ASM Online Member Community

Expand all | Collapse all

Balancing strength and IACS requirements for 7175-T74 aluminum

  • 1.  Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 25 days ago
    We forge a 7175 reverse extrusion on the order of 2.5" thick which gets heat treated to the T74 condition. The requirements are 76k/66k UTS/YS in the longitudinal direction and 38% min IACS (AMS 4149). I have been playing around with the heat treatment, namely the first age temp (225F-250F), second age temp (325F-350F), and cycle times for both, but I have not yet obtained a good balance of properties - I can get either IACS to pass or strength to pass, but not both simultaneously. On several samples I was just barely passing on YS/UTS/IACS but not by enough margin that I'd consider it a repeatable process.

    I know this is a particularly tricky temper condition and some people resort to exotic heat treatments like 3 or 4 step ages to fine tune their properties - does anyone have any suggestions?

    Appreciated,

    ------------------------------
    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
    ------------------------------


  • 2.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 24 days ago
    Sean,

    Could you provide details on what kind of quench you're using (i.e. water, glycol)? Also, are the parts/samples moving from quench to age pretty quickly?

    ------------------------------
    Colin Fletcher
    Sr. Metallurgist
    Xerox R&D / 3D Printing
    Cary NC
    ------------------------------



  • 3.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 22 days ago
    We're quenching these in room temperature water out of a drop bottom furnace; the transfer is 15 seconds or less and the max peak water temperature is 100F. The load is actually not too large so the highest water temperature we witnessed was about 84F.​ We're following AMS 2772.


    ------------------------------
    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
    ------------------------------



  • 4.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 22 days ago
    Sorry, in regards to your seconds question - right now I'm running a trial where we solutioned some remnants and sent them to a lab to cut into tensile blanks, age in a lab furnace, and test so there is a long delay between solution annealing and aging. I know that some grades are more susceptible to natural aging than others - for 7175, do you think that would invalidate the experiment?​

    ------------------------------
    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
    ------------------------------



  • 5.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 21 days ago
    Edited by Colin Fletcher 21 days ago
    Alright - just thinking of the low hanging fruit before diving deeper.

    You mentioned that there is a delay between solution and aging. 7X75 alloys are sensitive to this and are best moved immediately to either age or cold storage (see AMS2770 for some recommendations on cold storage). Over time, the response to aging is adversely affected, with a dip in all properties occurring between a couple hours and roughly 3 days. By 3 days it seems to come back up and stabilize with slightly worse response compared to immediate aging. This is based on an old diagram in Hatch's Aluminum: Properties and Physical Metallurgy.

    If you're right on the edge, I suggest shuffling the samples to age ASAP or store in dry ice or other suitable means until it's convenient to begin aging. It might not be enough on its own, but it should help move you in the right direction.

    ------------------------------
    Colin Fletcher
    Sr. Metallurgist
    Xerox R&D / 3D Printing
    Cary NC
    ------------------------------



  • 6.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 20 days ago
    I agree that this is a confounding factor - right now I am running an experiment to flesh it out a bit more. I am taking two tensiles from the material that was solutioned a few weeks ago and I'm going to re-solution one of them in the lab furnace before aging them both to the T6 condition (250F for 24hr). It's not totally apples to apples since the quench transfer time will be better out of the lab furnace but this way I'll be able to directly compare two samples with different solution/age delays with a more established aging practice. 

    Do you have any suggestions as far as 3-4 step aging practices?            ​

    ------------------------------
    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
    ------------------------------



  • 7.  RE: Balancing strength and IACS requirements for 7175-T74 aluminum

    Posted 19 days ago
    Afraid I don't have any specific recommendations for 3-4 step aging practices, but I would check out literature on retrogression and reaging (RRA) treatments if you haven't already. It involves an intermediate treatment at a higher temperature than normal aging but lower than solution to re-dissolve certain precipitates, then another aging step to re-precipitate them.

    RRA has a fairly long history (since 1970s?) and is capable of tweaking properties and corrosion resistance for 7000 series alloys, but treatments aren't documented in any industry heat treat standards that I'm aware of.

    ------------------------------
    Colin Fletcher
    Sr. Metallurgist
    Xerox R&D / 3D Printing
    Cary NC
    ------------------------------