Crystallization and Hardness Change of the Ti-Based Bulk Metallic Glass Manufactured by a Laser Powder Bed Fusion Process
Round 1
Reviewer 1 Report
The manuscript by Jang et al. reports on the crystallization behavior and hardness of a Ti-based bulk metallic glass synthesized by laser powder bed fusion at varying process conditions. In order to justify publication, more data must be incorporated. Therefore, a major revision is necessary to recommend publication.
The present submission needs severe language editing and more references must be listed. I have not seen a submission with only nine references, so far.
Furthermore, following requirements should be fulfilled, so that it can be published:
(1) The title is not correct. Please remove the word “alloy”. Furthermore, please use the terms according to ISO/ASTM nomenclature. Instead of SLM, you should use “laser powder bed fusion” (LPBF). Replace “mechanical properties” by “hardness”.
(2) I suggest to add “additive manufacturing” to the list of key words
(3) Introduction, first sentence: “BMG (Bulk Metallic Glass) alloys are an amorphous alloy…” This is not correct. A material or structure can be amorphous, but not an alloy. The respective alloy is termed “glass-forming alloy”.
(4) Please improve the introduction and cite the most important publications of the last years starting with Pauly et al. Materials Today 16 (1-2), 37-41, 2013. I expect at least 35 references. Following review was recently published on LPBF of glass-forming alloys: Zhang et al. Mater Sci Eng R 145, 100625 2021. The authors are encouraged to read this review.
(5) Quantify particle size distribution. Give D50- and D90-values for powder after sieving, if possible.
(6) The big advantage of LPBF or other techniques is that only small powder volumes are melted and subsequently solidified at very high rates entailing vitrification. Thus, very large BMGs can be produced, since the cooling rate is decoupled from specimen geometry. This is not the case for conventional preparation methods like casting. Overcoming the size restriction, is the strong selling point in using LPBF or other AM-techniques for the synthesis of BMGs. This aspect is not present in the introduction and must be added.
(7) Except of microhardness no mechanical properties determined from uniaxial loading in compression or tension were determined. Please then remove mechanical properties from the title and subtitle 3.2.
(8) In order to quantify the amorphous volume fraction of the specimens synthesized by LPBF, isochronal DSC measurements must be carried out for them as well as from the respective cast material which should serve as reference value. Please also show a DSC curve as well as a XRD pattern of the sieved powder. The volume fraction of the amorphous phase can be easily determined from the crystallization enthalpy.
Author Response
Comments and Suggestions for Authors:
- The revised manuscript can be improved, compared to its original version. We appreciated that the manuscript has been improved by the Reviewers’ comments.
(1) The title is not correct. Please remove the word “alloy”. Furthermore, please use the terms according to ISO/ASTM nomenclature. Instead of SLM, you should use “laser powder bed fusion” (LPBF). Replace “mechanical properties” by “hardness”.
- According to the Reviewer’s comment, we revised the wrong expressions and modified the manuscript. Thank you for your kind comments.
(2) I suggest to add “additive manufacturing” to the list of key words
- According to the Reviewer’s comment, we added the keyword of ‘additive manufacturing’.
(3) Introduction, first sentence: “BMG (Bulk Metallic Glass) alloys are an amorphous alloy…” This is not correct. A material or structure can be amorphous, but not an alloy. The respective alloy is termed “glass-forming alloy”.
- According to the Reviewer’s comment, we removed the ‘alloy’ expression and revised the manuscript.
(4) Please improve the introduction and cite the most important publications of the last years starting with Pauly et al. Materials Today 16 (1-2), 37-41, 2013. I expect at least 35 references. Following review was recently published on LPBF of glass-forming alloys: Zhang et al. Mater Sci Eng R 145, 100625 2021. The authors are encouraged to read this review.
- According to the Reviewer’s comment, we improved the introduction part, cited more and recent references (total 36 references).
(5) Quantify particle size distribution. Give D50- and D90-values for powder after sieving, if possible.
- According to the Reviewer’s comment, we revised the manuscript. D50- and D90-values for powder after sieving, are 25 ㎛ and 40㎛, respectively.
(6) The big advantage of LPBF or other techniques is that only small powder volumes are melted and subsequently solidified at very high rates entailing vitrification. Thus, very large BMGs can be produced, since the cooling rate is decoupled from specimen geometry. This is not the case for conventional preparation methods like casting. Overcoming the size restriction, is the strong selling point in using LPBF or other AM-techniques for the synthesis of BMGs. This aspect is not present in the introduction and must be added.
- According to the Reviewer’s comment, we revised the introduction of the manuscript. Thank you for your kind comments.
(7) Except of microhardness no mechanical properties determined from uniaxial loading in compression or tension were determined. Please then remove mechanical properties from the title and subtitle 3.2.
