Evaluating the Influence of Tool Material on the Performance of Refill Friction Stir Spot Welds in AA2029

Round 1

Reviewer 1 Report

The work of this paper is of interest, but the manuscript needs some work for publication.

1.Compare the microstructure and hardness of the RFSSW joints and then please further investigate the effect of the tool materials on the hardness of the joint.

2.The conclusion should be further revised.

Good English expression.

Author Response

Comments and Suggestions for Authors

The work of this paper is of interest, but the manuscript needs some work for publication.

1. Compare the microstructure and hardness of the 
2. The conclusion should be further revised.

Author’s Response:

1. Thank you for the comment on the link between tool material and hardness and microstructure. We’ve added content addressing this, and, per your advice, we’ve added a new figure showing the HAZ size in an optical cross-section and the indicated the size of the HAZ by microhardness as a function of tool material.
2. I’ve revised the conclusions, per your suggestion.

Reviewer 2 Report

This research addresses the complex and challenging task of welding AA2029-T8 joining high-strength skin-reinforced structures and recommends the use of filled friction stir spot welding. This article provides a comprehensive description of the welding process, discusses cycle times from 1 to 3 seconds, and demonstrates process forces in the range of 8 to 14 kilonewtons. A comparative analysis of RFSSW welds made with different tool materials such as H13 tool steel, tungsten carbide and MP159 was performed. The research provides valuable insights into the complex process of joining high-strength aluminum alloys through an innovative filler friction stir spot welding method. The research results have practical significance for the industry and can guide further research and development in this field. The comments are as follows,

1>First, tungsten carbide is widely employed as a tool material in Friction Stir Welding (FSW) due to its extensive benefits, in addition to its high thermal conductivity. What other advantages does tungsten carbide offer as a tool material? Moreover, considering its brittleness, is there a risk of tungsten carbide being entrained into the weld during the welding process?

2> How to solve the Tool-Al wear problem of friction stir welding and whether there is slag during the welding process?

3>How does the fatigue properties of friction stir welds compare with other fusion welds (MIG, MAG), etc. or traditional friction stir welding?

4>There is no acknowledgment, Author Contributions  section in this paper

5>The format of the references needs to be modified.

Author Response

Comments and Suggestions for Authors

This research addresses the complex and challenging task of welding AA2029-T8 joining high-strength skin-reinforced structures and recommends the use of filled friction stir spot welding. This article provides a comprehensive description of the welding process, discusses cycle times from 1 to 3 seconds, and demonstrates process forces in the range of 8 to 14 kilonewtons. A comparative analysis of RFSSW welds made with different tool materials such as H13 tool steel, tungsten carbide and MP159 was performed. The research provides valuable insights into the complex process of joining high-strength aluminum alloys through an innovative filler friction stir spot welding method. The research results have practical significance for the industry and can guide further research and development in this field. The comments are as follows,

1. First, tungsten carbide is widely employed as a tool material in Friction Stir Welding (FSW) due to its extensive benefits, in addition to its high thermal conductivity. What other advantages does tungsten carbide offer as a tool material? Moreover, considering its brittleness, is there a risk of tungsten carbide being entrained into the weld during the welding process?
2. How to solve the Tool-Al wear problem of friction stir welding and whether there is slag during the welding process?
3. How do the fatigue properties of friction stir welds compare with other fusion welds (MIG, MAG), etc. or traditional friction stir welding?
4. There is no acknowledgment, Author Contributions section in this paper
5. The format of the references needs to be modified.

Author’s response:

1. Thanks for your thoughts on WC. I did address advantages of WC in lines 100 to 117, and, per your advice, I’ve clarified further about the risk of brittle fracture with WC tooling.
2. Thanks for considering this. I, too, am interested in the question of tool wear in RFSSW, but that was determined to be out of the scope of this study. Furthermore, FSW is a solid-state welding process, so slag created in traditional fusion welding is not of concern here.
3. Thanks for the curiosity. Fatigue was of interest, but ultimately, considering our scope and resources, dynamic testing & analysis of the spots was not part of the comparison.
4. I’ve updated the acknowledgements and author contributions. Thank you for catching it.
5. Thank you for your attention to detail. I’ve modified the citations to align with the MDPI style guide.

