Effects of Gas Pressure during Electron Beam Energy Deposition in the EBM Additive Manufacturing Process

Round 1

Reviewer 1 Report

Good approach, however there are few points to improve. 

ARCAM MACHINES ARE VERY EXPENSIVE…ok, however the paper must address real new knowledge. The relationship between melt pool morphology (MPM) and grain size can be understood from the cooling rate. The classical theory of melting and solidification of metals defines that if the energy density (VED) increases, the volume of melted material(MPM) is greater, decreasing the cooling rate, which reduces the number of particles in the nucleation process, the development of constitutional undercooling is affected and the size of the grains increases. In your case, did you monitor the melting pool size and depth? 

In an EBM process, a pre-determined and generic pressure is usually applied for all kinds of materials printed by the Arcam Q20+ machine. Increasing the gas pressure will  help to increase the vapor pressure in the built chamber, which could reduce the volatility of the solid and melt pool. It is nice, I am sure how much is new or not, perhaps it would be better to describe the conclusions as points: each one aimed at a new contribution. 

Gas is also key in LMD and in SLM. It is nice to see the annex, useful for others. This is not only happening in EBM but in the related and coarse application of LMD (see DOI 10.1007/s00170014-6057-3 ) in which along with feed and five-axis problems, some ideas about the material structure is given. Also people of laser are studying complex problems in modelling. When process change direction machine and EBM must be linked, as it happen in laser processes as well, see and include above idea. 

Last reference is weird. Is it the machine manual, perhaps you can eliminate? 

Author Response

Dear reviewer, thank you for your comments, addressed below (in green).

Reviewer 1

Good approach, however there are few points to improve.

ARCAM MACHINES ARE VERY EXPENSIVE…ok, however the paper must address real new knowledge. The relationship between melt pool morphology (MPM) and grain size can be understood from the cooling rate. The classical theory of melting and solidification of metals defines that if the energy density (VED) increases, the volume of melted material (MPM) is greater, decreasing the cooling rate, which reduces the number of particles in the nucleation process, the development of constitutional undercooling is affected and the size of the grains increases. In your case, did you monitor the melting pool size and depth?

This is an important issue to be addressed in future work. Changing cooling rates will affect the microstructure and eventually the mechanical properties of the AM samples. In this current work, we did not monitor melting pool size and depth, since the focus was on the macro heating effect, measured by local thermocouples. We intend to incorporate an IR camera in the foreseeable future for this exact purpose.

In an EBM process, a pre-determined and generic pressure is usually applied for all kinds of materials printed by the Arcam Q20+ machine. Increasing the gas pressure will help to increase the vapor pressure in the built chamber, which could reduce the volatility of the solid and melt pool. It is nice, I am sure how much is new or not, perhaps it would be better to describe the conclusions as points: each one aimed at a new contribution.

Done (line 301-308)

Gas is also key in LMD and in SLM. It is nice to see the annex, useful for others. This is not only happening in EBM but in the related and coarse application of LMD (see DOI 10.1007/s00170- 014-6057-3 )  in which along with feed and five-axis problems, some ideas about the material structure is given. Also people of laser are studying complex problems in modelling. When process change direction machine and EBM must be linked, as it happen in laser processes as well, see and include above idea.

Done (line 46-48)

Last reference is weird. Is it the machine manual, perhaps you can eliminate?

Done

Author Response File: Author Response.pdf

Reviewer 2 Report

The article is written correctly. The scientific challenge was not ambitious, but thorough research is interesting and its results are interesting for the reader. 

Author Response

Dear reviewer, Thank you for your kind comments.

Reviewer 3 Report

The authors present a new way to calculate the energy deposition of electrons to a metal alloy using a Monte Carlo code. The calculations of the electrons’ energy deposition efficiency to the metal alloy are very important for the electron beam processes. This paper revealed that the increasing gas pressure in the vacuum chamber by one order of magnitude did not significantly reduce energy deposition efficiency. There are some un-clear questions or some suggestions as follow:

• In the part “Motivation”, there is no explanation that how many influence factors will be considered in this investigation and how important are those factors.
• I believe there are outdated sources in the list of publications. For a highly rated journal Metals, there should be only up-to-date sources. Therefore, it is necessary to replace references before 2000 with modern ones, with the corresponding correction of the text of the article.
• Authors may use the following publications as replacements for references and to improve the introduction: DOI: 1088/2053-1591/aaec1f; 10.1021/acs.langmuir.0c01912; 10.4028/www.scientific.net/MSF.992.940
• As for function (1), each symbol with the annotation in the paragraph is not consistent with the function. And the advantages of the Monte Carlo simulations should be given in this part.
• About the discussion of energy deposition with different materials, should the factor of differences in materials be considered?
• The chemical contents of each material before and after treatments is better to be listed in the table.
• In the analysis of gas pressure, there is no clear relationship between gas pressure and energy deposition efficiency.

Author Response

Response to Reviewer 3 Comments

Dear reviewer, thank you for your comments, all addressed below in green. 

Reviewer 3 comments
The authors present a new way to calculate the energy deposition of electrons to a metal alloy using a Monte Carlo code. The calculations of the electrons’ energy deposition efficiency to the metal alloy are very important for the electron beam processes. This paper revealed that the increasing gas pressure in the vacuum chamber by one order of magnitude did not significantly reduce energy deposition efficiency. There are some un-clear questions or some suggestions as follow:

• In the part “Motivation”, there is no explanation that how many influence factors will be considered in this investigation and how important are those factors.

Done, please see lines 123-128.

• I believe there are outdated sources in the list of publications. For a highly rated journal Metals, there should be only up-to-date sources. Therefore, it is necessary to replace references before 2000 with modern ones, with the corresponding correction of the text of the article.

We have removed the 1977’s reference (it used to be reference # 33). Since the Al volatility was explicitly discussed in this paper, we suggest keeping ANL-75-55 (1975) article.

• Authors may use the following publications as replacements for references and to improve the introduction: DOI: 1088/2053-1591/aaec1f; 10.1021/acs.langmuir.0c01912; 10.4028/www.scientific.net/MSF.992.940

Done

• As for function (1), each symbol with the annotation in the paragraph is not consistent with the function. And the advantages of the Monte Carlo simulations should be given in this part.

Done

• About the discussion of energy deposition with different materials, should the factor of differences in materials be considered?

Agreed, we revised the article (lines 153-154)

• The chemical contents of each material before and after treatments is better to be listed in the table.

Indeed, line 123 refers to Appendix B, which is listed as a table.

• In the analysis of gas pressure, there is no clear relationship between gas pressure and energy deposition efficiency.

Done. Explained in (line 223-226)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It is a good versio

Reviewer 3 Report

Authors have made all corrections. No new comments.