Mechanical Property Comparison of Ni–Cr–Mo Alloys Fabricated via One Conventional and Two New Digital Manufacturing Techniques
Round 1
Reviewer 1 Report
The topic chosen by authors sounds interesting and for sure is not enough presented in the literature.
However, the results are merely described and are limited to comparing the experimental observation. It would be helpful if the authors add information about the commercial name of the alloy (or the closest commercial alloy to the chosen one) and the standard properties for the selected alloy (for example, in Table 1).
The level of porosity of the SLM samples suggests that process parameters weren't optimized; therefore, it's not fair to compare the results of the conventional technique with AM.
The authors are encouraged to widen the discussion section and critically discuss the results from their investigation with existing data.
Conclusions must be improved as well.
Author Response
The topic chosen by authors sounds interesting and for sure is not enough presented in the literature.
- We have made every possible effort to improve the revision according to your valuable suggestions and comments.
However, the results are merely described and are limited to comparing the experimental observation. It would be helpful if the authors add information about the commercial name of the alloy (or the closest commercial alloy to the chosen one) and the standard properties for the selected alloy (for example, in Table 1).
- Thank you for your suggestion. We have added some information about the single Ni-Cr-Mo alloy used in our study. However, detailed information (including the standard properties) of the alloy is not available at present and is not disclosed by the manufacturer.
The level of porosity of the SLM samples suggests that process parameters weren't optimized; therefore, it's not fair to compare the results of the conventional technique with AM.
- We agree with your comment. At present, the optimal SLM process parameters for Ni-Cr-Mo alloys are not completely standardized. In our study, despite multiple cave-like pores, the SLM alloy still showed superior mechanical properties and satisfied the ISO 22674 criteria for type 5 materials. This aspect has been further discussed in the revised manuscript, with additional references.
The authors are encouraged to widen the discussion section and critically discuss the results from their investigation with existing data.
- According to your suggestion, we have revised the Discussion section, interpreting the experimental results in more detail.
Conclusions must be improved as well.
- We have also revised the Conclusions section. Once again, thank you very much for all your comments.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper performs a fully experimental study and comparison of the microstructure and mechanical properties of nickel-chromium alloys produced by three different manufacturing processes, i.e., selective laser melting (SLM), soft metal milling (SMM), and conventional lost-wax casting (LWC). The paper is very well written, and many experimental details have been provided. However, to further improve the paper, the following need to be addressed:
- Why the SLM AM process has been selected for comparison purposes and not any other metal-based AM process like electron-beam melting (EBM) or directed energy deposition (DED)? In case there is a reasoning behind this selection, it has to be further clarified in the manuscript.
- What are the advantages and disadvantages of the SLM and SMM techniques compared to the conventional LWC approach? Also, based on the paper conclusion and given that the specimens from all three processes met the required ISO standard as mentioned by the authors, why should one choose SLM or SMM over LWC or vice versa? Moreover, how would the cost of manufacturing be affected? A discussion of the above would be necessary.
- A more detailed description of the 3 processes (SLM, LWC, and SMM) could help the readers to better understand the experimental observations and the different nature of formed pores.
- In the results and discussions section, the authors have mainly presented the results of the experiments and their observations from figures/tables rather than providing useful discussions. A more detailed discussion would be needed to further solidify the contribution of this work.
- The quality of the figures should be improved.
- In subsection 2.1: I think the authors meant layer thickness of 35 microns and not mm? 35 mm seems too large for layer thickness in AM.
- How were the sintering conditions decided in subsection 2.1? is this based on a standard or was it decided by the authors? This is because similar to the selected process parameters, the sintering conditions can affect the microstructures and mechanical properties.
- In Figure 4 and 5 captions, the SMM process has been written as SM and thus is not consistent with the rest of the manuscript.
- In table 1, are the average values presented? Did the authors check for any possible outliers among the 6 studied samples as no discussion on this has been provided?
- At the top of page 5: I think the authors meant to refer to Figure 3 and not Figure 1?
- What are the limitations of this study? The authors are encouraged to add a brief discussion on this in the conclusions section.
