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Heat treat 6061 T6 for grain structure

January 10, 2012

Q. Maybe this question belong in a another category - kind of crosses several categories.

My theory is that extruded bar stock, 6061 T6 in this case has grain structure 'anomalies' particularly in the exterior regions of the bar (the first .100" of the bar?) that perhaps are the result of deformation related to the extrusion process. Add to this the likelihood that the mill heat treatment process may not be robust, or consistent and you have material that has a multitude of grain structure possibilities, some of them problematic when cosmetics are concerned if the product is to machined and hard black type III anodized.

For the practical machinist - who cares? the stuff machines and bends and anodizes. But if you are stuck using 6061 T6 and want a cosmetic grade Type III hard black anodize?? perhaps a fresh re-heat treatment may work to bring the bar stock (its grain) into control? the idea also here is to avoid post machine polishing... i.e., use the as machined surface as the final surface. We get exceptional smooth shiny as machined surfaces, but the anodize process pulls the grain out.

If this works... then what heat treat parameters within the standard 6061 T6 parameters might work best to yield a clean fresh grain?

What we have done so far to help with this issue is machine heavier bar stock, basically removing the exterior 1/4" of material and eliminating the exterior grain, which works to some degree actually. Be nice to return to thinner stock and it would be nice to actually improve or perfect the process, if possible. Solution temp and time - water or oil quench, time and temp of age??

Goal is to produce cosmetic hard black type III anodize with 6061 T6 bar stock

thank you - David A.

David Aldridge
- Everett, Washington, USA

May 1, 2012

I am in this exact boat, bailing water hand over fist, and I'd like to know how this turned out. Anyone have an idea how this would work?

Michael Vroegop
- Fort Lauderdale, Florida, USA

May 7, 2012

A. Hi!
Can you please clarify what exactly you mean by,
"anodising pulls the grains out..." ?
Do you get wrinkles, blisters, or similar blemishes?
We machine and hard anodise the same grade all the time. Usually mill tested and certified material does come with pretty consistent heat treatment and grain structure. Occasionally a very small section (of maybe one rod) does show some wrinkles, blisters, etc. which are (according to me) normally traceable to the billet stage prior to the extrusion process of the mills.
If you can maybe put up a picture of the defects you experience or describe it more clearly, will try to come up with my 2 cents worth!

Ravi Rao
- India

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