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Aluminum T6 vs. T6511

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What is the difference in 6061 aluminum tempers stamped T6 and T6511? Most handbooks have the same properties, so why the 2 numbers?

Linda Mayer
- Cedar Grove, New Jersey


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Hi Linda!

To get to the -T6 temper, the 6061 is heated to about 990F, then quenched in water, then aged at about 350F 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.

Lee Gearhart
metallurgist
East Aurora, New York



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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!

Milt Stevenson, Jr.
Syracuse, New York


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"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 etc as Milt pointed out?

Thanks,

James Tinker
- Denver,Colorado


October 13, 2010

How about the tonal qualities? Looking to use aluminum for bars in a musical instrument. Is there a difference between them in tone if you strike them?

Tom DuBois
- Palo Alto, California

May 18, 2011

Hi there. I work in the extrusion of 6XXX alloys for many years. Indeed, I have a question regarding the difference between the T6 and T6511 temper as well. Stretching between 1-3% is performed immediately after extrusion and cooling and prior to ageing. Stretching helps to minimise residual stresses from the extrusion process and give the final extrudate the necessary straightness.
So, can we use the T6511 in this case or do we need to perform extra stretching after the ageing process?
Residual stresses are mostly seen on the medium strength alloys like the 6061 or our equivalent in Europe 6082.
Thank you for your help.

Benedict Iroidis
Extrusion - Greece



July 27, 2011

Surface finish difference between 6061T6 and 6061-T651. I am interested in using the outside surface for sealing purposes. Is there a significant difference in the surface roughness between the two specs?

Bill Jaques
engineer - Perry, Ohio, USA


August 2, 2011

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.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom


August 5, 2011

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.

Bill Jaques
elastomers - Perry, Ohio, USA

May 17, 2012

Q. Hi I have recently welded two 6082 T6 sample together and after this noticed that the hardness of the heat affected zone greatly decreased and am hoping to regain a hardness equivalent to that of a 6082 T6 sample. What heat treatment procedure would you propose?

Irvin Vilakazi
- Pretoria, South Africa

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