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Does Aluminum, after machining, continue to dimensionally change over time?

  1. Jun 26, 2015 #1

    Ranger Mike

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    Does Aluminum , after machining, continue to dimensionally change over time?
    A large (1000 mm x 500 mm x 300 mm ) aluminum casting is clamped and machined. Various large diameter hole and drilled. Faces are milled. The casting is stress relieved in the normal manner. Next the milled faces are coated to reduce wear by the air bearing that traverse the face. I have read that the aluminum continues to change both in surface hardness ( Rockwell) and by creep deformation by residual stress. Can one confirm this? I have seen tech papers stating that the dimensions continue to change after 1500 hours.
    Any input is appreciated.
     
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  3. Jun 26, 2015 #2

    Baluncore

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    All metals change dimension before, during and after machining. Distortion is highly dependent on alloy composition, stress and thermal history.

    Traditionally, critical castings were pre-machined and then stored outside for a year. During that period of thermal cycling most of the expected changes took place. The final accurate machining, grinding and scraping was then done. Since minimal material was removed in the final process, there was little more creep after finishing. The process of coarse machining, followed by a relaxation process or time, before final machining, may be necessary to meet the final dimensional specifications over time.

    If the hardness of a surface changes it is probable that the dimension also changes since both involve a change of crystal structure. Surface hardening that does not require bulk heating leads to more dimensionally stable components. Chemical nitriding and induction hardening are both used on steel because they generate minimal distortion during the surface hardening process.
     
  4. Jun 27, 2015 #3

    Ranger Mike

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    Baluncore, thank you for taking the time and giving me some much needed confirmation. This is as I suspected. Attached are charts from a tech paper,

    The Effect of Aging on the Relaxation of Residual Stress in Cast Aluminum

    LARRY A. GODLEWSKI, XUMING SU, TRESA M. POLLOCK, and JOHN E. ALLISON

    DOI: 10.1007/s11661-013-1800-1

    The Minerals, Metals & Materials Society and ASM International 2013

    the original purpose of this paper was "

    There is some speculation that during the aging cycle, the residual stress is relaxed after a certain time at temperature. It is understood that there are a number of features that can influence relaxation in aluminum and redistribution of residual stresses. Increasing the aging temperature lowers the yield stress and can introduce time-dependent (creep) deformation."

    This was confirmed with experiments of Aluminum specimens subject to differing temperatures after machining. Both hardness and strain (stress) were measured over time.
    Rockwell Hardness reduced over time as did residual stress.
    My conclusion is you must be aware of the manufacturers machining processes and practices in order to get the best dimensionally stable aluminum casting.
     

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