Cold Spray Additive Manufacturing of Ti6Al4V: Special Nozzle Design Using Numerical Simulation and Experimental Validation

Round 1

Reviewer 1 Report

The cold spray (CS) deposition process of powders can successfully be used as an additive manufacturing process, providing dense and wear-resistant deposits. 

This paper optimizes the convergent length of the nozzle of CS gun. I consider that the research is original and well-conducted but, in my opinion, the coating process and the quality of the coating were not fully described.

My suggestions are as follows:

1. Results regarding the hardness of the obtained coatings (at least for those presented in Figure 13)  should be provided.
2. In the CS process, the impact forces between the powder particles and the target are dictated by their kinetic energy, depending also on their mass and not only on their velocity. Figures 10 and 11 must present also the variation of the kinetic energy. This new interpăretation may change the results, that is particles of higher volume could provide better results or the same as obtained for the smaller ones.
3. In Figures 8b, the particle temperature during impact must be correlated with their kinetic energy again.
4. What can the authors assert about the complex influence of surface quality (roughness), porosity, hardness and wear resistance of the obtained optimized coatings?! 
5. At line 127, the powder feeding rate is 3 rpm?! Usually, rpm is an abbreviation of rounds per minute.
6. The most important parameter in the evaluation of the quality of a deposited material (coating) is the adherence of the coating to the substrate. For a complete characterisation of the coatings, results of the scratch tests are requested, besides roughness and hardness measurements. This could change the interpretation of the results presented in Figure 13, as we can see that some deposits present a clear delimitation from the substrate, e.g., 1.5-30, 2.7-30, and 2.7-90, while no delimitation line can be observed for 1.5-65 and 2.7-40. To summarize, complete optimization of the nozzle geometrical parameters must be interpreted from the viewpoint of complete characterization of coating quality, but the authors presented no results on coating roughness, adherence and hardness.
7. In Figure 12-f, 15 is 1.5.

Author Response

Dear Assistant Editor Ms. Ana Kovačević and reviewers, 

Thank you for your encouraging and constructive comments on our manuscript (Cold Spray Additively Manufacturing of Ti6Al4V: Special Nozzle Design Using Numerical Simulation and Experimental Validation, 1529126). We have carefully and fundamentally modified the full manuscript accordingly. In addition, we have added the micro-hardness of the single path coating. The main changes are marked in yellow in the revised version.

The responses to your comments are as follows:

Response to the First Reviewer's comment NO.1:

The cold spray (CS) deposition process of powders can successfully be used as an additive manufacturing process, providing dense and wear-resistant deposits.

This paper optimizes the convergent length of the nozzle of CS gun. I consider that the research is original and well-conducted but, in my opinion, the coating process and the quality of the coating were not fully described.

My suggestions are as follows:

1 Results regarding the hardness of the obtained coatings (at least for those presented in Figure 13)  should be provided.

Response: Many thanks for your encouraging comments. We have made up the hardness test experiment and added the hardness of the coatings. Please see Figure 12 in the revised version.

Response to the First Reviewer's comment NO.2:

2 In the CS process, the impact forces between the powder particles and the target are dictated by their kinetic energy, depending also on their mass and not only on their velocity. Figures 10 and 11 must present also the variation of the kinetic energy. This new interpăretation may change the results, that is particles of higher volume could provide better results or the same as obtained for the smaller ones.

Response: Many thanks for your constructive comments on our paper. It is better to use kinetic energy to explain the state of particles before they impact the substrate. We have modified Figure 10 and Figure 11 and the corresponding interpretation in the paper.

Response to the First Reviewer's comment NO.3:

3 In Figures 8b, the particle temperature during impact must be correlated with their kinetic energy again.

Response: Figure 8b describes the relationship between the instantaneous temperature of the particles and the flight time. Figure 8c is the relationship between impact temperature and particle diameter. We focus on gas flow, gas temperature, and particle temperature in the 3.1 section. The kinetic energy of the particle is related to the velocity, so it remains the same here.

Response to the First Reviewer's comment NO.4:

4 What can the authors assert about the complex influence of surface quality (roughness), porosity, hardness and wear resistance of the obtained optimized coatings?