- According to the Reviewer’s comment, we revised the manuscript.
(8) In order to quantify the amorphous volume fraction of the specimens synthesized by LPBF, isochronal DSC measurements must be carried out for them as well as from the respective cast material which should serve as reference value. Please also show a DSC curve as well as a XRD pattern of the sieved powder. The volume fraction of the amorphous phase can be easily determined from the crystallization enthalpy.
- Ti-2.5Zr-5.0Hf-37.5Cu-7.5Ni-1.0Si-5.0Sn BMG is a well-known amorphous material and its BMG powder with sphere shape was successfully manufactured and prepared using this material in this study. In the case of LPBF additive manufacturing using these BMG powders, we tried to confirm that internal defects could be minimized, amorphous maintenance or some crystallization could occur depending on the LPBF process conditions. When the crystallization behavior occurs, the hardness which is the mechanical characteristic of the LPBF additive manufacturing part, can be changed.
It is intended to provide basic research data for the optimal process conditions for LPBF AM of Ti-based BMG powders. Accordingly, as can be seen in Fig.3(a)-(c), we hope that the Reviewer will understand that the crystallization intensity of CuTi and Ti3Cu4 crystalline phases is evaluated and analyzed. The correlation between crystallization and hardness property was evaluated by the microstructure observation, XRD analysis, and micro-hardness measurement for the specimens manufactured under different conditions.
* According to the Reviewer’s comment, we revised it and improved the quality of paper. Thanks for your helpful comments.
Reviewer 2 Report
Dear Editor: I would like to express my deep thanks for inviting me to review the manuscript ID: metals-1273996
Title: Crystallization and Mechanical Properties of the Ti-based Bulk Metallic Glass Alloy Manufactured by SLM Additive Manufacturing
Authors: Ji-Hoon Jang, Hyung-Guin Kim, Hwi-Jun Kim and Dong-Geun Lee
Comments:
Abstract:
Please rewrite the abstract according to the experimental results.
Introduction part:
Please discuss in novelty and aim of this work.
Materials and Methods:
Please mention how many samples used for hardness test. Explain in detail the hardness test.
Results and discussion:
- Figure 2 is not clear and provide clear images.
- Please provide high resolution xrd profile that can provide clear information.
- Figure 3 (b and d) does not provide any formation. Please mention how many samples are tested for calculation the peak intensity.
- Please add the standard deviation in Figure 4.
- It is important to correlate the structure and mechanical properties. Throughout the manuscript does not have any clear investigation. Please provide clear SEM, EBSD and TEM images
Conclusion
Please write conclusion in bullet point
RECOMMENDATION
After reviewing the enclosed manuscript for “Metals”, the present manuscript contains some kinds of scientific analysis but it is mandatory required to modify according to the preceding remarks. So, the manuscript can be accepted for publication after major revisions have been made.
Author Response
Comments and Suggestions for Authors:
- The revised manuscript can be improved, compared to its original version. We appreciated that the manuscript has been improved by the Reviewers’ comments.
Abstract:
Please rewrite the abstract according to the experimental results.
- According to the Reviewer’s comment, we revised the abstract. Thank you for your kind comments.
Introduction part:
Please discuss in novelty and aim of this work.
- According to the Reviewer’s comment, we revised the introduction and discussed the aim of this work.
Materials and Methods:
Please mention how many samples used for hardness test. Explain in detail the hardness test.
- According to the Reviewer’s comment, we modified the hardness testing method.
(~ a micro-Vickers hardness tester (HM-200, Mitutoyo, Takatsu, Japan) was used to evaluate the hardness and to verify crystallized region according to the process conditions. The hardness properties were evaluated by pressing at the conditions of load 200 gf, keeping time 10 sec, and 11 points from the center to the side of the fabricated specimens were tested three times. And the mean was calculated excluding the maximum and minimum values ~)
Results and discussion:
Figure 2 is not clear and provide clear images.
- According to the Reviewer’s comment, we provided much clearer Figure 2.
Please provide high resolution xrd profile that can provide clear information.
- According to the Reviewer’s comment, we provided high resolution XRD profile in Figure 3.
Figure 3 (b and d) does not provide any formation. Please mention how many samples are tested for calculation the peak intensity.
XRD tests of the fabricated specimens were conducted about three samples each condition.
Please add the standard deviation in Figure 4.
- According to the Reviewer’s comment, we added the standard deviation in Figure 4.
It is important to correlate the structure and mechanical properties. Throughout the manuscript does not have any clear investigation. Please provide clear SEM, EBSD and TEM images
- According to the Reviewer’s comment, we revised the manuscript. Thanks for your comment.
