In-Depth Thermal Analysis of Different Pin Configurations in Friction Stir Spot Welding of Similar and Dissimilar Alloys

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper effectively analyzes thermal behavior in FSSW for Al-Al and Al-Cu joints using experiments and FEM, highlighting higher heat in dissimilar welds and threaded pins. Strengths include real-time thermal monitoring and clear temperature-dwelling time correlations. However, there are still some issues that need to be addressed.
1、The paper compares experimental results with FEM simulations, the validation process lacks depth. The paper does not provide sufficient statistical analysis (e.g., error margins, standard deviations) to quantify the agreement between experimental and simulated data. This weakens the reliability of the conclusions. The study only examines threaded and non-threaded pins, ignoring other potentially influential pin geometries (e.g., tapered, fluted, or hybrid designs). This narrow focus limits the generalizability of the findings to broader FSSW applications.
2、The thermal analysis is thorough, but the paper does not adequately link temperature profiles to material flow dynamics or mechanical properties (e.g., tensile strength, hardness) of the welded joints. This omission leaves a gap in understanding how pin configurations affect weld quality beyond temperature. The FEM model assumes homogeneous and isotropic materials, rigid tool behavior, and negligible heat loss to fixtures. These assumptions may oversimplify real-world conditions, especially for dissimilar alloys with anisotropic properties and complex interfacial reactions (e.g., intermetallic compound formation).
3、For Al-Cu dissimilar joints, the paper briefly mentions IMCs but fails to analyze their composition, distribution, or impact on joint integrity. This is critical, as IMCs often dictate mechanical performance and corrosion resistance. The study uses dwelling times of 4, 6, and 8 seconds, but the rationale for selecting these specific durations is not justified. A broader range or optimization study might reveal more nuanced thermal and mechanical effects.
4、Tool wear, especially in dissimilar welding (Al-Cu), can significantly alter thermal profiles and joint quality over time. The paper does not address this, despite using a carbon steel tool prone to wear under high temperatures. The cooling phase is mentioned briefly, but its implications for residual stresses, microstructure evolution, and joint performance are not explored. This is particularly relevant for dissimilar alloys with differing thermal conductivities.
5、Figures and tables (e.g., temperature distributions for similar/dissimilar alloys) are redundant in places, with overlapping information that could be consolidated to improve clarity. The study does not sufficiently compare its findings with prior work on pin configurations or Al-Cu FSSW. For instance, how do the results align or contradict existing studies on threaded vs. non-threaded pins? This limits the paper’s contribution to the field.
6、The paper concludes that threaded pins generate higher temperatures, it does not translate this into actionable recommendations for industrial applications (e.g., optimal parameters for specific strength requirements). 
These issues highlight gaps in methodological rigor, analytical depth, and contextual relevance that should be addressed to strengthen the paper’s scientific contribution.

Author Response

Reviewer # 01

Comments and Suggestions for Authors

The paper effectively analyzes thermal behavior in FSSW for Al-Al and Al-Cu joints using experiments and FEM, highlighting higher heat in dissimilar welds and threaded pins. Strengths include real-time thermal monitoring and clear temperature-dwelling time correlations. However, there are still some issues that need to be addressed.

Ans: The paper effectively analyzes thermal behavior in FSSW for Al-Al and Al-Cu joints using experiments and FEM, highlighting higher heat in dissimilar welds and threaded pins. Strengths include real-time thermal monitoring and clear temperature-dwelling time correlations. However, there are still some issues that need to be addressed.

1. The paper compares experimental results with FEM simulations; the validation process lacks depth.

Ans: Thank you very much for this valuable insight. We use two different thermal measurement techniques that were employed for the FEM validation process. Firstly, a thermal camera was used to record the temperature distribution across the weld region during the friction stir welding (FSW) process. This method allowed us to obtain a detailed thermal map along the welding line. Secondly, an infrared (IR) thermometer was utilized to specifically monitor the temperature at the pin tip—a critical location for heat generation and material softening during FSW. The experimental data collected from both tools showed good agreement with the FEM simulation results. The consistency between measured and predicted temperature profiles supports the reliability of the numerical model. We have added a clarifying paragraph in the revised manuscript to highlight this validation approach more clearly.

Experimental temperature measurements served to validate the FEM simulation, utilizing two distinct tools. During the FSW process, a thermal camera recorded the real-time temperature distribution along the whole weld line, providing a comprehensive thermal profile of the heat-affected zone.

