Small- to Large-Scale Electron Beam Powder Bed Fusion of Functionally Graded Steels

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript introduces a straightforward and effective PBF-EB method for fabricating functional graded steels. Multiple characterizations are conducted to demonstrate the existence of material gradient transitions. The presentation is generally well-structured, although it could benefit from a more detailed discussion on the gradient control precision.

1. Can this method precisely control the spatial distribution of material ratios by design? For example, is it possible to achieve a desired composition at a certain position of the part? If so, such capability should be characterized.
2. How do you achieve smooth and repeatable gradient changes from 100%SS to 50%SS+50%TS, given that the mixing mechanism seems uncontrolled?
3. Along the building direction, there is a gradient. However, along the building planes perpendicular to the building direction, are there undesirable composition changes within that plane? How are these changes  controlled?
4. The color code used in Figure 3 should be labeled for clarity.
5. It would increase the readability by briefly discussing the state-of-the-art in   introduction section and highlighting the contributions of this work at the end of it.

 

Author Response

The authors would like to acknowledge the comments and suggestions by the Reviewers. The correspondent amendments and modifications included in the revised version have (in the authors opinion) significantly improved the quality of the manuscript. Specific comments and responses regarding the questions raised by each respective reviewer is correspondingly answered in below, and changes in Figures and text are included in the revised manuscript and marked with yellow.

 

The authors hope that the revised and resubmitted version is now suitable for publication and thank once more the reviewers for valuable observations.

.

 

 

R1.1: Can this method precisely control the spatial distribution of material ratios by design? For example, is it possible to achieve a desired composition at a certain position of the part? If so, such capability should be characterized

A: No, it is not possible to precisely controll the gradients obtained to design for an exact and specific ratio in a position in the approached proposed, as the intermixing of powders is randomly occuring by the action of gravity. The authors do not claim it is possible in the manuscript; however, it is demonstrated that transitions between 100%A to 100%B are accomplished in a gradient fashion.

 

R1.2: How do you achieve smooth and repeatable gradient changes from 100%SS to 50%SS+50%TS, given that the mixing mechanism seems uncontrolled?

A: Once more, it is not the claim of the authors that controlled 50% SS + 50% TS are achieved in the consolidated material. This propotions are discussed in the manuscript in terms of the propotions of each powder in the blend, previous to consolidation. Due to density differences during powder feeding it cannot be guaranteed that the intermixing of powders is consistent, and therefore one of the proposed approaches was to fill the middle part of the hoppers with a 50%/50% proportion, to somehow guarantee a more gradual transition between the pure (100%) powders during the processing.

 

R1.3: Along the building direction, there is a gradient. However, along the building planes perpendicular to the building direction, are there undesirable composition changes within that plane? How are these changes controlled?

A: In XY the composition changes cannot be controlled, as there is no mechanism to tailor the powder raking within each layer. As discussed in the introduction, this is possible nowadays for laser systems using AEROSINT technology, which is not possible practically in EB systems due to challenges with vacuum management. The authors has some future plans to investigate a method for compositional control in XY, but this approach is not similar to what is presented in this work.

 

R1.4: The color code used in Figure 3 should be labeled for clarity.

A: The different colours in figures (b) and (c) correspond to “levels” or “segments” done in the STL files to account for eventual changes in parameters needed under processing. A text explaining this was included in the caption of the figure in the ammended manuscript.

 

R1.5: It would increase the readability by briefly discussing the state-of-the-art in introduction section and highlighting the contributions of this work at the end of it.

A: Thanks to the reviewer for the comment! Authors agree and have added a concluding paragraph with discussion at the end of the seccion in the ammended manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

I found the article entitled “Small-to-large scale Electron Beam Powder Bed Fusion of functionally graded steels” interesting, the topic is timely, the research was planned correctly and described in detail. My comments are more often on the editorial side than on the merits.

1. In the methodology in sections 3.4 and 3.5 of the research, the parameters of hardness measurement should be given.

2.  The abstract should be shorter. The first part of it can be shortened slightly.

3.The summary of section 2 should end with the conclusion of what research the authors have done to answer the problems (lack of knowledge) signaled at the end of this section.

