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51027
Causes of streaking in machined 7075-T6
aluminum
January 21, 2009
We are a supplier of AL7075 bar. Some of our prospective customers
(both local & int'l) have streaking problems on turned faces with
their current supply (Drawn T6). It appears foggy on clear ano, and
lackluster and unimpressive on dye-colored parts.
I would like to help them solve their problem, but realize that there
are many causes, not necessarily all related to initial material
supply. We have narrowed down a few causes and would like to hear
opinions and / or ideas. Some of our possible causes:
-Dirty machining fluid
-Cutting speed
-T6 not uniform / not done to spec
-Residual stress not relieved
-Other finishing processes like bead blasting, pickling not done
properly
-Anodizing temperature and time
-Concentration of ano dye
-Gear grease mixing into cutting fluid
-Surface porosity damaged in one of the processes
Thank you all for your opinions.

Graham Sills
Alu Raw Mat'l Supplier - Fengyuan City, Taiwan

January 23, 2009
Without seeing several parts, it is not possible to do anything
but guess. Since you have a problem with several customers in widely
separated locations, I have to go with the problem being a non
homogeneous alloy or possibly the heat treat.
If it were only one or two customers, I would guess that cutting
fluid was the problem and possibly the cutting
tool/feeds/speeds.
7075 does not take anodizing well because of the high amount of alloy
material and thus requires different chemistry in the anodizing
process line to cope with it.(when compared to 6061)
James Watts
- FL
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