In this study, a basic study was conducted on the correlation between the microstructure change of Ti-BMG sample and its mechanical properties (hardness change) during LPBF additive manufacturing. We tried to convey these research results simply and clearly, so please understand that the analysis was conducted based on the microstructure observation, XRD and micro-hardness analysis for the specimens manufactured under various LPBF processing conditions.
(~ The hardness values in the dark region have decreased due to the crystallization of the BMG powder because of the refusion and re-solidification by the laser output of the LPBF process. Based on these results, it was verified that the dark region was consistent with the microstructural analysis, which predicts that the dark region would be a crystallized region in the BMG. The hardness values in the region of crystallization can be apt to be lower than the non-crystallized region ~)
Conclusion
Please write conclusion in bullet point
- According to the Reviewer’s comment, we modified the manuscript.
RECOMMENDATION
After reviewing the enclosed manuscript for “Metals”, the present manuscript contains some kinds of scientific analysis but it is mandatory required to modify according to the preceding remarks. So, the manuscript can be accepted for publication after major revisions have been made.
* According to the Reviewer’s comment, we revised it and improved the quality of paper. Thanks for your helpful comments.
Round 2
Reviewer 1 Report
The authors significantly enhanced the manuscript which still must undergo minor revision, based on the following comments:
(1) Now the reference list consists of an adequate number of references. I am still missing following reference which reports for the first time of additively manufactured Ti-based SLM BMGs:
Deng, L. et al, Selective laser melting of a Ti-based bulk metallic glass, Materials Letters 212 (2018), pp. 346-349.
Besides the introduction is more convincing now. One further remark on the first sentence of the introduction "Amorphous BMGs (Bulk Metallic Glasses) are solidified without crystallization even in slow cooling after melting". Bulk metallic glasses are always amorphous. Therefore, I suggest to remove the word "amorphous".
(2) I would have been happy to see DSC curves or results obtained from them. However, I can understand that they could not be carried out in time. Therefore, they are not mandatory.
(3) The XRD results are still very disapponting. One cannot recognize the broad maxima typical for BMGs. Furthermore, I have the impression to see artefacts in each pattern resulting from the sample holder. One cannot clearly identify crystalline phases based on only one reflection when no complementary results from e.g. EDXS analysis are presented. Instead of the absolute values of intensity, intensity ratios should be used in the discussion.
(4) I highly welcome that the term mechanical properties was replaced by only hardness.
For publication, I expect the authors to incorporte the literature reference listed in (1). Furthermore, they must present XRD patterns free from artefacts and use the intensity ratio for discussion.
Author Response
The authors significantly enhanced the manuscript which still must undergo minor revision, based on the following comments:
- We appreciate that the manuscript has been improved by the Reviewer’s comments.
(1) Now the reference list consists of an adequate number of references. I am still missing following reference which reports for the first time of additively manufactured Ti-based SLM BMGs:
Deng, L. et al, Selective laser melting of a Ti-based bulk metallic glass, Materials Letters 212 (2018), pp. 346-349.
- We added the above reference as [16] Ref. Thank you for your kind comment.
Besides the introduction is more convincing now. One further remark on the first sentence of the introduction "Amorphous BMGs (Bulk Metallic Glasses) are solidified without crystallization even in slow cooling after melting". Bulk metallic glasses are always amorphous. Therefore, I suggest to remove the word "amorphous".
- According to the Reviewer’s comment, we removed and revised the expressions.
(2) I would have been happy to see DSC curves or results obtained from them. However, I can understand that they could not be carried out in time. Therefore, they are not mandatory.
- We appreciate for the Reviewer’s understanding. We agree with your comment.
(3) The XRD results are still very disapponting. One cannot recognize the broad maxima typical for BMGs. Furthermore, I have the impression to see artefacts in each pattern resulting from the sample holder. One cannot clearly identify crystalline phases based on only one reflection when no complementary results from e.g. EDXS analysis are presented. Instead of the absolute values of intensity, intensity ratios should be used in the discussion.
- According to the Reviewer’s comment, we inserted the additional analysis results in the discussion part and revised the manuscript.
(4) I highly welcome that the term mechanical properties was replaced by only hardness.
- Thank you for your kind comment.
For publication, I expect the authors to incorporte the literature reference listed in (1). Furthermore, they must present XRD patterns free from artefacts and use the intensity ratio for discussion.
* According to the Reviewer’s comment, we improved the quality of this paper. Thanks for your helpful comments.
Reviewer 2 Report
Authors addressed all of my comment in the revised manuscript.
Author Response
Comments and Suggestions for Authors:
Authors addressed all of my comment in the revised manuscript.
- We appreciate that the manuscript has been improved by the Reviewer’s comments.
