Stress relief procedure for Al 7075-T6 1

[dculp1]

What is the stress relief procedure for Al 7075-T6 including:
1. Temperatures
2. Furnace time at each temperature (perhaps based on inches of thickness or diameter)
3. Furnace atmosphere
4. Cooling procedure
5. Other?

Thanks,
Don Culp

 

[swall]

A stress relief will destroy your T6 condition. What type of component do you have and what are you trying to do?

 

[dculp1]

The parts are cylindrical with radial slots. When the part diameter is less than 4" there are no problems. However, for larger diameters the parts fail by fatigue, even though finite element analysis indicates that the stresses should be acceptable. One possibility is that residual stresses may be present, either from the original material processing or from machining. Stress relieving should eliminate these residual stresses.

Don Culp

 

[Kwan]

so what kind of Kt did you get? what's your ref stress? It might be that you have a genuine fatigue problem. What's the slot geometry? width, depth, radii?

It also could be that you are experiencing stress corrosion cracking, scc. I would recommend shot peening due to the scc suseptability of T6. It's also a good idea to provide a coating such as anadizing or at least alodine.

It's also wise to send your failed part to a lab for testing. You will gain vast knowledge from the results (cycle counts, etc).

 

[CoryPad]

According to the ASM Handbook Volume 4 Heat Treating:

"In general, the precipitation treatments used to obtain the T6 tempers provide only modest reduction in stresses, ranging from about 10 to 35%."

Other options are mechanical stress relief (using tension or compression prior to age hardening), or cold stabilization (cycling of parts above and below room temperature).

You can consider an overage condition (T7) for improved performance.

You can look at geometry changes and surface modification (peening, burninshing) to improve fatigue resistance.



Regards,

Cory

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[swall]

I have observed a reduction in residual stresses by changing from a T6 condition to a T7, as suggested by CoryPad, although it was on a 355 casting, not a 7075 part.

 

[Kwan]

Yeah you can artificially age to T76 which will also provide better stress corrosion properties. But will also decrease your allowables. Not sure of the process, but suspect you have to artificial overage right after heat treat????

 

[dculp1]

Cory --

What is the process for cold stabilization? Does it improve the fatigue properties? Any particular on-line references?

I looked at matweb.com for 7075-T7. It listed 7075-T73 and 7075-T735x. The listed fatigue strengths are both 21,800 psi versus 23,000 psi for 7075-T6. Is there some other property for which you feel that the T7 would have better performance?

Don Culp

 

[dculp1]

What about 7075-T651? My understanding is that the T651 indicates that this material has already been stress relieved. Is this true?

Don Culp

 

[CoryPad]

Cold stabilization is cycling the temperature - for example, immersing in a water:alcohol solution below room temperature, then immersing in boiling water, then repeat. This can remove residual stresses. No comment regarding fatigue - such a complex issue, it is impossible to generalize. I have no idea if there are Internet information sources.

The biggest reason to use T7 is for improved Stress Corrosion Cracking resistance.

T651 means that mechanical stress relief was used between quenching and artificially aging. Usually done on semi-materials (sheet, bar, etc.) with constant cross section, not on final parts.

Regards,

Cory

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