The Influence of Absorbing Coating Material on the Efficiency of Laser Shock Peening

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor, 

This paper aims to examine the effect of different absorbing coatings on the efficiency of LSP under identical loading conditions where the residual stress profiles was utilized as a measure of the LSP efficiency.  Although, the results presented here are interesting, some major revisions are required before the manuscript must be considered for publication in Metals as discussed below.

As general comments, the quality of most of figures have to be improved. In particular, the color codes of Fig. 5c and 7 b and c are blurred.  In addition, the error bars are missing for almost all of the reported data, except for Fig. 11.

Specific comments:

1) Lines 60-61 : « In the initial condition the alloy had a globular 60 and a lamellar microstructures to establish  

 the effect of microstructure on residual stress generation.” Seems Misplaced at the end of this   

  paragraph

2) Lines : 63-65: “A comprehensive study of the processes accompanying laser shock peening was 63     

     conducted by investigating the microstructure and microhardness of the specimens after 64 treatment.” 

    This text is confusing. It’s not clear if the authors write about the reported work or the literature. 

    Please consider improving and add refs it concerns the literature.

3)    Line 71: “An increase in dislocation density is often observed in both the [15, 17-20] 71 and [15, 21] phases”. Please correct appropriately this text.

4)    Line 72 : “Of particular interest was the detection of mechanical twinning in the phase [13-16, 19, 22]” same comment as above

5)    Lines 74-75 : “Although significant progress has been made in previous microstructural studies, 75 the literature data are very inconsistent.” please add references

6)    Lines 98-99: “After LSP plates with different initial microstructures were sectioned through the 98 middle of the treated area. After sectioning, the samples were mounted into a conductive phenolic thermoplastic resin with ...” a lot of “after”. Consider improvement.

7)    Line 104: “The abrasive paper with gradual reduction of the abrasivity down 104 to P2000 (the abrasive particle size 7 microns) was used.” Which abrasive paper? May be a formulation such as  “an abrasive paper with gradual reduction of the abrasivity down to P2000 (the abrasive particle size 7 microns) was used” is better.

8)    Line 129 : “The residual stresses were quantified using the hole drilling method,.. “ consider adding references for the drilling method.

9)    Lines 133-134 “The сcurrent strains were quantified using a specially configured three-element strain gauge socket, after which they were utilized to calculate the residual stresses using a specialized software.”  Information about the software or references are required.

10) Line 139: “The residual stresses were then calculated using special calculation algorithms.” Same comment as above.

11) Line 143 : refs are required for the formula (1). What are ,  and E in this formula. Also, at line 144, ajk and bjk need to be corrected

12) Fig 2 and 3, the legends in Figs. 2 and 3 are not readable

13) Fig. 9  What the error bars are?

14) Lines 188-189: the authors state “Moreover, the IQ and KAM maps did not even show a significant change in contrast, which could indirectly indicate a sharp increase in dislocation density (Fig. 5a and b).” Again, at lines 206-207, it is stated “However, this map shows some darkening the contrast (Fig. 7a), which can be interpreted as an increase in the overall density of latticedislocations.” Both change and absence of change in the contrast are interpreted as indication of the dislocation’s density increases. This is confusing for me.

15) In fig. 10.  What the error bars are?  

16) Lines 267-268: “whereas in the case of black paint, this value is 32.08 MPa*mm, in the case of tape 17.64 MPa*mm, and in the case without ab orbing layer 22.37 MPa*mm.” lot of “in case”. Please consider reformulation.

17) Line 268-270 “The observed reduction in integral values for all types of protective coating, with the exception of aluminum foil, can be attributed to the presence of significant tensile stresses (up to 200 MPa) at a depth of 0.5 mm. In the case of tape, this depth is approximately 0.7 mm.” This affirmation can hardly be read in Fig. 10. 

18) Line 177:  Equation (2). What the parameters in the parenthesis mean?

19)  Line 470: ref to be completed

20) Line 475: ref to be completed

Author Response

Comments 1: Lines 60-61 : « In the initial condition the alloy had a globular 60 and a lamellar microstructures to establish  the effect of microstructure on residual stress generation.” Seems Misplaced at the end of this paragraph

Response 1: Thank you for pointing this out. I have moved this sentence to section 2.1.

