Production of Ti–1.5Al–1Mn Titanium Alloy Butt Joints by Friction Stir Welding

Round 1

Reviewer 1 Report

The paper “Production of Ti-1.5Al-1Mn Titanium Alloy Butt Joints by Friction Stir Welding” is an interesting peace of study in the area of titanium-based alloys and friction stir welding process (FSW). The study analyzes in depth the influence of FSW regimes on the structure and properties of a promising Ti-1.5Al-1Mn alloy. The presented results are of a good quality and have significant scientific and practical value. I recommend the study for publishing in Metals after proper revision. The comments and advice are drawn as follows:

• The figure 1 is not informative. It is better to demonstrate the scheme of the welding process with indicated axis and operating forces.
• The photos in the figure 2 are similar for chosen regimes. I advise the authors to compare the surface of chosen regimes with that for not appropriate regime. Moreover, some quantitative study of surface grooves is required. At least SEM analysis in secondary electrons or the analysis of the weld profile could significantly strengthen this part.
• What is the value Rs in the formula (2)?
• Lines 156-158: “The same is confirmed by optical images of weld cross sections, exhibiting slight deformation of the material under the shoulders. In this case, an average friction coefficient may indicate the state of the entire system.” Which images? The reference for the figure or literature is expected.
• Lines 177-182: “As the frictional force increases, more heat is generated, the material 177 heats up more, and the longitudinal force and friction coefficient decrease. However, with 178 increasing load, the torque also increases. This means that the material is less resistant to 179 the translational motion of the tool, but more resistant to the rotational motion as a larger 180 volume is involved in deformation. In addition, an increased load can improve adhesion 181 between the tool and the material [19].” Is it logical that friction coefficient decreases the adhesion between the tool and the material increases simultaneously?
• For comparing studied regimes, it is desirable to put all values of measured and calculated parameters (torque, longitudinal force, friction coefficient and heat input) in one table.
• In figure 4 the letters (a), (b) …etc should be assign to each graph. The corresponded references on the graphs should be given in the text. All axis should be descripted.
• Lines 198-200: “In addition, due to high pressure in the weld zone (up to 81 MPa along the tool axis and up to 868 MPa along the welding axis), the alloy has a higher melting temperature” What is the value of the melting point for the studied alloy?
• The obtained values of YS and UTS values is of a high accuracy ~0.1-3 MPa. How many samples were tested for each point? This information can be added in Methods section.
• The description of TMAZ abbreviation must be moved from the Line 285 to the Line 252.
• Line 252: “Most of the specimens are fractured along the boundary between the stir zone (SZ) and TMAZ.” Both these zones should be indicated on the Figure 7 or in a scheme.
• Line 275. Mode # 9
• Lines 286-290: “The material of the SZ, as can be seen from the next section, undergoes recrystallization; therefore, it deforms during tension by other mechanisms, for example, by the grain boundary sliding mechanism. That is why a mode # 9 specimen with the highest microhardness in the SZ has the lowest fatigue strength” If I understand well all regimes except of #9 provide a recrystallization in SZ zone. This should be confirmed by microstructure analysis.
• Lines 310-313: “With adhesive wear of 311 the tool, its particles are mechanically mixed into the joint, diffuse and form new com- 312 pounds, which are well detected by etching.” Which compounds? This is not obvious from the Figure 9. The SEM-EDS analysis is required.
• Lines 343-344: “Twins are found in some grains, which are indicative of deformation in this zone.”
• This can be suggested but not obvious from the figure 11. To confirm the presence of twins the TEM study is desirable.

Author Response

The paper “Production of Ti-1.5Al-1Mn Titanium Alloy Butt Joints by Friction Stir Welding” is an interesting peace of study in the area of titanium-based alloys and friction stir welding process (FSW). The study analyzes in depth the influence of FSW regimes on the structure and properties of a promising Ti-1.5Al-1Mn alloy. The presented results are of a good quality and have significant scientific and practical value. I recommend the study for publishing in Metals after proper revision. The comments and advice are drawn as follows:

• The figure 1 is not informative. It is better to demonstrate the scheme of the welding process with indicated axis and operating forces.

Answer: The figure has been revised.

• The photos in the figure 2 are similar for chosen regimes. I advise the authors to compare the surface of chosen regimes with that for not appropriate regime. Moreover, some quantitative study of surface grooves is required. At least SEM analysis in secondary electrons or the analysis of the weld profile could significantly strengthen this part.

Answer: At your suggestion, there is an example of a defective weld in Figure 2c. Visual inspection of welds is done just to find the welding parameters. It does not make sense to do SEM analysis for this purpose, as it will not provide the required information. The surface topography of these joints has already been studied previously in https://doi.org/10.1063/5.0034108.

• What is the value Rs in the formula (2)?

Answer: R_s – shoulders radius. Explanation added.

• Lines 156-158: “The same is confirmed by optical images of weld cross sections, exhibiting slight deformation of the material under the shoulders. In this case, an average friction coefficient may indicate the state of the entire system.” Which images? The reference for the figure or literature is expected.