Author Response
The paper performs a fully experimental study and comparison of the microstructure and mechanical properties of nickel-chromium alloys produced by three different manufacturing processes, i.e., selective laser melting (SLM), soft metal milling (SMM), and conventional lost-wax casting (LWC). The paper is very well written, and many experimental details have been provided. However, to further improve the paper, the following need to be addressed:
1. Why the SLM AM process has been selected for comparison purposes and not any other metal-based AM process like electron-beam melting (EBM) or directed energy deposition (DED)? In case there is a reasoning behind this selection, it has to be further clarified in the manuscript.
- Thank you for your suggestion. In the fourth paragraph of the Introduction section, we have added some descriptions about several AM methods including EBM and DED. In addition, the merit of SLM over EBM has been addressed in the paragraph.
2. What are the advantages and disadvantages of the SLM and SMM techniques compared to the conventional LWC approach? Also, based on the paper conclusion and given that the specimens from all three processes met the required ISO standard as mentioned by the authors, why should one choose SLM or SMM over LWC or vice versa? Moreover, how would the cost of manufacturing be affected? A discussion of the above would be necessary.
- We have added one paragraph at the end of the Discussion section to compare the three techniques comprehensively, following your suggestion.
3. A more detailed description of the 3 processes (SLM, LWC, and SMM) could help the readers to better understand the experimental observations and the different nature of formed pores.
- The Specimen Preparation subsection has been largely revised to cover the three processes in more detail.
4. In the results and discussions section, the authors have mainly presented the results of the experiments and their observations from figures/tables rather than providing useful discussions. A more detailed discussion would be needed to further solidify the contribution of this work.
- Following your suggestion, we have largely revised the Discussion section, interpreting our results and observations in more detail.
5. The quality of the figures should be improved.
- We have carefully checked all figures again and modified Figs. 1, 2, and 5.
6. In subsection 2.1: I think the authors meant layer thickness of 35 microns and not mm? 35 mm seems too large for layer thickness in AM.
- We have corrected the unit (mm to μm). Thank you for pointing it out.
7. How were the sintering conditions decided in subsection 2.1? is this based on a standard or was it decided by the authors? This is because similar to the selected process parameters, the sintering conditions can affect the microstructures and mechanical properties.
- The sintering conditions of the Ni–Cr–Mo alloy were decided based on those for an SMM Co–Cr alloy in a previous study. This has been added as a reference in the revised text.
8. In Figure 4 and 5 captions, the SMM process has been written as SM and thus is not consistent with the rest of the manuscript.
- We have corrected the mistakes in the two figures (SM to SMM). Thank you.
9. In table 1, are the average values presented? Did the authors check for any possible outliers among the 6 studied samples as no discussion on this has been provided?
- The mechanical property data were examined for the normality of distribution using a Shapiro-Wilks test and the equality of variances using a Levene test. As the data were normally distributed and exhibited equal variances, they were analyzed using a one-way ANOVA and Tukey post hoc test. This has been added in the revised manuscript.
10. At the top of page 5: I think the authors meant to refer to Figure 3 and not Figure 1?
- Thank you pointing it out. The mistake has been corrected.
11. What are the limitations of this study? The authors are encouraged to add a brief discussion on this in the conclusions section.
- The main limitation of this study was that the single Ni–Cr–Mo alloy particle fabricated for SLM was also used for the casting and SMM in order to focus on the different fabrication methods. This has been added to the end of the Discussion section, instead of the Conclusion, considering the flow of context. Once again, thank you very much for all your valuable comments and suggestions to improve the quality of our manuscript.
Author Response File: Author Response.docx
Reviewer 3 Report
Review of the manuscript "Mechanical Property Comparison of Ni – Cr Alloys Fabricated via One Conventional and Two New Digital Manufacturing Techniques" by the authors Kyung Ran Yang, Takao Hanawa, Tae-Yub Kwon, Bong Ki Min and Min-Ho Hong.
In the manuscript submitted for review, a study of the structure and properties of Ni-Cr-Mo alloy castings made by three methods is carried out: SLM, SMM, LWC.
There are comments on the work:
1. In the section material and research methods, the composition of the alloy is indicated - three-component (Ni-Cr-Mo). However, everywhere in the text and in the name, only two components of Ni-Cr appear. Why?
