6061 Aluminum: Difference between T6 vs. T6511

"The Surface
Treatment &
Finishing of
Aluminium and
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A. Hi Manjiri. I can start the discussion by telling you that anodization is an electrochemical process that converts some of the aluminum on the surface of the item to aluminum oxide and similar compounds and that it will always weaken the item, never strengthen it. You can review our Introduction to Anodizing if you wish.

Anodization might also increase the warping if the article is very thin or it may have no measurable effect if the item is more substantial.

Increasing the strength of the plate and freedom from warping by using a different heat treatment (you haven't said what the current heat treatment, if any, is), or by switching to a different alloy and heat treatment, or by changing from aluminum to stainless steel when we have no idea what the item is, what the needs are, or why it warps, is a hopelessly broad subject.

Both aluminum and stainless can be mechanically and/or electrolytically polished for reflection, but again, how much reflectivity and why? Aluminum can ideally reflect across a broad spectrum of wavelengths whereas stainless can look shiny while only reflecting in a narrow spectrum ... either can be a fine bathroom mirror, but only aluminum can be a telescope mirror. But anodization will reduce the reflectivity a little for some alloys (usually the not-so-strong ones) and hopelessly for others (usually including the stronger ones).

Please tell us what you are trying to do, then we may be able to help you get there :-)

Luck & Regards,

A. Hi Linda!

To get to the -T6 temper, the 6061 is heated to about 990 °F, then quenched in water, then aged at about 350 °F for around 8 hours. That changes the typical yield strength from 8 ksi to about 40 ksi- fairly substantial.

But that quenching in water puts residual stresses in the aluminum, since there is a surface-to-center cooling gradient. The -T651 designation means the mill took that extrusion and gave it a 1% to 3% stretching, or permanent set, to get rid of some of those residual stresses. Now we can machine it and it shouldn't distort.

Finally, the final digit in the -T6511 designation (and how I know it was an extrusion, since this only applies to extruded stock) means that the mill is allowed to straighten the extruded bars, like in a press, to get them to meet the straightness tolerances.

A. To expand on what Lee so correctly pointed out, if you mix alloys of different tempers, while they compositionally might be the same, they can be separated by measuring their surface conductivity. From a finishing or chemical processing standpoint, this means they have different anodizing rates, appear differently when exposed to various etchants (e.g. nitric-HF or caustic) and may even appear different upon chromating. Chemically speaking, they are indeed quite different!

Q. "To expand on what Lee so correctly pointed out, if you mix alloys of different tempers, while they compositionally might be the same, they can be separated by measuring their surface conductivity. From a finishing or chemical processing standpoint, this means they have different anodizing rates, appear differently when exposed to various etchants (e.g. nitric-HF or caustic) and may even appear different upon chromating. Chemically speaking, they are indeed quite different!"

-When you say mix alloys of different tempers, what do you mean exactly. Mix in a kiln, use various tempers on the same assembly?

And following on from the initial question by Linda, Lee pointed out the manufacturing differences, but what are the actual differences in properties, e.g., machinability, weldability, strength, etc etc. Could T6511 and T6 be so different as to have different anodizing rates, etc., as Milt pointed out?

Thanks,

A. Hi Bill,

The T number is the temper designation, so this should not have any impact on the surface finish, that will depend on how the material has been treated or handled.

Q. My understanding 6061-T6 tubing is drawn or mandrel drawn and 6061-T651 is extruded tubing. So the inquiry related to a possible difference in surface finish between the two.

A. Hello Irvin Vilakazi,

One option would be anneal and re-age so as to attain the required T6 temper.

T6 /T6511 are the temper designations only. Hence final finishing operations (such as anodizing) would be same in both the cases. There would be absolutely no change in either chemical or surface properties.

A. Hi Arthanari. I believe that Lee answered that question --

" ... the final digit in the -T6511 designation ... means that the mill is allowed to straighten the extruded bars, like in a press, to get them to meet the straightness tolerances."

Regards,

A. As the question is rightfully answered for the difference of AA6061-T6 Vs AA6061-T6511. If the T6 material is used as stock for such a geometry where exists very small micron size geometric features, there well not be dimensional stability of the geometric feature due to the residual stress in material -- and it will be ok with T6511.

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Aluminum: its forms, alloys, and tempers"

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A. Hi Varma. For the benefit of other readers, the "AA" refers to The Aluminum Association alloys and tempers designation system. But if you are trying to use ASTM specs instead of AA specs, you might see if ASTM B221 conveys what you are trying to cover. Good luck.

Regards,

Hi ioannis. Lee and Benedict have told us that to make T6511 from T6 involves controlled stretching and straightening, so it doesn't sound like something which could happen spontaneously. But please detail your situation rather than posing it in the abstract, and then people can probably be more helpful. Thanks.

Regards,

A. 7075 turning black after a sodium hydroxide based etch is normal. It's a high zinc alloy, if I recall correctly. They'll need a good desmutting agent to remove it (the "smut" from the etching process). The alloy itself tends to anodize to a yellowish color, which may affect the dying process, but it should dye to a red color without problems. The color hue will probably be a little different than the color of a 6000 series aluminum.

A. Hi Darren. Hydro.com and other sites say "mechanical property level higher than T6 achieved through special control of the process", and several sites offer those values for mechanical properties of T66 vs. T6, but I don't quickly see any place that tells you exactly how to obtain them.

Luck & Regards,

Thanks Ted - All I found was the same verbiage. "Special control of the process" I'll keep digging.

A. Good luck! But 'special control' doesn't always mean secret, proprietary, exotic technology; it might simply mean holding the variables within very tight limits.

Luck & Regards,