Microstructural Characterization of Friction Stir Welds of Aluminum 6082 Produced with Bobbin Tool

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper investigates the effects of different process parameters on the microstructure and mechanical properties of bobbin friction stir welded aluminium 6082. Two sets of welding parameters were used: a low energy condition (280 RPM, 280 mm/min) and a high energy condition (450 RPM, 450 mm/min). The microstructural characterisation involved optical microscopy, TEM, SEM, EDS, and EBSD. The hardness was evaluated through Vickers hardness tests and tensile strength was assessed using the tensile test. The paper also includes a numerical simulation to support the experimental findings. While some results are presented clearly, the paper could benefit from a more in-depth discussion of the underlying mechanisms. For example, the reasons behind the higher tensile strength of the 280/280 welds despite their lower hardness are briefly mentioned but not fully explored. The correlation between microstructure and tensile strength should be described accurately. Below are my comments:

1.     In line 60, please specify the appropriate distance range between the shoulders, considering the workpiece thickness (t) or allowable clearance. For instance, in your paper, this distance appears to be 1 mm

2.     At the end of the introduction, please describe in a paragraph that highlights the novelties, objectives, and main contributions of your paper. This paragraph should emphasise the unique aspects of your research, clearly state the primary goals, and summarise the significant work and findings presented.

3.     In Table 1, please include the relevant reference for the elements of Aluminium 6082.

4.     Please Identify in numerical simulation, the analysis type, mesh sizes in various areas, element type, analysis time.

5.     Line 107: The microstructure analysis (Fig. 3) should be moved to the Results and Discussion section.

6.     In Fig. 4, it appears that different temperatures result in varying colours in the weld zone. Are there any other differences between 280 rev/min and 450 rev/min that can be identified from these images? For instance, consider comparing the sizes of the HAZ and TMAZ at 280 rev/min and 450 rev/min.

7.     Please bring metallography images of different areas of the welded zones (SZ, TMAZ, HAZ) for both 280 rev/min and 450 rev/min. Then, compare the average grain sizes for these areas at each manufacturing process.

8.     Fig. 8 is unclear; please bring a better graph.

9.     Please analyze the correlation between microstructure of welded zone and hardness with tensile strength for various manufacturing process (280 rev/min 450 rev/min). Some similar studies as follwoing [*.**] showed that tensile properties of friction stir welded joints without defect were proportional to hardness values.

*Farhang, M., et al. (2022). Experimental Correlation Between Microstructure, Residual Stresses and Mechanical Properties of Friction Stir Welded 2024-T6 Aluminum Alloys. International Journal of Advanced Design & Manufacturing Technology, 15(3).

**Borrego, L. P., Costa, J. D., Jesus, J. S., Loureiro, A. R., & Ferreira, J. M. (2014). Fatigue life improvement by friction stir processing of 5083 aluminium alloy MIG butt welds. Theoretical and Applied Fracture Mechanics, 70, 68-74.

Author Response

Thank you to the reviewers and the editor for considering our manuscript and for the thoughtful comments and suggestions to improve its clarity, rigor, and quality. The following are our responses to the comments from Reviewer 1, and the text has been revised as appropriate to reflect these changes. Revisions associated with comments from Reviewer 1 are highlighted in yellow in the revised manuscript, and revisions associated with Reviewer 2 are highlighted in blue. Overlapping comments from the reviewers are highlighted in grey. The comments and suggestions from the reviewers have strengthened the manuscript, and we appreciate their time and effort.

Comment 1: In line 60, please specify the appropriate distance range between the shoulders, considering the workpiece thickness (t) or allowable clearance. For instance, in your paper, this distance appears to be 1 mm.

Response 1: Thank you for the suggestion as that is good information to add to the manuscript. As you inferred from our paper, the workpiece is 1 mm thicker than the shoulder-to-shoulder spacing of the tool. This also represents a good guideline for the bobbin tool. To that end, the referenced sentence now reads:

In addition, the distance between the two shoulders should be slightly smaller (~1 mm) than the thickness of the workpieces to ensure a vertical friction force during processing and to contain the material within the process zone [11,12,13].

Comment 2: At the end of the introduction, please describe in a paragraph that highlights the novelties, objectives, and main contributions of your paper. This paragraph should emphasise the unique aspects of your research, clearly state the primary goals, and summarise the significant work and findings presented.

