Now and then during the course of my metal working I've come across aluminum alloys that react very strangely to surface work hardening, developing that scaly hammered look. Has happened on different pieces of the same alloy (but different batches).
No idea what the cause is, but in my case I suspect it's something to do with the tempering or hardening process. Doesn't seem to effect strength in any way but does effect how it welds ... kind of like anodizing. Could also be due to something I've done to it but didn't remember ... heating etc.
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