Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigated systematically the fatigue behavior of steel ring structures reinforced by WAAM process. The experimental design was systematically done and the authors presented detailed and in-depth analysis results. The statistical analysis showed meaningful results and the authors provided detailed discussions on the results. The conclusions are supported by the discussions. In the reviewer's opinion, the paper can be recommended for the publication when following minor changes are made:

1. In page 2 and thereafter, the authors used the terms O and CO for the oxigen and carbon dioxide gases. These designations are confusing since CO usually means carbon monoxide. O₂ and CO₂ instead of O and CO would much better represent the type of gases and are recommended.
2. Reference order should be adjusted thoughout the paper. In current form, the reference number sometimes jump from 12 to 29. There are many references cited in the wrong order in the paper. 
3. In line 110, there is a typo ([8?].
4. Exceptionally long paragraph from line 67 to 114 needs to be split into several paragraphs. In the reviewer's opinion, this paragraph can be divided into at least three paragraphs.

Author Response

Comment 1: In page 2 and thereafter, the authors used the terms O and CO for the oxygen and carbon dioxide gases. These designations are confusing since CO usually means carbon monoxide. O2 and CO2 instead of O and CO would much better represent the type of gases and are recommended.

Response 1: Thank you for pointing this out. We agree with this comment. The cause of the error was due to incorrect LaTeX syntax, such as using "CO2" instead of the correct "CO$_2$", which resulted in the compiled text rendering as "CO" rather than "CO₂" as intended. Therefore, we have corrected all instances of "O" to "O$_2$" and "CO2" to "CO$_2$" throughout the manuscript to accurately represent oxygen and carbon dioxide gases, avoiding confusion with carbon monoxide. This change can be found – page 2, paragraph 3, line 59 (replacing "O" with "O$_2$"); page 2, paragraph 3, line 60 (replacing "CO2" with "CO$_2$"); and subsequently wherever these terms appear in the revised manuscript.

Comment 2: Reference order should be adjusted throughout the paper. In current form, the reference number sometimes jump from 12 to 29. There are many references cited in the wrong order in the paper.

Response 2: Agree. We have, accordingly, revised the manuscript to ensure the reference order is corrected throughout the paper. To address the reviewer's concern, we have reorganized the references in the revised manuscript to follow a sequential numbering system consistent with the initial citation order in the text, aligning with the MDPI class file's default behavior for the "submit" option. This change ensures no jumps in reference numbers (e.g., from 12 to 29) and maintains a logical sequence. The changes made, providing the necessary explanation/clarification, can be found exactly in the revised manuscript – throughout the Reference section and corresponding citations in the text.

Comment 3: In line 110, there is a typo ([8?].

Response 3: Agree. We have, accordingly, revised the manuscript to correct the typo in line 110. The original text contained an incomplete citation "[8?" which has been corrected. This change can be found – page 3, paragraph 6, line 98.

Comment 4: Exceptionally long paragraph from line 67 to 114 needs to be split into several paragraphs. In the reviewer's opinion, this paragraph can be divided into at least three paragraphs.

Response 4: Agree. We have, accordingly, modified the manuscript to split the exceptionally long paragraph from line 67 to 114 of Introduction section into three smaller paragraphs for improved readability and structure. The original paragraph (line 67 to 114) in the Introduction section has been divided based on thematic content: the first paragraph now covers the introduction to WAAM and its optimal parameter (lines 72-84), the second paragraph highlights the novelty (lines 85-91), and the third paragraph highlights WAAM applications (lines 92-102).

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article provides a valuable study on the fatigue performance of WAAM-fabricated metal shafts with stacked ring substrates and highlights key process parameters. However, some details remain insufficient, which merit further clarification.

1. The authors should rich the Introduction part by comparing and reviewing more articles about mechanical properties of the additive manufactured metallic materials to highlight the significance of the ANOVA method, e.g., https://doi.org/10.1016/j.apmate.2022.100097 and so on.
2. The Introduction section currently includes excessive descriptions of the research methods, such as the stacked-ring method. The authors are advised to relocate these methodological details to the Methods section. The Introduction should primarily emphasize the research background, the significance of the study, and the innovative aspects of the work. A clearer focus in this section will improve the logical structure and readability of the manuscript.
3. The manuscript examines various process parameters, with each parameter varying across different levels. The authors should clarify why this approach was chosen. It may be more reasonable to select parameters based on energy calculation formulas, considering energy changes. This could enhance the rationale for the experimental design.
4. Tables 3 and 4 contain largely overlapping information. The authors are advised to merge these tables or remove the redundant content to avoid unnecessary repetition and improve clarity.
5. The F-value plot in Figure 10 appears to be inconsistent with the data presented in Table 5. The authors are advised to clarify this discrepancy and provide an explanation to ensure consistency between the figure and the table.
6. The ANOVA results are mostly descriptive. The authors should add explanations in the Results section and provide deeper analysis in the Discussion section to better interpret the findings.
7. The Conclusion section requires revision. The authors are advised to restructure it in a clear, point-by-point format that highlights both the innovative contributions and the main findings of the study. This will help emphasize the novelty and significance of the work.
8. The authors are advised to improve the English language quality of the manuscript, preferably with assistance from a native English speaker, to enhance clarity and readability.