Additionally, an infrared (IR) thermometer measured the temperature at the critical tool pin tip to understand peak temperatures and thermal input. When results from both the thermal camera and IR thermometer were compared with the FEM simulation outputs, a strong correlation emerged. This confirms the FEM model's accuracy in predicting the thermal behavior of the welding process, and this dual-method validation reinforces the numerical model's reliability.

2. The paper does not provide sufficient statistical analysis (e.g., error margins, standard deviations) to quantify the agreement between experimental and simulated data. This weakens the reliability of the conclusions. The study only examines threaded and non-threaded pins, ignoring other potentially influential pin geometries (e.g., tapered, fluted, or hybrid designs). This narrow focus limits the generalizability of the findings to broader FSSW applications.

Ans: We sincerely thank the reviewer for their insightful comment. In this study, three independent tests were conducted for each specimen configuration, focusing on the maximum temperature distribution. The mean values were used in the analysis, and the results showed minimal variation, meeting the general expectations for standard deviation and consistency. Regarding the pin geometries, we intentionally limited the study to classical threaded and non-threaded pins, as these are still widely used in industrial FSW applications. While more complex geometries (e.g., tapered, fluted, hybrid) are indeed of interest, they fall outside the scope of this work and are considered promising directions for future studies. The text below is already added to the manuscript:

For each welding condition, three experimental trials were conducted to ensure the repeatability and reliability of the temperature measurements. The peak temperatures were recorded using thermal imaging and infrared thermometry, and the mean values from each set of tests were used for comparison with the FEM simulations.

3. The thermal analysis is thorough, but the paper does not adequately link temperature profiles to material flow dynamics or mechanical properties (e.g., tensile strength, hardness) of the welded joints. This omission leaves a gap in understanding how pin configurations affect weld quality beyond temperature. The FEM model assumes homogeneous and isotropic materials, rigid tool behavior, and negligible heat loss to fixtures. These assumptions may oversimplify real-world conditions, especially for dissimilar alloys with anisotropic properties and complex interfacial reactions (e.g., intermetallic compound formation).

Ans: We sincerely thank the reviewer for this valuable observation. The primary focus of this study was to investigate the influence of pin configuration on heat generation and temperature distribution in FSSW joints. While we acknowledge that linking thermal behavior to material flow and mechanical properties is critical for a full understanding of weld quality, such analysis lies beyond the current scope and will be explored in future work. Nonetheless, the observed temperature variations—especially the higher peak temperatures associated with threaded pins—are widely recognized in the literature as key contributors to enhanced material flow and improved joint consolidation. Regarding the FEM assumptions, we agree that simplifications such as homogeneous and isotropic material properties, rigid tool behavior, and limited heat loss modeling are idealizations. These assumptions were adopted to reduce model complexity and computational cost, and they are commonly used in early-stage thermal analysis. However, we have added clarifications in the methodology section to explicitly acknowledge these limitations and their potential effects, particularly for dissimilar joints where anisotropy and interfacial reactions (such as intermetallic compound (IMC) formation) may play a role.

4. For Al-Cu dissimilar joints, the paper briefly mentions IMCs but fails to analyze their composition, distribution, or impact on joint integrity. This is critical, as IMCs often dictate mechanical performance and corrosion resistance. The study uses dwelling times of 4, 6, and 8 seconds, but the rationale for selecting these specific durations is not justified. A broader range or optimization study might reveal more nuanced thermal and mechanical effects.

Ans: We sincerely thank the reviewer for highlighting these critical points, which help strengthen the scientific depth of our work. Regarding the analysis of intermetallic compounds (IMCs) in the Al-Cu dissimilar joints, we acknowledge that while IMCs were briefly mentioned, their detailed characterization (e.g., composition, morphology, and distribution) was not included in the current study. This work focused primarily on thermal behavior and mechanical performance rather than metallurgical phase analysis. As for the dwelling times (4, 6, and 8 seconds), the selection was based on preliminary trials and literature references where such short durations were found to be effective for friction stir spot welding (FSSW) in similar material systems.

5. Tool wear, especially in dissimilar welding (Al-Cu), can significantly alter thermal profiles and joint quality over time. The paper does not address this, despite using a carbon steel tool prone to wear under high temperatures. The cooling phase is mentioned briefly, but its implications for residual stresses, microstructure evolution, and joint performance are not explored. This is particularly relevant for dissimilar alloys with differing thermal conductivities.

