Tensile Properties of 21-6-9 Austenitic Stainless Steel Built Using Laser Powder-Bed Fusion

Round 1

Reviewer 1 Report

Based on the analysis of the paper, the following comments suggestions were made to the authors:
In the paper bar graphs with a double y-axis were presented ( fig 3 and fig 7). Linking results for individual specimens by line can be misleading and suggest some continuity of the characteristic.  
The graphs presenting the tensile curves seem to be unreadable. I suggest changing the thickness of the lines. 

Author Response

Dear reviewer, thank you for taking your time to review our paper, it is much appreciated. 

Comments on your comments: 

1) We have now removed these lines 

2) These aren't really lines, it's symbols for each measurement point (together they'll form a line though). What we've done (with the additional imput from another revieweer) is to mark the different symbols with different colors, this should make it easier to distinguish. 

 

Reviewer 2 Report

The manuscript can be published after major revision.

1. The abstract should be rewritten.
2. Do not use abbreviations in the title.
3. Why are tensile tests performed at two temperatures, including room temperature and 750 degrees? Is there a specific application for the material at a temperature of 750 degrees? Please explain.
4. Why did you use the full factorial method to perform the test? While there are different and well-known methods in the Design of Experiments (DOE) such as the Taguchi algorithm. The most important advantage of the Taguchi algorithm is that it provides the minimum number of experiments required to investigate the effects of different parameters on the response.
5. please check the error text "Error! Reference source not found" in the whole text.
6. In section 2.2, what is the meaning of "This too was performed …"? I think that this sentence should be edited.
7. Related to Figure 2, please check the result of sample No. 19.
8. The quality of Figure 8 is not good and the curves in it have to be separated in another way to be recognizable.
9. I believe that the novelty of this research is low and it is better to bold the innovation in the paper.

Author Response

Dear Reviewer, thank you for taking the time to review our paper, it is greatly appreciated. Here are our responses to your comments:

1. We have reworked parts of the abstract.
2. Fixed
3. We are working together with industrial partners in this project and they find this elevated temperature (750 °C) interesting for their applications, which is why it was included in our research.
4. Unfortunately, our technician who is the expert on DOE optimization is away on vacation and is unreachable before the deadline for this resubmission, we will however take this advice into consideration for future DOE-related projects.
5. Fixed
6. Fixed
7. These findings have been double-checked and we are confident that the results are correct.
8. Several reviewers have commented on this, and this has been rectified by coloring the different lines in the graph for better visibility.
9. We respect your opinion, however we do not agree given that no similar work has been published on this alloy (21-6-9) manufactured using laser powder bed fusion coupled with tensile testing at both room and elevated temperature.

Reviewer 3 Report

Attached

Comments for author File: Comments.docx

Author Response

Dear Reviewer, thank you for taking the time to review our paper, it is greatly appreciated. Here are our responses to your comments:

1. Several authors have re-read the manuscript and the software grammarly has been utilized as well, thanks to this, several minor errors have been rectified.
2. Please see page no. 2 (Introduction). This alloy is mainly used within the aerospace industry due to its cryogenic and high temperature properties (thermal stability).
3. As it is described in section 2.1 Material design of experiment, the process parameters were generated by the software MODDE, we cannot reach the technician who is the specialist on this topic due to him being on vacation past the deadline for this re-submission. This is regrettable and we will look into this for future knowledge.
4. Please see page no. 2 (2.1 Material design of experiment), where it is specified that the samples were manufactured using a SLM Solutions 125 machine.
5. Fixed
6. Unfortunately we cannot mention this since we have not conducted any research on the influence of aspect ratio on mechanical properties. Probably some type of modeling would give answers to this question, or if one designs a fatigue experiment with built-in defects with fixed aspect ratios.
7. Please see page no. 10, where LPBF built 21-6-9 is compared to wrought (with regard to strength).

Reviewer 4 Report

I would like to highlight a few points:

1. The error of reference to a table or a drawing is repeated many times in the work – “Error! Reference source not found”.
2. There is no information in the manuscript why the temperature of 750 °C was used during the tests. There is also a lack of information on the speed of heating (type of heating) to this temperature, immediately before the tests.
3. Due to the fact that before the tests at elevated temperature, the samples were annealed at the temperature of 600 °C, the lack of an exemplary image of the microstructure after this operation is clearly visible in the manuscript. This is of particular importance due to the subsequent application of slow cooling, favoring the activation of the precipitation processes.
4. Due to the relatively small value differences, for the hardness curve shown in Fig. 7, it would be more advantageous to use a scale, e.g. 200-300 HV. Such an approach would greatly facilitate the correlation of hardness with a selected sample representing grain size.
5. Using different colors for the strength-stress curves shown in Fig. 8 would make it much easier to interpret.
6. The fractographic analysis included in the manuscript was carried out very laconically. I understand that the images of fractures presented in Fig. 9, both at room and at elevated temperatures, are representative of all samples used in the research !? Moreover, it is limited to a general image of the fracture surfaces (using relatively low magnification) without even general description of the fracture type, e.g. on the basis of ASM Handbook - Fractography. In my opinion, the fractures presented in Fig. 9 are characterized by many features that distinguish them from each other, the identification of which with the use of a larger magnification scale would allow considering the loss of plastic properties (elongation) of the tested samples at elevated temperatures.

Author Response

Reviewer 4.

Dear Reviewer, thank you for taking the time to review our paper, it is greatly appreciated. Here are our responses to your comments:

1. Fixed
2. This has been rectified and has the information has been added to section “2.4 Tensile strength measurements”.
3. The microstructure of the built blanks have been investigated and no obvious changes have been recorded, according to literature, assuming we have pure austenite phase there should not be any significant precipitation of secondary phases occurring at the dwell time of 2h at 600 °C or during the subsequent furnace cooling.
4. Rectified, and thank you for this valuable comment.
5. Fixed
6. The fractography was not a main focus of the work carried out in this paper, and due to the findings at low magnification we did not find anything that warranted a deeper characterization at this stage but could be something to be addressed in future work. We have added to the manuscript, that we found that these two fracture surfaces to be representative for the investigated material.

Round 2

Reviewer 2 Report

The authors tried to edit the article based on the reviewer's comments. Also, they answer all cases. However, I strongly believe that it is necessary to use new techniques of DOE such as the Taguchi method or response surface method instead of the full-factorial test.

In summary, the manuscript can be published after minor revision.

Author Response

Dear reviewer, thank you once more taking your time to improve our manuscript by giving us suggestions of improvements. 

Could you please clarify what you would like us to do? We interpret it as if you would like us to do another DOE i.e. build new material using the taguchi method. This however isn't possible due to monetary issues, already the work we have done has been expensive (around 10 000 euro) and we have no budget to expand this work at this stage.