Material Behavior around the FSW/FSP Tool Described by Molecular Dynamics

Round 1

Reviewer 1 Report

The manuscript applied molecular dynamics simulation to study the Friction Stir Welding and processing (FSW/FSP). Authors describes material´s plastic flow at different conditions with regard to temperature and advancing and rotating speeds. They also display typical microstructural zones of the FSW/FSP materials and their crystallographic characteristics. The MD method shows good potential in the micro FSW/FSP. It’s a useful study. However, there are some questions.

1. In the molecular dynamics simulations, the number of atoms varies from 27k to 1.27 million. Why do the authors use so many different atom systems? And is there relative error in the calculation with different systems?

2. Which material is used in the molecular welding model? How many kinds of atoms are used?

3. Details about embedded atom method used in the study should be given. It’s the basis of the study.

4. There are different colors in Figure 1 and Figure 2, what do they represent, respectively?

5. Welding directions in the figures should be marked.

6. As shown in Table 2, pin radius in experiment is 2 mm, while it is 12 nm in the simulation. There is huge difference. How to compare them?

7. Different welding conditions cause different temperatures, In figure 2 how to maintain a same T_max=744K at different conditions?

8. Which point or zone does the temperature values Refer to in Figure 5?

9. Where is (a)(b)(c) in Figure 6?

10. Many numbers Lack of superscript. Such as 5 x 104 – 5 x 105 mm/s, 2.8 x 101 - 2.0 x 102.

Author Response

Dear Reviewer,

thank you very much for your report to our above paper "Material behavior around the FSW/FSP tool described by molecular dynamics" (jmmp-2107035) by Bentejui Medina and Ricardo Fernandez. The authors gratefully acknowledge the positive evaluation of the paper. We appreciate your comments which pinpointed significant points in the presentation of the data and allowed us to make the manuscript clearer. We have read very carefully all the concerns raised by you and, as requested, have amended our work accordingly. The modifications introduced are marked by changes tracking markup in the revised version of our work. The responses to your questions and comments are summarized in the following.

 

1. In the molecular dynamics simulations, the number of atoms varies from 27k to 1.27 million. Why do the authors use so many different atom systems? And is there relative error in the calculation with different systems?

We use different quantities of atoms to show the effect of the pin diameter. The number of atoms in the simulation, that is proportional to the pin size, also has a major influence on the microstructural characteristics obtained by MD simulation of the FSW/FSP process. In particular, to obtain a certain atomic mobility that generates crystal defects, it is necessary to consider a high number of atoms. In the present case, systems with 27k, 678k and 1.27M atoms have been considered. The system dimensions are 4 times bigger than the pin radius to avoid excessive constriction of the material. We saw that with 1.27 million atoms the flow becomes geometrically similar to simulations on real size welding. In these simulations there is no possibility to calculate errors (%) because the outcome of the simulations is a distribution of atoms around the pin. We have included one sentence in the revised manuscript about the relative error in the simultations.

 

2. Which material is used in the molecular welding model? How many kinds of atoms are used?

The material considered in the present simulations is aluminum. This point was not clearly described in the original submission. In the revised version, it is already written from the abstract.

In the simulations developed in the present work there are three types of atoms. The main one is that corresponding to aluminum free atoms. The second type corresponds to atoms at the external boundary of the system. They are used to generate a symmetric boundary to extend the system size. Finally, the atoms of the pin are aluminum atoms insensitive to the external force and are grouped having an invariant speed among them.

 

3. Details about embedded atom method used in the study should be given. It’s the basis of the study.

Many thanks to emphasize this point. The details of the method are included in the revised version of the manuscript. The embedded atom method is the pair style EAM/FS, as in: Finnis, Sinclair, Philosophical Magazine A, 50, 45 (1984).

 

4. There are different colors in Figure 1 and Figure 2, what do they represent, respectively?

The colors in figures 1 and 2 are used to visually track the flow dynamics of the material during the simulated welding process. Each individual color just represents the original position of the atoms prior to the weld process. A sentence explaining this has been added in the revised manuscript.

 

5. Welding directions in the figures should be marked.

Thank you for your comment. This certainly will improve the understanding of the figures. We have included some arrows indicating the main welding directions in the FSW process.

 

6. As shown in Table 2, pin radius in experiment is 2 mm, while it is 12 nm in the simulation. There is huge difference. How to compare them?

As it is stated by the reviewer, the dimensions of the tools in the experimental tests are certainly huge in comparison to the ones used in the MD simulations.

In the present case, there are two ideas that justify the comparison.

1. In the case of FSW/FSP process material deforms around the tool under such severe temperature and constriction conditions that it behaves like a viscous liquid, which minimizes scaling effects [24] in the reviewed version.
2. Moreover, as it has been previously reported, tiny volumes of a few atoms along the edge obey the classical laws of continuum mechanics when the surface energy effects are adequately considered [24] in the reviewed version.