4. The name of the types of configurations a, b, c causes a large accumulation of "a, b, c" designations in Fig. 2, Table 2, Fig. 3 - this makes it difficult to read and understand the text.

5. In Table 2, units of volume should be added.

6. The lettering, scales and markings in some figures are illegible: Fig. 4, Fig. 6.

7. Fig. 9 should be enlarged.

8. In Fig 8., the 500um scale refers to figures c-d rather than b.

 

9. In section 4.4.2 in 8th line from the end “(Figures 12 c,d respectively)” it probably should be Figures 12 d,e, and in the 3rd line from the end instead of Figures 12 a,b it should be b,c.

Author Response

The authors would like to acknowledge the comments and suggestions by the Reviewers. The correspondent amendments and modifications included in the revised version have (in the authors opinion) significantly improved the quality of the manuscript. Specific comments and responses regarding the questions raised by each respective reviewer is correspondingly answered in below, and changes in Figures and text are included in the revised manuscript and marked with yellow.

 

The authors hope that the revised and resubmitted version is now suitable for publication and thank once more the reviewers for valuable observations.

R3.1: In the methodology in sections 3.4 and 3.5 of the research, the parameters of hardness measurement should be given.

A: Agree with the reviewer. Profiles of indentations with at least 3 tests per level were performed across the cross-sections with a distance between indentations of at least 2 mm and a dwell time of 5s. A clarifying text is now included in the experimental part section 3.5 in the ammended manuscript. In the case of section 3.4., parameters for hardness are alredy included. Also note that eg. the spacing for each measurement is illustrated in each respective plot given in the results section.

 

R3.2 The abstract should be shorter. The first part of it can be shortened slightly.

A: Agree, first part has been shortened to make the abstract shorter. Thank to the reviewer for the suggestion

 

R3.3 The summary of section 2 should end with the conclusion of what research the authors have done to answer the problems (lack of knowledge) signaled at the end of this section.

A: Agree with the reviewer. A concluding paragraph is added accoridngly in the ammended manuscript.

 

 

R3.4  The name of the types of configurations a, b, c causes a large accumulation of "a, b, c" designations in Fig. 2, Table 2, Fig. 3 - this makes it difficult to read and understand the text.

A: Authors agree with the reviewer. Instead ”configuration I, II and III were used and changes in text were included in the ammended manuscript

 

 

R3.5  In Table 2, units of volume should be added.

A: Liters (L) are already included in the heading of the table.

 

R3.6  The lettering, scales and markings in some figures are illegible: Fig. 4, Fig. 6.

A: Authors agree with the reviewer. Figure 4 and 6 has been edited and included in the ammended version to account for this.

 

 

R3.7 Fig. 9 should be enlarged.

A: Agree with the reviwer’s comment. Figure 9 was redistributed and enlarged accordingly in the ammended manuscript

 

R3.8 In Fig 8., the 500um scale refers to figures c-d rather than b.

A: No. Scale refer to Figure b indeed. However, scales of Figures c-d were difficult to read and therefore the micrographs were edited and a larger and more visible scalebar was added to account for it.

 

R3.9  In section 4.4.2 in 8th line from the end “(Figures 12 c,d respectively)” it probably should be Figures 12 d,e, and in the 3rd line from the end instead of Figures 12 a,b it should be b,c.

A: Thanks to the reviewer for the observation. Text was changed accordingly in the ammended version of the manuscript.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Overall, the topic and results of the article are very interesting and the data will be beneficial for the industry in future. However, there are some edits required before the paper is publishable.

1- there are some minor grammatical errors throughout the paper that need to be revised.

2- In Figure 1, the magnification/ scale bar for parts a and b should be the same so it is easier to compare the powders. Also, it will be good if the cross- sectional SEM images and EDX maps of the powders are also added. Moreover, satellite particles are attached to almost all the particles in figure C. so using "only few" is not correct. 

 3- in Figure 6, the authors have provided the EDX data for 3 types of the samples that are fabricated.  However, there are not a lot of information regarding the sample preparation. Was EDX performed on etched sample or unetched? if unetched, were samples polished ? 

