Effect of Microstructure on High Cycle Fatigue Behavior of 211Z.X-T6 Aluminum Alloy

Round 1

Reviewer 1 Report

The article highlights peculiarities of evaluating the properties of an aluminum alloy under hot-rolling and as-cast stations. The authors conducted a detailed assessment of the microstructure of both variants of the aluminum alloy using SEM and TEM. They studied the mechanisms of initiation and propagation of fatigue cracks in these materials and evaluated their mechanical properties by static and cyclic tests. However, insufficient attention was paid to the analysis of the obtained results.

The article is interesting, but a number of shortcomings need to be corrected:

1. It is necessary to explain why tests were performed at different load frequencies (alloy W211Z.X at 20 Hz and alloy Z211Z. X at 120 Hz), because the load frequency can affect durability.
2. When analyzing the mechanical characteristics (Table 2), it is necessary to explain the untypical change of relative elongation and relative narrowing, because when the relative elongation decreases, then the relative narrowing should decrease.
3. Fig. 5 occurs earlier than the link to it.
4. The text in Figs. 4 and 5 is not visible. The magnification scale in Fig. 5 is not visible. Please also increase the font size on the graphs for Fig. 4 and Fig. 5.
5. The text is difficult to navigate in the marking W211Z.X and Z211Z.X. Perhaps, it would be better to make marking like R211Z.X (hot-rolling) and C211Z.X (as-cast).
6. In Fig. 13b, please add arrows that indicate secondary cracks.
7. The text of the article states “First, several factors would affect the properties of materials besides the grain size and size of the second phase. The PFZ, shape, volume percentage, and distribution of the second phase and inclusions, as well as other defects, such as holes, dendrite segregation, and reticular structure of the material, would greatly affect the fatigue strength and other properties of the material.”. However, when assessing the behavior of the studied alloys (W211Z.X and Z211Z.X), little attention was paid to this.
8. The statement “The fatigue lives of both Z211Z.X and W211Z.X alloys declined with the increase in cycle stress and survival probability.” is well known. Therefore, it cannot be given as a conclusion.
9. It is not clear from Table 1 what is the exact chemical composition of the aluminum alloy.
10. For a better analysis of Fig. 16, the axis scales should be the same, and the parameter values should be better represented as points rather than bars.

Author Response

Question 1. It is necessary to explain why tests were performed at different load frequencies (alloy W211Z.X at 20 Hz and alloy Z211Z. X at 120 Hz), because the load frequency can affect durability.

Response: As the reviewer said, the loading frequency does have a certain influence on the fatigue damage of the alloy. However, if the loading frequency difference is small, its influence is relatively small. Due to the reasons of raw materials of R211Z.X alloy, the variable shape and size is small, so it can only be processed into small samples, which cannot meet the sample size requirements of QBG high frequency fatigue experiment machine. Therefore, it can only be conducted on Instron fatigue experiment. Moreover, the maximum allowable frequency of the equipment is only 20 Hz. However, the cast state of alloy (C211Z.X) can be processed into large specimens, so it can be tested on QBG testing system with a higher frequency, up to 120 Hz, which can reduce the fatigue test time. Furthermore, the focus of this paper is on the influence of microstructure on fatigue damage of the 211Z.X aluminum alloy. Furthermore, although the frequency has a certain influence on fatigue experiment, however, the difference between 20 Hz and 120 Hz frequency is relatively small, and the influence of the frequency on fatigue damage of the alloy is very small compared with that of microstructure and other factors. Thus, the little influence of loading frequency in this work can be ignored.

Question 2. When analyzing the mechanical characteristics (Table 2), it is necessary to explain the untypical change of relative elongation and relative narrowing, because when the relative elongation decreases, then the relative narrowing should decrease.

Response: Generally, El and RA changing trend is representative of the material of plastic change trend, and the tendency in general is the same. However, in fact, the elongation (El) and reduction of area (RA) represent different meanings. The El represents a uniform axial plastic deformation. However, the RA mainly refers to the non-uniform deformation of radial size when the material gets to maximum tensile strength. Furthermore, there are not only the difference of microstructure between R211Z.X and C211Z.X alloys, but also different defects such as microholes in the microstructure which may also be the reason for this different phenomenon.

