Rheological Behavior of Inconel 718 Powder for Electron-Beam Melting
Round 1
Reviewer 1 Report
In this study, the authors compare in detail the properties of the two powder materials in many aspects. It needs a minor revise below:
1 The authors need to refer to some papers in the first paragraph when you introduce IN718 superalloy.
2 The authors are suggested to verify the presence of Al2O3 on the powder surface, i.e., Figures 2 and 3.
3 The authors are suggested to discuss or predict the printing quality using virgin and reused powder during PBF-EB even the there is no printing work in this study.
Author Response
Dear reviewer,
The authors would like to thank you for the constructive feedback that has improved the article's quality. Please find below the specific comments addressed with the changes made to the manuscript.
1 - The authors need to refer to some papers in the first paragraph when you introduce IN718 superalloy.
Answer: The authors added the following studies when introducing IN718 superalloy:
[1] J. E. Matz and T. W. Eagar, “Carbide formation in Alloy 718 during electron-beam solid freeform fabrication,” Metall. Mater. Trans. A Phys. Metall. Mater. Sci., vol. 33, no. 8, pp. 2559–2567, 2002, doi: 10.1007/s11661-002-0376-y.
[2] H. E. Helmer, C. Körner, and R. F. Singer, “Additive manufacturing of nickel-based superalloy Inconel 718 by selective electron beam melting: Processing window and microstructure,” J. Mater. Res., vol. 29, no. 17, pp. 1987–1996, 2014, doi: 10.1557/jmr.2014.192.
2 - The authors are suggested to verify the presence of Al2O3 on the powder surface, i.e., Figures 2 and 3.
Answer: The presence of Al2O3 highlighted in fig. 2 and 3 is verified in Figures 4 and 5 (In XPS analysis). The occurrence of Al peak in reused powder in contrast to virgin powder highlights the enrichment of Al on the surface. The peak positioning of Al spectra in Fig. 5 indicates that the Al on the surface is present in the form of Al-based oxides.
The size of Al-based oxide particulates is very small. Due to the possibility of inaccuracy in EDS data, the results are not presented here in the study. In the attached figure, an EDS composition map on the surface of the reused powder is showing the presence of Al-based oxides. However, the addition of this figure in the presence of XPS data is deemed unnecessary.
(figure added to the pdf. file "Review 1 Metals article"
3 -The authors are suggested to discuss or predict the printing quality using virgin and reused powder during PBF-EB even the there is no printing work in this study.
Answer: As suggested by the reviewer the authors have added a section in the discussion linking this study to the available literature on printing quality using virgin and reused powder:
“4.2 Challenges in processibility and mitigation actions
During six build cycles of powder reuse, Chandrasekar et al. [32] found additional powder spreading operations with increased reuse using rake position and rake sensor pulse data rake. In particular, Inconel 718 tends to agglomerate over multiple cycles of reuse, Additionally, Nandwana et al. [33] studied flowability using hall flowmeter but did not detect changes in the free flow behavior of the Inconel 718 powder during six cycles of re-use. However, this does not indicate that the changes in powder morphology have a large influence, due to PBF-EB machine log data indicating variations in the spreading operations. In this study, the flow behavior of the powder upon reuse shows variations in comparison to the virgin powder. This variation could influence the spreading operations during PBF-EB process. On the quality of the builds, the study by Gruber et al. [12] demonstrated a clear correlation between oxide inclusions with the increase in oxygen content due to powder reuse. Lack of fusion defects in samples built from reused powder couldn’t be consolidated by hot isostatic pressing, this could affect the structural integrity of the parts [13].”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Authors did a good experimental study on the effect of the Rheological behaviour of Inconel 718 powder for EB melting. This manuscript is recommended for publication with the following minor corrections or modifications.
1) in the experimental section: use the standard way of representation for used equipment or instruments (model, make, city, country)
2) Please check the Figure numbers: for e.g. Fig. no 7 missed and Fig. no 1 appeared two times.
Author Response
Reviewer 2:
Dear reviewer,
The authors would like to thank you for the constructive feedback that has improved the article quality. Please find below the specific comments addressed with the changes made to the manuscript.
Authors did a good experimental study on the effect of the Rheological behaviour of Inconel 718 powder for EB melting. This manuscript is recommended for publication with the following minor corrections or modifications.
- in the experimental section: use the standard way of representation for used equipment or instruments (model, make, city, country)
Answer: The authors have corrected the representation for used equipment or instruments as suggested by reviewer.
2) Please check the Figure numbers: for e.g. Fig. no 7 missed and Fig. no 1 appeared two times.
Answer: The authors have checked all Figures in the text and make corrections accordingly.
Reviewer 3 Report
More tests are suggested for the indentification of the morphology of different reused powder.
