JPH05320950A - Aluminum sheet modified in surface - Google Patents

Abstract

PURPOSE:To improve the press formability, spot weldability and filiform rust resistance by providing the surface of an Al sheet with a surface layer constituted of a Zn layer or which Al is diffused and Al for which Zn is diffused. CONSTITUTION:The surface of an Al sheet is coated by galvanization so as to be the total Zn coating weight to 0.5 to 10g/m<2> by a substitution plating method, an electroplating or deposition plating method. Then, thermal diffusion treatment is applied to form a surface layer having the double layer structure of a Zn layer for which Al is diffused and an Al substrate therebelow for which Zn is diffused. Heat treating conditions are preferably selected so as to regulate the coating weight of Zn in the Zn layer for which Al is diffused after the thermal diffusion treatment to be >=0.01g/m<2> and the coating weight of Zn in the Al substrate to be >=0.1g/m<2>. In such a manner, its lubricity at the time of press forming and continuous spotting properties at the time of spot welding, etc., are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to aluminum and aluminum alloy plates (hereinafter collectively referred to as "aluminum plates") which are used for press forming, joint assembly and painting in the fields of automobiles, building materials and the like. The present invention relates to an aluminum plate surface-modified for the purpose of having both lubricity in press molding, spot weldability, and thread rust resistance after coating.

[0002]

2. Description of the Related Art The application of aluminum plates to automobile body panels has been studied in recent years by taking advantage of their light weight. However, there are the following disadvantages, and their improvement is strongly desired. In press molding, the surface has a high sliding resistance and is easily plastically deformed so that it is easily cracked and is a material that is extremely difficult to mold. Although it is possible to improve it to some extent by applying a solid lubricant or the like to the surface, there is a drawback that it adversely affects adhesion and painting. In spot welding, aluminum is also an electrical conductor and has a low melting point, so it is also a difficult material to handle. Furthermore, aluminum has an inherent drawback that thread rust easily occurs in the usage environment after painting,
There is a strong demand for improvements in automobile bodies, etc. where aesthetics are important. In order to improve the yarn rust resistance, for example, JP-A-61-157693 discloses that
1 to 1 Zn or Zn-based plating layer on the aluminum plate surface
A coating of 5 g / m ² is disclosed. However, this method has almost no effect of improving press formability and spot weldability.

[0003]

In order to apply an aluminum plate to an automobile body panel or the like, it is important to impart properties having both press formability, spot weldability, and thread rust resistance after painting. It is an issue. The present invention aims to solve these problems by modifying the surface of aluminum. That is, it is an object of the present invention to provide a surface layer that improves lubricity in press molding, surface electric resistance in spot welding, and thread rust resistance.

[0004]

The features of the present invention are as follows.
Ro is a Z with aluminum diffused on the aluminum plate surface.
Zn layer diffused into the n-layer and the aluminum substrate below it.
Surface-modified aluminum plate consisting of layers, and Zn
The total Zn coating amount of the system plating is 0.5 to 10 g / m ²And
Z in the Zn layer in which aluminum has diffused after thermal diffusion treatment
n is 0.01 g / m²Above, Zn in aluminum substrate
Is 0.1 g / m²The above is the aluminum plate. A
Zn-based plating on a aluminum plate, 200 ℃, 10:00
When heat-treated at a temperature of 550 ° C for about 1 second,
Al diffuses into the n-based plating layer, and at the same time aluminum base
Zn diffuses into the plate. Depending on heat treatment conditions, plating
The majority of the Zn formed diffuses into the aluminum substrate,
The penetration depth reaches several tens of μm. Zn in the depth direction and
An example of the distribution state of Al is shown in FIG. Thus, the present invention
The feature of is that the plated Zn is separated deeply into the surface and below the surface.
I had it clothed. Zn layer with aluminum diffused
Is a surface layer that dissolves in sulfuric acid or nitric acid.
It is considered to correspond to a Zn hill of 1. Arminiu
Zn in the aluminum substrate is aluminum based on the total amount of Zn plated.
Is measured for convenience by subtracting the amount of Zn in the diffused Zn layer.
Can be set.

