Anodize Color issues

[Eng5100]

I have found that aluminum extrusion vendors have a hard time controlling the anodizing process on seperate mating parts even when those parts are anodized at the same time. I believe this is due to the variance in the actual aluminum alloy billet (percent of each element in the alloy) rather than a careless vendor issue. Does anyone else find this to be true?

 

[TVP]

The short answer is yes. Microstructure inhomogeneity is one of the causes of this, of which chemical composition variation is only one piece. Microstructure evolution during casting is a complex phenomenon, and this becomes more complicated by the thermomechnical processing that occurs during extrusion. Nevertheless, your vendor(s) should be able to document their understanding of this issue, and how they minimize the problem by proper control of their process, control of the extrusion raw material, etc.

 

[Eng5100]

Ah...words I haven't been around since I finished my materials engineering degree! So, there is no great way to control color variation unless the extrusion vendor is also the anodizing vendor. I suppose even then you can't always guarantee the alloy homegeneity from billet to billet. Thanks!

 

[KENAT]

Eng5100, something they try here, though I don't know with how much success, is they have samples of acceptable 'color' and require parts to be a close match. It's a bit of a soft requirement and I'm not sure we do it well, but it's an idea.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[Eng5100]

We do typcially have an approved color range. That is fine, but what I have found is that even within the same batch (all anodized at once) there is a variation. For instance an assembly with A, B, and C anodized parts. The vendor will anodize A, B , and C all together, but we will still see the color variation due to the different sizes or more importantly, different billets used for the extrusion.