Ans: We appreciate the reviewer’s valuable observations. The tool used in this study is a high-alloy steel containing 12.00 wt% Cr and 1.00 wt% Mo, which significantly enhances wear resistance and thermal stability, minimizing tool degradation during welding of Al–Cu dissimilar joints. Regarding the cooling phase, natural air cooling was adopted. While its full impact on residual stresses and microstructure was not detailed in this work, we acknowledge its importance and plan to address it more thoroughly in future investigations. The below text already addresses:

The specific chemical composition of a high-alloy carbon steel containing 12 wt.% chromium and 1 wt.% molybdenum enhances the tool’s resistance to wear and corrosion, particularly under elevated thermal and mechanical loads.

6. Figures and tables (e.g., temperature distributions for similar/dissimilar alloys) are redundant in places, with overlapping information that could be consolidated to improve clarity.

Ans: Thank you very much for this truly valuable and constructive comment. The authors fully agree that consolidating some of the figures and tables—particularly those presenting temperature distributions for similar and dissimilar alloys—could help improve clarity and streamline the presentation of results. However, during the preparation of the manuscript, we carefully evaluated this option and encountered a challenge: merging figures or tables led to reduced visibility and resolution, especially when multiple data sets were combined. In particular, merging tables such as Tables 5 and 6 significantly affected font size and legibility, potentially compromising the reader's ability to interpret the data clearly. For this reason, we decided to retain the current separation, prioritizing readability and the clarity of each individual result.

7. The study does not sufficiently compare its findings with prior work on pin configurations or Al-Cu FSSW. For instance, how do the results align or contradict existing studies on threaded vs. non-threaded pins? This limits the paper’s contribution to the field.

Ans: Thank you for raising this important point. We fully agree that positioning our findings within the context of existing literature strengthens the contribution of any study. However, as comparisons are only meaningful when experimental conditions, materials, and tool parameters are consistent, our primary focus was to ensure convergence between the FEM simulations and the experimentally measured temperature profiles using thermal imaging and infrared thermometry. This approach provides a reliable foundation for validating thermal behavior under our specific set of parameters. Nonetheless, we have taken care to align our interpretations with prior studies wherever applicable. For instance, Section 3.1.1 (Thermal Analysis in Similar Alloys) builds upon the findings reported in references [34]–[37], while Section 3.1.2 (Thermal Analysis in Dissimilar Alloys) is supported by relevant literature in [40]–[48]. These references help explain the observed behaviors and trends, including the influence of pin geometry (threaded vs. non-threaded) on heat generation and flow patterns.

8. The paper concludes that threaded pins generate higher temperatures, it does not translate this into actionable recommendations for industrial applications (e.g., optimal parameters for specific strength requirements).  These issues highlight gaps in methodological rigor, analytical depth, and contextual relevance that should be addressed to strengthen the paper’s scientific contribution.

Ans: We sincerely thank the reviewer for this insightful and valuable comment. It is well taken that the study should go beyond identifying the thermal effects of pin geometries and provide actionable implications for practical applications. Threaded pins enhance material stirring and heat input, which can be advantageous when targeting improved joint strength or better material mixing, especially in challenging dissimilar material combinations such as Al-Cu. The text below is already added to conclusions:

For industrial applications prioritizing maximum joint strength and reliability, like in automotive lightweight structures or aerospace skin–stringer assemblies, threaded pins are generally recommended. Operating them at moderate rotational speeds (around 1500 rpm) with controlled dwelling times (around 6–8 seconds) effectively balances heat input with mechanical integrity. Conversely, non-threaded pins are more suitable where minimal thermal exposure is crucial. This makes them a good option for joining heat-sensitive materials or when preserving the original microstructure is a primary concern.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Reviewer Comments:

The manuscript presents a thermal analysis of welding pin geometry, comparing two configurations - threaded and non-threaded - through finite element simulations. It further evaluates the thermal behaviour in both similar and dissimilar joints. The simulation results are validated experimentally using thermal imaging to measure temperature distribution in the weld zone. Several important issues must be addressed before it can be considered for publication. In my view, these concerns require more thorough clarification to ensure the work meets the standards expected for acceptance.

Abstract:

The abstract needs to be rewritten to clearly articulate the motivation behind the study and the specific problem it seeks to address. Currently, the objective is vague, and the conclusions lack clarity. It is essential to highlight what the study aims to achieve and how it contributes to the existing body of knowledge. Moreover, the innovative aspect of the work should be explicitly stated - what sets this research apart and why it matters?