Next, the effect of the present invention on the press formability will be described. Since the aluminum plate is soft and susceptible to plastic deformation, it is extremely important to reduce the frictional resistance at the die shoulder and the bead portion where surface pressure is applied in order to flow the material into the mold. Even if a Zn plating layer is applied,
Since Zn itself is a soft metal, it has no effect. Even if a relatively hard Zn-Fe alloy plating layer is applied, the effect of reducing the friction coefficient is insignificant. Since the aluminum substrate is soft, even if only the thin surface is hardened, the effect will not appear. In the present invention, the Zn layer in which aluminum is diffused is the hardest layer and suppresses the adhesion with the mold, but the Zn layer in which Zn that supports this layer is also cured.
Fig. 2 shows the results of examining the change in hardness in the depth direction when press-fitting from the surface by changing the indenter load for Vickers hardness measurement. Compared with a Zn-plated aluminum plate,
It is clear that the hardened layer of the aluminum plate of the present invention reaches deep into the substrate. Therefore, the friction coefficient is sufficiently reduced even under processing conditions such as a flat plate pulling test in which the aluminum substrate is plastically deformed. Thus, in press molding, the range of surface pressure that flows into the mold without breaking the aluminum plate is expanded, the degree of freedom in panel design is expanded, and the stability of press work is enhanced.

Next, the effect on the spot weldability will be described. Since aluminum is a good conductor of electricity and heat, a considerable amount of Joule heat is required to form the nugget, which is problematic in that it accelerates the wear of the electrode. If a resistive film is added to the surface, Joule heat can be earned, but de-energization and fusing are likely to occur and it is not practical. Zn
Electrode life cannot be improved by plating. This is Zn
It is thought that the reason is that since it is soft and has a low melting point, it is well adapted to the tip of the electrode during the energization process and the contact resistance becomes too low. Electrode life cannot be improved even if relatively hard Zn-Fe alloy plating is applied. This is because the aluminum substrate is deformed when the electrode is brought into pressure contact with the aluminum plate.
On the other hand, in the present invention, the electrode life can be greatly extended.
This is because the hard aluminum-diffused Zn layer develops a contact resistance between the electrode tip and the plate-plate under the support of the hardened Zn-diffused aluminum layer thereunder, and a relatively thick Zn layer is formed. Nugget formation is also advantageous due to the Joule heat effect of the diffused aluminum layer, and as a result, fusing and blowholes are suppressed, the load applied to the electrode is reduced, and the number of consecutive dots at the same electrode can be greatly extended. Is recognized.

Next, the effect on the thread rust resistance after coating will be described. Aluminum is a material that easily causes thread rust, and it has been proposed to coat a Zn plating layer to suppress it. Since Zn is a metal that does not generate thread rust, it has some effects, but thread rust is not an essential solution because aluminum under the plated layer is corroded and grows. In addition, Zn easily causes blister corrosion under the coating film, which is disadvantageous in that blisters are likely to occur.
The present invention essentially suppresses the growth of thread rust by diffusing the plated Zn deep into the aluminum matrix. That is, the thread rust of aluminum grows while etching at a depth of several μm at the anode part of the tip of the thread rust, and the composition of the aluminum substrate involved in the anode reaction has a dominant influence on the growth rate of the thread rust. .. The presence of Zn in the aluminum substrate is considered to act to reduce the activity of the anode by neutralizing the highly acidic anode. Further, it is presumed that the Zn layer in which aluminum is diffused promotes the formation of a phosphate film in the pretreatment of coating, enhances the adhesion of the coating film, and contributes to delay the growth of thread rust. Further, it is considered that the diffusion of aluminum into the Zn layer lowers the activity of Zn and suppresses the formation of blisters.

As described above, the present invention has a two-layer structure of a Zn layer in which aluminum is diffused and an aluminum layer in which Zn is diffused, and each of them has a function.
Is characterized by forming a thick surface modification layer by widely distributing in the depth direction. By doing so, press formability of aluminum plate, spot weldability, and thread rust resistance after painting are simultaneously improved. The effect is obtained.
The aluminum plate to which the present invention can be applied may be pure aluminum or an alloy additive such as Cu, Si, Mn, Mg, Cr, Zn or Fe. These alloying elements diffuse more or less during the thermal diffusion treatment, but the effect thereof is incidental, and the basic effect of the present invention is maintained. Zn
In addition to pure Zn, the system plating layer includes Fe, Mn, Cr, Al,
Even if the metal such as Ni is contained in 20% or less, the metalloid such as P, B, C, N, and O is contained in 1% or less, and the dispersoid such as alumina or silica is contained in 5% or less, the effect of the present invention is maintained. It In this case, the structure of the surface modification layer of the present invention described above is Z
Only n shall be counted.