Response 2: As suggested by the reviewer, a new paragraph has been added at the end of the Introduction. Please note that the first sentence of the previous last paragraph was pulled to the beginning of the new paragraph as its tone fit much better in the new location. The added paragraph reads as follows:

The goal of this proposed research, therefore, is to analyze the microstructure and mechanical properties of aluminum 6082 joints fabricated with the BTFSW method. Two unique combinations of rotation and welding speeds are considered: tool rotation speed 280 rev/min with weld velocity 280 mm/min and 450 rev/min with weld velocity 450 mm/min. Fine, equiaxed grains characterized the stir zones of each joining condition with hardness recovery occurring in relation to the TMAZ. The hardness of the stir zone for the 450/450 condition was higher than that of 280/280 and even exceeded the baseline hardness of the alloy. Such behaviors are correlated to the microstructural, precipitation and temperature characteristics during BTFSW. Ultimately, for either set of process parameters, the study demonstrates the efficacy of BTFSW to join 6082 workpieces, thus more confidently availing the method to industry.

Comment 3: In Table 1, please include the relevant reference for the elements of Aluminium 6082.

Response 3: The following information has been added to the first paragraph under Materials and Methods:

The concentration of main alloying elements (determined by the Perkin Elmer OP-TIMA 7300 DV ICP Optical Emission Spectrometer) of base alloys is presented in Table 1.

Comment 4: Please Identify in numerical simulation, the analysis type, mesh sizes in various areas, element type, analysis time.

Response 4: The following sentences have been added to the end of the first paragraph under Materials and Methods:

The simulation in Comsol couples the Heat Transfer in Solids and Non-Isothermal Flow studies to obtain the temperature distributions during welding. The mesh is comprised of 25433 tetrahedral, 7128 triangular, 936 edge and 68 vertex elements. In the area around the tool/workpiece interface and within the flow capable region, a fine mesh is applied that ranges in size from 1.19 mm near the tool center to 6.29 mm near the inlet and outlets of the flow region. A mesh study demonstrated that a finer mesh did not substantially change the simulation results.

Comment 5: Line 107: The microstructure analysis (Fig. 3) should be moved to the Results and Discussion section.

Response 5: Lines 107 – 110 from the original draft were moved from the Materials and Methods to the first paragraph of the Results and Discussion section, along with Figure 3. However, a few lines were added to the Materials and Methods section to describe the TEM methodology. The following sentences were added to the start of the paragraph just after Figure 2:

Transmission electron microscopy (JEOL-200CX TEM microscope) was employed to assess the precipitation characteristics of the baseline material. Thin foils were prepared from the material by excising 3 mm discs and then electropolishing in a solution of HNO₃ and CH₃OH (1:2) at -30^oC and 12 V.

Comment 6: In Fig. 4, it appears that different temperatures result in varying colours in the weld zone. Are there any other differences between 280 rev/min and 450 rev/min that can be identified from these images? For instance, consider comparing the sizes of the HAZ and TMAZ at 280 rev/min and 450 rev/min.

Response 6: The different colors in the macrostructures result from the etching process of the samples. The specifics of color etching of aluminum do not always allow for achieving the same color shade across all samples. The visible differences in color shades on individual samples are related to the appearance of so-called “onion rings”, which are described in the literature and also in the manuscript. However, these images do provide additional information concerning the hourglass shape of the weld in relation to the process parameters. To that end, the paragraph leading into Figure 4 has been amended and now reads:

The cross-sections of the weld microstructures (in macro scale) from the BTFSW joints are shown in Fig. 4 for both tool rotation speeds, i.e., 280 rev/min and 450 rev/min. An important difference between the weld microstructure produced during conventional FSW and BTFSW is the shape of the stir zone itself. The use of two shoulders produces a weld with a shape that resembles an "hourglass" as shown in the figure. The weld width measured at the mid-thickness of the cross-section for 280 rev/min is 13.95 mm and that for 450 rev/min is 14.54 mm. The examined joints exhibit three characteristic areas, i.e., heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and stir zone (SZ) as indicated in Figure 4.

Comment 7: Please bring metallography images of different areas of the welded zones (SZ, TMAZ, HAZ) for both 280 rev/min and 450 rev/min. Then, compare the average grain sizes for these areas at each manufacturing process.

