1 Introduction Cast aluminum alloys are one of the widely used engineering materials. They can be divided into aluminum-silicon, tungsten-copper, aluminum-magnesium and aluminum-zinc, among which aluminum-silicon cast aluminum alloys are widely used. Cast aluminum alloys are widely used in the automotive and motorcycle industries, aerospace industry, ships, and submarine industries, especially as structural parts such as frames and brackets and exterior parts such as casings. After cast aluminum parts are shaped, they often require decorative surface treatment. The surface conditions of cast aluminum alloy parts vary from place to place, and some parts are non-machined surfaces. Oxide film is formed on the surface, oil is heavy, some parts are machined surfaces, and the surface condition is good. Cast aluminum alloys, especially cast aluminum alloys with high aluminium-copper content, the addition of silicon copper greatly increases the strength of the aluminum alloy, but increases the surface finishing process difficulties. Some types of cast aluminum alloys cannot be plated or anodized. For example, for cast aluminum alloys containing Cu2% to 2.5% Si 7.5% 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. A product of crust-type parts is a die-cast aluminum alloy with a high silicon-copper content and is formed by machining. According to product part design requirements, product parts need surface finishing and strengthening. After repeated process tests, we chose a colorless chemical oxidation method to obtain a good quality film. 2 Process The chemical oxidation method is mainly divided into 4 steps: 1 shot peening. 2 activation treatment. 3 colorless chemical oxidation film. 4 inspection storage. The process flow is as follows: Acceptance before shot peening, shot peening, activation treatment, flow washing, colorless chemical oxidation, flow washing, blow drying, inspection and storage. 2.1 Shot peening 1 Shot peening and pretreatment for chemical oxide film preparation Sand blasting on the surface of cast aluminum parts, although it can remove grease, scale and burr on the surface of cast aluminum parts, and make the surface produce compressive stress It is strengthened. But the surface is rough and there is no metallic luster. Although the surface of the cast aluminum parts is treated with alkaline degreasing corrosion solution at 70-90°C, the surface of the parts can be effectively cleaned. However, 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 significantly 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. Attention is paid to avoid long-time injection of a part of the surface of the part. 2.2 Activation treatment 2 In order to maintain the compressive stress layer surface of the part free from damage, the surface of cast aluminum parts that have been shot peened is subjected to activation treatment. Activation treatment solution ingredients: HNO3(d=1.42) 60% HF (40%) 20% Room temperature activation immersion time 1～5s 2.3 Chemical Oxidized Film Cast aluminum alloy parts shall be chemically oxidized immediately after shot peening and activation. To produce a uniform, colorless, transparent oxide film. Chemical oxidation has the following A, B formulas, either: Formula A: Sodium dichromate (Na2Cr2O7) 3 ~ 3.5g/L Chromium (CrO3) 3~5g/L Sodium fluoride (NaF) 0.5-0.8g/L Oxidation temperature room temperature oxidation time 3 ~ 5min Formula B: Potassium dichromate (K2Cr2O7) 0.8-lg/L Hydrofluoric acid (HF40%) 0.25-0.5ml/L Non-ionic surfactant suitable pH 2.7-3.5 Oxidation temperature 20～40°C Oxidation time 30～90s In the formulation, dichromate, chromic anhydride, potassium dichromate, sodium fluoride, and hydrofluoric acid provide dichromate ion and fluoride ion, which play an important role in the process of chemical oxide film formation. Dichromate ion is an oxidant and is the main component that promotes the formation of oxide film. Fluoride is an activator. It works together with dichromate ions, which is conducive to the formation of a dense oxide film layer. The content and ratio of the two must be strictly controlled. The pH of the chemical oxidation solution should be controlled within the specified range and adjusted with dilute HNO3 or NaOH solution. The addition of an appropriate amount of non-ionic surfactant in the oxidizing solution is beneficial to enhance the wettability of the surface of the solution and parts, and effectively improves the quality of the oxide film on the surface of the product parts. When the temperature is low, the film formation reaction is slower and the reaction rate is increased when the temperature rises. When the temperature of the solution exceeds 40°C, the oxide film will be powdered. Therefore, the film temperature for the oxidation is preferably 25 to 30°C. When the content of aluminum ions in the oxidizing solution continuously rises and the pH value rises beyond the control range, the quality of the oxide film is poor. At this time, the oxidation solution should be replaced partially or completely. 2.4 Inspection and acceptance After the chemical oxidation treatment of the cast aluminum parts, it shall be washed and dried and then the oxide film shall be inspected. Oxide film is incomplete or film loose, ash parts, to be repaired, reoxidation.