Characterization of the Microstructure and Surface Roughness Effects on Fatigue Life Using the Tanaka–Mura–Wu Model

Round 1

Reviewer 1 Report

Comments to the Author

In this manuscript, the modified Tanaka-Mura-Wu (TMW) model, taking into account the microstructure, surface roughness factors, material’s elastic modulus, surface energy and Burgers vector, was developed to investigate the fatigue behavior of Additive manufacturing (AM) materials. Based on the large sets of fatigue data for various typical AM materials, the effects of surface roughness, microstructure and post-heat treatment on fatigue life are studied for to both low cycle fatigue and high cycle fatigue. Considering the importance of discussions on the fatigue behavior of AM materials, I believe it will attract great attention of the materials and mechanics community of researchers. Overall, the manuscript presented an interesting topic and is as such certainly suitable for publication. However, there are some major issues that need to be resolved first.

1. It would be ideal if the title could better summarizes the contents of this paper. The current one is a little confusing for readers not very familiar with fatigue fields.
2. The authors make claims without providing sufficient explanation and references to support their statements. Please give strong explanations for the following statements:

“The bulk material of SLM AlSi10Mg contains voids, which grow in both horizontal and vertical directions, but more numerous in the material grown in the vertical orientation. In addition, voids are more prevalent in the surface layers.”

“The vertical direction has a slightly higher microstructural factor (M = 2.08), but all within the range as caused by round ellipsoidal voids.”

3. In Table 11, the distribution functions of fatigue endurance limit are distinct from each other, what’s the reason?
4. It would be ideal if the authors discuss briefly the difference in microstructural and mechanical properties between AM and conventional materials. Some examples which may help include: Mater. Res. Lett., 2020, 8(10): 357-363; Wear 474–475 (2021) 203880; Int. J. Plast. 142 (2021) 102997.
5. In Table 2, the same surface energy W_s values were used in subsequent calculations for each set of materials without considering their composition and structural features. Specially, it seems that the surface energy value of AlSi10Mg alloys in section 4.3 is taken from pure aluminum but it’s obvious that the alloying elements Si and Mg should have an impact on the surface energy.

Author Response

The authors greatly appreciate the reviewer's comments. Revisions are made according to the reviewer's suggestions:

1. The title has been changed to:

Characterization of the Microstructure and Surface Roughness Effects on Fatigue Life using Tanaka-Mura-Wu Model

2. The description of the microstructure was referenced from the same paper the raw fatigue data were taken. Now, the references are given at the end of those sentences.

3. The reviewer raised an interesting question. Unfortunately, even with all the physics revealed for the Nature, people still have little understanding how the Nature makes up the statistical distribution for particular things. The authors added some reasoning and remarks In the revised manuscript. "Generally, the wrought material has gone through complex thermomechanical processing, which may skew the property distribution. During the AM process, the material’s microstructure is formed from powders under high energy input by a laser, which reasonably creates a new microstructure with normal (symmetrical) distribution of properties. Other examples of statistical distribution of properties can be found in metallic glasses [29,30]. It is beyond the scope of the current study to completely characterize the statistical distribution of processing-microstructure-property relationships for AM materials, but it is definitely an interesting point of future study. "  

4. The differences in microstructures and mechanical properties of conventional and AM materials are a huge topic. The reviewer gave some examples for metallic glasses in their suggested reference, which are included, and briefly discussed above. 

5. The authors totally agree with the reviewer on this question in general. But, unfortunately, surface energies for complex engineering alloys are rarely available in the literature. By the nature of these alloys with more than 80% of the base metal element and other major elements with comparable surface energies, it is reasonable as first approximation to use the pure metal surface energy. The text is revised to reflect this. "Note that the surface energy values are evaluated from the base metals of the corresponding alloys at ambient temperatures, based on the surface energy and entropy information given in Ref. [15], as the composition contains more than 80% of the base metal element and other major elements with comparable surface energies. Surface energies for complex engineering alloys are rarely available in the literature."

Reviewer 2 Report

The manuscript is focused on the application of modified Tanaka-Mura-Wu model to various AM materials. The factors of surface roughness and microstructure were established for these (and their counterpart wrought) materials using appropriate approximations of the fatigue data. It is a well-written manuscript which brings some new scientific knowledge. It needs only a formal revision of Fig. 4.: the scientific notation of numbers should be used on the x-axes (instead of the decimal one).

Author Response

The authors would like to thank the reviewer for the careful review. Fig. 4 is revised.