Response 7: Figure 4 has been revised as suggested. In addition, a discussion paragraph following Figure 4 has been added. The new paragraph reads as:

The analysis of the weld zones at tool rotation speeds of 280 and 450 rev/min did not reveal significant differences in grain size across the three characteristic areas. However, a detailed examination of the stir zone showed that at 280 rev/min (Fig. 4a), there is a slightly smaller grain size compared to the stir zone at 450 rev/min (Fig.4b) (though the differences are within the limits of measurement error). This phenomenon can be explained by the fact that at a higher tool rotational speed (450 rev/min), a higher welding temperature is observed, promoting slightly more grain growth than at the lower speed (280 rev/min) and yielding the minor differences in the microstructure of the stir zones.

Comment 8: Fig. 8 is unclear; please bring a better graph.

The resolution of Figure 8 has been improved as suggested.

Comment 9: Please analyze the correlation between microstructure of welded zone and hardness with tensile strength for various manufacturing process (280 rev/min 450 rev/min). Some similar studies as following [*.**] showed that tensile properties of friction stir welded joints without defect were proportional to hardness values.

*Farhang, M., et al. (2022). Experimental Correlation Between Microstructure, Residual Stresses and Mechanical Properties of Friction Stir Welded 2024-T6 Aluminum Alloys. International Journal of Advanced Design & Manufacturing Technology, 15(3).

**Borrego, L. P., Costa, J. D., Jesus, J. S., Loureiro, A. R., & Ferreira, J. M. (2014). Fatigue life improvement by friction stir processing of 5083 aluminium alloy MIG butt welds. Theoretical and Applied Fracture Mechanics, 70, 68-74.

Response 9: In our case, it was not possible to analyze the correlation between microstructure of welded zone and hardness with tensile strength due to the premature rupture of the 450RPM sample. An undiscovered defect may have been responsible for such behavior. Also, the location of rupture at the center of the test piece but not at the location of the smallest hardness indicates that the tensile curve for this sample is not complete and thus it was not deeply discussed. As such, our discussion was based on the hardness measurements only. 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents the double-side FSW on the AA6082 alloy. The work includes numerical simulation, microstructure observation, and mechanical testing as the procedures in the manuscript. In general, the manuscript shows a completed work to the audience with respect to the most of other FSW works. But some issues are existing, which needs a little bit additional input. The reviewer has detailed comments listed below:

1. Numerical simulation: Based on the mesh map showed in Figure 2c, the reviewer thinks the mesh size is not small enough. 

2. Numerical simulation: The authors related the hardness maps to the numerical simulation predicted temperature distributions. However, the reviewer thinks the hardness maps should be more suitable to be related to the simulated strain rate maps or effective strain maps, which refers the progress of grain fragmentation and dynamic recrystallizations. But, also, temperature maps also needs to be included in this work, which also is a factor of dynamic recrystallizations.

3. Fracture surface: the welding parameter induced ductile to brittle shift is very interesting to the reviewer, which in the fracture surfaces, the images shows significantly strong difference between the 2 parameters. The reviewer wondering if the authors could explain more and detailed the physical reasons why the 450RPM sample is that brittle. 

Author Response

Thank you to the reviewers and the editor for considering our manuscript and for the thoughtful comments and suggestions to improve its clarity, rigor, and quality. The following are our responses to the comments from Reviewer 2, and the text has been revised as appropriate to reflect these changes. Revisions associated with comments from Reviewer 1 are highlighted in yellow in the revised manuscript, and revisions associated with Reviewer 2 are highlighted in blue. Overlapping comments from the reviewers are highlighted in grey. The comments and suggestions from the reviewers have strengthened the manuscript, and we appreciate their time and effort.

Comment 1: Numerical simulation: Based on the mesh map showed in Figure 2c, the reviewer thinks the mesh size is not small enough.

Response 1: At the tool/workpiece interface and within the flow capable region, a fine mesh is applied. The mesh size is allowed to increase away from the tool in the flow capable region, and within the workpieces, away from the tool, the mesh is permitted to be much coarser. The authors did perform a mesh study and found that the simulation results did not appreciably change for finer meshes, especially at the tool/workpiece interface. As such, the authors feel that the mesh presented gives the best balance between reliability of results and computation time. To that end, the following sentences have been added to the end of the first paragraph under section 2. Materials and Methods:

The simulation in Comsol couples the Heat Transfer in Solids and Non-Isothermal Flow studies to obtain the temperature distributions during welding. The mesh is comprised of 25433 tetrahedral, 7128 triangular, 936 edge and 68 vertex elements. In the area around the tool/workpiece interface and within the flow capable region, a fine mesh is applied that ranges in size from 1.19 mm near the tool center to 6.29 mm near the inlet and outlet of the flow region. A mesh study demonstrated that a finer mesh did not substantially change the simulation results.

