Round 1

Reviewer 1 Report (Previous Reviewer 1)

The authors having responded to all of my comments, I therefore recommend their article for publication in Journal of Composites Science.

Just improve the quality of your figures 4, 7, 9 and 10 which appear blured in your pdf file.

Author Response

Figures 4, 7, 9 and 10 have been improved to make the font bolder and more legible. 

Reviewer 2 Report (Previous Reviewer 3)

No comments

Author Response

No comments identified

Round 2

Reviewer 1 Report (Previous Reviewer 1)

No comments. Figures have been improved as requested.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Review jcs-1898604 of « The Effect of Laser Shock Peening Treatment on the anti-Corrosion Performance of LENS built Ti-6Al-4V alloy » authored by Sharlotte Mamatebele Kubjane, Patricia Popoola, Sisa Pityana and Nana Arthur.

 

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This manuscript reports about the application of laser shock peening treatment to improve the anti-corrosion performance of a Ti-6Al-4V alloy. An interesting feature of this work is the use of a specific AM-processed material, produced by a 3D printing Laser Engineered Net-Shaping system (LENS). Overall, I think the work is interesting and could be publised in Journal of Composites Science. However, I do have a number of concerns and suggestions for improvement, which are listed below, and should be addressed before publication.

 

Comment 1:

When you first describe the Laser Shock Peening (LSP) process in your introduction (from line 67 to line 71), you mention that the laser pulse is focused on a ablative layer (black tape or aluminium foil). Later in your manuscript, when you describe the laser setup and parameters used in your experiments (in section 2, from line 134 to line 138), you mention that no protective coating was used. I think that this inconsistency might be confusing for people which are not familiar with the LSP process. You should write in your introduction that LSP can be performed in two different configurations. In the original one, the process is performed with a sacrificial protective overlay used to protect the sample from the thermal effects of the confined plasma, and in this case, LSP is a pure mechanical process. This configuration uses large focal spots (³1 mm) with no overlapping, or very low overlapping. In the second configuration, which is now often called Laser peening without coating (LPwC), the process is performed without any protective coating, using small focal spots and very large overlapping rates. In this case, LSP is a thermo-mechanical process.  

For the origial configuration, you can cite the following references:

P. Peyre and R. Fabbro, Optical and Quantum Electronics 27, 1213 (1995); doi:10.1007/BF00326477

A. H. Clauer, Metals 9, 626 (2019); doi:10.3390/met9060626

For LPwC, you can cite the following references:

Y. Sano et al., Journal of Laser Micro/Nanoengineering 1, 161 (2006) ; doi:10.2961/jlmn.2006.03.0002

Y. Sano, Metals 10, 152 (2020) ; doi:10.3390/met10010152

 

Comment 2:

At line 95, you write : Kanjer et al. [19] investigated …

This is not the right reference. It should be replaced by Guo et al. [15].

 

Comment 3:

At lines 99 and 100, you write : … as a result of the formation of an aluminium-rich layer thet prevented oxygen diffusion. Confirming that LSP provides improved oxidation protection.

It should be written in the form: … as a result of the formation of an aluminium-rich layer thet prevented oxygen diffusion, confirming that LSP provides improved oxidation protection.

 

Comment 4:

At line 101, you write : Karthik et al. [20] investigated …

This is not the right reference. It should be replaced by Kanjer et al. [19].

 

Comment 5:

In section 2, at lines 116 and 117, you mention that laser energy density plays a crucial role in the development of the microstruture, and in your Table 1, you describe four different sets of parameters that were used for the preparation of LENS-built samples. However, later in your paper, you only show results for the 249 J/mm³ sample, and do not talk about the influence of the laser energy density on your results. Could you please comment on this.

 

Comment 6:

In section 2, from lines 134 to 138, when you describe the laser parameters used for the LSP treatment, the pulse duration (and temporal profile) is missing.  You specify a power intensity of 4.46 GW/cm². What is the loading pressure imparted to your sample with this power density ?

