DonkeyPhysics
New member
- Jul 16, 2009
- 41
Is anybody here aware of a national standard, or at least an accepted corporate standard (i.e. from Boeing) for the cold stabilization of aluminum for the purpose of relieving internal stresses?
The process I'm talking about is the cyclic (or not) temperature soaking of aluminum blocks at sub-zero temperatures (i.e. -120°F) followed by a soak at +250°C in order to relieve residual internal stresses leftover following the tempering process.
The aluminum temper I'm starting with is 6061-T651. The intended purpose is to create a more stable material block from which to machine tightly-toleranced profile cuts.
After asking around, searching google heavily, and speaking with several metalurgists, machine shops and heat treat houses that actually DO this process, nobody can point me to a common standard, and in fact one place actually said flat out that no such national standard exists.
Further, neither could anybody actually explain to me in technical terms exactly WHY the process works, what the optimal cycle time, temperature or order might be (i.e. hot first or cold first, how cold, and for how long each); or how to quantify the net benefit.
The best description I've seen so far comes from page 381 of the ASM Specialty Handbook: Aluminum and Aluminum Alloys (
An earlier thread in this forum from 2002 quotes the same line from a different book (though I wonder if the info may have just been mis-attributed).
The quote is: "Other thermal stress-relief treatmens, known as subzero treatment and cold stabilization, involve cycling of parts above and below room temperature. The temperatures chosen are those that can be readily obtained with boiling water and mixtures of dry ice and alcohol-namely 100 and -73°C (212 and -100°F) - and the number of cycles ranges from one to five. The maximum reduction in residual stress that can be effected by these techniques is about 25%. The maximum effect can be obtained only if the subzero step is performed first, and immediately after quenching from the solution treating temperature while yield strength is low. No benefit is gained from more than one cycle."
The next paragraph then goes on to describe a more extreme process that may be more effective, but which is beyond what I'm trying to quantify here.
Now, that does indeed appear to address the issue of how many cycles, which goes first, hot or cold, and what's the best I can expect from the treatment.
What it doesn't address is, if I'm not doing the cold stabilization process immediately after solution heat treatment, how does that affect the results? If my machine shop sends out their material for this process 2 weeks after the metal was certified to its temper, how much of an effect can I expect? If I soak the material for 4 hours at each temp, will that give me better results than flashing at each temp for only 30 minutes? Does the ramp rate matter? How much?
Should I calculate the minimum soak time based on the maximum thickness of the material?
From what I've been able to gather, the only standards for this process that exist are all proprietary corporate processes, and that the efficacy of those processes is all anecdotal.
My assumption is that Boeing, being a very picky aerospace manufacturer who works a LOT with high tolerance aluminum parts, and a lot of subcontractors, I'm assuming a standard MUST exist out there somewhere. Does anybody here know where I might find one?
Thanks a lot.
-Alex-
The process I'm talking about is the cyclic (or not) temperature soaking of aluminum blocks at sub-zero temperatures (i.e. -120°F) followed by a soak at +250°C in order to relieve residual internal stresses leftover following the tempering process.
The aluminum temper I'm starting with is 6061-T651. The intended purpose is to create a more stable material block from which to machine tightly-toleranced profile cuts.
After asking around, searching google heavily, and speaking with several metalurgists, machine shops and heat treat houses that actually DO this process, nobody can point me to a common standard, and in fact one place actually said flat out that no such national standard exists.
Further, neither could anybody actually explain to me in technical terms exactly WHY the process works, what the optimal cycle time, temperature or order might be (i.e. hot first or cold first, how cold, and for how long each); or how to quantify the net benefit.
The best description I've seen so far comes from page 381 of the ASM Specialty Handbook: Aluminum and Aluminum Alloys (
An earlier thread in this forum from 2002 quotes the same line from a different book (though I wonder if the info may have just been mis-attributed).
The quote is: "Other thermal stress-relief treatmens, known as subzero treatment and cold stabilization, involve cycling of parts above and below room temperature. The temperatures chosen are those that can be readily obtained with boiling water and mixtures of dry ice and alcohol-namely 100 and -73°C (212 and -100°F) - and the number of cycles ranges from one to five. The maximum reduction in residual stress that can be effected by these techniques is about 25%. The maximum effect can be obtained only if the subzero step is performed first, and immediately after quenching from the solution treating temperature while yield strength is low. No benefit is gained from more than one cycle."
The next paragraph then goes on to describe a more extreme process that may be more effective, but which is beyond what I'm trying to quantify here.
Now, that does indeed appear to address the issue of how many cycles, which goes first, hot or cold, and what's the best I can expect from the treatment.
What it doesn't address is, if I'm not doing the cold stabilization process immediately after solution heat treatment, how does that affect the results? If my machine shop sends out their material for this process 2 weeks after the metal was certified to its temper, how much of an effect can I expect? If I soak the material for 4 hours at each temp, will that give me better results than flashing at each temp for only 30 minutes? Does the ramp rate matter? How much?
Should I calculate the minimum soak time based on the maximum thickness of the material?
From what I've been able to gather, the only standards for this process that exist are all proprietary corporate processes, and that the efficacy of those processes is all anecdotal.
My assumption is that Boeing, being a very picky aerospace manufacturer who works a LOT with high tolerance aluminum parts, and a lot of subcontractors, I'm assuming a standard MUST exist out there somewhere. Does anybody here know where I might find one?
Thanks a lot.
-Alex-
