Microstructure, Porosity, and Bending Fatigue Behaviour of PBF-LB/M SS316L for Biomedical Applications

Comments 1: The discussion of the results looks speculative, since convincing experimental evidence is often not provided. Moreover, there is no convincing evidence at all about the influence of such low porosity on fatigue. For example, there are no direct observations of the sites where fatigue crack was initiated. In addition, the authors did not actually observe the mentioned carbides in the steel microstructure.

Response 1: Thank you for highlighting this issue. Major revisions have been made to address your concerns. Higher-magnification SEM images have been added (Figure 18), and the descriptions of fracture surfaces have been revised to align clearly with these new observations (Pages 17-18). Additionally, the discussion has been updated accordingly, with mentions of carbides significantly reduced due to the lack of direct observation, and clearly identified as an area requiring future investigation (Pages 18-20).

Comments 2: In describing Figure 10, the authors report a grain structure, although this is a cellular structure that has nothing to do with grains, as also evidenced by the EBSD results. By the way, what are the cell boundaries?

Response 2: Thank you for this comment. We have revised the description in the manuscript to avoid confusion. The previous mention of "grain boundaries" has been corrected to clearly indicate "cellular boundaries," which are formed by rapid solidification due to constitutional supercooling and solute partitioning [35] (Page 12).

Comments 3: The authors consider only grains as a microstructural characteristic affecting fatigue. In doing so, the authors ignore that the structure of the printed steel is multi-level: melt pools, grains and cells

Response 3: The discussion has now been expanded to acknowledge the multi-level microstructural features, stating clearly: "SLM as a manufacturing method produces a hierarchical structure comprising melt pools, grains, and the solidification structure." (Page 18).

Comments 4: EDS analysis is weakly sensitive to light elements such as C, O, S. Special analysis methods should be used to determine these elements. Thus, all EDS results for these elements look unconvincing.

Response 4: We have removed the EDS results and discussion related to the lighter elements. A clarifying statement was added to the manuscript: “EDX analysis has inherent limitations in accurately detecting and quantifying lighter elements, such as carbon and oxygen. Accurate determination of these elements would require specialized analytical methods (e.g., XPS), which are beyond the scope of this study. Therefore, the reported results primarily represent the heavier metallic alloying constituents.” (Page 12).

Comments 5: Did the authors attempt to calculate the fraction of phases based on the XRD data? Judging by the intense peaks of the alpha phase and Mn3Ni2Si, their content is significant. Thus, there may be a contradiction with the EBSD data.

Response 5: Quantitative phase analysis from XRD data was not performed in this study, but we will include it in future work. The absence of the Mn₃Ni₂Si phase in EBSD results is explained in detail in the manuscript (Page 13, Paragraph 1).

Comments 6: The description of fatigue fractures largely contradicts the images in Fig. 16. Such a low magnification does not allow one to see either dimples or fatigue striations (if they are there at all). The fracture surface looks predominantly flat and brittle, although with ductile bridges. Thus, I see almost no evidence of ductile fracture.

Response 6: Thank you for pointing this out. Please see Response 1. Figure 18 has been added, providing higher-magnification SEM images of fracture surfaces, clearly showing fatigue-related features such as micropores, unmelted powder particles, and fatigue striations. The manuscript text (Page 18) has been updated to reflect these observations, while noting the absence of significant ductile fracture features, in line with your comments.

Comments 7: The conclusions are poorly supported by experimental data

Response 7: Thank you for highlighting this. The conclusions have been fully revised to better reflect the experimental data and clearly align with the improved results and discussion. The updated conclusions are provided on Page 19.

Comments 8: The reference list should be formatted according to the journal guidelines

Response 8: The reference list has now been formatted according to the journal guidelines.

Comments 9: All measured melt pool dimensions should be rounded to the nearest whole number or at least to one decimal place

Response 9: The dimensions have been rounded accordingly (Page 8).

Comments 10: Page 10. This should refer to Figure 8(c) instead of Figure 6(c).

Response 10: Thank you for pointing this out. This has been corrected.