2. Not all characteristics of sample making processes are listed: How was SLM carried out? Powder? What size? At what pressure were samples pressed with SMM?
3. In the microstructure section. It is indicated that the final polishing was carried out on a colloidal suspension of 0.04 mm. This is a very coarse slurry, rather for grinding. Is the size exactly specified?
4. What was the purpose of ultrasonic cleaning?
5. It is not indicated (Fig. 2) in which plane of the workpiece the crossection was made. And how the direction of the pores in the SLM sample relates to the orientation of the sample.
6. In the captions of all figures of microstructures, the magnification and size of the scale bar are indicated. This is unnecessary information, since it is in the drawings themselves.
7. Fig. 3. How do the authors explain the different intensities of the lines of the gamma-phase spectrum for the LWC and SLM samples?
8. Fig. 5. According to X-ray phase analysis, there are two phases only in the SMM sample. However, in Fig. 5, two phases are visible only on the phase map for the SLM sample. Is there only one phase on the map for SMM?
9. In the caption of figure 5 there is a typo - instead of SMM it is written SM.
10. Table 1. The heading states, "Mechanical properties of three different alloys". Why? The alloy is the same. Only the technologies for making samples differ.
11. How were the specimens cut for mechanical testing? Why is there no dip in the mechanical properties in the sample, but does it have colossal porosity?
12. The conclusion section needs to be expanded, supplemented with research results.
In general, I believe that the manuscript requires serious revision.
Author Response
Review of the manuscript "Mechanical Property Comparison of Ni – Cr Alloys Fabricated via One Conventional and Two New Digital Manufacturing Techniques" by the authors Kyung Ran Yang, Takao Hanawa, Tae-Yub Kwon, Bong Ki Min and Min-Ho Hong.
In the manuscript submitted for review, a study of the structure and properties of Ni-Cr-Mo alloy castings made by three methods is carried out: SLM, SMM, LWC.
There are comments on the work:
1. In the section material and research methods, the composition of the alloy is indicated - three-component (Ni-Cr-Mo). However, everywhere in the text and in the name, only two components of Ni-Cr appear. Why?
- Thank you for pointing it out. We agree with you and have used the term “Ni-Cr-Mo alloy” rather than “Ni-Cr alloy” throughout the revised manuscript.
2. Not all characteristics of sample making processes are listed: How was SLM carried out? Powder? What size? At what pressure were samples pressed with SMM?
- We have reviewed and revised the specimen preparation subsection, providing the processes in more detail, despite the limitations that all information was not fully disclosed by the manufacturers.
3. In the microstructure section. It is indicated that the final polishing was carried out on a colloidal suspension of 0.04 mm. This is a very coarse slurry, rather for grinding. Is the size exactly specified?
- This careless mistake has been corrected. Thank you.
4. What was the purpose of ultrasonic cleaning?
- The purpose of the ultrasonic cleaning step has been added.
5. It is not indicated (Fig. 2) in which plane of the workpiece the crossection was made. And how the direction of the pores in the SLM sample relates to the orientation of the sample.
- Thank you for your suggestion. We have indicated the SLM building direction using an arrow in Fig. 2.
6. In the captions of all figures of microstructures, the magnification and size of the scale bar are indicated. This is unnecessary information, since it is in the drawings themselves.
- We agree with you and have modified the figure captions.
7. Fig. 3. How do the authors explain the different intensities of the lines of the gamma-phase spectrum for the LWC and SLM samples?
- We have added the reason within the text.
8. Fig. 5. According to X-ray phase analysis, there are two phases only in the SMM sample. However, in Fig. 5, two phases are visible only on the phase map for the SLM sample. Is there only one phase on the map for SMM?
- Thank you for pointing it out. We have added a higher magnification phase map image clearly showing the two phases of the SMM alloy in Fig. 5. The unit of the scale bars has also been corrected.
9. In the caption of figure 5 there is a typo - instead of SMM it is written SM.
- We have corrected the careless mistakes in Figs 4 and 5. Thank you for pointing it out.
10. Table 1. The heading states, "Mechanical properties of three different alloys". Why? The alloy is the same. Only the technologies for making samples differ.
- The table caption has been modified according to your comment.
