Microstructure and Mechanical Properties of P21-STS316L Functionally Graded Material Manufactured by Direct Energy Deposition 3D Print

Round 1

Reviewer 1 Report

The article presents a study of the structural features and the mechanical properties of a gradient material produced by an additive manufacturing technology. It is shown that the technology used in this work allows obtaining a material whose chemical composition and the mechanical properties dependent on the composition gradually change from a ferrite steel to an austenite steel. Some conclusions are made about a possibility of application of such material in nuclear materials technologies.

The results obtained in this work present interest to investigators involved in the development of additive manufacturing technologies.

Thus, the article can be recommened to publishing in the "Metals" journal.

The text of the article and the figures are carefully prepared and easy to understand.

There are some small questions, the answers to which can be useful to a reader. These questions are placed as comments into the original text of the article.

The article does not need to be re-reviewed if minor changes are made.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this research article, the authors have demonstrated the fabrication of P21-STS316L functionally graded material (FGM) using the direct energy deposition (DED) process. The gradual composition change from P21 ferritic steel to 316L stainless steel concerning the location was incorporated using a multi-hopper DED system. The microstructure of the as-fabricated FGM of P21-STS316L was analyzed using advanced microstructural characterizations. Moreover, the structure-property correlation was obtained by performing mechanical property testing such as hardness, tensile, and impact tests. The author has conducted extensive testing and characterization to give insights into the application of FGM in nuclear power plants. However, there are further improvements and clarifications required before considering it for publication:

1.     In the title of the paper author used “FGM as functionally gradient material,” while in the first line of the introduction, they defined “FGM as functionally graded material.” Using the same terminology throughout the text is suggested to make it more understandable to the readers.

2.     The writing logic in the introduction is very unclear. I would suggest the authors change the entire introduction section and elaborate the discussion in this format: FGM definition, its application, methodologies to fabricate the FGM (why the DED AM process is preferred in this study), the current state of the art in the DED of FGM and the novelty of their research work.

3.     In section 2, the author mentioned that the “FGM block was laminated with a thickness of 0.3 mm and 100 layers while changing the P21 and STS316L by 1% each”. Can you please elaborate on how you ensured that a change of 1% from P21 to STS316L was there during the deposition process? Did you change the powder feed rate? If yes, then it is essential to highlight that information in Table 2.

4.     In Figure 4 (b), the color change after etching was observed due to the difference in the material properties of the base plate and deposited material. Therefore, it is suggested to add text in Figure 4 (b) which clearly defines base plate length and deposited layer length. Although the author mentioned it in the text, it is always desirable that Figures be self-explainable.

5.     Figure 5 (b) shows the inclusion distribution in DED samples. Are these inclusions represent AM defects like porosity and lack of fusion? If yes, then AM community refers them to AM defects. Using inclusion terminology is confusing as it can also be understood as nano-inclusions intentionally added to the material to enhance its mechanical properties.

6.     How many planes were considered to calculate the inclusion distribution in the DED sample qualitatively? It is suggested to consider the plane with the maximum inclusion distribution.

7.     In the section on the uniaxial tensile test (page 13 and paragraph 2), the author said that the tensile strength of FGM material and STS316L is almost similar to 616-619 MPa. The author must correct it with yield strength (YS) since the tensile strength is considered the ultimate tensile strength (UTS). Also, in the explanation author represented the elongation in percentage, while in Figure 15, it is described as a stroke (mm). Therefore, it is suggested to plot the total elongation in terms of strain as it will give a better understanding. Else, make the explanation consistent with Figure 15.

8.     The Charpy impact test shows that FGM and P21 have similar impact energy, which is larger than the STS316L sample. While based on the tensile stress-strain curve, the FGM material and STS316L have similar YS and UTS. However, the total elongation for STS316L is more significant, around 60%. Therefore, concerning the higher ductility and equal strength of STS316L with the FGM sample, the impact energy of the STS316L sample should be higher than the FGM sample; however, based on impact energy results, this is not the case. Could you please explain the reason behind the difference in the impact energy while considering the tensile results?

9.     The reviewer suggests extending the discussions on the observed results with the support of appropriate literature. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this manuscript, using a multi-hopper direct energy deposition 3D printer, an FGM material whose composition changes gradually from P21 ferritic steel to stainless steel 316L austenitic steel was fabricated over 100 layers with a thickness of 0.3 mm. Based on the results of microstructure and properties, factors to consider when applying FGM to improve the properties of ferrite and austenite welds in nuclear power plants were identified and FGM evaluation techniques were suggested. This work is a very deeply-analysis and organized systematically work, making me impressive. I think this is an interesting work and deserves to be published in the journal of Metals. As far as I am considered, Only a few comment about it.

1 introduction part is too wordy, should be shortened.  

2 the abstract part and conclusion part contained too many experimental processes but few results. the results of the work should be added in the two parts.

Author Response

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Author Response File: Author Response.pdf