Anodizing Die Cast Aluminum Alloys

Question:

I am interested in finding some information about anodizing an aluminum die cast parts. The two alloys I am looking at are 360°F and 380°F. Are there any aluminum die cast alloys that would take an anodized finish better than others? I am planning on having the parts polished before the finishing operation. I’m not real familiar with the anodizing process so I would appreciate any information that you can provide. The parts I am looking at are produced in China. T.P.

Answer:

Both 360 and 380 die cast alloys can be successfully anodized. The 360 alloy is high in silicon and low in copper while 380 is fairly high in both of these alloying elements. The “A” versions of either are lower in iron, which helps produce a better anodized finish. Whichever alloy you anodize, the quality of the die casting is critical to achieving a good anodized finish. I am not a metallurgist, so bear with me. Grain size, crystal size and phase distribution are the important factors affecting casting surface soundness and uniformity. In high-silicon alloys, such as 360 and 380, the silicon crystals are hard, plate-like particles and can be fairly large. Since silicon does not anodize, you can end up with an irregular, blotchy surface appearance after anodizing. Crystal size and distribution can be controlled by the addition of sodium or strontium. This is done in the melting stage. Your metallurgists probably already know this.

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In any case, anodizing of either of these alloys will give a rather dull gray appearance. If the silicon is not homogeneous, it wants to migrate to the metal surface and will appear as brownish, blotchy areas after anodizing. Anodizing should be done in a bath of 200–250 g/liter sulfuric acid at 70° F,voltage should not exceed 15 volts.

Polishing of the castings may result in the “high” points of the metal surface being smoothed over resulting in “voids” in the metal that may be revealed in the anodizing process. You will have to experiment to see what polishing techniques produce the best finish after anodizing.