An aluminum plate is alkali-cleaned and / or acid-cleaned, and Zn-based plating is replaced by plating, electroplating,
And / or the total Zn coating amount is 0.5 to 1 by the vapor deposition plating method.
It is preferable to adhere it in the range of 0 g / m ² . 0.5
When it is less than g / m ² , the effect of the present invention after the thermal diffusion treatment becomes virtually unclear. Even if it exceeds 10 g / m ² , the effect of the present invention is exhibited, but it is not economical. Next, a thermal diffusion treatment is performed at 200 ° C. for 10 hours to 550 ° C. under a soaking and retaining condition for about 1 second. Treatment in a reducing or non-oxidizing atmosphere is preferred, but treatment in air is also possible. In the latter case, it is preferable to remove the oxide film with an alkali or an acid after the heat treatment. Zn in the Zn layer having aluminum diffused after the thermal diffusion treatment is 0.01 g / m ² or more,
The heat treatment conditions are preferably selected so that Zn in the aluminum substrate is 0.1 g / m ² or more. If the amount of Zn in the Zn layer in which aluminum is diffused is less than 0.01 g / m ² , the Zn concentrated surface layer as shown in FIG. 1 disappears, which is not preferable. 0.1g Zn in aluminum substrate
If it is less than / m ² , the diffusion treatment is insufficient and the effects of the present invention do not remarkably appear, which is not preferable. In addition,
Although the case where the operation and effect of the present invention is applied to an aluminum plate has been described, the present invention should not be limited to only a plate material if the principle of the present invention is applied to an aluminum cast material, an extruded material, and a drawn material. Absent.

[0010]

The embodiments and effects of the present invention will be clarified with reference to Examples and Comparative Examples. Table 1 shows the manufacturing conditions and the constitution of the surface modified layer, and Table 2 shows the results of the quality performance test. An aluminum plate having a plate thickness of 1 mm was subjected to alkali cleaning and acid cleaning by a usual method, and Zn-based plating was carried out by the following method. Displacement plating is NaOH 75g / l, ZnO 15g /
1 and 50 g / l of Rochelle salt were added to cause precipitation. Electroplating was carried out by electrodeposition at a current density of 20 A / dm ² from a sulfuric acid acid bath containing ZnSO ₄ 150 g / l and, if necessary, other metal ions or colloids. Vapor plating is 10
Zn was evaporated in a vacuum vessel of ^-3 Torr and evaporated on an aluminum substrate. Then, heat treatment was performed in an N ₂ atmosphere under the conditions shown in Table 1.

The total Zn coating amount and the Zn amount of the Zn layer in which aluminum was diffused were determined by ICP analysis of the solution, with the time point when the dissolution of sulfuric acid and the suppression of bubble generation as the end point. In addition,
The Zn amount of the Zn-diffused aluminum layer was determined by subtracting the Zn amount of the Zn-diffused aluminum layer from the total Zn coating amount. Flat plate pull-out test: Washing oil (Preton R303 manufactured by Sugimura Chemical Co., Ltd.
20) while applying P) and pressing both sides with a load (N) of 450 kgf with a tool with a contact area of 30 mm × 30 mm.
It was pulled out at a speed of 0 mm / min. When the pull-out load is F, the friction coefficient μ is obtained from F = 2 μN.

Continuous spot welding test: tip diameter 6 mm, 4
Continuous spot welding was performed using a 0R Cu-Cr electrode under the conditions of a welding current of 27 kA, a pressing force of 2950 N, and a welding time of 8 cycles, and evaluation was made by the number of spots at which the nugget diameter was maintained at 5 mm or more. Thread rust resistance test: A 70 mm x 150 mm test piece was used to form a phosphate film in a fluoride-containing phosphate treatment bath, and cationic electrodeposition coating was performed at 20 µm, and intermediate coating and top coating were applied to give a total coating thickness of 80 µm. did. After making a knife cut to reach the aluminum substrate, salt spray (5% NaCl, 35%
C.) 1 day, wet (85% relative humidity, 40.degree. C.) 5 days, and allowed to stand in a room for 1 day.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

EFFECTS OF THE INVENTION According to the present invention, the lubricity at the time of press forming, which is a drawback of the aluminum plate, the continuous spotting property at the time of spot welding, and the thread rust resistance after coating are remarkably improved, and the automobile body panel It can be advantageously applied to applications such as molding, joining, and painting.

[Brief description of drawings]

FIG. 1 is a Z in the depth direction from the surface of an aluminum plate surface-modified under the conditions of Example 1 by glow discharge spectroscopy.
The distribution state of n and Al is shown.

FIG. 2 shows hardness changes when press-fitted from the surface of an aluminum plate surface-modified under the conditions of Example 1 and Vickers hardness measurements of Comparative Examples 1 and 2 while changing the indenter load.

Claims (2)

[Claims] 1. A surface-modified aluminum plate comprising a Zn layer in which aluminum is diffused on the surface of the aluminum plate and a surface layer in which Zn is diffused on an underlying aluminum substrate. 2. The total Zn coating amount is 0.5 to 10 g / m.
² and Zn with aluminum diffused after thermal diffusion treatment
The surface-modified aluminum plate according to claim 1, wherein Zn in the layer is 0.01 g / m ² or more and Zn in the aluminum substrate is 0.1 g / m ² or more.