Review Reports

Comments 1: Page 4

What was the delivery status of alloy 5052? – no information in the text.

The information in Table 1 indicates delivery status T6. Alloy 5052 belongs to the group of non-heat-treatable alloys, so in my opinion, it is not true. Usually, it is H32 or H321.

Response 1: A drafting error occurred. I mistakenly wrote AA 5052-T6 instead of AA 5052-H321. I wanted to choose a material with superior corrosion resistance. The error has been corrected in the final version of the paper.

Comments 2: There is no information on whether the chemical composition is by weight or volume. Is the given chemical composition of the alloys derived from standards, a catalogue, or from melt analysis of the specific sheets used in the tests?Please provide this information.

Response 2: The chemical compositions presented in Table 1 are expressed in weight percent (wt%).

The composition of AA 5052-H321 is based on the standard ASTM B209, which defines the nominal ranges for Al–Mg alloys in the 5xxx series and the composition of AA 6061-T6 is based on ASTM B209 and Aluminum Association Alloy Designations, representing typical nominal values for the alloy in the T6 temper condition. The document has been updated with the requested information.

Comments 3: There is no information on the material of the welding tool.

Response 3: The tool was made by modifying a milling cutter whose material comes from the high-speed steel class. The document has been updated.

Comments 4: Line 146 - heat-affected zone (HAZ) – the authors have explained the abbreviation; there is no need to explain it further in the article (e.g., lines 304, 316, 442, etc.)

Response 4: Thank you for the observation. The document has been updated and the explains have been removed.

Comments 5: In the text, the penetration speed varied between 40 and 72 mm/min, while in Table 2, the range of this parameter is 22-42 mm/min. Please verify which value is correct.

Response 5: Thank you for the observation. The correct domain is the one shown in Table 2, and the paper has been updated accordingly.

Comments 6: Table 2 - is: pine diameter - should be: pin diameter. The table indicates a pin diameter of 3 mm, while the text specifies 6 mm.

Response 6: Thank you for the observation. The pin diameter is 6 mm. The error has been fixed and the paper has been updated accordingly.

Comments 7: I'm not sure whether the choice of a stereomicroscope was the best for determining grain structure. A stereomicroscope is typically used to assess surface condition. In my opinion, a metallographic microscope is better suited for this purpose.

Response 7: Thank you for the observation regarding the microscopic analysis of the welded joint. We would like to mention that we considered performing this type of metallographic analysis; however, for the present paper, we limited our investigation to the macroscopic analysis only.

Comments 8: Page 6, Line 182 – missing beginning of sentence.

Response 8: The error has been fixed and the paper has been updated accordingly.

Comments 9: Line 190 – is: mbH – should be: GmbH

Response 9: The error has been fixed and the paper has been updated accordingly.

Comments 10: Table 3 – correct the unit – it is Mpa but should be MPa.

Response 10: The error has been fixed and the paper has been updated accordingly.

Comments 11: Table 3 - For Vickers hardness, remove the MPa unit – it is not a hardness unit!!!!!!!!  Add the indenter load value, e.g., HV10. Change it.

Response 11: Thank you for the observation. In this paper, hardness testing was not performed; the information in Table 3 regarding the Vickers hardness of the materials was taken from the specialized literature. The corresponding column in the table has been removed.

Comments 12: All units should be placed in square brackets – check it in whole paper.

Response 12: The paper has been updated accordingly.

Comments 13: Line 204 – is: tool steel – the authors probably meant HSS (High Speed Steel).

Response 13: I agree with you, it's about HSS. The document has been updated.

Comments 14: Table 6 – The table description should be more precise. Units should be placed in square brackets.

Response 14: The document has been updated.

Comments 15: Fig. 13 and 14 – The description should indicate what a, b, and c mean (there is an explanation in the text, but there should also be a brief description of what is shown there in the description itself).

Response 15: We have tried to respond to your request as we understood it. The document has been updated.

Comments 16: Table 7 – Units should be placed in square brackets.

Response 16: The document has been updated.

Comments 17: Fig. 16 – The scale bars are illegible, almost invisible – add legible markings.

As mentioned earlier, choosing a metallographic microscope would be a better solution, which could result in higher-quality and resolution images.

Response 17: Thank you for your observations. We have re-done the photographs by clearly highlighting the specific scales of each optical image. We also considered the option of taking microscopic images, but for this paper we decided to investigate with macroscopic images.

Comments 18: If possible, it is worth adding average grain sizes in individual zones – this would increase the value of the article.

Response 18: Thank you for this valuable suggestion. We fully agree that including the average grain size in the individual zones would add value to the paper. However, such measurements were not performed within the scope of the current study, as the experimental setup was focused on macroscopic and mechanical analyses rather than detailed metallographic characterization. Therefore, the corresponding data are not available at this stage.

Comments 1: Minor editorial comments concern the caption under the Figure 16, where they should have included descriptions for all presented cases from I to VI. In the text of subchapter 4.4, the figure should be cited by number Figure 16 not Figure bellow and reference should be made to specific parts of the photo (I – VI).

Response 1: We appreciate the reviewer’s careful observation. The caption and description of Figure 16 have been revised accordingly. A detailed explanation of each labeled area (I–VI) has been added in the Results and Discussion section (subchapter 4.4), specifying the macrostructural characteristics of each zone: HAZ, TMAZ, TMAZ/SZ interface, SZ, nugget pull-out defect, and base material.

In addition, the figure citation has been corrected to “Figure 16” throughout the text.
These changes enhance the clarity of the figure and improve the understanding of the macrostructural features discussed in the study.

Comments 1: Introduction: you should state clearly which is the problem or the question that you are trying to solve. The author should present more clearly the novelty and scope of the present study.

