Effect of T6 and T8 Ageing on the Mechanical and Microstructural Properties of Graphene-Reinforced AA2219 Composites for Hydrogen Storage Tank Inner Liner Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is significant for the field of composite materials, especially in the context of application in the space industry. The combination of graphene and AA2219 alloy, with the application of modern processing techniques (ultrasonic mixing, ball milling, squeeze casting), as well as the analysis of the effects of different aging regimes (T6 and T8), provides a relevant contribution. However, paper requires additional elaborations and clarifications in certain segments, in order to be completely clear, convincing and consistent.

1. In the abstract there are some complex sentences that make it difficult to understand. Missing quantification in UTS summary in as-cast condition. Only the T8 value is given. The purpose and novelty of the paper are not clearly stated.
2. Aluminum and aluminum alloys are increasingly used in various fields of technology. They are applied in the aircraft, space, military, automotive and electronic industries. Alloys and composites have special applications in engines, pistons, cylinders, car parts, etc. Better explain the areas of application of these materials. View papers https://doi.org/10.46793/aeletters.2024.9.1.5 https://doi.org/10.46793/adeletters.2024.3.4.5 https://doi.org/10.30657/pea.2018.18.03
3. Could the authors include a brief comment on the significance of this paper in the context of industrial application?
4. Explain in more detail the reasons for choosing exactly 0.5 wt.% graphene? Have multiple concentrations been tried?
5. Why was a speed of 450 RPM selected during mixing? Has the effect of higher/lower speeds been tested?
6. Why was this fabrication method chosen, what are it’s advantages when compared to other ones?
7. Can the authors clarify why 3% cold rolling was used and how that value was optimized?
8. Are there quantitative data on the thickness of the zone with fine precipitation in T6 and T8 samples?
9. Quantitative analysis of grain size is missing.
10. Can you quantify the proportion of fine microstructure in the T8 samples?
11. What exactly is the trace oxygen content and can it negatively affect the mechanical properties?
12. Give standard deviation of UTS?
13. Why is Young's modulus not shown? This would further complement the mechanical analysis.
14. Have you quantified dimple sizes and density?
15. Can you cite additional studies that used a combination of T8 processing and graphene in an Al matrix, to compare the results?
16. Can you explicitly indicate potential drawbacks or limitations of the proposed method (eg cost, reproducibility under industrial conditions)?
17. Tables with mechanical results next to graphs (for better readability).
18. Analysis of process costs compared to conventional methods.
19. Give chemical composition of base alloy.
20. Text on the figures should be the same size and type as in the rest of the manuscript.
21. Before a figure appears in the manuscript it should be referred to in the text and after the figure, it should be explained what is presented on it.
22. Check rows164-169 there is some literature cited [111], [200] etc
23. Compare the obtained results with the results of other researchers.
24. Expand the discussion based on the analysis 27. Based on the extended analysis and discussion, expand the concluding considerations.
25. The literature is not well cited. Names of journals and certain elements (like authors) of references are missing. Unify the citation (APA or another style according to the guidelines of the journal.)
26. English language needs proofreading.

Comments on the Quality of English Language

needs proofreading

Author Response

Journal of Composites Science

Manuscript ID: jcs-3706576

Dear Reviewer,

We would like to thank you for your valuable time and detailed comments. Below are our point-by-point responses to the comments. The revisions in the manuscript are highlighted in yellow. We have also made necessary English language revisions for grammar and punctuation. We hope these revisions and responses address the concerns raised and make our manuscript suitable for publication.

Author’s responses to the reviewer comments:

Comment 1: In the abstract there are some complex sentences that make it difficult to understand. Missing quantification in UTS summary in as-cast condition. Only the T8 value is given. The purpose and novelty of the paper are not clearly stated.

Response:

We thank the reviewer for this constructive feedback. In response, we have revised the abstract to enhance clarity and readability by simplifying complex sentences. We have also included the ultimate tensile strength (UTS) value for the as-cast condition, alongside the T8 condition, to provide complete quantification. Furthermore, the purpose and novelty of our research have been explicitly stated in the revised abstract. The updated version is provided below for your consideration. Please see the revised abstract on Page 1 of 15, lines 13–29.

Comment 2: Aluminum and aluminum alloys are increasingly used in various fields of technology. They are applied in the aircraft, space, military, automotive and electronic industries. Alloys and composites have special applications in engines, pistons, cylinders, car parts, etc.  Better explain the areas of application of these materials. View papers https://doi.org/10.46793/aeletters.2024.9.1.5,https://doi.org/10.46793/adeletters.2024.3.4.5 https://doi.org/10.30657/pea.2018.18.03.

