Hard Coating A380 die casting

Letter 2053

Hard Coating A380 die casting

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I am looking for a hard anodize coating or an equivalent that is aesthetically clean that can be applied to an A380 die cast aluminum part. It is my experience that the high silicon level in the A380 works against us when trying to hard anodize leaving a swirling visual affect on the surface that is not random and looks crappy. I need some guidance concerning a good hard anodize or an equivalent that can be attempted. Need is current and urgent.

Brian Kemp
- Dayton, OH

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Alloy 380 and similar die cast alloys are an anodizers' worst nightmare. You were right in stating that the silicon is causing your problem- this alloy has 8.5% Si, 3-4% Cu and 1-2% Fe, all of which can interfere with a good anodize. The problem is compounded as the parts are cast in the mold and cannot really solidify all at once, resulting in striation of the alloying elements and a resultant mottled apprearance. Since the surface is only about 85% aluminum, it is a challenge to get a uniform anodic coating at all, let alone one with good appearance. In the job shop where I once worked we had our best luck using organic additives in our anodize bath to reduce oxide dissolution, and an even better hard coat using a pulse current rectifier. Appearance may still be a problem, depending how much of the as-cast surface is left on the part, and how thick your coating is. Dying the parts black may help to hide the variegated coating.

Phil Johnson
Madison Heights, Michigan

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Brian:

One way to help with anodizing die castings is to ball burnish with stainless steel balls prior to Hardcoating. Dying black also helps reduce swirling surface look.

Bob Bramson
B&M Finishers / Prismatic Stainless Steel

Kenilworth, NJ

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Although alloy A380 can be difficult to anodize, a little care in the pretreatment cycle will go a long way in giving you a better looking part.

After cleaning, the casting should be given a short etch in a mixed acid (nitric/sulfuric/ammonium bifluoride). This will give you a "clean" surface to begin your anodize.

A typical anodize solution will consist of:

Sulfuric acid - 180-200 g/L
Al- 6-12 g/L
Temperature: 32-45 deg F (depending on the organic additive added)

A slow ramp up to your current density will help build a good base for subsequent anodizing.

If you have any more questions please let me know.

Charlie Grubbs
- Alpharetta, Georgia

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