Response: The surface roughness and surface quality are shown in Figure 12. And the maximum width of the single-pass coating ranges from 4.1 to 8.9 mm.The friction and wear specimens are square, with a size of at least 12 mm by 12 mm. Because the single-pass coating is too narrow, it does not meet the conditions for friction and wear experiments.

Response to the First Reviewer's comment NO.5:

5 At line 127, the powder feeding rate is 3 rpm?! Usually, rpm is an abbreviation of rounds per minute.

Response: The unit of rate is correct. It is “rpm”, meaning “rounds per minute”.

Response to the First Reviewer's comment NO.6:

6 The most important parameter in the evaluation of the quality of a deposited material (coating) is the adherence of the coating to the substrate. For a complete characterization of the coatings, results of the scratch tests are requested, besides roughness and hardness measurements. This could change the interpretation of the results presented in Figure 13, as we can see that some deposits present a clear delimitation from the substrate, e.g., 1.5-30, 2.7-30, and 2.7-90, while no delimitation line can be observed for 1.5-65 and 2.7-40. To summarize, complete optimization of the nozzle geometrical parameters must be interpreted from the viewpoint of complete characterization of coating quality, but the authors presented no results on coating roughness, adherence and hardness.

Response: The traditional test sample for testing the adhesion strength of the coating is a disc with a coating and a substrate, the diameter of the disc is 25mm. The area of the single-pass coating is too small to test the adhesion strength. We have supplemented the hardness test and added hardness data and analysis to the paper.

Response to the First Reviewer's comment NO.7:

7 In Figure 12-f, 15 is 1.5.

Response: Thank you again, we have changed “15” into “1.5” in Figure 12-f.

Author Response File: Author Response.docx

Reviewer 2 Report

The work is solid, but poor in the verification results.
I suggest supplementing the article with the results of the hardness and modulus of elasticity of the obtained coatings.

The main topic of the article is the simulation of the influence of the spray nozzle geometry on the quality of the applied coatings. The obtained results were validated by an experiment. Simulation results show that particle impact temperature increases remarkably for a long convergent length, while particle velocity slightly increases, which is validated by experiments. The relationship between the particle impact temperature and practice diameter shows the first increase and then decrease. The experimental results show that as the nozzle convergent section becomes long, the edges of the single-pass deposits become smooth, the width, density, and deposition efficiency of the single-pass deposits increase. The strength of the work is a concise description of the results of the experiment and a summary. The presented drawings are clear and legible.

The value of the work will increase significantly if the results of hardness H and modulus of elasticity are added to the work. Presentation of the results will increase the value of the work and broaden the group of people interested in the practical application of Ti6Al4V in Cold spray technology.

 

Originality/Novelty: 

Research problem posed correctly. The presented results cover current issues.

 Significance: 

The presented results are correct.

Quality of Presentation: 

The article is legible. Data in tables and graphic presentations are presented correctly.

Scientific Soundness: 

Analyzes of the results were carried out on the basis of solid data.

Nevertheless: in Chapter 2. Numerical Modeling and Experimental Methodology the description of the methodology of numerical analyzes was reduced to line 107-111. It is advisable to devote a little more attention to the description of the simulation methodology.

Section 3.3 focuses on the microstructure of the resulting coatings. The microstructure is an important parameter of the resulting coatings butTi6Al4V coatings are used for their mechanical properties. It is recommended to expand the article with the analysis of hardness H and modulus of elasticity E of the obtained coatings.

Interest to the Readers: 

Expanding the work to discuss the results of H and E will significantly increase its value for the readers.

 Overall Merit:

The work presents solid research. The research are typical, but necessary for the proper preparation of devices for industrial applications.

 

Author Response

Dear Assistant Editor Ms. Ana Kovačević and reviewers, 

Thank you for your encouraging and constructive comments on our manuscript (Cold Spray Additively Manufacturing of Ti6Al4V: Special Nozzle Design Using Numerical Simulation and Experimental Validation, 1529126). We have carefully and fundamentally modified the full manuscript accordingly. In addition, we have added the micro-hardness of the single path coating. The main changes are marked in yellow in the revised version.

The responses to your comments are as follows:

Response to the Second Reviewer's comment NO.1:

The work is solid, but poor in the verification results. I suggest supplementing the article with the results of the hardness and modulus of elasticity of the obtained coatings.