 

 

Response:

We thank the reviewer for this helpful suggestion. In response, we have expanded the introduction to provide a more comprehensive overview of the applications of aluminum and its alloys, citing the suggested references to highlight recent advances and diverse industrial uses. Please see the revised paragraph on Page 1-2, Lines 34–42.

Comment 3: Could the authors include a brief comment on the significance of this paper in the context of industrial application?

Response:

We thank the reviewer for this insightful suggestion. In response, we have added a statement highlighting the industrial significance of our research. This addition can be found in the introduction section on Page 3, Lines 98-106.

Comment 4: Explain in more detail the reasons for choosing exactly 0.5 wt.% graphene? Have multiple concentrations been tried?

Response:

We thank the reviewer for the comment, and the selection of 0.5 wt.% graphene was based on our previous studies with AA6061, AA2024, AA7075, and AA2195 alloys, where this concentration consistently yielded optimal improvements in mechanical properties without agglomeration or processing issues. Both lower and higher concentrations proved less effective. This observation is strongly supported by relevant literature, which also identifies 0.5 wt.% as an optimal threshold for property enhancement in aluminum composites. The relevant references are cited to support this statement.

Comment 5: Why was a speed of 450 RPM selected during mixing? Has the effect of higher/lower speeds been tested?

Response:

We thank you for your valuable comment, and a mixing speed of 450 RPM was chosen because, in our trials, 400 RPM did not achieve proper mixing and resulted in lower hardness (45–50 HV). At speeds above 500 RPM, excessive turbulence caused agglomeration and oxidation. Therefore, 450 RPM was found to provide optimal dispersion without these drawbacks. Relevant references are cited to support this selection.

Comment 6: Why was this fabrication method chosen, what are it’s advantages when compared to other ones?

Response:

We chose the ultrasonic-assisted stir and squeeze casting method because it helps mix the graphene evenly throughout the aluminum and keeps the material strong. The ultrasonic stirring breaks up clumps of graphene, so it spreads out better. Squeeze casting reduces air bubbles and makes sure the bond between the graphene and aluminum is strong. Compared to standard stir casting, this method gives a more uniform material with fewer defects. In contrast, powder metallurgy or in-situ techniques may achieve good dispersion but often lack scalability or involve more complex processing steps. Thus, the selected method combines efficient dispersion, high material quality, and industrial scalability, making it especially suitable for advanced structural applications

Comment 7: Can the authors clarify why 3% cold rolling was used and how that value was optimized?

Response:

We thank you for the valuable comment. For the T8 condition, it is standard practice to use a cold rolling reduction in the range of 3–5%. We chose 3% based on the existing literature, as it provides the desired effect of increasing dislocation density for enhanced precipitation during aging, without causing excessive deformation or processing issues. This approach is well supported by previous studies on similar aluminum alloys.

Comment 8:  Are there quantitative data on the thickness of the zone with fine precipitation in T6 and T8 samples?

Response:

Thank you for your question. In this study, we did not perform quantitative measurements of the thickness of the fine precipitation zones in the T6 and T8 samples. Our microstructural characterization focused primarily on grain size evolution, precipitate distribution, and qualitative assessment of microstructural refinement using optical microscopy and FE-SEM imaging. While the images clearly indicate improved precipitate refinement and uniformity after T6 and T8 treatments, determining the precise thickness of the precipitation-enriched zone would require higher resolution analysis, such as TEM. We agree that quantitative data on the thickness of these zones would provide valuable additional insight, and we plan to address this aspect in future work.

Comment 9:  Quantitative analysis of grain size is missing.

Response:

Thank you for pointing this out. In the manuscript, we have included quantitative grain size measurements for the as-cast, T6, and T8 samples, obtained from optical micrographs using the linear intercept method. The average grain size is approximately 75μm for the as-cast sample, 50μm after T6 heat treatment, and 21μm after T8 treatment. These results have been incorporated into the Results and Discussion section.

Comment 10: Can you quantify the proportion of fine microstructure in the T8 samples?

Response:

We thank the reviewer for highlighting the need to quantify the proportion of fine microstructure in the T8 samples. As per our quantitative image analysis, the T8 sample exhibited a grain boundary area fraction of 84.2% and a fine microstructure area fraction of 23.4%. These results confirm the effectiveness of the applied processing route in producing a highly refined and elongated grain structure, with a significant proportion of fine microstructural regions. The quantification underscores the microstructural refinement achieved through the selected processing parameters, as discussed in the revised manuscript (see Page 10, lines 274-277).

Comment 11: What exactly is the trace oxygen content and can it negatively affect the mechanical properties?

