MICRESS Forum

dear bernd,
Sorry to bother you again,I built a simple model myself.First, the specified grains are three, and the positions are (10,0),(20,0), and (30,0), but why is the calculated model not like this?Why is it (0,0),(10,0),(20,0)?
Secondly, I observed a particularly thick dendrite spacing, which I felt was very bad. Do you have any good suggestions?
Thank you very much for your reply in your busy schedule. Thank you very much. Have a nice life.
Lv Shaojie

Hi Lv Shaojie,

The reason why you get unexpected positions is the following: (30,0) is just outside the domain, so it is skipped. Instead you get nucleation at (0,0) according to your nucleation type at the defined region...

The primary dendrite spacing you observe is defined by these initial positions. However, this microstructure is not dominated by cooling rate and gradient, but by the initial undercooling, and thus is much too fine. At the present starting conditions it would take very long time to reach stationary growth, which you could verify by a constant tip temperature (last column of .TabL output).
Practically, it will be necessary to start at significantly higher temperature and with a coarser grid if you want to approach stationary conditions.

Bernd

Dear Bernd,
Thank you very much for your letter. I have basically understood what you said, but I still don't understand some parts of it. Is the initial undercooling you mentioned here used to control the thickness of primary dendrite? How should the parameters be set? To tell you the truth, I did not find the control parameter for the initial overcooling.We hope to get your help.
Lv Shaojie

Dear Lv Shaojie,

During instationary dendrite growth, the current front undercooling determines the growth velocity, the microstructure length scale and thus the primary (PDAS) and secondary arm dendrite spacing (SDAS). The front undercooling is the difference between the front temperature and the equilibrium (liquidus) temperature. You control it by setting the initial bottom temperature and the cooling rate.

If dendrites grow in a temperature gradient, they can adapt their front temperature to a stationary value. However, this may require long time and a long growth length, especially if the initial front undercooling is far from the stationary one.

Bernd

Dear Bernd,
Thank you very much for your timely reply and your valuable suggestions. I will set the program according to your suggestions. Wish you a good mood every day.finger heart.
Lv Shaojie

Dear Bernd
Hello, I have been calculating some cases in MICRESS recently, and I have encountered some problems in case T003. Attachment is the program I used in the calculation. I would like to know why secondary dendrites no longer appear in the growth process of dendrites.In this case, the cooling rate is 10. For example, there are secondary dendrites at time 0.5 seconds, and only primary dendrites at time 1 second.
In addition, other conditions remain the same, when the cooling rate is 100, the secondary dendrite will basically always exist (although the secondary dendrite will be a little less when the time is longer), which may be because the evolution time is too short.
Looking forward to your reply, I wish you a happy life and smooth work.
Lv Shaojie

Dear Lv Shaojie,

You should allow the dendrites more time to select their primary distance. As you can see in your simulation after 1s (cooling rate of 10K/s) 2 of the 7 dendrites already have fallen back considerably and will probably soon be overgrown. The resulting bigger distances certainly will allow formation of secondary arms. Generally, you reached stationary growth when the primary spacing and the tip temperatures don't change any more.

With 100K/s everything goes much faster. However, it seems that microstructures are already too fine for the given resolution (Δx=2µm). Here you should increase resolution accordingly.

Bernd

Dear Bernd

First of all thank you very much for your advice, then in order to verify the dendrite spacing, I will change the original time t = 2 s to time t = 30 (last time), there is shown in the figure, so I don't know if my calculation results conform to the normal rules, hope to receive your further answers, I speak very grateful to you.
Finally, I would like to express my gratitude once again.
Lv Shaojie

Dear Lv Shaojie,

As you can observe in your results, dendrites grow very fine at the beginning, and get much coarser afterwards. This indicates that initial undercooling and thus the initial growth velocity is high, while stationary growth occurs at a higher temperature with a lower stationary velocity (defined by cooling rate and temperature gradient).
I would advice you to make a plot of the tip temperature (column 9 in .TabL) vs. time to visualize this process. In order to reach stationary conditions more quickly, you should start your simulation with an initial temperature (at the bottom) close to this stationary tip temperature.

Bernd