Materials and Methods:

The Materials and Methods section should be restructured to incorporate the suggestions listed below. It is essential that the authors address these points to clarify the methodology and reinforce its scientific validity:

1. Materials and Methods (1st paragraph): What was the criterion for selecting the rotational speed? Was it based on previous literature or the authors’ prior work? This should be clearly stated and properly referenced.
2. Materials and Methods (Figure 2): Although the dimensions of the FSSW tools are described in the text, Figure 2 should include a technical drawing of the tools, clearly and accurately illustrating their geometry.
3. Materials and Methods (4th paragraph): The emissivity value used with the thermal camera is not specified in the text. While Table 4 provides this value for simulation purposes, it is crucial to indicate the actual emissivity considered during the experimental measurements.
4. Materials and Methods (5th paragraph): There is no mesh dependency study presented. The authors should include a mesh sensitivity analysis to support the reliability of the simulation results.

Results:

1. Section 3.1.1: The authors state: “With longer dwelling times, heat builds up faster than it dissipates, leading to higher maximum temperatures. The heat generated is proportional to the duration of contact, leading to higher temperatures as dwelling time increases.” - Given that this work aims to provide an in-depth thermal analysis, it is important to define the maximum temperature limit achievable and identify the corresponding dwelling time at which it occurs. Furthermore, the claim that “the heat generated is proportional to the duration of contact” is questionable. Such a proportional relationship is not self-evident and lacks supporting data. To substantiate this statement, the authors should provide a graph showing dwelling time versus temperature, which would help to clarify and support their conclusions.
2. Section 3.1.2: The same concerns raised in Section 3.1.1 also apply here. Additionally, the authors employ finite element modeling (FEM), which is an ideal tool for analyzing how temperature evolves as a function of dwelling time. Therefore, it is essential that the authors leverage the capabilities of FEM to provide a more detailed and quantitative assessment of this relationship. Specifically, they should present data or plots illustrating how temperature varies with different dwelling times, in order to strengthen the analysis and support their conclusions.
3. Section 3.3: The authors present only a qualitative analysis. A more comprehensive quantitative analysis is needed. This should include a characterization of the joint shape, cross-sectional examination of the samples, and an investigation into how the different tool geometries affected the resulting welds.
4. General Comments: Overall, the results section relies heavily on qualitative descriptions. The authors are encouraged to complement their discussion with more quantitative data and measurements, which would significantly enhance the scientific rigor of the study.
5. General Comments: In the Materials and Methods section, the use of a load cell is mentioned; however, no force data are presented. Including this information is crucial, as the forces applied during the FSSW process significantly influence heat generation and, consequently, the results obtained. Presenting these measurements would meaningfully reinforce the analysis and improve the completeness of the study.

Conclusion:

The manuscript does not clearly convey the innovative contribution of the work. As it stands, it is difficult to identify what sets this study apart from existing research in the field. Furthermore, the conclusions presented are mostly straightforward observations that one might reasonably expect, rather than insights that advance current knowledge. To strengthen the impact of the work, the authors should clearly articulate the novel aspects of their study and ensure that the conclusions reflect meaningful and original findings.

Comments for author File: Comments.pdf

Author Response

Reviewer # 02

The manuscript presents a thermal analysis of welding pin geometry, comparing two configurations - threaded and non-threaded - through finite element simulations. It further evaluates the thermal behaviour in both similar and dissimilar joints. The simulation results are validated experimentally using thermal imaging to measure temperature distribution in the weld zone. Several important issues must be addressed before it can be considered for publication. In my view, these concerns require more thorough clarification to ensure the work meets the standards expected for acceptance.

Ans: We sincerely thank the reviewer for the thoughtful evaluation and constructive feedback. We appreciate the recognition of our work and fully acknowledge the importance of the concerns raised. We will carefully address each point in detail and revise the manuscript accordingly to meet the standards required for publication.

Abstract: The abstract needs to be rewritten to clearly articulate the motivation behind the study and the specific problem it seeks to address. Currently, the objective is vague, and the conclusions lack clarity. It is essential to highlight what the study aims to achieve and how it contributes to the existing body of knowledge. Moreover, the innovative aspect of the work should be explicitly stated - what sets this research apart and why it matters?

Ans: Many thanks for this valuable and constructive comment. In response, the abstract has been thoroughly revised to clearly state motivation, define the specific problem addressed, and highlight the objectives and innovative contributions of the study. We have also clarified how the findings enhance the current understanding of thermal behavior in friction stir welding using different pin geometries, particularly for similar and dissimilar nonferrous alloys. We trust that the revised abstract now meets the clarity and depth expected.

