PDA

View Full Version : Aluminum: When, or is, stress relief needed?



gfphoto
08-25-2009, 02:40 PM
I'd prefer to at least try to make accurate parts that won't change shape, is this something I should to worry about? If so, what should I be doing?

Thanks,

Gary

winchman
08-25-2009, 03:27 PM
When I was working for Avco-Lycoming in the late '60s, largish aluminum pieces were always sent to heat treatment several times during processing. The greater the amount of material being removed, the more often they went.

It may have been overkill, but we rarely had problems with things fitting together after they were done.

I have no idea what they were doing in "heat treatment", but I'm sure someone here knows what's required for the specific material you're working with.

Roger

Evan
08-25-2009, 04:37 PM
When I was working for Avco-Lycoming in the late '60s, largish aluminum pieces were always sent to heat treatment several times during processing. The greater the amount of material being removed, the more often they went.


That would be to remove strains caused by work hardening when machined. It's only an issue on the really high strength alloys such as 2024 and 7075. If the material is being formed rather than machined it will be heat treated to take it down to a soft condition for forming so that it doesn't lock in strains that could cause it to eventually crack or even crack during forming.

Just as with steel when aluminum is quenched from solution heat treating of the thicker sections it is possible for internal strains to develop that may nearly exceed the tensile strength of the material because of the differential cooling rates on thick sections. That is why the "largish" pieces will benefit from additional heat treatment.

Heat treating aluminum properly isn't a home shop process. It requires very accurate temperature control and heating and cooling rate control. Temperatures must be held to within a few degrees of the target and then stepped down in a controlled fashion to avoid unpredictable changes in properties. It also is not a realistic "send it out for treating" process unless you live next door to the heat treat shop. When aluminum is treated it immediately begins to age and within a few hours is well on it's way to regaining whatever temper it will end up with depending on alloy and previous condition. If treated to allow cold working you have a few hours when it is in optimum condition and by 8 hours it is already half hard.

Of course, this only applies to the heat treatable series which are the 2000, 6000 and 7000 alloys. The other alloys will work harden and may be annealed by heat treatment but are not heat treatable in order to improve strength. They gain strength only by the action of cold forming which produces strain hardening.

Rich Carlstedt
08-25-2009, 05:05 PM
Gary
Since you use 'Photo" in your tag, I assume you are into camera's
and maybe adapter rings
6061 is notorious for warping in very thin cross-section rings.

If you run into this, take a roughed out ring, just before final bore and thread, and mark it with a felt tipped marker.
Then heat it up with your propane torch, until the marker mostly dissappears.
stop and let it cool.
you can naturally age it in 30 days , when you can machine it to finish , as the temper will return during that time to a T6 condition ( 30 days at 68 F)
If you raise the temp to 260 degree for 2 days, you can machine it then.
Rich

Glenn Wegman
08-25-2009, 05:37 PM
Quite some time ago,(late 1980's) I was curious as to stress relieveing some machined 2024-T4 parts so I contacted the local aerospace certified Heat Treater and asked what the process would involve and they said they would call me back with the info. I recieved a call from the Metallurgist and he stated that the process was referred to as an "uphill quench" and it involved soaking the parts in liquid nitrogen and then "quenching" them back to room temperature.

Has anyone heard of this process or did I forget something/misunderstand?

Just curious!

Thanks,

Glenn

toastydeath
08-25-2009, 05:57 PM
We heat treat aluminum if we've removed a lot of metal, or want the material to be very stable for finishing.

Long term stability hasn't been much of an issue for us.



Quite some time ago,(late 1980's) I was curious as to stress relieveing some machined 2024-T4 parts so I contacted the local aerospace certified Heat Treater and asked what the process would involve and they said they would call me back with the info. I recieved a call from the Metallurgist and he stated that the process was referred to as an "uphill quench" and it involved soaking the parts in liquid nitrogen and then "quenching" them back to room temperature.

Has anyone heard of this process or did I forget something/misunderstand?

Just curious!

Thanks,

Glenn

It's usually called cryotempering. In any material that isn't monocrystalline, amorphous, or one of the other "interesting" phases in between, there's going to be some junctions between crystals and atoms in crystals themselves that are bonded to a neighbor that isn't the "right" one - not the lowest energy state.

By drawing energy out of the material, the atoms become closer packed. As the energy drops, the unfavorably-bonded atoms will become close enough to their lowest-energy neighbor for that attraction to overcome the existing bond.

The process isn't quite as the gentleman described, because just throwing it in the nitrogen will induce more stress cracks than it solves.

There are also more nuanced effects in many materials, such as steels. Cryotempering a steel will not only re-order the grains, but cause the alloy to undergo various phase changes (such as austenite decomposing into martensite).

gfphoto
08-25-2009, 07:07 PM
Thank you all. Very interesting.

I'd like to ask about some specific cases:

Say I turn and bore a 1 1/4" eyepiece barrel from a piece of 2" 6061 round rod. It will have several diameters and fairly thin walls.

Or I cut pieces for a laminated follower rest from 3/16" or 1/4" sheet?

Or make a diagonal cut in a 10"x25"x3/4" plate?

Or drill weight relief holes in that cut 10"x25" plate?

Or turn an 18"x2" shaft that I want to remain straight?

Or boring a hole for a 2" bearing I want to be a snug, but not hammered in, fit.

Should I expect any significant distortion? I'm not talking space shuttle specs, but movement of a thousandth or two might be meaningful for me.

