Stress Relieving 7075-T6511 2

We currently have a 7075-T6511 component that requires stress relief after rough machining. What is the correct stress relief temperature and soak time for this material. I have several industry standards for heat treatment but this material and temper is not covered. We were considering 300 F for 2 hours (cryogenics is not an option as we have an urgent requirement for the component).

 

[diamondjim]

I would be more curious about why they
are asking for a stress relief after
machining? How much are you reducing the
part by this machining? I think 7075 was
developed by the aircraft industry. The old
standard was 7079 which supposedly is now
obsolete.

 

[Kenneth]

7075-T6 is a very high strength, solution treated and "artificially" aged (i.e. furnace-aged at a temperature above room temperature) aluminum alloy. The Tx51 tempers are subjected to a mechanical "stress relief" that can be thought of as "stress aligning". The material is stretched (at the mill) during the straightening after solution treatment and prior to aging. The stretching operation aligns the hot/cold working, straightening and quenching stresses in the direction of the stretch. While this method does not eliminate the stresses, it does allow for control of those stresses. During machining the "movement" of the machined surface becomes more predictable since the randomness of the residual stresses have virtually been eliminated.

The temperature required to provide any meaningful stress relief would be greater the aging temperature originally used to achieve the T6 strength level, and would therefore result in "overaging" of the material -- significantly decreasing the mechanical properties. This is the reason that you are having a hard time finding stress relief schedules for solution-treated-and-aged aluminum alloys, and that thermal stress relief is not recommended for heat-treated aluminum alloys. Reheating of previously heat-treated aluminum alloys is also subject to other potential problems.

As an item of information, the T6x tempers of 7075, despite their high strengths, have limited aerospace application because of the potential for catastrophic failure due to a relatively low stress corrosion-cracking threshold, and poor exfoliation corrosion characteristics, of this alloy in these tempers. It is more typically used (at a 10%-15% reduction in strength) in one of the "overaged" T7x tempers, which provide an increased stress corrosion-cracking threshold and superior exfoliation corrosion resistance.

Are you really experiencing distortion to the extent that requires stress relief? The stresses involved in machining aluminum alloys (even "high strength" ones) are generally low. You may want to take a look at the control on the specifics of your machining operations, with an eye toward reducing machining-induced stresses. "Vibratory" stress relief (see

http://www.bonal.com/Home/Literature/atlantis.pdf)

also provides a potential alternative to thermal stress relief.

 

[Kenneth]

The link in the above post doesn't work anymore.
Bonal has moved/changed their vibratory stress relief info to:

http://www.bonal.com/meta-lax/Home/Literature/atlantis.pdf

 

[brin]

On a very similar topic, I am considering replacement of 7075-T6 die forging with a "hog-out" part machined from 7075-T651 plate (QQ-A-250/12). The plate would start at 2.0 inch thick and there would be considerable material removal. Note this is an old design (mid 50's) and we are stuck with 7075-T6.

I was concerned about residual stress induced into the part by the machining process and increase of potential for SCC. ARP 823 (minimizing stress corrosion cracking) lists stresses from machining as a factor in SCC.

How do I evaluate the degree of residual stress induced into the part by the machining process? Is this directly related to the amount of distortion; eg: no significant distortion = no significant residual stress?

I looked at the link to vibratory stress relief. Are there any guidelines for when to use stress relief?

 

[bklauba]

The correct time to use the VSR Process, based upon the description, would be after rough machining. This would be the point when stresses from rough machining could be addressed, and the workpiece would also likely have resonances with both the lowest frequencies and highest amplitudes, i.e., it would be most responsive to vibration treatment.

I can send you a copy of a paper on vibratory stress relieving, of which I was one of three coauthors. This is being published by the ASM, and is being presented this next week at the ASM's Trends in Welding Research Conference, being held SW of Atlanta, GA. The paper will be presented on 5/19/5.

BK

 

[israelkk]

livelyka

The best and safest way to avoid distorsion is to let the part rest for about 2 weeks before final machining. Then the final machining should be done with sharp tools and small feed. This is the original recommendation from the ASM metal handbook (revision 8) that I used more than 30 years ago and it worked fine for very accurate parts.

 

[bklauba]

I agree with the concepts expressed by israelkk; the part should be given ample opportunity to find its own equilibrium.

What the vibration treatment would do (providing the geometry of the workpiece would be suitable; something not discussed yet) is hasten the equilibrium state to occur.

What can you tell us about the geometry of the workpiece?

BK