Friction Stir Welding of Ti-6Al-4V Using a Liquid-Cooled Nickel Superalloy Tool

Round 1

Reviewer 1 Report

The author of this manuscript aims to provide the feasibility of a liquid-cooled superalloy tool for joining Ti alloy by friction stir welding. It is a interesting work, however, the manuscript should be revised based on the comments listed as follows:

1)      Introduction: This work focuses on studying the effect of the superalloy tool cooling on the FSW joint. However, no comparative study was carried on to reveal the positive effect of the liquid cooling condition, such as the counterpart without cooling or with different flow rates. In addition, no experimental data was provided to show the improvement of the wear resistance and strength of the tool when using the liquid-cooled tool.

2)      Materials and Methods: how to ensure a stable tool plunging when using a constant plunging force? Why not increase the force gradually to the maximum level?

3)      Twice writing of the subheading 2.1. Materials and Experimental Set-up.

4)      The standard complied during the tensile test should be provided, including the specimen size.

5)      Figure 5: Check the congruent relationship between the unit and the numerical value of the vertical coordinate.

6)      The caption of figure 7: “Green lines identify the lines of microhardness profiles.” However, no green line was found in the figure.

7)      The author states that the microstructrure (Figure 9) of the as-received base metal is characterized by α-Ti and β-Ti grains of the mean size 4.1 ± 1.5 μm and 1.2 ± 0.3 μm, respectively. What’s the method to calculate the grain sizes based on the optical image?

8)      The caption of figure 12: “This is a figure. Schemes follow the same formatting.” This may be a misdescription.

9)      Why is the strength of the joint higher than that of the base metal which is a part of the joint?

Author Response

Dear Mr/Ms Editor

We apologize for submitting the revised version of the manuscript that late. However, we did our best to answer all questions and comments from the reviewers and this took us some time required to perform additional research. Please, find below our replies to all the comments.

My best regards,

Andrey Chimaevskii

1)      Introduction: This work focuses on studying the effect of the superalloy tool cooling on the FSW joint. However, no comparative study was carried on to reveal the positive effect of the liquid cooling condition, such as the counterpart without cooling or with different flow rates. In addition, no experimental data was provided to show the improvement of the wear resistance and strength of the tool when using the liquid-cooled tool.

A: The fact is that using the non-cooling superalloy FSW tool on a titanium alloy results in fast and extremely severe deterioration of the tool’s pin even after welding a 1 meter long seam. Water cooling allows welding about 3 m long sheets, however, diffusion-controlled wear is still a problem and therefore should be investigated.  

2)      Materials and Methods: how to ensure a stable tool plunging when using a constant plunging force? Why not increase the force gradually to the maximum level?

A: Since heat is generated during FSW tool rotation it serves to soften the welding zone and constant force is enough to provide penetration as required. Maintaining the plunging force constant is one of the methods practically used to avoid excess penetration and welding the workpiece to a supporting substrate. However, there is an alternative method for controlling the FSW by keeping constant the tool penetration, and, in this case, the plunging force can be varied during the FSW. Our FSW machine was designed in accordance with the first approach when previously selected FSW parameters can not be varied during the welding.

3)      Twice writing of the subheading 2.1. Materials and Experimental Set-up.

A: Thank you. The subheadings have been changed

4)      The standard complied during the tensile test should be provided, including the specimen size.

A: Thank you. The dog-bone specimens were prepared according to  ISO 6892-84

5)      Figure 5: Check the congruent relationship between the unit and the numerical value of the vertical coordinate.

A: Arbitrary units are usually used to represent the XRD peak intensity as obtained from an diffractometer

6)      The caption of figure 7: “Green lines identify the lines of microhardness profiles.” However, no green line was found in the figure.

A: Thank you. It was a mistake. Revised to read :” Red lines….”.

7)      The author states that the microstructrure (Figure 9) of the as-received base metal is characterized by α-Ti and β-Ti grains of the mean size 4.1 ± 1.5 μm and 1.2 ± 0.3 μm, respectively. What’s the method to calculate the grain sizes based on the optical image?

A:

8)      The caption of figure 12: “This is a figure. Schemes follow the same formatting.” This may be a misdescription.

A; Thank you. The caption has been added as appropriate

9)      Why is the strength of the joint higher than that of the base metal which is a part of the joint?

A: Such a result was obtained in testing samples obtained according to regime 3 with the highest tool rotation rate 380 RPM. More intense stirring might result in better structural homogenization and less stress concentrators.

Author Response File: Author Response.docx

Reviewer 2 Report

In this study, the authors applied a liquid-cooled FSW tool to friction stir welding of Ti-6Al-4V alloy and evaluated mechanical properties and microstructure. However, there are some ambiguities and many mistakes in the manuscript.

(1) It is not theta of the X axis of XRD.

(2) Why did many IMCs form on the AS side?

(3) The hardness units in Fig. 11 differ from those in the text.

(4) I did not understand what the figure caption for Fig. 12 means.

(5) The authors say that the increase in a-phase improved strength, but is there no effect of grain refinement?

(6) The authors say that IMCs caused a reduction in ductility. However, In Figs. 7-8, IMCs were observed in large quantities on the AS side. Furthermore, the fracture area appeared on the RS side (approximately 2.3-3.1 mm from the center of the tool) of tensile specimens No. 4 and 5 in Fig. 13. In the optical micrographs, no IMCs were observed on the RS side of 2.3-3.1 mm from the center of the tool. 