4- Regarding EDX, how many points per data point is shown in Figure 6? Why EDX mapping was not selected? As it is mentioned in the paper EDX spectrum points are not accurate enough for ensuring the compositional changes. 

Moreover, even if the point spectrum is used, the exact points should be shown in the corresponding images. In these data (Figures a, b) it appears that the points were located equally distanced which is hard to achieve when using SEM.

5- Figure 6 b and c should be edited and SEM or macroscopic images should be used instead of images that are taken with phone. Even if camera images are used, they should be edited to fully show the surface of the sample where the EDX is performed on.

6- what parameters were used for hardness testing? Load? Dwell time? number of  indentations? space between indentations?

7- The acronyms should be defined when they first introduced in paper and after that should be used instead of full phrase. in this paper EDX was never defined and FGM was defined but was not used later in the text. same for V and Mo.

8- Figure 4a should be edited to be more neat and visually appealing  by cropping certain end sections to align the edges of the images or adjusting the brightness to achieve a more consistent contrast across the images. Also, what are the arrows showing in Figure 4b, c. They should be defined in the capture.

9- Figure 5 could be rearranged to enlarge images so the features can be noticed better. Moreover what are arrows showing? the color of arrows for each specific feature should be different and they should be explained in the capture. 

10- Regarding Figure 13, Figure d can be presented in another figure, alloying the enlargement of the rest of the figures. It is very difficult to see the various microstructural features in Figures a, b, c. Also it seems that the size and magnification of images are not the same. 

Moreover, What is the black and grey image in the middle? microCT? it should be labeled and explained in capture.

11- There is no scale bar in  Figure 14 (b-f).  Also, figures b-f could have been taken in a way that the images edges align. Or stitching tool available on most optical microscopes could have been used. 

 

 

Author Response

The authors would like to acknowledge the comments and suggestions by the Reviewers. The correspondent amendments and modifications included in the revised version have (in the authors opinion) significantly improved the quality of the manuscript. Specific comments and responses regarding the questions raised by each respective reviewer is correspondingly answered in below, and changes in Figures and text are included in the revised manuscript and marked with yellow.

 

The authors hope that the revised and resubmitted version is now suitable for publication and thank once more the reviewers for valuable observations.

 

R2.1: there are some minor grammatical errors throughout the paper that need to be revised

A: Thank you, we have carefully looked through the paper and fixed any mistakes that we could find.

 

R2.2a In Figure 1, the magnification/ scale bar for parts a and b should be the same so it is easier to compare the powders..

A: Agree with reviewer. Figure 1 has been edited in the ammended version with new images at same magnification for easiness of comparison.

 

R2.2b Also, it will be good if the cross- sectional SEM images and EDX maps of the powders are also added.

A: Authors agree in that CS images and EDX of powders could be interesting; however, we do not consider that such information is necessary to support the results and claims of the investigation. Original feedstock powder microstructures and features are ”Deleted” when the beam melt and consolidate the powder, and so focus here is to analyse the solid material after manufacturing. CS/EDX of powders are not often reported in metal AM articles to the best of our knowledge.

R2.2c Moreover, satellite particles are attached to almost all the particles in figure C. so using "only few" is not correct

A: Authors agree with reviewer. New text is written in the ammended version to account for this.

 

R2.3 in Figure 6, the authors have provided the EDX data for 3 types of the samples that are fabricated. However, there are not a lot of information regarding the sample preparation. Was EDX performed on etched sample or unetched? if unetched, were samples polished ?

A: Authors agree with the reviewers on this point. EDX was performed on the polished cross section of all three specimens. A clarification note is added in the experimental section on the ammended manuscript.

 

 

R2.4 Regarding EDX, how many points per data point is shown in Figure 6?

A: At least in 3 points per sample which has been added to figure 6 description now. We do have SEM images of each point measurement but decided that it is already so many figures in this manuscript and it would not add significant information to the reader.

 

R2.4b Why EDX mapping was not selected?

