Peening Natural Aging of Aluminum Alloy by Ultra-High-Temperature and High-Pressure Cavitation

Round 1

Reviewer 1 Report

Authors present a novel method using MFC. From the introduction, I see that this method is only posted by authors. I doubt the validity of this method.  However, if the author can analyze the sound pressure field in the working gap, I would like to recommend this paper to publish.

Important: the ultrasonic cavitation bubbles are generated by sound pressure. Thus, the analysis of the sound pressure is necessary in this paper.

Author Response

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Reviewer 2 Report

Dear authors,

the article is well written and clearly present the results. I find them interesting and this work deserves publication.

As far as this is a special issue on cavitation and not an issue dedicated to material science, I would suggest you to develop a little bit in the introduction the aim of material peening. I think you could also cite reference textbooks about mettalurgy such as this chapter (and maybe the entire book): https://doi.org/10.1016/B978-008044495-6/50008-2

I also think the title of figure 7 is not correct: it should be LPTC process and not UTPC if I refer to the parameters (20 MPa, 100W).

 

 

Author Response

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Reviewer 3 Report

In my opinion, this manuscript needs major improvement. Too many assumptions are given as dogmas in the paper. Below are my comments:

1. line 69-71: As the Authors have written "In this study, the outermost surface of a material was solution-treated using UTPC processing to raise the temperature to at least 500 °C higher than the equilibrium phase diagram of the aluminium alloy", please write in detail how the temperature of the outermost surface was measured. 
2. Table 2: Why are Cu and Zn listed in Table 1 when these elements are not present in this alloy?
3. line 97-98: Please describe in detail the measurement of the pressure inside the bubble during collapse using the pump discharge pressure. 
4. Please describe in detail how the ultrasonic output controls the temperature inside the bubble.
5. lines 105-107: Because the Authors have used ultra-high-temperature and high-pressure cavitation please write the exact temperature of this process. What temperature is when low-temperature and high-pressure cavitation is used. How the temperature was measured?
6. As the Authors have written "The processing time was changed to 2 min, 5 min, 10 min, 20 min, and 30 min to increase the outermost surface temperature to the solution heat temperature and clarify the time required for the solution treatment", please write in detail how the temperature of the outermost surface was measured. 
7. What was the standoff distance between the sample and the water jet nozzle?
8. line 128: As Ref.[22] is the work of Yoshimura, T. and Ijiri, M. please add the temperature of the hot spots and temperature inside the bubble when LTPC s applied. 
9. Please add the scale bars to all images in Figures 3 and 4.
10. Please show the place of roughness measurements and the sampling and evaluation length. In cavitation, these values are very important. As Figs 3 and 4 show, the place of roughness measurements determines the roughness. 
11. Lines 159-161: Please show these results.
12. Figures 6 and 7: The location where the hardness is measured is very important. Please indicate where the hardness was measured. How many indenters/measurements have been taken?
13. Was the temperature measured during the experiments? On what basis the authors write about the increase/decrease of the temperature of the outermost surface. These are just their speculations. 
14. lines 182-184: As X-ray diffractions were performed, they must be added to prove the phase change over time.
15. lines195-196: Since the temperature was not measured, such a statement cannot be written. There is no direct evidence of an increase in temperature. Suggestions for the effect of temperature exist, but these are still just suggestions. 
16. Figures 6 and 7: Please add the error (or standard deviation) bars.
17. line 211-212: What temperature was reached at the surface during the LTPC process.
18. Please add an image showing the region of interest on the surface after UTPC and LTPC processing.
19. Please describe what you mean by "the frequency distribution of the ROI analysis intensity". I think that the presentation of the content of individual elements is more representative and legible than the used "frequency distribution of the ROI analysis intensity".
20. line 271: please provide the temperature and describe the mechanism of the temperature increase 72 days after the UTPC processing. How the temperature was measured?
21. Table 2: Please describe what UTPC (1), UTPC (2), UTPC (3) and UTPC (4) mean? 
22. line 309: What do you mean by the surface roughness was small. Please add the value of the Ra (Sa) parameter, for example.
23. lines 310-312: I do not understand this sentence: "In contrast, in UTPC 60-min processing, peening solution processing was performed up to 30 min; however, as the surface roughness increases with processing, the microjet impingement is not always vertical, and despite processing under UTPC conditions, UTPC processing does not occur." Please clarify why it is written: In UTPC processing, UTPC processing does not occur. 
24. and the next sentence: "This is because the processing was performed at low temperature and low pressure" UTPC is, according to the Authors "  ultra-high- temperature and high-pressure cavitation (UTPC) processing.

Author Response

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Round 2

Reviewer 1 Report

You said;"the sound pressure of the ultrasonic output 100 W is 5 mV under the LTPC condition." So you mean the sound pressure is 5mV? The unit of the pressure is Pa! I do not know what you described.

Author Response

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Reviewer 3 Report

The explanation given by the Authors regarding the temperature is not convincing. For this reason, I still believe that this is over-interpretation and speculation. 

According to Surface Engineering and Applied Electrochemistry journal website, the latest issue is Volume 56, Issue 6, November 2020, so the paper "T. Yoshimura, D. Shimonishi, D. Hashimoto, N. Nishijima, and M. Ijiri, Effect of Processing). Degree and Nozzle Diameter, on Multifunction Cavitation, Surface Engineering and Applied Electrochemistry, 2021, Vol. 57, No. 1, pp. 106–116.)." does not exist.

Authors have said that they have calculated the collapse pressure of individual bubbles using the Rayleigh-Plesset equation. Thus, I suppose that they know the exact number and size of cavitation bubbles just before the collapse and at the last stage of the collapse during your experiments and the exact collapse time. Thus, please add the distribution of the cavitation bubbles (correlation between their number and size before the collapse and just after the collapse, as well as the location of the collapse). I recommend the papers of W. Lauterborn, Ultrasonics Sonochemistry 14 (2007) 484–491 and in J.Fluid Mech. (1975) vol.72, part 2, 391-399, and Experimental Thermal and Fluid Science 84 (2017) 179–189.

No X-ray diffraction has been presented to prove that aging has taken place. I expect the presentation of X-ray diffractions with phase composition before and after cavitation. If aging has taken place, precipitations should appear. Hardness measurements are too little. However, they show evident changes in the aluminium alloy after cavitation erosion studies. They show that for all the tests performed at the UTPC, the trend of changes in hardness was the same, and in all the tests performed at the LTPC, the trend of changes was the same, but different than at the UTPC. The variation in hardness may, but not have to be related to a temperature effect. Clear and unequivocal evidence of an increase in temperature on the surface of the eroded material would be an important contribution to the knowledge of cavitation erosion. Unfortunately, so far there is only circumstantial evidence. The high temperature inside the bubble, determined as presented by the authors of this manuscript, does not prove that such a temperature arises on the surface of the tested material. In the cavitation cloud, there are millions of bubbles of different sizes and filled with different levels of vapour. According to the Rayleigh-Plesset theory and equation, the bubble size and the vapour level in the bubbles influence the theoretical temperature generated at the final stage of cavitation bubble implosion. In addition, the bubbles implode at different distances from the surface of the solid, so the water film affects the final temperature. Water immediately reduces the generated temperature. 

Author Response

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Round 3

Reviewer 1 Report

I do not agree what you answered.  It's not rigorous!

Reviewer 3 Report

I have no comments.