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Aluminium impurities makes preparing parts for anodising impossible, is there a cure?

June 4, 2009

Hi,

I recently purchased and setup my anodising equipment. I am new to this so apologise if I am asking something blindingly obvious to the experienced.My fledgling company makes high-power LED lights for commercial, domestic and transportation purposes.

For my projects in aluminium, I purchased some aluminium tube and rod of various sizes, the company advise the aluminium supplied is either 6082 or 6063, depending on size (though they do not specify any further)

I have found on machining that the finish straight off the lathe is excellent. However, when I hold the part under light, I can see very small imperfections that run the length of the part, not around the circumference. These "marks" are not surface irregularities (by this I mean they are not a result of an uneven surface finish) and cannot be removed by sanding, etching in caustic or polishing. The best I can describe them is they look like tiny blotches, generally around 1 to 1.5mm across and in random shapes. I have tried making repeated passes with the lathe to see if the artifacts are only at a certain depth in the material, but they seem to appear regardless of how much I machine off the surface.

My best guess is that these are impurities in the aluminium and that I should be using a different grade of material for my work, as obviously these marks are equally visible after anodising (as expected)

I have mailed the supplier to this question, but to date they have failed to reply (so I will not be using them again)

So I politely ask if anyone here has experienced this too, or perhaps a recommendation on the grade of aluminium I should be using.

Thanks
Dave

David Blakelidge
Product design, R&D - Lancashire, UK

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June 9, 2009

Hi Dave,

I suspect, these could be metallurgical flaws (not caused through your operation) and will get highlighted in further surface treatments like chemical etching, chemical brightening and or anodising.
The material resourcing is very important in these cases and the alloy selection as well. Certain items to be taken care as listed below:
1. Select good extruders who are experienced in producing for anodising quality.
2. Select only 6063 alloy.
3. During machining take care of cooling to avoid heating of the extrudate.
4. Material should be stored with care and protection prior anodising
I would appreciate if I could see a photograph of this defect.

Best of luck,
Basu

Srimay Basu
- Dubai, U. A. E.

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Thanks for your reply,

I have since also had a response from my supplier. He advises that the lateral marks are as a result of the welding process of the extrusions (caused by the spider-die) He also confirmed the alloy supplied is not ideal for anodising, it's better suited to powder coat.

So it seems I should be looking for 6063 and specifying that it must be suitable for anodise?

I will google this to see if there is anyone local who can supply such.

Thanks for your help

David Blakelidge
- Accrington, Lancashire, UK
June 12, 2009