Question 3. Fig. 5 occurs earlier than the link to it.

Response: We are sorry for negligence that this is a mistake made by ourselves. This mistake has been revised and other similar problems also have been checked and revised carefully in the revised manuscript.

Question 4. The text in Figs. 4 and 5 is not visible. The magnification scale in Fig. 5 is not visible. Please also increase the font size on the graphs for Fig. 4 and Fig. 5.

Response: We are sorry for negligence that this is a mistake made by ourselves. This mistake has been revised and other similar problems also have been checked and revised carefully in the revised manuscript.

Question 5. The text is difficult to navigate in the marking W211Z.X and Z211Z.X. Perhaps, it would be better to make marking like R211Z.X (hot-rolling) and C211Z.X (as-cast).

Response: Thank you for reviewer’s valuable suggestion. According to the reviewer’s suggestion, we have revised all marking W211Z.X and Z211Z.X to R211Z.X (hot-rolling) and C211Z.X (as-cast) in the revised manuscript.

Question 6. In Fig. 13b, please add arrows that indicate secondary cracks.

Response: We are sorry for negligence that this is a mistake made by ourselves. This mistake has been revised and other similar problems also have been checked and revised carefully in the revised manuscript.

Question 7. The text of the article states “First, several factors would affect the properties of materials besides the grain size and size of the second phase. The PFZ, shape, volume percentage, and distribution of the second phase and inclusions, as well as other defects, such as holes, dendrite segregation, and reticular structure of the material, would greatly affect the fatigue strength and other properties of the material.”. However, when assessing the behavior of the studied alloys (W211Z.X and Z211Z.X), little attention was paid to this.

Response: Thank you for reviewer’s valuable suggestion, this section has been revised carefully. In the latter part of the article, in addition to the different grain morphology in the microstructure was analyzed from the fracture morphology and profile. The influence of size and distribution of secondary phase, porosity and other defects on fatigue property of alloy was also explained from page 10 to page 17 in revised manuscript. Furthermore, there is a special part at the end of the article, which analyzed the influence of the size and distribution of the secondary phase on the fatigue property of the alloy.

In addition, according to the opinions of the reviewers, we have added some parts to supplement the influence of other factors on the fatigue properties of the alloy, please see the revised manuscript.

Question 8. The statement “The fatigue lives of both Z211Z.X and W211Z.X alloys declined with the increase in cycle stress and survival probability.” is well known. Therefore, it cannot be given as a conclusion.

Response: According to the suggestion of reviewer, this mistake has been revised and this sentence has been removed in conclusions in the revised manuscript.

Question 9. It is not clear from Table 1 what is the exact chemical composition of the aluminum alloy.

Response: Thank you for reviewer’s valuable suggestion. According to the suggestion of reviewer, we have detected the exact chemical composition of the 211Z.X aluminum alloy by the chemical method combined with EDS, as shown in Table 1 in the revised manuscript.

Question 10. For a better analysis of Fig. 16, the axis scales should be the same, and the parameter values should be better represented as points rather than bars.

Response: Thank you for reviewer’s valuable suggestion, this section has been revised carefully, and the modification has been completed according to the author's opinion in the revised manuscript.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Last sentence in line 311 seems to be incomplete.

In line 473 it is said: "the fatigue strength of the alloy decreased with the increase in grain size (table 3 and fig. 17)" but it seems just the opposite from fig. 17, while table 3 does not say anything about the grain size.

The conclusions need to be completely rewritten (e.g. "the fatigue lives of the alloy decreased with the increase in cycle stress and survival probability" is the normal behavior of any material tested in fatigue and certainly does not need to be evidenced here).

Author Response

Question 1. Last sentence in line 311 seems to be incomplete.

Response: We are sorry for negligence that this is a mistake made by ourselves during adjusting the format and position of pictures and text. We have completed this sentence and checked the whole manuscript carefully to avoid such mistakes in the revised manuscript.

Question 2. In line 473 it is said: "the fatigue strength of the alloy decreased with the increase in grain size (table 3 and fig. 17)" but it seems just the opposite from fig. 17, while table 3 does not say anything about the grain size.

