Characteristics of Microstructure Evolution during FAST Joining of the Tungsten Foil Laminate

Round 1

Reviewer 1 Report

This manuscript presents the optimization of the production of W foil laminate by the field-assisted sintering technique. This optimization is complex because the authors try to avoid recrystallization to ensure better low-temperature mechanical behavior, but recrystallization is essential in diffusion bonding processes (crucial to ensure good bonding between the 21 foils of W). The results are interesting, and the manuscript can be published after the following aspects are answered:
1. In the manuscript, and particularly in the abstract, the authors must clarify what they mean by microstructure deterioration, which, in this case, means a reduction in the defects introduced by plastic deformation and recrystallization.
2. The hardness results and Figure 7.a add nothing to the discussion. If the authors intend to keep these results, they should state in which equipment they were determined, in which plane of the W foil, and how many indentations correspond to the results presented in the Figure.
3. In lines 176-178, it is said that "The typical rolling texture of plate texture can be discovered. After measurement from Figure 5c, the size of plate texture viewed from transverse direction (TD) is up to tens of microns in RD and approximately 0.5 μm in ND." In this context, the texture can be confused with crystallographic orientations that have not been measured. This statement must be clarified.
4. It is stated in the manuscript that thermal diffusivity is more sensitive to macro-defects, such as a pore or crack. Consequently, the samples subject to FAST joining optimization (point 3.4) must be characterized using this technique, and the results compared with those in Figure 7.b. This is particularly important because, in Figure 9.b, it appears to be a discontinuity in the bonding zone.

Author Response

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Reviewer 2 Report

In this research work, the authors reported the microstructure evolution during FAST joining of W foil laminate. The authors presented very interesting results that may be of interest to the joining community; however, the manuscript needs some major improvements before considering for publication. My recommendation would be to revise the manuscript. Some of the comments are listed below:

1. There are too many authors are listed for such a short manuscript. The authors list may be revised.
2. In lines 29-30, it has been mentioned that the critical current for mitigating the microstructure deterioration is evaluated by changing the sample size? How does this investigation can be related to the bulk component in practical application?
3. The optimized joining process highlighted in line 106 is not fully understood.
4. There are some typographical mistakes such as “Janpan” in line 126, and “adjacent between” in line 127. Line 231, “Figure 3b, the Ø13 mm sample under…”, should be Ø20 mm. Please check the entire manuscript for such kind typos.
5. No descriptions of metallographic sample preparation methods and OM/SEM imaging are provided. How microhardness measurements were performed? Is the hardness plot shown in Figure 7(a) for the bulk materials or at the interface? It is suggested that the authors evaluate the microhardness distribution across the interface as well.
6. Line 172-174, What does it mean by erosion traces? How can dislocation be identified using SEM? Is it just an assumption?
7. There are numerous times where the authors concluded that there is a good interfacial bonding without quantifying the bond strength or interfacial strength by means of experiments. It is strongly recommended that the authors estimate the interfacial bonding or interfacial strength quantitatively.
8. Line 203-204, hardness measurements can not evaluate microstructure evolutions, it actually measures the strength of the resultant microstructure. The sentence may be revised.
9. Line 225-226, Why the current density optimization was performed at 800°C as 1400°C samples showed good interfacial bonding?
10. The discussion section needs to be more developed.
11. Line 328-329, “The main challenges for the production of the W foil laminate using FAST are microstructure deterioration and weak interfacial bonding”. The authors did not address this issue well as interfacial strength never been estimated.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Authors,

Congratulations on your work, which is about an interesting subject.

