solidification of Al-alloy (Equiaxed crystal)

dendritic solidification, eutectics, peritectics,....
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Xianyi Kuang
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solidification of Al-alloy (Equiaxed crystal)

Post by Xianyi Kuang » Sun Jan 23, 2022 4:03 pm

Dear Bernd,

I am using MICRESS for research on the solidification process of aluminum alloys. Also encountered some problems. First of all, I think that the solidification process should be accompanied by the release of latent heat. I set up several heat extractions, whether it is at 7J/(s·cm3) (fig.a) or 50J/(s·cm3) (fig.b), the results of simulation are all cellular equiaxed crystals, which are inconsistent with the experimental results. In the experiment, I used a cast iron mold capable of casting about 1 kg of castings. The results of changing different processes are the coexistence of equiaxed crystals and dendrites. I believe that a wide solidification temperature range of metals should be cast as I experimented, but I don't know what went wrong with the simulated process.
Attached is a metallographic photo of me.
Looking forward to your reply and wishing you a Happy Chinese lunar New Year in advance.

XianYi Kuang.
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Bernd
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Re: solidification of Al-alloy (Equiaxed crystal)

Post by Bernd » Mon Jan 24, 2022 11:44 am

Dear Xianyi Kuang,

The results you get are as to be expected. You are using latent heat coupling in the "DTA" approximation, i.e. latent heat is released in form of an average temperature increase without having a temperature gradient. Having heat extraction without temperature gradient is only possible in case of very small samples (as typical for Differential Thermal Analysis, hence the name "DTA" approximation), while being unrealistic for most casting processes. Without temperature gradient you cannot have columnar dendrites...

The other extreme situation is having a very high thermal gradient, which makes it possible to neglect the effects of latent heat (which we call "Bridgman Approximation" because of its relevance in the related casting processes).

Unfortunately, most casting processes are between the two extremes, which makes it necessary to explicitly solve the thermal problem. This is not trivial because latent heat release depends on the microstructure formation process, so that the two scales (microstructure scale and temperature solution scale) are strongly coupled. We have developed a simulation scheme which allows for iterative solution of this problem with MICRESS using in internal macroscopic 1d-temperature solver for plate, cylinder or spherical approximations of the casting. We called this approach "Homoenthalpic Approach":

B. Böttger, J. Eiken, M.Apel, Phase-field simulation of microstructure formation in technical castings – A self-consistent homoenthalpic approach to the micro–macro problem J. Comput. Phys. 228 (2009), 6784-6795.

An example for Al-alloys can be found in the MICRESS application examples (A002_AlCu_Temp1d_dri, in versions 6.x just AlCu_Temp1d_dri). You can find more discussions about the problem of the temperature boundary condition in this Forum, e.g. here.

Another question is why you get equiaxed cellular and not equiaxed dendritic microstructures. This is only a question of the applied seed density: If you use a complex seed density distribution, the transition of cellular to dendritic with varying heat extraction rate will depend on the exact shape of this distribution.

Best wishes for 2022 as well as for the Chinese lunar New Year!

Bernd

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