Comments 11: Page 12. “Microporosity was observed but less extensive than on the top face, again primarily concentrated near grain boundaries”. This is a repetition of the previous phrase.

Response 11: The redundancy has been removed (Page 12).

Comments 12: Page 15. What is “sample (24)”?

Response 12: Thank you for highlighting this. "(24)" was an incorrect citation format, mistakenly used instead of "[24]". The referencing has now been corrected and updated throughout the manuscript.

Comments 13: Page 19. What are “cracks (3)”?

Response 13: As with Comment 12, this was an incorrect citation format. The referencing has now been corrected and updated throughout the manuscript.

Comments 14: There is a problem with subscripts in the designation of chemical compounds in the text.

Response 14: Thank you for highlighting this. We have carefully checked and corrected all subscripts in chemical compound designations throughout the manuscript.

Comments 1: The paper addresses how microstructure and porosity can affect the fatigue behavior. The title refers to biomedical application – however, everything investigated also counts for other applications, I am not sure if the addition of biomedical application has to be made.

Response 1: Thank you for your comment. We decided to retain the mention of biomedical applications in the title because the study directly addresses issues relevant to biomedical implants. For instance, bending fatigue significantly impacts the performance and lifespan of implants like bone plates and orthopedic fixation devices, as mentioned in the abstract. Additionally, our previous work (Reference 27) provides further background on the biomedical relevance, while this manuscript focuses specifically on microstructure, porosity, and fatigue properties relevant to these biomedical devices.

Comments 2: However, the paper lacks on support for the discussion of fatigue behavior regarding images of microstructure and fracture surfaces, especially at higher magnification (for example Figure 14, no relation of the microstructure and crack initiation and propagation can be seen).

Response 2: Thank you for this comment. We agree with your point about insufficient support from the fracture images. We have added new, higher-magnification SEM images (Figure 18) and revised the text (Pages 18-20) to better align with these fractographs. Regarding Figure 14, we acknowledge your concern. Although the external cracking pattern could be observed from optical images, internal crack paths were not clearly visible. We plan to conduct a more detailed internal crack analysis in future work.

Comments 3: First of all, please check that you are consistent through the paper, what you really mean: stainless steel (SS) 316L … 316L stainless steel … SLM 316L … 316L … SLM-printed SS316L … SS316L, especially you also refer to SLM-printed and conventional SS316L.

Response 3: Thank you for highlighting this issue. We have standardized the terminology throughout the manuscript, consistently using "SLM-printed SS316L" when referring specifically to the printed material. The term "SS316L" alone is used in sections referring more generally to the alloy.

Comments 4: Abstract: following sounds confusing: “Four-point Bending fatigue tests conducted under elastic and early plastic regimes” … do you mean the initial elastic regime before you started the fatigue test or is the test all the time within the elastic regime (than it would not fail or fail much later, isn’t it?)

Response 4: We have revised the abstract to clarify the fatigue testing conditions (Page 1). The updated sentence reads: "Four-point bending fatigue tests, conducted under two loading conditions: below and slightly above the yield point, demonstrated that defects inherent to the SLM process, particularly micropores and unmelted powder particles, strongly influence fatigue crack initiation."

Comments 5: Abstract: so far, I would not support that “These findings clarify how microstructure…”, here images with higher magnification are needed (see further comments)

Response 5: We have adjusted the abstract to soften this claim, now stating: "These findings indicate the influence of microstructural defects…" (Page 1). Additionally, we have updated the results, discussion, and conclusion sections (Pages 17-23) to provide clearer evidence regarding fracture surfaces and their associated features.

Comments 6: Introduction: following sentence needs references: “Conventionally manufactured (CM) implants usually come in standardized sizes and shapes, often requiring surgical adjustments.”

Response 6: Thank you for noting this oversight. We have added three references [1-3] to support the mentioned claim.

Comments 7: Introduction: please check, you have used the abbreviations before “Additive Manufacturing (AM) implants generally demonstrate comparable static strengths to Conventionally Manufactured (CM) implants.”