Materials and Methods:

1. The Materials and Methods section should be restructured to incorporate the suggestions listed below. It is essential that the authors address these points to clarify the methodology and reinforce its scientific validity:

Ans: Thank you for your valuable notice and constructive feedback. We appreciate your attention to the clarity and structure of the Materials and Methods section. In response, we have carefully revised and restructured this section to incorporate your suggestions and improve the overall transparency and scientific rigor of our methodology.

2. (1st paragraph): What was the criterion for selecting the rotational speed? Was it based on previous literature or the authors’ prior work? This should be clearly stated and properly referenced.

Ans: Thank you again for this valuable comment. The selection of the rotational speed and other welding parameters was based on a series of studies conducted as part of a broader five-year research project at our university focused on FSW and FSSW of non-ferrous alloys and polymers. This long-term investigation has led to the publication of over 15 peer-reviewed articles in reputable journals, the most recent of which was published this week.

https://doi.org/10.3390/jmmp9050159

The text below is already addressed to the manuscript.

This study is part of a long-term research project initiated at our university over the past five years, focusing on FSW and FSSW of non-ferrous alloys and polymeric materials. The selection of welding parameters—particularly the tool rotational speeds—was based on a series of experimental investigations previously conducted.

3. (Figure 2): Although the dimensions of the FSSW tools are described in the text, Figure 2 should include a technical drawing of the tools, clearly and accurately illustrating their geometry

Ans: Thank you very much for your precise observation. Figure 2 has been revised for improved clarity, with all components clearly labeled and the necessary dimensions added.

4. (4th paragraph): The emissivity value used with the thermal camera is not specified in the text. While Table 4 provides this value for simulation purposes, it is crucial to indicate the actual emissivity considered during the experimental measurements.

Ans: Thank you for your valuable comment. We have clarified this point in the revised manuscript.

The surface emissivity used during the experimental measurements with the TC002 thermal imaging camera (TOPDON, USA) was 0.3, consistent with the value specified in Table 4 and used in the COMSOL Multiphysics 6.2 simulation. This emissivity set-ting was applied based on the welded region's surface characteristics for AA6061 and C1100 materials, which were considered homogeneous and isotropic in the simulation.

5. (5th paragraph): There is no mesh dependency study presented. The authors should include a mesh sensitivity analysis to support the reliability of the simulation results.

Ans: Thank you for this critical observation. We have already added the following text:

A mesh sensitivity analysis was conducted to ensure the reliability of the simulation results. Three mesh densities—coarse (~45,000 elements), medium (~92,000 elements), and fine (~160,000 elements)—were applied and evaluated in terms of the resulting temperature distribution in the weld zone. The comparison showed that the peak temperature varied by less than 2% between the medium and fine meshes, indicating that further refinement had minimal effect on the accuracy of the thermal predictions while significantly increasing computation time. So, a medium-density mesh with 92,000 elements, including triangular and tetrahedral elements, was chosen because it offers a good mix of accuracy and calculation speed.

Results:

1. Section 3.1.1: The authors state: “With longer dwelling times, heat builds up faster than it dissipates, leading to higher maximum temperatures. The heat generated is proportional to the duration of contact, leading to higher temperatures as dwelling time increases.” - Given that this work aims to provide an in-depth thermal analysis, it is important to define the maximum temperature limit achievable and identify the corresponding dwelling time at which it occurs. Furthermore, the claim that “the heat generated is proportional to the duration of contact” is questionable. Such a proportional relationship is not self-evident and lacks supporting data. To substantiate this statement, the authors should provide a graph showing dwelling time versus temperature, which would help to clarify and support their conclusions.

Ans: Thank you very much for this valuable observation. The relationship in question is clearly demonstrated in Table 5, which directly supports the authors’ claim. As such, the information presented in Tables 5 and 6 sufficiently conveys the intended message without the need for further elaboration. Nevertheless, to improve clarity and coherence, the accompanying text has been revised, and the tables have been repositioned accordingly in the updated manuscript, as shown below:

