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ã€China Aluminum Network】Aluminum castings have loose structure and high porosity, containing a variety of metal and non-metallic impurities, and the quality of the anodized film is difficult to guarantee. In order to obtain a normal oxide film quality and avoid white spots after dyeing of the anodized film, the following measures can be taken.
(1) High voltage and high current density impact method. In the initial stage of anodization, high voltage and large current surges are applied, so that the “islands†of the size that were originally divided by the impurities are connected by a large current impact to form a sheet. The specific process is as follows:
Initial voltage of anodic oxidation is adjusted to about 30V. At this time, the current density is about 2 to 2.5 A/dm2. After 3 to 5 minutes, it is transferred back to the normal anodic oxidation voltage, anodized for 50 minutes, and then can be dyed after sufficient cleaning. Satisfactory oxide film, white spots disappeared.
A slight disadvantage of the above method is that if the screw hole of the casting is slightly enlarged, it is possible to control the large current density, high voltage anodizing time, and prevent the temperature of the anodizing solution from rising too fast. In continuous production, the anodizing solution should take cooling measures.
(2) casting surface grinding method. Grinding can make the ground aluminum fill the pores of the casting and act as a bridge to connect the islands that are separated by impurities. The actual effect of this method is slightly inferior to method (1), because the polished aluminum end sometimes does not achieve true filling, bridge action, and some parts of the filler will be eroded in the process of alkali corrosion and anodization and fall off.
(3) Shot blasting on the casting surface. Before the shot peening trial, the author used the method of knocking with a round head hammer. The original intention was to make the gap blocking the “islands†closed by tapping, and to achieve the purpose of connecting into pieces. The result was significant. After considering that the shot peening efficiency will be greatly improved, the effect after anodizing and dyeing is better than the above two methods.
Generally, the wall thickness of the cast aluminum parts can meet the pressure requirements during shot peening. The use of greater pressure will improve the shot peening quality requirements, and the effect of closing the voids and gaps through the extrusion of the pellets can increase the surface strength of the cast aluminum parts. And cleanliness.
It has been mentioned why white spots do not appear after the anodized dyeing of unprocessed castings because of the dense oxide film on the surface of the castings.
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