Comment 2: Numerical simulation: The authors related the hardness maps to the numerical simulation predicted temperature distributions. However, the reviewer thinks the hardness maps should be more suitable to be related to the simulated strain rate maps or effective strain maps, which refers the progress of grain fragmentation and dynamic recrystallizations. But, also, temperature maps also needs to be included in this work, which also is a factor of dynamic recrystallizations.

Response 2: This is a very important point to recognize in the discussion. To provide information on the strain rates in the stir zone, rather than showing full strain rate maps, the authors present the average strain rate along a reference line for the advancing and retreating sides. This can facilitate the comparison between the two sets of weld parameters coupled with the temperature distributions. The reference line, located at mid-thickness and 6 mm behind the tool, extends from the TMAZ/SZ boundary to the weld center on each side of the weld. These average strain rates have been added to Figure 9, and the revised figure now appears in the revised text. The paragraph leading into Figure 9 has also been amended and now reads:

Differences in the hardness profiles result from the temperature differences between the welding parameter sets in relation to the precipitation/dissolution behavior of the aluminum alloy, as well as the dynamic recrystallization behavior. Included in Figure 9 above the hardness profiles are the predicted temperature distributions from the simulation on the plane 5 mm from the tool shoulder for each parameter set. The average strain rate on a mid-thickness reference line (6 mm behind the tool) extending from the TMAZ/SZ boundary to the weld center are also presented for the AS and RS. Due to the resulting heat input and temperatures, dissolution and ripening of the strengthening phases in the alloy can contribute to the decrease in hardness within the thermo-mechanically affected zones. In this region the grains are also larger and longer than in the SZ indicating that the temperature was insufficient for dynamic recrystallization to occur. The temperatures in the heat affected zone and the lack of plastic deformation from the tool causes ripening of the strengthening phases, thus the decrease in hardness is largest in this zone. The large differences in hardness in the stir zones of both samples are caused by the higher temperature in the 450/450 sample compared to the 280/280 sample, approximately 460 ºC versus 430 ºC, and by the higher strain rates promoting recrystallization. At these higher temperatures, the precipitates dissolve to a greater extent, and, therefore, the effect of post-weld natural aging in the 450/450 sample is stronger. The influence of natural aging on hardness in the stir zone has been described in detail in [22,23] for aluminum alloys of the 7000 series.

Comment 3: Fracture surface: the welding parameter induced ductile to brittle shift is very interesting to the reviewer, which in the fracture surfaces, the images shows significantly strong difference between the 2 parameters. The reviewer wondering if the authors could explain more and detailed the physical reasons why the 450RPM sample is that brittle.

Response 3: Thank you for your question and interest in this aspect of the work. The final paragraph of the Results and Discussion has been amended to address your question. The paragraph now reads:

Tensile tests were also carried out on the welded panels. The tensile strength of the welded samples was less than that of the base material as summarized in Table 3. For the 280/280 parameters, the tensile strength was 238 MPa, and for the 450/450 sample, the tensile strength was 172 MPa. The large difference in the tensile strength of the tested samples is reflected in the nature of the fracture in both variants as highlighted in Figure 10. For the lower rotational speed, the fracture was of a ductile nature, and the breakthrough of the sample took place in the heat affected zone on the retreating side. For the higher rotation speed, the breakthrough occurred in the middle of the weld, and the fracture was quasi-brittle nature. The physical reason for the brittleness of the 450/450 sample is associated with the higher processing temperatures. The temperature rise during FSW promotes complex changes in the morphology of the strengthening phases, β'' and β'. The precipitates in the stir zone are partially or completely dissolved and then, upon cooling, they reprecipitate and are subjected to ripening. With this reprecipitation, new secondary phases appear on grain boundaries and contribute to the brittle behavior. The stir zone hardness of the 450/450 sample is higher than for the 280/280 sample (Fig. 9), indicating that a lower ductility in 450/450 sample would be expected.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept in present form

Reviewer 2 Report

Comments and Suggestions for Authors

The comments were addressed well.