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Casting a metal (Enthalpy of Fusion Question)

  1. Oct 24, 2009 #1
    I want to take a liquid metal, Aluminum at 700oC and pour it into a steel mold starting at 20oC. For the purpose of the question let’s say that the heat transfer is 100% with no losses to the surroundings.

    Here are some numbers:

    Al
    Mass = 200 g = 0.2 Kg
    Ti, Al = 700oC
    Cp = 900 J/Kg-K
    ΔHf = 10.67 kJ/mol
    Melting point of Al = 660oC

    Steel
    Mass = 3Kg
    Ti, st = 20oC

    So the liquid aluminum will cool once it hits 660, but the steel mold should never change state due to its higher melting point.

    At this point I want to find the final temperatures of the Aluminum and the Steel mold.

    Here's what I have so far can someone please check my theory/number placement:

    (900 J/Kg-K)(0.2Kg)(700oC-660oC)+(200g)(1 mol/26.96g) + (900 J/Kg-K)(0.2Kg)(Tf, Al - 20oC)

    = (486 J/Kg-K)(3Kg)(Tf, St - 20oC)

    So my guess is both final temperatures will be the same, I just wanted to know if this work looks correct or if I need to change/move some numbers.
     
  2. jcsd
  3. Oct 24, 2009 #2

    Astronuc

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    Staff: Mentor

    This part (900 J/Kg-K)(0.2Kg)(Tf, Al - 20oC) is not correct. One is still cooling from Tmelt to Tf. The Al decreases in temperature while the Fe heats up.

    At equilibrium Tf(Al) = Tf(Fe)
     
  4. Oct 24, 2009 #3
    That's actually why I posted this, I had a feeling that using the 20oC on the Aluminum side didn't make sense, so I think the left hand side should be changed to (900 J/Kg-K)(0.2Kg)(Tf, Al - 660oC).

    Is that correct, or am I still missing something else?
     
  5. Oct 24, 2009 #4

    Astronuc

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    Staff: Mentor

    Try (660°C-Tf(Al)). The Al is cooling from 660°C to Tf, 660°C > Tf.
     
  6. Oct 24, 2009 #5
    Ok, I reworked it again let me know what you link:

    LHS (Aluminum)
    (900 J/Kg-K)(0.2Kg)(700oC-660oC)+(200g)(1 mol/26.96g)(10.67 kJ/mol)+(900J/Kg-K)(0.2Kg)(660oC - Tf)

    RHS (Steel)
    (486 J/Kg-K)(3Kg)(Tf - 20oC)

    I took the sum of the LHS and the RHS and put them both into excel

    I set Tf as my variable on both sides of the equation and used goal seek to find Tf = 94.77oC

    With that LHS=RHS=109019.9
     
  7. Oct 25, 2009 #6

    Integral

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    Staff Emeritus
    Science Advisor
    Gold Member

    It is my experience that molten Al can do nasty things to Stainless. Even though you are well below SS melting point reactions can still occur. The fact that your steel is cool should help.

    Good luck and dry molds.
     
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