A: The areas in the samples across which the compositional changes take place are too large to be mapped using one EDX map (or even a few stitched maps). Beyond this, since in the specimen ”spring” we have a ”discrete” evolution of the composition in each ”turn” or ”strut”, the authors decided to do point measurements to dispose of consistent data for comparison. It is also significantly easier to export the composition from a point measurement in our version of Aztec as the composition from a map has to be reconstructed from the individual spectrums. We do agree with the reviewer that a map can yield a truer composition but would like to keep the point EDX (please note the discussion of EDX validity given in section 4.2).

 

2.4c As it is mentioned in the paper EDX spectrum points are not accurate enough for ensuring the compositional changes.

Moreover, even if the point spectrum is used, the exact points should be shown in the corresponding images. In these data (Figures a, b) it appears that the points were located equally distanced which is hard to achieve when using SEM.

A: Authors think that it is already to saturated with images to add more images of exact places for EDX analysis. The measurements were aquired in a region around the points marked in the plots. Scale bars are now added for Figures 6 a and b and so the reader has a reference to locate the measurement points. A clarifying text was added in the Figure caption.

 

R2.5 Figure 6 b and c should be edited and SEM or macroscopic images should be used instead of images that are taken with phone. Even if camera images are used, they should be edited to fully show the surface of the sample where the EDX is performed on

A: The size of the images of the entire specimens are too big to SEM or even optical microscope. They are used here just as a guideline to orient the reader on where the detailed experiments were taken. However, authors agree with reviewer that a more focused images are preferred and therefore Figures 5 and 6 were edited to account for this.

 

 

R2.6 what parameters were used for hardness testing? Load? Dwell time? number of indentations? space between indentations?

A: Thanks to the reviewer. Parameters are now included in the experimental seccion of the ammended manuscript; specifically in 3.5.

 

R2.7 The acronyms should be defined when they first introduced in paper and after that should be used instead of full phrase. in this paper EDX was never defined and FGM was defined but was not used later in the text. same for V and Mo.

A: Agree with the reviewer and thanks for the comment. Acronyms have accordingly being defined in the ammended version of the manuscript

 

R2.8 Figure 4a should be edited to be more neat and visually appealing by cropping certain end sections to align the edges of the images or adjusting the brightness to achieve a more consistent contrast across the images. Also, what are the arrows showing in Figure 4b, c. They should be defined in the capture.

A: Authors agree with the reviewer. Figure 4 has been edited and included in the ammended version to account for contrast differences and irregular edges. Arrows in the images point to different carbide formations. Indications of the arrows are now clarified in the figure captions.

 

R2.9 Figure 5 could be rearranged to enlarge images so the features can be noticed better. Moreover what are arrows showing? the color of arrows for each specific feature should be different and they should be explained in the capture.

A: Authors agree with reviewer. The arrows here once more are pointing to carbides. Molybdenum (Mo-C) and Vanadium (V-C) rich carbides are specified within the insert in Figure 5 (e) and just pointed in Figure 5 (d). They are pointing a change in the contrast within a grain in Figure 5 (i). A clarification note to explain this is also included in the Figure 5 caption, and the colour of arrows is also changed as proposed by the reviewer.

 

R2.10 Regarding Figure 13, Figure d can be presented in another figure, alloying the enlargement of the rest of the figures. It is very difficult to see the various microstructural features in Figures a, b, c. Also it seems that the size and magnification of images are not the same.

Moreover, What is the black and grey image in the middle? microCT? it should be labeled and explained in capture.

A: Fully agree, and thanks to the reviewer for the comment. Figure was separated so that Figure 13b became Figure 14, and Figure 14 became Figure 15. Changes in text were done accordingly.

 

R2.11 There is no scale bar in Figure 14 (b-f). Also, figures b-f could have been taken in a way that the images edges align. Or stitching tool available on most optical microscopes could have been used.

A: Agree for the reviewer and thanks for the observation. Scale was now added to this Figure and the stitched images were adjusted to present a more aligned profile.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have adequately addressed or responded to my previous comments. 

There is one thing that I prefer to be addressed before publishing the paper: in Figures 5 and 13, each carbide should be marked with a different color, and the color should be explained in the caption. E.g., the blue arrows show MoC and the yellow arrows show VC.