Effects of Heat Treatment on the Microstructure Evolution and Mechanical Properties of Selective Laser Melted TC4 Titanium Alloy

Round 1

Reviewer 1 Report

• Technical suggestion. The article should be entirely in accordance with the instructions (https://www.mdpi.com/journal/materials/instructions).
• At conclusion add a comparison between the results obtained by you and another previous research.
• Discuss in introduction more about the applications of the studied materials?
• Add chemical analysis of alloys (EDS).
• On Figure 5 a, add the Miller indices.
• What is novel, in theory, in experimental techniques, or a combination of both. Try to highlight this at the end of the Introduction section.
• Indicate the location of the manufacturer of all equipment’s used.
• Analyze and discuss possibilities of practical application.
• In the conclusions, state the scientific contribution, the shortcomings of your methodology and future research.
• Generally, the quality of the writing could be improved.

Author Response

Detailed responses to reviewer No.1 comments

Dear Editor and Reviewer No.1:

Thank you very much for your interest in our paper, and the comments you give are very helpful to improve this manuscript. We have exactly followed your suggestions and revised the manuscript thoroughly according to your comments and requests. We attached the revised manuscript in “Revised Manuscript-Unmarked” for your approval. The revised manuscript, marked with “track changes” feature in “Revised Manuscript-Marked”, was also attached as the supplemental material and for easy check/editing purpose.

We hope you are satisfied with the revised version, however, if there is more question, we are willing to revise it again.

The responses to the reviewer’s comments are as following:

 

Reviewer #1:

Point 1: Technical suggestion. The article should be entirely in accordance with the instructions (https://www.mdpi.com/journal/materials/instructions).

Response 1: Thank you very much for pointing this out. We have made changes according to journal requirements in the revised manuscript.

 

Point 2: At conclusion add a comparison between the results obtained by you and another previous research.

Response 2: Thank you for your pertinent advice. We have added the comparison between the results obtained by this paper and another previous research to the beginning of the Conclusion section.

 

Point 3: Discuss in introduction more about the applications of the studied materials?

Response 3: As Reviewer suggested, the applications of SLM TC4 were written at the end of the second paragraph of the Introduction section.

 

Point 4: Add chemical analysis of alloys (EDS).

Response 4: As Reviewer suggested, the chemical analysis of initial TC4 powder by EDS was illustrated in Fig.2.

 

Point 5: On Figure 5 a, add the Miller indices.

Response 5:

We are very sorry for our negligence of the Miller indices. They were added in Fig.7-(a).

 

Point 6: What is novel, in theory, in experimental techniques, or a combination of both. Try to highlight this at the end of the Introduction section.

Response 6: Considering the Reviewer’s suggestion, we have re-written this part at the end of the Introduction section.

 

Point 7: Indicate the location of the manufacturer of all equipment’s used.

Response 7: We are very sorry for our negligence of the location of the manufacturer of all equipment’s used. We have added them after the type of used equipment.

 

Point 8: Analyze and discuss possibilities of practical application.

Response 8: Thank for Reviewer’s suggestion. The discussion of the possibilities of practical application was written in the end of the Introduction section.

 

Point 9: In the conclusions, state the scientific contribution, the shortcomings of your methodology and future research.

Response 9: Thank for Reviewer’s suggestion. The contributions, shortcomings and future research were written at the end of the Conclusion section.

 

Point 10: Generally, the quality of the writing could be improved.

Response 10: We have tried our best to revise the manuscript, if it still does not meet your requirements, we will be happy to revise it further until it meets your requirements.

 

Special thanks to you for your good comments again.

Reviewer 2 Report

This work investigates the microstructure formation in a TC4 titanium alloy obtained by laser powder bed fusion and post thermal treatments. I highly recommend considering the following points before publication:

 

The diffractograms and their phase-peaks are not properly visible in Fig. 5 a. please provide insets where the peaks can be visualized and compared.

 

The scattering of the hardness data shown in Fig. 6 does not allow establishing a trend of microstructure evolution. The microstructure must be studied in more detail along the building direction and new insights must be provided.

 

„..the most mature technology is Selective Laser Melting (SLM), which melts metal powder layer by layer with a high-power laser beam..“ --- the introduction must include prior references not only about SLM of commercial alloys but also the insights of tailoring alloys to this technique: see eg doi:10.1016/j.apmt.2020.100767 and doi:10.1002/adma.202105096.

