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View Full Version : Interesting Surface Finish on Aluminum - Looks Galvanized



Fasttrack
10-17-2014, 12:42 PM
Unfortunately, I cannot post pictures of the piece I was machining but I was turning the OD of a large chunk of cored aluminum. Nominal 12" OD and 8" ID, 6061 - nothing special. For the first ~ 0.25" of OD, I was getting a pattern that looked the flake finish you see on galvanized sheet metal. It was very smooth and highly reflective; under 10x magnification I could not see obvious tool marks and the pattern was hard to make out. It only appeared when light was hitting it at the correct angle. I noticed the same phenomenon on the ID.

We had a thread here earlier about porosity in metals and the effects of extruding/rolling metal. What says the braintrust? Was I seeing the "bark" from the extruding/rolling process?

Tool bit was HSS, ground properly. Tried various speeds from about 100 SFM up to about 450 SFM. DOC ranged from 0.01" off diameter to 0.1" off diameter. ID was boring bar with HSS bit, ground properly. Lathe is in great shape - a little small for the task but not terrible by any means. (Clausing-Metosa 13" by 40" gap bed lathe) Once I got in deeper, the pattern disappeared.

lbender
10-17-2014, 02:25 PM
Was I seeing the "bark" from the extruding/rolling process?



Sounds like you have extruded stock with recrystallized surface grain. Once you have cut deep enough you are into the core where there isn't any more large grain. All aluminum produced by the direct extrusion process will have some surface grain, but 6061 tends to have larger grains and a thicker layer.

http://s1371.photobucket.com/user/michael_schwarz1/media/IMG_4122_zpsfcb64f24.jpg.html

Fasttrack
10-17-2014, 02:32 PM
That is exactly what it looks like. Very interesting - thank you!