Author Response

Comment 1: The authors should enrich the Introduction part by comparing and reviewing more articles about mechanical properties of the additive manufactured metallic materials to highlight the significance of the ANOVA method, e.g., https://doi.org/10.1016/j.apmate.2022.100097 and so on.

Response 1: Agree. We have revised the manuscript to enrich the Introduction by incorporating a comparative review of additional articles on the mechanical properties of additively manufactured metallic materials, emphasizing the significance of the ANOVA method. We have included references such as https://doi.org/10.1016/j.matpr.2022.06.490 and other relevant studies to provide a broader context and underscore the robustness of ANOVA in optimizing process parameters for fatigue performance. This change can be found – page 12, lines 301, reference 34.

Comment 2: The Introduction section currently includes excessive descriptions of the research methods, such as the stacked-ring method. The authors are advised to relocate these methodological details to the Methods section. The Introduction should primarily emphasize the research background, the significance of the study, and the innovative aspects of the work. A clearer focus in this section will improve the logical structure and readability of the manuscript.

Response 2: Agree. We have revised the manuscript by relocating the detailed descriptions of the stacked-ring method and other methodological details from the Introduction to the Methods section. The Introduction has been refocused to emphasize the research background, the significance of studying fatigue performance in WAAM-fabricated shafts, and the innovative aspects of the stacked-ring configuration. This change can be found – page 1, Introduction section, paragraphs 5-6 (removed method details); page 7, Specimen Fabrication Process sub section, paragraph 2, lines 201-205 (added method details). The Methods section now includes: " “The substrate’s stacked-ring method was employed to fabricate the shafts, this technique, illustrated in Figure 4, allows the creation of complex internal profiles, such as hollow features or cooling channels, by securing the rings by MIG welding in a layer-by-layer process...."

Comment 3: The manuscript examines various process parameters, with each parameter varying across different levels. The authors should clarify why this approach was chosen. It may be more reasonable to select parameters based on energy calculation formulas, considering energy changes. This could enhance the rationale for the experimental design.

Response 3: Agree. We have revised the manuscript to clarify the rationale for selecting the process parameters and their varying levels. The original approach utilized a Taguchi L25 orthogonal array to systematically evaluate parameters based on their influence on fatigue life, as informed by preliminary experiments and literature review. To enhance the experimental design rationale, we have added an explanation considering energy calculation formulas, noting that parameters like current intensity and torch speed directly affect heat input and energy distribution, which were optimized to balance weld quality and fatigue performance. This change can be found – page 4, Process Parameters sub section, paragraph 2, lines 130-143, lines 144-145, lines 150-152, lines 154-155,

Comment 4: Tables 3 and 4 contain largely overlapping information. The authors are advised to merge these tables or remove the redundant content to avoid unnecessary repetition and improve clarity.

Response 4: Agree in part. Instead of fully merging Tables 3 and 4, we have retained both tables but adjusted Table 3 to include only a brief illustration of a few representative rows, avoiding complete overlap with Table 4. This approach maintains Table 3 as a summary illustration while Table 4 provides the full detailed data. This change can be found – page 10, Data Analysis and Method  sub section, Table 3 adjusted to be concise; Table 4, retained with full data.

 

Comment 5: The F-value plot in Figure 10 appears to be inconsistent with the data presented in Table 5. The authors are advised to clarify this discrepancy and provide an explanation to ensure consistency between the figure and the table.

Response 5: Agree. We have revised the manuscript to address the inconsistency between the F-value plot in Figure 10 and the data in Table 5. Upon review, the primary cause was an error in processing the Table 5 data during integration into the LaTeX template, leading to inaccuracies, rather than an issue with Figure 10. We have corrected Table 5 to ensure its data (e.g., F-value for ring diameter as 446.87) aligns with Figure 10. This change can be found – page 12, Results section, Table 5 from line 306.

Comment 6: The ANOVA results are mostly descriptive. The authors should add explanations in the Results section and provide deeper analysis in the Discussion section to better interpret the findings.