Answer: Reference added.

• Lines 177-182: “As the frictional force increases, more heat is generated, the material 177 heats up more, and the longitudinal force and friction coefficient decrease. However, with 178 increasing load, the torque also increases. This means that the material is less resistant to 179 the translational motion of the tool, but more resistant to the rotational motion as a larger 180 volume is involved in deformation. In addition, an increased load can improve adhesion 181 between the tool and the material [19].” Is it logical that friction coefficient decreases the adhesion between the tool and the material increases simultaneously?

Answer: No, of course not. In this case we are talking about competing processes and their balance. Explanation added.

• For comparing studied regimes, it is desirable to put all values of measured and calculated parameters (torque, longitudinal force, friction coefficient and heat input) in one table.

Answer: In the form of graphs, information about the pattern is presented visually. Duplicating the same information in the form of a table actually makes no sense. To make it easier to navigate through the modes, notations have been added.

• In figure 4 the letters (a), (b) …etc should be assign to each graph. The corresponded references on the graphs should be given in the text. All axis should be descripted.

Answer: Corrected as suggested.

• Lines 198-200: “In addition, due to high pressure in the weld zone (up to 81 MPa along the tool axis and up to 868 MPa along the welding axis), the alloy has a higher melting temperature” What is the value of the melting point for the studied alloy?

Answer: The temperature is specified in point 2.1.

• The obtained values of YS and UTS values is of a high accuracy ~0.1-3 MPa. How many samples were tested for each point? This information can be added in Methods section.

Answer: For each welding mode, at least 3 samples were tested for each type of test.

• The description of TMAZ abbreviation must be moved from the Line 285 to the Line 252.

Answer: Corrected as suggested.

• Line 252: “Most of the specimens are fractured along the boundary between the stir zone (SZ) and TMAZ.” Both these zones should be indicated on the Figure 7 or in a scheme.

Answer: A diagram of fracture localization is given and an explanation is added.

• Line 275. Mode # 9

Answer: Corrected.

• Lines 286-290: “The material of the SZ, as can be seen from the next section, undergoes recrystallization; therefore, it deforms during tension by other mechanisms, for example, by the grain boundary sliding mechanism. That is why a mode # 9 specimen with the highest microhardness in the SZ has the lowest fatigue strength” If I understand well all regimes except of #9 provide a recrystallization in SZ zone. This should be confirmed by microstructure analysis.

Answer: Yes, this is true. During the FSW , the grain size decreased by about 4 times in all samples in the mixing zone. The results of the microstructure study are shown in Table 5, Figure 11 and 12. In addition, in fact, recrystallization in the FSW is well known.

• Lines 310-313: “With adhesive wear of 311 the tool, its particles are mechanically mixed into the joint, diffuse and form new com- 312 pounds, which are well detected by etching.” Which compounds? This is not obvious from the Figure 9. The SEM-EDS analysis is required.

Answer: EDS results have been added.

• Lines 343-344: “Twins are found in some grains, which are indicative of deformation in this zone.” This can be suggested but not obvious from the figure 11. To confirm the presence of twins the TEM study is desirable.

Answer: Unfortunately, twins in this area are very rare. As can be seen in the figure, only a few grains with these objects are observed in the whole field of view. Therefore, it will be very problematic to search for them in the TEM. Actually, their presence or absence in the context of the paper is not so important, so it is marked in glimpses.

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The writing team thanks you for your comments and questions.

Reviewer 2 Report

In this paper, Ti-1.5Al-1Mn Titanium Alloy was successfully welded by Friction Stir Welding. The welding process (process modes, torque, and axial force) were investigated. Welded joints were analyzed from a microstructure. From the experiment, it found that the strength of the joints up to 92% of base material strength. This is an interesting research work. However, the manuscript needs to be revised before publication and my comments are listed as follows:

Reviewer’s comments as follows:

• In the introduction, the introduction to dissimilar joining technique needs further refinement. Here, I list several closely-related articles for consideration as references:1) comparative study on joining quality of electromagnetic driven self-piecing riveting, adhesive and hybrid joints for Al/steel structure. Thin-Walled Structures 164 (2021) 107903. 2) Effect of locking mode on mechanical properties and failure behavior of CFRP/Al electromagnetic riveted joint. Composite Structures. 2021; 257: 113162.How to ensure HSS prob-less tools and prefabricated hole coaxial in the experiment?
• Why did you use ZhS32 alloy to make the tool in this experiment?
• Is there any literature or theoretical basis for the visual inspection of weld defects described in chapter 3.1?
• Does cutting the joint out of the dumbbell during the mechanical properties test in chapter 3.2 affect the performance of the joint?
• Does the selection of the cutting area affect the test results in mechanical properties?
• Table headings in line 292 on page 9 should be on the same page as the table.
• Tool debris can affect the quality of the joint, how to avoid it?