Response:

We appreciate the reviewer’s observation regarding trace oxygen content. In our study, any trace oxygen present is primarily due to the use of alumina (Al₂O₃) suspension during the polishing process for microstructural analysis. This can result in minimal surface-level oxygen, but it does not affect the bulk material. Furthermore, X-ray diffraction (XRD) analysis of the samples did not reveal any oxide peaks, confirming that no significant oxide formation occurred due to the polishing process. Therefore, we can conclude that the trace oxygen introduced during sample preparation does not negatively impact the mechanical properties of the material.

Comment 12: Give standard deviation of UTS?

Response:

Thank you for your valuable comment regarding the reporting of the standard deviation for the ultimate tensile strength (UTS). In our study, tensile tests were performed on three independent specimens for each condition (as-cast, T6, and T8) in accordance with ASTM E8 standards. These revised values have been included in Section 3.5 (Ultimate Tensile Strength) of the manuscript.

(see Page 11, line 326 and Page 12, lines 329,333).

Comment 13: Why is Young's modulus not shown? This would further complement the mechanical analysis.

Response:

Thank you for your suggestion regarding Young’s modulus. We have now included the Young’s modulus values for the graphene-reinforced AA2219 composites in the revised manuscript for each condition. Please refer to page 12, lines 335–342, for the updated details. We appreciate your input, which has helped strengthen the mechanical analysis in our study.

Comment 14: Have you quantified dimple sizes and density?

Response:

Thank you for your valuable suggestion regarding the quantification of dimple size and density. We have now included a detailed quantitative analysis of dimple size and density for the as-cast, T6, and T8 samples in the revised manuscript. Please see page 13, lines 368-381, for the updated results and discussion.

Comment 15: Can you cite additional studies that used a combination of T8 processing and graphene in an Al matrix, to compare the results?

Response:

We thank the reviewer for this suggestion. To our knowledge, direct studies on AA2219 composites processed with both T8 aging and graphene reinforcement are very limited. While recent literature highlights the benefits of T6/T8 heat treatments and graphene addition in Al-based composites, most report on T6 or hybrid post-processing rather than T8 specifically. Our work addresses this gap by systematically investigating the combined effect of T8 processing and graphene reinforcement in AA2219. Relevant literature has been cited and discussed in the revised manuscript (see References [6, 7, 11, 12, 24, 27, 28, 31–34].

Comment 16: Can you explicitly indicate potential drawbacks or limitations of the proposed method (eg cost, reproducibility under industrial conditions)?

Response:

We thank the reviewer for raising this important point. While the proposed processing route increases cost relative to traditional casting methods, it is necessary for achieving uniform graphene dispersion and the associated improvements in composite properties. We acknowledge that repeatability studies were not performed in this work; however, our future research will explicitly address reproducibility and process optimization for industrial applications.

Comment 17: Tables with mechanical results next to graphs (for better readability).

Response:

We thank the reviewer for this helpful suggestion. In the revised manuscript, we have included tables presenting the detailed mechanical results alongside the corresponding graphs for improved clarity and readability. Please see the updated figures and associated tables in the Results section (page 10, line 286 and page 12, line 321).

Comment 18: Analysis of process costs compared to conventional methods.

Response:

            We thank the reviewer for this important suggestion. At present, our work is focused on laboratory-scale studies aimed at achieving improved material strength and uniform graphene dispersion within the aluminum matrix. Comprehensive process cost analysis and industrial-scale costing were not within the scope of this research. However, we recognize the importance of such analysis for future industrial implementation, and we plan to address detailed process costing and economic feasibility in our future work.

Comment 19: Give chemical composition of base alloy.

Response:

We thank the reviewer for this suggestion. The chemical composition of the base AA2219 aluminum alloy has now been included in the revised manuscript as a horizontal table for clarity and ease of reference (see page 4, line 124).

Comment 20: Text on the figures should be the same size and type as in the rest of the manuscript.

Response:

We appreciate the reviewer’s comment regarding the consistency of text size and type in the figures. In preparing the revised manuscript, we initially attempted to use the same font size and style as the main text (size 10) in all figures. However, this resulted in reduced readability, especially for complex graphs and image labels. Therefore, we opted for a slightly larger and clearer font within the figures to ensure that all figure elements are easily legible to readers. We trust this approach maintains overall visual consistency while prioritizing clarity.

Comment 21: Before a figure appears in the manuscript it should be referred to in the text and after the figure, it should be explained what is presented on it.

Response:

We thank the reviewer for this valuable suggestion. In the revised manuscript, all figures are now referred to in the text before they appear, and each figure is followed by a clear explanation of its content and relevance. This ensures the logical flow of information and improves the clarity and readability of the manuscript, as per the reviewer’s recommendation.

