The Influence of Pre- and Post-Heat Treatment on Mechanical Properties and Microstructures in Friction Stir Welding of Dissimilar Age-Hardenable Aluminum Alloys

Round 1

Reviewer 1 Report

In this study, an Al-Mg-Si alloy 6061 and an Al-Zn-Mg alloy 7A52 were joined by friction stir welding followed by a variety of pre- and post-heat treatments to improve the strength of the weld. The best weld joint with the lowest hardness of 100 HV in 6061 matrix, achieved by post solid-solution and subsequent two-stage artificial aging for the whole weld joint of the 7A52 and 6061 solid solution where the strength of the weld nugget was between the strength of each side and was achieved by precipitation hardening through η' and L precipitation where η' precipitation is originated from the 7A52 chemistry and the L phase from the 6061.

The introduction needs improvement. Some of the references have been placed out of context. Such instances are highlighted within the text, which can be found in the manuscript file attached to this review. The authors should rewrite the parts that are related to the FSW of dissimilar Al alloys. There is no need to cite works on FSW of other groups of materials, as the focus has to be on the dissimilar joints between 6xxx and 7xxx series Al alloys.  In the current format, the introduction section does not prepare the background for the rest of the article. The authors claim that no work has been done on the effect of heat treatment of the FSW joint at the atomic level. This claim needs more clarification. Moreover, the authors suggest that the present work explains "the underlying physics of precipitation kinetics at nano, even atomic level." Though this study presents some results in the "nano" scale, it fails to give a deep understanding of the physics of precipitation kinetics at the atomic level.

The hardness results shown corresponding to different heat treatment regimes seem sound and interesting.

The SEM image and corresponding maps shown in Figure 5 could be removed as they do not add any new data to the text and are of low quality as well.

In terms of grain size evolution after heat treatment (section 3.2.2.), authors pointed out an abnormal grain growth in WNZ without elaborating on why the growth is considered abnormal.

In the microstructure section 3.2.2, the authors claim that all TEM images for the 6062 alloy are obtained with the beam parallel to [001] zone axis. However, the HRTEM images seem to be completely off-axis, as is evident from the FFT patterns.

It is not clear why all images of the 7A52 are shown in [112] zone axis of the Al matrix.

In general, the TEM work presented is weak, and except for Fig 8a, Fig 10a, and Fig 10C, the rest, which includes the HRTEM work, are not suitable for publication.

Overall, this paper presents sound data regarding the welding and heat treatment of the dissimilar FSW joints. The hardness tests and EBSD work are valid and interesting. However, TEM work is flawed and not suitable for publication. The TEM work needs to be appropriately redone before considering for publication.

Comments for author File: Comments.pdf

Author Response

Thanks for your kind comments. We have reply your comments point to point as the attached world file. Looking forward to your further good suggestion.

Author Response File: Author Response.pdf

Reviewer 2 Report

The submitted manuscript deals with a dissimilar friction stir welding process and the analysis of the effects induced by pre and post welding heat treatments. Aluminum alloys 6061 and 7A52 were successfully joined, treated and inspected.

In referee's opinion, there is something valuable in this paper, in particular regarding the microstructure characterization, nevertheless, it is evident that the paper cannot be accepted in this form due to relevant drawbacks, as indicated in what follows:

- overall, the paper is too long considering its actual content. The introduction is neither focused nor appealing. Most of this section is composed assembling very general info about FSW that are redundant to specialists. Authors should consider that FSW is nowadays a well known welding technology, that detailed description of its fundamentals is not needed. On the other hand the analysis of dissimilar friction stir welding process is not comprehensive. The papers are simply recalled without highlighting any relevant findings useful to go through this work. Finally, authors completely ignored several contribution to dissimilar FSW (check, for instance the work done by A. Astarita et al. or K.P. Mehta et al. and revise accordingly).

- The experimental procedure should be more precisely reported. It is not clear why authors cared about indicating the full rotational speed of the used machine without explaining the rationale behind the selection of the actually used parameters. A doublecheck of this section should be done, considering that all relevant details must be provided in order to make the work replicable by other researchers;

- The performed analysis is not satisfactory since very basic tests (tensile tests for instance) were not even considered by the authors. In this regard authors should also be more precise in the definition and description of mechanical properties, since, it is not straightforward to state that higher hardness implies higher resistance. In particular, in the case of dissimilar joining, it is typically observed that hardness peaks induce a brittle behavior reducing the mechanical properties of the joint. A more detailed investigation and a general revision is mandatory. 

- The reported results are well presented and described, however, at the moment, the discussion is definitely poor. Please consider to merge sections 3 and 4, dramatically improving the quality of the discussion (providing mechanisms, comparing data with literature analysis).

- The bibliography should be revised as indicated above. Furthermore, it seems that there is multiple citation of the same work (see refs 39 and 40). It is surprising that that work was authored by some of the present authors!

- Last but not least. The article is hardly readable due to the very poor language and style. Authors are invited to take advantage of some professional editing service before resubmitting this work.

The rejection of the paper, with the option of resubmission, is recommended.

Author Response

Thanks for your kind comments. We have reply your comments point to point as the attached world file. Looking forward to your further good suggestion.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I want to thank the reviewers for taking the time and going through the revision process. All in all, the quality of the paper has significantly improved; however, there are still some issues that need to be addressed, especially in the discussion and conclusion sections.

I have provided my comments and suggestion through the manuscript in the attached pdf document that the authors should carefully review and address. 

Comments for author File: Comments.pdf

Author Response

Many thanks reviewer for the micromesh comments. We have improved the paper point by point according to the comments. We are looking forward to your kind and positive reply. 

Reviewer 2 Report

The originally submitted paper has been partially revised by the authors as per referee's suggestions.

Nevertheless the authors failed in revising the introduction and the state of the art analysis as well as in providing stronger evidence of the mechanical characterization of the joints, therefore they are warmly invited to:

- make a proper selection of the bibliography. 66 references for this kind of work is hardly acceptable and not all the cited work are fundamental to this submission;

- extend the experimental analysis including at least some tensile testing (microhardness measurements is definitely not enough).

Another round of major revision is recommended.

Author Response

Thanks reviewer for the kind comments. We have replied point by point. Looking forward to your further good suggestions.

Round 3

Reviewer 2 Report

The reference list is acceptable now.

Regarding the data from tensile tests, they sould somehow discussed in the text and not only send to the referee. Readers will be interested in having a detaied and comprehensive analysis from this study and there is no reason to exclude inherent data. Please revise accordingly

Author Response

Many thanks for the kind suggestions. We have added the tensile results and fracture analysis in the revised edition. After double-check the tensile results, we found a group of tensile properties in the response is wrong, we have corrected that. Thanks again for the precious review time.

Round 4

Reviewer 2 Report

The paper is now eligible for publication.