Author Response

In this paper, Ti-1.5Al-1Mn Titanium Alloy was successfully welded by Friction Stir Welding. The welding process (process modes, torque, and axial force) were investigated. Welded joints were analyzed from a microstructure. From the experiment, it found that the strength of the joints up to 92% of base material strength. This is an interesting research work. However, the manuscript needs to be revised before publication and my comments are listed as follows:

Reviewer’s comments as follows:

• In the introduction, the introduction to dissimilar joining technique needs further refinement. Here, I list several closely-related articles for consideration as references:1) comparative study on joining quality of electromagnetic driven self-piecing riveting, adhesive and hybrid joints for Al/steel structure. Thin-Walled Structures 164 (2021) 107903. 2) Effect of locking mode on mechanical properties and failure behavior of CFRP/Al electromagnetic riveted joint. Composite Structures. 2021; 257: 113162.How to ensure HSS prob-less tools and prefabricated hole coaxial in the experiment?

Answer: Thank you for your suggestion, we have read these articles. Unfortunately, we cannot refer to these papers as they are not relevant to our article. Our paper studies friction stir welding of a homogeneous titanium alloy. The papers you cited investigate very different technologies and materials.

• Why did you use ZhS32 alloy to make the tool in this experiment?

Answer: At this stage, we are searching for a more durable alloy for welding. Previously, we tried the alloy ZhS6U, which is less heat-resistant. Thus, we wanted to increase the durability of the tool. In general, this class of alloys was chosen because of the high hardness values and high heat resistance. In addition, nickel superalloys are hardly studied in this context. These considerations are explained in the "introduction" and "Experimental Procedure".

• Is there any literature or theoretical basis for the visual inspection of weld defects described in chapter 3.1?

Answer: In this case, general considerations for any welding are used - the weld should not show defects, such as surface grooves and excessive flash. Both of these defects reduce the cross-section of the weld and make it less strong.

• Does cutting the joint out of the dumbbell during the mechanical properties test in chapter 3.2 affect the performance of the joint?

Answer: Samples were cut by electrical discharge method. It is widely accepted that this method does not significantly affect the mechanical properties.

• Does the selection of the cutting area affect the test results in mechanical properties?

Answer: The samples were cut from different parts of the welds, so the small error in the strength test indicates a high stability of the properties throughout the weld.

• Table headings in line 292 on page 9 should be on the same page as the table.

Answer: Corrected.

• Tool debris can affect the quality of the joint, how to avoid it?

Answer: Avoiding the influence of tool debris is possible by selecting a more resistant tool that will not wear out as much. Nevertheless, the work has produced joints that are as strong as the initial material. It can also be assumed that the tool debris even strengthens the weld. Explanation added to the text.

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The writing team thanks you for your comments and questions.

Reviewer 3 Report

The present manuscript is dedicated to production of Ti-1.5Al-1Mn titanium alloy butt joints by friction stir welding. The following questions and raised concerns are:

• First sentence is unclear. Rewriting is needed.
• I am not sure if titanium alloys are difficult to weld. Ti-alloys are relatively easy weldable with fusion welding as well. Challenges must be indicated.
• Why the studied Ti-alloy is special?
• 15-15^th lines are vague.
• 32th lines. The term ‘’high-tech alloys’’ is not scientific. It must be changed to conventional terms.
• 35^th Which strengths? Yield, tensile?
• 38^th Sentence is vague. Is not vanadium beta stabilizer?
• 42-49^th Repeating sentence explaining the same thing.
• Is the studied alloy being commercial? Where it has been used? Is it ‘’cheaper’’ substitution for the most popular Ti-6Al-4V alloy? If yes, more comparisons should be made to understand why it was chosen (produced) and how specifically FSW benefits for this compared to arc/laser welding?
• Figure 1 must include description of component and most important indicated forces in FSW. What is longitudinal force? Where is axial/feeding force?
• 136^th What is ‘flashing’?
• Figure 2. Figures must contain welding parameters. What is the appearance of the root?
• Number of welds should be as “no. X” not # sign.
• 138^th How ‘’acceptable appearance ‘’ is judged? There are standards? Acceptance of weld cannot be judged making 30-50 mm long welds.
• What is meant by longitudinal force?
• Not all terms in equations 1 and 2 are explained.
• Figure 4 is not understandable and must be updated containing more information. Added processes parameters are not explained explicitly. Subfigures must be added (a,b,c, etc.).
• Line 165^th. It is not intuitively clear because how some parameters were calculated/estimated is not clear.
• Line 171-172th. What is meant by saturation and overheating if material should not reach melting point?
• Line 172th. What is transfer layer?
• Line 175^th. All defect should have link to figures and shown appropriately.
• Line 195^th. Measuring flash temperature is irrelevant. It needs temperature values within the weld nugget. Such text can be removed from article completely.
• Line 197^th. How temperature can be very short?
• Lines 203-204^th. These results should be indicated in figures.
• Figure 5 lack of welding parameters.
• Line 203th. Which data is meant? Form which figure?
• Line 204^th. What means undulation? Can it be shown in Figure?
• Line 212. Wording “dumbbells’’ must be changed to scientific term.
• ‘‘Initial material’’ is wrong name for ‘’base metal’’.
• Lines 217-219^th. Vague results and explanations.
• Strength efficiency in % (compared to base metal) must be presented in Table 2.
• Is it possible to visualize effect of welding/process parameters on strength? Recheck line
• 226^th How can strength vary along the weld? How was it measured?
• 239-240^th It is irrelevant to mention aluminium alloys.
• 243th line. The problem is that the roots are not shown within this work.
• Paragraph 242-249^th has no meaning since results were not presented visually or compiled as table.
• Figure 7, Welding parameters should be presented.
• Reduced fatigue life can be related to poor surface upper quality, as shown in Fig. 2, and hard phases introduced through the tool, judging from Fig. 12, there is visible wear. Based on this, it seems friction stir welding seems not advantageous to laser/EB welding considering the latter processes are significantly faster and have no wear issues providing narrow weld beneficial for fatigue life.
• Line 255^th. What is residual deformation and why it is related to fatigue strength?
• Figure 8. Welding/process parameters should be indicated explicitly in figure's caption.
• Table 3. Hardness values are very wrong. How it can be 2-3 HV for Ti-alloys?
• Line 299. Unclear sentence. Are aluminum alloys providing a HAZ after FSW? References are needed. How it can be explained that Ti-alloys do not have any HAZ after FSW? Can it be compared with already published work on other Ti-alloys (there are many of them)?
• Line 303. Some number must be presented.
• Line 309^th. This is very doubtful that there is not macro defects looking at Figure 2, otherwise it should be proven (all macrosection should be shown both transversal and longitudinal). The next sentence stated about tool debris which is a very serious macro defect in FSW.
• Please do not always use SZ abbreviations in the text (especially in tables and figures), stir zone is more understandable and short enough.
• Weld mode is wrong term. Process parameter set is more appropriate.
• Based on theory smaller grain must provide improved mechanical properties. Considering that grain size in stir zone was reduced by a factor of four, why strength and fatigue life was lower than base metal? Further investigation should be made.
• Line 341th. TMAZ was not defined.
• Lines 341-344^th. The lines are vague, why it is compared to aluminium alloys, and no references? I believe twins are not recognizable with SEM (especially with such magnification), TEM is more appropriate with resolution of nanometers.
• Reference list must be expanded and contain more references to high-quality articles from higher impact journals. Comparison with fusion welding and other Ti-alloys must be made.

Most important notes: all terminology must be revised as well as grammar since much of the text is ambiguous. Most of figures needs major revision and improvement. Results must be better explained and visualized.

Author Response

The present manuscript is dedicated to production of Ti-1.5Al-1Mn titanium alloy butt joints by friction stir welding. The following questions and raised concerns are:

• First sentence is unclear. Rewriting is needed.

Answer: Sentence revised.

• I am not sure if titanium alloys are difficult to weld. Ti-alloys are relatively easy weldable with fusion welding as well. Challenges must be indicated.

Answer: In the first case, a vacuum is required, which makes it difficult to weld large sheets. In the second case, there is grain growth, after which heat treatment is required. FSW can avoid these disadvantages, although it has its disadvantages. Nevertheless, the interest in FSW titanium has grown a lot in recent years, judging by the number of publications.

• Why the studied Ti-alloy is special?

Answer: The main feature of the alloy is easy deformability, which facilitates the structure production. In addition, the alloy is not thermally hardened, which also facilitates production. In other words, despite its lower strength than Ti-6-4 alloy, it is much cheaper to produce structures from it. These features are mentioned in the introduction.

• 15-15^thlines are vague.

Answer: Sentence revised.

• 32th lines. The term ‘’high-tech alloys’’ is not scientific. It must be changed to conventional terms.

Answer: Changed to «workable».

• 35^thWhich strengths? Yield, tensile?

Answer: Tensile strength.

• 38^thSentence is vague. Is not vanadium beta stabilizer?

Answer: This paragraph discusses alloys like Ti-1.5Al-1Mn, as stated at the beginning of the paragraph. In these alloys the manganese is the beta stabilizer. A clarification is added.

• 42-49^thRepeating sentence explaining the same thing.

Answer: Line 42 discusses the disadvantages of titanium alloys. Then the difficulties in welding are discussed. Line 49 discusses FSW and its advantages. These are all different things.

• Is the studied alloy being commercial? Where it has been used? Is it ‘’cheaper’’ substitution for the most popular Ti-6Al-4V alloy? If yes, more comparisons should be made to understand why it was chosen (produced) and how specifically FSW benefits for this compared to arc/laser welding?