 

Comment 7:

In section 2, at lines 135 and 136, you mention that you used flowing water and no protective coating for your LSP treatment. The choice of this LPwC configuration rather than the origial LSP configuration with a protective overlay, is rather surprising for an anti-corrosion application, given that the direct ablation of the sample surface is known to promote the oxidation of the sample surface. Could you please comment on this and justify the choice of your LSP configuration without protective overlay.

 

Comment 8:

In section 2, at line 138, you mention that you use 90, 95 and 99% overlapping, but you do not give any detail about the overlap path on the sample. Do you use the same overlap rate on both x and y directions ? Did you moved the sample in zigzag direction ?

 

Comment 9:

In section 3.1, at lines 198 and 199, you write: Figure 3(a-b) displays … The XRD peaks at both graphs clearly show …

There is only one graph in your Figure 3, so you should either add the missing graph or correct your text and figure’s legend consequently.

 

Comment 10:

In section 3.1, Figure 3 should me modified. First, te XRD spectrum should be flattened, and given that there are no usefull data below 15° and above 100°, please restrict the graph in the range 15-100°. Replace 2q by 2q (deg.) and replace Intensity by Intensity (a.u.).

 

Comment 11:

In section 3.2 you write at lines 223-225 : Since LSP generates shock waves, the hardness is expected to decrease with depth for treated samples as compared to untreated samples.

I do not understand this statement. After LSP, the evolution of the hardness as a function of the thickness seems to be material dependent. Kanjer et al. [19] found no gradient in the hardness profiles from the surface to the core of the material for their cp-Ti samples. So you should remove this sentence, unless you present in depth measurements corroborating this statement.

 

Comment 12:

In section 3.2 (lines 228 to 233) you mention that roughness of the treated samples increases when the overlap rate increases from 90 to 99%, and you conclude that this implies that a decrease in properties such as corrosion rate and fatigue life could thus be observed when overlap above 95 % is used. I think this statement should be mitigated, given that other processes can balance the increase in roughness (see comment 15).

 

Comment 13:

In section 3.3 (lines 258 to 260) you write: The corrosion rate is seen to improve significantly from 0.063 mm/year for un-peened samples to 0.003 mm/year when a 90% overlap was used, …

The value of 0.003 mm/year is for 95% overlap in your Table3. It is 0.004 mm/year for 90% overlap. Please correct the value.

 

Comment 14:

In section 3.4, at line 297, replace of which by which.

 

Comment 15:

In section 3.4, at lines 310-312, you write: Sample A1-95% had better oxidation resistance as compared to the other samples with minimal weight gain. The sample also showed better corrosion resistance.

This is in contradiction with your conclusion in section 3.2, where you claim that overlap rate should be kept below 95 % to limit the surface rougthness (see comment 12).

 

Comment 16:

In figure 6, there is no curve for 99% overlap, whereas you always compare the results for the three overlap rates in all the previous sections. Please add the corresponding curve.

 

Comment 17:

In section 4, at lines 324-326 your write: The energy input, measured as the energy density has a positive effect on the microstructural evolution as it reduces the formation of defects, which results in improved mechanical properties.

All along your paper, you only showed results for the 249 J/mm³ sample, and never addressed the evolution of the results as a function the energy density (see comment 5). You should thus provide some results before concluding that it improves mechanical properties.

 

Comment 18:

In section 4, at line 327 your write: The ED range of 200 to 250 J/mm3 used to fabricate Ti6Al4V samples …

This range is different than the one shown in Table 1 which goes up to 332 J/mm³. Why ?

 

Comment 19:

In section 4, at lines 336-338 your write: The use of LSP treatment with high overlaps more than 95% induced an increase in surface roughness (greater than 4 Ra), which could potentially reduce the fatigue life of the Ti alloy.

You give no proof of this statement. Even at 95% overlap, you get a rougthness of 4.6 mm, and despite this relatively high value, this samples has the best oxidation and corrosion resistance properties.

 

General comment :

You should improve the quality of your Figures which is really low. They all appear to be blured which makes their reading quite difficult for the reader.

