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Aluminum: When, or is, stress relief needed?

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  • Michael Moore
    replied
    http://www.eurospares.com/graphics/m...IL-H-6088G.pdf

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

    cheers,
    Michael

    Leave a comment:


  • Evan
    replied
    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.

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  • David Powell
    replied
    A lifetime of work.

    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.

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  • oil mac
    replied
    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.

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  • Evan
    replied
    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.

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  • Your Old Dog
    replied
    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.

    Leave a comment:


  • Glenn Wegman
    replied
    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
    Last edited by Glenn Wegman; 08-28-2009, 08:14 AM.

    Leave a comment:


  • Evan
    replied
    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.

    Leave a comment:


  • Glenn Wegman
    replied
    Originally posted by Evan
    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

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  • gfphoto
    replied
    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

    Leave a comment:


  • rantbot
    replied
    Originally posted by gfphoto
    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.

    Leave a comment:


  • Evan
    replied
    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.
    Last edited by Evan; 08-26-2009, 12:38 AM.

    Leave a comment:


  • lazlo
    replied
    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.

    Leave a comment:


  • gfphoto
    replied
    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

    Leave a comment:


  • toastydeath
    replied
    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.


    Originally posted by Glenn Wegman
    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).

    Leave a comment:

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