Investigation on Ti-6Al-4V Microstructure Evolution in Selective Laser Melting

Round 1

Reviewer 1 Report

Dear authors,

 thanks for this interesting contribution. The research is well presented and written (only a couple of typos but nothing major). The research has its merits and i am recommending for publication after addressing a couple of minor issues:

please add more references in the introduction to relevant researchers present in the literature; please explain why we see the discontinuities of the temperature gradient at the junction between powders and substrate in Figure 6. Aren't powders modelled as a continuum with porosity? I suggest to remove the interpolation in Figure 7. I can notice no obvious changes in slope - so i would delete those remarks too. Figure 11: are these dendrites observed at the same time after the start of the solidification? I suggest to include videos to show the entire predicted solidification.

Author Response

Response to Reviewer 1 Comments

Thanks for this interesting contribution. The research is well presented and written (only a couple of typos but nothing major). The research has its merits and i am recommending for publication after addressing a couple of minor issues::

Point 1: please add more references in the introduction to relevant researchers present in the literature.

Response 1: I have added three references in the third paragraph of the introduction to describe the development of titanium alloy produced by additive manufacturing technology.

Razavi, S.-M.-J.; Ferro, P.; Berto, F. Fatigue assessment of ti–6al–4v circular notched specimens produced by selective laser melting. Metals 2017, 7, 291.

Zhang, P.; He, A.N.; Liu, F.; Zhang, K.; Jiang, J.; Zhang, D.Z. Evaluation of low cycle fatigue performance of selective laser melted titanium alloy ti–6al–4v. Metals 2019, 9, 1041.

Salsi, E.; Chiumenti, M.; Cervera, M. Modeling of microstructure evolution of ti6al4v for additive manufacturing. Metals 2018, 8, 633.

Point 2: please explain why we see the discontinuities of the temperature gradient at the junction between powders and substrate in Figure 6. Aren't powders modelled as a continuum with porosity ? 

Response 2: The thermal conductivity between the powder bed and the substrate is very different, even the thermal conductivity of the powder can be simply estimated as 1% of that of the same material substrate. At the junction of the powder bed, the substrate and the molten pool, the temperature is high (melting point). Due to the huge difference of the thermal conductivity, a significant temperature gradient drop occurs in the very small area(one or several grids). Limited by the display size of Fig. 6, it is difficult to display the temperature gradient drop directly. In fact, the temperature gradient of simulation results is continuous.

Point 3: I suggest to remove the interpolation in Figure 7. I can notice no obvious changes in slope - so i would delete those remarks too.

Response 3: Thank you for the suggestion, I have removed the corresponding content about the slope.

Point 4: Figure 11: are these dendrites observed at the same time after the start of the solidification? I suggest to include videos to show the entire predicted solidification.

Response 4: I am sorry that it is difficult to observe the dendrites formed in the early stage of solidification due to the excessive cooling rate during additive manufacturing. I used 10 time nodes to extract data in my calculation, the main results have been shown in the paper. Due to the small number of frames, it may not be suitable for videos.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript gives a very good account of the solidification phenomena that takes place during Selective Laser Melting of Ti6Al4V alloy. It is well written and explains with sufficient detail all the aspects related to how the simulation of the process is carried out with adequately chosen Finite Elements. There are minor corrections needed, particularly where the text has been highlighted in the enclosed, corrected pdf. In addition, there are two comments that are suggested for the authors to be addressed, in order to make the paper even more complete. For these reasons, and because the paper will make a very good contribution to this field of study and technology, I recommend it for publication after very minor revision.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 2 Comments

This manuscript gives a very good account of the solidification phenomena that takes place during Selective Laser Melting of Ti6Al4V alloy. It is well written and explains with sufficient detail all the aspects related to how the simulation of the process is carried out with adequately chosen Finite Elements. There are minor corrections needed, particularly where the text has been highlighted in the enclosed, corrected pdf. In addition, there are two comments that are suggested for the authors to be addressed, in order to make the paper even more complete. For these reasons, and because the paper will make a very good contribution to this field of study and technology, I recommend it for publication after very minor revision:

Point 1: What about thermal diffusivity? How relevant is it? Please comment if an effect is expected within the model results, or if it may be neglected.

Response 1: In this study, heat conduction plays a leading role, and heat convection and diffusion occur on the upper surface of the molten pool, so the effect on the solidification behavior at the bottom of the molten pool can be ignored. So when it comes to thermal conductivity, only the thermal conductivity of heat conduction is considered. This method has also been used in the following references. Some modifications have been made to the description in this paper, changing the ‘thermal conductivity of the heat flux’ to the ‘laser heat source’.  