Answer: This alloy is a commercial alloy. A note in section 2.1 has been added. Alloy Ti-1.5Al-1Mn was used civil aircrafts, in body armor and cruising missiles. The alloy is not cheaper, but it is more workable, as stated earlier. Laser welding causes grain growth in titanium alloys, which is widely known. Arc welding is rarely used at all for welding such alloys. We could not find information on its application on the alloy Ti-1.5Al-1Mn. In addition, the alloy itself is quite poorly studied, which motivated our choice. In FSW, there is no melting of the metal, so the method is free of the disadvantages of fusion welding: high residual stresses, grain growth, gas pores, and shrinkage.

• Figure 1 must include description of component and most important indicated forces in FSW. What is longitudinal force? Where is axial/feeding force?

Answer: The designations are given.

• 136^thWhat is ‘flashing’?

Answer: Flash is a well-known defect of FSW. For example, this is how it is described by the creators of the technology: «Friction heating is generated by rubbing the faying surfaces to be joined together, under an axial force. This rubbing action causes softening of a region either side of the weld interface which is gradually extruded or expelled away from the interface as the welding force is maintained. The circumferential collar of extruded material from either component rolls back from the weld interface forming a rams horn configuration, known as a bifurcated flash formation. In production, this flash formation is very often sheared off whilst still hot and soft. The appearance of an abnormal flash can indicate that the parameters used for a particular weld are not ideal.» https://www.twi-global.com/technical-knowledge/faqs/faq-what-does-the-term-flash-refer-to-when-friction-welding . For better understanding this defect is shown in Figure 2 a.

• Figure 2. Figures must contain welding parameters. What is the appearance of the root?

Answer: The figure presents the welding modes. Visually, the appearance of the root did not differ in any way at the different welds, so it was not recorded.

• Number of welds should be as “no. X” not # sign.

Answer: Revised.

• 138^thHow ‘’acceptable appearance ‘’ is judged? There are standards? Acceptance of weld cannot be judged making 30-50 mm long welds.

Answer: The main criterion for these welds is the absence of surface grooves and excessive flash. As a rule, if the weld is of poor quality, this is already evident in the first 30 mm of the weld (Figure 2 c). In this work, welds with a length of 100 mm were produced that showed high stability (e.g., Figure 2 a). Due to the low thermal conductivity of titanium, conditions no longer change after 100 mm of welding.

• What is meant by longitudinal force?

Answer: The longitudinal force is the force with which the tool moves along the welding direction. The direction of the force is shown in Figure 1.

• Not all terms in equations 1 and 2 are explained.

Answer: Revised.

• Figure 4 is not understandable and must be updated containing more information. Added processes parameters are not explained explicitly. Subfigures must be added (a,b,c, etc.).

Answer: More information has been added to the figure.

• Line 165^th. It is not intuitively clear because how some parameters were calculated/estimated is not clear.

Answer: The torque and longitudinal force are controlled by a welding machine. Specified in point 2.1. Heat input and friction coefficient are calculated by (1) and (2) equations.

• Line 171-172th. What is meant by saturation and overheating if material should not reach melting point?

Answer: By saturation, we meant a plateau on the graph. The term has been changed in the revised version. Further it is exactly discussed that overheating should not occur, otherwise defects will appear.

• Line 172th. What is transfer layer?

Answer: During the FSW process, the tool transfers material layer by layer. These layers are called transfer layers. Explanation added.

• Line 175^th. All defect should have link to figures and shown appropriately.

Answer: Defects are indicated and described in the revised version.

• Line 195^th. Measuring flash temperature is irrelevant. It needs temperature values within the weld nugget. Such text can be removed from article completely.

Answer: The temperature was measured only to observe the stability of the welding process when adjusting the mode. For example, the first modes had unstable temperatures. Another graph is shown for comparison.

• Line 197^th. How temperature can be very short?

Answer: Short is not the temperature, but the temperature flash, as written. When one rough surface slides over another, the contact occurs only at particular points, which are in contact for a very short time. At the spots of actual contact, there are temperature flashes of 10^-3 to 10^-8 s duration. The flash temperature may reach the melting point of the contacting metals. Flash temperature has a great influence on the occurrence and development of tribochemical reactions. This phenomenon actually is the basis of all tribology. You can learn more about it in the following papers:

https://doi.org/10.1098/rspa.1954.0098

https://doi.org/10.1016/0956-716X(90)90116-X

• Lines 203-204^th. These results should be indicated in figures.

Answer: Obviously, we can not show all thermograms, as it would increase the paper's length and make it difficult to read. Therefore, only typical graphs are given. For a better understanding a more illustrative case of an unstable regime is given.

• Figure 5 lack of welding parameters.

Answer: The revised version contains the mode numbers.

• Line 203th. Which data is meant? Form which figure?

Answer: The figure number is indicated.

• Line 204^th. What means undulation? Can it be shown in Figure?

Answer: Term changed to «irregular heat input».

• Line 212. Wording “dumbbells’’ must be changed to scientific term.

Answer: Term changed to «samples».

• ‘‘Initial material’’ is wrong name for ‘’base metal’’.