Response: We have checked the whole manuscript carefully to avoid such mistakes. Table 3 mainly presents the relationship between the tensile and fatigue properties of the alloy and emphatically describes the variation trend between the fatigue and tensile properties. Fig. 17 describes the relationship of the grain size of two different microstructures to mechanical properties and the explanation has been shown in the revised manuscript.

Question 3. The conclusions need to be completely rewritten (e.g. "the fatigue lives of the alloy decreased with the increase in cycle stress and survival probability" is the normal behavior of any material tested in fatigue and certainly does not need to be evidenced here).

Response: Thanks for the suggestion of reviewer. The conclusions have been completely rewritten in the revised manuscript according to the comments of reviewer.

Author Response File: Author Response.pdf

Reviewer 3 Report

1. Please explain all abbreviations and markings at work, e.g. FEI, R, and others.
2. Fig. 1 – in figure the values ​​are given, but in which units (in millimeters?). Please specify the unit in the caption Fig. 1.
3. In figs. 6-8 the stress amplitude is denoted Sa, and in formula (2) and in Fig. 9 it is sigma_a - this needs to be made uniform.
4. Whether the cracks were brittle or ductile?
5. It would also be worthwhile to quote the following paper: 1) Rozumek D., Faszynka S., Surface cracks growth in aluminum alloy AW-2017A-T4 under combined loadings. Engineering Fracture Mechanics 2020, 226.

Author Response

Question 1. Please explain all abbreviations and markings at work, e.g. FEI, R, and others.

Response: Thank you for reviewer’s valuable suggestion, most of the markings are abbreviations of companies of laboratory equipment, such as FEI Tecnai G2 F20, KYKY-2008B and QBG-100 High-frequency Fatigue Testing Machine, etc. Where R is stress ratio, which stands for the ratio of the maximum stress to the minimum stress within a cycle. Furthermore, other abbreviations have specific explanations in revised manuscript.

Question 2. Fig. 1 – in figure the values ​​are given, but in which units (in millimeters?). Please specify the unit in the caption Fig. 1.

Response: Thank you for reviewer’s valuable suggestion, we are sorry for negligence that this is a mistake made by ourselves. The units (in millimeters) hae been given in the caption Fig. 1. in revised manuscript.

Question 3. In figs. 6-8 the stress amplitude is denoted Sa, and in formula (2) and in Fig. 9 it is sigma_a - this needs to be made uniform.

Response: Thank you for reviewer’s valuable suggestion. According to the suggestions, we have revised the manuscript carefully and uniformed the marking of stress amplitude as σ_a in the revised manuscript.

Question 4. Whether the cracks were brittle or ductile?

Response: In fact, high cycle fatigue fracture is a kind of brittle fracture under low stress, but some local plastic deformation is produced for materials with good plasticity, such as 211Z.X aluminum alloy with good elongation (＞10%). Therefore, the HCF fracture surface of 211Z.X aluminum alloy mainly display the brittle features. However, some local micro zone in the HCF fracture surface of this alloy show some ductile features.

Question 5. It would also be worthwhile to quote the following paper: 1) Rozumek D., Faszynka S., Surface cracks growth in aluminum alloy AW-2017A-T4 under combined loadings. Engineering Fracture Mechanics 2020, 226.

Response: Thank you for reviewer’s valuable suggestion. According to the suggestions, we have carefully revised the manuscript and the paper. However, the manuscript mainly studies the effect of microstructure on high cycle fatigue behavior of 211Z.X-T6 aluminum alloy. Which explains the effect of microstructure and distribution of precipitated phases and impurities on the mechanical properties. But the reference suggested by the Reviewer, “1) Rozumek D., Faszynka S., Surface cracks growth in aluminum alloy AW-2017A-T4 under combined loadings. Engineering Fracture Mechanics 2020, 226.” focuses on the tests results of the growth of fatigue cracks subjected to torsion and bending with torsion specimens at the constant amplitude loading and with zero or non-zero mean load. Which deals with the effect of combined loadings on the surface cracks growth in aluminum alloy AW-2017A-T4. Therefore, we think this article is not appropriate for reference in this work.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors took into account the comments of the reviewer and made appropriate corrections to the manuscript. The article is interesting and can be accepted in the present form.