Thanks,

Gary

lazlo
08-25-2009, 11:09 PM
If you're really worried about the aluminum warping from internal stress, just use tooling plate. It's cast (not rolled) aluminum that's stress relieved. Very, very stable, and not very expensive.

Note that it has substantially lower tensile strength than 6061, for example.

Evan
08-26-2009, 12:35 AM
Cryotempering has no application to aluminum. Aluminum doesn't undergo a glassy transition at any temperature and actually becomes slightly more malleable at cryogenic temperatures. The crystal structure remains the same all the way to absolute zero.

In answer to your questions, I do a lot of work in aluminum of all alloys and warping or strain induced distortion just isn't an issue.

rantbot
08-26-2009, 03:37 AM
I'd prefer to at least try to make accurate parts that won't change shape, is this something I should to worry about?
Depends on that pesky word "accurate."

If you're cutting a chunk out of, say, an optical flat polished onto an aluminum surface, you will almost certainly have a problem maintaining flatness.

Otherwise, no, nothing to worry about.

gfphoto
08-26-2009, 09:39 AM
lazlo wrote
If you're really worried about the aluminum warping from internal stress, just use tooling plate. It's cast (not rolled) aluminum that's stress relieved. Very, very stable, and not very expensive.

Not worried, just wondering if I should be...

I'll keep the "tooling plate" in mind though mostly get scrap metal so can't be too choosy.

rantbot wrote
Depends on that pesky word "accurate."
If you're cutting a chunk out of, say, an optical flat polished onto an aluminum surface, you will almost certainly have a problem maintaining flatness.
Understood. I'll stick to glass flats.

Evan wrote
In answer to your questions, I do a lot of work in aluminum of all alloys and warping or strain induced distortion just isn't an issue.
Thanks, that's what I needed to know.

Gary

Glenn Wegman
08-27-2009, 11:20 PM
Cryotempering has no application to aluminum. Aluminum doesn't undergo a glassy transition at any temperature and actually becomes slightly more malleable at cryogenic temperatures. The crystal structure remains the same all the way to absolute zero.


http://www.croucher.us/uphillqnch-1.htm

Evan
08-28-2009, 01:07 AM
That has all the hallmarks of a scam. It's "little known". It's attributed originally to a well recognized source a long time ago. It's difficult to do and the results are hard to replicate (therefor you can't do it but we can). There is only a single outside reference to a document that is unavailable.

NASA showed it to be ineffective but I won't quote it as you must be a registered member to download the document (I am) and as far as I am concerned the study methodology was flawed as it didn't test the "uphill quench" by itself. Sounds like the "special treatment" that somehow makes "FORTAL" different from 7075-T651 aluminum even though the alloy and properties are identical.

You can find even more such proprietary nonsense in the various types of die making aluminum alloys.

Glenn Wegman
08-28-2009, 08:00 AM
Evan,

If you read my original post, I am not claiming that it is the way to go for stress relieving. I simply pointed out that it was recommended to me quite some time ago.

Here is the entity that I was using for heat treat at the time that recommended the process to me. They seemed to know what they were doing, but perhaps not! At the time they were doing work for outside vendors, but have since stopped that and only do their own in house heat treating/metallurgy.

http://www.heico.com/fsg/fsProd.htm

Again, just providing a source for the info I was given.

Thanks,

Glenn

Your Old Dog
08-28-2009, 08:21 AM
As in another thread I mentioned this may not answer your question directly but....

I have had more trouble from finishing a project in one sitting from heat. The aluminum heats up and expands blowing your critical measurements all to hell. I've since learned to let the aluminum cool in the chuck for an hour or so before taking my final passes.

Evan
08-28-2009, 09:28 AM
Unless you are machining Invar that is a problem that affects nearly all materials. Plastics are far worse as they often have coefficients of linear expansion ten times greater than metals.

oil mac
08-28-2009, 06:53 PM
Hi Folks,
When i was serving my apprenticeship in the late 1950/s, We were making aluminium castings in various specifications, I remember one particular component, where the customer specified that the castings be put in water & brought up to boiling temperature, and boiled for a specific period, Then allowed to cool down,
does anyone have any ideas, what this "heat treatment" was supposed to achieve for this particular component? sorry i cant remember the metal spec, Was it some form of stress relieving? Wish i could recall the details of the casting etc.

David Powell
08-28-2009, 11:27 PM
I once met and did a little work for a University Professor who spent literally a lifetime investigating how to improve the strength of aluminium castings. Apparently early helicopters had many unexpected failures of aluminium castings, despite great care having been taken in their manufacture, and this got the fellow interested. The "secret" to make better castings apparently was in keeping temperatures of the melting and pouring cycles very tightly controlled. There was no mention of later heat treatments of the castings. Hope this is of interest. Regards David Powell.

Evan
08-29-2009, 01:17 AM
Casting alloys are an entirely different category of aluminum alloys compared to the heat treatable and fabrication alloys. I am not as familiar with the casting alloys since they are not much used in aircraft construction. It's a lot harder to inspect and qualify a casting for soundness than it is a sheet or plate product and the alloys are very different with most commonly much larger percentages of silicon to enhance the pourability and liquidity of the molten metal. The silicon content of the fabrication alloys may be as high as a couple of percent but the casting alloys may go as high as 15 percent. That makes a big difference to the properties including especially the response to temperature variations.

Michael Moore
08-29-2009, 12:39 PM
http://www.eurospares.com/graphics/metalwork/MIL-H-6088G.pdf

is a milspec doc on the heat treatment of aluminum alloys that may be of interest to some.

cheers,
Michael