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Spotting in 6061-T6

Spotting in 6061-T6

Spotting in 6061-T6

We are experiencing spotting in turned 6061-T6 bar stock.  The bar stock is turned and either anodized or polished with a high cosmetic requirement. In the past, our only resolution has been to use increasingly difficult to obtain "control grain" products (Alcoa's 6061-T6511X or Kaiser's 6061-T6511C for example).  We have had some relief by using a minumum amount of etching on the anodized pieces, but this proves only slightly better.

I was hoping someone could comment on:

1) the possible alternatives to these products and/or

2) the wishful possibility of using a standard off-the-shelf 6061 product and somehow treating it to acheive a clearer finish in both anodized and polished situations.

Thanks in advance for any assistance you can give.


RE: Spotting in 6061-T6

Why is anodizing required?  Can an organic coating (paint) be used?  Ken Vlach will be along shortly to discuss anodizing in more detail, and why it is important to use the controlled grain products.  What is the end application?

RE: Spotting in 6061-T6

I had a similar problem about 20 years ago with "streaks" or mottling in hardcoated 6061 alloy. We found that the problem was most prevalent when extruded bar was used and hardly ever occurred when rolled bar was used. We then specified only rolled bar for the application.

RE: Spotting in 6061-T6

Thanks for your reply ...

TVP, basically the parts are anodized as a customer  requirement.  The short answer is it's a price justification issue, the end customer is essentially paying more for high quality materials and our customer doesn't want to cover that up with just paint.

swall, I'm not exactly sure what you mean by rolled bar?  If you are referring to a cold finished product, then yes I would suspect that it would have a much cleaner material than standard extruded, but the cost increase is significant.

RE: Spotting in 6061-T6


What is Your material spec??

Regards, Wil Taylor

RE: Spotting in 6061-T6

Well, the material specs are 6061-T6, but the customer is aware of the difficulty we are having obtaining the grain control products.  I know they would be open to changes.  The basic requirements are mostly cosmetic, even more so than dimensionally.  I believe they settled on the current spec because it "generally" has good machineability and finish results.

RE: Spotting in 6061-T6


6061-T6 is the alloy and temper.

6061-T6xxx can be procured per the following specifications [just a few listed].

AMS-QQ-A-250/11, AMS4025, AMS4027 [sheet/plate]

AMS-QQ-A-225/8, AMS4117, AMS4128 [rolled bar]

AMS-QQ-A-200/8, AMS4160, ASTM B308 [extruded bar & shapes]

AMS-WW-T-700/6, ASTM B429 [extruded tube/pipe]

Now… Seco1… what is the material spec Your 6061-T6 was procured per?

Note: there are may other "specs" for this material... with far fewer controls for grain size, cleanlieness, material properties, heat treatment, etc... such as "commercial or "architectural" grade material... that may be causing Your problem.

Regards, Wil Taylor

RE: Spotting in 6061-T6

I agree that cold-finished bar is better than (most) extruded.  Extruded material has a 'skin effect' because of the die friction; the skin doesn't move as fast or as uniformly as the bulk.  Any etching reveals the grain structure, so minimize or eliminate. Material that is clean and polished doesn't require etching, just a non-etch soak clean & deox prior to anodizing.  

So, preferable to order material per ASTM B211-03 Standard Specification for Aluminum and Aluminum-Alloy Bar, Rod, and Wire
-- rolled or cold-finished bar -- than per ASTM B221-05a Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes.

Also, try different sources for extruded material; the stuff is normally pretty good.

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