Response 6: Agree. We have revised the manuscript by enhancing the Results section with explanatory text for the ANOVA results, detailing the statistical significance of each parameter (e.g., ring diameter’s 85.79% contribution). Additionally, Both of Results section and the Discussion section now includes a deeper analysis, exploring the implications of these findings on weld quality and fatigue life, such as the interaction between current intensity and torch speed. This change can be found – page 12, Results section, paragraph 2, lines 307-317; – page 14, Results section, paragraph 2, lines 335-346; – page 15, Results section, paragraph 2, lines 354-365;  page 17, Discussion section, paragraph 1, lines 150-165 the Results now include: "Ring diameter has the most substantial impact on fatigue life, as evidenced by its high F-value (446.87) and low p-value (<0.001) in the ANOVA results, explaining 85.79% of the variance…. "

Comment 7: The Conclusion section requires revision. The authors are advised to restructure it in a clear, point-by-point format that highlights both the innovative contributions and the main findings of the study. This will help emphasize the novelty and significance of the work.

Response 7: Agree. We have revised the manuscript by restructuring the Conclusion section into a clear, point-by-point format to highlight the innovative contributions and main findings. The revised section now emphasizes the novelty of the stacked-ring configuration and the predictive accuracy of the regression model. This change can be found – page 20, Conclusion section, lines 529-536.

Comment 8: The authors are advised to improve the English language quality of the manuscript, preferably with assistance from a native English speaker, to enhance clarity and readability.

Response 8: Agree. We have revised the manuscript to improve the English language quality. This change can be found – throughout the manuscript, all sections. The revised text reflects improved sentence structure and word choice, ensuring a professional and coherent presentation.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1. The manuscript repeatedly states ‘rotating bending in accordance with ISO 1143:2021,’ which is unnecessarily repetitive. In addition, please convert the applied load of 87 kg into a nominal stress amplitude (MPa).

 

2. Table 5 reports a total factor contribution of ≈98.5%, with only 1.5% error; however, the Abstract and Section 4.4 state that “Approximately 50% of fatigue life variability remains unexplained”. Please verify/correct this inconsistency.

 

3. The Introduction reviews the influence of shielding gas on the tests, and the stated research objective includes comparing pure Ar and Ar-CO/O mixtures. However, the Materials and Methods section employs only Ar and CO₂ mixture (8-10 L/min) and does not treat gas composition as an experimental factor. Moreover, the Conclusions do not compare different gases. This inconsistency needs further revision.

 

4. “Throughout the manuscript, the improvement in service life is attributed to ‘ferrite refinement, reduced interfacial defects, lower stress concentration, and enhanced interlayer bonding’ (Abstract; Sections 4.1, 4.2), yet the main text provides no microstructural or fractographic evidence to substantiate these claims.”

 

5. I recommend incorporating more recent references in the Introduction to support the authors’ arguments; please provide two references for the authors to cite. Suggested reference: “doi.org/10.1016/j.msea.2024.147422” and “doi.org/10.1016/j.ijmachtools.2023.104061”.

Author Response

Comment 1: The manuscript repeatedly states ‘rotating bending in accordance with ISO 1143:2021,’ which is unnecessarily repetitive. In addition, please convert the applied load of 87 kg into a nominal stress amplitude (MPa).

Response 1: Agree. We have revised the manuscript to remove the repetitive phrase ‘rotating bending in accordance with ISO 1143:2021. Regarding the conversion of the applied load of 87 kg into a nominal stress amplitude (MPa), we have calculated it based on the uniform machined gauge diameter of 17 mm and a moment arm of 60 mm from the test gauge, using the bending stress resulting in a nominal stress amplitude of approximately 106 MPa. We have updated the relevant sections accordingly. This change can be found – page 6, Fatigue Testing Setup subsection, line 168-175.

Comment 2: Table 5 reports a total factor contribution of ≈98.5%, with only 1.5% error; however, the Abstract and Section 4.4 state that “Approximately 50% of fatigue life variability remains unexplained”. Please verify/correct this inconsistency.

Response 2: Agree. We have reviewed and revised this inconsistency in the manuscript. The discrepancy arose from a misinterpretation in the Abstract and Section 4.4 of the previous version, where the statement “Approximately 50% of fatigue life variability remains unexplained” was based on an initial analysis that was not updated. The model explains 96.5% of the variability, with 3.5% remaining unexplained due to parameter interactions or experimental error, as noted in the Abstract and Section 4.4. We have adjusted the Abstract and Section 4.4 to align with Table 5 and the updated data. This change can be found – page 1, Abstract, lines 12; page 19, Section 4.4, paragraph 1, lines 498.

Comment 3: The Introduction reviews the influence of shielding gas on the tests, and the stated research objective includes comparing pure Ar and Ar-CO/O mixtures. However, the Materials and Methods section employs only Ar and CO₂ mixture (8-10 L/min) and does not treat gas composition as an experimental factor. Moreover, the Conclusions do not compare different gases. This inconsistency needs further revision.

