Investigation of the Formability of Cryogenic Rolled AA6061 and Its Improvement Using Artificial Aging Treatment

Round 1

Reviewer 1 Report

This manuscript studied the mechanical properties (i.e. FLD) of AA6061 alloy by artificial ageing treatment (AAT). Frankly, AAT of Al alloy is very important for expanding its application. The formability is enhanced after AAT is impressive. In addition, it also shown some points of interests, but also need to address following weak points:

1.     The originality of this research should be indicated more clearly.

2.     It is better to provide hardness curve in terms of aging time, only 25h or 50h is not enough.

3.     Why these is no microstructure of such alloys in various conditions?

4.     The scalebar is missing in Fig. 1, Fig. 3 and Fig. 4.

5.     According to Fig. 5a and b, why the elastic modulus is different after AAT?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

It contains innovation in terms of subject. However, the article needs further development in terms of writing. I think that a major revision is necessary for the article.

My detailed comments were given below.

·         In the abstract, there are two different terms expressing the same situation as artificial aging and heat treatment. I think it could use only artificial aging.

In addition, in the conclusion part of the summary, information should be given about which conditions the best formability is achieved and the optimum formability conditions. It is not necessary to give minor strain range values.

·         The introductory part should be fully developed. In the first paragraph of the introduction, the importance of aluminum alloy, the sectors in which it is used, and the properties of the 6000 series should be given in more detail.

·         In addition, a literature review was not presented. There are different studies in the literature about cryogenic rolling, artificial aging, and shaping conditions of 6061 aluminum alloy. After the literature review, the literature review and the novelty content of the study should be specified.

·         Who is the manufacturer of the 6061 aluminum alloy plate? How was its chemical composition determined?

·         One of the most important shortcomings I see in the article is that there are no microstructure photographs of the material depending on the conditions. Microstructure photographs of the base material as rolled and subjected to different artificial aging processes should be attached.

·         Figure 1 shows tensile test specimens cut in three different directions, but the photograph is missing on the right.

·         Crosshead speed is given for tensile tests. However, the strain rate value must be given in addition to this value.

·         The schematic representation of the Nakajima test is given in Figure 2, but this figure does not belong to the authors. It is 90% similar to the figure used in the article titled "Forming-limit diagrams and strain-rate-dependent mechanical properties of AA6019-T4 and AA6061-T4 aluminum sheet materials". Therefore, a reference should be given. In addition, this article can be referenced for literature research in the introduction.

·         Figure 4 is not clear enough. Therefore, it needs to be updated.

·         The tensile curve of the base material can also be added to the article for comparison.

·         Commas are used on the X-axis of Figure 5b graph. The comma should be replaced with a period.

·         For Table 3, it can be written for 0,45,90 degrees as rolling direction, diagonal direction, and transverse direction.

·         The conclusion section can be written in more detail.

·         When the references are examined, there are old references. It should be renewed with newer references.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Line 32: Could you please provide an example of cryogenic temperature?

Line 41:Precipitation of alloying elements is not correct as an expression. Precipitation of a phase is more appropriate.

Line 50: "Researchers have addressed several materials in this sense". Please include the references of the aforementioned studies. Do these studies refer to Al alloys in general, 6xxx Al alloy or other materials?

Lines 51 - 52: "Still, there is ... of age hardening". Please clarify the gap that your research is going to fill because from the references it seems that similar studies have already been conducted.

Lines 52 - 53: "Therefore, the present study ... and its corresponding FLD." Please elaborate more the scope of this study because it is not clear.

CR procedure: A schematic representation of the CR procedure would be helpful.

Line 73: "... are kept below 150C ..." this part needs rephrasing since the peak temperature for artificial aging is 150C.

Line 76: According to reference [14], the cryogenic rolling accelerates precipitation of metastable phase β' and β''. The authors claim that the beneficial phases are GP zones and β'' phases whereas β' is more stable and thus results in strength reduction which is contradictory to the finding of reference [14].