Answer: In the context of welded joints, "base metal" refers to the material outside the weld. "Initial material" usually refers specifically to the original material that was cut from the sheet. This distinction is made to avoid confusion because they are different things.

• Lines 217-219^th. Vague results and explanations.

Answer: We literally say that there are no macro defects in the welds, so the test diagrams are similar. In the presence of macrodefects, the appearance of the curves can vary significantly, like in our previous work. Explanation added.

• Strength efficiency in % (compared to base metal) must be presented in Table 2.

Answer: Values are given.

• Is it possible to visualize effect of welding/process parameters on strength? Recheck line

Answer: Unfortunately, no relationship was detected.

• 226^thHow can strength vary along the weld? How was it measured?

Answer: This is indicated by a low error value. Since the test specimens were cut from different parts of the welds, the measurement error may indicate the stability of the mechanical properties. The weld strength can vary along the weld when the welding process is unstable. Explanation added.

• 239-240^thIt is irrelevant to mention aluminium alloys.

Answer: Aluminum alloys are most often welded by FSW. In this work, we aimed to get quality welds and investigate them. Therefore, throughout this paper we repeatedly compare titanium welding with aluminum welding to show the features. As noted in the paper, there are differences as well as similarities. This is very important to us because we have welded a lot of aluminum too, and we can share this knowledge.

• 243th line. The problem is that the roots are not shown within this work.

Answer: Unfortunately, observation of the root side does not provide any information. Because of the high welding pressure, it is not possible to visually detect an incomplete penetration. This can only be done with a microscope. Figure 9 shows one such example.

• Paragraph 242-249^thhas no meaning since results were not presented visually or compiled as table.

Answer: The results of the bend test are shown in Table 2.

• Figure 7, Welding parameters should be presented.

Answer: The parameters are given in the revised version.

• Reduced fatigue life can be related to poor surface upper quality, as shown in Fig. 2, and hard phases introduced through the tool, judging from Fig. 12, there is visible wear. Based on this, it seems friction stir welding seems not advantageous to laser/EB welding considering the latter processes are significantly faster and have no wear issues providing narrow weld beneficial for fatigue life.

Answer: Table 2 shows that the two modes produced joints comparable in durability to the initial material. Regarding the advantages or disadvantages - this is a question of targeting and priorities, which is subject to debate in the scientific community. It is unlikely that a single paper can completely close the issue. Therefore, at this time FSW titanium is actively developing in the world.

• Line 255^th. What is residual deformation and why it is related to fatigue strength?

Answer: Typos. It was referring to residual stresses. Corrected.

• Figure 8. Welding/process parameters should be indicated explicitly in figure's caption.

Answer: Revised.

• Table 3. Hardness values are very wrong. How it can be 2-3 HV for Ti-alloys?

Answer: Microhardness of the specimens is measured using a Duramin5 microhardness tester (Struers, Ballerup, Denmark) at the indentation load 100 g and time10 s. The device is calibrated and has a certificate of verification. The commercial alloy was shipped in a rolled condition. The composition of the alloy corresponds to the grade. This was checked during purchase. The paper also contains the results of the EDS analysis.

• Line 299. Unclear sentence. Are aluminum alloys providing a HAZ after FSW? References are needed. How it can be explained that Ti-alloys do not have any HAZ after FSW? Can it be compared with already published work on other Ti-alloys (there are many of them)?

Answer: Yes, it is well known that this zone is often observed in aluminum alloys. It is supposed that the absence of the thermal affected zone is due to the low thermal conductivity of titanium alloys. Added explanations and references to the most important sources on the subject.

• Line 303. Some number must be presented.

Answer: Values are given.

• Line 309^th. This is very doubtful that there is not macro defects looking at Figure 2, otherwise it should be proven (all macrosection should be shown both transversal and longitudinal). The next sentence stated about tool debris which is a very serious macro defect in FSW.

Answer: The sentence is corrected. Still, there were weld with a lack of penetration, which was not immediately noticed. The figure is also corrected. As for the tool debris: there is currently no understanding as to whether this should be considered a defect. In our opinion, it can be considered a matrix composite, which can only go to good effect. But this is a topic for another research.

• Please do not always use SZ abbreviations in the text (especially in tables and figures), stir zone is more understandable and short enough.

Answer: The full title is given in the text and tables. It would have taken up too much space in the figures.

• Weld mode is wrong term. Process parameter set is more appropriate.

Answer: Feed rate, rotation speed and load are process parameters. Their combination is called the welding mode. These terms are separated to avoid confusion.

• Based on theory smaller grain must provide improved mechanical properties. Considering that grain size in stir zone was reduced by a factor of four, why strength and fatigue life was lower than base metal? Further investigation should be made.

Answer: Figures 6 and 7 show images of tensile samples. You can see that the samples contained not only SZ, but also BM. That is, the deformation occurred not only in SZ. In many cases the fracture occurred in BM. So there is nothing unusual about such strength values. Cutting specimens only from the Stir Zone, of course, will result in a much stronger specimen. This is shown in our previous work DOI: 10.3390/met10060799

• Line 341th. TMAZ was not defined.