 

 

Reviewer 2 Report

The current manuscript is a study on the effect of laser shock peening on the anti-corrosion performance of LENS-built Ti-6Al-4V alloy. Some microstructural analyses were conducted as well as some simplified electrochemical measurements. The topic is interesting to readers in a relevant field, however, the study does not seem to be well completed. There is a significant lacking of proper discussion on the current results, which makes the manuscript read like an experimental report. Some interpretations and conclusions could not be fully supported by the presented results and need further experimental proof (e.g. pitting could not be seen from the current characterization results). Unfortunately, the reviewer could not recommend it for publishing. Some detailed comments are listed below:

- figure 2 does not have the proper resolution to show the microstructural features, and even the scale bar could not be seen clearly. Please provide better-quality images.

- figure 3 has only subfigure (a) but it has been mentioned a (b) in line 198.

- line 283 to 288: there are no characterization results on the pitting of the samples.

- line 290: should be Figure 6.

- conclusions 1 to 3 seem to be irrelevant to the presented results. The effect of energy density was not presented nor analyzed in the context.

 

Reviewer 3 Report

The manuscript deals with the surface modification by laser peening of Ti6Al4V additively processed by LENS, with special emphasis on the effect on its corrosion and oxidation behaviour, which are surface related properties. In addition, subsurface hardness profiles were performed on cross-sections. Overall the document fails to correlate results with conclusions and therefore I recommend to reject it in its present state. The following comment are relevant:

- according to the abstract and the introduction, the problem-solving approach focus on the mechanical failure of the components at high temperatures, but this topic is not directly addressed in this study. All references in the manuscript (results and discussion, conclusions) to the surface modification induced effects on the fatigue strength is then highly speculative and should be eliminated, especially in the conclusions.

- page 3 line 121; check the description of “t”

- page 4, lines 181-184. Red arrows reveals a zone with a minor contrast but, why it is correlated with the presence of compressive residual stresses? (it is a speculation)

-page 4, line 188. Hardness profiles are not reported in the manuscript, then it is a speculative

- page 5, lines 201-203. Ref 24 study the system Al-Cu-Fe/Ti alloy, thus it make sense to speak of dilution of Ti, buy this is not applicable to explain the formation of intermetallic phases in this study. Are intermetallic phases visible under SEM analysis?  Ti6Al4V is a biphasic alloy (alfa/beta), however, the spectrum does not reveal  the beta phase, why? XRD results must be reviewed.

- page 5 lines 213-215. Correlation between hardness and corrosion resistance is meaningless

- page 6 lines 219-220. The presence of residual tensile stresses will contribute to decrease hardness, no to increase them . Overall this paragraph is misleading, what does mean closer to the substrate?.

- Page 6, line 237. Table 2 for hardness is confuse. What does mean maximum and minimum values? STD deviation must be associated to each value. It seems that overlapping differences has not statistical meaning

-Page 6, line 241. Please add the standard deviation of the measurement of roughness. Have the differences statistical meaning?

-Page 7 line 260. Data of corrosion rate does not correspond to that of table 3.

- Page 7 Table 3. Polarization resistance values are not described. Differences are relevant and should be explained.

- Page 7 line 279 . Passive layer (usually of few nm in thickness) form spontaneously. On the other hand, why correlate hardness with corrosion? It is meaningless

-  Page 8 line 289. Oxidation behaviour of an alloy is scientifically studied by considering the time and temperature dependence of the mass gain (i.e. scale thickness) by means of isothermal tests. TGA is useful to follow the mass variation but is not appropriated to study the oxidation kinetic, thus any comment on the parabolic or linear kinetic is speculative.

-Page 8 line 303. Is there any evidence of the scale spallation? Mass gain increase could correspond to an increase in scale thickness… Overall the study of the oxidation behaviour is uncomplete.

-since corrosion and oxidation resistance are surface related properties, surface examination is very important. Relevant chemical changes on the outer most surface has been reported after LSP of Ti6Al4V alloy [Lara Crespo et al . On the interactions of human bone cells with Ti6Al4V thermally oxidized by means of laser shock processing 2016 Biomed. Mater. 11 015009 ].

-A more detailed analysis is required to explain the different behaviour of the treated samples.