Comments 2: Lines : 63-65: “A comprehensive study of the processes accompanying laser shock peening was 63 conducted by investigating the microstructure and microhardness of the specimens after 64 treatment.” 

Response 2: In the text of the work, I explained that I was talking about the work described in the article (line 61).

In this work a comprehensive study of the processes accompanying laser shock 62 peening…

Comments 3: Line 71: “An increase in dislocation density is often observed in both the [15, 17-20] 71 and [15, 21] phases”. Please correct appropriately this text.

Response 3: The necessary characters were added to the article text (line 70, 72).

…often 70 observed in both the α [15, 17-20] and β [15, 21] phases.

Comments 4: Line 72 : “Of particular interest was the detection of mechanical twinning in the phase [13-16, 19, 22]” same comment as above

Response 4: This point has also been corrected (line 72).

Of particular interest was the detection of mechanical twinning 72 in the α phase…

Comments 5: Lines 74-75 : “Although significant progress has been made in previous microstructural studies, 75 the literature data are very inconsistent.” please add references

Response 5: Yes, you are right, we will definitely add a link to the works confirming our words a little later.

Comments 6: Lines 98-99: “After LSP plates with different initial microstructures were sectioned through the 98 middle of the treated area. After sectioning, the samples were mounted into a conductive phenolic thermoplastic resin with ...” a lot of “after”. Consider improvement.

Response 6: The text has been corrected (line 99-100).

After LSP plates with different initial microstructures were sectioned through the 100 middle of the treated area. Then sectioning, the samples

Comments 7: Line 104: “The abrasive paper with gradual reduction of the abrasivity down 104 to P2000 (the abrasive particle size 7 microns) was used.” Which abrasive paper? May be a formulation such as  “an abrasive paper with gradual reduction of the abrasivity down to P2000 (the abrasive particle size 7 microns) was used” is better.

Response 7: Thanks for the example, we accepted your wording and included it in the article (line 105).

Comments 8: Line 129 : “The residual stresses were quantified using the hole drilling method,.. “ consider adding references for the drilling method.

Response 8: A reference to the standard has been added to line 130.

Comments 9: Lines 133-134 “The сcurrent strains were quantified using a specially configured three-element strain gauge socket, after which they were utilized to calculate the residual stresses using a specialized software.”  Information about the software or references are required.

Response 9: Added a ref to a work devoted to the methodology for calculating residual stresses [26] (line 135), and also indicated the software (line 136).

Comments 10: Line 139: “The residual stresses were then calculated using special calculation algorithms.” Same comment as above.

Response 10: Reference [26] was added in response to question 9.

Comments 11: Line 143 : refs are required for the formula (1). What are ,  and E in this formula. Also, at line 144, ajk and bjk need to be corrected

Response 11: A ref to the work from which the formula was taken has been added, and the missing description has also been added.

Comments 12: Fig 2 and 3, the legends in Figs. 2 and 3 are not readable

Response 12: Agreed, thank you for bringing this to our attention. Unfortunately, we haven't had time to add clearer pictures yet, but we'll definitely add them in the next edits.

Comments 13: Fig. 9  What the error bars are?

Response 13: There are no error bars in this graph because the residual stress measurements were performed for one sample of each microstructure.

Comments 14: Lines 188-189: the authors state “Moreover, the IQ and KAM maps did not even show a significant change in contrast, which could indirectly indicate a sharp increase in dislocation density (Fig. 5a and b).” Again, at lines 206-207, it is stated “However, this map shows some darkening the contrast (Fig. 7a), which can be interpreted as an increase in the overall density of latticedislocations.” Both change and absence of change in the contrast are interpreted as indication of the dislocation’s density increases. This is confusing for me.

Response 14:

The authors would like to apologize for this controversy. In order to avoid misunderstanding, the text of the revised manuscript has been rephrased as follows (lines 188-189):

…Moreover, the IQ and KAM maps did not even show a significant change in contrast, thus exhibiting no clear evidence of an increase in dislocation density (Fig. 5a and b)… 

Comments 15: In fig. 10.  What the error bars are?

