6061 stress relief time

[JimEF]

We have an aluminum welded structure consisting of .38 6061 plate with 3x3x.38 angles on one side for stiffness. This assembly will typically be used as part of a mold in an oven operating at 415°F for 3 hours, then cooling for an hour to about 300°F. At this point it is taken out an air cooled with fans. With the residual stresses in the weld as well as the stressed due to the thermal cycling welds will often crack. I found the stress relief temperature for aluminum to be 650°F, but our ovens will only do 550F. My thought is to reduce the residual stress by heat soaking the part at 550°F. My question is will this method work and how long do I need to soak the part at 550°F to get an effective stress relief.

 

[CryogenIndustries]

From my experience 6160 can be tempered at 350 F for 8 hours then air cooled to give a good final stress relieve. You can also cryogenically treat it then stress relieve if you wanted to take an extra step that might help reduce fracturing

Shannon Mafodda

http://www.cryogen.com.au

 

[NickE]

CryogenIndustries-

Could you please explain in technical terms how cryogenic processing affects the micro-structure of aluminium alloys? I have gone to your website and find very little hard data on aluminium alloys. I am specifically interested in clarification of the following statements:

"Cryogenic tempering of aluminium and aluminium alloys will stress relieve your aluminium components and increase their load bearing capacity, making them stronger and last longer.

In addition to sporting equipment, aluminium engine components such as heads and blocks when treated cryogenically will exhibit lower warping and distortion and enhanced heat dissipation properties.

Thank you for your time.

Nicholas J. Enright

 

[swall]

JimEF--what condition was the 6061 aluminum in prior to welding? Secondly, how do you know that the weld cracks found after the furnace cycle were due to thermal fatigue? It is possible that the welds had cracks prior to the furnace cycle and the furnace cycle merely made them visible or propagated to where they became visible.

 

[JimEF]

The aluminum are in as rolled and as extruded condition. What I am trying to accomplish is to relieve the residual stress from the welding shrinkage. It is the residual stress coupled with the thermal stress causing the welds to crack.

As to your second question, of course there could be micro cracks due to welding. Again, these would presumably propogate during stress relieving and could be repaired afterward.

 

[metengr]

One solution to this problem could be the filler metal used for welding. What filler metal did you select?

Perhaps you can use one of the lower strength weld filler metals that would have increased ductility, and better accommodate residual stress from welding. You might be able to eliminate this problem without even having to perform a separate post weld stress relief.

 

[metengr]

NickE;
The web site below is one that I had found regarding improved dimensional stability of 6061 plate. The process is to cycle between low temperature (liquid nitrogen)and moderate temperature to improve dimensional stability. Besides this web site, several others popped up with similar but unexplained results. There was nothing mentioned about increased strength only improved dimensional stability by reduction of residual stresses. Some interesting reading anyway

http://www.noao.edu/ets/gnirs/SDN0013-07.htm

 

[SVanden]

To anneal any 6061 we heat it to around 985° F throughout, then slow cool down to 500° at 50°/hr. You may need to use a 6XXX series filler, then anneal the assembly, then heat treat it thru T-4 and T-6 cycles. T-6 is 350° for 8 hrs, so cycling from ambient to 415 and back multiple times may be over-aging the material, making it brittle.

svanden

 

[unclesyd]

SVanden,
I used a bit different procedure to anneal 6061.
To full anneal:
Heat to 775°F hold for 2-3 hrs depending on size and cool @ 50°F/Hr until 500°F.

To just remove cold work heat to 650°F for 2 Hrs then air cool.

 

[CryogenIndustries]

NickE,

The reason their isn't any hard data with relation to stress relieving of aluminium and aluminium alloys on my site is that there hasn't been any conclusive studies to definitively explain the results of the process with this type of material. We have relied on our field testing do develop our results (hence our statement), and from experience the process works very well. Its not a miracle cure-all but it does work, remember that commercial cryogenics is still in its infancy and when more technical data is released I will certainly make it available.

Shannon Mafodda

http://www.cryogen.com.au