The effect of (v,w) have been normalized by the pin size in the MD simulations to provide a dimensionless weld pitch. This normalization process allows to properly comparing the experimental results and simulations.

 

7. Different welding conditions cause different temperatures, In figure 2 how to maintain a same Tmax=744K at different conditions?

We want to thank the referee for his/her highly detailed revision. In the case of figure 2, we have made a mistake and the same temperature was copied to the figure caption for the three conditions. As it is seen in Table 1, these temperatures are 744, 938 and 1600K. The right data has been copied in the figure caption of the revised version of the manuscript.

 

8. Which point or zone does the temperature values Refer to in Figure 5?

We thank the reviewer for this comment. We have modified the Figure 5 by including the exact zones representing the different temperatures determined around the tool.

 

9. Where is (a)(b)(c) in Figure 6?

The a), b) and c) are indicated in the right side of the figure.

 

10. Many numbers Lack of superscript. Such as 5 x 104 – 5 x 105 mm/s, 2.8 x 101 - 2.0 x 102.

Thanks, we have amended these mistakes.

Reviewer 2 Report

It seems that the unit for tool radius in the text and Table 1 does not match.

English: line 39, "on one hand"

line 104, "has been considered"

Author Response

Dear Reviewer,

thank you very much for your report to our above paper "Material behavior around the FSW/FSP tool described by molecular dynamics" (jmmp-2107035) by Bentejui Medina and Ricardo Fernandez. The authors gratefully acknowledge the positive evaluation of the paper. We appreciate your comments which pinpointed significant points in the presentation of the data and allowed us to make the manuscript clearer. We have read very carefully all the concerns raised by you and, as requested, have amended our work accordingly. The modifications introduced are marked by changes tracking markup in the revised version of our work. The responses to your questions and comments are summarized in the following.

 1. It seems that the unit for tool radius in the text and Table 1 does not match.

Thank you for this appreciation. The reason of the difference in the units for the tool radius in the text and in Table 1 is because in Table 1 we need to compare the dimensions of the tools in different systems and we use the experimental tools as a reference. For this reason, the units are in mm in the Table. However, in the text, the typical units for the pin radius in the MD simulations are nm.

2. English: line 39, "on one hand"

Thanks, we have solved this mistake.

3. line 104, "has been considered"

Thanks, we have amended this mistake.

Reviewer 3 Report

The manuscript presented an interesting research topic on FSW/FSP simulation. It is not clear whether the proposed methodology is also applicable to polymeric materials.

The paper should be improved with some more appropriate references about FSW of polymers, such as the following valuable references:

https://doi.org/10.1016/j.mtcomm.2019.100796

https://doi.org/10.1177/0954405414546705

https://doi.org/10.1016/j.matdes.2014.07.047

https://doi.org/10.1016/j.matdes.2014.02.036

https://doi.org/10.3390/polym14122474

Sidahmed Ameur, Chikh Ould El Bahri, Abdelkader Djebli, Mostefa Bendouba, Abdelkrim Aid, Benattou Bouchouicha, “ANALYSIS OF FSW PARAMETERS: NUMERICAL SIMULATION OF HDPE PLATE,” STRUCTURAL INTEGRITY AND LIFE, Vol. 21, No 3 (2021), pp. 301–308

 

The writing should be polished, check lines: 62 (fsw/fsp); 88 (over- representation); etc.

Throughout the manuscript it is not clear what "system dimensions" means, as in line 398?

The figure captions should be improved with more information, such as scale and region identification, tool identification and color coding.

Author Response

Dear Reviewer,

thank you very much for your report to our above paper "Material behavior around the FSW/FSP tool described by molecular dynamics" (jmmp-2107035) by Bentejui Medina and Ricardo Fernandez. The authors gratefully acknowledge the positive evaluation of the paper. We appreciate your comments which pinpointed significant points in the presentation of the data and allowed us to make the manuscript clearer. We have read very carefully all the concerns raised by you and, as requested, have amended our work accordingly. The modifications introduced are marked by changes tracking markup in the revised version of our work. The responses to your questions and comments are summarized in the following.

 

 

The manuscript presented an interesting research topic on FSW/FSP simulation. It is not clear whether the proposed methodology is also applicable to polymeric materials.

The paper should be improved with some more appropriate references about FSW of polymers, such as the following valuable references:

https://doi.org/10.1016/j.mtcomm.2019.100796

https://doi.org/10.1177/0954405414546705

https://doi.org/10.1016/j.matdes.2014.07.047

https://doi.org/10.1016/j.matdes.2014.02.036

https://doi.org/10.3390/polym14122474

Sidahmed Ameur, Chikh Ould El Bahri, Abdelkader Djebli, Mostefa Bendouba, Abdelkrim Aid, Benattou Bouchouicha, “ANALYSIS OF FSW PARAMETERS: NUMERICAL SIMULATION OF HDPE PLATE,” STRUCTURAL INTEGRITY AND LIFE, Vol. 21, No 3 (2021), pp. 301–308

 

We completely agree with reviewer 3. It is a good idea to include some references about FSW/FSP in polymers. As the present work is not focused on polymers and some of the references proposed by the reviewer do not use MD simulations, we have included only some them in the introduction section.

