Anodising of Aluminium Castings is a simple process in principle but can be difficult in practice due to the nature of the alloy and the casting process itself. In contrast to wrought or extruded aluminium, castings have characteristics which cannot be influenced by the anodiser.
Silicon Content Affects Finish Colour
Aluminium casting alloys generally contain between 5 and 12% silicon. This silicon does not anodize well. The silicon will appear as dark grey or black patches in the oxide layer. The higher the silicon the darker and more mottled the surface will be. This is a chemical problem and cannot be cured by changing the anodizing process.
Alloy Grade Makes a Real Difference
In general LM25 anodises better than LM6 due to the lower silicon content, producing a more even, pale finish. If you are selecting an alloy for casting, it is worth discussing with your foundry whether LM25 would be more suitable for the part than LM6, even at the design stage.
Porosity Must Be Controlled Beforehand
Open cavities within a casting (porosity) can cause problems for the anodising process. Pores on the surface of a casting can cause localised blistering and pitting on the finished coating. Subsurface pores can be filled by impregnation agents before anodising to prevent such problems.
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As-Cast Skin Versus Machined Surfaces
It’s also worth noting that as-cast skins have segregated alloy phases and surface oxides that anodising will highlight unevenly. Fully machined surfaces give the best finish. Parts with some machined and some as-cast skins will have noticeable colour and texture variation.
Set Realistic Cosmetic Expectations
Anodised castings do not have the same finish quality as anodised extrusions. When specifying anodised castings it is important to set a realistic specification for the finish quality. A good starting point would be to agree a sample finish with the anodiser prior to commencement of production. This will highlight any potential problems prior to inspection.
Get the specification right early on to save time later!
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