Questions regarding new/modified inputs for AM in Micress 7.1

technical aspects of .dri file generation (e.g. debug mode ) etc...
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Atur
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Questions regarding new/modified inputs for AM in Micress 7.1

Post by Atur » Tue Nov 09, 2021 3:15 pm

Dear Bernd,

I would first like to say, it is so nice to see two examples on simulation of SLM processes (A017 and A018 applications), which serves as a solid bases for reference while we are constructing our simulations! However, there are some inputs/assumptions which I actually didn't understand what is the reason behind them. You answers will be much valuable for me to improve my simulations:

1-When I start my simulations, I receive:
# In phase BCC_B2 component CO is defined as interstitial.
# In phase BCC_B2 component NI is defined as interstitial.
# In phase BCC_B2 component AL is defined as interstitial.

As far as I understand this is automatically detected by Micress. Does that mean this elements will diffuse faster in B2_BCC phase but not in other phases? Is these selections composition dependent ?

2-I would also like to ask what is the factor 0.7 standing next to atc mob_corr refers to in A017_M247 example? I realized it is only there for FCC_L12 phase, which is the initial pahse to solidify.

3- What happens when we categorize a seed type? What is the number 11 is standing for?
# Shall categorization be applied to this seed type?
# Options: categorize {Number} no_categorize
categorize 11

4- What does the number 111 stands for in shield phase or group number?
# Shield effect:
# Shield time [s] [shield phase or group number] ?
5.E-6 111

5-What is the difference between shield and nucleation distances?
# Shield distance [micrometers] [ nucleation distance [micrometers] ] ?
0.15 0.80

6- Can you also briefly explain what does giving a penatly to a certain component in certain phase stands for in concentration solver?

7- In concetration solver I saw order/disorder term with defining sublattices. Does it somehow helps to define the so called ingredients of the ordered or disordered phases?

8. There is also diff_comp_sets and vol_comp_sets option where L12 and L12#2 phases are selected. I would also like to understand where this input can be useful.

I would appreciate your answers and thank you again for your efforts to prepare such nice application examples!

Regards,
Ahmet

Bernd
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Re: Questions regarding new/modified inputs for AM in Micress 7.1

Post by Bernd » Tue Nov 09, 2021 7:55 pm

Dear Ahmed,

I am pleased to hear that you like my SLM examples. Let me try to answer your questions (or refer to already existing answers):

1.) Elements are assumed to be "interstitial" as soon as they are on a common sublattice with 'VA' species. In typical steel phases (FCC_L12, BCC_A1) these are C, N, B, etc., the typical interstitial elements which can diffuse faster due to their small atom sizes. However, some phases like BCC_B2 this is also the case for normal sized atoms.
The important issue with "interstitial" is not the diffusivity but the fact that they can change place without the necessity that another element goes in the opposite direction because the vacancies can compensate. Hence, the important thermodynamic variable for their movement or interchange with other phases is the chemical potential and not the diffusion potential.
If in MICRESS an element is manually or automatically assigned as "interstitial", this currently has consequences in three places:

a) If the volume solver ('volume_change') is activated, interstitial elements are assumed to diffuse without compensation by the matrix element.
b) If TQ-coupling is used, quasi-equilibrium is calculated by assuming the chemical potential to be identical in both phases. Please note that manually defining an element as "interstitial" only has effect on the linearisation parameters in case of 'diagonal' or multiternary extrapolation, while calculation of quasi-equilibrium cannot be altered.
c) In case of stress coupling, the calculation of the eigenstrain from the molar volume does not include the interstitial elements into the frame of reference. Furthermore, in case of mechanochemical coupling, the driving force for diffusion by stress is done using the chemical potential instead of the diffusion potential.

2.) When performing MICRESS simulations with different grid resolution, in the ideal case the resulting phase transformation should be the same. This is difficult to achieve under conditions of SLM, thus some "calibration" can be used to get better scaling with resolution. In this case, it was observed that growing dendrites at coarser resolution slightly fell behind those at higher resolution. This can be corrected for by a prefactor on the mob_corr function.
I am currently working on an improved formulation of the mobility correction for such cases.

3.) Categorizing seed types means to assign a number of fixed classes of orientations instead of the assignment of completely random orientations. The advantage is that after growing to full size and passing the shield time, they can be grouped accordingly to reduce the number of existing grains (if "categorize" is defined for the respective phase, see also here).

4.) Seed types with the same seed group number will interact via shielding (see here).

5.) While the seed distance defines the radius around a previously set grain where no further nuclei (of the same phase or shield group before end of the shield time) are allowed to be set, the nucleation distance is the distance between the points to be checked for nucleation. If no nucleation distance is specified, the value of the shield distance is used instead.

6.) see here

7.) Similar to concentration limits, these options define that a certain phase should always stay ordered or disordered (in e.g. FCC_L12 this means that sublattices 1 and 2 have different or identical constitution), otherwise equilibria are rejected with an error message (see also here). This is important for handling composition sets.

8.) see here

Best wishes
Bernd

Atur
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Re: Questions regarding new/modified inputs for AM in Micress 7.1

Post by Atur » Thu Nov 25, 2021 3:07 pm

Dear Bernd,

Thank you for your reply!

Do you have any estimations regarding the pre-factor if we would like to simulate different additive processes such as LMD? The cooling rates are lower compared to L-PBF and I would like to compare two processes. Would using the given pre-factor (0.7) would effect segregation and solidificaiton profiles significantly? Or should I directly ignore this pre-factor during LMD?

Regards,
Ahmet

Bernd
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Joined: Mon Jun 23, 2008 9:29 pm

Re: Questions regarding new/modified inputs for AM in Micress 7.1

Post by Bernd » Thu Nov 25, 2021 7:42 pm

Dear Ahmet,

As I said, the prefactor of 0.7 was specifically calibrated for the example. If you do not want to calibrate it for your cases, you should just omit it. It won't make a great difference, especially if you work with sufficiently high resolution.
Calibration makes sense if you want to tweak the range of the grid size which still gives quantitatively correct values, to the upper limit. To do so, you need to perform representative simulations (e.g. 1/2 dendrite in 2D in longitudinal direction) at different grid resolutions (including much finer ones), and calibrate the prefactor for getting exactly identical tip temperatures. Please note that the prefactor to the mobility correction mainly affects the tip temperature, which can not be correct in 2D anyway...

Bernd

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