Reviewer 3 Report

The manuscript under consideration entitled „Evaluating the Influence of Tool Material on the Performance of Refill Friction Stir Spot Welds in AA2029” by Belnap et al. raises important and current issues. However, there is some questionable information in the paper, and some important information is also missing. In order to recommend this manuscript for publication, the following comments should be considered:

1. The article is not prepared in accordance with the journal's guidelines. Among others affiliations are written incorrectly.

2. In the introduction, it should be more strongly articulated what is a scientific novelty in this manuscript.

3. There is no uniform system of units, the authors use metric dimensions, e.g. for tool dimensions, while the joint sample is dimensioned in inches. This is unacceptable, it should be standardized, it is suggested that all dimensions be standardized to metric.

4. The authors write the units in words "kilonewtons" instead of the accepted standard notation "kN".

5. The authors wrote "...some welds were performed with embedded into thermocouples." It should be precisely specified how many samples were prepared for each test. Samples with thermocouples should absolutely not be used for other tests.

6. Although the authors explained in section 3.1. why a different rotation speed was used for the WC material. However, it should be noted that when comparing materials, the same parameters should be used for all materials and only after drawing conclusions from the comparison should the tests be repeated for a different rotational speed. The comparison should be made with the same parameters.

7. Figure 6 shows two different issues, which is unjustified. Weld forces should be presented on a separate graph and discussed independently. Similarly, strength should be presented on a separate chart. Then, as part of the discussion, the correlation of welding forces and strength should be made. Additionally, it should be noted that average values of forces are presented here, so the dispersion of results should be presented, e.g. providing standard deviation on the charts.

8. In the discussion and in the chart (Figure 6), the authors write about Tensile Strength, and the results are given as Force (N). If Strength is mentioned, the results should be presented in MPa. And if the authors decide to leave Force (N), then it is Load Capacity. This is an lap joint, so it should rather talk about Shear Strength.

9. What parameters were used for strength tests, were they based on the guidelines of any standard?

10. In Figure 4, individual zones should be marked (TMAZ, HAZ, SZ, BM). It would also be valuable to enlarge the microstructure to present the size and arrangement of grains for individual variants.

11. Figure 10a is unclear, it would be necessary to describe what exactly the authors mean with the marked dimensions.

12. A more scientific discussion of the obtained research results should be carried out, which should include an in-depth analysis of the reasons for the occurrence of specific phenomena that are presented. The authors only state that a given phenomenon has been observed without wider discussion about reasons. The authors only determined the differences between joint variants made with different tools. Therefore, this is only a form of research report. Meanwhile, a scientific article must contain a broader discussion explaining the presented phenomena.

13. Discussion is too general. It is necessary to compare results obtained with results from references.

14. Discussion should contain plans for further research.

15. Editing of English language and style required.

16. The introduction should be extended to include a review of additional references and a thematic discussion. Example of recent paper:

Kubit, A., Faes, K., Trzepieciński, T. et al. The effect of RFSSW parameters on load capacity of EN AW-6082-T6 aluminum alloy and AlCu bimetallic joints. Int J Adv Manuf Technol 127, 1703–1719 (2023). https://doi.org/10.1007/s00170-023-11598-5

Moderate editing of English language required.

Author Response

Comments and Suggestions for Authors

The manuscript under consideration entitled „Evaluating the Influence of Tool Material on the Performance of Refill Friction Stir Spot Welds in AA2029” by Belnap et al. raises important and current issues. However, there is some questionable information in the paper, and some important information is also missing. In order to recommend this manuscript for publication, the following comments should be considered:

1. The article is not prepared in accordance with the journal's guidelines. Among others, affiliations are written incorrectly.
2. In the introduction, it should be more strongly articulated what is a scientific novelty in this manuscript.
3. There is no uniform system of units, the authors use metric dimensions, e.g. for tool dimensions, while the joint sample is dimensioned in inches. This is unacceptable, it should be standardized, it is suggested that all dimensions be standardized to metric.
4. The authors write the units in words "kilonewtons" instead of the accepted standard notation "kN".
5. The authors wrote "...some welds were performed with embedded thermocouples." It should be precisely specified how many samples were prepared for each test. Samples with thermocouples should absolutely not be used for other tests.
6. Although the authors explained in section 3.1. why a different rotation speed was used for the WC material. However, it should be noted that when comparing materials, the same parameters should be used for all materials and only after drawing conclusions from the comparison should the tests be repeated for a different rotational speed. The comparison should be made with the same parameters.
7. Figure 6 shows two different issues, which is unjustified. Weld forces should be presented on a separate graph and discussed independently. Similarly, strength should be presented on a separate chart. Then, as part of the discussion, the correlation of welding forces and strength should be made. Additionally, it should be noted that average values of forces are presented here, so the dispersion of results should be presented, e.g. providing standard deviation on the charts.
8. In the discussion and in the chart (Figure 6), the authors write about Tensile Strength, and the results are given as Force (N). If Strength is mentioned, the results should be presented in MPa. And if the authors decide to leave Force (N), then it is Load Capacity. This is a lap joint, so it should rather talk about Shear Strength.
9. What parameters were used for strength tests, and were they based on the guidelines of any standard?
10. In Figure 4, individual zones should be marked (TMAZ, HAZ, SZ, BM). It would also be valuable to enlarge the microstructure to present the size and arrangement of grains for individual variants.
11. Figure 10a is unclear, it would be necessary to describe what exactly the authors mean with the marked dimensions.
12. A more scientific discussion of the obtained research results should be carried out, which should include an in-depth analysis of the reasons for the occurrence of specific phenomena that are presented. The authors only state that a given phenomenon has been observed without wider discussion about reasons. The authors only determined the differences between joint variants made with different tools. Therefore, this is only a form of research report. Meanwhile, a scientific article must contain a broader discussion explaining the presented phenomena.
13. Discussion is too general. It is necessary to compare results obtained with results from references.
14. Discussion should contain plans for further research.
15. Editing of English language and style required.
16. The introduction should be extended to include a review of additional references and a thematic discussion. Example of recent paper:

Kubit, A., Faes, K., Trzepieciński, T. et al. The effect of RFSSW parameters on load capacity of EN AW-6082-T6 aluminum alloy and AlCu bimetallic joints. Int J Adv Manuf Technol 127, 1703–1719 (2023). https://doi.org/10.1007/s00170-023-11598-5

Author’s response:

1. Thank you for pointing this out. I have updated the affiliations, as well as other details throughout the paper, to align with the journal’s guidelines.
2. Thank you for this insight. I agree that it was missing that emphasis, and it has now been updated.
3. I’ve converted all instances of in. to mm. Thank you for catching this.
4. I changed kilonewtons to the standard notation: kN.
5. Thank you. I’ve now specified how many thermocouple welds we did, and I clarified that they were only used for temperature data.
6. I appreciate your consideration of this issue. Although we were able to make welds at the lower RPM, they demonstrated ULSS of around 8.5kN, which was around 2kN less than the ULSS we were able to achieve at the higher RPM. We decided to examine the higher ULSS weld, rather than comparing it at equal parameters, because analysis of a stronger weld was of greater interest to us than analysis of the weaker weld. I’ve noted details associated with the reasons for this decision in the manuscript.
7. You’re right that it’s more appropriate to separate the weld strengths and the weld forces into distinct figures. I’ve also added information about standard deviation to my charts. Thank you for making this point.
8. I’ve corrected all instances of “tensile strength” to “lap-shear strength” to be more accurate. Thank you for your attention to this.
9. Thank you for catching this. The test parameters have now been further specified, which were aligned with ASTM standard D1002-01.
10. I’ve added a figure to include an indication of the SZ, TMAZ, and HAZ. Thank you for this comment.
11. Thank you for this feedback. I agree that further specification was necessary, which I’ve now added.
12. Thanks for this comment. Additional discussion was provided throughout the entire results and discussion section increase the overall work count by a factor of 2.
13. Thank you again for this comment. Several specific references to other work have been added to provide more clarity to our results.
14. I’ve added a section addressing future work. Thank you for the reminder.
15. Per your advice, we looked through the paper and revised the use of the English language.
16. Thank you. We are familiar with Dr. Kubit’s work and have referenced him elsewhere, but this specific paper is not particularly applicable to this research as it doesn’t disclose tool material and is rather interested in bi-metallic joints. Nonetheless, we did cite additional papers in the introduction as part of our discussion, as you suggested.

Round 2

Reviewer 3 Report

The authors significantly supplemented the manuscript and corrected inaccuracies in accordance with the recommendations, therefore I recommend the article for publication in its current form.

Moderate editing of English language required