Results and Discussion

For overall better understanding you should either split results and discussion into two different section or entail discussion in every result. Apart from this observation, the discussion part lacks in the analysis of the results. Below you will find several points that need to be addressed:

Figure 5: Why stress strain curves are presented in different positions (parallel, 45 degrees, perpendicular to the rolling direction)? How does temperature and time affect precipitation and dislocation density?

Lines 149-150: "The Stress - Strain curves ... in the CR condition". There are cases (Figure 5a and b) in which the samples heat treated after CR have lower strength than CR condition. Could you please explain why is this happening?

Lines 154 - 155: "The strength of the CR samples ... high dislocation density [26]." CR samples are in the solid state condition since they are processed at cryogenic temperatures. I do not see how this statement justifies the findings of your results.

Lines 159 - 160: 100B does not decrease significantly in strength as shown in Figures 5a and b. In Figure 5a the strength in 100B is slightly lower compared to CR condition whereas in Figure 5b the strength in 100B is slightly higher compared to CR condition. 

Lines 164 - 167: "This behavior is ... strengthening precipitate". These statements require references.

Lines 174 - 178: "Solid solution treatment ... 0.32% weight percent respectively". This is a rough approximation since the reference are not for the exact chemical composition of the current samples. Weight percent in solid solution is directly correlated with the chemical composition. A phase diagram for the specific chemical composition is advised to used.

Lines 180 - 181: "Simultaneously ... to be predominately β'' ''. Please check again the phase at 150C from reference [14] because I understand that β' phase forms at 150C

Lines 181 - 183: "Furthermore, ... at about 150C". For this statement either reference is required or evidence based on experimental results.

The conclusions are basically the experimental results of this study. It is not clear what conclusions have been derived regarding precipitation and dislocation rearrangement and how to exploit precipitation and dislocation movement/recovery in order to tailor the strength and formability of the alloy.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revised manuscript looks much better now and I am satisfied with it.

I suggested to accepted as it is.

Author Response

Special thanks to you for your good comments.

Reviewer 2 Report

The article is highly improved by revision. It is acceptable for publication.

Author Response

Special thanks to you for your good comments.

Reviewer 3 Report

Lines 58 - 59: The scope of this study is given with more clarity than previously. However, in this study various post - CR heat treatments have tested and their corresponding FLDs in order to evaluate which heat treatment is more appropriate. If authors agree with the previous sentence, please modify the final sentence accordingly.

Lines 139 - 154: This paragraph is more suitable for the discussion section. The results of each figure must be referenced in the results section.

Lines 181 - 185: In this paragraph, the results are described therefore is more suitable at the results section.

Line 187: Please add in text the Taylor equation as it explains the increase in yield strength due to the presence of dislocations.

Line 188: the what does the high dislocation density do? The sentence stops abruptly.

Lines 218 - 223: As mentioned in the previous comment, this is a rough approximation since not all 6061 alloys have exactly the same chemical composition and this has an impact on the content of Mg and Si in solid solution. So you can't use the exact amount of Mg and Si in solid solution found in other studies also time and temperature play significant role in the amount of Mg and Si in solid solution.

Lines 227 - 231: Since both GP zones and β'' phase are favorable why the superior balance of strength and formability is achieved at 150C for 50h?

The overall text has been improved. However, since the explanation about the increase in strength relies on the presence of precipitates, experimental evidence of precipitation is required in order to show the effect of the particular aging in the specific alloy. The explanation relying solely on literature is not adequate since the precipitation is affected by soaking time and the specific chemical composition of the alloy.

Author Response

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Author Response File: Author Response.docx

Round 3

Reviewer 3 Report

Overall, the paper has improved significantly. The introduction provides an overview of the topic and state of the art. The scope of this study is clear in this version while the paper is well structured, results are presented with clarity and discussion remains within topic and supports the conclusions. The conclusions rely on results and address the scope. Regarding the metallography, now that the scope is defined  in this version and it is addressed adequately with the current results, discussion and conclusions, it is not necessary to include metallography. Therefore, the overall recommendation is accept in the current form.