, also on the electrode contact surface side, the aluminum oxide layer of the aluminum plate is removed in advance. In the degreasing step as a pretreatment for coating, the chromate is dissolved in the (alkali) degreasing solution and contaminates the (alkali) degreasing solution, but if the chromium fixing ratio of the chromate film is 95% or more, it does not cause a problem. Therefore, it is preferable to carry out the chromate treatment so that the chromium fixing rate in the treatment with the degreasing liquid becomes 95% or more.

The method of increasing the chromium fixing rate to 95% or more is as follows.
After the chromate treatment, a method of washing with water or hot water and then drying, a method of drying at a high temperature of 150 ° C. or higher, and the like can be adopted.

As the chromate treatment method, a reactive chromate method, a coating chromate method, or an electrolytic chromate method can be adopted. As a method for uniformly forming a chromate film on an aluminum plate, a reactive chromate method and a coating chromate method are excellent and can be preferably used.

Lubricating Inorganic Compound Layer The presence of this layer imparts slidability to the surface of the aluminum plate and improves the press workability of the aluminum plate. Moreover, since this layer is formed of a water-soluble inorganic compound, it can be practically removed in the degreasing step as a pretreatment for coating, that is, the washing step with water or alkaline water, so that the coating property after pressing may be impaired. Absent.

Examples of the inorganic compound constituting the lubricative inorganic compound layer include alkali metal borates, phosphates, carbonates and the like, which are remarkably effective in improving slidability and are easily soluble in water. Of these, Na salts, K salts and Li salts of metaboric acid, tetraboric acid, pentaboric acid and the like are preferable, and the water-containing crystals of these salts are more preferable because they are more excellent in solubility in water or alkaline water used for degreasing. An inorganic compound layer can be provided by bringing an aluminum plate into contact with an aqueous solution obtained by dissolving such a compound in water and drying it at a temperature of usually 50 to 100 ° C. By appropriately controlling the temperature and time during drying, a layer composed of water-containing crystals of the above salt can be formed.
In industrial practice, it is recommended that an inexpensive aqueous solution of borax is used to form a layer of sodium tetraborate decahydrate (Na ₂ B ₄ O ₇ · 10H ₂ O).

The layer of the inorganic compound is 10 to 1000 mg / m ² , particularly 50 to 1000 for the purpose of sufficiently obtaining the effect of improving the slidability and minimizing the residual amount after the degreasing treatment step.
It is preferably 500 mg / m ² .

[0029]

EXAMPLES Next, the present invention will be specifically described based on examples. (Example) A 5 mm alloy having a thickness of 1 mm, which is a typical aluminum alloy plate (alloy plate of aluminum and magnesium) used as a material for automobile bodies, was used. First, the aluminum alloy surface was vapor-degreased with trichlorethylene, An oxide layer removal treatment was performed using a caustic soda solution and a nitric acid solution, the chromate treatment shown in Table 1 was performed, and subsequently an aqueous solution of borax was applied, followed by drying at 60 to 70 ° C to prepare a test piece. The amount of chromium deposited and the amount of borate deposited were determined by fluorescent X-ray analysis.

The chromates used are as follows. Chromate Reactive Chromate A Phosphoric Acid Chromate: Alchrome 3701 (Nippon Parker Rising) B Chromic Acid Chromate: Alchrome 713 (Nippon Parker Rising) Coating Chromate C No silica added: 4513H (Nippon Parkerizing) D Silica addition: Cosma- 150 (made by Kansai Paing)

Evaluation of workability The workability of the aluminum alloy plate after the surface treatment is 70 mmφ.
Each sample punched out was subjected to high-speed cylindrical drawing with a diameter of 33 mmφ (processing speed: 500 mm / sec), and the film was removed or the side surface of the cylinder had galling was defective (marked with X). These defects did not occur. Those that were good (marked with ◯) were visually judged.

