Letter 23085

Problems anodizing 6061-T6 extrusion [California] 

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My company designs and manufactures powered surgical hanpdieces. The housings for these handpieces are often machined from extruded aluminum shapes. We are struggling with getting to the root cause of a "streaking" problem that the anodize process brings out. The parts are machined and the remaining extruded surfaces are buffed, polished and glassbeaded. When they leave the plant, they look very uniform cosmetically. After anodize, which is a type II clear coat, they have axial (along the axis of the extrusion) streaks in the part and we loose very expensive parts at the last operation. Our anodizer says that the etching is brings the lines out, but they cannot offer any solution. Does anyone have any ideas about what this may be?

Thanks for any help.

Roger McPherson
- Santa Ana, California, USA

We are both a CNC machine shop and anodizing job shop. I agree with your anodizer that the etching process does tend to enhance the grain structure, but one option we've used is to not etch the part at all, but leave it in the alkaline cleaner slightly longer. Instead of testing a whole run of your very expensive parts, test one piece and see if the finish improves.

Bill Smith
- Trinity, NC

Send a small lot of parts to an anodizer that uses non etch cleaners and preps. Very few anodizers use this, because it is not needed for 99% of their parts. The quality of the extrusion depends on the extruder, not the anodizer. Extrusions are only a tiny bit better than casting in many cases. I have seen absolute garbage extrusions that customers were irate because the anodize brought out the flaws that you could see before you started if you washed the part and looked closely.

James Watts
- FL

Make the last polishing or buffing operation ACROSS at 90 degress from the direction of the streak. Cut the alkaline etch concentration down to 6 oz/gal and 135 Degrees F, transfer to rinsing rapidly. Discontinue glass beading altogether. If the glass beads are used on other metals tehen you are pounding non-aluminum into aluminum, better, just skip the glass beading whichi also work hardens the surface, more in some areas than others.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

The above ideas are good. 6061 T6 is sensitive in the quenching operation after extrusion. Improper quenching will bring out Mg silicides. This is a long shot as you machine the metal before anodize. Try another extruder.

Jon Quirt
- Minneapolis, MN

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