However, as any other paper, there are improvements those must be done, namely:

1. The contextualization of the main gap that has originated your work is not clear. The novelty is not stated at all.
2. English: the level of English use is under the standards for METALS. Thus, please use an edditing service in order to correct several grammatical errors contained in the paper. Some sentences as example: "Each samples of the W 120 foil laminate in this study composes of 21 W wafers"; "a Mo foils"; "Janpan"; "a good contact adjacent between", only in part of the page 3.
3. In page 3, please explain hor the optical pyrometer has been calibrated.
4. In 3.1, who were prepared the samples for metallurgical observations? What kind of microscope was used? Which analysis mode was selected?
5. In 3.1, there is a lack of goals to be achieved. Just a comment on the observation. Nothing more.
6. In Figure 7 (left), there are some incoherences in the hardness values for different temperatures, those are not properly explained.
7. In page 6, in the sentence (line 217): "The medium thermal diffusivity of the sample joined at 1000 °C 217 in Figure 7b is also related to the existence of some micro gaps at interface between W foil 218 as shown in Figure 6c.", are you comment the thermal diffusivity or the hardness? Because, the graph is about hardness.
8. Discussion doesn't compare your results with others.
9. The conclusions are disappointing.

Based on theses reasons, I cannot suggest your paper for publication.

Kind regards,

Reviewer

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Dear authors,

after the first reading of your manuscript, I must state that you have chosen interesting area of research by using of FAST joining of tungsten foils. It is a modern technology that, among other aspects, can be used for creation the composite structures. It is a pity that you did not take into account this possibility when designing your experiment and all foils were placed on top of each other in the same rolling direction. In addition to that, also discussion summarizes the achieved results rather than comparing the results with similar works.

Below are mentioned various comments which should be useful for authors:

Line 4-5 and 20-23. It is obvious that this is a cooperative result, which is good, but lines 20-23 mention two institutions, to which none of the authors belong.

Introduction

Introduction is written comprehensibly with a sufficient number of relevant citations.

 Materials and Methods

Line 116-117: It is not clear from the text why the EDM method was chosen for sample preparation, after which the removal of residual dirties had to be performed. Why there was not use a simple cutting by means of punch and die.

Line 121-122: Please explain why was W wafers putted in the same rolling direction during stacking process. This could be an ideal way to optimize mechanical properties in different directions. In this way, it is possible to use rolled foils having larger thicknesses with similar results.

Line 134-135: It is not clear from the text to which place the pyrometer was focused. It couldn't be on the W foils, because they were wrapped in Mo and graphite foil. Please add and explain whether there is the same temperature at the location of joined W foils and at the location of the mould when the current passes through.

Results

Line 172: Please add information about etching.

Line 175: It is not clear from the text what the designation RD-ND means. This RD-TD designation must be taken by the reader from Fig. 1. Add it to the text.

Line 197-198: Statement “It indicates that the W foil laminates have a good interfacial bonding” is somewhat bold, because in the details of Figure 6d, e discontinuities are still evident, although significantly smaller than at lower temperatures. In diffusion welding, this would be considered as an insufficient joint. Please edit the text

Fig. 7 (b) It is not clear from the figure or the text that the thermal diffusivity curves overlap each other. Use Either a larger image size or describe it better in the text.

Line 243: You state, that “process was designed with a maximum current density of 380 242 A / cm2”. Add to the text why right this value was used when on line 236 you state “… optimum current density for the FAST joining of W foil laminate is below 418 A/cm2”.

Discussion

Chapters 4.1. - 4.3. rather summarize the achieved results than discuss and compare these results with similar works. In addition to that, in the discussion is not stated whether the hardness and thermal diffusivity values were achieved at a current density of 380 A/cm 2 or higher. If this was done under higher current value, it would be appropriate to add whether and, if so, how the given values change for 380 A/cm2. Furthermore, it would be appropriate to discuss a two-fold increase both in heating time and the maximum achievable temperature for the selected current density.

Conclusion

I propose to mark the most important conclusions in points, supported with values from tests. It would be much more clearer.

References

There are 14 self-citations from 31 references.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors addressed all the comments and carefully revised the manuscript. The current manuscript is acceptable for publication. 

Reviewer 3 Report

No comments.

Author Response

Dear reviewer,

Thank you for your comments.

Best regards,

Xiaoyue