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