Aluminum stress relief anneal 1

[dwillms]

We're doing some machining to a beefy 6061-T6 extrusion and are VERY concerned about residual stresses in the material afterwards(as a result of extruding and machining). Two questions:

1. Any idea if dimensional creep is likely as a result of machined in stresses over the life of the part? For this application 5 micron is too much! In general, can someone describe the mechanics of micro creep as a result of residual stress? Does such a thing occur??

2. What heat treatment would remove residual stresses but preserve the T6 temper? What exactly would this heat treat do to the micro structure of the material?

Thanks for your help!

 

[metman]

Your situation might requre a procedure as follows:
1) Full Anneal.
2) Rough machine within 1/8" or less of finish dimensions.
3) Stress relief anneal 650F.
4) Solution treat using holding fixture (die quench).
5) Cold stabilze as discussed below.
6) Maintain in W temper (cold storage) until finish machined.
7) Age harden to T6.

Stress relief annealing is accomplished by heating to 650F which will dramatically reduce tensile strength.

Quoting from 8th Edition of Metals Handbook volume 2 (Heat Treating, Cleaning and Finishing) page 281: Numerous attempts have been made to develop a thermal treatment that will romove, or appreciably reduce, quenching stresses. normal precipitation heat treating temperatures aare generally too low to provide appreciable stress relief and exposure to higher temperatures (at which stresses are relieved more effectively) results in lower properties. However, such treatments are sometimes utilized when even a moderate reduction in residual stress levels is important enough so that some sacrifice in mechanical properties can be accepted. The T7 temper, mentioned previously, for castings is a typical example of this kind of treatment.

Other thermal stress relief treatments known as "subzero treatmtnt" and "cold stabilization" involove the cycling of parts above and below room temperature. The temperatures chosen are those that can be readily attained with boiling water and mixtures of dry ice and alcohol - namely, 212F and -100F - and the number of cycles ranges from one to five. The maximum reduction in the residual stress made possible by these techniques is about 25%. The maximum effect can be obtained only if the subzero step is performed first, immediately after quenching from the solution treating temperature while yield strength is low, and no benefit is gained from more than one cycle.

The next paragraph gives a patented process by which 83% relief of residual stress is possible. If you want I could fax some of these pages to you which also talk about effects on microstructure. Jesus is The TRUTH

 

[TVP]

metman,

I was getting ready to post the exact same excerpt-- you just beat me to it!

 

[WKTaylor]

dwillms...

Another Idea if you need heat-treated 6061-T4 or T6 material...

Try using 6061-T4510 or 6061-T6510 extrusion [AMS-QQ-A-200/8]. This material has been solution heat-treated and is then stretched for stress-relief... then age hardened. NOTE: 6061-T6510 and 6061-T6511 are simmilar, but they have had post heat-treatment straightening applied... so it may tend to have low and slighy irregular, residual stresses. Regards, Wil Taylor

 

[ellev0]

Dear Metman,

I am interested in the "83%" stress relief mentioned, could you give more details or how to request it?

Reply:
lvesier@hotmail.com
Thank you

 

[metman]

ellev0,
From 8th Edition of Metals Handbook volume 2 (Heat Treating, Cleaning and Finishing) page 281.

This is a patented process U.S. Patent 2,949,392 which I think you can get from the internet. If you have access to a fax machine, let me know the number. I can photocopy the article and fax it to you.

Jesus is THE life,
Leonard

 

[ellev0]

Al anneal stress relief
L Vesier
Fax number is 302 453 1302

Thank you for any info on this.

 

[metman]

ellev0

faxed

Jesus is THE life,
Leonard