Microstructure Evolution of Inertia Friction Welded Joints of TC21 Titanium Alloy

Round 1

Reviewer 1 Report

The extensive research on the microstructure of the welded joint of the TC21 titanium alloy sufficiently demonstrates the advantages of the welding method used. A small-size joint with fine-grained microstructure is obtained. This is determined by the pressing force, high temperature and rapid cooling in the welding method used.
The fine-grained microstructure has a very positive effect on the strength properties of the joint - which can undoubtedly be demonstrated by carrying out further strength tests.

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Reviewer 2 Report

The research showed interesting results. In addition to this research, one question can be asked. Inertial friction welding is a specific process with high metal deformation. In this regard, it is very interesting to study local residual deformations in a welded joint. These deformations can influence the initiation of crack formation and the failure of welded joints.

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Reviewer 3 Report

1. It is very difficult to see the deformed β phases in TMAZ from Fig. 2(a). Photos with better resolution are required.
2. No crystallization happened in TMAZ and HAZ because of low temperatures. However, no temperature data were presented in this work. What is the β-transformation temperature? The welding temperature has to be measured in order to confirm this explanation.
3. What are the mechanisms or reasons causing the formation of α+β lamellar microstructure in the WZ, TMAZ and HAZ regions? Does this lamellar microstructure deteriorate the properties of TC21?
4. Different microstructures were observed in different zones of the IFW joints. Authors presented the experimental observations, but failed to evaluate or analyze their effects on the joint properties.
5. No comparisons between IFW and other welding joints are presented in this work. Authors are suggested to make a comprehensive comparison and summarize the differences and advantages or disadvantages in a Table. 

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Reviewer 4 Report

Reviewed article is interesting and wrote at proper scientific level. Presentation method is mostly good and in accordance with generally accepted standards in that area. Figures, tables as well as terminology are clear and precise.

The conclusion part seems as results; the sentences are too long and have many non-necessary details. The conclusion part must be re-written to show only your unique findings. It should highlight the novelty and contribution to the state of the knowledge in the paper area.

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Round 2

Reviewer 3 Report

Authors have properly replied the comments made by the reviewer. This paper can now be accepted for publication.