With longer dwelling times, heat builds up faster than it dissipates, leading to higher maximum temperatures. The heat generated is proportional to the duration of contact, leading to higher temperatures as dwelling time increases. As the dwelling time increases from 4 to 8 seconds, the maximum temperature increases for threaded and nonthreaded pins. The maximum temperature obtained from the threaded pin was 219 °C (experimentally) at maximum dwelling time (8s) for the threaded pin corresponding with 208 °C nonthreaded pin, as shown in Table 5. The temperature increase was due to extended frictional heating, improved heat penetration, and enhanced material plasticization. As the material softens and becomes more plastic, the resistance to deformation decreases, which can lead to increased heat generation due to viscous dissipation. In addition, this increase can be explained by looking at phase changes, grain composition, and grain boundaries. As the temperature increases, some materials may undergo phase transformations (e.g., recrystallization or grain of threaded and nonthreaded pins, but threaded pins will generate more heat overall due to their design and material stirring capabilities. Figure 5 a-f indicates the temperature distribution map obtained from the thermal camera, while Figure 6 a-f explores temperature distribution from FEM simulation.

Table 5 Maximum welding temperature during FSSW for similar alloys

2. Section 3.1.2: The same concerns raised in Section 3.1.1 also apply here. Additionally, the authors employ finite element modeling (FEM), which is an ideal tool for analyzing how temperature evolves as a function of dwelling time. Therefore, it is essential that the authors leverage the capabilities of FEM to provide a more detailed and quantitative assessment of this relationship. Specifically, they should present data or plots illustrating how temperature varies with different dwelling times, in order to strengthen the analysis and support their conclusions.

Ans: Thank you very much for your insightful comment. As previously addressed in our response to a related inquiry, Table 6 will be relocated in the revised manuscript to improve clarity. Given that this table already provides the maximum temperatures recorded for both threaded and non-threaded pins across all dwelling times—alongside the corresponding experimental and FEM values—we believe that adding additional figures would offer limited benefit and could potentially complicate the presentation without adding substantial value. The text below will be revised in the manuscript.

It is worth noting that a similar temperature trend observed in Section 3.1.1 for similar alloys also applies here to the dissimilar Al–Cu joints: as dwelling time increases, the maximum temperature increases accordingly. This consistent behavior is quantitatively captured in Table 6, which serves to reinforce the FEM’s capability in capturing the thermal evolution across varying dwelling times.

3. Section 3.3: The authors present only a qualitative analysis. A more comprehensive quantitative analysis is needed. This should include a characterization of the joint shape, cross-sectional examination of the samples, and an investigation into how the different tool geometries affected the resulting welds.

Ans: Thank you very much for this valuable and constructive suggestion. We fully agree that including a cross-sectional analysis would enhance the clarity and completeness of the results. In response, a detailed cross-sectional view has been incorporated, illustrating the different zones of the weld joint. Additionally, microstructural images of these zones have been added to provide a more comprehensive understanding of how the different tool geometries influence weld formation. The following descriptive text, along with newly added Figures 11 and 12, has been included in the revised manuscript to address this point.

Figure 11 presents a cross-sectional view of the weld joint, illustrating the bonding between the upper and lower sheets. The bonded region is located centrally, followed symmetrically by the Stir Zone (SZ), Thermo-Mechanically Affected Zone (TMAZ), and Heat-Affected Zone (HAZ) on both sides.

Figure 11. Cross-sectional view of the weld joint showing bonding characteristics and distinct weld zones (SZ, TMAZ, and HAZ)

To study the effects of heat generated during the welding process, a weld section was analyzed with the following parameters: a rotational speed of 1500 rpm and a dwell time of 6 s. The weld pin geometry was threaded. The samples were cut and wet-grinded using Emery SiC paper (320, 400, 600, 800, 1000, 1200, and 2000). The samples were polished, cleaned with water and alcohol, and air-dried. The samples were immersed in a clear reagent solution (1.5% HCl + 2.5% HNO₃ + 1% HF + 95% H₂O) [1] and then washed with distilled water. A metallurgical microscope, NJF-120A (Nanjing Leochang International, Jiangsu, China), was used to investigate the microstructure of different welding regions in the joint.

Intense stirring and plastic deformation occurs in the stir zone (SZ), the central region directly beneath the tool pin. The material in this zone undergoes severe plastic deformation and dynamic recrystallization, forming fine, equiaxed grains, as shown in 12 a. The TMAZ Surrounds the stir zone and is affected by both the heat and the mechanical action of the tool shoulder and pin. The material experiences plastic deformation without complete recrystallization, and the grains become elongated and distorted, often showing a shear texture due to the rotational movement of the tool, as shown in Figure 12b. The HAZ, shown in Figure 12c, is next to the TMAZ and is only affected by the heat from welding, not by any shape changes, so its structure stays the same, but the heat changes the size of the grains and how the particles are spread out.

