Effect of Ultrasonic Nanocrystal Surface Modification Treatment at Room and High Temperatures on the High-Frequency Fatigue Behavior of Inconel 718 Fabricated by Laser Metal Deposition

Round 1

Reviewer 1 Report

The manuscript entitled “Effect of ultrasonic nanocrystal surface modification at room and high temperatures on the high-frequency fatigue behavior of Inconel 718 fabricated by laser metal deposition” provided a novel design to produce improved fatigue behaviors. However, revision is required before the manuscript can be accepted, that I have several concerns and questions that need proper attention and should be well addressed. 

1. For experimental parameters, “scanning speed of 900 mm/s” was mentioned. Is the scanning speed described accurately? What is the value of Frequency in Table 2? Please verify.
2. To present the clear experimental results, it is suggested to use different legends with clear contrast in Figure 4 to clearly distinguish USNM at RT and USNM at HT.
3. Line 208 “It should be noted here that after USNM technique at RT and HT the surface of the samples still has some valleys and incompletely closed pores and defects (mentioned by yellow arrows), which may lead to an increase in surface roughness” It is mentioned here that valleys, pores and defects will increase the surface roughness, while the samples processed by USNM can effectively reduce surface roughness as displayed in Table 3. This appears as an contradiction for the reviewer.
4. I cannot catch the meaning of English expression and there are some mistakes in the manuscript. For example, Section 3.4. “It obvious from Figures 7(b and c) that the depth of number of high-angle grain boundaries increased after UNSM at RT and HT up to a depth of 90 µm and 100 µm, respectively.” “Attarilar and co-authors concluded that high severe surface plastic deformation also might be responsible for the development of necklace structure”. Other expression errors exist elsewhere, please reading carefully and modifying.
5. In Figure 7, the nanostructured surface layer with a thickness of about 130 and 150 µm was confirmed; the depth of high-angle grain boundaries are 90 µm and 100 µm, respectively. For directly observing these results, please plot the surface and relevant reference line in figures, instead of a simple description.
6. For the necklace structure in Figure 7(c), it looks different with the cited reference. For me, they are the unrecognized areas which origin form cracks or severe deformation, instead of boundaries. Check the results.
7. References are required to support the following description.Line 275: “a sliding of grain boundaries might be difficult with the presence of low-angle grain boundaries leading to a high strain hardening”;
8. Line 279: “responsible factor for formation of low-angle grain boundaries might be crystal slip in the structure and increasing hardness, however formation of high-angle grain boundaries mostly occurs due to significantly accumulated strains”.
9. Line 155: “LMD fabricated materials tend to have high surface roughness along with unsatisfactory mechanical properties due to the nonequilibrium nature of LAM processing”;
10. It is suggested for the citation of the latest articles which may be helpful to the manuscript, e.g., Int. J. of Plast. 142 (2021) 102997; ACS Appl. Mater. Interfaces 2021, 13, 55712−55725.
11. Is there any dynamic recrystallization observed in your EBSD results? Please point it out if necessary.

Author Response

We would like to thank the reviewers for their time and effort to review the manuscript and provide us with valuable comments and suggestions. We have revised the manuscript according to all the comments to the best of our ability. We have outlined revisions point by point, following attached WORD file are the details of the revisions

Author Response File: Author Response.pdf

Reviewer 2 Report

The current manuscript is a study on the high-cycle fatigue behavior of Inconel 718 fabricated by laser metal deposition and modified by ultrasonic nanocrystal surface modification (UNSM). The topic is interesting, but unfortunately, I cannot recommend it for publication due to several reasons. Some detailed comments are listed below:

• A major problem is that the proposed treatment does not have significant effects on the properties. The fatigue limit was increased from 233 MPa to 253 MPa, which is only 20 MPa or less than 10%. And the reasoning in the discussion is not convincing enough.
• There is possible misleading information in the results. The microstructure shown by the EBSD does not show the nanocrystalline structure close to the surface (probably the step size of the EBSD scan is larger than the nano-scale). The authors need to use other characterization techniques to support their rationales.
• The claimed "necklace structure" does not appear as a proper microstructural feature, but more like an artifact. The cited references in the manuscript all showed the "necklace structure" as a result of dynamic recrystallization, and the "necklaces" are formed close to the preferred nucleation sites (grain boundaries in most cases). But the present result shows the structure inside a large grain and no clear indication of dynamic recrystallization can be found.
• The language needs strong polishing. There are a lot of grammatical errors and complex sentences that are difficult to understand.
• The information in table 2 must be wrong. Please double-check.
• Although the topic is interesting, the quality of the current paper is not as expected. The authors are suggested to dig more into the physical reasons behind the performance of the tested materials, even though the treatment cannot provide a striking improvement.

Author Response

We would like to thank the reviewers for their time and effort to review the manuscript and provide us with valuable comments and suggestions. We have revised the manuscript according to all the comments to the best of our ability. We have outlined revisions point by point, following attached WORD file are the details of the revisions

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript “Effect of ultrasonic nanocrystal surface modification at room and high temperatures on the high-frequency fatigue behavior of Inconel 718 fabricated by laser metal deposition” points to a significant improvement in fatigue strength for the surface modifications used. The data and analysis contained are valuable. The structure of the manuscript is clear. The study description is properly implemented. Please consider the following comments:

Page 3, lines 105-106: Are all surfaces of LMD fabricated components under high frequency vibration always machined? The testing of the untreated surface seems particularly appropriate in view of the practical use of the components. The condition of the unmachined surface can significantly determine the fatigue behaviour of the real objects.

Fig. 1: Incorrect caption of figure (a) -> (c).

Table 2: Check the Frequency (kHz).

Page 4, Section 2.3: What is the end of fatigue test criterion for fractured and run-out specimens?

Fig. 3: The diameter is missing.

Page 5, Section 2.4: Have the tests been carried out according to the standard?

Page 10, lines 305-307: It was concluded that “An insignificant improvement in fatigue life of the LMD fabricated samples in the LCF regime may result of the improved yield strength.” The mechanical properties data should improve the conclusions.

Page 10, lines 307-308: Was the fatigue limit estimated for run-out specimens? What method was used to determine the fatigue limit?

Page 11, lines 344-345: Can the measured pore size be related to the fatigue limit?

Page 11, lines 349-352: Can the crack initiation location for UNSM-treated at RT and HT specimens be related to hardness?

Fig. 9: What is the type of subsurface crack initiation defect for UNSM specimens treated at RT and HT? Was the failure mechanism the same for other stress amplitude levels?

Pages 12-13, Section 4: The conclusions concern the surface condition of the specimens after machining, which significantly affects the fatigue behaviour. The statement should be added.

Author Response

We would like to thank the reviewers for their time and effort to review the manuscript and provide us with valuable comments and suggestions. We have revised the manuscript according to all the comments to the best of our ability. We have outlined revisions point by point, following attached WORD file are the details of the revisions.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The author has addressed my concern and it can be accepted.

Author Response

We greatly appreciate Reviewer insightful and helpful comments on our manuscript.

Reviewer 2 Report

The categorization of "high-angle grain boundaries" and "low-angle grain boundaries" layers in the EBSD analysis does not appear to be reasonable and usefu. The black regions in Fig.7b and c result from non-indexed points which can be from many reasons, not necessarily high-angle grain boundaries. And the "low-angle grain boundaries" layer might also be a combination of non-indexed points, HAGBs, and LAGBs. Basically, the features in the statements cannot be resolved in the presented results. The readers might be able to imagine what is the logic behind it, given that they are familiar with the deformation mechanisms of this material during the treatment, but only based on the results presented in the manuscript, this description lacks solid support. I suggest the authors remove these unnecessary or even misleading descriptions.

Line 273: dislocations are reasons for strain hardening but not results from strain hardening. 

Author Response

We greatly appreciate Reviewer insightful and helpful comments on our manuscript.

Author Response File: Author Response.pdf