11. How were the specimens cut for mechanical testing? Why is there no dip in the mechanical properties in the sample, but does it have colossal porosity?
- For the tensile test, the dumbbell-shaped SLM specimens were prepared in a horizontal direction, and the tension load was applied perpendicular to the building direction (please see Fig. 2(b)). This has been more clearly described in the revised manuscript.
12. The conclusion section needs to be expanded, supplemented with research results.
- We have revised the Conclusions section according to your suggestion.
In general, I believe that the manuscript requires serious revision.
- We highly appreciate all the kind suggestions from the reviewer, which helped to improve our manuscript significantly. We have made every possible effort to make the revisions according to your comments.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Dear Authors,
There are still few issues in the revised version. So please, find details below:
1) Abstract. Line 21 - the single alloy mentioned, then in lines 27 and 30, it is mentioned as three alloys. It seems that readers could be misled.
2) Abstract. Line 32-33, the conclusion that microstructure and mechanical properties are depended on the fabrication method is obvious and well-known.
3) Introduction. Line 60 - DED and PBF are not an option; they are categories. There are seven process categories of AM technologies according to the ISO / ASTM52900 - 15. Standard Terminology for Additive Manufacturing – General Principles – Terminology. Therefore, it is not clear why mentioned only PBF and DED.
4) Introduction. It is not clear why the authors mentioned the difference between the properties of materials produced in SLM and EBM cause those properties are different for a different group of materials.
5) Introduction. Lines 75-76: "...Ni-Cr alloys containing molybdenum". Please, precise, is it different alloys, or is it one single alloy with identical composition. In the title, it was mentioned as Ni-Cr-Mo, and here are Ni-Cr alloys...
6) Specimen Preparation. From the technological point of view (in SLM technology), powder morphology is essential. Please, add the image of powder particles, which were used in the research. Describe the shape of the powder - is it spherical, non-spherical, etc.
7) Mechanical Properties. Line 127, it is not clear how many samples were prepared in each technology for the testing. Please specify.
8) Results and discussion. It would be helpful to evaluate the level of porosity, additionally to the density analysis.
9) Results and discussion. Line 143, the description of porosity in the SLM sample as cave-like, is misleading. It is a lack-of-fusion type of pores.
8) Results and discussion. Figure 2. The red arrow on the image of the SLM sample is misleading; it should be shown as a vector. Moreover, it would be helpful to specify which plane (XY, XZ, or YZ) is presented on the image. It is essential for the description of porosity.
9) Result and discussion. Figure 3. It would be helpful and clearer for readers to put all three XRD spectra on one diagram; therefore, it would make the difference between peaks visible.
10) Common notice. Within the text, authors overuse the words "probably" and "likely", suggesting the lack of confidence in obtained results.
11) Lines 222-224 suggest that obtained Ni-Cr-Mo alloys have potential as new dental materials are an overstatement. The authors did not present at least corrosion test, not even mentioned clinical testing.
12) Conclusions are apparent. The statement that properties depend on the manufacturing method is well-known and does not bring any new valuable change to the research field.
Author Response
Dear Authors,
There are still few issues in the revised version. So please, find details below:
1) Abstract. Line 21 - the single alloy mentioned, then in lines 27 and 30, it is mentioned as three alloys. It seems that readers could be misled.
- Thank you for your comment. We have added the term “resultant,” to avoid any misunderstanding.
2) Abstract. Line 32-33, the conclusion that microstructure and mechanical properties are depended on the fabrication method is obvious and well-known.
- We agree with you and have deleted the last sentence from the Abstract.
3) Introduction. Line 60 - DED and PBF are not an option; they are categories. There are seven process categories of AM technologies according to the ISO / ASTM52900 - 15. Standard Terminology for Additive Manufacturing – General Principles – Terminology. Therefore, it is not clear why mentioned only PBF and DED.
- Thank you for your comment. Thus, before introducing the two metallic AM technologies (DED and PBF), we have added a short sentence explaining that AM technologies have seven categories according to ISO/ASTM 52900 (Page 2, Lines 61-62).
4) Introduction. It is not clear why the authors mentioned the difference between the properties of materials produced in SLM and EBM cause those properties are different for a different group of materials.