Figure 12. Microstructure of the welded joint: (a) SZ, (b) TMAZ, (c) HAZ.

General Comments:

1. Overall, the results section relies heavily on qualitative descriptions. The authors are encouraged to complement their discussion with more quantitative data and measurements, which would significantly enhance the scientific rigor of the study.

Ans: Overall, the results section relies heavily on qualitative descriptions. The authors are encouraged to complement their discussion with more quantitative data and measurements, which would significantly enhance the scientific rigor of the study.

2. In the Materials and Methods section, the use of a load cell is mentioned; however, no force data are presented. Including this information is crucial, as the forces applied during the FSSW process significantly influence heat generation and, consequently, the results obtained. Presenting these measurements would meaningfully reinforce the analysis and improve the completeness of the study.

Ans: Thank you very much for your valuable observation. We fully agree that the force applied during the FSSW process plays a critical role in heat generation and, consequently, the overall results. In response to your comment, additional details regarding the load cell and its connection setup have been included in the revised manuscript, as outlined below:

A load cell SS300 (Sewha CNM Co., Bucheon-si, South Korea) was used to monitor the axial load during the FSSW process. This stainless steel load cell has a capacity of 50 kgf/tf and a combined error of 0.03%. It was connected to an analog-to-digital converter (U3-LV, LabJack, USA), which interfaced with LabVIEW software for real-time data acquisition and monitoring.

Conclusion:

The manuscript does not clearly convey the innovative contribution of the work. As it stands, it is difficult to identify what sets this study apart from existing research in the field. Furthermore, the conclusions presented are mostly straightforward observations that one might reasonably expect, rather than insights that advance current knowledge. To strengthen the impact of the work, the authors should clearly articulate the novel aspects of their study and ensure that the conclusions reflect meaningful and original finding.

Ans: Thank you very much for your thoughtful and constructive comment.

A key innovation is the comparative analysis of threaded vs. non-threaded tool pins. While previous studies often focused on mechanical properties, the current study delved deeply into thermal behavior, showing quantitatively how threaded pins significantly enhance heat generation, an insight with direct process optimization value. Moreover, the study combines finite element modeling and thermal imaging to investigate temperature distribution during FSSW for both similar (AA6061–AA6061) and dissimilar (AA6061–C1100) joints, providing high accuracy by correlating FEM predictions with actual thermal camera measurements.

Overall, the conclusion was revised to be compatible with your valuable notices.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Presented review on “In-Depth Thermal Analysis of Different Pin Configurations in Friction Stir Spot Welding of Similar and Dissimilar Alloys” in my opinion covers an interesting topic. This article presents the topic of friction stir spot welding (FSSW). Thermal analysis of the welds was performed using a thermal imaging camera and an infrared thermometer for different welding variants and then the experiment was compared to finite element simulations for two types of welds, similar and dissimilar, using threaded and non-threaded pins. I recommend its publication in this journal. However, there are still some tips I want to offer:

• Why do temperature differences between the simulation model and the temperatures obtained from a thermal imaging camera, especially for similar alloys, reach up to 18 degrees Celsius?

• In the materials and methods section, please provide the name of the welding machine.

• Please provide the source in table 2.

• In figure 2, please provide a description of the tool.

• In figures 5 and 7 the temperature shown is illegible.

• Please check the bibliography for consistency with the journal format.

• I suggested that a microstructural analysis of the joints would be a good complement to the article.

Author Response

Reviewer # 03

Presented review on “In-Depth Thermal Analysis of Different Pin Configurations in Friction Stir Spot Welding of Similar and Dissimilar Alloys” in my opinion covers an interesting topic. This article presents the topic of friction stir spot welding (FSSW). Thermal analysis of the welds was performed using a thermal imaging camera and an infrared thermometer for different welding variants and then the experiment was compared to finite element simulations for two types of welds, similar and dissimilar, using threaded and non-threaded pins. I recommend its publication in this journal. However, there are still some tips I want to offer:

Ans: Thank you very much for your positive and encouraging feedback, as well as for recommending our work for publication. We sincerely appreciate your thoughtful review and your recognition of the study’s contribution to the field of friction stir spot welding (FSSW).

We also thank you for the valuable suggestions provided. We are carefully addressing each of your tips to further enhance the clarity, depth, and scientific quality of the manuscript. We are confident that the revised version will better reflect the rigor and significance of our study.

1. Why do temperature differences between the simulation model and the temperatures obtained from a thermal imaging camera, especially for similar alloys, reach up to 18 degrees Celsius?