Comment 22: Check rows164-169 there is some literature cited [111], [200] etc

Response:

We thank the reviewer for pointing out the citations such as [111], [200], etc., in rows 164–169. We would like to clarify that these are not literature references but denote specific XRD plane orientations (Miller indices) commonly used in crystallography to identify crystallographic planes. This has now been clarified in the revised manuscript for better understanding. (see page 4, lines 183-187)

Comment 23: Compare the obtained results with the results of other researchers.

Response:

We appreciate the reviewer’s valuable suggestion to compare our results with those from other studies and acknowledge that such comparisons are often important for contextualizing new findings. However, in this work, the direct, quantitative comparison of our results with existing literature is not entirely appropriate due to significant differences in our experimental approach. Our study employs a novel processing route, distinct material parameters, and specific experimental conditions that differ substantially from previous studies, leading to inherently different expected outcomes. Our primary aim was to understand the effects of these unique processing steps, rather than to achieve direct comparability with standard methods. While we have considered relevant literature in designing our experiments and interpreting the results, we believe that direct numerical comparison could be misleading without extensive contextual analysis, which is beyond the present manuscript’s scope. We hope the reviewer understands our rationale.

Comment 24: Expand the discussion based on the analysis 27. Based on the extended analysis and discussion, expand the concluding considerations.

Response:

We thank the reviewer for the valuable feedback. We have expanded the discussion and conclusions based on the reviewer’s comments, highlighting the key findings and implications of our study. We hope these additions sufficiently address the reviewer’s suggestions and provide a more comprehensive summary of our results.

Comment 25: The literature is not well cited. Names of journals and certain elements (like authors) of references are missing. Unify the citation (APA or another style according to the guidelines of the journal.)

Response:

We thank the reviewer for highlighting the need to improve the literature citations. In the revised manuscript, all references have been updated and unified according to APA style, in line with the journal’s guidelines. Author names, journal titles, and all required reference elements have been included to ensure consistency and completeness.

Comment 26: English language needs proofreading.

Response:

We appreciate the reviewer’s observation regarding the English language. The manuscript has been carefully proofread and revised to improve clarity, grammar, and overall readability.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the mechanical and microstructural properties of graphene-reinforced AA2219 composites fabricated through a novel ultrasonic-assisted stir and squeeze casting method, complemented by tailored T6 and T8 aging treatments. The authors reinforced 0.5 wt.% of graphene with aluminum 2219 87 alloy composite fabricated via ultrasonic-assisted stir and squeeze casting which was 88 subjected to mechanical and microstructural property evaluation.

This original paper can be published. However, it is really surprising that the authors did not provide sufficient information on the surface properties of their materials. No information is given on the surfaces and interfaces between aluminum and graphenes. The surface energy, the Lewis acid-base properties of the different materials alone and after incorporation of graphenes. The work of adhesion and the crack propagation observed through the microstructure should be more detailed and theoretically explained as well as the mechanical properties (Stress and deformation).

Author Response

Journal of Composites Science

Manuscript ID: jcs-3706576

Dear Reviewer,

We would like to thank you for your valuable time and detailed comments. Below are our point-by-point responses to the comments. The revisions in the manuscript are highlighted in yellow. We hope these revisions and responses address the concerns raised and make our manuscript suitable for publication.

Author’s response to the reviewer's comment:

This original paper can be published. However, it is really surprising that the authors did not provide sufficient information on the surface properties of their materials. No information is given on the surfaces and interfaces between aluminum and graphenes. The surface energy, the Lewis acid-base properties of the different materials alone and after incorporation of graphenes. The work of adhesion and the crack propagation observed through the microstructure should be more detailed and theoretically explained as well as the mechanical properties (Stress and deformation).

Authors Response:

We sincerely thank the reviewer for their positive evaluation and for recommending our manuscript for publication. We greatly appreciate your constructive feedback and valuable suggestions, which have helped us to further improve the quality of our work. Also, we thank the reviewer for highlighting the importance of surface and interfacial properties. In the revised manuscript, we have substantially expanded the discussion on the interface between graphene and the aluminum matrix, the surface energy and Lewis acid-base properties of both the AA2219 alloy and graphene, both individually and after composite formation. This information has now been included and discussed in detail in the revised manuscript. (See section 3.7 and page 10, lines 286-305)

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have put a lot of effort to adjust the paper. There are some comments:

Size of text on the figures should be the same as in the rest of the manuscript. For equations give literature sources. Why are there dashes between equation and it's number?

Comments on the Quality of English Language

proofreading needed

Author Response

File Attached

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The corrected version is good, the paper can be now accepted for publication.

Author Response

File Attached

Author Response File: Author Response.docx