I think that Ti₂Ni is not the reason for the reduction of ductility.

Author Response

Dear Mr/Ms Editor

We apologize for submitting the revised version of the manuscript that late. However, we did our best to answer all questions and comments from the reviewers and this took us some time required to perform additional research. Please, find below our replies to all the comments.

My best regards,

Andrey Chimaevskii

1) It is not theta of the X axis of XRD.

A: It is standard 2Q abscissa axis

(2) Why did many IMCs form on the AS side?

A: Such an intermixing pattern is common with the FSW when most defects such as wormholes form on the advancing sides where stagnant stirring conditions exist. At the same time an adhesion-diffusion tribological layer that generated on the tool surface during FSW might delaminate here because of higher mechanical resistance in opposing motion. 

(3) The hardness units in Fig. 11 differ from those in the text.

A: Thank you . Corrected

(4) I did not understand what the figure caption for Fig. 12 means.

A: The caption was missing. Corrected.

(5) The authors say that the increase in a-phase improved strength, but is there no effect of grain refinement?

A: The stir zone grains were of the same sizes for all FSW regimes. However, grain refining provided the strength gain as compared to the as-received alloy characteristics.

(6) The authors say that IMCs caused a reduction in ductility. However, In Figs. 7-8, IMCs were observed in large quantities on the AS side. Furthermore, the fracture area appeared on the RS side (approximately 2.3-3.1 mm from the center of the tool) of tensile specimens No. 4 and 5 in Fig. 13. In the optical micrographs, no IMCs were observed on the RS side of 2.3-3.1 mm from the center of the tool. I think that Ti2Ni is not the reason for the reduction of ductility.

A: Thank you.  In fact, Figure 13 micrographs 4 and 5 demonstrate that fracture occurred closer to the RS. This fact means that Ti₂Ni IMC structures formed in the welds have no detrimental effect of the weld joint strength. We made revisions to the text and explained the increased strength by forming more α-Ti as a result of FSW. 

Author Response File: Author Response.docx

Reviewer 3 Report

Manuscript should be accepted after a minor revision. Please see the attached file for comments. 

Comments for author File: Comments.pdf

Author Response

Dear Mr, Ms Reviewer

Thank you very much for the high appreciation of our work

1. Page 1, end of para 2, line 36. The literature review should be expanded by a couple of sentences to include some past work of FSW on other materials apart from Aluminum. Also, it would be good to mention some modeling efforts regarding FSW. You can add the following articles as a reference while discussing some other materials used in FSW [1] and some modeling work in FSW [2,3].
2. Li, G., Zhou, L., Luo, S., Dong, F. and Guo, N., 2020. Microstructure and mechanical properties of bobbin tool friction stir welded ZK60 magnesium alloy. Materials Science and Engineering: A, 776, p.138953.
3. Kulkarni, S.S., Truster, T., Das, H., Gupta, V., Soulami, A., Upadhyay, P. and Herling, D., 2021. Microstructure-Based Modeling of Friction Stir Welded Joint of Dissimilar Metals Using Crystal Plasticity. Journal of Manufacturing Science and Engineering, 143(12).
4. Kulkarni, S.S., Gupta, V., Ortiz, A., Das, H., Upadhyay, P., Barker, E. and Herling, D., 2021. Determining cohesive parameters for modeling interfacial fracture in dissimilar-metal friction stir welded joints. International Journal of Solids and Structures, 216, pp.200-210.

References added to the article.

2. Page 1, lines 37-39: Can you cite a reference for this statement? “Earlier experimenting with FSW on titanium alloys allowed revealing a number of problems among which the fast wear of the FSW tool and intermixing the wear particles into the weld joint was the most prominent”

References to such works are provided further on page 2.

3. Page 1, line 42: There is a typo where FSW is spelled as FSP. Unless you want to mention friction stir processing. In that case, please add the full form when mentioning FSP for the first time.

Added full form

4. Page 10, figure 15: “and of” should be corrected as “of”.

Corrected

Best regards

Chumaevskii Andrey

Reviewer 4 Report

The authors prepared a manuscript that may be of use to scientists dealing with friction stir welding of Ti-6Al-4V using a liquid-cooled nickel superalloy tool.

It seems to me that there are interesting scientific papers of this type that have been published in last time. I am curious, did you find similar research recently carried out by scientists?

The manuscript is after one review. The corrections made improved the scientific quality of the article. It can be stated that this article is within the scope of Technologies (ISSN 2227-7080). The title is satisfying. The abstract covers appropriate points. The scientific quality of the paper is good. Conclusions are adequately supported by the data. The presented data are reliable and useful.

I am convinced that the paper can be published.

Author Response

Dear Mr./Ms. Reviewer
Thank you very much for the high appreciation of our work
Best regards
Chumaevskii Andrey

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors state that he has made corrections to comment 3, but I do not believe they have been corrected in the text. Furthermore, It is also difficult to see where he corrected to my comment.

Author Response

Dear Reviewer,
Thank you for your comments
The article has been corrected according to remark No. 3

my best regards,
Andrey Chimaevskii

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

I think it can be judged as Accept.