The main topic of the article is the simulation of the influence of the spray nozzle geometry on the quality of the applied coatings. The obtained results were validated by an experiment. Simulation results show that particle impact temperature increases remarkably for a long convergent length, while particle velocity slightly increases, which is validated by experiments. The relationship between the particle impact temperature and practice diameter shows the first increase and then decrease. The experimental results show that as the nozzle convergent section becomes long, the edges of the single-pass deposits become smooth, the width, density, and deposition efficiency of the single-pass deposits increase. The strength of the work is a concise description of the results of the experiment and a summary. The presented drawings are clear and legible.

The value of the work will increase significantly if the results of hardness H and modulus of elasticity are added to the work. Presentation of the results will increase the value of the work and broaden the group of people interested in the practical application of Ti6Al4V in Cold spray technology.

Response: Many thanks for your encouraging comments. We have made up the hardness test experiment. However, due to the limitations of the hardness testing equipment, we only obtained the hardness of the coating, but not the elastic modulus data. We have added the micro-hardness of the coatings. Please see Figure 12 in the revised version.

Response to the Second Reviewer's comment NO.2:

Originality/Novelty:

Research problem posed correctly. The presented results cover current issues.

Significance:

The presented results are correct.

Quality of Presentation:

The article is legible. Data in tables and graphic presentations are presented correctly.

Scientific Soundness:：

Analyzes of the results were carried out on the basis of solid data.

Nevertheless: in Chapter 2. Numerical Modeling and Experimental Methodology the description of the methodology of numerical analyzes was reduced to line 107-111. It is advisable to devote a little more attention to the description of the simulation methodology.

Response: Thank you for your affirmation of our research work. We have added content related to numerical simulation.

Response to the Second Reviewer's comment NO.3:

Section 3.3 focuses on the microstructure of the resulting coatings. The microstructure is an important parameter of the resulting coatings butTi6Al4V coatings are used for their mechanical properties. It is recommended to expand the article with the analysis of hardness H and modulus of elasticity E of the obtained coatings.

Interest to the Readers:

Expanding the work to discuss the results of H and E will significantly increase its value for the readers.

Response: Thanks for your constructive comments. We have added the micro-hardness of the coatings.

Response to the Second Reviewer's comment NO.4:

Overall Merit:

The work presents solid research. The research are typical, but necessary for the proper preparation of devices for industrial applications.

Response: Thank you for your good suggestions. In the future, we are committed to improving the cold spray device so that the device can bear higher pressure and higher temperature to promote the industrialization of cold spray technology.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have provided answers to all questions and have supplied additional microhardness test results and particle kinetic energy values. Their results have not changed. However, I do not agree that scratch tests can be performed on single-pass deposits or that tribological tests cannot be carried out for light loads, but I believe this paper already contains many original contributions, so I recommend its publication in its current form.

Author Response

Dear Assistant Editor Ms. Ana Kovačević and reviewers, 

Thank you for your encouraging comments on our manuscript (Cold Spray Additively Manufacturing of Ti6Al4V: Special Nozzle Design Using Numerical Simulation and Experimental Validation, 1529126). We have carefully modified the full manuscript. The main changes are marked in yellow in the first revised version. Similarly, the main changes are marked in green in this revised version.

The responses to your comments are as follows:

Response to the First Reviewer's comment NO.1:

English language and style are fine/minor spell check required.

Response: Many thanks for your comments. We have carefully revised the English of the full manuscript, correcting errors such as grammar and misuse of words.

Response to the First Reviewer's comment NO.2:

2 The authors have provided answers to all questions and have supplied additional microhardness test results and particle kinetic energy values. Their results have not changed. However, I do not agree that scratch tests can be performed on single-pass deposits or that tribological tests cannot be carried out for light loads, but I believe this paper already contains many original contributions, so I recommend its publication in its current form.

Response: Many thanks for your constructive comments on the manuscript. We have added the micro-hardness of the coatings. Please see Figure 12 in the revised version. In the future, the wider and thicker coating will be fabricated, so that scratch tests and tribological tests can be performed to characterize the performance of the coatings.

Author Response File: Author Response.docx