- To clarify why we selected SLM, rather than EBM, as an AM approach, we have combined the two paragraphs explaining AM and SM technologies and modified the last sentence.
5) Introduction. Lines 75-76: "...Ni-Cr alloys containing molybdenum". Please, precise, is it different alloys, or is it one single alloy with identical composition. In the title, it was mentioned as Ni-Cr-Mo, and here are Ni-Cr alloys...
- Thank you for pointing this out. The revised manuscript now consistently uses the term “Ni–Cr–Mo alloy” throughout.
6) Specimen Preparation. From the technological point of view (in SLM technology), powder morphology is essential. Please, add the image of powder particles, which were used in the research. Describe the shape of the powder - is it spherical, non-spherical, etc.
- Thank you for your suggestion. A SEM image showing the powder morphology used in our study has been added (see Figure 1). The term “spherical,” indicating the powder shape, has also been added in the revised text (Page 2, Line 88).
7) Mechanical Properties. Line 127, it is not clear how many samples were prepared in each technology for the testing. Please specify.
- The sample number has been more clearly expressed following your suggestion (Page 3, Line 134).
8) Results and discussion. It would be helpful to evaluate the level of porosity, additionally to the density analysis.
- In the present study, μCT images were used to estimate the overall trend of the internal porosity of the alloys. Since no protocols have yet been established for precisely calculating the porosity and density of an alloy from a μCT image, we obtained the alloy densities using the Archimedes method.
9) Results and discussion. Line 143, the description of porosity in the SLM sample as cave-like, is misleading. It is a lack-of-fusion type of pores.
- Thank you for pointing this out. We agree, and have modified the paragraph following your comment (Page 4, Lines 152-157).
8) Results and discussion. Figure 2. The red arrow on the image of the SLM sample is misleading; it should be shown as a vector. Moreover, it would be helpful to specify which plane (XY, XZ, or YZ) is presented on the image. It is essential for the description of porosity.
- Following your comments, we tried to find the best way to indicate the directions appropriately. Thus, the SLM building direction has been indicated in relation to the XYZ axis in this revision (see Figure 1(b)).
9) Result and discussion. Figure 3. It would be helpful and clearer for readers to put all three XRD spectra on one diagram; therefore, it would make the difference between peaks visible.
- We agree, and have modified the figure to include all three spectra in one diagram. In addition, we have added the shift of the (111) plane in the SMM alloy.
10) Common notice. Within the text, authors overuse the words "probably" and "likely", suggesting the lack of confidence in obtained results.
- Thank you for pointing this out. We have eliminated these two terms throughout the text.
11) Lines 222-224 suggest that obtained Ni-Cr-Mo alloys have potential as new dental materials are an overstatement. The authors did not present at least corrosion test, not even mentioned clinical testing.
- We agree, and have further revised the last two paragraphs of the Results and discussion section to state the limitations of this study more clearly.
12) Conclusions are apparent. The statement that properties depend on the manufacturing method is well-known and does not bring any new valuable change to the research field.
- We have deleted this sentence from the Conclusions section to match the Abstract. Once again, thank you very much for all your valuable comments and suggestions to improve the quality of our manuscript.
Author Response File: Author Response.docx
Reviewer 2 Report
The authors have put forward efforts to improve the manuscript, therefore the paper can be considered for publication after minor revisions, i.e., after better highlighting the contributions of the work and adding better discussions on the experiment outcomes and their implications for engineers, manufacturers, or healthcare professionals in the field of dentistry.
Author Response
The authors have put forward efforts to improve the manuscript, therefore the paper can be considered for publication after minor revisions, i.e., after better highlighting the contributions of the work and adding better discussions on the experiment outcomes and their implications for engineers, manufacturers, or healthcare professionals in the field of dentistry.
- We appreciate your valuable suggestion to improve our manuscript. According to your comments, we have further revised the Results and Discussion sections and added some references. The Conclusions section has also been modified.
Reviewer 3 Report
I believe that the authors have significantly improved the content of the manuscript. In this form, it can be washed published.
Author Response
I believe that the authors have significantly improved the content of the manuscript. In this form, it can be washed published.
- Thank you very much for your encouraging comment. Once again, thank you very much for all your valuable comments and suggestions.