Response 3: Agree. We acknowledge the inconsistency and have revised the manuscript to address it. The mention of comparing pure Ar and Ar-CO₂/O mixtures in the Introduction was intended to provide broader context but was not fully implemented in the experimental design. We have updated the Introduction to clarify that the study focuses on optimizing the Ar-CO₂ mixture (8-10 L/min) as the primary shielding gas, removing the implication of a comparative analysis with pure Ar (page 2, Introduction section, paragraph 3, lines 69-70). The Materials and Methods section now explicitly states the use of the Ar-CO₂ mixture as the sole gas composition (page 7, Methods section, paragraph 1, line 198). The Conclusions have been adjusted to reflect this focus, omitting any comparison with other gases (page 20, Conclusion section, lines 532).

Comment 4: “Throughout the manuscript, the improvement in service life is attributed to ‘ferrite refinement, reduced interfacial defects, lower stress concentration, and enhanced interlayer bonding’ (Abstract; Sections 4.1, 4.2), yet the main text provides no microstructural or fractographic evidence to substantiate these claims.”

Response 4: Agree. We recognize the lack of direct evidence and have revised the manuscript to address this by incorporating citations from related studies to explain and support these claims, rather than adding new data from the current study (as no microstructural or fractographic analyses were conducted). We have updated the Section 4.1, and Section 4.2 to reference studies such as [ref15] and [ref35] on ferrite refinement and reduced defects, as well as [ref21] on lower stress concentration and enhanced interlayer.

 

Comment 5: I recommend incorporating more recent references in the Introduction to support the authors’ arguments; please provide two references for the authors to cite. Suggested reference: “doi.org/10.1016/j.msea.2024.147422” and “doi.org/10.1016/j.ijmachtools.2023.104061”.

Response 5: Agree. We have revised the manuscript to incorporate more recent references in the Introduction to strengthen the supporting arguments. We have added the some suggested references, e.g., Ref1-“DOI:10.1016/j.rineng.2021.100330.”  ref-39 “doi.org/10.1016/j.msea.2024.147422” which provide insights into recent advancements in WAAM and fatigue performance optimization. This change can be found on reference section. 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Most issues have been addressed, but there are a few points still need attention:

1. The authors should rich the 'introduction' part after reading more high-level SCI papers about the mechanical properties of additive manufactured metallic materials, like response surface method (e.g., https://doi.org/10.1016/j.apmate.2022.100097), as it offers useful insights that would enhance the context of the study.
2. Table 3 still largely overlaps with Table 4. Retaining Table 3 adds limited value, as Table 4 already provides the full data. Thus, I still insist that the authors need remove Table 3 to avoid redundancy.
3. The revised conclusion has improved, but it still lacks clarity and logic. The authors should further revise the conclusion to a more concise, point-by-point format (e.g., 1, 2, 3), clearly highlighting the key contributions, main findings, and novelty of the study. This would help make the conclusions more direct and easier to follow.

Author Response

Comment 1: The authors should rich the 'introduction' part after reading more high-level SCI papers about the mechanical properties of additive manufactured metallic materials, like response surface method (e.g., https://doi.org/10.1016/j.apmate.2022.100097), as it offers useful insights that would enhance the context of the study.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, We have expanded the introduction to include a detailed discussion on the application of Response Surface Methodology (RSM) in optimizing WAAM parameters, with references to recent studies. This change can be found – page 2, paragraph 2, lines 50-79, page 18, lines 494-499.

Comment 2: Table 3 still largely overlaps with Table 4. Retaining Table 3 adds limited value, as Table 4 already provides the full data. Thus, I still insist that the authors need remove Table 3 to avoid redundancy.

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have removed Table 3 from the manuscript to eliminate redundancy, as Table 4 now fully encompasses the data.

Comment 3: The revised conclusion has improved, but it still lacks clarity and logic. The authors should further revise the conclusion to a more concise, point-by-point format (e.g., 1, 2, 3), clearly highlighting the key contributions, main findings, and novelty of the study. This would help make the conclusions more direct and easier to follow.

Response 3: Thank you for pointing this out. We agree with this comment. Therefore, We have revised the conclusion section into a concise, point-by-point format, emphasizing key findings and novelty. This change can be found – page 18-19, lines 534-549.

Comments 4: “The English could be improved to more clearly express the research.”

Response 4: Thank you for pointing this out. We agree with this comment. Therefore, We have collaborated with a native English speaker to revise the manuscript, improving clarity and precision in expressing the research.

Author Response File: Author Response.pdf