Answer: Revised.

• Lines 341-344^th. The lines are vague, why it is compared to aluminium alloys, and no references? I believe twins are not recognizable with SEM (especially with such magnification), TEM is more appropriate with resolution of nanometers.

Answer: Unfortunately, twins in this area are very rare. As can be seen in the figure, only a few grains with these objects are observed in the whole field of view. Therefore, it will be very problematic to search for them in the TEM. Actually, their presence or absence in the context of the paper is not so important, so it is marked in glimpses.

• Reference list must be expanded and contain more references to high-quality articles from higher impact journals. Comparison with fusion welding and other Ti-alloys must be made.

Answer: A comparison with other types of welding is added. References to journals Q1-Q2 are given.

Most important notes: all terminology must be revised as well as grammar since much of the text is ambiguous. Most of figures needs major revision and improvement. Results must be better explained and visualized.

Answer: Terminology, grammar, figures, and explanations have been checked and revised.

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The writing team thanks you for your comments and questions.

Reviewer 4 Report

In this manuscript, the author study microstructure and mechanical properties of friction stir welded Ti-1.5Al-1Mn titanium alloy. They have discussed the macrostructure, microstructure and some mechanical properties of the joint. My recommendation is major revision The manuscript was abundant in experimental data. Some comments are provided below:

1. The design details of the stirring tool are not shown in Materials and Experimental Set-Up, so the actual or schematic image of the stirring tool need to be added. Also, the cooling device and gas protecting device are necessary in this part. In this paper, the mechanical properties of the joints are analyzed, but it is not mentioned in the methods of testing in 2.2.
2. Basic marks, such as advancing side, retreating side, welding direction, should be marked in Fig. 2. In this paper, the hardness data are lacked and it would be better for the authors to provide the microhardness contour map or the line graph. In addition, the microhardness analysis related to the microstructure should be clarified.
3. As mentioned in page 5, 6^th line, “same is confirmed by optical images of weld cross sections,” the optical microscope pictures of weld section tissue must be provided to justify the interpretation.
4. According to the article, the 13# pattern is failed in the BM, while the tensile strength displayed in Table 2 is quite lower than that of the BM. The proper explanation should be clarified.
5. What is the basis for selecting 9# and 13# in Fig. 5? The welding thermal cycles at other parameters are suggested to be added to establish the relationship between the peak temperature and welding parameters. The relevant results should be added.
6. This paper studies the influence of welding process on the microstructure and mechanical properties of the joints, but the conclusion about the optimal welding parameters is absent. The corresponding part is suggested to be revised.
7. The English writing deserves being further polished. Most sentences contain grammatical and spelling mistakes or are not complete sentences. For example, “Among visual defects are a reduction in thickness of the retreating side and flash (page4,line12) ,”In most cases, fracture is ductile”(page9,line2) ,”

Author Response

In this manuscript, the author study microstructure and mechanical properties of friction stir welded Ti-1.5Al-1Mn titanium alloy. They have discussed the macrostructure, microstructure and some mechanical properties of the joint. My recommendation is major revision The manuscript was abundant in experimental data. Some comments are provided below:

1. The design details of the stirring tool are not shown in Materials and Experimental Set-Up, so the actual or schematic image of the stirring tool need to be added. Also, the cooling device and gas protecting device are necessary in this part. In this paper, the mechanical properties of the joints are analyzed, but it is not mentioned in the methods of testing in 2.2.

Answer: The welding diagram has been changed. The diagram of the tool is given.. Tensile and three-point bending tests are performed using a UTC 110M-100 testing machine (Test systems, Ivanovo, Russia) at room temperature at the rate 1 mm/s and 5 mm/min, respectively. The bending tests are carried out with the root turned upward to identify incomplete penetration. Fatigue tests are carried out using a BiSS UTM-100 testing machine (Bangalore Integrated System Solutions (P) Ltd., Bangalore, India) at the frequency 20 Hz and the 60% stress of the ultimate strength of the base metal. For each welding mode, at least 3 samples were tested for each type of test. All mechanical test specimens are cut across the weld. Microhardness of the specimens is measured using a Duramin5 microhardness tester (Struers, Ballerup, Denmark) at the indentation load 100 g and time10 s. It is specified in section 2.2.

2. Basic marks, such as advancing side, retreating side, welding direction, should be marked in Fig. 2. In this paper, the hardness data are lacked and it would be better for the authors to provide the microhardness contour map or the line graph. In addition, the microhardness analysis related to the microstructure should be clarified.

Answer: Figure 9 shows the Microhardness line graph. Table 3 shows the results of microhardness measurements. The relationship between microhardness and structure (discussed in section 3.3) has not been detected.

3. As mentioned in page 5, 6^thline, “same is confirmed by optical images of weld cross sections,” the optical microscope pictures of weld section tissue must be provided to justify the interpretation.

Answer: A link to the image has been added.