The effect of treatment of reused powder such as screening and drying should be considered .
The present paper didn't connect the physical properties of the virgin and reused powders with the printed quality or the selection of printing parameters.
How about the influence of repeated use on thermal-physical properties? how do you evaluate the repeated times on the powder and printed quality?
The conclusions should be more specific on the effect of reused powder on the formation quality of EB-PBF printed parts with more quantatitive results.
Author Response
Reviewer 3:
Dear reviewer,
The authors would like to thank you for the constructive feedback that has improved the article's quality. Please find below the specific comments addressed with the changes made to the manuscript.
1-More tests are suggested for the indentification of the morphology of different reused powder.
Answer: The images in Figure 2 are a representation of a large compilation of images taken from the studied powders. While Figure 2 focused on changes to the powder surface and slight morphological changes as deformation due to blasting, Figure 3 shows agglomerates that could go through the mesh and formation of oxides on the particle neck formed during the PBF-EB process.
2-The effect of treatment of reused powder such as screening and drying should be considered .
Answer: Sample drying was performed ex-situ at 100ºC for 2h to assess differences between as received and dried powder. The differences in physical behavior were mainly attributed to the reuse as shown in the graphs of Figure 7.
3-The present paper didn't connect the physical properties of the virgin and reused powders with the printed quality or the selection of printing parameters.
Answer: As suggested by the reviewer the authors have added a new section for discussion of the physical properties of the powder and another on the challenges in processability and mitigation actions. Available literature on print quality and current study are also added:
“During six build cycles of powder reuse Chandrasekar et al. [32] found additional powder spreading operations with increased reuse using rake position and rake sensor pulse data rake. In particular, Inconel 718 tends to agglomerate over multiple cycles of reuse, Additionally, Nandwana et al. [33] studied flowability using hall flowmeter but did not detected changes in the free flow behavior of the Inconel 718 powder during six cycles of re-use. However, this does not indicate that the changes in powder morphology have a large influence, due to PBF-EB machine log data indicated variations in the spreading operations. In this study, the flow behavior of the powder upon reuse shows variations in comparison to the virgin powder. This variation could influence the spreading operations during PBF-EB process. On the quality of the builds, the studied by Gruber et al. [12] demonstrated a clear correlation between oxide inclusions with the increase in oxygen content due to powder reuse. Lack of fusion defects in samples built from reused powder couldn’t be consolidated by hot isostatic pressing, this could affect the structural integrity of the parts [13].”
In the section about mitigation actions different strategies on printing parameters are discussed to minimize the issues of reuse.
4-How about the influence of repeated use on thermal-physical properties? how do you evaluate the repeated times on the powder and printed quality?
Answer: Thermal and physical properties of powders depend on powder properties and particle surface. Increasing oxide inclusions on the powder surface has a large impact in the charging behavior of the powder particles as shown in Figure 8 and supported by available literature:
- Chiba, Y. Daino, K. Aoyagi, and K. Yamanaka, “Smoke suppression in electron beam melting of inconel 718 alloy powder based on insulator–metal transition of surface oxide film by mechanical stimulation,” Materials (Basel)., vol. 14, no. 16, 2021, doi: 10.3390/ma14164662.
- C. Cordero, H. M. Meyer, P. Nandwana, and R. R. Dehoff, “Powder bed charging during electron-beam additive manufacturing,” Acta Mater., vol. 124, pp. 437–445, 2017, doi: 10.1016/j.actamat.2016.11.012.
Quantifying the prints quality on reuse cycles has not been the scope of this study, as there has been previous work*,** on this topic using PBF-EB and Inconel 718. This study completes such studies with a deeper understanding on variations in powder flow behavior and particle charging linked to high resolution surface characterization. However, this study has included discussion on previous study established link between repeated used with printed quality.
* H. Gruber, C. Luchian, E. Hryha, and L. Nyborg, “Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718,” Metall. Mater. Trans. A Phys. Metall. Mater. Sci., vol. 51, no. 5, pp. 2430–2443, 2020, doi: 10.1007/s11661-020-05674-8.
** H. Gruber, M. Henriksson, E. Hryha, and L. Nyborg, “Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder,” Metall. Mater. Trans. A, vol. 50, doi: 10.1007/s11661-019-05333-7.
5-The conclusions should be more specific on the effect of reused powder on the formation quality of EB-PBF printed parts with more quantatitive results.
Answer: Following the suggestion of the reviewer, the authors have added to the conclusions: “When tapping the packed volume fraction increased to 60% in virgin powders and to 57% in reused powder, the particulates and different surface roughness of the reused powder could be hindering a further packing which could affect the thermal properties and therefore quality of the prints.”
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
My questions and concerns were solved, thus it can be accepted as it is.