Response 1: Thank you for this observation. The Introduction section has been revised to clearly state the research problem and objective. A new paragraph has been added to emphasize the current gap in understanding the correlation between process parameters, temperature distribution, and joint strength in dissimilar RFSSW of aluminum alloys. The research question: how these parameters influence the thermal and mechanical response of the joint, has been explicitly formulated, and the purpose of the study is now clearly defined.

Comments 2: Introduction: The RFSSW is so spread out, industrially, so try to describe the advantages with conventional methods and show that your results are better, are they?

Response 2: Thank you for this valuable suggestion. The Introduction section has been revised to emphasize the industrial relevance of the Refill Friction Stir Spot Welding (RFSSW) process and to clearly describe its advantages compared with conventional joining techniques such as Resistance Spot Welding (RSW), Friction Stir Spot Welding (FSSW), and fusion-based methods (MIG/TIG).

A new paragraph has been added (after the second paragraph of the Introduction) highlighting that RFSSW eliminates keyhole defects, reduces residual stresses, minimizes the heat-affected zone, and enables the joining of dissimilar aluminum alloys without filler material or shielding gas. These advantages contribute to improved joint quality, mechanical strength, and process sustainability.

Comments 3: Materials and Methods, chemical composition of the experimental aluminum alloys are measured or derived from literature?

Response 3: Thank you for your question. The chemical compositions of the aluminum alloys AA6061-T6 and AA5052-H321 were taken from the specialized literature, as indicated in Table 2. The corresponding reference has been added to clarify that these data were not measured experimentally but derived from standard material specifications.

Comments 4: Materials and Methods: Which were the conditions used for shear test analysis?

Response 4: We appreciate this comment. The experimental conditions for the shear test have been added in the Materials and Methods section. The specimens were tested at room temperature using a universal testing machine with a crosshead speed of 1 mm/min. The fixture setup and loading configuration were described, and Figure 13 illustrates the main phases of the shear test procedure. The paper has been revised accordingly.

Comments 5: Materials and Methods: Check the text on Finite element modeling section. (The continuity is loss in line 182)

Response 5: Thank you for noticing this. The Finite Element Modeling section has been revised to improve text continuity and logical flow. The sentence interruption near line 182 has been corrected.

Comments 6: Materials and methods: Add the reference of Eq. 1

Response 6: Thank you for the observation. The reference corresponding to Equation (1) has been added in the text.

Comments 7: Results and conclusions: It is not clear what is what you found in this paper, not found before or different from before.

Response 7: Thank you for this insightful remark. The Results and Discussion and Conclusions sections have been revised to clearly highlight the novel contributions of this work. A new paragraph has been added emphasizing that the originality of this study lies in establishing a direct correlation between the experimentally measured temperature distribution and the shear strength of dissimilar RFSSW joints; developing and validating a finite element model capable of accurately predicting thermal field evolution in AA6061-T6/AA5052-H321 welds and demonstrating that the optimized process parameters (plunge depth = 2.6 mm, welding time = 6 s) lead to improved joint strength and consistent agreement between experimental and numerical results.

Comments 8: Results and discussion: Compare your findings with other previously published results to provide better correlations.

Response 8: Thank you for this valuable suggestion. The Results and Discussion section has been expanded to include a detailed comparison between the findings of this study and previously published results.

New paragraphs have been added to discuss how the experimentally obtained shear strength values and temperature distributions compare with those reported by Tier et al. [15] and Xu et al. [3]. The revised text highlights that the present results are consistent with the general trends observed in the literature, such as the increase in joint strength with higher plunge depth and moderate heat input, but also provide new insights through the combined experimental and FEM-based analysis.

This comparison helps establish stronger correlations with previous research and reinforces the validity of the proposed thermal–mechanical model for dissimilar RFSSW joints.

Comments 9: Results and discussion: Fig. 7. Why does the sensor detect an equal temperature at time of 2 s and this behavior is not detected by FEM simulation? What is happening in the material?

Response 9: Thank you for this insightful question. The apparent temperature stabilization observed at approximately 2 s in the experimental data (Figure 7) is due to a temporary thermal equilibrium occurring during the plunge phase of the RFSSW process. At this point, the heat generated by friction and plastic deformation is balanced by the heat dissipated through conduction and convection in the surrounding material, resulting in a short plateau in the measured temperature.

In contrast, the FEM model assumes continuous heat generation and uniform heat transfer properties, and therefore does not reproduce this transient equilibrium. Moreover, the thermocouple’s thermal inertia and the delay in heat propagation at the sensor location contribute to the apparent constant temperature.

This localized difference does not affect the overall correlation between experimental and simulated results, as both approaches show consistent temperature evolution and similar peak temperature values.

Comments 10: Results and discussion: Fig 16 is not mentioned in the text.

Response 10: Figure 16 has now been properly cited in the text, and the paper has been revised accordingly.

Comments 11: Conclusion: The conclusions also need re-writing concisely as they portray the total outcome of the study.

Response 11: Thank you for your suggestion. The Conclusions section has been rewritten concisely to better reflect the overall outcomes of the research. The revised version now summarizes the main experimental and numerical findings, the validation of the FEM model, and the significance of the observed correlations between temperature field evolution and joint performance.

Comments 12: The conclusions section should highlight the unique contributions of the paper, limitations of the research and some future research directions.

Response 12: We appreciate this valuable feedback. The Conclusions section has been further improved to highlight the unique contributions of this work—namely, the combined experimental–numerical approach to RFSSW of dissimilar aluminum alloys. The limitations of the current study (such as the absence of microstructural characterization and grain size measurement) have been acknowledged, and directions for future work are proposed, including detailed metallographic analysis and optimization of process parameters using artificial intelligence methods.