 

Please, provide the microstructures for each heat treatment condition using scanning electron microscopy. Those provided in fig. 4 do not allow estabishing the different features between conditions and do not present differences.

 

 

Author Response

Detailed responses to reviewer No.2 comments

Dear Editor and Reviewer No.2:

Thank you very much for your interest in our paper, and the comments you give are very helpful to improve this manuscript. We have exactly followed your suggestions and revised the manuscript thoroughly according to your comments and requests. We attached the revised manuscript in “Revised Manuscript-Unmarked” for your approval. The revised manuscript, marked with “track changes” feature in “Revised Manuscript-Marked”, was also attached as the supplemental material and for easy check/editing purpose.

We hope you are satisfied with the revised version, however, if there is more question, we are willing to revise it again.

The responses to the reviewer’s comments are as following:

 

Reviewer #2: This work investigates the microstructure formation in a TC4 titanium alloy obtained by laser powder bed fusion and post thermal treatments. I highly recommend considering the following points before publication:

Point 1: The diffractograms and their phase-peaks are not properly visible in Fig. 5 a. please provide insets where the peaks can be visualized and compared.

Response 1: Thank you very much for pointing this out. We have added an inset in Fig.7-(b).

 

Point 2: The scattering of the hardness data shown in Fig. 6 does not allow establishing a trend of microstructure evolution. The microstructure must be studied in more detail along the building direction and new insights must be provided.

Response 2: Thank you for your pertinent advice. We are very sorry for our negligence of building direction and our incorrect writing in the relationship between the hardness and the area of samples. We have re-analyzed the relationship between the hardness and the area of samples, and added an explanation to the building direction in the caption of Fig. 3. Besides we added the measurement error to Fig. 3-(b) and Table 3 to show a clear relationship between hardness and heat treatments.

 

Point 3: „..the most mature technology is Selective Laser Melting (SLM), which melts metal powder layer by layer with a high-power laser beam..“ --- the introduction must include prior references not only about SLM of commercial alloys but also the insights of tailoring alloys to this technique: see eg doi:10.1016/j.apmt.2020.100767 and doi:10.1002/adma.202105096.

Response 3: Thanks for Reviewer’s suggestions. We have added prior references such as doi:10.1016/j.apmt.2020.100767 and doi:10.1002/adma.202105096 to this statement.

 

Point 4: Please, provide the microstructures for each heat treatment condition using scanning electron microscopy. Those provided in fig. 4 do not allow estabishing the different features between conditions and do not present differences.

Response 4: Thank you for your constructive suggestions. We have added the SEM tests and multiplied pictures of microstructure to show more details, as illustrated in Figs. 5-7.

 

Special thanks to you for your good comments again.

Reviewer 3 Report

The authors of the work presented study of the Ti-6Al-4V alloy, manufactured by laser powder-bed fusion after different heat treatments. There are too many inaccuracies in the work that the authors should clarify.

Comments for author File: Comments.pdf

Author Response

Detailed responses to reviewer No.3 comments

Dear Editor and Reviewer No.3:

Thank you very much for your interest in our paper, and the comments you give are very helpful to improve this manuscript. We have exactly followed your suggestions and revised the manuscript thoroughly according to your comments and requests. We attached the revised manuscript in “Revised Manuscript-Unmarked” for your approval. The revised manuscript, marked with “track changes” feature in “Revised Manuscript-Marked”, was also attached as the supplemental material and for easy check/editing purpose.

We hope you are satisfied with the revised version, however, if there is more question, we are willing to revise it again.

The responses to the reviewer’s comments are as following:

 

Reviewer #3: The authors of the work presented study of the Ti-6Al-4V alloy, manufactured by laser powder-bed fusion after different heat treatments. There are too many inaccuracies in the work that the authors should clarify.

Point 1: The authors should use the ASTM standard for 3D printing of the materials (not SLM but laser powder-bed fusion).

Response 1: Thank you very much for pointing this out. It is really true as Reviewer suggested that we should use the ASTM standard for the samples. However, due to the limitation of the 3D printing materials’ size, the samples couldn’t meet the ASTM standard size. And the size of samples would only affect the elastic period, which meant it would not have an effect on the results of our research. And regarding to the type of SLM process, thank you for your constructive comments, we have made some changes at the beginning of the Materials and experiments section.