Evaluation of Weldability The electrode life of the aluminum alloy plate after the surface treatment was evaluated. That is, the following electrodes and welding conditions were evaluated using an inverter DC welding machine. In the evaluation, the effect was judged by either the number of dots until welding or the number of dots until the nugget diameter falls below 4√t (t: plate thickness), whichever is smaller. (Electrode) -Shape: Cone head (CF) type-Tip diameter: 5.0 mmφ-Material: Cr-Cu (welding condition) -Pressure: 150 kgf-Initial pressurization time: 20/50 seconds-Electrification time : 6/50 seconds ・ Holding time: 5/50 seconds ・ Welding current: 15 kA Weldability evaluation criteria are as follows: the number of dots is less than 500 (poor), 500 to 1000 is normal (△), 1000 to 1000
1500 points are good (○), 1500 points or more are excellent (◎)
Evaluated as.

Evaluation of Corrosion Resistance In order to evaluate the corrosion resistance after welding, two 50 × 100 mm plates were joined under a new electrode under the above conditions, and then a salt spray test (5% NaCl, 35 ° C., 2000 hours) was performed. After that, the test piece was separated, and the corrosion resistance on the joint surface side (gap structure side) was evaluated. The evaluation standard is the maximum erosion depth, which is 0.05
Less than mm was evaluated as good (◯), 0.05 to 0.1 mm was evaluated as normal (Δ), and 0.1 mm or more was evaluated as poor (x).

Content of removal of the lubricating inorganic compound layer in the degreasing step
Ease (Degreasing solubility) The degreasing liquid FC- manufactured by Nippon Parkerizing Co., Ltd.
Spraying 4460 at 40 ° C for 30 seconds (water pressure 1 kg
/ cm ² ) and the residual amount of borate was determined by fluorescent X-ray analysis.

Chromium Fixation Ratio The residual amount of chromium after spraying was determined by fluorescent X-ray analysis in the same manner as above.

The above results are summarized in Table 2.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

As is clear from the results shown in Table 2, Example 1 of the present invention
~ 7 shows excellent weldability, workability, and corrosion resistance,
The borate layer is easily removed with a degreasing solution. On the other hand, in Comparative Example 8, since the oxide layer removal treatment was not performed, chromate unevenness occurred, and the weldability and corrosion resistance were insufficient. In Comparative Example 9, the amount of chromium deposited on the electrode contact surface side was 2 on the welded joint surface side.
Since it exceeds / 3, sufficient weldability cannot be obtained. Comparative Example 1
No. 0 does not have sufficient corrosion resistance and weldability because the amount of deposited chromium is small. In Comparative Example 11, the chromium deposition amount was too large, and the weldability was rather deteriorated. Comparative Example 12 is inferior in workability because it has no borate treatment.

[0041]

As described above, according to the present invention, the oxide layer on the surface of the aluminum plate is removed, then the chromate treatment of the thickness difference is performed, and the lubricating inorganic compound layer is provided thereon to improve the weldability and workability. The corrosion resistance can be remarkably improved and the paintability of the aluminum plate is not impaired. By spot welding with the aluminum plate of the present invention,
Since the productivity of processed automobile parts is improved and the corrosion resistance of the hem portion formed by the welding and bending is improved, it is expected that the quality stability is also improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Uesugi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor Masaki Mabuchi, Chuo-ku, Chiba-shi, Chiba 1st Kawasaki-cho, Technical Research Division, Kawasaki Steel Co., Ltd. (72) Inventor, Rin Tadashi, 1st Kawasaki-cho, Chuo-ku, Chiba, Chiba Inside Technical Research Division, Kawasaki Steel Co., Ltd.

Claims (3)

[Claims] 1. A chromate film having a chromium adhesion amount of 5 to 200 mg / m ^{2 in} terms of metallic chromium is formed on the surface of the aluminum plate where the oxide film has been removed, on the surface on the spot welding joint surface side, and lubrication is performed thereon. Chromate coating is formed on the surface of the electrode contact surface side so that the adhesion amount of chromium is 100 mg / m ² or less and 2/3 or less of the adhesion amount of chromium on the weld joint surface side. Excellent in spot resistance weldability, workability, and corrosion resistance, characterized in that a water-soluble lubricating inorganic compound layer is formed directly on the aluminum surface without forming a chromate film Aluminum plate. 2. The lubricating inorganic compound layer is 10 to 1000.
The aluminum plate according to claim 1, which has an amount of mg / m ² . 3. The aluminum plate according to claim 1, wherein the lubricating inorganic compound is a hydrous borate of an alkali metal.