Response 15: Sorry for not adding error bars right away. Replaced fig. 10 (page 8).

Comments 16: Lines 267-268: “whereas in the case of black paint, this value is 32.08 MPa*mm, in the case of tape 17.64 MPa*mm, and in the case without ab orbing layer 22.37 MPa*mm.” lot of “in case”. Please consider reformulation.

Response 16: The sentence has been corrected (line 273).

whereas in the case of black paint, this value is 32.08 MPa*mm, with tape 17.64 MPa*mm, and without absorbing layer 22.37 MPa*mm.

Comments 17: Line 268-270 “The observed reduction in integral values for all types of protective coating, with the exception of aluminum foil, can be attributed to the presence of significant tensile stresses (up to 200 MPa) at a depth of 0.5 mm. In the case of tape, this depth is approximately 0.7 mm.” This affirmation can hardly be read in Fig. 10. 

Response 17: Yes, you have noticed it correctly. The wording has been adjusted based on the graph in Figure 10.

The observed reduction in integral values for all types of protective coating,  with the exception of aluminum foil, can be attributed to the presence of significant ten sile stresses (up to 200 MPa) at a depth of 0.4 mm. In the case of foil, this depth is approximately 0.6 mm.

Comments 18: Line 177:  Equation (2). What the parameters in the parenthesis mean?

Response 18: The dimension P in the original language was given in brackets. We translated it into English (page 9).

Comments 19: Line 470: ref to be completed

Response 19: The ref has been corrected.

Comments 20: Line 475: ref to be completed

Response 20: The ref has been corrected.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the effect of different absorption layers on the residual stress field generated by laser shock peening (LSP) and has obtained some useful results. However, the current manuscript lacks some experimental details and in-depth discussion. After careful review, the following issues need further improvement:

1. In the second section, what is the basis for the authors' choice of an energy density of 10 GW/cm²?

2. Based on my experience, using a higher energy density can lead to the rupture of thin aluminum foil (such as the 80 μm thick foil used in this study) more easily compared to black tape, thereby reducing the maximum value of compressive residual stress. The authors need to clarify whether the aluminum foil or black tape was damaged during the LSP process at the same high energy density.

3. Please include relevant international standards for testing residual stress and microhardness to facilitate replication of the study by other researchers.

4. In line 207, the authors claim that the overall density of lattice dislocations increased, but why did the KAM value not increase? Please explain.

5. The results in Fig. 10 show that in some samples, the compressive residual stress appears below the surface. Please explain this phenomenon.

6. For example, the authors may refer to the following sources:

https://doi.org/10.1016/j.jmst.2024.03.051

https://doi.org/10.1016/j.jallcom.2024.173664

https://doi.org/10.1016/j.matdes.2021.109626

7. The depth of the compressive residual stress generated by different absorption layers is about 0.4 mm, but the hardening layer in samples using tape and aluminum foil is about 0.8 mm thick. Please explain the reason for this phenomenon.

8. In line 362, the authors claim that aluminum foil offers the advantage of rapid application, but in actual production, closely adhering aluminum foil to samples with large surface areas (even very flat surfaces) is still time-consuming. Please rephrase this statement.

9. There are obvious errors in the subscripts in formulas (2), (5-8). Please review and correct them.

Comments on the Quality of English Language

 Moderate editing of English language required.

Author Response

Comments 1: In the second section, what is the basis for the authors' choice of an energy density of 10 GW/cm²?

Response 1: This value was chosen based on our experience in processing this material, as well as on the assumption that at high power densities parasitic breakdown phenomena occur [1], which causes a decrease in processing efficiency [2].

[1] Berthe, L. et al. Shock waves from a water-confined laser-generated plasma. Journal of Applied Physics, 1997, 82, 2826-2832.

[2] Mazhukin V. I., Nosov V. V., Smurov I. Yu. Air plasma expansion during impact laser processing of materials. Mathematical modeling, 2003, 15, 23-42.