Response 7: Thank you for highlighting this. The abbreviations were previously defined, so the repeated full terms ("Additive Manufacturing" and "Conventionally Manufactured") have now been removed.

Comments 8: Materials and Methods: is there a distribution of particle size provided (10 to 45 µm)?

Response 8: We performed additional particle size analysis using a Malvern Morphologi 4 particle analyser. The manuscript now includes the volume-based statistical values (D10, D50, and D90) and the volume-based particle size distribution plot (Figure 1) to clearly describe the particle size distribution (Page 3)

Comments 9: Materials and Methods: check the paper for µm, min rather than minutes and h instead of hours

Response 9: All units have now been standardized throughout the manuscript to µm, min, and h, as suggested.

Comments 10: Materials and Methods: 15 µm2

Response 10: Thank you for pointing this out. A space has now been added.

Comments 11: Materials and Methods: why did you analyze exactly 18 pores?

Response 11: We clarified in the manuscript why exactly 18 pores were analyzed. The following explanation was added: "These pores were within the resolution limits of OM and clearly distinguishable from other microstructural features or potential contamination." (Page 4)

Comments 12: Materials and Methods: please let the reader know, why you have used a 4-point-bending test and why the samples have a notch

Response 12: We added text to explain the selection of the four-point bending test: "A four-point bend test was selected to simulate bone plate fracture fixation conditions, as tibial fracture fixation implants experience bending loads with simultaneous tensile and compressive stresses [31]." The rationale for including a notch, namely, to localize crack initiation and facilitate crack monitoring, was already described in the manuscript (Page 5).

Comments 13: Materials and Methods: I would recommend to flip the image in 2(c) – then the notch is also at the bottom

Response 13: Thank you for the suggestion. The image has been flipped for consistency, placing the notch at the bottom (Page 5).

Comments 14: Materials and Methods: Figure 3 is rather a “Result”

Response 14: We agree that the figure presents results. However, it has been included in the Methods section because it directly determines the displacement limits for the fatigue test conditions. Without this figure, the fatigue test parameters cannot be properly defined in the methodology.

Comments 15: Materials and Methods: can you add the stress ratio for the fatigue test

Response 15: In displacement-controlled fatigue testing, the stress ratio (R) continuously changes as the sample deforms with successive cycles, so only the initial R-value is meaningful. We have added these initial stress ratios to the manuscript: "For the elastic regime, the initial stress ratio was R = 0.01, and for the early plastic regime, R = 0.8." (Page 6).

Comments 16: Materials and Methods: please check carefully, what you mean with elastic regime (see comment to that within the abstract: will it become less elastic or already plastic during fatigue)

Response 16: We clarified how the loading regimes were defined by adding the following text to the manuscript: "Elastic-regime tests employed displacement cycles of 0.1-1.2 mm, cycling within the elastic region of the quasi-static test, while early plastic-regime tests employed displacement cycles of 1.3-2.1 mm, cycling within the early plastic region." (Page 6).

Comments 17: Results:  “chess board” morphology rather in quotation marks?

Response 17: We agreed and have added quotation marks around both "chess board" and "fish scale" morphologies for consistency (Page 7).

Comments 18: Results: space between symbol and number ± 5.56 µm / ± 6.37 µm

Response 18: We agreed and added spaces between the symbols and numbers (± 5.56 µm / ± 6.37 µm) on Page 8.

Comments 19: Results: Figure 7(b): also here label columnar grains and fish-scale morphology, more common are fine and coarse grains and epitaxial growth of elongated columnar grains

Response 19: Thank you for your comment. We prefer not to modify this figure, as this section specifically addresses porosity and defects rather than grain and melt pool morphologies, which were already discussed in a previous figure in Section 3.1. However, we appreciate your suggestion and will consider performing a more detailed analysis of grain morphologies in future work.

Comments 20: Results: Figure 10: the scale bar is very difficult to see, please magnify and decide if you need both - x10.000 and x14.000 in the image and the figure caption)

Response 20: We agree. The scale bar has been magnified, and the magnification levels (×10,000 and ×14,000) have been removed from the figure caption for clarity (Page 12).