 

Point 2: The name of company city and country should be added to the description of used equipment.

Response 2: Thank you for your pertinent advice. We are very sorry for our negligence of the name of company city and country. We have added them after the type of used equipment.

 

Point 3: What kind of the 3D printer was used for the sample producing? What was the orientation of the sample inside of the 3D printer chamber?

Response 3: We are very sorry for our negligence of the type of the 3D printer and the building orientation. The type of the 3D printer we used for this research was EOS M28, Germany, as included in the first line of the Materials and experiments section. And the building orientation of samples inside of the chamber was added to Table 1, as well as illustrated in Fig.3 to make a better explanation.

 

Point 4: Add please the study of the initial powder (X-ray, microstructure).

Response 4: We agree that more study of the initial powder would be useful to our research on SLM TC4. The microstructure of the initial TC4 powder was illustrated in Fig. 1-(a), and we could also know the distribution of its size in Fig. 1-(b). The XRD result of initial TC4 powder was added to Fig. 7-(a).

 

Point 5: What was the density of the manufactured 3D samples?

Response 5: Thank you very much for pointing this out. The density of the manufactured 3D samples was 4400 kg/m³, as mentioned in the second line of the Materials and experiments section.

 

Point 6: In all microstructure pictures, please, indicate the growth direction of samples in the 3D printer camera, as well as the scale-bar should be increased and put on the pictures. The quality of the pictures should be improved.

Response 6: We are very sorry for our negligence of the growth direction of samples in the 3D printer camera in the pictures. We have added the scale-bar of the growth direction to Figs. 5-7.

 

Point 7: Figure 1 does not correspond to Figure 5(a). According to Figure 5a the X-ray analysis does not show the presence of the beta phase in as-built sample, however, the highest volume fraction of beta phase is pointed in as-built sample (Figure 5b). Check please these results. All literature data indicate the absence of beta phase in the as-built samples manufactured with 3D laser printer.

Response 7: We are very sorry for our incorrect writing of the volume fraction of β phase in the as-built samples. We have made correction according to the XRD analysis with MDI Jade. And actually there was an absence of β phase in the as-built samples, as illustrated in Fig. 9.

 

Point 8: The hardness value is statistical, so Figures 6-7 should be removed, and the measurement error should be added to Tables 3-4; the values should be rounded to an integer value.

Response 8: Thanks for your excellent advice. We have removed the needless pictures of hardness and nanoindentation. And the measurement error was added to Figs. 9-10-(b) and Tables 3-4. The values were rounded to an integer value in Tables 3-4.

 Table 3 Hardness of SLM TC4 under different heat treatments

Sample	Heat treatments	Average hardness/HV
0	/	374±7
1	600℃-2H-AC	360±11
2	600℃-4H-AC	357±5
3	800℃-2H-AC	330±5
4	930℃-1H-WC+600℃-8H-AC	352±6
5	960℃-1H-WC+600℃-2H-AC	359±5
6	960℃-1H-WC+600℃-8H-AC	363±6

 Table 4 Nanoindentation hardness and modulus of elasticity of SLM TC4 under different heat treatments

Sample	Heat treatments	Hardness/HV	Modulus/GPa
0	/	475±66	136±11
1	600℃-2H-AC	459±24	147±7
2	600℃-4H-AC	465±65	157±11
3	800℃-2H-AC	408±25	165±2
4	930℃-1H-WC+600℃-8H-AC	459±13	156±13
5	960℃-1H-WC+600℃-2H-AC	472±20	153±7
6	960℃-1H-WC+600℃-8H-AC	461±7	155±8

 

Point 9: Table 5. The literature data (including the data for conventional Ti-6Al-4V) and the measurement error should be added to values in Table 5 for comparison. Add please the Young’s modulus; the elongation values should be rounded to an integer value.

Response 9: We are very sorry for our negligence of the literature data, measurement error and the Young’s modulus. And they were added to Fig. 12 and Table 5. The elongation values have been rounded to an integer value.