Gong, X.; Chou, K. Phase-Field Modeling of Microstructure Evolution in Electron Beam Additive Manufacturing. JOM - Journal of the Minerals, Metals and Materials Society 2015, 67, 1176-1182.

Point 2: Please specify what is "Too Large" for the scanning speed. Is there a value of energy density when the model starts to deviate or loose accuracy?

Response 2: This conclusion is based on the experience of many simulations, but the energy density can not be accurately calculated by software. I decided to delete this statement due to the rigor of the paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper studies the effect of the selective laser melting (SLM) on the microstructure evolution of Ti-6Al-4V titanium alloy. The research is interesting. However, to be accepted for publication the following comments need to be addressed

The proficiency of the language needs a more improvement in the manuscript. The introduction section needs to be improved by citing new and related articles of the current journal. The specialized word should be unified. Such as "Figure" and "Fig" (some but not all). Please read the journal guide for authors. The author is recommended to move the part of research content (Fig. 1) from introduction to the methodology part. In introduction you should clearly state the problem then the main aims of your research. In Fig. 9, the actual dendritic morphology not clear by the SEM you can use a color etching for titanium alloys (for example Weck’s Reagent) and use a polarized light optical microscopy. To prove your claim, only one experiment not enough. you should make more experiments at different scanning speeds, then compare the results with the simulated ones on the bases of the size and shape of the dendrites. The conclusions section should be clear and concise.

Author Response

Response to Reviewer 3 Comments

This paper studies the effect of the selective laser melting (SLM) on the microstructure evolution of Ti-6Al-4V titanium alloy. The research is interesting. However, to be accepted for publication the following comments need to be addressed:

Point 1: The proficiency of the language needs a more improvement in the manuscript.

Response 1: I have revised the grammar and tense in the manuscript and some places where the descriptions are repeated. Please refer to the revision location marked in the manuscript for details.

Point 2: The introduction section needs to be improved by citing new and related articles of the current journal.

Response 2: I have added three references in the third paragraph of the introduction to describe the development of titanium alloy produced by additive manufacturing technology.

Razavi, S.-M.-J.; Ferro, P.; Berto, F. Fatigue assessment of ti–6al–4v circular notched specimens produced by selective laser melting. Metals 2017, 7, 291.

Zhang, P.; He, A.N.; Liu, F.; Zhang, K.; Jiang, J.; Zhang, D.Z. Evaluation of low cycle fatigue performance of selective laser melted titanium alloy ti–6al–4v. Metals 2019, 9, 1041.

Salsi, E.; Chiumenti, M.; Cervera, M. Modeling of microstructure evolution of ti6al4v for additive manufacturing. Metals 2018, 8, 633.

Point 3: The specialized word should be unified. Such as "Figure" and "Fig" (some but not all).

Response 3：According to your requirements, I have unified the corresponding description in the article. Please refer to the position marked in the manuscript for details.

Point 4: Please read the journal guide for authors. The author is recommended to move the part of research content (Fig. 1) from introduction to the methodology part. In introduction you should clearly state the problem then the main aims of your research.

Response 4:  I have moved Figure 1 and the corresponding description to methodology part as you requested.

Point 5: In Fig. 9, the actual dendritic morphology not clear by the SEM you can use a color etching for titanium alloys (for example Weck’s Reagent) and use a polarized light optical microscopy.

Response 5: As you can see, the dendrite structure under SEM is not very clear. I tried to deal with it with Mipar software, but there are many very small acicular martensite phases in the structure, which is difficult to accurately calibrate even with EBSD. Whether ordinary metallographic photos can be used, as shown in the figure.

 Point 6: To prove your claim, only one experiment not enough. you should make more experiments at different scanning speeds, then compare the results with the simulated ones on the bases of the size and shape of the dendrites.

Response 6: However, this paper mainly focuses on simulation. Through macro finite element simulation and micro phase field simulation, we try to bring the influence of macro parameters into the microstructure. The experiment is mainly to prove that SLM can obtain columnar dendrites. As for the experiment of columnar  dendrites obtained under different scanning speeds, another article may be needed to elaborate.

Point 7: The conclusions section should be clear and concise.

Response 7:According to your request, I will delete the part about the research content in the conclusion, and only retain the final research results.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Thank you for taking my comments in your considerations.