Comments 21: Results: the chemical composition is provided twice: Table 1 and text (I would rather keep the Table, also the text only uses % and not wt. %)

Response 21: The chemical composition is now presented only in Table 1, and the duplicate description in the text has been removed (Page 12).

Comments 22: Results: check second phase, for example Cr7C3  … lower numbers

Response 22: Thank you for pointing this out. The subscripts in the chemical formulas (e.g., Cr₇C₃) have been corrected throughout the manuscript.

Comments 23: Results: Figure 14 (a) to (f): in this magnification the image do not say anything – here micrographs around the crack tip and along the crack growth would be needed to support the discussion

Response 23: We agree with your comment. However, obtaining detailed micrographs around the crack tip and along the crack growth path would require dedicated efforts beyond the scope of the current study. We plan to address this in future work.

Comments 24: Results: description to Figure 16 on page 17: the SEM images do not show, or it is not highlighted in an appropriate magnification, what the text is presenting: (a) where is a pore initiating the crack? Which irregularities? Ductile tearing at the fatigue crack region? Label the fine ridges! the striations are not clearly shown! label the extensive tearing and enlarged dimples! As well as the network of fine ridges, extensive dimples, and microvoids – here you need to provide higher magnifications

Response 24: We agree with your comments. Major revisions, including the addition of higher-magnification SEM images and substantial updates to the corresponding text, have been made to address these issues. Please see our detailed explanation in Response 1 to Reviewer 1.

Comments 25: Results: and: what is the tensile surface and the compressive bottom side – I guess you mean the side of the sample exposed to tensile stress and the side exposed to compression stress (to start with)

Response 25: The text has been revised to clarify the terminology, indicating that the tensile surface refers to the outer surface initially under tension, while the compressive surface is the inner surface initially under compression (Page 17):"…from the outer surface (initially under tension) toward the inner surface (initially under compression)."

Comments 26: 3.9: comprehensive microstructural and chemical analyses

Response 26: We agree. The heading has been modified to "Comprehensive microstructural and chemical analyses" (Page 18).

Comments 27: Discussion: please provide somewhere more detailed information on the fatigue crack propagation - you refer to OM images in 3.9.1 ... but they do not show the crack path!!!

Response 27: We agree with your comment. The discussion has been fully revised in response to your feedback, including higher-quality SEM images clearly illustrating fracture features, an updated discussion to align with these images, and more precise, less speculative conclusions. These changes appear on Pages 18-20.

Comments 28: Conclusions: again (see above) these conclusions need to be based on results presented: “Fatigue crack initiation and propagation are primarily influenced by pore-induced stress concentrations.” …. “Under these conditions, pore morphology critically affects fatigue behaviour.”

Response 28: The conclusions have been thoroughly revised based on the updated results and discussion sections, directly addressing your concerns and ensuring they align clearly with the presented evidence.

Comments 1: Can the authors comment on the fatigue test results themselves? Is this high fatigue strength or not? This point would also be desirable to reflect in Conclusions.

Is (%) on the X-axis of Figure 1 correct?

Response 1: Thank you for your valuable comment. We have attempted to determine whether our observed fatigue strength can be considered high. However, direct comparisons remain challenging due to substantial differences across published studies, including variations in sample geometry, surface finishing, processing parameters, fatigue loading conditions, and testing methods. A new paragraph discussing this issue has been added to the revised manuscript, supported by two additional literature references. Additionally, a fatigue-related conclusion has been included in the manuscript. Regarding Figure 1, thank you for highlighting this point. The axis label has been revised to Volume Frequency (%), indicating the volume-based measurement.

Comments 1: Thank you for addressing the comments and suggestions! I still think cross-sectional micrographs on the crack growth would improve the paper - but I leave this decision to the Editor.

Response 1: Thank you for your suggestion. We fully agree that cross-sectional micrographs would enhance the manuscript, and we have already planned detailed microstructural analyses on crack growth. However, as these investigations require significant additional experimental work, we intend to address this comprehensively in future studies rather than in the current manuscript.