As aluminum alloy consisting essentially of 2.3 to 5.0 wt.% of Mg, 0.03 to 0.15 wt.% of Fe, 0.05 to 0.15 wt.% of Cu, and the balance of Al. An anodic oxide film in a thickness of 5 to 25 µm sealed by pressurized steam method may be formed on the surface of the aluminum alloy. The aluminum alloy contains an appropriate amount of Mg content, and has a sufficient strength and a favorable metallic gloss. Since nickel is not contained, metal allergy is not induced.
The present invention relates to an aluminum alloy, and more particularly to an aluminum alloy preferably used in goods or articles which often directly contacts with the human skin, such as necklace, bracelet, and band of wristwatch.
Generally, the watch case, back lid, hand, base plate, bezel, watch band, push-button, winding crown, assembling screws, and other watch parts are called watch external parts. These watch external parts are conventionally formed by using stainless steel, brass, zinc, titanium or its alloy, aluminum or its alloy, and other metals.
These metallic materials used for watch external parts have the following problems.
Having sufficient strength and corrosion resistance, but the weight is heavy. Formed by cold forging, its forming property is inferior to that of other metals. Containing nickel, it is likely to cause metal allergy when wearing the watch.
An alloy mainly composed of copper and zinc, it requires surface plating process. In this plating process, a plating film containing nickel is formed, and the nickel causes metal allergy.
Same as brass, it requires surface plating process, and the nickel contained in the plating film causes metal allergy.
No problem of metal allergy, but it is inferior in forming property. Besides, it is expensive, and the price of the watch soars.
To resist corrosion, in both, the surface is treated by anodic oxidation to form an anodic oxide film. In this anodic oxidation process, since nickel acetate is used as sealing promoter, nickel is contained in the anodic oxide film, which causes metal allergy. In aluminum alloy, since impurities such as iron and copper are contained, the gloss of the anodic oxide film is poor, which is a restriction in design. If, as countermeasure, an aluminum alloy of high purity is used, the strength is lowered, and the strength required for watch case or watch band cannot be maintained.
The present invention is intended to solve the above problems of metal materials.
It is, therefore, an object of the invention to provide an aluminum alloy having sufficient strength and gloss without causing metal allergy.
To achieve the object, the invention provides an aluminum alloy consisting essentially of 2.3 to 5.0 wt.% of magnesium, 0.03 to 0.15 wt.% of iron, 0.05 to 0.15 wt.% of copper, and the balance of aluminum. In this composition, the aluminum or the balance contains a trace amount of inevitable or unavoidable impurities such as manganese and zinc.
As defined above, the alloy of the present invention contains magnesium in an amount of 2.3 to 5.0% wt.%, and magnesium in this range offers sufficient strength required for watch external parts. The aluminum alloy of the invention having the composition defined above does not contain any nickel, and thus metal allergy due to nickel is not induced.
The surface of the aluminum alloy of the invention can be covered with an anodic oxide film sealed by pressurized steam method. The anodic oxide film can be formed by electrolysis, using the aluminum alloy as the anode, and is formed on the surface of the anode. The anodic oxide film can preferably be formed by electrolysis in sulfuric acid at a temperature of about 15 to 25°C and a voltage of about 10 to 20 volts for about 35 to 55 minutes. This anodic oxide film is a dense film and protects the underlying aluminum alloy. In the invention, using the aluminum alloy of the specified composition as the anode, an anodic oxide film free from cloud spots can be formed due to the characteristic of this composition. As a result, watch external parts having a sufficient surface gloss can be obtained. The thickness of the anodic oxide film is preferably 5 to 25 µm. The anodic oxide film having this range of thickness has a sufficient durability to maintain a favorable film thickness for a long period, in addition to the advantages noted above.
This anodic oxide film is sealed, and the invention is characterized by effecting the sealing by the pressurized steam method. The pressurized steam does not require nickel, and hence the anodic oxide film does not contain nickel. Therefore, the anodic oxide film does not cause metal allergy due to nickel.
The sealing treatment may preferably be effected by placing the alloy covered with the anodic oxide film in a steam at about 3 to 7 atmospheric pressures for about 5 to 15 minutes.
Using aluminum alloys having compositions 1 to 10 expressed in wt.% in Tables 1 and 2 wristwatch cases of 5.5 mm in thickness were fabricated. The tensile strength was measured on wristwatch cases made of the alloys of compositions 1 to 10 in Table 1 and Table 2. As a comparative example, the tensile strength was also measured on the material specified as JIS 5 NO1 in Japanese Industrial Standards (a material consisting of 0.15% of Si, 0.25% of Fe, 0.20% of Cu, 0.20% or Mn, 0.03% or Zn, and the balance of aluminum). The tensile strength of JIS 5 NO1 material was 85 N/m2. The wristwatch cases of the compositions 2 to 5 and 7 to 10 exhibited a tensile strength of 195 N/m2 or more, which is higher by more than two times that of the comparative example. This strength was sufficient for wristwatch case. The tensile strength of wristwatch case of the compositions 1 and 6 was 80 N/m2 or less, and the mechanical strength was insufficient. It is considered because the content of magnesium is smaller. Meanwhile, in all the compositions 1 to 10, manganese, silicon and zinc were inevitable impurities in aluminum alloys, and were not added intentionally, but were found to be present by analysis of the prepared aluminum alloys.
The watch cases made from aluminum alloys of the compositions 1 to 10 were worn by ten subjects for three days. Then patch test was conducted, and metal allergy was not found in the skin of any subject.
Next, using the wristwatch cases of the compositions 1 to 5 in Table 1 as the anodes, an anodic oxide film was formed on the surface in a thickness of 15 µm by electrolysis in sulfuric acid at 20°C and 14 volts for 44 minutes. Then, the anodic oxide film was sealed by pressurized steam method by placing the wristwatch cases in steam at 5 atmospheric pressures for 10 minutes. The metallic gloss of the anodic oxide films was observed visually, and the wristwatch cases of the compositions 2 to 4 had a sufficient gloss. By contrast, the compositions 1 and 5 were slightly inferior in gloss. It was considered because the magnesium content was too small and too much, respectively.
The wristwatch cases covered with such anodic oxide films were similarly worn by ten subjects for three days. Then, patch test was conducted, and metal allergy was found in none of the subjects. The tensile strength of these wristwatch cases was measured, and the results were same as the values before coating with the anodic oxide film.
The invention has been described particularly on the wristwatch cases, but the invention may be similarly applied in other watch external parts such as back lid, hands, bezel, winding crown, push-button, watch band, and assembling screws, and also other merchandise than watch external parts such as necklace and bracelet.
As has been described above, the aluminum alloys of the invention have a sufficient strength and do not contain nickel, and hence will not induce metal allergy if worn directly on the skin.