 

H.A. Derazkola, A.Eyvazian and A. Simchi. Modeling and experimental validation of material flow during FSW of polycarbonate. Materials Today Communications 2020, 22; 100796.

 

N.Mendes, A. Loureiro, C.Martins, P.Neto and J.N.Pires. Effect of friction stir welding parameters on morphology and strength of acrylonitrile butadiene styrene plate welds. Materials & Design 2014, 58; 457-464.

1. The writing should be polished, check lines: 62 (fsw/fsp); 88 (over- representation); etc.

Thank you very much for your detailed revision of the manuscript. We have carefully revised the full manuscript and we have polished the document by correcting all these mistakes stated by the referees and some other minor points found along the text.

 

2. Throughout the manuscript it is not clear what "system dimensions" means, as in line 398?

Thank you very much again for your detailed revision of the manuscript. The term “dimensions” was wrongly used sometimes in the original version of the manuscript. The right term in these cases is “size”.

We have change the term “dimensions” by “size” in these cases along the text.

 

3. The figure captions should be improved with more information, such as scale and region identification, tool identification and color coding.

We have included the scales and the color codes in the figure captions.

Reviewer 4 Report

Dear Authors: “Material behavior around the FSW/FSP tool described by molecular dynamics” - congratulations on your interesting research study.

 

-   The abstract and introduction are correct in my opinion

-   Please underline what is the main scientific goal of the work

-   In subchapters 2.1 to 2.3, I propose to include, apart from descriptions, a diagram, an illustrative drawing ... In my opinion, it will enrich the article

-   Section 3.1 presents the results of numerical modeling and a brief analysis. Table 1 shows the results of various works. In my opinion, you should refer to these works and state whether the obtained research results are cumulative with them. If not, how are they different and what is the reason? Please comment or complete the work,

-   The discussion presented in point 4 should be a continuation of the presented results of research 3.1, 3.2, 3.3 Discussion. This subchapter should refer to the published results with reference to specific results of the work.

-   In my opinion, the conclusions are properly described

 

Author Response

Dear Reviewer,

thank you very much for your report to our above paper "Material behavior around the FSW/FSP tool described by molecular dynamics" (jmmp-2107035) by Bentejui Medina and Ricardo Fernandez. The authors gratefully acknowledge the positive evaluation of the paper. We appreciate your comments which pinpointed significant points in the presentation of the data and allowed us to make the manuscript clearer. We have read very carefully all the concerns raised by you and, as requested, have amended our work accordingly. The modifications introduced are marked by changes tracking markup in the revised version of our work. The responses to your questions and comments are summarized in the following.

 1. Please underline what is the main scientific goal of the work.

Thank you for your comment. We have underlined the main scientific goal of the work in the abstract.

2. In subchapters 2.1 to 2.3, I propose to include, apart from descriptions, a diagram, an illustrative drawing ... In my opinion, it will enrich the article.

Thank you for your suggestion. Other reviewers have also recommended the inclusion of some illustrative drawings along the document. We have modified figures 1-4 to improve these descriptions.

3. Section 3.1 presents the results of numerical modeling and a brief analysis. Table 1 shows the results of various works. In my opinion, you should refer to these works and state whether the obtained research results are cumulative with them. If not, how are they different and what is the reason? Please comment or complete the work.

The results obtained in the present work are complementary to those previously obtained by FEM simulations such as those in references [8-11]. The MD technique allows obtaining a level of detail in aspects such as material flow that cannot be achieved by FEM. In relation to the few works published to date describing the FSW process by MD, in the present work we have established the minimum conditions of the system to ensure the goodness of the results. These minimum conditions are a minimum of one million atoms (4 times the pin diameter), a pin size >10nm and the use of a normalized weld pitch to compare with experimental values of the process, such as the maximum temperature reached. In this way it is possible to compare systems of very different dimensions such as those simulated in the present work and the experimental systems or those simulated by FEM. This is the first time that a strategy has been established to validate MD simulations of the FSW/FSP process.

4. The discussion presented in point 4 should be a continuation of the presented results of research 3.1, 3.2, 3.3 Discussion. This subchapter should refer to the published results with reference to specific results of the work.

In the discussion section we have compared our results to those of previous woks in the literature. In particular, we have compare different specific results to those in references [10, 19, 20, 33, 35-53].  A total of 22 references have been cited in the discussion section.