4. According to the article, the 13# pattern is failed in the BM, while the tensile strength displayed in Table 2 is quite lower than that of the BM. The proper explanation should be clarified.

Answer: Although many of the samples fractured over the base metal, their strength was lower than that of the initial material. This may be due to the fact that the weld sample is a multilayer composite that consists of the base material, the TMAZ, and the stir zone. Each of these zones has different characteristics, which can lead to stress redistribution. Explanation added.

5. What is the basis for selecting 9# and 13# in Fig. 5? The welding thermal cycles at other parameters are suggested to be added to establish the relationship between the peak temperature and welding parameters. The relevant results should be added.

Answer: The figure has been changed and modes 5 and 8 have been selected. Mode 8 was chosen as the most stable. Mode 5 was chosen because it showed a clear instability of the process and further on in the article its mechanical characteristics are given. No regularity between peak temperatures and mode was found, so no data are given. Thermography was performed only to check the stability of the welding process. The value of this temperature itself says nothing about the process, especially with such an error.

6. This paper studies the influence of welding process on the microstructure and mechanical properties of the joints, but the conclusion about the optimal welding parameters is absent. The corresponding part is suggested to be revised.

Answer: The conclusions are expanded according to your suggestion.

7. The English writing deserves being further polished. Most sentences contain grammatical and spelling mistakes or are not complete sentences. For example, “Among visual defects are a reduction in thickness of the retreating side and flash (page4,line12) ,”In most cases, fracture is ductile”(page9,line2) ,”

Answer: The text was checked for grammar and revised.

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The writing team thanks you for your comments and questions.

Round 2

Reviewer 2 Report

This manuscript has met the requirements for publication and is recommended for acceptance.

Author Response

Thanks again for your comments and suggestions.

Reviewer 3 Report

1. Introduction must be rewritten. Wording "we.... " must be removed and more neutral point of view must be used.
2. It is unlikely that presented Ti alloy is commercially available worldwide. Analogous Ti-alloy in western market should be presented if possible.
3. Microhardness should be converted to HV standard values, not GPa's.
4. Figure 2. Previous figure with more welds was more representative for the work. Otherwise explanation why only exp. no 8 was presented. Caption should be revised. It should be better more welds represented with indication of defects/imperfections.
5. A grammar before re-submission must be re-checked.

Author Response

1. Introduction must be rewritten. Wording "we.... " must be removed and more neutral point of view must be used.

Revised.

2. It is unlikely that presented Ti alloy is commercially available worldwide. Analogous Ti-alloy in western market should be presented if possible.

The alloy has no analogues in ASTM standards. A similar Japanese alloy ST-A90 is known. 

3. Microhardness should be converted to HV standard values, not GPa's.

Revised.

4. Figure 2. Previous figure with more welds was more representative for the work. Otherwise explanation why only exp. no 8 was presented. Caption should be revised. It should be better more welds represented with indication of defects/imperfections.

The picture was shortened at the suggestion of another reviewer. It was thought that the joints were too identical. We added another representative weld to the figure.

5. A grammar before re-submission must be re-checked.

Revised.

Reviewer 4 Report

The revised manuscript includes all the suggestion and questions raised by reviewers. These changes have improved the quality of the work presented to a great extent. It is well revised and could be accepted in the current form if the following problem could be solved.

1. About the mechanical properties part, the author states that the tensile fracture was attributed to the presence of cracks. It’s strongly suggested that formation mechanism of the cracks should be further clarified.
2. In general, the Vicker hardness (HV) is adapted to describe the variation of hardness, not GPa. The corresponding data in Fig. 9 should be revised.
3. About the microhardness analysis part，it’s suggested to establish the relationship between the grain size and microhardness.

Author Response

The revised manuscript includes all the suggestion and questions raised by reviewers. These changes have improved the quality of the work presented to a great extent. It is well revised and could be accepted in the current form if the following problem could be solved.

1. About the mechanical properties part, the author states that the tensile fracture was attributed to the presence of cracks. It’s strongly suggested that formation mechanism of the cracks should be further clarified.

Joint specimens did not contain cracks before testing. This can be verified by looking at the light microscopy images. Some samples (e.g., no. 4) contained a defect - incomplete root penetration. This defect is not a crack, but is caused by insufficient stirring force or insufficient tool plunge. Explanation added.

2. In general, the Vicker hardness (HV) is adapted to describe the variation of hardness, not GPa. The corresponding data in Fig. 9 should be revised.

There is no error in this case. The microhardness was measured using the Vickers method, but the values were converted to GPa. These values are shown in the graph and in the table. Explanation added.

3. About the microhardness analysis part，it’s suggested to establish the relationship between the grain size and microhardness

The relationship between microhardness and structure (discussed in section 3.3) has not been detected. In particular, in the stir zone, the grain size is 4 times smaller than in the base metal. This may explain the higher microhardness of the stir zone. However, the grain size in the stir zone does not differ in all samples, and the microhardness is different. Explanation added.

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Thank you for your comments and suggestions.