Aluminum castings have a loose structure, high porosity, and contain various metal and non-metal impurities, making the quality of the anodized film difficult. In order to obtain normal oxide film quality and avoid white spots after anodic oxide film staining, the following measures can be taken.
(1) High voltage and high current density impulse method. In the early stage of anodizing, high voltage and high current are applied to connect the small and large "islands" that were originally divided by impurities into pieces through high current impact. The specific process is as follows:
At the initial stage of anodizing, the voltage is adjusted to about 30V, and the current density is about 2~2.5A/dm2. After 3~5min, the normal anodizing voltage is adjusted back, anodizing for 50min, and then after full cleaning, a satisfactory oxide film can be dyed, and the white spots basically disappear.
The slight drawback of the above method is that if the casting has screw holes, it may slightly enlarge. Therefore, it is necessary to control the high current density, high voltage anodizing time, and prevent the temperature of the anodizing solution from rising too quickly. During continuous production, cooling measures should be taken for the anodizing solution.
(2) Surface polishing method for castings. Grinding can fill the pores of castings with aluminum powder, serving as a bridge connecting the isolated "islands" of impurities. The actual effect of this method is slightly inferior to method (1), because the polished aluminum powder sometimes does not play a real filling and bridging role, and some parts of the filling material will be corroded and fall off during alkali corrosion and anodizing processes.
(3) Shot peening method on the surface of castings. Before using the shot blasting method, the author used a round headed hammer to strike, with the original intention of closing the gaps that block the "islands" by striking, achieving the goal of connecting them into pieces. The result was significant. Considering that the use of shot peening can greatly improve the efficiency, the effect after anodizing and dyeing is better than both of the above methods.
If the wall thickness of general cast aluminum parts can meet the pressure requirements during shot peening, using a higher pressure is beneficial for improving the quality requirements of shot peening. The squeezing of shot particles can achieve the effect of closing pores and gaps, and can improve the strength and cleanliness of the surface of cast aluminum parts.
Someone mentioned why white spots do not appear on the surface of unprocessed castings after anodizing and dyeing, because the surface of the castings has a dense oxide film.