Table 5 Tensile properties of SLM TC4 under different heat treatments

Sample	Heat treatments	Tensile strength /MPa	Elongation /%	Modulus/GPa
0	/	1079±12	19±1	116±4
1	600℃-2H-AC	996±9	17±3	136±8
2	600℃-4H-AC	1006±3	23±4	122±6
3	800℃-2H-AC	991±19	22±2	104±2
4	930℃-1H-WC+600℃-8H-AC	1011±14	17±3	121±11
5	960℃-1H-WC+600℃-2H-AC	1048±8	17±2	124±13
6	960℃-1H-WC+600℃-8H-AC	1044±1	18±0	134±8
Forged	/	975±14	16±2	114±3 [1]

 

Point 10: Because the sample size and the deformation rate were not chosen according to ASTM recommendations, the obtained results of the mechanical test were difficult to compare with the literature data. Please explain high elongation value in the sample 800-2H-AC.

Response 10: It is really true as Reviewer suggested that we should use the ASTM standard for the samples. However, due to the limitation of the 3D printing materials’ size, the samples couldn’t meet the ASTM standard size. And the size of samples would only affect the elastic period, which meant it would not have an effect on the results of our research. And thank you for your constructive comments, we have made some changes to the manuscript. Regarding to the high elongation value in the sample 800-2H-AC, we are very sorry for our incorrect writing, for we had had a misunderstanding of the elongation in the sample 800-2H-AC. We have re-analyzed the raw data of this sample and get the new elongation value according to the tensile curve, updated and added the measurement error to Table 5.

Sample	Heat treatments	Tensile strength /MPa	Elongation /%	Modulus/GPa
3	800-2H-AC	991±19	22±2	104±2

 

Special thanks to you for your good comments again.

 

Reference:

 

[1] B. Wang, L. Cheng, W. Cui, X. Chen, C. Wang, D. Li, Effect of forging process on high cycle and very high cycle fatigue properties of TC4 titanium alloy under three‐point bending, Fatigue & Fracture of Engineering Materials & Structures 44(8) (2021) 2054-2069.

 

Round 2

Reviewer 1 Report

The presented data are original and interesting. The manuscript has been significantly improved and is suitable for publication in the present Journal. 

Author Response

Detailed responses to reviewer No.1 comments

Dear Editor and Reviewer No.1:

Thank you very much for your interest in our paper. It is an honor to have your approval of our work. And thank you for your previous comments to improve the manuscript quality.

Special thanks to you for your good comments again.

Author Response File: Author Response.docx

Reviewer 2 Report

the reviewer's points have been addressed by the authors

Author Response

Detailed responses to reviewer No.2 comments

Dear Editor and Reviewer No.2:

Thank you very much for your interest in our paper. It is an honor to have your approval of our work. And thank you for your previous comments to improve the manuscript quality.

Special thanks to you for your good comments again.

Reviewer 3 Report

Please change the selective laser melting with powder-bed fusion.

Author Response

Detailed responses to reviewer No.3 comments

Dear Editor and Reviewer No.3:

Thank you very much for your interest in our paper, and the comments you give are very helpful to improve this manuscript. We have exactly followed your suggestions and revised the manuscript thoroughly according to your comments and requests. We attached the revised manuscript in “Revised Manuscript-Unmarked” for your approval. The revised manuscript, marked with “track changes” feature in “Revised Manuscript-Marked”, was also attached as the supplemental material and for easy check/editing purpose.

We hope you are satisfied with the revised version, however, if there is more question, we are willing to revise it again.

The responses to the reviewer’s comments are as following:

 

Reviewer #3: Please change the selective laser melting with powder-bed fusion.

Point 1: Please change the selective laser melting with powder-bed fusion.

Response 1: Thank you very much for pointing this out. It is really true as Reviewer suggested that we should change the selective laser melting with powder-bed fusion. We have looked up some descriptions of the process of selective laser melting (SLM) and laser-based powder-bed fusion (LPBF) in the literature [1], where we could know the SLM technology is also known as LPBF. And we have made some explanations to the SLM process with LPBF, where we mentioned the SLM process in the Introduction section for the first time. Besides, if it’s necessary to change all the SLMs with LPBFs, please let us know, and we would be glad to make more changes.

 

Special thanks to you for your good comments again.

 

Reference:

[1] K.G. Prashanth, Selective laser melting: materials and applications, Multidisciplinary Digital Publishing Institute, 2020, p. 13.