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Chemical Oxidation—Decorative surface treatment of cast aluminum parts

Cast aluminum alloy is one of the widely used engineering materials, and can be divided into aluminum-silicon, tungsten-copper, aluminum-magnesium and aluminum-zinc, among which the aluminum-silicon cast aluminum alloy is the most widely used. Cast aluminum alloys are widely used in the automotive, motorcycle industries, aerospace industry, ship and submarine industries, specially as frames, brackets and exterior parts such as casings.
After casting aluminum parts are shaped, they often required decorative surface treatment.
The surface conditions of casting aluminum alloy parts are different, some parts are non-machined surface, oxide film is formed on the surface, oil is heavy; and some parts are machined surfaces, the surface condition is good.
Casting aluminum alloys, especially with high aluminum-copper content, the addition of silicon copper greatly improves the strength of the aluminum alloy, but increases the surface process difficulties. Some types of cast aluminum alloys cannot be plated or anodized. For example, for cast aluminum alloys containing Cu2% to 2.5% Si7.5% to 12%, whether it is electroplating or anodizing, it is very difficult. To ensure the smooth progress of electroplating or anodization, special pretreatments are often required.
The product of crust-type is casting aluminum alloy with high silicon-copper, shaped by machine processing. According to product design, surface finishing and strengthening requirements, we choose a colorless chemical oxidation method to obtain a good quality film.
Aluminium extrusion company tell you the technology processing
The chemical oxidation methods is mainly divided into 4 steps:
1. Shoot peening
2. Activation treatment
3. Colorless chemical oxidation film
4. Inspection and storage

Shoot peening
Shot peening is prepared for chemical oxidation film. Sand blasting is applied to the surface of the casting aluminum parts. Although the surface of the casting aluminum parts can be degreasing, oxide scales, burrs, etc., and the surface is compressed, it is strengthened. But the surface is rough and there is no metallic luster.
The surface of the cast aluminum parts at 70-90°C alkaline degreasing corrosion solution can effectively clean the surface of the parts, but due to the different surface conditions of the parts, there are non-machined surfaces and processed surfaces, and the degreasing corrosion time is not easy to control. If the degreasing corrosion time is too long, uneven corrosion tends to occur. In addition, excessive corrosion of the oil can also make the surface of the part rough and non-metallic luster, can not produce compressive stress and strengthen the surface.
The process uses glass pellets for shot peening. The injection of spherical glass pellets onto the surface of cast aluminum parts can not only effectively remove oil stains, oxide scales, etc., but also form many tiny semi-circular surfaces on the surface of the parts, which are reflected by light and exhibit metallic luster, and the surface roughness is obviously improved. In addition, shot peening can also generate compressive stress on the surface, increase the fatigue life of the surface, reduce the sensitivity of the surface to stress corrosion, and greatly strengthen the surface of the part. The chemically oxidized surface can produce a dense and bright oxide film that fully meets product design requirements. Therefore, we used shot peening as a pre-treatment process for chemical oxide film formation.
The glass ball shot peening process generally controls the injection distance of 200-350 mm, the injection angle of 60-70° C., and the injection pressure of 5 kg/mm2, taking care to avoid long-time injection of the local area of the part surface.