Comments 2: Based on my experience, using a higher energy density can lead to the rupture of thin aluminum foil (such as the 80 μm thick foil used in this study) more easily compared to black tape, thereby reducing the maximum value of compressive residual stress. The authors need to clarify whether the aluminum foil or black tape was damaged during the LSP process at the same high energy density.

Response 2: Yes, we absolutely agree with you. At high densities, aluminum foil can be destroyed. However, based on our experience and this study, this coating can withstand 10 GW/cm2, maintaining its integrity. We wrote about this in lines 362-363, but for the sake of specificity, we have adjusted this sentence.

All used coatings withstanded processing with energy density of 10 GW/cm2, maintaining its integrity, and can be used in industry.

Comments 3: Please include relevant international standards for testing residual stress and microhardness to facilitate replication of the study by other researchers.

Response 3: In the article, we briefly described the methodology for measuring residual stresses in section 2.3. Additionally, we added references to measurement and calculation standards [25-28].

Comments 4: In line 207, the authors claim that the overall density of lattice dislocations increased, but why did the KAM value not increase? Please explain.

Response 4: According to the comment, appropriate explanation has been provided in the revised manuscript (Lines 207-***):

However, this map shows some darkening of the contrast (Fig. 7a), which can be interpreted as an increase in the overall density of lattice dislocations. In contrast, the KAM map does not show any significant anomalies (Fig. 7b). This observation can be explained as follows. The dislocations in metallic materials can be broadly categorized into two groups, viz., geometrically-necessary ones and statistically-stored ones [Ashby]. The geometrically-necessary dislocations produce orientation gradients, and thus they may give rise to a contrast in the KAM maps. On the other hand, the statistically-stored dislocations result only in the degradation of the crystal lattice structure (thereby becoming visible in the IQ maps) but provide no orientation gradients. In view of the above issues, the experimental observations in the present work can be attributed to an accumulation of the statistically-stored dislocations.   

[1] ASHBY M F. The deformation of plastically non-homogeneous materials, The Philosophical Magazine, 1970, 21: 399−424.

Comments 5: The results in Fig. 10 show that in some samples, the compressive residual stress appears below the surface. Please explain this phenomenon.

Response 5: The shock waves caused by the laser induce plastic deformation that changes the surface morphology and underlying dislocation microstructure, as well as high residual compressive stresses in near-surface regions [1]. 

In the introduction we described the process itself and the creation of residual stresses.

[1] Kattoura, M., Mannava, S. R., Qian, D., & Vasudevan, V. K. Effect of laser shock peening on residual stress, microstructure and fatigue behavior of ATI 718Plus alloy. Int. J. Fatigue, 2017, 102, 121-134.

Comments 6: For example, the authors may refer to the following sources:

https://doi.org/10.1016/j.jmst.2024.03.051

https://doi.org/10.1016/j.jallcom.2024.173664

https://doi.org/10.1016/j.matdes.2021.109626

Response 6: The answer was written above.

Comments 7: The depth of the compressive residual stress generated by different absorption layers is about 0.4 mm, but the hardening layer in samples using tape and aluminum foil is about 0.8 mm thick. Please explain the reason for this phenomenon.

Response 7: 0.4 mm depth – this is the case with the tape. We associate this phenomenon with a large difference in the acoustic impedances of titanium and the tape, due to which most of the energy does not pass into the sample, thereby reducing the processing efficiency.

Comments 8: In line 362, the authors claim that aluminum foil offers the advantage of rapid application, but in actual production, closely adhering aluminum foil to samples with large surface areas (even very flat surfaces) is still time-consuming. Please rephrase this statement.

Response 8: Yes, you are right, the wording was not entirely clear. In the text of the article we explained that the foil is applied to the sample quickly compared to black paint, which has to be applied in several layers, which significantly increases the preparation time.

Comments 9: There are obvious errors in the subscripts in formulas (2), (5-8). Please review and correct them.

