Aluminum castings have loose structure, high porosity, and many kinds of metal and non-metallic impurities, so it is difficult to guarantee the quality of anodic oxide film. In order to obtain normal oxide film quality and avoid white spots after anodizing film dyeing, the following measures can be taken.
(1) High voltage and high current density impulse method. In the early stage of anodic oxidation, high voltage and large current impact are adopted to connect the large and small "islands" originally separated by impurities into pieces through large 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 disadvantage of the above method is that if the casting has screw holes, it may be slightly expanded. Therefore, it is necessary to control the anodic oxidation time of high current density and high voltage, and prevent the temperature of anodic oxidation solution from rising too fast. During continuous production, cooling measures shall be taken for anodic oxidation solution.
(2) Casting surface grinding method. Grinding can make the ground aluminum powder fill the holes of the casting, and play a bridge role to connect the block "islands" separated by impurities. The actual effect of this method is slightly inferior to that of method (1), because the polished aluminum powder sometimes cannot play the role of real filling and bridge, and the filler in some parts will be corroded and fall off during the process of alkali corrosion and anodic oxidation.
(3) Shot peening of casting surface. Before the trial of shot peening method, the author used a round headed hammer to knock, which was intended to close the gap blocking "islands" by knocking, so as to achieve the purpose of connecting into pieces. The result was remarkable. Taking into account that the efficiency of shot peening will be greatly improved, the effect after anodizing and dyeing is better than the above two methods.
In general, if the wall thickness of aluminum castings can meet the pressure requirements during shot peening, the use of larger pressure is conducive to improving the quality requirements of shot peening. Through the extrusion of shot particles, the effect of closing pores and gaps can be achieved, and the strength and cleanliness of the surface of aluminum castings can be improved.
Some people mentioned why white spots would not appear on the surface of the unmachined casting after anodizing and dyeing, because there is a dense oxide film on the surface of the casting.
