Aluminum base part and manufacturing method

The present invention relates to a manufacturing technique for an aluminum base part.

An aluminum base part is sometimes used for a handle holder in a motorcycle or a door knob and emblem in a passenger car. In this case, the aluminum base part becomes an exterior part exposed to a person\'s sight, and a good appearance of the aluminum base part is therefore desired.

Various techniques for improving the appearance of an aluminum base part have been proposed (see PCT Patent Publication No. WO 00143153 pamphlet (page 22) and Japanese Patent Laid-Open No. 2004-17738, for example).

The invention disclosed in PCT Patent Publication No. WO 00143153 discloses a surface treating method for a light metal casting including a casting step for applying a casting pressure of greater than about 50 megapascals from an injection plunger to a molten metal of light metal material supplied into a die to thereby form a casting so that pin holes generated on its casting surface are suppressed to meet predetermined conditions; a polishing step for polishing the casting surface so that the surface roughness of a polished surface becomes a predetermined value or less; a coating step for forming a first resin coating layer on the polished surface; and a plating step for forming a metal or metal compound layer on the surface of the first resin coating layer by dry plating.

According to this method, the size and number of pin holes are sufficiently reduced by squeeze casting. Further, barrel polishing is next performed to reduce the surface roughness. Thereafter, resin is coated, and plating is next performed. As a result, a light metal cast product having a bright design surface can be provided.

However, the coating step for coating the resin is indispensable, thus the number of man-hours increases, causing an increase in production cost.

Japanese Patent Laid-Open No. 2004-17738 discloses a surface brightened light alloy wheel for an automobile including a light alloy wheel body having a minutely rough surface, a resin coating formed on the surface of the light alloy wheel body, a titanium alloy thin film formed on the resin coating and having a chromium plating appearance, and a transparent resin protective layer formed on the titanium alloy thin film. Thus, the resin coating is formed on the surface of the light alloy wheel body. Also in this technique, a coating step for forming the resin coating is indispensable, thus the number of man-hours increases, causing an increase in production cost.

Accordingly, if the resin coating step can be eliminated, the number of man-hours can be reduced, thus reducing a production cost.

It is accordingly an object of the present invention to provide a technique which can improve the appearance of an aluminum base part without applying a resin as an undercoat.

There is provided an aluminum base part adapted to be provided on the exterior surface of a vehicle, including a forging cast product produced by a forging cast process and a nickel chromium plating layer formed on the forging cast product.

Further, the nickel chromium plating layer includes a semibright nickel plating layer formed on the forging cast product and having a thickness of 5 to 10 μm, a bright nickel plating layer formed on the semibright nickel plating layer and having a thickness of 5 to 10 μm, and a chromium plating layer formed on the bright nickel plating layer and having a thickness of 0.5 to 3 μm.

There is provided a manufacturing method for an aluminum base part adapted to be provided on the exterior surface of a vehicle, including preparing a pressing device including a die having upper and lower openings, a lower punch for closing the lower opening of the die, an upper punch opposed to the lower punch and adapted to be inserted into the die from its upper opening, a first cylinder unit for raising and lowering the upper punch, a partial punch formed as a divisional part of the upper punch, and a second cylinder unit for raising and lowering the partial punch; supplying an aluminum alloy molten metal into the die to which the lower punch is fitted; a primary pressing step performed by collectively inserting the upper punch and the partial punch into the die from the upper opening of the die to compress the molten metal; a secondary pressing step performed by increasing a pressing pressure of the upper punch but not the partial punch, generated by the first cylinder unit to further insert the upper punch into the die, thereby further compressing the molten metal; releasing a forging cast product from the die after solidification of the molten metal; and a plating the forging cast product with a nickel plating layer and a chromium plating layer.

The manufacturing method further includes polishing the surface of the forging cast product between the forming step and the plating step so that the average surface roughness of the forging cast product becomes 6 μm or less.

According to the invention, a forging cast product rather than a cast product is adopted. The forging cast product has a smooth surface, so that a plating layer can be directly formed on the surface of the forging cast product without forming an undercoat resin layer. Accordingly, the appearance of the aluminum base part can be improved without applying a resin as an undercoat, and the number of man-hours can therefore be reduced.

According to the invention, a semibright nickel plating layer having a thickness of 5 to 10 μm is first formed on the forging cast product, a bright nickel plating layer having a thickness of 5 to 10 μm is next formed on the semibright nickel plating layer, and a chromium plating layer having a thickness of 0.5 to 3 μm is finally formed on the bright nickel plating layer.

Thus, the semibright nickel plating layer having a thickness of 5 μm or more and the bright nickel plating layer having a thickness of 5 μm or more are formed as an undercoat layer, so that a sufficient corrosion resistance can be maintained. Furthermore, the maximum thickness of each nickel plating layer is set to 10 μm, so that the consumption of nickel can be suppressed.

The two nickel plating layers having a total thickness of 10 μm at a minimum are formed as an undercoat on the forging cast product. Accordingly, the thickness of the chromium plating layer as a top coat can be sufficiently reduced, and although it is reduced to 0.5 μm, a metallic luster can be produced to thereby maintain a good appearance. Further, a maximum thickness of the chromium plating layer is 3 μm, and thus the consumption of chromium can be suppressed.

According to the invention, the forging cast product is produced by a two-stage process composed of the primary pressing step and the secondary pressing step. By strongly compressing the molten metal, the density of the forging cast product can be increased. In the secondary pressing step, only the partial punch, which is independent of the upper punch, is allowed to be raised so that an excess amount of molten metal can escape toward the partial punch. Accordingly, even when the amount of molten metal supplied into the die is varied, this variation in supply amount can be absorbed by the amount of the rise of the partial punch. Accordingly, the finished shape of the forging cast product can be improved.

According to the invention, the forging cast product obtained by the forming step is polished so that the average surface roughness of the forging cast product is 6 μm or less. By setting the average surface roughness to be 6 μm or less, the appearance of the aluminum base part can be further improved.