Response 9: All the formulas mentioned have been adjusted.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Modern production, in particular mechanical engineering, requires parts and products with improved performance characteristics. The manufacture of such products by traditional methods often causes significant difficulties. In this regard, the development of new methods of strengthening technologies, an important place among which is occupied by laser shock wave treatment, is of particular relevance. The authors of the article investigated the effect of absorbing coatings, namely black PVC tape with an adhesive layer, aluminum foil and black alkyd paint on the properties of three-millimeter-thick plates of titanium alloy Ti-6Al-4V with globular or lamellar microstructures during laser shock wave processing. LSP of one side of the plates was carried out with a power density of 10 GW/cm2. The hole drilling method was used to estimate the residual stresses. It was found that aluminum foil is the optimal option for LSP of titanium alloy Ti-6Al-4V. The authors' microstructural studies carried out using EBSD analysis showed that LSP did not result in a significant decrease in grain size, twinning or an increase in dislocation density, regardless of the original structure. The obtained results are important for science and practice, but there are several questions:

1. In formula (2), ГПа should be replaced on GPa.

2. The commas in the decimal fraction should be replaced with a period.

3. I suggest that one of the overall dimensions or a scale mark be indicated in Fig. 8. This will allow one to visually assess the scale of what is seen.

4. What is the accuracy of residual stress determination?

5. What was the surface roughness after laser shock wave treatment? Did it change?

6. It is known that during operation of parts after Laser Shock Peening (for example in cyclic loading conditions) relaxation processes will occur. How stable will the treatment effect be?

7. In the reference [21] the number of pages of the book must be indicated, and the place of its publication "Moscow" must be written in full. The list of references must be formatted according to the requirements of the journal.

8. It is necessary to more clearly define the scientific novelty of the article and describe it in the conclusions.

9. I propose to supplement the review with an analysis of the paper: https://www.scientific.net/KEM.592-593.346

Author Response

Comments 1. In formula (2), ГПа should be replaced on GPa.

Response 1: Yes, you are right. The units of measurement have been corrected.

Comments 2. The commas in the decimal fraction should be replaced with a period.

Response 2: Commas were replaced with periods.

Comments 3. I suggest that one of the overall dimensions or a scale mark be indicated in Fig. 8. This will allow one to visually assess the scale of what is seen.

Response 3: Thank you for your comment. The dimensions of the processing area have been added to Figure 8.

Comments 4. What is the accuracy of residual stress determination?

Response 4: Nominal measurement accuracy is 15 MPa.

Comments 5. What was the surface roughness after laser shock wave treatment? Did it change?

Response 5: This is an interesting question. When processing without coating, the surface roughness changes significantly, this is visible even without measurements, when hardening with coatings, the changes are less significant. However, we did not conduct measurements as part of this study. In the future, we plan to do them.

Comments 6. It is known that during operation of parts after Laser Shock Peening (for example in cyclic loading conditions) relaxation processes will occur. How stable will the treatment effect be?

Response 6: Another actual question that we plan to explore in the future. However, at this stage we have not conducted research in this area.

Comments 7. In the reference [21] the number of pages of the book must be indicated, and the place of its publication "Moscow" must be written in full. The list of references must be formatted according to the requirements of the journal.

Response 7: References to sources have been corrected.

Comments 8. It is necessary to more clearly define the scientific novelty of the article and describe it in the conclusions.

Response 8: We have summarized a little what was done in the work in the last paragraph of conclusions (lines 396-399).

Comments 9. I propose to supplement the review with an analysis of the paper: https://www.scientific.net/KEM.592-593.346

Response 9: Thank you very much for the advice. We have reviewed this article and included it in the citation list.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors have satisfactorily incorporated corrections

Author Response

Comments 5: Lines 74-75 : “Although significant progress has been made in previous microstructural studies, 75 the literature data are very inconsistent.” please add references

Response 5: References 24-25 have been added to the text of the paper (line 75).

Comments 12: Fig 2 and 3, the legends in Figs. 2 and 3 are not readable

Response 12: The images indicated have been replaced with more readable ones. We have also replaced fig. 5 and 7.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Accept.

Author Response

In the last round we answered all the reviewer's questions. We did not see any new comments in the second round.