Ans: Thank you for your insight comment.

The temperature differences—up to 18 °C—between the simulation results and the thermal imaging measurements, particularly in the case of similar alloys, can be attributed to several factors: The accuracy of thermal imaging heavily depends on correct emissivity settings. Even slight deviations in surface finish or oxidation can alter emissivity and cause discrepancies in measured temperatures. Moreover, the TC002 thermal imaging camera, while suitable for general thermal analysis, has limitations in spatial and temperature resolution. This can lead to minor deviations, especially when capturing localised temperature peaks. Finally, the FEM model assumes that everything is perfectly connected, that the materials are the same throughout, and that the conditions around it don’t change, which is not how things work in real experiments. In practice, slight air gaps, surface roughness, and environmental heat losses can affect actual temperature readings.

2. In the materials and methods section, please provide the name of the welding machine.

Ans: Thank you very much for your valuable information. The information below was added to the revised manuscript.

A radial drilling machine R915L (Breda, Treviso, Italy) of 40 mm capacity and a table of size 600 mm x 450 mm was used as an FSSW machine.

3. Please provide the source in table 2.

Ans: Thank you for your insight point. The required references were added to table 2 as below:

Table 2. Mechanical composition of AA 6061 aluminium and C11000 copper [10], [28]

4. In figure 2, please provide a description of the tool.

Ans: Thank you very much for your valuable notice. Figure 2 was modified, more details added included dimensions of each part.

5. In figures 5 and 7 the temperature shown is illegible.

Thank you very much for this valuable comment.

We agree that the temperature at the tool–workpiece interface may not appear clearly in the thermal images. To ensure the authenticity and reliability of the results, the thermal images were not post-processed and are presented exactly as captured by the thermal camera. However, the camera was configured to record the maximum temperature, which is typically located at the tool–workpiece contact area. Therefore, the highest temperature visible in the image represents the actual temperature at this interface, as shown below.

6. Please check the bibliography for consistency with the journal format.

Thank you very much for this important point. The references have been revised and formatted using the Mendeley reference management tool in accordance with the IEEE citation style.

7. I suggested that a microstructural analysis of the joints would be a good complement to the article.

Thank you very much for your valuable comment. We added the required microstructural analysis at the end of results and discussion section as below.

Figure 11 presents a cross-sectional view of the weld joint, illustrating the bonding between the upper and lower sheets. The bonded region is located centrally, followed symmetrically by the Stir Zone (SZ), Thermo-Mechanically Affected Zone (TMAZ), and Heat-Affected Zone (HAZ) on both sides.

Figure 11. Cross-sectional view of the weld joint showing bonding characteristics and distinct weld zones (SZ, TMAZ, and HAZ).

To study the effects of heat generated during the welding process, a weld section was analyzed with the following parameters: a rotational speed of 1500 rpm and a dwell time of 6 s. The weld pin geometry was threaded. The samples were cut and wet-grinded using Emery SiC paper (320, 400, 600, 800, 1000, 1200, and 2000). The samples were polished, cleaned with water and alcohol, and air-dried. The samples were immersed in a clear reagent solution (1.5% HCl + 2.5% HNO₃ + 1% HF + 95% H₂O) [1] and then washed with distilled water. A metallurgical microscope, NJF-120A (Nanjing Leochang International, Jiangsu, China), was used to investigate the microstructure of different welding regions in the joint.

Intense stirring and plastic deformation occurs in the stir zone (SZ), the central region directly beneath the tool pin. The material in this zone undergoes severe plastic deformation and dynamic recrystallization, forming fine, equiaxed grains, as shown in 12 a. The TMAZ Surrounds the stir zone and is affected by both the heat and the mechanical action of the tool shoulder and pin. The material experiences plastic deformation without complete recrystallization, and the grains become elongated and distorted, often showing a shear texture due to the rotational movement of the tool, as shown in Figure 12b. The HAZ, shown in Figure 12c, is next to the TMAZ and is only affected by the heat from welding, not by any shape changes, so its structure stays the same, but the heat changes the size of the grains and how the particles are spread out.

Figure 12. Microstructure of the welded joint: (a) SZ, (b) TMAZ, (c) HAZ

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

ok

Reviewer 2 Report

Comments and Suggestions for Authors

The submission of the revised article and the detailed responses to the reviewers' comments are appreciated. It is evident that all suggestions have been incorporated and the raised concerns have been adequately addressed. Acceptance in its current form is recommended.