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Streaking problems in anodizing 6061-T6 extrusions

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Q. My company designs and manufactures powered surgical hand-pieces. The housings for these hand-pieces 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 glass beaded. 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


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A. 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, North Carolina


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A. 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
- Navarre, Florida

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Wernick, Pinner & Sheasby
Surface Treatment & Finishing of Aluminium and Its Alloys


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

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note:    
   Mr. Probert is the
   author of



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A. 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, Minnesota


March 29, 2012

Q. Dear All,

Recently we have started Anodizing on pre fabricated Aluminium profiles and are facing the problem of caustic stains. These are not going even after doing proper desmutting.

Arun Nath
- Bangalore,Karnataka, India


March , 2012

A. Hi, Arun.

Where are the caustic stains coming from ... your own process? If so, start with Bob Probert's advice about caustic concentration, temperature, and quick transfer. Please get back to us with more detail.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey



April 24, 2013

Q. We purchase and machine 6061-T6 extruded bar in 1.25" diameter. The finished product is Type II anodized and has a high requirement for cosmetics (color match, consistency, etch, etc.). Recently we've come across (for a second time) lots of aluminum that have what look like water spots in them. This is best described as a spot that has a slightly different reflectivity. Our guess is that it is material that wasn't alloyed properly but we're not intimately familiar with the extrusion/ingot process.

These spots are noticeable before anodizing and very noticeable afterwards. This may yield thousands upon thousands of dollars in scrap.

What is causing these material flaws? How can we avoid them? Should we pursue the material supplier?

Rob F
- Atlanta, Georgia
  ^- Privately contact this inquirer -^


April 24, 2013

A. Hi Rob. Hopefully you'll get some clues in this public forum, but when thousands upon thousands of dollars is at risk, you should immediately hire an aluminum finishing consultant to see the situation firsthand.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


April 25, 2013

A. What Ted Mooney said plus if the spots are elliptical, the extruded bar is probably "cold drawn", in which case, get some bar that is not cold drawn.

Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina


April 29, 2013

A. Can you please post a photograph of the defective sample?

Thanks.

Srimay Basu
- Dubai, U. A. E.



March 5, 2014

Q. We are having problems with extruded 6063 T6 aluminum tubes for which we specified and purchased custom sized, extruding dies. Is there anything that can be done with the extruder to reduce streaks and spots in our extruded metal after anodizing? We are machining 1.5 mm off the extruded surface presently; would we get better results machining off a greater amount? Aside from specifying AQ material and temper, is there anything we should require of our extruder in way of extrusion specifications that would reduce the streaks and spots? Thanks for any advice regarding.

Scott Thompson
- Broomfield, Colorado


March 7, 2014

A. Scott

I would get samples from your extruder of the same alloy drawn through other dies that are comparable (as best possible) and have them anodized to your current process. By comparable, I mean similar wall thickness.

Without seeing pictures, its hard to venture a guess if root cause is the extrusion process or the anodize process. Both can produce similar results.

There are variables the anodizer can control to minimize the appearance of the streaks & spots, but not eliminate.

There are numerous threads here discussing this issue. I suspect it has to do with the cooling rate / method once drawn.

Good luck.

Willie Alexander
- Colorado Springs, Colorado

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