6061-T6 CTE and grain direction

[flyerfly]

Hi

Does anyone know if there is a difference in CTE for large sheets of rolled 6061-T6 between the grain direction and the transverse direction?

I have some large sheets (288" long x 120" wide x .5" thick) that do not behave the way I would expect. The expansion seems different perpendicular to the grain then with the grain. I called Alcoa and nobody responds so I looking elsewhere.

Thanks in advance.

Jon

 

[cloa]

I`d say he means Coefficient of Thermal Expansion

 

[Ron]

The CTE is probably going to be greater in the rolling direction than in the transversed direction. Is this what you're seeing? Are you measuring the CTE or the gross expansion?

 

[metengr]

I have some large sheets (288" long x 120" wide x .5" thick) that do not behave the way I would expect. The expansion seems different perpendicular to the grain then with the grain. I called Alcoa and nobody responds so I looking elsewhere.

How did you verify the above? Did you use strain gages to measure thermal expansion? Are the sheets warping resulting in your conclusion?

If not, movement could be caused by residual stress from forming or handling.

 

[jedward]

Some SPIE papers(Jacobs spie 1335(1990)) imply 6061-T6 Al has a thermal expansion homogeniety of about 60 parts per billion with a CTE of 23 ppm.
The implication could be that it is also isotropic.
This material, when thermally stabililized, is good for large optical elements which are very sensitive to anisotropic expansion

 

[flyerfly]

My observations were not from a strain gage but in the size of the composite part that came out of the tool. The sheet is not warped and I do not feel that the residual stesses are the culprit since it is repeatable and we do not heat the sheet above the heat treat temp.

The sheet is only getting heated to about 195 F so that should not be enough to do anything to the heat treat. Basically it seems like the CTE is more in the grain direction then the transverse direction (which is logical to me) but I would like to know if there is some real data on this someplace. At this point I do not feel that this sheet (or any of the sheets, we have many) are thermally isotropic.

Ron I am measuring the gross expansion. The CTE in the grain directtion is pretty much as expected at 13.1x10-6 in/in/F

 

[Compositepro]

Composite materials can have directionally variable CTE and/or cure shrinkage.

 

[maya04]

jedward
Do you know of any difference between 6061 T6 to 6061T6511 for optical elements?

 

[flyerfly]

Yes I know that the CTE for composites can have directional variables. I have calculated that out and because of the layup schedule and the use of carbon fiber (primarily) the CTE is very close to zero in both directions. Theoretically the total expansion in any direction (for this layup) is around 0.0007 of an inch so it is not worth looking at any more. I know there are other things at play as well like gel temp etc. etc. but my primary question is about the Aluminum.

I do not know of any differences between the optical elements aluminum. This does not really affect us though because we do not have that, we have the ordinary 6061-T6 tooling plate...large ones that I doubt are available in optical form anyway in that size!

 

[jedward]

a search found:

http://www.engineersedge.com/aluminum_tempers.htm

for a comparison

I have only see the -T6 designation used for aluminum to be used for optical elements and is commonly used for diamond machining where precison and stability is required.
Nothing definitive in MILHDB 5

 

[flyerfly]

My mistake... the heat treat is 6061-T651 NOT just -T6

 

[unclesyd]

if you could use cast Al tooling plate you would virtually eliminate your problem.
A cast tooling plate such as ALCOA MIC 6 can take more heating and cooling cycles than the a wrought version of tooling or jig plate no matter the heat treatment.
We used oodles and gobs of Al tooling plate mostly of the cast variety for it's dimensional stability in every direction. The wrought, believe it was ALCOA 200, was used for machines where a little movement could be tolerated. For both materials I am talking of movements under 0.0005" for cast Al tooling plate and slightly higher for wrought tooling plate.

We were looking mainly at center distances at approximately 18" on production equipment.

http://www.tquigley.com/mic-6.htm

http://www.alcoa.com/mill_products/north_america/en/product.asp?cat_id=1481&prod_id=619

 

[flyerfly]

Please keep in mind folks I am not looking for material alternatives...just if anyone knows about the possibility of a transverse CTE or not.

I know there are other materials out there but a cast sheet that is 288" long x 120" wide x .5" would cost $$$$$$$$$$$$$$(if anyone would even make it for you). I know that I could use INVAR or other exotic tooling alloys but I am trying to solve an existing problem not purchase a new material. Thanks and I really do appreciate those who take time to look at this thread